TL: DUMPING SEWAGE SLUDGE AT SEA 
SO: Greenpeace UK (GP)
DT: May 1989
Keywords: toxics oceans sewage sludge waste disposal ocean
dumping greenpeace uk reports gp /

REPORT FOR GREENPEACE UK

BY

MIRANDA MACQUITTY Ph.D.

May 1989


TABLE OF CONTENTS

1. Origin and composition of sewage sludge

2. UK disposal of sewage sludge

     2.1 Sewage sludge production and disposal routes

     2.2 UK policy on dumping sewage sludge at sea

3. Disposal of sewage sludge by other European countries

     3.1 Sewage sludge production and disposal routes

     3.2 European view of the acceptability of dumping sewage  
         sludge at sea

4. Disposal of sewage sludge by the USA

5. Alternatives to marine disposal

     5.1 Productive use of sewage sludge

     5.2 Unproductive disposal of sewage sludge

     5.3 Cost considerations

6. Sewages sludge quality

     6.1 Contamination

     6.2 Controls and improvements

     6.3 Monitoring for marine dumping

7. Fate of sewage sludge dumped at sea

     7.1 Site characteristics

     7.2 Choice of sites - accumulating versus dispersing sites

     7.3 Recovery of sites

8. Environmental impact of dumping sewage sludge at sea


     8.1 Effect on the benthos

     8.2 Assessment by biological response tests

          8.2.1 Bioassays
          8.2.2 Stress tests

     8.3 Contaminants

          8.3.1 Field assessment
          8.3.2 Laboratory assessment

     8.4 nutrients

     8.5 Fish and crustacean diseases

     8.6 Bacteria, viruses and protozoans

9. Sewage sludge dumping by Ireland

10. Summary

11. Conclusions

References


1. Origin and composition of sewage sludge

Sewage sludge is the residue left after the treatment of the
liquid and solid wastes discharged into sewers. Sewage is either
of domestic origin or a mixture of both domestic and industrial
wastes. During the first stage of treatment, sewage is screened
to remove grit and large solids and then is settled in
sedimentation tanks. The solids which settle-out are known as
primary or raw sludge and are disposed of separately to the
liquid supernatant. In addition to these solids, secondary
sludges are produced from the further treatment (secondary
treatment) of the liquid fraction of sewage. (1,2,3). Secondary
treatment increases the amount of sludge produced by a factor of
two compared to primary treatment solely, as more particulates
are extracted (123). Secondary sludges (usually surplus
activated sludge) are often added to primary sludges before
disposal (4).

The composition of sewage sludge varies according to its origin
and treatment but all sludges have a high water content (about
93-97%). The solids consist mostly of organic matter like
cellulose fibres, fatty acids and oils. Sludge contains traces
of toxic substances including metals, organochlorines,
pesticides, detergents,and petroleum hydrocarbons (2, 5, 83,
84). The degree of contamination of sludge depends upon the
extent and type of industrial wastes entering the sewers in the
catchment area. Where storm water runoff contributes to the
inflow of sewers, it is also a source of contaminants,
particularly lead and petroleum hydrocarbons (35). Some sludges
contain high levels of dissolved ammonia which may be toxic to
marine life. Nutrients, like nitrogen and phosphorous, are
another component of sewage sludge which may be considered
pollutants if they contribute to eutrophication or excessive
algal growth. The solid phase of the sludge contains 95% of the
nutrients and toxic contaminants (15).

The bacterial content of sludge varies according to treatment
but includes methogenic bacteria (which help to degrade the
sludge), faecal bacteria and sometimes pathogenic bacteria, like
Salmonella. Viruses are also present and may include pathogenic
enteroviruses. Other pathogens, such as the eggs of tapeworms
and roundworms, fungi, and cysts of protozoans, may be present
(6, 123). The long term survival of bacteria, viruses and
protozoa in the marine environment is not well understood (5).

Sewage sludge may be digested at sewage treatment works before
disposal so that the organic material is broken down by natural
fermentation of the sludge. Digestion reduces the numbers of
pathogens and the Biological Oxygen Demand (BOD) which could
significantly depress dissolved oxygen levels (1, 3, 7). In
addition, digestion reduces the organic matter by 30-40% so
there is less sludge for disposal (6).

In general, sewage treatment is not an effective way to bring
about reduction in the total amount of contaminants through
biological, physical or chemical breakdown processes (123).
However, chromium can be reduced to a relatively insoluble and
less toxic form (+3 valence form). Some organic contaminants
undergo microbial transformation during sewage treatment.
Digestion results in the degradation of phenols, some phthalates
and chlorinated pesticides (83). Other contaminants, such as
some of the PCBs, are more resistant to breakdown [see section
6.1]. While the degradation products of some contaminants are
more toxic that the original compounds (123) (See section 6.1]. 

2. UK disposal of sewage sludge

2.1 Sewage sludge production and disposal routes

The amount of sludge produced by a country depends on the size
of the population and the degree of development of the sanitary
system (7). The UK has about 95% of its population served by
sewers and is the second largest producer of sludge in Europe
(8). Per head of population the UK produces 27 kg/year of sewage
sludge (dry weight) in comparison to Belgium where 7 kg/year per
person is produced as only 55% of the population is served by
sewers (7). In total the UK produces about 30 million tonnes
(wet weight) of sewage sludge annually which represents about
1.5 million tonnes (dry weight) (8, 9). After treatment of the
sludge 50% is used as soil conditioner and fertiliser on
agricultural and other land, 16% is tipped (landfill), 4% is
incinerated and the remaining 30% is disposed at sea. Most of
the sludge disposed at sea is dumped by vessels (28% of the
total) and the remainder (2%) is discharged by pipeline (10).

Approximately 9 million tonnes of sewage sludge is dumped at 14
sites around the UK (Table 1). Over 75% of this sludge is
disposed at three major sites: Barrow Deep, Thames Estuary;
Liverpool Bay; and Garroch Head, Firth of Clyde and arises
respectively from the three population centres of London, the
Mersey basin and Strathclyde. The total amount of sewage sludge
dumped at sea has remained fairly stable since 1981 (11, 85).
The amount of sewage sludge dumped by Thames Water Authority
(TWA) reached a maximum of 5 million tonnes wet weight in 1979
(23) but declined to 4.2-4.5 million tonnes per year (1983 to
1986). Latest figures indicate a substantial reduction from 4.5
million tonnes in 1986 to 3.9 million tonnes wet weight in 1988
(85, pers.comm. McEvoy, 127). This drop is also reflected in the
dry weight figures 99 000 dry tonnes dumped in 1985 and 89 000
tonnes dumped in 1988 (86, 127) and is due to changes in the
amount of sludge stored at the treatment works concerned (127).
TWA are installing centrifuges at Crossness and Beckton sewage
works to thicken the sludge from about 2.5% dry solids to 6-6.5%
dry solids in order to reduce its shipping costs. It is
anticipated that not all the sludge will have to be centrifuged
to reach the required volume reduction (127). As the centrifuges
become fully operational wet weight figures for TWA sludge
dumped will continue to fall but the dry weight figures are
expected to stabilize in the long term (7, 127). The current
licence permits TWA to dump 3.6 million tonnes wet weight at
4.1% solids.

The amount of sludge dumped by North West Water Authority (NWWA)
in Liverpool Bay is increasing due to the new Sandon Dock sewage
treatment scheme where sewage formerly discharged to the Mersey
Estuary is being treated. The amount of sewage sludge produced
will exceed the amount currently licensed for disposal in
Liverpool Bay by the mid 1990s (12, 13)1. Lothian Council
anticipates that its sewage sludge production will also increase
as untreated discharges are diverted to treatment works (88).

The majority of sewage sludge dumped at sea is digested at
treatment works and includes about 3.6 million tonnes (wet
weight) from Thames Water Authority and about 1.6 million tonnes
(wet weight) from NWWA. The rest of the sewage sludge dumped at
sea is mainly undigested primary sludge.

Sewage sludge dumping comes under the Food and Environment
Protection Act 1985, Part II (which replaces the Dumping at Sea
Act 1974) and requires licences to be issued. These designate
the boundaries of the dumping grounds and dictate the quantity
of sludge that ray be dumped. The licensing authorities are the
Ministry of Agriculture, Fisheries and Food (MAFF) in England
and Wales, Department of Agriculture, Fisheries and Food for
Scotland (DAFS) and Department of the Environment (DOE) for
northern Ireland. The Food and Environment Protection Act 1985,
Part II implements the provisions of the London Dumping
Convention (a global agreement) and the Oslo Dumping Convention
North Sea and North East Atlantic). (North Sea and North East
Atlantic).

1 NWWA currently dumps approximately 67,000 dry tonnes per year
into Liverpool Bay and is licensed to dump up to 80,000 dry
tonnes. It is anticipated that by the mid 1990s dumping may
increase to 90,000 dry tonnes (13, 87).

Dumping sites off the coasts of England and Wales are surveyed
by MAFF and the water authorities, and those off the coast of
Scotland by the Scottish Marine Biological Association and the
Forth River Purification Board under requirements set out by
DAFS (14, 15). Additional surveys have been carried out by the
DOE (especially of Liverpool Bay) and by consultants under
contract to the water authorities. Despite the imminent
privatisation of the water industry MAFF is passing on some of
the responsibility of field monitoring to the water authorities
(89). MAFF anticipates that this will generate more
environmental impact data. It remains to be seen if this
information will be published as is the case for the major MAFF
surveys which are documented in the fisheries technical report
series (albeit up to ten years after the survey). By comparison
there is virtually no published information available on the
impact of dumping liquid industrial wastes at sea by UK
companies.

It is unclear if the National Rivers Authority will have any
role in monitoring sewage sludge disposal. It is envisaged that
the NRA will be concerned with tidal waters and coastal waters
up to 3 miles from land. 

Table 1. Sewage sludge dumped dt sea by the UK in 1987

(Omitted .. unscannable)

2.2 UK policy on dumping sewage sludge at sea

Since its inception in the 1890s and early 1900s, marine dumping
has been regarded as a convenient way to get rid of the large
quantities of sewage sludge arising from cities such as London,
Manchester and Glasgow with easy access to the sea. This
attitude has continued, typified by the report from a government
working party entitled 'Out of Sight, Out of Mind' (16) although
with increasing awareness of the environmental aspects of sewage
sludge disposal. In 1981 the Department of the Environment's
Standing Committee on the Disposal of Sewage Sludge (4) reported
that:

"on operational and economic grounds and also on the basis of
current known environmental effects, deposition of sewage sludge
at sea is an acceptable disposal method particularly for large
urban areas with ready access to the sea."

The UK generally follows the 'Best Practicable Environmental
Option' (BPEO) approach in its waste disposal strategy where the
option that has the least environmental impact is selected (3,
17). In evaluating the BPEO scientific, technical, practical,
economic and geographic actors are also taken into consideration
(11).

The Royal commission on Environmental Pollution (90) emphasised
that BPEO:

"is a procedure that would lead, if properly implemented, to
reductions in environmental pollution and to improvements of the
environment as a whole."

However, the Royal Commission noted that BPEO may be
misinterpreted, and used to describe:

"any course of action which takes some account of environmental
factors".

Marine disposal of sewage sludge is considered by the UK water
authorities, such as Thames Water Authority as the BPEO (19,
although this is yet to be established as the House of Commons
Environment Committee (20) pointed out in May 1987:

"MAFF and the DOE need to demonstrate by published reports a
thorough investigation of the best practicable environmental
option for sewage sludge and sewage disposal, and should report
their findings to Parliament."

This long-awaited information should be included in the first
annual report on disposal of waste at sea by MAFF which is due
to be published sometime this year (91).

Other European nations adopt the 'Precautionary Principle' which
may preclude the use of certain disposal routes, such as dumping
at sea, because of the uncertainties regarding their
environmental effects (18, 92, 93).

The UK now accepts that in certain instances a precautionary
approach may be required. In the North Sea Declaration (94), the
UK agreed that:

"to protect the North Sea from possibly damaging effects of the
most dangerous substances, a precautionary approach is
necessary".

The UK is obliged under its commitments to the Oslo Convention
and the London Dumping Convention to consider the practical
availability of alternative means of disposal. This is
implemented under the Food and Environment Protection Act 1985,
Part II (FEPA) which requires the licensing authority to have
regard to the practical availability of alternative means of
disposal or treatment when determining whether to issue a
licence for sea disposal of wastes.

However, since the North Sea Conference the UK's commitments
concerning dumping of wastes at sea are more stringent than
those required by the Oslo and London Dumping Conventions.

The Ministerial Declaration (94) states that:

"as from 1 January 1989, no material should be dumped in the
North Sea unless there are no practical alternatives on land and
it can be shown to the competent international organisations
that the materials pose no risks to the marine environment." 
[emphasis added]

Furthermore, these changes are not restricted to the North Sea
as the Department of the Environment's guidance note (95) on the
Ministerial Declaration states that:

"the changes of policy implied by the Declaration ....will in
general be applied consistently throughout the United Kingdom."

Despite considerable opposition at the North Sea Conference, the
sea disposal route for sewage sludge is preserved as an option.
However the Declaration (94) required:

"urgent action, in the case of those countries that dispose of
sewage sludge in the North Sea, to reduce the contamination of
such sludges by persistent, toxic or bioaccumulable materials,
so that they pose no hazard to the marine environment, and
ensure that the quantities of such contaminants disposed to sea
by this pathway in the immediate future do not increase above
1987 levels."

A definitive list of contaminants is still to be published but
an action list including Annex I and Annex II metals from the
Oslo Convention, and a range of organic contaminants is being
considered by the regulatory authorities in consultation with
the water industry (88, 95). These may be supplemented by other
substances on the Department of the Environment's Red List as
necessary. Monitoring of organic contaminants in English and
Welsh sludges disposed to sea has proved to be inadequate and a
crash programme to analyse sludges was instigated last year in
order to establish "1987" baseline levels. (88) [see section
6.3]. OSCOM had already recognised the same problem in compiling
data for 1985 and concluded that:

"The organohalogen data available is inadequate to represent a
realistic assessment of the total amounts contained in sewage
sludge dumped." (21)

In order to impose limits and reductions of the quantities of
contaminants disposed to sea by sewage sludge dumping the UK
authorities are planning to divide the marine disposal
operations into two groups: those that dump sludge into the
North Sea and those which dump elsewhere (88). Limits will be
set for each group as a whole rather than for a particular
sludge dumping operation (88). This means that disposal
authorities effectively will be able to trade either quantities
of sludge or contaminants so that the entire quantity of
contaminants dumped for that group does not exceed 1987 levels.
This gives disposal authorities which are increasing their
sludge production (Lothian and NWWA - see section 2.1) room to
manoeuvre. On the other hand some sludge disposers could merely
take advantage of another's improvements in sludge quality.

3. Disposal of sewage sludge by other European Countries

3.1 Sewage sludge production and disposal routes

The UK and Ireland are the only European countries to dump
sewage sludge at sea. Ireland dumps 234,320 tonnes compared to
9 million tonnes (wet weight) dumped each year by the UK (85,
96). However, the Netherlands discharges 9% of its sewage sludge
into the sea by pipeline (1.8 million wet tonnes) but this
practice will be terminated by 1990 on completion of new
treatment works, when the sludge will be disposed on land (20,
22).

During the period from 1974-1980 (in addition to the countries
above) two other European countries dumped sewage sludge at sea;
the Federal Republic of Germany (FRG) dumped 2% of its annual
sludge production (approximately 34 000 tonnes dry weight) and
Norway dumped 5% (approximately 2 500 tonnes dry weight) (23).
FRG dumped sewage sludge into the German Bight from 1961 to 1980
but due to adverse environmental impact the dump site was
shifted to the Atlantic Ocean (18). In 1983 FRG stopped dumping
at sea entirely.

Where the marine disposal route is not considered an option,
sewage sludge is for the most part used on agricultural land or
disposed in landfill. The FRG which ranks as Europe's largest
producer of sewage sludge (2.2 million tonnes dry weight in
1977, 8) disposes of 15-20% on agricultural land, 60-70% in
landfill, and 15-20% by other routes such as incineration (24).
The amount disposed on agricultural land has decreased since
1974 due to the stricter quality standards set for
concentrations of heavy metals. Sweden in comparison produces
200 000 tonnes (dry weight) of sludge per year, of which 60% is
used as a soil conditioner and 40% is disposed in landfill (25).
Overall EC countries (excluding Portugal) produced 5 564 000
tonnes of sludge (dry solids) in 1984 (97). Forty-four percent
was disposed of to landfill, 37% was used productively in
agriculture, land reclamation, horticulture, forestry and park
land, 9% disposed of to sea and 3% went to a variety of minor
uses such as processed products.

3.2 European view of the acceptability of dumping sewage sludge
at sea

The dumping of sewage sludge at sea is regarded at least as a
cause for concern by other European nations and for many the
practice is unacceptable. Proposals for new legislation have
been put forward which would require the UK to reduce the
quantity of sewage sludge dumped at sea or to stop dumping
entirely. The proposal for a Council directive by the Commission
of European Communities [com (85) 373 final 12/7/85] on the
dumping of waste at sea was interpreted as requiring either a
ban on marine sludge dumping or a 10% reduction per year for
five years from 1990 (17, 19, 26).

In June 1987 the Nordic Nations (Denmark, Finland, Iceland,
Norway and Sweden) submitted a proposal (OSCOX 13/3/2-E) at the
thirteenth meeting of the Oslo Commission for a recommendation
on the reduction and cessation of dumping sewage sludge at sea.
This recommendation was that sewage sludge dumping should be
gradually reduced and should cease altogether by 1991. Although
the proposal was not passed, the Nordic countries themselves
have a policy not to dump sewage sludge and industrial wastes at
sea which they consider an internationally shared natural
resource. A similar proposal is being made to this year's OSCO
meeting in Dublin (12 -14 June) (Booth, pers. comm.).

The strength of European concern has been noted ln the third
report by the House of Commons Environment Committee (20) in May
1987:

"UK policy of dumping sewage sludge at sea is regarded as
unacceptable by other nations and the UK is becoming ever more
isolated regarding this."

The report went on to state that:

"It seems unlikely that the UK can count indefinitely on sea
dumping as an open-ended option for disposal of sewage sludge." 

The state of the North Sea is of major concern, especially
regarding the build-up of pollutants in the water and sediments
in areas such as the Wadden Sea and German Bight and build-up of
nutrients in the coastal belt, from the Netherlands to north of
Denmark, and in the Skagerrak and Kattegat (27,28). Sewage
sludge dumping adds to both input of contaminants and nutrients
in the North Sea [see 8.3, 8.4]. Further research is required to
establish the significance of sewage sludge dumping but there is
concern that wastes dumped off the British coast not only
disperse into North Sea but could reaccumulate in areas such as
the Wadden Sea. [see 7.2] (17).

The UK is the only country to dump sewage sludge into the North
Sea, with Thames Water Authority being responsible for 80% of
the sludge dumped (19). The Netherlands Ministry of Transport
and Water officials voiced their concern over this practice in
1987 (20).

4. Disposal of sewage sludge by the USA

The majority of US sludge production is disposed on land (31% on
agricultural land and 24% on other land) or incinerated (21%).
The remainder is disposed at sea through pipelines and dumping
(18%) (8). The sewage sludge dumped at sea arises from the New
York and New Jersey metropolitan areas. Until 1986 this sludge
was dumped at a shallow (approximately 27 metres in depth)
nearshore site within the New York Bight Apex known as the 12-
Mile Site (29). But due to the adverse environmental effects of
sludge dumping this site is no longer in use.

Since 1973, the US Environmental Protection Agency (EPA)
designated the New York area only as an interim site and ln 1981
the EPA refused to renew dumping permits. This was contested by
the dumping authorities in New York and New Jersey so that the
final denials of their petitions for redesignation of the 12-
Mile site occurred in 1985.

The main reasons for the EPA closing the 12-Mile site are (29)

1. Increase in the volume of sewage sludge (from 4.5 million
tonnes in 1972 to 7 million tonnes in 1985) would preclude
dumping in a manner that would meet the water quality standards
without causing navigational hazards.

2. Sewage-related contaminants had spread along the ocean floor.

3. Elevated levels of sludge-associated bacteria contributed to
the closure of 150 acres for commercial shell fishing.

4. Heavy metals and organohalogens had accumulated in bottom
sediments.

Sewage sludge is now dumped at a more dispersive deep water site
off the continental shelf known as the 106-Mile Deepwater
municipal Sludge Disposal Site. This site is 170 km from the
nearest landfall and 1 432 - 2 800 metres in depth. Currently,
almost half of the 8.13 million tonnes of sludge dumped at this
site arises from New York City (122).

Following recent state and federal legislation both sewage
sludge and industrial waste dumping should stop by 1991 (122).
Under the amendment to the federal marine Protection, Research
and Sanctuaries Act (Ocean Dumping Act) interim dumping permits
must be approved by the EPA and dumpers also have to work with
the EPA to produce a schedule to end dumping. If any of the nine
New York and New Jersey municipalities that dump sewage sludge
fail to meet the 1991 deadline they will be faced with a rising
scale of fines. A provision adopted from the New Jersey state
bill on dumping means that any money collected will be put in a
trust fund for dumping alternatives.

It is EPA policy to encourage the recycling of sewage sludge
[see section 5.].

5. Alternatives to marine disposal

5.1 Productive use of sewage sludge


Almost half the sewage sludge in the UK is used productively by
spreading on agricultural land. The value of this is two-fold,
the nitrogen and phosphorous present in the sludge fertilise the
soil and the need for artificial fertilisers is reduced. The
organic material in sludge also conditions the soil. Potential
savings in fertiliser costs have been estimated as over L15 m
(9)2. Total UK sewage sludge production represents about 4-5% of
the national agricultural requirement for fertiliser. Sewage
sludge is only used on 1% of agricultural land. Water
authorities, like Thames Water (TWA), with large urban centres
do not consider this particular route as an alternative to
marine disposal because of the distances between treatment works
and agricultural land (19).

Levels of contaminants such as heavy metals pose further
limitations on the use of sludge on agricultural land. New
statutory controls for sewage sludge application to agricultural
land are being introduced further to the EC directive
86/278/EEC. The UK has opted to set limits for the concentration
of heavy metals in sludge amended soils and for the annual
average rate of addition of heavy metals in sludge applications.
(98) Sewage sludge disposed to sea could be used in agriculture,
although application of sludges with relatively high levels of
heavy metals could rapidly exceed the permitted concentration in
the soil. Approximately 10% of UK sludge is currently too highly
contaminated for use on agricultural land and is incinerated or
disposed to landfill.

2 Phosphorous is a limited and non-renewable world resource so
recycling should be encouraged.

Sewage sludge is also used productively in land reclamation
schemes. Only 7% of UK sludge disposed to land is used in this
way (30). In 1984 Environmental Data Services Ltd (31) pointed
to the enormous potential of this disposal route; if sludge was
used on all the land reclaimed each year this would provide an
outlet for half of the UK's annual sludge production.

A study by the Water Research Centre (WRC) estimates that 30% of
sludge disposed on land (238 000 dry tonnes) potentially could
be used in land reclamation schemes. The potential is greatest
in the north of England where 40% of the total sludge production
of Northumbrian Water Authority, North West Water Authority, and
Yorkshire Water Authority could be used on derelict land in need
of reclamation. All these authorities dump sludge at sea (Table
1.)

Land reclamation schemes are often not considered a viable
alternative to sea disposal by the authorities because of
operational and economic considerations. The WRC noted the main
constraint is the need for long-term planning by the disposal
authorities and reclamation industries in order that sludge
production is matched against the availability of reclamation
sites.


The WRC is also investigating the use of sewage sludge as a
forest fertiliser (32). The results indicate that sludge
application can increase the growth rate of trees and
investigations are continuing to establish guidelines for the
effective use of sludge. Latest reports show that the growth of
trees treated with sludge may exceed those treated with
inorganic fertilisers (124). Furthermore, unlike some
agricultural use of sewage sludge, application is possible
throughout the year as coniferous forests take up nutrients in
the winter months. In Scotland there is potential for 30% of the
total annual sludge production to be used as a forest
fertilisers. About 77% of Scottish sludge is currently dumped at
sea by Strathclyde Regional Council (SRC). Operational trials
are underway by SRC to recycle some sludge to forestry. Fifty
hectares of sitka spruce growing on an former open cast coal
mine have been treated with sludge (124).

In addition to use on land there are a variety of processes
where useful products could be extracted from sludge. The 1984
ENDS (31) noted that:

"Sewage sludge, in fact, is a large secondary resource whose
constituents have a notional value of L200 million per year."

Potential products include: composts, building materials, and
use of fats and proteins (99, 100). A number of recent technical
developments indicate that thermal conversion of sewage sludge
to fuel oil can be cost-effective (101, 102). If the
technologies for these products are actively developed sewage
sludge may no longer be regarded as waste but as a valuable
commodity. The House of Commons Environment Committee (20)
stressed in their third report (1987) that:

"....other disposal routes which convert sludge into useful
products will seem more desirable and their viability should be
researched and reviewed thoroughly."

There have been some reports that sewage sludge enhances the
productivity of the sea and could be deliberately used for this
purpose (33). Parker and McIntyre (1987, 7) reviewed this
concept and concluded:

"..in view of the lack of understanding of the eutrophication
processes, marine disposal of sludge could not yet be carried
out with the precision of agricultural fertilisation, not least
because it is difficult to calculate and control the dosage
required."

The US Environmental Protection Agency emphasises the importance
not only of the beneficial reuse of sewage sludge but also the
possibility to reduce the amount of sludge produced by reducing
volume of domestic waste discharged to sewer. For example,
vegetable wastes could be used in home composts rather than
putting them down a home waste disposal unit (126). There are
also new sewage treatment methods under development that
generate less sludge than traditional methods (101). 

5.2 Unproductive disposal of sewage sludge

Landfill

Landfill is considered a possible alternative to sea disposal by
water authorities, such as Thames Water (TWA), providing
additional sites could be acquired (19). About 16% of the UK's
sludge production already is disposed in landfill but there is
substantial competition for landfill sites in urban areas for
other types of waste, such as refuse, making increase in sludge
tipping problematical. One of the problems with landfill is the
possible leaching of contaminants into groundwater.

Incineration

Incineration is also proposed as an alternative for sludge
currently dumped at sea. The House of Commons Environment
Committee (20) considered that incineration may increasingly
become the preferred option for sludge disposals. Currently 4%
of the UK's sludge production is incinerated. In some heavily
industrialised regions such as those of Severn Trent Water
Authority and Yorkshire Water Authority sewage sludge is
incinerated because high levels of contaminants prohibit
disposal by other routes (30). Incineration still leaves some
30% the original dry weight to be disposed of as ash (6, 103).
It can also contribute to air pollution, releasing highly
volatile metals such as mercury and cadmium into the atmosphere
(6, 103). Incineration has come under increasing criticism due
to the environmental impact of aerial emissions (including those
contributing to the "greenhouse effect"), high energy costs, and
problems of disposal of the products of incineration (126).

5.3 Cost considerations

Cost is obviously of prime importance to the authorities when
evaluating disposal routes. The marine disposal route is
particularly attractive because for authorities with easy access
to the sea it is cheapest with operating costs running at about
L1 per wet tonne (19, 34). Generally, it can be seen that sea
disposal is the cheapest and incineration the most expensive
(6). In a 1987 evaluation by Thames Water Authority (TWA), use
of land (tipping of sludge cake) as alternative to sea disposal
would cost approximately 1.5 times as much and incineration
would cost twice as much. However, after payment of initial
capital costs of L7.6 - L15.8 m, annual operating costs for land
disposal (L3.6 - L5.0 m per annum) were similar to TWA's
existing marine disposal operations (L4.6 m per annum). TWA is
reducing the costs of marine sludge disposal by thickening the
sludge so fewer dumping trips are required.

The House of Commons Environment Committee (20) emphasised that
the high degree of investment of the water authorities in the
marine disposal routes meant "they are resistant to any
suggestion of change". North West Water Authority has invested
L23 m in the new Sandon Dock development therefore increasing
the amount of sludge they dump at sea (albeit improving the
condition of the Mersey Estuary by treating the crude sewage
which was formerly discharged). Indeed, the total cost of
abandoning the sea disposal route would be high. In 1984 the
Water Authorities Association calculated the rise in national
costs would be L85 m in initial capital costs and then L32 m per
year for operating costs.

With the imminent privatisation of the water authorities it is
hardly surprising that the UK government is so keen to protect
the sea disposal route.

6. Sewage sludge quality

6.1 Contamination

Sludge quality is dependent on the origin and treatment of the
material entering the sewage works. The two main sources of this
material are domestic sewage and industrial discharges.
Stormwater run-off also contributes to the inflow to the sewers
in some areas and may be an important source of contaminants
such as lead and petroleum hydrocarbons (35). The concentration
of metals is higher in sewage works receiving domestic sewage
mixed with industrial discharges than in works treating only
domestic sewage (35). During primary sedimentation metals that
exist in particulate form and those adsorbed onto settleable
suspended solids will accumulate in sewage sludge (36). Between
40-70% of metals such as cadmium, chromium, copper and lead are
removed from liquid fraction of sewage to the sludge at this
stage of treatment.

Other contaminants such as industrial PCBs (polychlorinated
biphenyls)3 and pesticides also end up in sludge. Of the total
inflow into a Los Angeles sewage plant 6% of PGBs, 12% of DDTs
and 14% of aldrins accumulated in digested sludge (37). Other
studies indicate that up to 70% of PCBs are removed during
sewage treatment and substantial quantities of both PCBs and
organochlorine pesticides are adsorbed by sludge (83). There is
some evidence that the lesser chlorinated biphenyls are more
likely to be degraded by sludge bacteria but tri- and
pentachlorobiphenyls are resistant to attack (83, 84).

3 PCBs and pesticides such as DDT, BHC and aldrins are
organochlorine compounds (one group of organohalogens) which are
persistent, man-made chemicals that can bioaccumulate in animals
and transfer through the food chain to exert harmful effects on
birds and mammals.

Sewage sludge arising from the treatment of industrial wastes
and domestic sewage generally has higher levels of contaminants.
Table 2. compares the levels of metals in sludges which are
dumped at sea from the main London sewage works with those of
purely domestic sludge. The highest levels of heavy metals are
for the Thames Water Authority (TWA) sludge from Beckton sewage
works and are indicative of the extent that industrial wastes
enter the sewers in this region. TWA treats more trade effluent
than any other authority except for North West Water Authority
(Table 3).

Published data on organochlorine levels in sludge dumped at sea
are scarce, apart from in Scottish sludges (table 4.). This is
because analysis is difficult and regular monitoring is not
undertaken by MAFF or by the water authorities (10). TWA
historically have had composite bulked samples of sludge
analysed for a range of organochlorine pesticides but the method
used (DOE SCA standard method) is subject to considerable
interference and the compounds were not verified by GC/MS (pers.
comm McEVoy, 127). According to TWA in general only gamma-HCH
and dieldrin have been found above the detection limits of this
method, typical values are <0.07 mg kgm dry matter and <0.15 mg
kgm dry matter respectively. Because of the inadequacy of the
detection method TWA point out that these results must be
treated with some caution. Work is underway to provide more
reliable quantitative information. 

Table 2. Quality of sewage sludge

(Omitted .. unscannable)

Table 3. Volume of trade effluent discharged to sewer 1987/88
(Megalitres/day)

North West Water Authority         258.4

Thames Water Authority             221.8

Severn Treat Water Authority       204.0

Yorkshire Water Authority          152.2

Anglian Water Authority             92.8

Welsh Water Authority               73.6

Southern Water Authority            39.8

Northumbrian Water Authority        45.1

Wessex Water Authority              33.1

South West Water Authority           9.7

Source - Water Facts 1988


Table 4. Organochlorines in sewage sludge dumped in
Scottish waters 1984-1985 (14).


                          organochlorines mg kgm dry weight

Authority                 PCB              BHC     Dieldrin

Lothian Regional          0.2              0.04    0.22
Council                   (Arochlor 1242)

Strathclyde Regional      0.32             0.94    0.14
Council


While there is some information on organochlorine pesticides and
PCBs in sludges, data on other contaminants such as the newer
pesticides (eg eulans and mitins used in the woollen industry)
and surfactants used in detergents are rarer still. A Water
Research Centre report undertaken for the DOE in 1987 emphasised
that:

"little is known about the diversity of xenobiotic organics that
enter sewage treatment works, the efficiency with which they are
degraded during sewage and sludge treatment processes and their
concentrations in sludge for disposal" (83).

For example, substances used in detergents have caused concern.
Alkylphenols and linear alkylbenzene sulphonates (LAS) are
biodegradable substitutes used in detergents to reduce the
problem of foaming on surface water from the synthetic
detergents* (83, 104). During anaerobic digestion of sewage
sludge alkylphenols are transformed into 4-nonylphenol which is
known to be toxic to aquatic organisms. Furthermore
concentrations of nonylphenols reach exceptionally high levels
in sewage sludge (up to 4 mg gm dry weight) (105). Preliminary
investigations by MAFF indicate that the acute toxicity of
nonylphenols to marine life is probably avoided due to its
dilution when sewage sludge is dumped. Further investigations
are underway to assess the chronic toxicity of nonylphenols and
potential for bioaccumulation by fish and shellfish.

*Since 1976 The Soap and Detergent Industry Association (UK) has
had a voluntary agreement not to use alkylphenol ethoxylates in
domestic detergents because of problems which might occur as a
result of its widespread use. (106).

LAS also occurs in high concentrations in digested sludges (1.2%
dry matter or 11.9 g kgm) but are less toxic than nonylphenols.
LAS do at least degrade under aerobic conditions when sludge is
applied to land (106).

6. Controls and improvements

Major industrial discharges to the sewer system are usually
controlled by the authorities (England and Wales) and regional
councils (Scotland) who issue consents for this practice.
Substances that are banned or controlled include corrosive
chemicals which could damage the structure of sewers,
potentially dangerous chemicals such as inflammable substances,
toxic chemicals that could impair the biological processes at
the treatment works (such as cyanide) and potential pollutants.
There has to be co-operation between the discharger and the
authority in order that the consent specifies the toxic
chemicals expected to be present in the effluent. However, it is
not considered possible to specify every substance which could
have an adverse effect on sewers, sewer disposal systems and the
receiving water course (38). Consents and monitoring data on
industrial discharges to sewers are confidential. After
privatisation consents will be made available to the public but
monitoring data will remain confidential (128).

Despite the authorities' control efforts, industrial wastes may
also enter sewers from unauthorised discharges and chemicals may
be spilled by accident into the sewer without the authorities
being alerted. Industries with consents may also discharge
illegal amounts of effluent. It is difficult to trace such
pollution incidents back to the originator once they have
occurred. So the inflow into the sewage works could contain a
number of unknown and possibly toxic substances. Some toxic
chemicals, such as cyanide, will deactivate the bacteria
required for the biological treatment of sewage in which case
they should be easily detected at the sewage works. Other
chemicals do not have this affect so that they will only be
detected if specifically included in the sewage works monitoring
programme. Therefore pollutants could be retained in sludge and
disposed at sea.

For example MacKay (39) describes how at Garroch Head, Firth of
Clyde:

"large quantities of PCLs were discharged unwittingly before
they were recognized as a major pollutant, or even known to be
a component of sludge."

Sewage sludge dumping in the 1960s contributed about one tonne
per year of PCBs to the Clyde dumping site until controls were
introduced (2).

It is generally regarded as impractical, too expensive and in
some cases not technically possible to improve sludge quality by
the removal of contaminants from sludge (19). Metal removal from
sludge is however technically possible and TWA is developing the
use of solubilisation agents but processes such as these have
yet to be applied to large scale operations (34). Over the past
ten years, metal levels in sewage sludge have fallen
substantially due to tighter controls on industrial discharges
into sewers and due to the decline of heavy industry (34, 40).
As metals also come from numerous diffuse sources (including
domestic sewage) it is often stated by the disposal authorities
that there is little scope for further reductions. However,
judging by the fall in metals levels in London sludge from 1987
to 1988 (Table 2.) improvements are clearly still possible.
Furthermore, controls on the use of metals, such as lead in
petrol and paints, should also lower the amounts present in
sludge (7). As more environmentally safe products become
available to the consumer and if householders are educated not
to dispose of potentially toxic household wastes down drains
further reduction in contaminant levels should occur.

In order to meet its commitment in the North Sea Declaration the
UK should reduce the contamination levels of sewage sludge
dumped at sea. Further efforts to clean up industrial effluent
discharged into sewers or even separate treatment of industrial
wastes will be necessary to bring about a reduction in
contaminant levels. OSCOM regards the reduction of contamination
at source is the "most effective means of avoiding pollution
from dumping sewage sludge". (21). Indeed, TWA are currently
undertaking a survey to identify point sources so that the
concentrations of metals can be reduced "wherever practicable"
(pers. comm. McEvoy, 127). It is more equitable that
contaminants be reduced at source whereby the clean-up costs are
borne by the producer rather than by the sludge disposer.

However, there is obviously a financial incentive for sewage
authorities to treat industrial wastes and some sewage treatment
works, such as Ellesmere Port, are run as joint sewage schemes
(20). North West Water encourages traders to divert discharges
to sewer and generally regard the treatment of industrial wastes
in admixture with domestic sewage as the best practicable
environmental option. (pers. comm Harper, 125). Where sewage
treatment works are heavily subsidized by industry and with the
privatisation of the water authorities there will be concern
whether quality standards are strictly maintained.

Contaminant inputs to the North Sea and other seas around the UK
from sewage sludge dumping, obviously, could increase if there
is an increase in the quantities of sludge dumped. The disposal
authorities are to include ln their action plans, produced as a
result of the North Sea Declaration, forecasts of the total
quantities of sewage sludge planned for sea disposal [see
section 2.1 & 2.2]. Increases in the amount of sewage sludge
produced are anticipated from authorities such as NWWA and
Lothian Council where untreated sewage discharges are being
diverted into sewage treatment works. The plan to regulate
contaminant levels to a total input for North Sea sludge dumpers
and a total input for other dumpers would accommodate an
increase in sludge dumping by one authority if balanced by a
decrease by another, especially if there is improvement in
sludge quality. However, it appears from NWWA's lack of success
in obtaining the MAFF's approval for a new dumping site that UK
government is not keen to encourage further dumping. (12, 107)

There is a possibility that contaminant levels in sewage sludge
could increase if the reduction of input of dangerous substances
into rivers and estuaries, as specified by the Ministerial
Declaration (94), results in more industrial wastes being
discharged to sewers or further treatment of sewage effluent.
This means merely transferring contaminants from one part of the
environment to another, the solution to such a problem is
reduction of contamination at source.

6.3 Monitoring for marine dumping

Licences issued by MAFF, DAFS, and DOE (northern Ireland) for
the marine dumping of sewage sludge should comply with the
requirements of the Annexes of the London Dumping Convention and
the Oslo Convention. Annex I of both conventions list substances
for which dumping is prohibited and Annex II list substances for
which dumping is controlled by special permit (26).

Sewage sludge contains Annex I substances, such as
organohalogens, mercury and cadmium but these are only
considered to be 'trace contaminants' and therefore under the
Oslo Convention sewage sludge dumping is acceptable. The exact
level of 'trace contaminants' allowed has yet to be defined.
There is a prior consultation procedure if substances containing
more than 0.3 mg kg-1 total mercury and more than 1 mg kg -1
total cadmium are to be dumped (21). Sewage sludge containing
Annex II substances in 'significant quantities' requires prior
notification of all the contracting parties of the Oslo
Convention before dumping is permitted. 'Significant quantities'
are defined under the convention (500-1000 ppm) and according to
MAFF concentrations are usually well below the specified levels
(10). The UK licensing authorities negotiate the actual levels
of metals with each disposal authority; higher levels are
permitted in regions which have a problem with contamination
from industrial wastes.

Organohalogens which are also Annex I substances are not usually
specified in MAFF licences possibly because MAFF considers that
organohalogen contamination "is not normally a problem in sewage
sludge" (10). The increased screening of sludge for the presence
of harmful organics (such as organohalogens) is one of the
recommendations in a report circulated to the water authorities
in 1987 and shortly to be published by the Marine Pollution
Monitoring Management Group (15). Research into the levels of
organohalogens in sludge from selected catchment areas was
planned by MAFF in 1986 but no results have been published. Due
to inadequate monitoring of organic contaminants in sewage
sludges a new survey was instigated in 1988 to establish "1987"
baseline levels to comply with the North Sea Declaration
requirements. The action list of sewage sludge contaminants
currently under consideration by the disposal authorities
include: mercury and cadmium (Annex I); arsenic, chromium,
nickel, copper, lead, zinc (Annex II); PCBs, DDT, dieldrin,
hexachlorobenzene, and HCH (including lindane) (88) [see section
2.2].

Annual checks on sludge quality are made for GAFF when the
licences are renewed. These may be done either by MAFF or more
usually by the water authorities themselves. All Annex I and
Annex II metals are analysed. (10). Random checks on the quality
of sludge destined for sea disposal are carried out by MAFF.
However, only 7% of all sailings of vessels dumping wastes to
sea are checked by MAFF and fewer inspections are made of sludge
dumping vessels because priority is given to liquid industrial
wastes. Despite the fact that disposal authorities make their
own checks on sludge quality MAFF did detect "elevated metal
levels in sludge at one sewage works" in 1985 (10). It should be
noted that the levels of metals in sludges do vary especially
where pulses of industrial discharge are treated by sewage works
(41).

TWA take a representative sample of sludge from each load for
dumping at sea and these are bulked over a weekly or monthly
period and then analysed for a range of metals (127). Analyses
are carried out after the sludge is dumped at sea so this is not
a precautionary measure to prevent the dumping of sludge loads
which could perhaps contain unusually high levels of
contaminants. This may have happened during a MAFF survey of the
TEA dumping ground at Barrow Deep when an exceptionally high
reading for the cadmium concentration in the suspended
particulate phase was found (42).

NWWA take samples of sludge for analysis from the loading arm
conveying sludge to the dump ship at Sandon Dock (pers. comm
Harper, 125). The following determinants are made: total
nitrogen; total phosphorous; total and mineral solids; percent
dry solids; cadmium; chromium; copper; mercury; nickel; lead;
zinc; BOD; mineral oil; di-and tri-chlorobenzenes;
hexachlorobutadiene; hexachlorocyclohexane; DDT and derivatives;
drins; PCBs; carbon tetrachloride; pentachlorophenol.


7. Fate sewage sludge dumped at sea

7.1 Site characteristics

The three major UK sludge dumping sites (Thames Estuary, Firth
of Clyde, and Liverpool Bay) were all established by the early
1900s when practical criteria, such as the presence of a
convenient navigational mark, determined the choice of site.
Scant attention was paid to possible environmental effects of
sludge dumping until the 1950s and 1960s. Scientific surveys
revealed two categories of dumping sites: accumulating sites
(also called containment or sacrificial sites) where the sludge
quickly reaches the bottom and mostly remains in the vicinity of
the dumping ground and dispersing sites where water movements
are stronger and sludge is carried away and dispersed (3). The
Garroch Head site in the Firth of Clyde is accumulative while
all other UK sites are considered dispersive, although there may
be some areas of sludge accumulation within these sites (15,
43).

The impact of dumping is marked at accumulating sites. At
Garroch Head 10 km2 of seabed are severely affected by sludge
dumping with the greatest impact at the centre of the site where
there is a 15 cm deep layer of sludge. The oxygen level of the
sediments is very low at the centre of the site and an area of
about 10 km2 are permanently reduced. Carbon levels are elevated
as the input from sludge is estimated to be one or two orders of
magnitude higher than normal. Contaminants also accumulate in
the sediments and the centre is a 'hot spot' with elevated
levels of metals (copper is up to ten times and lead up to five
times background levels) and other anthropogenic material such
as PCBs. For the most part background levels of metals are found
at about 3 km from the centre. The fauna of the seabed (benthos)
is severely modified with normal communities occurring at 6 - 8
km from the centre of the dumping ground [see 8.1]. The total
area of effect is confined to about 95 km2. It is assumed that
transference of potentially hazardous materials beyond the
identified contaminated area does not take place. (44-47).

In comparison to accumulating sites the impact of sludge dumping
at dispersing sites is more difficult to detect. Tracers, such
as radioactively labelled sludges and faecal bacteria, are used
to track the dispersion of the sludge particles. The dump site
in the Thames Estuary at Barrow Deep is considered highly
dispersing as tidal induced currents continually erode the area
(6). Sludge remains in the water column for up to 8 hours after
dumping and has been traced up to 9 km away (43, 48). Some areas
of sludge accumulation occur at the dumping ground (Barrow Deep)
and at nearby Middle Deep (42). Sludge also can be detected
within days of dumping in sediments about 10 km away (42, 43).
Where sludge particles are detected in the sediments the
concentrations of carbon are elevated and metal levels in the
fine sediment fractions are five to eight times higher than
background levels (42). It is notable that similar levels of
contaminants are found in the sediments at both 'accumulative'
and 'dispersive' sites. The nature of the sediments appears to
be the most important variable determining the species
composition of the benthic fauna. However, areas of organic
enrichment have been found and presence of polluter-indicator
species detected (42, 43). Sludge dumping may also be
responsible for the impoverishment of the fauna at Barrow Deep
and Middle Deep which occurred between surveys in 1972 and 1977
(44)

Liverpool Bay is considered a dispersive site where strong
currents should rapidly disperse the sludge. Deposition of
sludge occurs 50% of the time during calm weather and neap
periods but in the vicinity of the dumping ground the capacity
for accumulation is limited. Sludge particles tend to move
inshore towards the Mersey where accumulation may take place in
muddy areas. Surveys have shown elevation of carbon and metal
levels in the sediments which are associated with sludge dumping
(44, 49>.

7.2 Choice of sites - accumulating versus dispersing sites

Both types of sites are licensed by the Department of
Agriculture and Fisheries for Scotland* and are apparently
considered acceptable options for sludge disposal (14) despite
the different impact on the marine environment. There has been
some debate by scientists as to which type of site is really the
best environmental option.

* The Firth of Clyde site at Garroch Head is accumulating and
the Firth of Forth sites at St Abb's and Bell Rock are
dispersing (14). 

The proponents of accumulating sites admit that the
environmental impact is high but argue that it is limited to a
small area of the seabed where the effects can be easily
assessed (39, 47). Such sites are considered by some to be
comparable to landfill or waste tips on land (2, 39). It has
been suggested that if harmful substances were detected
subsequent to dumping, the area could be sealed-off or capped
with clean sediments (39, 44). However, recolonisation of clean
sediment would be slow and then there could be a danger of
disturbing the wastes through bioturbation.

Even with accumulating sites there is still lack of knowledge as
to the fate of potentially dangerous materials in the sludge as
Pearson (44) emphasised:

"The final fate of the hazardous material in the sludge is not
known for certain however. It is assumed that the bulk is
inactivated, buried, or degraded in the central area, but this
has not been conclusively demonstrated. The possibility that
there may be transfer out with the area through food webs cannot
be dismissed."

And went on to state:

"Moreover should containment on that type of ground be
considered, then extensive assessment should be made of the
eventual fate of all toxins and pathogens in the dumped
material. Ignorance in this respect is the weakest link in
assessing the possible options for marine disposal."

Dispersing sites have also been criticised because the eventual
fate of the dumped material is unknown. It has been found that
sludge derived material does accumulate at some distance from
the dumping site and it could reaccumulate outside the area
being monitored (44, 47). The potential hazard of such localized
areas of accumulation is unknown and difficult to assess (44).
Undoubtedly some of the sludge disperses over considerable
distances. The finest sludge particles from the Barrow Deep
dumping ground, for instance, are carried out of the Thames
Estuary into the southern North Sea (42) [see section 8.3].

Another problem is that with long-term dumping there is
accumulation of contaminants even in dispersing sites whereas
ideally there should be no measurable changes with continued
dumping. For example, after surveying the Barrow Deep dumping
ground in 1976 and 1977 MAFF (42) concluded that:


"The good dispersive characteristics of the area have thus
prevented the occurrence of seriously detrimental accumulations
of metals and organic substances. However, it appears that
current rates of dumping have somewhat exceeded the dispersive
capacity of the area, resulting in readily identifiable areas
where organic matter and metals have accumulated."

Similar effects have been reported by MAFF (49) for the
Liverpool Bay sludge dump:

"In the region of the dumping ground there also appears to have
been a steady increase in the degree of contamination of the
fine fraction of the sediments between 1975 and 1980 .........
This suggests the natural dispersive processes are incapable of
adequately dispersing sludge particles at the rate of dumping
which has taken place since 1975."

This may be one of the reasons why North West Water Authority
proposed dumping the increased quantities of sludge which arise
from the Sandon Dock development at a new site (12).

Other sludge dumping sites such as the Tyne, Tees and Lyme Bay
sites are also noted for being less dispersive and more likely
to accumulate contaminants (7, 50).

7.3 Recovery of sites

There is some evidence that once the dumping stops the
communities of benthic animals may recover. The original dumping
ground in the Firth of Clyde was about two km south of Garroch
Head, dumping stopped here in 1974 when DAFS officially
designated a new dumping site about 4 km further south. While in
1970-71 very few species existed in the old dumping site by 1983
there were some 25 to 38 taxa (39). However there are still
elevated metal levels present and the biomass is higher than in
other areas on the edge of the present dump site (46, 47). Metal
levels are still elevated ln Black Deep in the Thames Estuary
where dumping ceased in 1967 (43). So it therefore appears that
the long-term effects of sludge dumping continue for at least
ten years. Sludge derived bacteria, viruses and protozoans have
also been found at sewage sludge dumping sites a year or more
after dumping had ceased [see 8.6]. 

8. Environmental impact of dumping sewage sludge at sea

8.1 Effect on the benthos

The impact of sludge dumping on the benthos (bottom-living
organisms) in less dispersive sites is severe and has been well-
documented in sites such as the Firth of Clyde (44-47). In the
New York Bight (51) and German Bight (18) sludge dumping has
also contributed to the overall decline of normal benthic marine
life. Dramatic changes in species composition, abundance and
biomass have occurred in all these areas. Such changes can be
correlated with the degree of organic enrichment of the seabed
on both temporal and spatial scales (ie a gradient across the
dumping ground). Initially, the numbers of most of the animals
at the site increase. Then some of sensitive species disappear
while the more tolerant species increase in numbers so the
biomass remains high. The numbers of species continue to decline
until the final stage is reached where the sediments are anoxic
and devoid of macrobenthic life (39).

Conditions are severe at Garroch Head (Firth of Clyde) where in
the centre of the Bite all but the final stage has occurred
(39). Biomass here is four times higher than normal background
levels and normal fauna is replaced by a few opportunistic
species of nematode worms and small polychaete worms such as
spionids and capitellids. At 1-2 km from the centre biomass and
abundance decline and more diverse fauna is present. At 6-8 km
normal unenriched communities are found. There has been little
change in this pattern over six years (44-47).

For many dispersing sites, the impact of sewage sludge dumping
is assumed to be negligible because gross changes in the benthos
are not apparent. Examples include the Liverpool Bay and
Plymouth dumping sites (49, 52). However, effects induced by
sludge dumping can be masked by natural fluctuations ln the
populations of benthic organisms (53). Other factors like
sediment type and depth may be the dominant influence on the
benthos so that it becomes almost impossible to determine if
sludge dumping is having an effect by conventional benthic
monitoring (53). In some areas like the Bristol Channel,
difficulty in sampling has prevented all but a qualitative
description of the benthos (54).

Sludge dumping began at many sites before any baseline studies
were conducted to establish the natural state of the benthic
communities so the degree of deterioration is hard to assess.
Since the 1970s MAFF have attempted to rectify this situation by
conducting intensive surveys of areas like the Barrow Deep in
order to set up some reference point or bench-mark against which
future surveys can be compared (55) but with ten years of
dumping in the Barrow Deep and ninety years in the adjacent
Black Deep substantial changes may have already occurred. Even
where baseline studies have been conducted the response of the
benthic community to sludge dumping initially may be slow
possibly giving a false impression of the assimilative capacity
of the site (56).

There are now more sensitive and direct biological tests that
demonstrate marine organisms can be stressed by sludge dumping
in areas where previous benthic surveys found no significant
effects [see 8.2]. A notable example is the Plymouth site where
300 tonnes of sludge are dumped daily (57). Stress can be taken
as an early warning of environmental deterioration.

8.2 Assessment by biological response tests

8.2.1 Bioassays

Marine organisms vary in their sensitivity to sewage sludge. In
toxicity tests with New York sludges mysids (crustaceans) were
more sensitive than a coastal fin fish (Nenidia menidia) with a
96 hour LCso (lethal concentration calculated to kill 50% of the
test organisms in 96 hours) ranging from 54 - 42 000 ppm (0.005%
to 4.2% dilution of whole sludge) (41). Copedpods have been
found to be six to nine times more sensitive than mysids (58).

Larvae of marine organisms are less tolerant to pollutants such
as heavy metals (59). In tests on brown shrimp (Crangnon
crangnon) larvae were 500 times more sensitive to contaminated
sewage sludge than adults with 24 hour LCso range from 3.3 - 10
ppm (dilution of whole sludge) (60.) Such highly contaminated
sludge could well exert a toxic effect on sensitive forms of
marine life because initial dilution of sludge discharged to sea
are in the order of 290:1 to 400:1 increasing to about 24 000:1
after 6 hours* (43) and to 2 000:1 in a less dispersive site
after four hours (43, 108). For mussel larvae exposed to the
soluble fraction of Thames Water Authority sludge their toxic
threshold corresponded to a dilution of 200:1 (43). It was noted
that the dilution factor was calculated using the solid phase of
sewage sludge and in the bioassay the soluble phase was used.

* 10 ppm is equivalent to a dilution of 100 000:1.

Although the levels of contaminants in sludge vary, correlation
of acute toxicity with specific chemical components of sludge
are often not significant. Toxicity is related to the combined
effect of all the toxic components of sludge as well as the
concentration of suspended particulates (41).

It is interesting to note that the levels of metals and
petroleum hydrocarbons in sewage sludge from the USA have been
found to be several orders of magnitude higher than acute
toxicity values for sensitive marine organisms (41). These
levels are assumed to drop below the acute toxicity threshold
when diluted on discharge.

Sludge concentrations in the water column after discharge are
more likely to reach the threshold for sublethal effects. Far
example, sludge and water samples from the Plymouth site
significantly inhibited the growth rate of hydroids (57).
Although the growth-inhibiting sludge concentration would occur
for only a short period of time following dumping. Bioassay on
contaminated sediment from the Thames estuary produced no effect
on mussel larvae (43).

The Marine Pollution Monitoring Management Group recommends that
further investigations into the use of field and laboratory
bioassays for assessing the impact of sludge dumping should be
made (15).

8.2.2 Stress tests


Biological testing has been used to assess stress in marine
organisms induced by sludge dumping. Scope for growth (SFG)
tests demonstrate the physiological well-being of marine
organisms. SFG of an individual animal is the net energy balance
or difference between the energy assimilated and the energy
respired and excreted. If the energy balance is positive then
growth and reproduction are possible and the population should
be maintained. Mussels are commonly used in this test and are
transplanted cages placed in the study area. After the exposure
period (usually a month or more) the mussels are collected and
the SFG measured. Tests on mussels placed along the dispersal
path of sludge in the Barrow Deep and mussels placed in the
vicinity of Plymouth site both showed signs of sublethal stress
or lower SFG (43, 57).

A variety of cellular detoxification mechanisms are also used as
an indication of stress. These include overloading of lysosomes
resulting in destabilization of the lysosomal membrane;
increased activity of the Mixed Function Oxygenases (MFO)
enzymes which metabolize organic xenobiotics such as aromatic
hydrocarbons and pesticides; and stimulation of the metal
detoxification systems in which metals are bound to low
molecular weight proteins as well as being incorporated into
vesicles (57).

Such cellular processes were examined in the digestive glands of
mussels transplanted to the Plymouth sludge dumping site.
Examination of the lysosomes revealed that mussels placed in the
centre of the dumping site, where exposure to particulate sewage
sludge solids would be highest, were the most severely stressed.
Levels of organic detoxification enzymes were elevated in some
mussels and the metal detoxification system induced. In these
incidences both land based or estuarine sources of pollutants
and sewage sludge may have been involved (57).

8.3 Contaminants

8.3.1 Field assessment

Sewage sludge dumping adds to the input of contaminants into the
marine environment. Monitoring of sewage sludge dump sites
usually involves examination of the metal levels ln the water
column and sediments but investigation of potentially harmful
organics are rarely carried out. A report undertaken for the
department of the Environment in 1987 by the Water Research
Centre stated that:

"virtually nothing is known about the behaviour of organic
micropollutants when sludge plumes disperse in seawater".(84)

The report emphasised that the fate of organic micropollutants
at sludge disposal sites should be investigated, including the
possibility of organics enriching the sea surface microlayer
above the dump sites.

Elevated levels of metals detected both in the water column
(mostly on suspended particles) and in the sediments at dump
sites are clearly the result of sludge dumping (42, 109, 110).
These metals may be available to the biota and could be passed
along the food chain. There is some evidence that the high
organic content of sludge may reduce the availability of metals
from contaminated sediments (59). Changes in the bioavailability
and chemical form of both metals and xenobiotics occur through
biological processes such as microbial degradation and
bioturbation as well as chemical processes such as
sorption/desorption, oxidation and dissolution (123).

In the 1970s chemical monitoring of commercial fish stocks
showed that levels of mercury were higher in fish caught in the
Thames Estuary than in other inshore areas and sewage sludge was
considered a major source of this contamination (60). More
stringent controls led to the reduction in mercury levels in
sewage sludge dumped in the Estuary and a concomitant fall in
mercury levels in fish (61). In view of this the conclusion of
a MAFF report (61) was that:

"....the experience with mercury which showed how the
assimilative capacity of the area can be exceeded indicates that
continued efforts are needed to reduce the inputs of persistent
substances via the sludge."

Sewage sludge dumping in Barrow Deep still represents a
significant input of metals into the Thames Estuary which
supports commercial fisheries (cod, plaice, sole, whiting,
herring) and shell fisheries (cockles, whelks, shrimps,
oysters). Inputs of zinc, copper, lead and chromium from sewage
sludge exceed those from other sources including rivers, direct
sewage and industrial discharge (43). In 1985 mercury in sewage
sludge represented 38% of the total mercury input into the
Thames Estuary.

Up to 1973 controls on the mercury content of industrial
discharges and sewage sludge dumped in Liverpool Bay also helped
to reduce mercury levels in commercial fish (61). However these
are still high (0.25 -0.29 mg per kg wet weight) and close to
the EEC standard (0.3 mg per kg wet weight) (34). mercury
accumulated in the sediments of Liverpool Bay from past inputs
and redistributed through disposal of dredgings may be
responsible but the actual route from mercury input to fish is
unknown (34).

Elevated levels of other metals have also been found in
shellfish and fish from Liverpool Bay. Samples of shrimps and
queen scallops from the vicinity of the dumping ground contained
twice the normal levels of cadmium and lead (49). Harper of NWWA
(34) noted that:

"The quality of fish and shellfish has been affected by inputs
of metals and other substances and sewage sludge is a relatively
minor input of such materials but it may make a significant
contribution through some route as yet unknown."

The Irish Sea Status report (111) noted that the mercury problem
in Liverpool Bay is:

"arguably the most important single problem in the Irish Sea."

Surveys at Garroch Head (Firth of Clyde) showed metal and
organochlorine levels were elevated in the benthic fauna. In
particular the whelk Buccinum undatum had high levels of zinc up
to (866 ppm dry weight) which it may have accumulated from the
dumping site (39, 44). Shellfish collected from the area showed
no increase in metal concentration (112). More data on metal
levels in biota from dumping grounds are needed before a
realistic assessment of the impact of sewage sludge dumping can
be made (112).

Far field effects of the input of contaminants from sludge
dumping are hard to identify. In the case of the North Sea it is
often assumed that because the input of metals from sludge
dumping is small the effects will be negligible (57). Sewage
sludge dumping contributes about 1-2% of total metal inputs
(including rivers, direct discharge, atmospheric deposition,
dumping of industrial waste and dredging) (27). Metals appear to
be bound to the smaller sludge particles ( <10 um) (62) and it
is the finest sludge particles which are carried in suspension
out of the Estuary (42). Computer models indicate that
pollutants originating from the NE and SE coast of the UK could
become concentrated near the UK-Dutch boundary of the
continental shelf in the North Sea (63). More research is
required to establish whether inputs from sewage sludge could
have significant far field effects.

8.3.2 Laboratory assessment

There have been few studies on the bioavailablilty of
contaminants in sewage sludge or attempts to identify possible
routes by which they could enter the food chain. Laboratory
tests have shown that lead, zinc and copper can be taken up by
fish, shrimps and mussels exposed to contaminated sewage sludge
for 60 days (60). In these tests there was no significant
bioaccumulation of mercury and cadmium probably because the
organism used would not have ingested the sludge (113). 

A possible pathway for metal transfer within the marine food
chain has been identified in laboratory experiments (64).
Experiments with sludge labelled with silver (110m-Ag) show that
there is potential for metals associated with sludge in
suspension to be taken in by filter-feeders (mysids) and passed
onto animals high up the food chain (shrimp) (113). Metals need
not be assimilated into the body tissues of the prey, but as in
the case of mysids, can merely be present in the gut. Predatory
fish could take in sludge by feeding on shrimp whose body
surfaces had become fouled with sludge particles. Laboratory
experiments also suggest that sludge may be utilised as a food
source by planktonic copepods (65). Copepods are an important
part of the diet of larval fish in the North Sea so this a
potential route whereby sludge and associated metals could be
incorporated into food chains.

8.4 Nutrients

Sewage sludge is a rich source of the nutrients nitrogen and
phosphorous hence its use as a fertiliser on land. In estuarine
and coastal waters increased input of nutrients from
anthropogenic sources is of particular concern as this may lead
to excessive production of phytoplankton or eutrophication.
Plankton blooms caused by elevated concentrations of nutrients
can seriously deplete oxygen levels resulting in fish kill and
death of benthic species. This has occurred frequently in last
few years in the North Sea especially in the German Bight and
adjacent Danish coasts (27, 28, 66). There has been a change in
the types of species in plankton blooms from diatoms to
flagellates (27). In addition some algal blooms produce toxins
which can kill marine life and contaminate shell fisheries.

Significant oxygen depletion is rarely recorded in UK waters.
But to meet the requirements of the North Sea Declaration a
review of UK coastal and estuarine waters is underway to
identify areas which could be adversely affected by the input of
nutrients. Plankton blooms do occur in areas such as Liverpool
Bay where sewage sludge is dumped. In both inshore and offshore
areas of Liverpool Bay there are extensive blooms of the
flagellate Phaeocystis during May and June each year which may
be toxic and cause beach amenity nuisance. The algal blooms
cannot be directly linked to sewage sludge dumping but the
additional input of nutrients could well exacerbate the
eutrophic trends in the Bay (44, 67). Apart from Liverpool Bay
changes in phytoplankton growth are rarely monitored over
dumping grounds.

Nutrient levels have increased substantially in parts of the
North Sea and along the eastern coastal belt, from the
Netherlands to Denmark, they have doubled (27, 28). It has been
estimated that rivers containing waste water emissions and
sewage sludge dumping transport four times more nitrogen and
seven times more phosphorous into the North Sea than can be
considered natural (68). Sewage derived nutrients also enter the
North Sea from treated and untreated direct discharges. In
comparison to the inputs from rivers, direct discharge, and the
atmosphere, that of sludge dumping appears small (2.8% for
phosphorous and 1% for nitrogen) (27). However out of all these
inputs sewage sludge dumping is perhaps the most easily
controlled point source.

8.5 Fish and crustacean diseases

The occurrence of diseases* in fish and shellfish frequently is
linked to pollution (69, 70, 114) and could prove to be a signal
of environmental deterioration. Fish stressed by the presence of
pollutants may be more liable to succumb to infectious diseases
and develop pathological conditions such as tumours and skeletal
abnormalities (70). In addition, pollutants can damage the eggs
and developmental stages of fish (71, 72).

* Defined as any abnormality in fish structure (70)

The development of fish disease is complex as a number of
environmental variables are involved, apart from stress from
pollution. For example, lower water temperatures can suppress
the immune system of fish and bacterial infections due to Vibrio
species are more likely to occur in estuaries where the salinity
is low (73). The nutritional and reproductive state of fish and
their age are also important (70). Attempts to find
uncontaminated control sites (ie a clean site) may be
unrealistic: as environmental conditions are rarely identical,
little may be known about the range of the fish, and the natural
distribution of diseases (107).

Sewage sludge has been implicated as a causative factor of fish
disease in several areas. An intensive study of Pacific Dover
sole off southern California showed that the incidence of fin
rot was greater in areas where the fine sediments contained high
levels of contaminants derived from discharged sewage sludge
(74). Laboratory experiments showed that fin rot in Pacific
Dover sole could be induced after long term exposure to
contaminated sediments and the presence of PCBs was indicated as
a causative factor (75). In the New York Bight where sewage
sludge and dredging spoils were dumped the incidence of fin rot
in the winter flounder was higher (14.1 %) there than in
adjacent clean areas during surveys in 1973 and 1974 (1.9%)
(69). Lobsters and crabs from the New York Bight were affected
by 'shell disease' which eroded their shells and by a
pathological condition of the gills (76). Healthy lobsters and
rock crabs kept in aquaria containing sediments contaminated
with sewage sludge or dredge spoils developed these conditions
within six weeks. Further surveys on rock crabs linked the
incidence of 'black gill' with sewage sludge dumping (77).

In 1980 MAFF conducted a survey of the outer Thames Estuary and
concluded that there was no evidence that sewage sludge dumping
had significantly affected the incidence of fish disease and
other abnormalities (73). Although, incidence of fin rot in cod,
flounder, plaice and dab appeared to be higher in the Thames
Estuary (0.8 - 3.8%) in comparison to the control areas of Rye
Bay and Southwold; the frequency of disease and abnormalities
did not appear to be higher over the dumping ground. As the
incidence of ulcers, haemorrhages and pigment abnormalities
appeared to be highest in The Warp, the sampling site highest up
the Estuary, input from riverine and estuarine discharges were
considered a more significant factor than sewage sludge dumping.

A Joint study by Thames Water and the Water Research Centre in
October 1986 found the incidence of fish disease from the
dumping ground was 0.6% and a site higher up the Estuary was
1.0% which was not significantly different. It was noted that
the incidence of disease could be expected to be at a minimum at
this time due to warm water and lack of reproductive activity in
the fish studied (15) and that the distance between the sites
may encompass the range of individual fish. Further studies are
in progress.

A working group from Greenpeace and University of London
discovered that of the 222 flounder taken from the intake screen
at West Thurrock Power Station in the inner Thames Estuary
during October and December 1986 about 24% were affected by fin
rot, skin lesions, petachiael bleeding, surface nodules and
lymphocystis (78). This contrasts with the 1980 MAFF study of
the outer Estuary where of the 501 flounder examined only 11%
suffered from a similar range of diseases (73). The Greenpeace
report pointed out that flounder from the inner estuary could be
exposed to higher pollutant levels.

Studies off the Firth of Forth (Bell Rock and St. Abb's Head)
apparently showed no indication of adverse effects from sewage
sludge dumping (114). But the authors emphasised that:

"great caution is required before differences in dab disease
levels are linked with pollution or dumping at sea, either in a
positive or negative manner".

Further investigations into the relationship between fish
disease and all forms of pollution is obviously required.
Especially in view of the high incidence of diseased fish found
in parts of the North Sea (18, 70). But as Simmonds et al, 1988
(107) point out it is an impossibility to prove beyond any doubt
that fish disease is caused by pollution in a wild population,
therefore a precautionary approach, limiting sludge dumping, is
desirable.

8.6 bacteria, viruses and protozoans

Sewage sludge can contain pathogenic bacteria, viruses and
protozoans. During sewage treatment a large proportion of the
bacteria and viruses are removed and accumulate in sewage
sludge. For example about 60% of the viral load of sewage
settles out in primary sludge (53) and the secondary treatment
of sewage removes over 70% of Salmonella bacteria (79).
Digestion of sewage sludge reduces the levels of bacteria and
viruses present. For example digestion reduces the numbers of
Coliform bacteria in sewage sludge by 90% (34). There is still
a substantial input of bacteria into the marine environment from
dumping even digested sludge and the presence of both bacteria
and viruses in marine sediments is actually one of the methods
used to trace the dispersal of sewage sludge (43, 80).

Marine disposal of sewage and sludge is generally regarded as an
acceptable method of treatment because bacteria are supposed to
die off rapidly in seawater (81). Studies now show that faecal
bacteria may be present up to 30 months after dumping has ceased
(82) and that pathogenic bacteria may survive for a long time in
seawater but cannot be cultured by traditional plating
techniques. Professor Colwell a pioneer of new microbiological
techniques recently reported (81) that:

"It can no longer be concluded that inability to culture a given
enteric pathogen from estuarine or marine water samples is
unequivocal evidence that the pathogen is either dead or absent
from the sample."

And went on to state:

"The apparent lack of human pathogens in the open ocean at, or
near, waste disposal sites probably reflects the ability of
pathogens to be viable, but non-culturable bacteria."

Viruses also can survive for extended periods of time in the
marine environment especially when associated with sediment
particles (80). Human pathogenic viruses have been detected in
the sediments at sewage sludge disposal site in the Mid-Atlantic
17 months after dumping was stopped (82).

Potentially pathogenic Amoeba have also been found in sewage
contaminated sediments at dump sites and these can persist for
two to five years after the cessation of dumping (115).

Adverse effects due to bacterial and viral contamination are
supposed to be avoided by selecting sewage sludge dumping
grounds which are remote from shell fisheries and public
amenities. But in the New York Bight 150 acres were closed for
commercial shell fishing due to elevated bacterial levels from
sewage sludge dumping (29). There is a potential risk that
sewage sludge with associated bacteria and viruses could be
transported to more sensitive areas (such as bathing beaches and
shellfisheries) especially from highly dispersive sites. Viruses
have also been detected in places where faecal indicator
bacteria were absent demonstrating that water quality and
shellfish quality cannot be judged by study of bacteria alone
(81). The significance of the prolonged survival of pathogens at
sludge dumping sites should be investigated in terms of possible
public health risks (81). 

9. Sewage sludge dumping by Ireland

Policy

Sewage sludge is dumped at sea by Ireland only at one site - the
outer limits of Dublin Bay. Ireland follows a policy of BPEO for
its waste disposal operations but also accepts that a
precautionary approach is necessary to control the inputs of the
most dangerous substances (116). Dumping is regulated by the
Department of the Marine (under the 1981 Dumping at Sea Act) and
is only permitted if 'there is no perceived risk to the marine
environment and where there is no other realistic disposal
operation' (117). The Department of the Marine apparently is
keen to encourage alternatives to dumping at sea. According to
Dickson and Boelens (1988, 118), and Green (1988, 119) there are
no plans for alternative means of disposal of sewage sludge so
dumping will continue for many years to come (118, 119).
However, there are recent reports that Dublin Corporation is
currently costing plans to set up a plant for the digestion and
solidification of sludge to prepare it for burial on land, due
to fears of an EC ban on sludge dumping in the Irish Sea (121).

Sludge quality and monitoring

The amount of sludge dumped has steadily increased from 97,000
tonnes (wet weight) in 1980 to 234,320 in 1987. The increase is
due to a rise in population and to the diversion to treatment
works of sewage formerly discharged through outfalls at the
shore (118). The sludge dumped is primary sludge which according
to Dickson and Boelens (118) contains metals in comparable
levels to those of UK primary sludges (comparison made to pre
1978 figures). The 1987 mercury level for Dublin sludge is
higher than that specified in the public register of 1988/89
licences for UK sewage sludges disposed to sea (3.5 mg/L mercury
for Dublin sludge compared to maximum of 0.26 ppm wet weight for
Northumbrian Water Authority sludge) (119).

The mercury concentration in Dublin sludge is also high enough
to require a prior consultation procedure by OSCOM*.

* The level set for PCP by OSCOX is 0.3 mg kg-1 total mercury in
wet weight of material dumped. (OSCOM 12th report)

Data on organic contaminants in Dublin sewage sludge is not
sufficient to establish temporal patterns (118). The Dublin
sewer system receives both domestic and industrial discharges so
some contaminants will be of industrial origin. In 1988 there
were 103 licences for industrial discharges to sewer (119).

The sewage sludge disposal operation is carried out by Dublin
Corporation. Three or four sludge samples are taken for analysis
every two weeks. Further to this the Department of Marine
inspects the dumping operation on a random basis about four
times a year when samples of sludge from the dump boat are also
taken for analysis (119).

Impact on the Marine Environment

Several surveys of the benthic fauna at the Dublin Bay dump site
have been carried out. Surveys in 1971-72 and in 1983 used
traditional methods concentrating on identifying benthic fauna
as indicators of environment conditions. The most recent survey 
in 1988 used a new optical coring device, REMOTS (remote
ecological monitoring of the seafloor) which is a
relatively quick method to assess the condition of the seabed.
These surveys revealed that Dublin Bay is in general a
dispersive site but there are localized accumulations of sludge
particularly in the west of the site. In this area organic
enrichment has led to an increase in numbers of benthic species
and individuals. The REMOTS survey showed qualitatively that the
benthic fauna appears not to have deteriorated since the 1983
survey (120).

The synthesis report of the 1988 survey produced by the
University College, Galway and the Department of the Marine
emphasised that there are large number of unknowns regarding the
impact of sewage sludge dumping on Dublin Bay and the
surrounding coastal area. A major concern is the ultimate fate
of the dispersed sludge. Unanswered questions include whether
the material accumulated west of the sludge dump site is
periodically washed away; whether sludge is flushed back into
the inner part of Dublin Bay or whether is it is concentrated
north of the dumping ground and whether the "hot spots" of
accumulated sludge (and associated contaminants) will become
enlarged as the input increases. The report also emphasised that
more information is required concerning the accumulation of
contaminants in sediments and their effects on the marine
environment. In addition, the report noted that studies are
needed on the effects of sludge dumping on the water column
especially in regard to the water quality of Dublin Bay and that
further north along the coast. The report stated that the future
of dumping depended on finding answers to such questions.

10. Summary

The UK and Ireland are the only European countries that dump
sewage sludge at sea. Currently, the UK dumps about 9 million
wet tonnes of sewage sludge and Ireland dumps 234,320 wet tonnes
at sea each year, a practice which is regarded by some other
European nations as unacceptable because of the uncertainties
regarding the environmental effects. Despite considerable
opposition at the North Sea Conference the UK is still fully
committed to the sea disposal option for sewage sludge.

Even though land-based alternatives for sewage sludge dumped at
sea exist, this disposal route has been preserved as an option
for the UK in the North Sea Declaration. Sewage sludge has a
variety of proven productive uses which include use as a
fertiliser and soil conditioner in agriculture, forestry and
land reclamation. Dumping sewage sludge at sea is a waste of
this resource. The 1984 Ends report noted that: 

"Sewage sludge, in fact, is a large secondary resource whose
constituents have a notional value of L200 million per year".

The North Sea Declaration does require the UK to reduce the
concentrations of harmful contaminants (those that are
persistent, toxic or bioaccumulable) in sewage sludge disposed
to sea and to ensure that quantities disposed to sea by this
pathway do not increase above 1987 levels. A provisional action
list includes Annex I and Annex II metals and a range of organic
contaminants. Monitoring for latter has proved to be inadequate
and results from a 1988 survey are to be taken as the "1987"
baseline.

The levels of contaminants in sludge are generally higher when
domestic sewage is treated along with industrial effluent.
Removal of these substances once they are incorporated in sludge
is difficult and expensive so improvements in sludge quality
must be made by reducing the input of contaminants from
industrial effluents discharged to sewers.

The UK licensing authorities permit dumping at sites with
totally different characteristics: accumulating sites where
sludge stays in the vicinity of the Bite and dispersing sites
where most of the sludge is carried away. It is debatable as to
which is the best environmental option. In accumulating sites
the impact on the marine environment is more easily assessed as
a limited area of seabed is affected. However, the eventual fate
of the potentially harmful materials in the dumped sludge has
not been conclusively established. Sewage sludge dumped at
dispersing sites is spread over a wide area. It can reaccumulate
at some distance from the site and the potential harm to marine
life of such localized patches of sludge is unknown. This is one
of the main areas of concern about the Dublin Bay dump site. Few
studies have been made on whether dumping sites can recover once
dumping has ceased. In one site some effects were still
detectable after ten years (46, 47).

The impact of sludge dumping cannot be accurately assessed in
some sites using conventional benthic surveys, especially where
dumping began before any baseline surveys were carried out. The
use of direct biological response tests indicate that sludge
dumping induces stress in marine organisms where no significant
effects were previously detected. Such stress can be taken as an
early warning of environmental deterioration.

Elevated levels of mercury in commercial fish have been linked
to input from sewage sludge dumping. Possible pathways for metal
transfer are unknown but laboratory studies indicate that there
is potential for metals associated with sludge particles to be
taken up by filter feeders and passed up the food chain.

Sewage sludge dumping increases the input of nutrients into the
marine environment and the risk of eutrophication. Although
dumping represents a small percentage of the total input in
areas, such as the North Sea, it is perhaps the most easily
controlled point source.

The incidence of fish and crustacean diseases from areas, like
the New York Bight, has been associated with sewage sludge
dumping. Some surveys indicate that levels of fish disease are
elevated in the Thames Estuary but these are not specifically
related to sludge dumping. 

Sewage sludge can contain pathogenic bacteria and viruses. Some
studies show that these may survive in the marine environment
for long periods of time. In view of this the potential public
health risks from sewage sludge dumping needs to be investigated
(81).

11. Conclusions

In continuing dumping sewage sludge at sea, the UK has
considerable difficulties in meeting its commitments as stated
in the North Sea Ministerial Declaration. The UK has yet to
demonstrate that sewage sludge poses no risk to the marine
environment.

Major areas of concern are:

1. The ultimate fate of contaminants present in sewage sludge
dumped at sea is unknown and their availability to the biota
poorly understood.

2. Accurate assessment of the impact of sewage sludge on marine
life is often not possible using conventional survey techniques.
Marine monitoring could be improved through the use of direct
biological response tests which could provide an early warning
of environmental deterioration.


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