Report No FR0086

S C Nixon, T ap Rheinallt, I D Codling, S G Wills

Oct 1990



To assess the environmental impact of the disposal of sewage sludge to sea by undertaking case-study investigations at the Thames estuary and Liverpool Bay disposal sites.


The Water Utilities want to be assured that their sludge disposal to sea operations are environmentally acceptable and what decisions over the future of marine disposal are made on a scientific basis. In addition there are concerns among a number of EC member states and environmental pressure groups*over the effects of sludge disposal to sea. It should be noted that since the study commenced the UK government has announced, in March 1990, that it has decided to phase out the disposal of sludge to sea by 1998.


Thames estuary case study

  1. Sewage sludge disposed of in the outer Thames estuary represents only a small proportion of the total inputs of nutrients and other contaminants into the North Sea.

  2. The Thames estuary disposal site was found to be highly dispersive: tracer studies showed that sludge was widely dispersed over the northern part of the estuary within days of release.

  3. Traditional techniques of impact assessment (such as physicochemical measurements), employed over four years of intensive surveys, showed only limited effects of sludge disposal- this was mainly manifested as an increase in macrobenthic species number and abundance in some areas to the south west and north east of the disposal site.d) The results from potentially more sensitive bioassays again only indicated limited impact: this was most clearly demonstrated by the effect of sediment extracts on the Microtox bioassay.

  4. Studies of the prevalence of disease in fish populations from the Thames estuary did not show a direct link with sludge disposal.

    Liverpool Bay studies

  5. Disease prevalence in dab was not significantly higher near the sludge disposal ground than elsewhere. For most diseases, significant spatial variation was not apparent.

  6. The concentrations of some metals in dab livers showed significant spatial variation. The pattern of Cd and Hg may be affected by sludge disposal.

  7. Several organic contaminants in dab livers showed a gradient of decreasing concentration in a westerly direction, suggesting that inputs via or near the Mersey are important.

  8. The prevalences of some diseases, and the concentrations of some metals in the livers, were higher than those observed in the outer Thames estuary at the same season. While direct comparisons of metal concentrations may be valid, this is probably not the case for disease prevalences which may vary with factors unrelated to pollution.

  9. Disease prevalences in 1989 were high compared to those in dab from the Irish Sea between 1971 and 1982, but were similar to those reported in 1987. Although the possibility of a long-term trend cannot be discounted, the validity of any comparisons is doubtful because of a lack of standardisation in the past.

  10. A bioassay test carried out on the aqueous phase of sewage sludge indicated that a 1000 fold dilution of sludge in seawater had no significant effect on oyster embryo development.

  11. Seawater samples taken in the wake of a tanker discharging sewage sludge showed levels of toxicity in the bioassay test which varied considerably on a small spatial scale around the sampling boat, and also relative to the time taken after the sludge disposal: some samples taken soon after sludge discharge were non-toxic compared to seawater control samples whereas samples taken an hour after sludge demonstrated a considerable toxic effect.


The Thames estuary case study has shown that it is difficult to detect any effects from sludge disposal to sea at dispersive sites. The application of toxicity tests and, to a lesser extent, sub-lethal stress measurements has shown these to be potentially more sensitive and to offer some promise for future monitoring programmes. However, development work is underway to enable these measurements to be related to pollution effects at the population or ecosystem level. It is, therefore, recommended that bioassays should be considered in any future monitoring programme.

Though the disposal of sludge to sea is to be phased out over the next eight years, the Water Utilities involved will have to continue to monitor the disposal grounds as a condition of their dumping licence. For the most part it is likely to be recommended by MAFF that the procedures outlined by the Marine Pollution Monitoring Management Group's Coordinating Group on Monitoring of Sewage Sludge Disposal Sites (CGMSD) should be followed.

However, if the prevalence of fish disease in the vicinity of sludge disposal grounds is to be monitored then it is recommended that the protocol adopted by the International Council for the Exploration of the Sea (ICES) is followed rather than that from the CGMSD. It is recommended that the sludge disposal sites are monitored after the end of sludge disposal to determine whether there are any detectable changes and/or improvements in the ecosystem.


This report describes the legislation and conventions that apply to the disposal of sewage sludge to sea, and the work undertaken by the Coordinating Group on Monitoring Sewage Sludge Disposal Sites in arriving at appropriate EQO/EQSs and protocols for monitoring programmes.

The main findings from a four year intensive study at the UK's largest sewage sludge disposal site in the Thames estuary are reported, and a detailed description of a study of fish disease undertaken in Liverpool Bay in 1989 is also given.

Copies of the Report are available from FWR, price 25.00 less 20% to FWR Members