Report No FR0031



Sept 1989



To assess the effects on the marine environment of discharge of sewage to coastal waters via correctly designed sea outfalls.


Coastal waters of the UK have traditionally been used extensively for the disposal of sewage. Until recently, the sea was used merely as a dumping ground for raw sewage, discharged via short outfalls at locations chosen for economic, rather than environmental reasons. For this reason, the use of the sea for sewage disposal has come under increasing attack from the public and from other countries bordering the North Sea. Most frequent demands are for land-based treatment of the sewage before discharge to sea. However, this is not necessarily the only solution to the problem.

Correctly designed and sited sea outfalls, discharging screened sewage into deep, dispersive waters, are now replacing the vast number of nineteenth century short outfalls in many coastal areas. Such outfalls have been shown (Cooper and Lack 1987) to be capable of achieving EC bacterial standards for bathing waters, but there is also a need to understand and quantify their environmental impact.

The UK water industry needs to demonstrate that the marine disposal of sewage is an environmentally sound option.


Broad-based surveys were carried out at two case-study outfalls over 3 years. Studies included: sediment sampling on coarse grids and transects; bioassays on mussel scope-for-growth, bivalve larval development and Microtox; uptake of taint compounds; sewage sampling; dispersion studies.


  1. At both Weymouth and Tenby, the whole survey area was affected to some extent by sewage-derived material, but both outfalls were operating within the dispersive and assimilative capacities of the environment.
  2. Within 50 m of the Weymouth outfall, levels of thermotolerant coliforms (TTC), coprostanol, metals and Microtox toxicity in were elevated significantly above background.
  3. The sewage discharges from the Weymouth outfall had no detectable impact on the surrounding sediments in terms of persistent organics compound concentrations, particle size distribution, organic content or macrobenthic communities.
  4. The sewage discharges from the Tenby outfall had no detectable spatial impact on any physical, chemical or biological characteristics of the sediment.
  5. Neither outfall had any significant effect on Scope-for-Growth (SFG) in mussels.
  6. Neither Weymouth nor Tenby sewage had effects on the development of mussel larvae at dilutions shown to occur in the receiving waters.
  7. Oysters held very near to a sewage discharge may become tainted, but only when so contaminated that they would need depurating for microbiological reasons.


Marine monitoring of sewage discharges has traditionally concentrated on annual analysis of sediments for parameters such as macrobenthic species distribution, metals, particle size distribution and organic content in samples taken on a coarse grid around an outfall. The case studies described in this report suggest that, although this may be sufficient for a baseline survey, it may not be capable of detecting minor environmental changes which could be an early warning that the dispersive and assimilative capacity of the receiving waters are being exceeded.

A suitable monitoring regime might be as follows:

  1. Baseline surveys

Annual sediment sampling, ideally for 3 years using a quantitative sampler, such as a Day grab. A minimum of three replicate samples should be taken on a coarse grid and analysed for macrobenthic species diversity, metals, particle size distribution, organic content and coprostanol.

  1. Post-construction surveys
  1. Annual surveys

Transects laid in the directions of net current flow, extending approximately 100 m either side of the outfall, to be sampled by divers. Pre-construction computer modelling and dispersion studies will help to select the length of the transects, but analysis of thermotolerant coliform bacteria in the sediments gives a rapid, inexpensive indication of whether the region of maximum impact has been sampled.

Triplicate sediment samples should be analysed for persistent organic compounds, coprostanol, metals, thermotolerant coliform bacteria and microtox toxicity. However, the frequency of analysis for persistent organic compounds may be reduced if early results indicate that levels are not increasing.

  1. Three-yearly surveys

Grid surveys at the sites used for the baseline survey, using identical sampling techniques and analyses.

  1. Bioassays

Sewage toxicity should be analysed using the 48 hour mussel larval development test. A protocol for measuring and controlling the toxicity of effluents is currently being developed by WRc.

Copies of the report are available from FWR, price 35.00, less 20% to FWR Members.