Report No FR0148



Dec 1990



To review the policies and practice relating to storm sewage discharges to coastal waters in key countries within the EEC.


To advise members of the FWR on the status of storm sewage control for coastal waters in other EC countries and the measures being considered to avoid aesthetic and bacteriological problems associated with such discharges.


  1. The proportion of combined sewers in each of the four countries was reported to be 90% in the Netherlands, 70-80% in France, ?0% in West Germany and 45% in Denmark. In coastal regions of Germany the sewerage systems are predominantly of the separate type.
  2. The advice of a national review study in the Netherlands is to build 'improved' separate systems when new sewers are required. Combined systems are preferred in France and Denmark and in inland cities in Germany.
  3. Sedimentation tanks are the most common form of storm overflow devices in all the countries. However, in Denmark, because of the stringent nitrogen limits for the discharge of sewage effluent into receiving waters, other devices are being considered, such as swirl concentrators, microfiltration and microstraining, in order to avoid the building of extensions to the sewage treatment plants. In the Netherlands the use of detention tanks is recommended; if this is not possible then swirl concentrators or high sided weir chambers should be considered. The preferred storm tank design in Germany consists of two chambers; the purpose of the first chamber is to collect the first flush, whereas the second chamber provides additional storage capacity for large storm events.
  4. In Denmark storm sewage overflows often discharge into small tributaries which are susceptible to pollution. At least 60% of combined sewer overflows in the Netherlands discharge into semi-stagnant watercourses or ponds. Overflows in the coastal regions of Germany usually discharge to rivers at points close to the sea.
  5. None of the four countries applies systematic controls to storm overflow discharges. Storm sewages and stormwaters are not routinely sampled. Monitoring of storm overflow operation is very limited.
  6. In terms of sewerage network design, the emphasis has been on the frequency of overflow operation and its relation to sewer and treatment capacity. This is now changing to an approach based on safeguarding receiving water quality, with associated requirements for regulating the polluting load discharged.
  7. Various models are in use for sewer design, evaluation of storm sewage quantity and quality, and assessment of the impact on receiving water quality. A number of new models are being developed or evaluated. However, there are no models which deal with aesthetic effects of storm sewage discharges. Also none of the models in use is specifically set up to assess the impact of overflow discharges on bacterial water quality.
  8. Disconnection of impervious surfaces is considered in the Netherlands as a means of reducing the frequency of operation of storm overflows. In some parts of France porous roads and permeable pavements are being used for similar reasons.
  9. The problems of bacterial contamination of receiving waters is expected to attract more attention in the future.
  10. Other proposals for future action include the use of 'improved' separate sewers in the Netherlands as well as the relocation of the overflows, research in Denmark on storm overflow devices, and reinforcement of the legislation in France. In German coastal towns work is expected to continue on building of separate sewers in new areas, replacement of combined systems by separate sewers and the building of storage tanks on extensive combined sewers.


Information is reviewed for four countries (The Netherlands, West Germany, Denmark and France) based on consultations with acknowledged experts in each country, supplemented where possible with published data. Summaries are provided to describe the sewerage systems and associated storm sewage overflow arrangements, regulation and monitoring, specific controls to avoid aesthetic and bacterial pollution of receiving waters, and modelling techniques. Characteristics of storm sewage systems which relate to particular countries are highlighted. Prospects for future controls on storm sewage overflows are considered.

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