The 1991 EU Directive on urban wastewater treatment sets standards for sewage works depending on the size of population served and the sensitivity of the receiving waters. It requires enhanced treatment for discharges to "sensitive areas" - including those which are eutrophic or at risk of becoming eutrophic.
The Directive required all sewage works serving more than 10,000 population equivalent discharging to sensitive areas to be fitted with appropriate additional treatment by 1998. In 2001, the European Commission published a report on how Member States had complied with these rules (ENDS Report 323, pp 9-10 ).
Although many inland sensitive areas have been designated in the UK, the Commission criticised the failure to designate many coastal waters which it regarded as at risk of eutrophication - including the Wash, Southampton Water and the Thames and Humber estuaries.
The reports on the Humber are Yorkshire Water's response to the threat of the estuary being designated a sensitive area. Designation would require phosphate and nitrogen removal at all larger sewage works in the Humber catchment - a massive area covering a fifth of England and major urbanised rivers like the Trent, Don, Aire and Rother. The cost of providing additional treatment would fall on Yorkshire, Severn Trent and Anglian Water and their customers.
One report by the Institute of Estuarine and Coastal Studies at Hull University reviews the estuary's nutrient status.1 It concludes that it shows few signs of eutrophication, although it is "hypernutrified" or contains high levels of nutrients.
Sewage effluent contributes 65% of phosphorus and 33% of nitrogen inputs. Nutrient stripping of sewage effluents would reduce total phosphorus inputs by 43% and nitrogen inputs by 17%. Agriculture is the other major nutrient source, contributing 27% of phosphorus and 64% of nitrogen.
Although nutrients are abundant, the estuary's turbidity limits algal growth, and toxic blooms or algal mats have not been observed. Dissolved oxygen levels are depleted in the upper estuary due to organic carbon inputs from rivers, but the problem is much reduced following improvements in sewage treatment.
The second report, by AEA Technology, covers the environmental, economic and social impacts of designating the estuary as sensitive.2 Removing nutrients from sewage would require capital investment of £1.1 billion and operational expenditure of £46 million over 20 years. The discounted cost over this period would be £1.39 billion.
External costs would be energy use of 118 gigawatt hours per year - about 0.03% of the total electricity supplied in Britain every year - the use of over 130,000 tonnes of treatment chemicals and generation of 86,000 tonnes of sewage sludge. Three sludge incinerators would be needed to cope with this, and some 2.8 million lorry kilometres per year involved in transporting chemicals and sludge.
The report says that there would be "little or no environmental benefit" and concludes that there is no case for designating the estuary. To do so might create "significant public hostility" and bring EU legislation into disrepute, it warns.
However, an independent marine consultant contacted by ENDS felt that the reports had underplayed several key points. These included signs of eutrophication such as high chlorophyll and phytoplankton levels occasionally found in the lower estuary. Nutrient impacts in rivers upstream or in coastal waters downstream were also not considered.
The report's failure to discuss eelgrass was a particular omission, the consultant said. The plant was once common in the estuary but has now all but disappeared. It forms a valuable wildlife habitat but is now absent from the Spurn Point nature reserve at the head of the estuary and will not re-establish under current conditions.
Tony Harrison, Yorkshire Water's environment manager, felt that the Commission was pushing the UK into a decision which was "blindingly wrong".
He also warned that lack of clarity about the final sewage treatment objectives was a major problem for the industry as it approaches its five-yearly investment review. Companies were facing significant investment to reduce ammonia levels to comply with the freshwater fish Directive, for example, but would make different investment decisions if they also had to fit nutrient removal at the same works.