Water companies are increasingly turning to UV sewage disinfection instead of long sea outfalls (ENDS Report 212, pp 11-12 ). However, the SWW case shows that such plans do not always find favour with the NRA - and illustrates the complex environmental factors to be considered in the design of new sewerage systems in sensitive areas.
The Trevone regional sewage treatment scheme is intended to replace five untreated discharges to the coast and the estuary of the river Camel, near Padstow in north Cornwall. The discharges have contributed to failures at three of the six local EC designated bathing waters. The estuary has five Sites of Special Scientific Interest, is an Area of Outstanding Natural Beauty and is important for its shellfisheries, water sports and the passage of salmonid fish into the river.
In December 1992, SWW submitted plans to the NRA to collect sewage from both sides of the estuary for treatment at Porthilly, with primary and secondary treatment and UV disinfection being provided before discharge into the estuary. The company says that its studies and models show that the scheme would ensure compliance with all the necessary standards, including the EC bathing water limits at Rock Beach, only 600 metres from the discharge point.
However, the NRA rejected the scheme last May because of its heavy reliance on UV disinfection and dilution in the estuary. UV treatment of secondary effluents "does not yet have a proven history of reliable microbiocidal performance", it concluded, noting that any temporary variation in performance would adversely affect microbiological quality in the shellfish and recreational waters.
Flows in the estuary give a dilution of only 100-fold at certain times of year. The NRA believes that any future reduction in flow or increase in discharges could lead to non-compliance with ammonia standards for migratory fish. It also objects in principle to the diversion of effluents from coasts to more sensitive estuarine waters. Although the NRA admits that there would be an overall improvement in the estuary, it says there would be a local deterioration in the channel receiving the discharge at low tide.
Further uncertainty is created by the unstable pattern of channels and sandbanks in the estuary. A change in the flow pattern could reduce dilution of the discharge and even remove the channel receiving the effluent. SWW counters that any change of flow could be accommodated by moving the discharge point.
The NRA favours an alternative scheme which would involve only primary treatment and discharge via a long sea outfall. This would avoid impacts on the estuary and allow greater flexibility for upgrading to meet any more stringent standards in future.
However, a long outfall would be more expensive. SWW puts the capital costs at £22.2 million, compared to £15.3 million for the estuarine scheme. Allowing for long-term running costs, its preferred option is almost £5 million cheaper than the long outfall.
SWW has appealed against the NRA's decision, claiming that its proposal would use proven technology and deliver the required standards within acceptable limits of risk. It accuses the NRA of inconsistency as it has given the go-ahead to similar schemes in the Dart and Salcombe estuaries.
The environmental group Surfers Against Sewage has, unusually, sided with the water company. It says that a long outfall would certainly have an impact on water quality in the estuary throughout the year, but that the chances of failure in the estuarine treatment would be slight.
The Trevone scheme was originally due for completion in January 1996, but a delay is now inevitable. Work has already been delayed by a year - and if a long sea outfall is required, plans would take at least a further 18 months to finalise.