In the UK there are over 7000 small works which treat the sewage from populations of less than 10,000. Many of these works are at risk of non-compliance with effluent quality consents and options for improving the standard of treatment are being pursued by many utilities.
WRc and Anglian Water Services have developed designs for packaged sewage treatment plants to serve populations in the range of 1000-10,000. A demonstration plant has been constructed at the Waterbeach STW of Anglian Water to serve a population of about 6,500. Target effluent quality is 15:20:5mg/l of BOD, SS and ammonia nitrogen respectively on a 95 percentile basis.
Following plant commissioning a process performance evaluation programme was commenced in February 1991. Nitrification was established after about 6 weeks of operation but suspended solids values have been affected by the presence of a stable foam on the surface of the aeration tank. Process modifications have reduced the effect of this phenomenon substantially and effluent quality has improved.