RECENT DEVELOPMENTS IN THE BIOLOGICAL FILTRATION OF SEWAGE TO PRODUCEHIGH-QUALITY NITRIFIED EFFLUENTS

The biological filtration of sewage remains the process which is most commonly used for small works in rural areas throughout the UK. In man y cases, the need to produce an effluent of high quality with full nit rification is essential in order to comply with the standards set by t he Environment Agency. Traditionally, design loading rates of biologic al filters have been based on historical experience of performance, an d on relatively short-term statistical evaluation of the performance o f a large number of treatment works, using average and maximum values far flow and loading rates. Many works which were designed on such inf ormation have failed to nitrify consistently, particularly during the winter and spring months. This paper describes the detailed evaluation of six full-scale, biological-filtration sewage-treatment works over a period of two years, in which there were significant changes in sewa ge flowrate, composition and temperature, from summer to winter. The k ey parameters which influence the production of high-quality nitrified effluents were determined. The conclusions are used to provide a deta iled guide on the design of conventional-media biological filters to p roduce, consistently, fully-nitrified effluents.