In the UK there are about 7500 sewage treatment works of which 85% serve populations of less than 5000. Many of the smaller works do not comply with effluent quality standards and options for improving treatment are being pursued by many water companies. WRc have developed designs for packaged activated sludge systems to serve populations in the range 100 - 1000 persons. A detailed design has been completed for a population equivalent of about 600. Target effluent quality is 15:20:5 mg/l of BOD, SS and ammonia nitrogen respectively on a 95 percentile basis. The activated sludge system is designed to operate as a batch process with aeration and sludge settlement both occurring in the same tank. Batch operated activated sludge plants are known to produce sludges with good settling properties providing the influent wastewater is admitted into the aeration tank in controlled manner. Therefore a specially designed holding tank has been included as the first stage in the treatment sequence.
All process tanks are specified as glass-coated steel installed on a flat concrete base. Target construction cost for 600 population is ₤350 per capita.