Design and evaluation pf a small package sewage treatment system

Abstract

The aim was to develop and subsequently evaluate a small package sewage treatment system that combined the commercial demands of the market with current and future environmental legislation in the UK. Questionnaires sent to key customers confirmed results from an initial literature review that non-process related features such as installation, plant price, aesthetic impact, maintenance requirement and operation have a similar impact on the final purchase decision than the achievable effluent quality. Conventional submerged aerated filter (SAF) technology was chosen for its simple operation over more advanced treatment technologies like sequencing batch reactors (SBR) and membrane bioreactors (MBR). The combination of SAF technology and jet aeration allowed the use of an exceptionally shallow tank structure. Two settlement stages for primary treatment and final clarification completed the new unitank design. Following successful clean water and field trials of a prototype unit, the new design was finalised, set-up for production and launched into the market. A second trial plant was permanently installed at a rural cottage in Cranfield, UK, with the objectives to validate and further improve the performance of the new design. Despite high variations in influent conditions, the trial unit produced a good effluent quality with average effluent BOD s , COD and SS levels of 16 mg rI, 100 mg rI and 30 mg rI respectively, thereby achieving overall removal efficiencies of 95.3%, 86.2% and 88.5% respectively. However, effluent ammonia nitrogen (~-N) levels were found to be inconsistent varying from as low as 9 mg rI to over 60 mg rI. Similar findings were revealed from other plants in the field serving domestic and non-domestic applications. The compact and exceptionally shallow tank structure combined with a competitive retail price met the commercial needs of the target market, which resulted in increased sales turnover and market share.