Comparison and performance evaluation of direct aerobic submerged attached growth and anaerobic cum aerobic submerged attached growth systems, for factory sewage

Abstract

Most factories located outside of BOI industrial zones in Sri Lanka do not have proper sewage treatment systems. Due to the high number of employees, these factories are always encountered with the problem of handling large quantities of low strength sewage generated from lavatories, canteens and kitchens. In such factories sewage is usually handled using conventional septic tank - soakage pit systems. Such systems are commonly found infested with insects, and promote further breeding. They cause nuisance due to obnoxious odor, and is a major cause for severe ground and surface water pollution. It has been noted that these factories have critical problems of handling sewage during rainy season, especially when the factory is located in areas with high ground water table such as a marshy land or near a surface water body. The activated sludge suspended growth aerobic systems designed to treat sewage in most of the factories have various operational problems which increase the operator involvement and therefore the plants performance are operator dependent. Activated sludge suspended growth aerobic systems are more susceptible for sludge bulking which leads to poor effluent quality and the unbulking process is very difficult and ~time consuming. In addition to this, the several parameters such as SVI, MLVSS, etc have to be monitored carefully in the conventional activated sludge systems for the proper operation and the maintenance which increase the operational and the maintenance cost. The maintenance departments of factories always request a trouble free sewage treatment plant to minimize their involvement in operation of the plant. Therefore the submerged attached growth aerobic systems are becoming the most appropriate system as an alternative of the activated sludge system for treating low strength effluent, sewage from factories. A leading company in Sri Lanka which provides turnkey solutions for waste water and sewage has introduced submerged attached growth aerobic and anaerobic treatment systems to overcome above issues ..Depending on the affordability of the client and the space availability they have introduce submerged attached growth aerobic systems in two ways. (a) Direct aerobic submerged attached growth systems where sewage is treated only with aerobic treatment (b) Anaerobic cum submerged attached growth aerobic systems where sewage is treated with anaerobic process prior to aerobic treatment. However no proper study has been carried out to find out the most appropriate submerged attached growth system to treat sewage generated from factories. Research suggested that the anaerobic cum submerged attached growth aerobic system is more suitable for treating factory sewage than direct aerobic submerged attached growth system in terms of quantity of sludge wasting and lower operational and maintenance cost which results to a lower unit cost per m3 of treated water.