An anaerobic side-stream reactor in wastewater treatment: A review

In the frame of a modern waste management, an important sector concerns the sewage sludge minimisation. In recent years a lot of techniques have been developed to reduce the sludge production such as biological, thermal, thermochemical, high temperature oxidation and mechanical treatments, ultrasonication and ozonation or by using chemical compounds. Some of these have been proven to be not energy saving technologies, while others could negatively affect the effluent quality of the process due to the formation of byproducts. Among those, the use of an anaerobic side-stream reactor (ASSR) in the conventional activated sludge line is considered a challenging biological technology to minimize the sludge production in wastewater treatment plants. The whole process is mainly based on the waste activated sludge recirculation between aerobic, anoxic and anaerobic conditions. The ASSR can be easily realized in both new and existing plants as it consists of an anaerobic side-stream reactor for sludge treatment and reduction where a portion or, in some cases, all the excess sludge of the activated sludge process is recycled. Studies show that combining a conventional activated process with an ASSR could reduce the sludge yield by up to 40–60% without any negative effects, neither on the effluent quality nor on the settling characteristics of the activated sludge. The process has been applied by using various configurations. Further, different explanations about the reduction mechanism behind the process have been provided. This paper is a review of the existing application of the ASSR, in order to describe the configurations implemented, the mechanisms of sludge reduction observed and the main parameters involved.