Abstract:
The upflow anaerobic sludge blanket (UASB) reactors are widely used for the sewage treatment in warm climate countries like Brazil, Colombia and India since early 1980's (van Haandel et al., 1994; Gomec, 2010). However, the UASB effluent quality rarely meets the discharge standards despite several modifications; such as settlers at the top of gas-liquid-solid-separator (GLSS), addition of anaerobic filter (AF), two UASB reactors placed one after the other and even the incorporation of an external sludge digester.
The inability of UASB process to meet disposal standards, owing to its anaerobic nature has given enough impetus for subsequent post treatment. Furthermore, the growing concern over the impact of sewage contamination on rivers and lakes and the increasing scarcity of water in the world along with rapid population increase in urban areas give reasons to consider appropriate technologies for the post treatment of anaerobic effluent in order to achieve the desired effluent quality and save receiving water bodies.
A variety of post treatment configurations based on various combinations with UASB have been studied by researchers such as aerobic suspended growth, aerobic attached growth, combined suspended and attached growth aerobic processes, anaerobic processes, natural treatment processes, physical processes and physico- chemical processes.
UASB followed by final polishing units (FPU) or polishing pond (PP) is a common process used at several sewage treatment plants (STPs) in India, Colombia and Brazil, since the technology is simple in operation (von Sperling and Mascarenhas, 2005; von Sperling et al., 2005). However, the final effluent is generally devoid of DO and rich in nutrient. Moreover, polishing ponds operate at HRT of I day, leading to a high land requirement.
In India, other post treatment options in use are aeration-ASP and aeration-polishing pond. A demonstration scale Downflow hanging sponge (DHS) is also in operation (Tandukar et al., 2005; 2006).
Several other options such as aeration, sequencing batch reactor
