The activated sludge process (ASP) is the most common method of secondary municipal wastewater treatment, and the key control parameter for this process is the solid retention time (SRT). Normally operating at long SRT is considered only to provide nitrification, but research has shown
more advantages of high SRT operations. This paper presents experimental and literature evidence to demonstrate three major, additional benefits of long SRT operation: enhanced removal of many emerging contaminants such as pharmaceuticals, personal care products and endocrine disrupting compounds;
increased oxygen transfer efficiency and denitrification to maintain energy conservation, and larger biomass particle size distribution, which results in more efficient clarification with fewer effluent particles and suspended solids. The paper presents evidence from the literature documenting
improved removal of emerging contaminants and biodegradable organic carbon, and a survey of a number of treatment plants, showing increasing oxygen transfer efficiency and particle size with increasing SRT. A long term survey of a treatment plant shows that operating at higher SRT is not as
energy intensive as conventionally assumed.
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