Toward long solids retention time of activated sludge processes: Benefits in energy saving, effluent quality, and stability

Shao Yuan Leu, Licheng Chan, Michael K. Stenstrom

Research output: Journal article publicationJournal articleAcademic researchpeer-review

47 Citations (Scopus)


The activated sludge process is the most common method of secondary municipal wastewater treatment; solids retention time (SRT) is the key control parameter for this process. Typically, operating at long SRT is considered only for nitrification, but there are additional benefits of high SRT operation. This paper presents experimental and literature evidence to demonstrate three major additional benefits of long SRT operation: increased oxygen transfer efficiency; improved biomass particle size distribution, which results in more efficient clarification with fewer effluent particles and suspended solids; and enhanced removal of many emerging contaminants such as pharmaceuticals, personal care products, and endocrine disrupting compounds. This paper presents experimental results from several treatment plants that showed increasing oxygen transfer efficiency and particle size with increasing SRT, and evidence documenting improved removal of emerging contaminants and biodegradable organic carbon. A long-term survey of three treatment plants concludes that operating at higher SRT is not as energy intensive as typically assumed.
Original languageEnglish
Pages (from-to)42-53
Number of pages12
JournalWater Environment Research
Issue number1
Publication statusPublished - 1 Jan 2012
Externally publishedYes


  • Activated sludge
  • Emerging contaminants
  • Energy
  • Particle size
  • Solids retention time
  • Wastewater

ASJC Scopus subject areas

  • Environmental Chemistry
  • Water Science and Technology

Cite this