Standard Article

Settling Tanks

Municipal Water Supply

  1. Nikolay Voutchkov

Published Online: 15 JUL 2005

DOI: 10.1002/047147844X.mw506

Water Encyclopedia

Water Encyclopedia

How to Cite

Voutchkov, N. 2005. Settling Tanks. Water Encyclopedia. 1:452–457.

Author Information

  1. Poseidon Resources Corporation, Stanford, Connecticut

Publication History

  1. Published Online: 15 JUL 2005


Settling tanks (clarifiers) are key wastewater treatment facilities used for gravity separation of settleable solids contained in the wastewater. Depending on their geometrical shape settling tanks could be rectangular or circular. Clarifier shape most suitable for a given application depends on a number of factors and has to be selected based on a cost-benefit analysis. Depending on their function, settling tanks are classified in two general categories: primary and secondary. The main purpose of the primary settling tanks is to remove suspended solids from the wastewater treatment plant influent. Secondary clarifiers are located downstream of the plant's biological treatment facilities, such as aeration basins or tricking filters, and are used to separate the biomass generated during the secondary treatment process from the treated plant effluent. Key design criteria used for sizing primary settling tanks are surface overflow rate and hydraulic detention time, while secondary settling tanks are usually designed based on surface and solids loading rates. Recommended values for these design criteria according to various design guideline sources are presented in a table format.


  • clarifiers;
  • settling tanks;
  • sedimentation;
  • activated sludge;
  • design;
  • surface overflow rate;
  • solids loading rate;
  • sludge;
  • solids;
  • performance