Activated sludge is the most commonly used biological process for treating municipal and industrial wastewaters. Although the following discussion focuses primarily on activated sludge and solids contact secondary clarifiers, much of the information is applicable to secondary clarifiers following other biological processes as well. The performance of all biological wastewater treatment processes is profoundly affected by the solids capture efficiency of the secondary, or final, clarifier. Solids escaping secondary clarifiers are primarily organic, contributing substantial oxygen demand when released to the receiving waters. In addition, these biological solids contain significant concentrations of the nutrients nitrogen and phosphorus. Heavy metals and some non-biodegradable toxic organics that partition to the solids phase are released into the environment associated with effluent suspended solids (ESS). Finally, ESS greatly impact the performance of downstream processes such as filtration and disinfection.