ABSTRACT
G RAVITY is the moving force in primary treatment. Primary treatment unit processes (screening, degritting, and primary sedimentation) remove pollutants that either float or settle out of the effluent, leaving behind about 50% of the raw pollutant load. Biological activity is the moving force behind secondary treatment. Secondary treatment includes methods that use biological processes to convert dissolved, suspended, and colloidal organic wastes to more stable solids, which can either be removed by settling or harmlessly discharged to the environment. Biological treatment (sometimes called secondary treatment, but in practice, biological treatment is part of secondary treatment processes) provides BOD removal well beyond the levels that primary treatment can achieve, producing an effluent with not more than 30 mg/L BOD5 and 30 mg/L total suspended solids to meet Clean Water Act (CAA) requirements (see Figure 16.1).
