Which modified activated sludge process does not require primary treatment?

The modified activated sludge process that does not require primary treatment is the contact stabilization process. In this method, an influent wastewater is mixed with settled activated sludge in an aeration tank where the microorganisms consume the organic pollutants. The key aspect of contact stabilization is that it allows for the immediate treatment of wastewater without needing to remove solids or settle effluent beforehand, which is what primary treatment typically entails.

In contrast, other processes like complete mix aeration and tapered aeration often involve some form of primary treatment, such as sedimentation, to reduce the solids load before biological treatment occurs. Step aeration also incorporates a primary treatment phase by allowing the settling of larger solids before aeration. Therefore, contact stabilization's design and operational characteristics make it unique among modified activated sludge processes for enabling direct treatment of raw wastewater without requiring prior primary treatment.