Activated sludge technology is a biological treatment process for treating blackwater and greywater. Activated sludge treatment removes organic wastes, and treats nitrogen and phosphorus. The process degrades organic matter and assimilates nutrients by using fixed film systems or using mechanical aeration and mixing and clarification to suspend bacterial communities in wastewater. Treatment typically provides secondary treatment and can be modified to provide tertiary treatment. Sludge systems can achieve enhanced phosphorus reduction by adding aerobic and anaerobic stages for removal of biological nutrients. Additionally, the biological process can be speeded up to achieve treatment goals within a few hours by adding aeration. Activated sludge plants are, generally, centralized systems that serve small to large cities.