Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various industrial, agricultural, and domestic purposes. Although there is a wide range of applicable technologies for treating different source of wastewater, the technologies most commonly used for performing main treatment stage (secondary treatment) of wastewater treatment plant rely on microorganisms suspended in the wastewater to treat it.
Primary treatment, which is often referred to as primary sedimentation, removes material that will either float or readily settle out by gravity. It includes the physical processes of sedimentation when gravitational forces allow for settleable and suspended solids to settle due to being heavier than water. Accumulated solids are removed and disposed, and primary treated wastewater continues to next treatment step.
Secondary treatment is the next step in the treatment process. Secondary wastewater treatment is a biological treatment process designed to reduce the number of organic materials in the wastewater before it is discharged to a disposal or sent to tertiary treatment and disinfection. Secondary treatment produces a much higher quality of effluent than a primary treatment. The process involves the removal of biodegradable organic matter, nutrients, and suspended solids (TSS) using aeration and clarification. Traditional secondary treatment is typically characterized as producing a treated wastewater effluent with a BOD of 25 mg/L or less and TSS of 30 mg/L or less.