Secondary treatment

Secondary treatment is a stage in wastewater treatment designed to substantially degrade the biological content of the sewage derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc.

Process

Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment. This is achieved by letting the water stand undisturbed for six to eight hours in a secondary clarifier, allowing the micro-organisms to settle to the bottom. The top water is then removed and treated for disinfection in a process known as tertiary treatment.

Types of Secondary Treatment

There are two main types of secondary treatment:

• Activated sludge process: This is a process dealing with the treatment of sewage and industrial wastewaters and developed around 1912-1914. There is a large variety of activated sludge processes available for use in secondary treatment.

• Trickling filter process: This is a type of wastewater treatment in which a bed of high surface area material (e.g. plastic or a type of stone) is used as a final stage of effluent treatment.

See Also

• Primary treatment
• Tertiary treatment
• Wastewater treatment
• Sewage treatment