Activated sludge

Sludge that has been aerated and treated with microorganisms in order to accelerate microbial activity and the oxidation (decomposition) of organic compounds.

How does it work?

• Activated sludge is a biological process that utilizes microorganisms to convert organic and certain inorganic matter from wastewater into cell mass.
• The activated sludge is then separated from the liquid by clarification.
• The settled sludge is either returned (RAS) or wasted (WAS).

Wastewater treatment

Activated sludge is commonly used as a wastewater treatment process because it is an effective and versatile treatment process and capable of a high degree of treatment.

Role of microorganisms

• The principle role microorganisms have in the activated sludge process is to convert dissolved and particulate organic matter, measured as biochemical oxygen demand (BOD), into cell mass.
• In a conventional activated sludge process, microorganisms use oxygen to break down organic matter (food) for their growth and survival.
• Over time and as wastewater moves through the aeration basin, food (BOD) decreases with a resultant increase in cell mass (MLSS concentration). The activated sludge wastewater treatment process must operate under proper environmental conditions to support a healthy, growing population of microorganisms.

Monitoring for conditions

The operator must monitor the activated sludge process to ensure the right environmental conditions are being provided for the microorganisms. Efficient wastewater treatment plant performance will then be achieved.

Organic matter

• Incoming wastewater to a treatment plant provides the food that microorganisms need for their growth and reproduction. This food is mostly organic material.
• The more soluble the organic material is, the more easily microorganisms can use it. Since the amount and type of organic loading in the treatment plant affects the growth of the microorganisms, influent total BOD and soluble BOD are measurements an operator can make to determine the organic loading and organic overload.