Activated Sludge | Activated Sludge Process | Wastewater Treatment Plant

Activated Sludge 

The growth of organisms (including bacteria) in aeration tanks with dissolved oxygen produces activated sludge, which is made up of sludge particles teeming with living organisms and created in either raw or settled wastewater. 

Activated Sludge Process

In Secondary Treatment, the Activated Sludge Process is one of several biological wastewater treatment options. The organisms in the mixed liquor breakdown the wastes in the treated wastewater quickly when Activated Sludge is added with wastewater. The mixed liquor generally flows to a separate tank called a clarifier after a required period of aeration and agitation in the aeration tank, where the activated sludge settles and the remaining liquid is discharged as effluent. The settled sludge is either disposed of as waste activated sludge or reused as return activated sludge in the aeration tank. To keep an adequate population of organisms, some sludge must always be returned to the aeration tanks.

Microorganisms are used in the process to accelerate waste decomposition.

• Food is referred to as Biochemical Oxygen Demand (BOD). BOD is a measure of the amount of oxygen in the incoming wastewater. A high demand will exist for a strong wastewater, whereas a low demand will exist for a weak wastewater. BOD is a measure of how much oxygen is required to stabilize the waste (or food) in wastewater.
• Mixed Liquor Volatile Suspended Solids is the name given to the mass of organisms (MLVSS).
• Mixed Liquor Suspended Solids refers to the total concentration of suspended solids in an aeration tank (MLSS). This is primarily made up of microorganisms and non-biodegradable suspended matter.

 

When wastewater is mixed with activated sludge, the following happens: 

• Microorganisms in wastewater feed and grow on waste particles.
• Waste is removed and wastewater is partially cleaned as organisms grow and reproduce.
• Organisms require a balanced supply of food (BOD) and oxygen. BOD is naturally present in wastewater, and oxygen is added by aeration equipment.
• The F/M ratio, or food to microorganism ratio, is the balance of food to organism mass. To achieve proper performance from the activated sludge process, an appropriate F/M ratio is required.

 

In waste treatment, the activated sludge process results in the oxidation and removal of soluble or suspended solids. 

• In an aeration tank, this treatment takes only a few hours.

When organisms partially oxidize solids, they produce stabilized soluble or suspended solids.

• Carbon dioxide, water, sulphate, and nitrate compounds are formed as a result of organism activity.
• During sedimentation, remaining solids are converted into a form that can be settled and removed as sludge.

For the activated sludge process to work properly, the operator must control the number of organisms and the dissolved oxygen level in the aeration tank, as well as the treatment time in the aeration tank. When these variables are under control, the process will run smoothly.