Aeration Tank | Activated Sludge Process | Wastewater Treatment Plant

 Aeration Tank

Operational Control Parameters

An operator can optimize the biological treatment process by monitoring the operational control parameters listed below.

MLSS/MLVSS

The mixed liquor suspended solids concentration (MLSS) is a measure of the total solids concentration in the aeration tank, which includes both inert and organic solids. 

• In conventional activated sludge plants, MLSS concentrations typically range from 2,000 to 4,000 mg/L. 

MLVSS (mixed liquor volatile suspended solids) is an indirect measure of the concentration of microorganisms in the aeration tank. 

• MLVSS should be between 70% and 80% of MLSS.

Residual Dissolved Oxygen

To oxidize organic waste, the microorganisms in the aeration tank require oxygen. To ensure that there is enough oxygen available, the residual dissolved oxygen (DO) concentration in the aeration tank must be monitored. A DO concentration of 2 to 4 mg/L is usually sufficient to produce high-quality effluent.

pH and Total Alkalinity

pH and total alkalinity, like residual DO concentration, are important parameters that can affect the health of activated sludge microorganisms. The optimal pH level for bacterial growth is generally between 6.5 and 7.5. Low pH values may inhibit nitrifying organism growth while promoting filamentous organism growth. 7.8 to 8.2 is the optimal pH range for nitrification.

Nitrosomonas bacteria oxidize ammonia to nitrite, and Nitrobacter bacteria oxidize nitrite to nitrate during the nitrification process.

Specific Oxygen Uptake Rate

The specific oxygen uptake rate (SOUR) is a relative measure of the rate of biological activity that measures the amount of oxygen consumed by microorganisms. The SOUR increases as microorganisms become more active, and vice versa. 

The SOUR is calculated by taking a sample of mixed liquor, saturating it with oxygen, and measuring the decrease in oxygen over time with a DO probe. The oxygen uptake rate (OUR), measured in mg O2/L-min, is divided by the MLVSS to produce the SOUR, measured in mg O2/g MLVSS-hr.

SOUR and final effluent COD can be correlated, according to research. As a result, changes in SOUR can be used to forecast final effluent quality. If SOUR rises, it indicates that the MLSS respiration rate has increased and that more oxygen is required to stabilize.

Color

• Reduce the sludge wasting rate if there is white, crisp foam on the surface of the aeration tank.
• The presence of a filamentous organism, most commonly Nocardia, is indicated by a thick, dark brown or grey, greasy foam.

Microscopic Examination of Biomass (Mixed Liquor)

• Qualitative assessments can be made based on observations such as the size and nature of floc particles, as well as the type and number of organisms.