Questions of Excess Sludge

Biochemical Treatment is to effectively remove the soluble and partially insoluble organics in wastewater by using the life activity process of microorganisms, so as to purify the water. Gather all the innumerable microorganisms in a pool to create an environment (such as temperature, pH value, oxygen, nitrogen, phosphorus and other nutrients) that is very suitable for microbial reproduction and growth, so that microorganisms can proliferate in a large number to improve the speed and efficiency of their decomposition of organic matter. Then the wastewater is pumped into the tank, so that the organic substances in the wastewater can be oxidized and degraded during the life activities of microorganisms, so that the wastewater can be purified and treated.

What is DO?

Dissolved Oxygen (DO) refers to the dissolved amount of oxygen in water, expressed in mg/L. Different Biochemical Treatment methods have different requirements for dissolved oxygen. In the facultative biochemical process, the dissolved oxygen in water is generally between 0.2-2.0mg/L, while in the SBR aerobic biochemical process, the dissolved oxygen in water is generally between 2.0-8.0mg/L. Therefore, the aeration volume shall be small and the aeration time shall be short during the operation of facultative tank; In SBR aerobic tank operation, the aeration volume and aeration time are much larger and longer, while we use contact oxidation, and the dissolved oxygen is controlled at 2.0-4.0mg/L.

Who provides the oxygen required by microorganisms in the biochemical process?

Oxygen required by microorganisms in biochemical process is mainly provided by Roots blower.

What is Excess Sludge?

In the process of Biochemical Treatment , microorganisms in activated sludge continuously consume organic substances in wastewater. Among the consumed organic substances, some organic substances are oxidized to provide energy for microbial life activities, while the other organic substances are used by microorganisms to synthesize new cytoplasm, so that microorganisms can reproduce. While microorganisms metabolize, some old microorganisms die, resulting in excess sludge.

How to Estimate the Production of Excess Sludge?

In the process of microbial metabolism, some organic substances (BOD) are used by microorganisms to synthesize new cytoplasm to replace dead microorganisms. Therefore, the amount of excess sludge generated is related to the amount of BOD decomposed, and the two are related. In the engineering design, it is generally considered that each kilogram of BOD5 is treated to produce 0.6-0.8kg of surplus sludge (100%), which is converted into 3-4kg of dry sludge with a moisture content of 80%.

How to Control the Discharge of Excess Sludge?

• Sludge Control:

If the fluctuation of inflow and organic concentration in the aeration tank is small, the discharge of excess sludge can be calculated only by the amount of mixed liquor sludge in the aeration tank.

Discharge amount of excess sludge=sludge amount of mixed liquor in aeration tank/(sludge age x return sludge concentration) sludge amount of effluent from secondary sedimentation tank. When the inflow fluctuates, the sludge amount of secondary sedimentation tank shall also be included.

• Sludge Concentration Control:

The sludge concentration of the mixed liquor in the aeration tank generally has an optimal value. If it is higher than this value, the sludge must be discharged in time.

Excess sludge discharge amount=difference between the concentration of mixed liquid in the aeration tank and the ideal concentration × Control of aeration tank volume/return sludge concentration

• Sludge Load Control:

According to the principle of keeping the sludge volume in the aeration tank unchanged, calculate the sludge output according to the sludge load, and discharge all newly generated sludge from the system.

Excess sludge discharge amount=(sludge amount of mixed liquor in the aeration tank - BOD5 amount of influent water/sludge load)/return sludge concentration,

• Sludge Sedimentation Ratio Control:

When the measured sludge sedimentation ratio SV increases, it may be caused by the increase of sludge concentration, or it may be caused by the deterioration of sludge sedimentation performance. In either case, the excess sludge should be removed in time to ensure the relative stability of SV.

It has been proved that SRT is an ideal Biochemical Treatment method to control excess sludge discharge (Q) for urban sewage with the emphasis on nitrogen and phosphorus removal.

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