Sludge Volume Index, also known as SVI, is another way for wastewater treatment operators to control the activated sludge process. It is a number that is used to determine if the activated sludge process is working correctly.
Simply by looking at the number, an operator will be able to tell if the aeration tank is operating abnormally. For example, if the SVI is too high, it means that the activated sludge is settling slowly and not compacting well. In these cases, the sludge usually looks fluffy. Often, this is a result of a young sludge, during the start-up of a new plant. SVI can be as high as 250 mL/g or more.
What happens when the sludge doesn’t settle well? Excessive amount of the microorganisms will leave the activated sludge process, and lead to a higher total suspended solids concentration in the effluent. Therefore, it’s important for operators to monitor the SVI and look for trends to prevent problems from occurring.
SVI is calculated using the following equation. Note that both the numerator (top of equation) and denominator (bottom of equation) are obtained from lab tests. The numerator is obtained from a 30-minute settleability test, while the denominator is the suspended solids concentration (MLSS) in the aeration tank. The SVI is always in mL/g format.
FORMULA FOR SLUDGE VOLUME INDEX – SVI
By looking at this number, the operator should control the wasting rate of the sludge to increase or decrease it, to ensure a proper amount of microorganisms in the aeration tank.
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If you’re preparing to take the Wastewater Treatment Operator CertificationTest, my suggestion is to solve a lot of practice problems. It’s a great way to test your knowledge before the actual test. It will also give you an idea of what to except on the exam.