The sludge volume index (SVI) is an indicator for measuring the settling performance of activated sludge. A high SVI usually means poor settling performance of sludge and is prone to problems such as sludge swelling. The following is the analysis of the reasons and solutions:
cause analysis
-Filamentous bacteria expansion: The proportion of nutrients in sewage is imbalanced, such as an inappropriate carbon nitrogen phosphorus ratio and a lack of nutrients such as nitrogen and phosphorus, leading to the proliferation of filamentous bacteria. The presence of filamentous bacteria can loosen the structure of sludge, making it difficult to settle, thereby increasing SVI.
-Excessive sludge age: If the sludge stays in the aeration tank for too long, it will cause sludge aging, reduce its activity, and deteriorate its adsorption and sedimentation performance, resulting in an increase in SVI value.
-Insufficient dissolved oxygen: If the concentration of dissolved oxygen in the aeration tank is too low, the metabolic activity of microorganisms will be inhibited, aerobic microorganisms cannot grow and metabolize normally, and some facultative anaerobic or filamentous bacteria may proliferate in large numbers, leading to deterioration of sludge settling performance and increase in SVI.
-Water temperature changes: Water temperature has a significant impact on the performance of activated sludge. Excessive or insufficient temperature can affect the growth and metabolism of microorganisms, thereby affecting the settling performance of sludge. For example, in low-temperature environments, the metabolic rate of microorganisms decreases, the settling performance of sludge may deteriorate, and SVI may increase.
Solution measures
-Adjusting nutrient ratio: detecting the content of nutrients such as carbon, nitrogen, and phosphorus in sewage, adding appropriate amounts of nitrogen, phosphorus, and other nutrients according to the actual situation to achieve a suitable range of carbon, nitrogen, and phosphorus ratio, generally BOD ₅: N: P=100:5:1, to suppress the excessive reproduction of filamentous bacteria and improve the settling performance of sludge.
-Control sludge age: Reasonably control sludge age according to the properties of sludge and the requirements of treatment processes. By implementing measures such as sludge discharge, the age of the sludge is kept within an appropriate range. Generally, the age of the activated sludge process is controlled at around 5-15 days to prevent sludge aging and improve its activity and settling performance.
-Increase dissolved oxygen: Optimize the aeration system, increase the aeration volume, and increase the dissolved oxygen concentration in the aeration tank. Generally, the dissolved oxygen is controlled at around 2-4mg/L to meet the growth and metabolic needs of aerobic microorganisms, inhibit the proliferation of harmful microorganisms such as filamentous bacteria, and improve the settling performance of sludge.
-Control water temperature: Install water temperature regulating devices in the sewage treatment system to keep the water temperature within the appropriate range for microbial growth, generally 20-30 ℃. When the water temperature is too high, cooling measures can be taken; When the water temperature is too low, heating measures can be taken.
In actual operation, various factors need to be comprehensively considered to determine the best solution through small-scale or pilot tests.
