MLSS is the mg/L of suspended solids in the aeration tank and is simply a measure of the solids themselves, having been filtered, and the water evaporated in a drying oven, then weighed. The SVI is a measure of how well the solids settle, or compact, in the quiescent environment of the clarifier and mimicked in a settleometer, in 30 minutes. A high SVI is a bulking or "fluffy" sludge that does not compact well. Since SVI is an expression of "how many mls of the settled sludge it takes to make a gram", a low SVI means it is compacting well and consequently takes less mls to make that gram. If you have a high SVI you can do a 50:50 test, using 1/2 plant effluent and 1/2 activated sludge and try that in the settleometer. If it settles much better, you are carrying too many solids and need to waste more sludge from the system. If it settles only slightly better, then you probably have an issue with long filament dominence and need to find the problem. It is often related to areas in the system with very low D.O. such as a recycle wastestream from an aerobic digester with only intermittent aeration or even scum build up on the basins, but there are various types of filaments, and even a long line feeding the plant can select for them as it loses oxygen.

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MLSS is the mg/L of suspended solids in the aeration tank and is simply a measure of the solids themselves, having been filtered, and the water evaporated in a drying oven, then weighed. The SVI is a measure of how well the solids settle, or compact, in the quiescent environment of the clarifier and mimicked in a settleometer, in 30 minutes. A high SVI is a bulking or "fluffy" sludge that does not compact well. Since SVI is an expression of "how many mls of the settled sludge it takes to make a gram", a low SVI means it is compacting well and consequently takes less mls to make that gram. If you have a high SVI you can do a 50:50 test, using 1/2 plant effluent and 1/2 activated sludge and try that in the settleometer. If it settles much better, you are carrying too many solids and need to waste more sludge from the system. If it settles only slightly better, then you probably have an issue with long filament dominence and need to find the problem. It is often related to areas in the system with very low D.O. such as a recycle wastestream from an aerobic digester with only intermittent aeration or even scum build up on the basins, but there are various types of filaments, and even a long line feeding the plant can select for them as it loses oxygen.