【Weekly Topic】-009- A decrease in the sludge volume index can render the sludge inactive
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The last edit to this post was made by zhaolijun on 2015-7-13 at 10:36. One of the operating parameters for the activated sludge process is SVI, which stands for Sludge Volume Index and is used as a measure of the settling properties of activated sludge； Can it be indirectly determined that the activity of activated sludge has declined by reducing SVI?
This post was last edited by CoCo070563 on 2015-7-13 11:05. When the sedimentation behavior remains essentially unchanged, it can be considered as such. . . However, sludge bulking cannot be tolerated. . . For example, when the amount of sludge remains roughly constant but the SVI increases, it indicates that sludge bulking has occurred, meaning its sedimentability has declined. . .
A low SVI can lead to sludge compaction, which reduces its activity; for example, a sudden increase in inorganic suspended solids in the influent results in a decrease in the inorganic components within the activated sludge, causing the MLVSS to drop； But as you said, this only holds true when SV or MLSS remains essentially unchanged.
A decrease in SVI may be caused by reduced organic nutrient availability, excessive aeration time, a low reflux ratio resulting in high amounts of excess sludge being discharged, etc. If the weight of the activated sludge remains unchanged while its volume decreases, this indicates that its specific surface area also decreases, which in turn means that the area of contact between the activated sludge and the wastewater is reduced; this suggests that the activity of the sludge has decreased.
“Can a low SVI cause sludge compaction, thereby reducing activity? ! In biochemical systems, sludge is generally in a suspended state, right? . . This situation occurs in sedimentation tanks, but the sludge from these tanks is generally discharged directly to a sludge thickening tank for treatment. . . The main purpose of using SVI is to track the sedimentation properties of sludge, right? . . In a stable system, the amount of sludge remains relatively constant; by monitoring SVI, it is possible to assess the sedimentation properties of the sludge and monitor its condition, thereby detecting sludge bulking at an early stage. . . The above are purely personal opinions. . .
If SVI is used solely to determine sludge bulking, its utility is rather limited. Sludge bulking is not something that occurs easily in most wastewater treatment facilities, and the proper use of SVI can be very helpful for the efficient operation of such facilities. For example: it is quite useful as an auxiliary method for judging sludge aging and over-aeration.
How can SVI be used to assist in determining sludge aging and over-aeration? ? ? Could you please explain it in detail? Thank you. . .
SVI is an indicator for measuring the sedimentation performance of sludge, as well as an indicator for assessing its adsorption capacity. Generally, the higher the SVI value, the worse the sedimentation performance, but the better the adsorption performance ； Conversely, the smaller the SVI, the better the sedimentation performance, while the adsorption performance is worse, that is, the adsorption activity decreases. In the conventional activated sludge process, it is generally believed that an SVI value of around 100 yields the best overall results; values that are too high or too low are not conducive to improving the quality of the effluent.
The SVI of 100 that you mentioned is presumably the value obtained from statistics on urban sewage treatment plants. For different water qualities, this SVI is no longer a very accurate value. In the activated sludge process, the condition of the sludge can be assessed using SVI or SV; however, in the contact oxidation process, since the sludge adheres to the biological fillers, SVI or SV is not very useful. Personal opinion, for reference only.