Why sludge volume index is important

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. A high SVI may also indicate filamentous sludge bulking. In this case, a microscopic exam is recommended and might show light floc particles that contain long filaments extending out of the particle and touching filaments from other particles.

Or, the filaments may be contained within the floc, causing a dispersed, open floc structure. In these cases, the liquid above the sludge blanket is usually very clear. The sludge can sit in the settleability test container for long periods and settle very little, or not at all. When choosing a method of filamentous control chlorination or other oxidizer , SVI should be calculated and used in trend charts to show the effectiveness of control.

Example 1. The SVI calculates to If the supernatant is very clear, then filamentous sludge bulking may be the cause of the high SVI.

In the actual clarifier, the sludge blanket might be seen below the surface. Example 2. The SVI is over Look for a young sludge condition, the result of toxic influent loading, or a clarifier solids washout event. Example 3. This SVI is 61 and indicates a rapid-settling sludge condition. The clarifier may be somewhat cloudy, and pinpoint floc particles might be seen in the settleability test container.

An old, over-oxidized MLSS may be the cause. Example 4. Some general SVI guidelines are given below:. An SVI of 80 or less usually indicates a sludge that is dense and has rapid settling characteristics. Most activated sludge plants seem to produce a clear, high-quality effluent with an SVI in the range of to The sludge typically settles slower and traps more particulate matter as it forms a uniform blanket before settling.

At this elevated SVI, the sludge settles very slowly and compacts poorly in the settleability test. The MLSS looks light and fluffy, not very dense. Raising the amount of MLSS reducing waste rates changes the density of the floc, creating a heavier sludge particle. The more dense the particle, the more likely it will settle faster. To increase the SVI, an operator would increase the waste sludge rate, effectively creating a less dense particle that settles slightly slower.

This same particle might also trap more fine suspended solids as it settles, clarifying the effluent even more. SVI is a very useful tool when using chlorine or other oxidizers to control sludge bulking conditions. Combining SVI with microscopic exam results can give the operator a great way to monitor the effect of oxidizers on overall sludge characteristics.

For instance, at certain SVI values, the operator knows the plant experiences sludge bulking and solids washout of the secondary clarifiers. Calculating the SVI for each MLSS sample and settleability test gives the operator of an activated sludge plant a valuable tool that can help prevent problems before they begin. Knowing what the SVI is for a given condition at a wastewater treatment plant and plotting the data on a trend chart, process control adjustments can be made before problems get out of hand.

Value of Sludge Volume Index can then be calculated from the formula given here. Basically what the value represents is the settling characteristics that have profound effect on the return rates and also the MLSS value. With this value in mind, generally the clarifier or settling basin has to be designed to accept higher solid loadings so that loss or carry over of sludge due to hydraulic displacement can be minimized. Activated sludge pond is designed to allow adjustments on the amount of sludge return and also the take off rate.

Regular desludging must be carried out to remove the aged sludge so that the new bacteria can regenerate and allowed to grow.