Sludge Volume Index (SVI)


Sludge volume index (SVI) is the most widely used indicator of sludge settleability, with a direct relationship acting between the bound water content of flocs and the SVI.

Sludge settleability (SS) is a measure of the ability of the activated sludge to separate from the mixed liquor in order to achieve adequate sludge recycle and a final effluent with a low suspended solids concentration. SS is dependent on the microbiological, biochemical, and physico-chemical properties of the sludge, as well as sludge loading.

The Sludge Volume Index, and by extension Sludge Settlebility, is an important empirical measurement that can be used as a guide to maintain a sufficient concentration of activated sludge in the aeration basin, where too much or too little can be detrimental to the system’s overall health.

SVI gives a more accurate picture of the sludge settling characteristics than settleability or MLSS alone. Most importantly, the SVI can indicate changes occurring in the activated sludge treatment process. The best SVI for each plant should be determined when the facility is running at optimum, and should be used as a benchmark.

Desludging, sometimes referred to as the recycling sludge process, actually plays a very important role in plant operation because the whole operation is needed to somehow strike a balance between removing dead or aged bacteria out of the systems or to determine how much goes back to the aeration pond.

Calculating SVI

The standard SVI test requires a 1-liter graduated cylinder for the MLSS settling test. A separate aliquot of mixed liquor is used for a total suspended solids (TSS) test. A fresh sample of mixed liquor should be used for the tests and is normally collected from the effluent end of the aeration system, just upstream of the secondary clarifier.

The sludge volume index (SVI) is the volume in milliliters occupied by 1 g of a suspension after 30 min settling. SVI typically is used to monitor settling characteristics of activated sludge and other biological suspensions.

SV       = volume of the settled sludge in the graduated cylinder (mL)

MLSS  = mixed liquor suspended solids (mg/L).

Effects on WWTPs

Implications of SVI on Plant Operation

Activated sludge systems are sensitive to fluctuations in the organic strength of the wastewater, which seriously affects sludge characteristics and the SVI, with carbohydrates in particular encouraging filamentous and zoogloeal growths in the sludge, causing a rapid increase in SVI.

SVI = 80 mL/g or less: Indicates a sludge that is dense and has rapid settling characteristics. 

Typically old, over-oxidized sludge, the floc particles would be dense and granular in appearance. As this type of sludge settles, it may leave a cloudy appearance in the supernate above the settled sludge blanket. This turbidity is called pinpoint floc (pin-floc). The sludge usually begins settling quickly after the start of the sludge settleability test, and it does not form larger particles before settling. Effluent BOD results may be below requirements, but TSS levels can still be high.

SVI = 100 to 200 mL/g. Most activated sludge plants seem to produce a clear, good-quality effluent with an SVI in this range.

The sludge typically settles more slowly and traps more particulate matter as it forms a uniform blanket before settling. Microscopic examination of this MLSS would show an irregularly shaped floc particle with some filaments forming a backbone for floc-forming bacteria to attach and colonize. The sludge first forms a blanket and seems to flocculate together before starting to settle usually within the first five minutes of the test. As the particles come together, they form larger particles that have a specific gravity greater than water. As the sludge settles, channels form in the sludge by the liquid being squeezed out of the sludge as it compacts.

SVI = 250 mL/g or higher. 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. A high SVI may 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.