The sludge volume index (SVI) is one of the most watched numbers in an activated sludge plant, and for good reason. It tells you how well your mixed liquor settles and compacts, and settling is what the secondary clarifier depends on. When SVI starts to climb, operators feel it before the lab report comes back: the settleometer looks fluffy, the clarifier blankets creep up, and on a wet day the weirs start to look nervous.
This article walks through what a rising SVI really means, how to confirm the number is real, what causes it, and how to keep your clarifiers in control while you find and fix the cause.
What SVI measures
SVI is the volume in milliliters occupied by one gram of mixed liquor solids after 30 minutes of settling. The formula is simple:
SVI (mL/g) = SSV30 (mL/L) x 1,000 / MLSS (mg/L)
If your 30 minute settled volume is 300 mL/L and your MLSS is 3,000 mg/L, the SVI is 300 x 1,000 / 3,000 = 100 mL/g. Typical guidance for conventional plants:
- Below about 80 mL/g: dense, fast settling sludge. Watch for pin floc and a hazy supernatant.
- About 80 to 150 mL/g: normal range for most plants.
- Above about 150 mL/g: slow settling and poor compaction. Clarifiers have less capacity.
- Above about 200 to 250 mL/g: bulking. Clarifier failure is likely at peak flows.
Every plant has its own "normal." A plant that runs 90 mL/g and is suddenly at 140 mL/g has a real change on its hands, even though 140 is "within range." Trend matters more than any single value.
Step one: make sure the number is real
Before you change anything in the process, confirm the SVI. A surprising number of SVI "problems" are sampling or lab problems.
Use matched samples. The MLSS used in the calculation must come from the same sample, at the same time, as the settleometer. Using yesterday's MLSS, or a composite MLSS with a grab settleometer, can swing SVI by 20% or more.
Watch for a crowded cylinder. When SSV30 is above about 800 mL/L, the sludge cannot settle freely and the test overstates SVI. Run a diluted SVI: mix one part mixed liquor with one part clarified secondary effluent, run the test, double the settled volume, and use the original MLSS. If the diluted SSV30 is still high after doubling, the sludge really is bulky.
Read the whole curve, not just 30 minutes. Record settled volume at 5, 10, 15, 20, 30 and even 60 minutes. A sludge that drops quickly then stalls behaves differently in a clarifier than one that settles slowly from the start. The settleometer article covers this in detail.
Note the supernatant. Clear supernatant with a fluffy blanket points to filaments or viscous bulking. Cloudy supernatant with a normal blanket points to dispersed growth or pin floc, which is a different problem.
Step two: look under the microscope
Once you trust the number, put a drop of mixed liquor on a slide and look at it at 100x. You are asking one question: is this filamentous bulking or something else?
Filamentous bulking shows long filaments extending out of the floc and bridging between flocs. The filaments hold the flocs apart so they cannot compact. Rate abundance on a 0 to 6 scale; at about 4 (very common) and above, filaments usually affect settling.
Viscous (non-filamentous) bulking shows floc that is spread out and loose, sometimes with a slimy, gelatinous appearance, but few filaments. An India ink stain shows large clear areas around the bacteria, which is extracellular slime holding water.
Dispersed growth shows many single bacteria and tiny clumps rather than true floc. SVI may not be very high, but effluent TSS will be.
If filaments are the problem, a Gram and Neisser stain will help narrow the type, and the type points straight at a probable cause. Our microscopy library has sketches and notes for common groups.
The common causes
Low dissolved oxygen
Low DO is the most common cause of filament growth in many plants. Filaments such as Sphaerotilus natans, Type 1701 and Haliscomenobacter hydrossis have a high surface area to volume ratio and grab oxygen more effectively than floc formers when DO is scarce. The problem does not need to be continuous. Two or three hours a day at low DO during the afternoon peak can be enough.
The fictional White River WRRF is a good example. On September 7, Blower B-2 tripped on a bearing fault, leaving only two blowers available until September 13. Average aeration DO fell from about 1.7 to 2.0 mg/L to about 1.0 mg/L, with lower values in the first grid at peak load. In six days the SVI rose from about 115 to about 150 mL/g, and S. natans and Type 1701 became common under the microscope. After the September 13 storm it reached 172 mL/g. That is the classic low DO pattern.
What to check: DO in every zone at 15 minute resolution, especially at peak load. Daily average DO hides low periods.
Low F/M and completely mixed conditions
Some filaments, such as Microthrix parvicella and Types 0041, 0675 and 1851, thrive when the food supply is low and evenly spread. Long SRT, low load seasons and completely mixed basins favor them. Plants with this problem often see bulking in winter.
What to check: F/M, SRT, and whether the plant has any high food zone (a selector) at the head of the basin.
Septic wastewater and sulfide
Thiothrix, Type 021N and Beggiatoa use reduced sulfur compounds. Septic influent, long force mains, hauled waste, septage and some industrial wastes deliver sulfide and organic acids that feed them.
What to check: influent ORP, sulfide, odor, and receiving logs for septage and hauled waste.
Nutrient deficiency
Bacteria need nitrogen and phosphorus in roughly a BOD:N:P ratio of 100:5:1. Industrial wastes high in carbohydrates and low in nutrients can starve the floc formers, leading to filament growth (Type 021N, Thiothrix) or viscous slime.
What to check: soluble ammonia and orthophosphate leaving the aeration basin. Aim for at least about 1 mg/L ammonia-N and 0.5 mg/L ortho-P in the effluent of the aeration basin (in a plant without nutrient limits).
Grease and oil
High grease loads support M. parvicella and Nocardioforms, which often show up as brown foam as well as bulking. See What Does Brown Foam Mean?.
Changes in wasting
A sudden SRT increase can shift the floc population toward low F/M filaments. A sudden SRT decrease can produce young, weak floc. Both change settling.
Protect the clarifiers while you fix the cause
Fixing the root cause takes two to three SRTs, which can be a month or more. Meanwhile you need to keep solids in the clarifier.
Run a state point analysis. Enter your current SVI, flow, RAS rate and MLSS into the state point tool. If the operating point is near or over the limiting flux, increase RAS until the underflow line clears the flux curve. More RAS is not always better: it adds hydraulic load to the clarifier and thins the RAS. The tool shows the balance.
Measure blankets every shift. A rising blanket is your early warning. At White River the blankets normally run 1 to 3 ft. During the September event they reached 6 to 7 ft at peak flow, and RAS was increased from 4.0 to 5.2 MGD to bring them down.
Put all clarifier area in service. A standby clarifier is worth more than any chemical.
Lower MLSS carefully, if SRT allows. Less mass going to the clarifier lowers solids loading. But do not cut SRT so far that you lose nitrification.
Consider short term tools. Targeted RAS chlorination, hydrogen peroxide, or a cationic polymer at the clarifier inlet can buy time. Chlorination needs careful dosing and microscope checks; it damages floc formers and nitrifiers if overdone. See the filamentous bulking guide.
Fix the cause, then be patient
Once you know the likely cause, fix it directly:
- Low DO: restore aeration capacity, fix diffusers, move air to the starved zone, and manage return streams.
- Low F/M: add or improve a selector, or adjust SRT.
- Septicity: control sulfide in the collection system, meter septage and hauled waste.
- Nutrients: add ammonia or phosphorus where needed.
- Grease: work with pretreatment on grease control.
Then hold steady. Big swings in wasting, RAS or DO setpoints make it hard to see what is working. SVI should begin to fall within one SRT of the fix and return to normal in two to three SRTs.
A simple SVI routine
- Run the settleometer daily, same time and location, with matched MLSS.
- Plot SVI, SRT, F/M, DO and temperature on one chart.
- Look at the microscope at least weekly, daily during an upset.
- Set an action trigger, for example SVI 30% above your 30 day average.
- When the trigger hits, confirm, look, run state point, and find the cause.
A rising SVI is not an emergency if you see it early. It becomes one when nobody connects the settleometer to the clarifiers and the clarifiers to the weather forecast.