High SVI

1Symptom

The 30 minute settled volume (SSV30) is high and the sludge volume index (SVI) has climbed above about 150 mL/g, or it has risen 30 to 50 points over a week or two. Clarifier blankets run deeper than normal, and at high flow you may start to see solids carry over the weirs.

2Check first

1. Run the settleometer yourself today. Read SSV at 5, 10, 15, 20 and 30 minutes and look at the supernatant: clear, cloudy, or full of straggler floc.
2. Confirm the MLSS used in the SVI is from the same sample and time. A bad MLSS number makes a bad SVI.
3. If SSV30 is above about 800 mL/L, repeat with a 1:1 dilution (diluted SVI). An undiluted test in a crowded cylinder overstates SVI.
4. Put a drop under the microscope at 100x. Are there filaments extending from the floc and bridging between flocs, or is the floc itself loose and spread out?
5. Measure the blanket in every clarifier and note which ones are deepest.

3Possible causes

• Filamentous bulking: low DO, low F/M, nutrient deficiency, septic influent or high grease feed favor filaments.
• Viscous (non-filamentous, zoogloeal) bulking: too much slime, often from nutrient deficiency or high F/M with readily degradable food.
• Low DO for several days, for example from a blower outage. At White River WRRF the SVI rose from about 115 to 150 in a week with one blower out.
• Septic or high sulfide influent, or septage and hauled waste dumps.
• Sudden SRT change, or an F/M that swings widely between weekday and weekend loads.
• Low pH or a toxic event that damaged floc formers more than filaments.

4Next investigation

• Trend SVI, MLSS, F/M, SRT, DO by zone and temperature for the last 30 days on one chart.
• Do a filament ID: Gram and Neisser stains, filament abundance (0 to 6 scale), and dominant type.
• Check nutrients in the aeration effluent: aim for at least about 1 mg/L ammonia-N and 0.5 mg/L orthophosphate.
• Check influent for septicity (sulfide, low ORP, black color, odor) and review hauled waste logs.

5Corrective actions

1. Protect the clarifiers first: run the state point tool and adjust RAS so the underflow line clears the flux curve.
2. Fix the cause the microscope points to: raise DO in the first zone, correct nutrients, stop or meter septage, or add a selector.
3. Hold SRT steady. Do not make large wasting swings to "fix" SVI. Changes of 10 to 15% per day are plenty.
4. If clarifiers are about to lose solids, consider targeted RAS chlorination (see the filamentous bulking guide) as a short term tool, not a cure.
5. Keep effluent TSS in check with more clarifier area in service if available.