Digester Souring After the Thickener Failed
A 1,800 m³ mesophilic anaerobic digester runs at 35.5 °C on 55 m³/d of 5.0% thickened primary plus waste activated sludge, 65% volatile solids, giving about a 33-day HRT. Normal gas is 3,200 m³/d at 64% methane; VFA runs 150 mg/L as acetic with 3,000 mg/L alkalinity as CaCO3. The dissolved air thickener failed two weeks ago, so the plant has been feeding raw waste activated sludge at 0.8% solids instead, about 160 m³/d. Gas is now 1,400 m³/d at 58% methane, VFA is 1,400 mg/L, alkalinity 3,000 mg/L, pH 6.8 and falling. The gas-mixing compressor has been down for ten days and the sludge recirculation pumps run intermittently. Effluent ammonia is 1,600 mg/L as N.
1. Interpret the numbers. What stage of failure is this digester in, and which single measurement would you ask the lab for next?
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The ratio is the headline. VFA/alkalinity = 1,400 / 3,000 = 0.47. Healthy is roughly 0.1–0.2; above about 0.3–0.4 indicates the acid formers are outpacing the methanogens. This is an established upset, not an early warning.
The gas confirms it. Gas volume down 56% and methane down from 64% to 58% means the acetoclastic methanogens are inhibited; the carbon is going to volatile acids and CO2 instead of methane.
The pH is a lagging indicator. Because the system is buffered at 3,000 mg/L, pH only starts falling once the bicarbonate is largely consumed. A pH of 6.8 and dropping means you are close to losing the buffer entirely, and methanogens slow sharply below about 6.6–6.7. Do not wait for a lower pH to act.
The cause is written into the feed data. Switching from 55 m³/d at 5.0% to 160 m³/d at 0.8% cuts the solids load but nearly triples the hydraulic load: HRT collapses from ~33 days toward roughly 11 days, well under the methanogen doubling time, and the cold, dilute feed drags temperature down. Add ten days without gas mixing and intermittent recirculation and you have washout plus stratification plus localized acid pockets.
Ask for the individual VFA speciation, especially propionate. Acetate can be consumed quickly; propionate above roughly 1,000 mg/L is the classic marker of impending or actual methanogenic failure and it recovers slowly. Also ask for a feed sulfide and a check on any industrial dischargers - a toxic shock would produce a similar picture and changes the response.
2. Give your immediate action list in priority order, and state explicitly what you would not do.
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Do, in this order:
- Stop or drastically cut the feed. Starve the digester rather than feed it. Hold the raw WAS in the thickener/storage or send it to an alternate route. The single most common operator error is to keep feeding a souring digester because the gas demand has not changed.
- Restore temperature to 35–37 °C and verify it at multiple depths - the dilute feed has been cooling the tank.
- Restore mixing. Repair the gas compressor, or run the recirculation continuously. Stratification creates acid pockets that the monitoring misses.
- Add alkalinity - sodium bicarbonate is preferred. It buffers without a pH spike. Lime will raise pH fast but can precipitate calcium carbonate, cause scaling on heat exchangers and mixers, and push pH past the point where you can read the trend.
- Notify your supervisor and begin planning the contingency: if gas production keeps falling and pH passes below about 6.5, you are looking at hauling sludge, and hauling capacity needs to be booked days ahead, not hours.
Do not:
- Do not add air or oxygen to "sweeten" it - you will kill the obligate anaerobes outright.
- Do not shock-dose a commercial bacterial product in place of stopping the feed.
- Do not drive pH up rapidly to 7.2+; a fast swing is its own insult.
- Do not resume full feed the moment gas ticks up - see the recovery criteria below.
3. What criteria would you use to prove recovery and resume feeding, and over what timeframe?
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Recovery criteria - all of them, not one:
- Gas production climbing steadily back toward the 3,200 m³/d baseline, with methane fraction rising above about 62%. Rising gas volume with poor methane content is fermentation, not recovery.
- VFA falling and VFA/alkalinity back below about 0.2, sustained over several consecutive days.
- Propionate trending down - it recovers last and is the most honest signal.
- pH stable at 7.0–7.3 without continued chemical addition. If pH only holds because you are dosing bicarbonate, you have not recovered.
- Temperature and mixing stable and verified.
Resumption schedule: restart at roughly 25% of design volatile solids load, and increase only after the VFA/alkalinity ratio has held steady for 3–5 days. Step up in increments of 10–20% of design load, keeping the same hold period between steps. Expect this to take four to eight weeks for a severely soured mesophilic digester - propionate degraders grow slowly and there is no shortcut.
Then fix the root cause in parallel: get the DAF thickener back in service or arrange an alternative thickening method, and write the rule into the operating procedure - never double the digester hydraulic load to compensate for a thickening failure. If the thickener is unreliable, the standby plan should be a dedicated holding tank, not raw feed to the digester.