Digester Foaming and Gas Production Drop
A mesophilic anaerobic digester receives primary and waste activated sludge at about 4 percent solids. For two weeks the gas production has fallen from 9,000 to 6,500 cubic feet per day, and the digester has developed a thick foam layer that is entering the gas line. The volatile solids destruction has dropped from 55 to 42 percent. The digester temperature is steady at 95 degrees F, but the pH has drifted from 7.2 to 6.7 and the volatile acids are 3,200 mg/L (normally 400 mg/L). The operator has been feeding the digester at the same rate but recently changed the sludge thickening practice, sending thinner waste activated sludge to the digester. The gas line has a flame arrestor and a pressure relief valve, and the foam is now visible at the sight glass.
1. Diagnose the likely cause of the foaming and gas decline, and explain what the volatile acids and pH trends indicate.
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The pattern is classic digester upset from organic overload or a change in feed characteristics. Volatile acids at 3,200 mg/L with pH falling to 6.7 means acid-forming bacteria are outpacing methane formers, so the buffer capacity is being consumed. The drop in gas production and volatile solids destruction confirms reduced methane formation.
The recent change to thinner waste activated sludge is a plausible trigger: thinner feed means more water and a different solids composition, and if the volatile solids loading per unit digester volume rose or the feed became more readily acidified, the acid formers surge. Foaming in digesters is often associated with filamentous organisms, high fatty acid concentrations, or rapid changes in feed. The foam entering the gas line is a safety concern because it can block the line and cause pressure problems.
The operator should stop or reduce feeding, check mixing, verify temperature uniformity, and test volatile acids and alkalinity to assess buffer capacity before resuming feed.
2. List the immediate safety and process actions, in priority order, and state what must be verified before returning to normal feeding.
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Priority actions: (1) Protect personnel and equipment: treat the gas line and digester headspace as a confined-space and explosion hazard, keep ignition sources away, and verify the flame arrestor and pressure relief valve are clear of foam. Do not open the digester without confined-space procedures. (2) Stop or sharply reduce feed to let the acid formers and methane formers rebalance. (3) Check and restore mixing, and verify temperature is uniform throughout the tank. (4) Measure volatile acids, alkalinity, pH, and gas composition (methane and carbon dioxide) to track recovery; a rising methane fraction and falling volatile acids indicate recovery.
Before returning to normal feed, confirm volatile acids are trending down, pH is recovering toward 7.0 or above, gas production is stable, and the foam layer has collapsed. Resume feed gradually and monitor daily. Document the event, the feed change that preceded it, and the corrective actions in the operating record.