Sour Digester After Septage Acceptance Begins
Your 500,000-gallon primary anaerobic digester (95°F, 20-day HRT, 2,800 lb VSS/day) has run stably for a year, producing 12,000 ft³/day of 62% methane gas. Over eight days gas production has fallen to 6,200 ft³/day and the boiler can no longer hold temperature; the digester is at 92°F. This morning's lab: volatile acids 3,600 mg/L as acetic acid, alkalinity 1,900 mg/L as CaCO3, pH 6.6. Supernatant is dark and odorous. The only upstream change is that the plant began accepting septage at the headworks three weeks ago - up to 8,000 gallons/day with no pretreatment. Solids loading to the digester has not otherwise changed.
1. Interpret the numbers. Calculate the volatile acid to alkalinity ratio, explain why pH 6.6 understates how far along the upset is, and classify the severity.
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Ratio VA/ALK = 3,600 / 1,900 = 1.9
Typical benchmarks for a healthy mesophilic digester: below about 0.1 is normal, 0.1-0.3 is a warning band, 0.3-0.5 means the acid formers are outrunning the methane formers, and above 0.5-1.0 is a serious upset. At 1.9 the digester is in advanced, severe inhibition - the methane formers are largely knocked back and the acid formers are still producing.
Why pH is misleading The digester's bicarbonate system absorbs volatile acids and holds pH near 7.0 until a large share of the buffering capacity is used up. When the alkalinity buffer is exhausted, pH falls steeply because it responds logarithmically to acid accumulation. So pH 6.6 with 1,900 mg/L alkalinity remaining means the buffer is being consumed now and the pH cliff is close. pH is a lagging indicator; the VA trend is the leading one.
Caution on the VA number Confirm the lab is reporting volatile acids as acetic acid on a filtered sample and not a total acidity or an unfiltered value. A single high result should be repeated before major decisions - but the gas drop, the temperature loss, and the dark supernatant all corroborate a real upset.
Working diagnosis The digester is unbalanced, not merely under-heated: the temperature fall is a consequence of losing methane to the boiler, not the initiating cause. Note that a temperature shock usually shows a sharp drop in gas within a day, whereas a feed-related inhibition tends to be slower - the eight-day pattern fits the septage start (three weeks earlier) better.
2. Give the first 72-hour response in order of priority. Where is the line between stabilizing the digester and masking the problem?
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1. Stop the cause first (immediately) Stop accepting septage at the headworks. Suspend, or cut to a trickle, digester feed. Do not feed a sour digester just because it is being seeded - the acid formers will respond faster than the methane formers and push the VA higher.
2. Protect the temperature (hours) Use whatever supplemental heat is available (a rental boiler, the second-stage gas, or the sludge heater on a separate fuel). Below about 90°F the methane formers slow further, and a cold digester takes weeks to recover. Keep it in the 95-98°F band with gentle mixing.
3. Restore alkalinity - carefully (hours to days)
- Preferred: sodium bicarbonate, dosed to bring the alkalinity back to 2,500-3,000 mg/L as CaCO3 and pH into the 7.0-7.4 range.
- Lime slurry is a second choice: it is cheap and fast, but it can spike the pH locally, scale the heater and piping with calcium carbonate, and add sulfide problems. Dose into a well-mixed point, never slug-loaded.
- The line between stabilizing and masking is this: adding alkalinity under a continued or resumed feed load just buys time against a rising VA. Treat the alkalinity dose as a stopgap that must be paired with a shrinking VA trend. If VA is still rising two days after the pH is restored, the cause has not been controlled.
4. Seed (days) Bring in 10-20% of digester volume of seed sludge from a healthy digester, or seed with the second-stage supernatant and solids. Seed is usually the fastest path back.
5. Reduce mixing intensity Gentle, intermittent mixing keeps the sludge from stratifying and forming a sour crust, but vigorous continuous mixing can keep re-suspending acid-rich material against the methane formers.
6. Monitoring cadence Twice daily: temperature, gas production, gas composition (methane and CO2). Daily: VA, alkalinity, pH. Also track ammonia and sulfide. Recovery shows first as rising methane percentage and falling CO2, then as a falling VA - not as a rising gas volume.
What not to do Do not raise pH and resume full feed on the same day. Do not dump a large lime charge. Do not assume the boiler is the problem just because the temperature dropped.
3. Septage is the only upstream change. What would you check, and how can 8,000 gpd matter at a plant with 2,800 lb VSS/day of digester feed?
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Quantify the load 8,000 gpd is only 0.008 MGD, but septage is not dilute. At a typical 12,000 mg/L TSS: 0.008 × 8.34 × 12,000 ≈ 800 lb/day of solids - nearly 30% on top of 2,800 lb VSS/day, and the VSS fraction can be high because septage carries grease and fecal solids. At BOD around 4,000 mg/L, the organic load is about 270 lb/day of BOD, plus a large ammonia load.
What to check, in order of how fast it can kill a digester
- Source and manifests. Portable-toilet and chemical-toilet waste carries quaternary ammonium disinfectants and other biocides that are directly toxic to methanogens at low concentrations. A single tanker can do this. Ask who hauled what, when, and from where, and pull the three-week receiving log against the day gas production started to fall.
- pH and sulfide of the septage. Low-pH loads and high-sulfide loads both inhibit; sulfide also strips iron and causes scaling.
- Ammonia. Septage is ammonia-rich; un-ionized ammonia above roughly 1,500-3,000 mg/L as N (temperature and pH dependent) becomes inhibitory, and this digester is already warm.
- Grit and sand. Septic tank pumpings are full of grit. Check the digester for accumulated grit reducing the effective volume - a sudden loss of active volume looks exactly like a shortened SRT.
- Grease and floatables. Fats form a crust and a foam layer that reduces gas release and can cause the gas line to carry over.
Corrective follow-through Do not simply ban septage and forget it. If the plant wants the revenue, require pretreatment and staged acceptance: a receiving station with screening, a holding tank, grab sampling of each load for pH, sulfide, and ammonia, a limit on quaternary ammonium sources, and a ramp schedule of no more than a few thousand gallons per day blended into the feed. Document the prohibited-discharge criteria in the hauler agreement so a bad load can be rejected at the gate.