Souring of a mesophilic anaerobic digester
An anaerobic digester at a 3,000 m3 mesophilic plant at 35 C receives 60 t/d of food waste and waste-activated sludge, giving an organic loading rate of 4.8 kg VS/m3-d against a design value of 3.0. Over three weeks, pH fell from 7.2 to 6.5, total VFA rose to 7,200 mg/L as acetic acid, alkalinity fell to 2,600 mg/L as CaCO3, and biogas yield dropped to 0.45 m3/kg VS with 48% methane; the design expectation is 0.90 m3/kg VS at 62% methane. Propionate is 1,800 mg/L, ammonia-nitrogen is 4,200 mg/L, and hydrogen sulphide is 4,500 ppm. The digester is still being fed at full rate because the biogas boiler supplies process steam and the plant has no backup fuel. The operator proposes to add 2 t of hydrated lime, raise pH to 7.2, and continue feeding.
1. Diagnose the failure using the VFA/alkalinity ratio, propionate, sulphide and free ammonia. Decide whether ammonia is the primary inhibitor at the current pH and whether liming to pH 7.2 is safe.
Read the worked answer
Acidification is primary. VFA/alkalinity = 7,200/2,600 = 2.77. A healthy digester is usually below 0.3-0.5, and values above about 0.8 indicate imminent souring. The OLR is 60% above design (4.8 versus 3.0 kg VS/m3-d), so acidogens are outrunning methanogens. Propionate at 1,800 mg/L is a strong stress indicator because propionate degradation is slow and is inhibited by low pH and high hydrogen partial pressure. The pH drop from 7.2 to 6.5 and the methane fall from 62% to 48% confirm a shift from methanogenesis to acidogenesis. Sulphide at 4,500 ppm also indicates sulphate reduction and toxicity to methanogens.
Ammonia is not primary at pH 6.5. At 35 C the pKa of NH4+ is about 9.0. The free NH3 fraction is 1/(1 + 10^(9.0 - 6.5)) = 0.0032, so free NH3 is about 4,200 x 0.0032 = 13 mg/L, below the 50-100 mg/L inhibitory range. At pH 7.2 the fraction rises to 0.016, giving about 65 mg/L; at pH 7.5 it is about 130 mg/L. Liming to 7.2 therefore converts a non-inhibitory total ammonia into a potentially inhibitory free ammonia concentration, while also risking calcium carbonate precipitation and scaling. Lime does not remove VFA; it only neutralises acid, so the underlying overload remains. Liming as the main response is unsafe.
2. Quantify the overload and the methane loss, and decide whether continuing full feed to protect the boiler is justified.
Read the worked answer
The digester volatile solids load is 4.8 x 3,000 = 14,400 kg VS/d. At 0.45 m3/kg VS, current biogas is 6,480 m3/d and methane is 0.48 x 6,480 = 3,110 m3/d. At the design 0.90 m3/kg VS and 62% methane, the same VS load would give 12,960 m3/d biogas and 8,035 m3/d methane. The shortfall is about 4,925 m3 CH4/d, or 4,925 x 35.8 MJ/m3 = 176 GJ/d, about 49 MWh of thermal energy per day.
That is a large loss, but it is a potential loss, not a reason to keep feeding. Continuing at 4.8 kg VS/m3-d will drive pH lower, increase propionate and sulphide, and risk a complete souring that produces no gas for weeks. The boiler is better protected by securing 3-5 days of backup fuel and reducing feed than by sacrificing the biomass. To return to the design OLR of 3.0, feed should be cut to 3.0/4.8 x 60 = 37.5 t/d, a 37.5% reduction, but the immediate recovery step should be a near-total feed stop for 48-72 h rather than a gradual cut, because the VFA pool must be consumed before methanogens can recover.
3. Give a staged recovery plan with an alkalinity dose. Explain why sodium bicarbonate is preferred to hydrated lime and how the feed should be restarted.
Read the worked answer
Stage 1: stop the acid load and protect the biomass. Stop feed for 48-72 h, keep mixing and heating at 35-37 C, and recirculate. Do not add 2 t of lime in one dose. If pH is below 6.6, add alkalinity in small controlled doses to bring pH to 6.8-7.0, not 7.2.
Stage 2: restore buffering. To raise alkalinity from 2,600 to 3,500 mg/L as CaCO3 requires 900 mg/L x 3,000 m3 = 2,700 kg as CaCO3. As sodium bicarbonate: 2,700 x 84/50 = 4,536 kg, added over 2-3 days. If lime were used, the stoichiometric CaO would be about 1,512 kg, but lime raises pH sharply, precipitates carbonate, and increases free ammonia; sodium bicarbonate or caustic is safer and easier to control. Add iron salt if sulphide remains above about 1,000 ppm, and dilute with 10-20% water or effluent if the digester is at the upper solids limit.
Stage 3: restart slowly. Once VFA is falling and pH is stable above 6.8, restart at 10-15% of the design OLR, then increase weekly while VFA/alkalinity stays below 0.5 and propionate stays below 500 mg/L. Seed with healthy digestate if available. Secure backup fuel for the boiler and install a flare so excess or off-spec gas can be burned safely. The lime proposal should be rejected as the primary response.