0.3–0.4 (VFA/alkalinity ratio 0.3–0.4)

The range 0.3-0.4 defines the healthy VFA-to-alkalinity ratio (also called FOS/TAC) for a stable anaerobic digester operating on typical mixed feedstocks. Within this band the digester is well-buffered, methanogens are consuming VFA at the rate it is being produced, pH is steady between 6.8 and 7.6, and gas yield is at design level. Below 0.3 the system is under-loaded and capacity is being wasted; above 0.4 stress begins and corrective action is needed.

The ratio is computed from a simple two-step titration: a digester slurry sample is titrated with sulphuric acid down to pH 5.0 (giving total alkalinity in mg CaCO₃/L) then continued down to pH 4.4 (the additional acid consumption gives VFA proxy in mg acetic acid/L). Dividing the second number by the first gives FOS/TAC. A typical healthy CBG digester might show alkalinity of 8,000-12,000 mg CaCO₃/L and VFA of 2,500-4,000 mg/L for a ratio of 0.3-0.4.

The reason this specific band represents the operating sweet spot is biochemical. Alkalinity above 8,000 mg/L provides robust buffering against the natural variation in VFA production rates, holding pH stable even when the upstream acidogen-methanogen balance fluctuates by 15-30%. VFA up to 4,000 mg/L represents the working stock of acetate, propionate and butyrate that methanogens are continuously consuming, signalling vigorous microbial activity. The product of these — high alkalinity and moderate VFA — defines a digester running near design capacity with margin for sudden feedstock shocks.

Operating below 0.3 (VFA artificially low) is wasteful because the methanogens have excess capacity that is not being used; OLR can be raised to capture more gas production. Operating above 0.4 risks rapid drift toward souring — typically a ratio of 0.4 today becomes 0.6 in two weeks and 0.9 in four weeks unless corrective action is taken (OLR reduction, alkalinity supplementation with sodium bicarbonate at 5-10 kg per 1,000 m³ slurry, verification of temperature and feedstock). The 0.3-0.4 range therefore functions as the green zone of the operational dashboard, with weekly monitoring as discipline and online sensors paying back capex through avoided downtime.