Alkalinity (alkaline conditions)

Alkalinity is the capacity of a solution to neutralise acids and resist pH changes, expressed in milligrams of calcium carbonate equivalent per litre (mg CaCO₃/L). In water-chemistry terms it is the sum of bicarbonate (HCO₃⁻), carbonate (CO₃²⁻) and hydroxide (OH⁻) species available to react with incoming acid. In an anaerobic digester, alkalinity is the buffer system that absorbs volatile fatty acids produced during acidogenesis and protects the methanogenic community from pH drops.

In CBG operations, alkalinity is one of the three core process-stability indicators alongside pH and VFA. A healthy digester runs with total alkalinity of 8,000-15,000 mg CaCO₃/L, depending on feedstock — plants on cattle dung sit at the higher end because dung is naturally rich in bicarbonate, while plants on lignocellulosic feedstock run lower. The buffer derives from bicarbonate equilibrium with dissolved CO₂ produced during fermentation; CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ holds pH in the 6.8-7.6 range as long as bicarbonate is available.

When alkalinity drops below about 3,000-4,000 mg CaCO₃/L the buffer reserve runs out, and any further VFA production causes pH to fall sharply — the classic souring trajectory in which pH drops from 7.0 to 6.4 in 5-10 days, methanogens are progressively inhibited, and gas production collapses. This is why the VFA-to-alkalinity ratio (FOS/TAC) — not VFA alone — is the right warning indicator: it is the buffer headroom that matters, not the acid concentration in isolation.

Operational management of alkalinity is supplementation when natural buffering falls short. Sodium bicarbonate (NaHCO₃) at 1-5 kg per cubic metre of slurry is the standard intervention; food-grade lime (Ca(OH)₂) is cheaper but precipitates with phosphate. Long-term alkalinity is sustained by maintaining adequate ammonium-nitrogen and avoiding feedstock acidification (silage-quality storage of paddy straw, for example, drops pH and consumes alkalinity before the feedstock even reaches the digester). Plants designed for high-protein feedstocks like food waste or distillery slop typically include continuous bicarbonate dosing to hold alkalinity at the upper end of the range, providing margin for the inevitable feedstock-quality variation in commercial operation.