Alkalinity Ratio (VFA to alkalinity ratio)

The alkalinity ratio — also called the FOS/TAC ratio in German notation widely adopted in European biogas practice — is the ratio of total volatile fatty acids (expressed as acetic acid equivalent) to total alkalinity (expressed as calcium carbonate equivalent) in a digester sample. It is the most widely used early-warning indicator of anaerobic digestion process stability, computed from a simple two-step titration that any plant lab can perform.

Interpretation follows a well-established threshold scale. Below 0.2 — the system is under-loaded; the methanogens have spare capacity and OLR can be raised. 0.2-0.3 — optimum operation; high gas yield, stable pH, full microbial activity. 0.3-0.4 — early warning; VFA is starting to accumulate or alkalinity is dropping, requiring closer monitoring but not yet corrective action. 0.4-0.6 — clear stress; cut OLR by 20-40% and investigate. Above 0.6 — imminent process failure; emergency load reduction, alkalinity supplementation with sodium bicarbonate or lime, and verification of temperature, feedstock composition and toxic-shock causes.

The ratio is more sensitive than pH alone because pH is buffered by alkalinity until it suddenly collapses. A digester moving from FOS/TAC 0.25 to 0.45 over two weeks may show pH falling only from 7.2 to 7.0 — within the healthy range — even though the underlying process is in clear stress. By the time pH drops below 6.8 visibly, the FOS/TAC ratio is already 0.8-1.0 and the methanogen population has been impaired; recovery takes 4-8 weeks during which 30-50% of normal gas output is lost.

Operational practice in well-run Indian CBG plants is weekly FOS/TAC measurement on grab samples from the main digester, with daily measurement when the ratio crosses 0.3, and continuous online monitoring on plants above 10 TPD capacity. The measurement uses titration with sulphuric acid down to pH 5.0 (giving alkalinity) and then to pH 4.4 (giving VFA proxy). Online sensors from vendors like Hach, Endress+Hauser and indigenous suppliers cost ₹3-8 lakh per installation and typically pay back in 6-12 months through avoided downtime — a high-leverage instrumentation investment for any commercial plant.