Digester Volatile-Acid to Alkalinity Ratio
The early-warning stability index the pH meter misses: ratio = volatile acids / alkalinity (common CaCO3 basis). Below ~0.1 stable, 0.1-0.25 acceptable, 0.25-0.4 begin corrective action (cut feed, add alkalinity), above ~0.4 souring. The bicarbonate buffer holds the pH steady until the alkalinity is consumed, so the ratio flags the upset days before the pH moves. A digester at VA 900 / alkalinity 3,000 (ratio 0.30) needs action now while its pH still reads normal. An operating aid, not a control setpoint.
Formula and source
ratio = volatile_acids / alkalinity (both to a common CaCO3 basis); buffer_margin = alkalinity - volatile_acids.
Volatile-acid to alkalinity ratio, anaerobic-digester stability index (WEF Manual of Practice; EPA operator practice), by name.
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