Digester Volatile-Acid to Alkalinity Ratio
The early-warning stability index the pH meter misses.
Example
You enter
- Volatile acids as acetic (mg/L) 180
- Total alkalinity as CaCO3 (mg/L) 2400
You get
- Ratio 0.075
- Bicarbonate buffer margin 2220 mg/L not yet consumed
Details, formula, and sources
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.
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.
The VA/alkalinity stability ratio is a public wastewater-operations index; the digester monitoring and the operator govern.
Estimate. AHJ and licensed professional govern.
Field names used by the API: volatile_acids_mgl, alkalinity_mgl, ratio, buffer_margin_mgl
- Common basis volatile acids and alkalinity must be expressed to the same CaCO3 basis for the ratio to be meaningfulWEF Manual of Practice
- pH lags the ratio rises days before the pH falls; the bicarbonate buffer masks the upset until it is consumedEPA operator practice