pH (pH)

pH is a logarithmic scale from 0 to 14 measuring the activity of hydrogen ions in a solution. A unit change in pH represents a tenfold change in hydrogen ion concentration: pH 6 is ten times more acidic than pH 7, and a hundred times more acidic than pH 8. Neutral water sits at pH 7; values below indicate acidity, values above indicate alkalinity. In anaerobic digestion, pH is the single most important indicator of biological process health because the methane-producing archaea operate within a narrow tolerance window of 6.8 to 7.5.

The four-stage anaerobic digestion process is acid-producing in its first three stages (hydrolysis, acidogenesis, acetogenesis) and acid-consuming only in the final methanogenic stage. When the system is balanced, acid generation and consumption stay in equilibrium and pH holds steady. When feeding outpaces methanogenic capacity — through overloading, temperature drop, toxic shock, or feedstock change — volatile fatty acids accumulate, alkalinity is consumed, and pH crashes from 7.0 toward 5.5 within days. Below pH 6.5, methanogens are inhibited; below pH 6.0, they are effectively dead, and recovery requires weeks of careful operator intervention.

Operators monitor pH continuously through inline probes and laboratory grab samples, paired with VFA-to-alkalinity (FOS/TAC) ratios for early warning. Key control levers include:

• Alkalinity supplementation with sodium bicarbonate or lime to buffer acid accumulation.
• Reducing organic loading rate temporarily to let methanogens catch up.
• Recirculating digestate from a healthy digester to dilute inhibitors.
• Co-digesting nitrogen-rich material (manure) to balance acidic carbon-rich feedstocks.

The trade-off is alkalinity cost: sodium bicarbonate dosing at 1-3 kg per tonne of feedstock adds 80-250 INR per tonne to operating cost. Indian plants typically prefer designing for stable feedstock and conservative OLR rather than chasing throughput with continuous chemical buffering, since the recovery cost from a souring event — lost gas, wasted feed, and inoculation — can exceed a month of normal operating margin.