acidity (acidity in AD)

Acidity, expressed through the pH scale, measures the concentration of hydrogen ions in an aqueous solution. The scale runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 representing neutrality. Because pH is logarithmic, each whole-number change represents a tenfold shift in hydrogen ion concentration — a digester drifting from pH 7.0 to pH 6.0 has become ten times more acidic, not marginally so. In anaerobic digestion, acidity is the most actively monitored process parameter because the microbial consortium is unforgiving of pH excursions.

Methanogenic archaea — the microorganisms responsible for converting acetate and hydrogen into methane — operate within a narrow band of pH 6.8 to 7.5, with peak activity around 7.0–7.2. Acidogenic and acetogenic bacteria upstream tolerate a wider range (pH 5.0–8.5), which creates the classic failure mode of anaerobic digestion: under organic overload, acid-formers continue producing volatile fatty acids while methanogens slow down, causing the digester to acidify in a self-reinforcing spiral known as digester souring.

Indian CBG plants typically maintain pH stability through three buffering mechanisms. The first is natural bicarbonate alkalinity generated by ammonia release from protein-rich feedstocks like poultry litter and slaughterhouse waste; alkalinity targets are 3,000–5,000 mg/L as CaCO3. The second is operator dosing of lime, sodium bicarbonate, or sodium carbonate when alkalinity drops, which costs roughly Rs 8–15 per kg of CaCO3 equivalent. The third is feedstock co-digestion — blending a high-VFA substrate like fruit waste with a buffer-rich substrate like cow dung. Daily pH measurement using a calibrated digital probe is standard; advanced plants also monitor the VFA-to-alkalinity ratio (target below 0.3) as an early-warning indicator that pH crashes are imminent.