In a biogas digester having biomass and animal wastes as feed, for optimum methane production, the value of pH should be such that initially

Correct option is A

Introduction
· Biogas production is a complex biochemical process called anaerobic digestion, where microorganisms break down biodegradable material in the absence of oxygen.
· The process occurs in four main stages: Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis.
· Each stage involves different groups of bacteria, but the methanogens (methane-producing archaea) are the most sensitive to environmental conditions, particularly the pH level.

Information Booster
· The correct answer is pH in the range 6.6 to 7, as this near-neutral range is critical for the survival and activity of methanogenic bacteria.
· During the initial stages (acidogenesis), organic acids are produced, which naturally lower the pH. If the pH drops below 6.2, the methanogens become inhibited, and the digestion process can "sour" or fail.
· Maintaining a stable pH between 6.6 and 7.0 ensures that the rate of acid consumption by methanogens matches the rate of acid production by acidogens.
· While the overall digestion can occur between pH 6.5 and 8.0, the optimum methane yield is specifically observed when the environment is slightly acidic to neutral (6.6–7.0).
· If the pH rises too high (above 8.0), it can lead to ammonia toxicity, which also halts methane production.

Additional Knowledge
· pH < 6.2 is the danger zone; at this level, the acidic environment becomes toxic to the methanogens, leading to a buildup of volatile fatty acids (VFAs).
· pH 7 to 7.2  is a healthy range for a stabilized digester, but for the initial delicate balance of methane production startup, the 6.6 to 7.0 range is the traditional target in environmental engineering.
· pH 7.6 to 8  is slightly alkaline; while some digesters operate here, it increases the risk of free ammonia formation ($NH_3$), which inhibits the microbial community.
· Buffering capacity, often provided by carbonates and bicarbonates naturally present in animal waste (like cow dung), helps the digester resist drastic pH changes during the high-acid-production phases.