Methanogenic Characteristics of Formate-Utilizing Sludge

The formate-utilizing sludge was first enriched in a chemostat reactor for over 90 days; at steady state, the sludge yield averaged 0.066 mg volatile suspended solids (VSS)/mg chemical oxygen demand (COD). The methanogenic characteristics of this sludge were then investigated in three series of batch experiments at 37°C using formate, acetate, and H2/CO2, individually, as substrates. At pH 6.4–8.0, the formate-degrading rate averaged 0.76 mg/mg VSS/h (6.35 mg COD/mg VSS/d). At pH 3.0, the sludge completely lost its bioactivity, and required a lengthy recovery period to regain a fraction of its bioactivity after the pH was adjusted to pH 7.1. The sludge was also able to utilize H2/CO2 as substrate at an average rate of 0.0167 mg H2/mg VSS/h (3.21 mg COD/mg VSS/d). At pH ≤ 8.0, the sludge degraded acetate at a very low rate of 3.0 μg/mg VSS/h (0.077 mg COD/mg VSS/d). The sludge exhibited a slight homoacetogenic activity at pH > 8.0 using formate as substrate; the homoacetogenic reaction using H2/CO2 as substrate was thermodynamically infeasible, according to chemical free energy analysis.