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 ch aracteristics of this sludge were then investigated in three series of batc h experiments at 37 degrees C using formate, acetate, and H-2/CO2, individu ally, as substrates. At pH 6.4-8.0, the formate-degrading rate averaged 0.7 6 mg/mg VSS/h (6.35 mg COD/mg VSS/d). Ar pH 3.0, the sludge completely lost its bioactivity, and required a lengthy recovery period to regain a fracti on of its bioactivity after the pH was adjusted to pH 7.1. The sludge was a lso able to utilize H-2/CO2 as substrate at an average rate of 0.0167 mg H- 2/mg VSS/h (3.21 mg COD/mg VSS/d). Al pH less than or equal to 8.0, the slu dge degraded acetate at a very low rate: of 3.0 mu g/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 H-2/CO2 as substrate was thermodynamically infeasible, according to chemical free ene rgy analysis.