Effect of nitrate on acetate degradation in a sulfidogenic staged reactor

The optimisation of acetate removal in sulfidogenic reactors by adding nitrate to the last compartment of a baffled reactor was evaluated. In a 5.4 l baffled reactor, containing three equal compartments, a VFA mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) was treated under mesophilic (30°C) and sulfidogenic (COD:SO2−4 ratio: 0.5) conditions. At a specific sludge loading rate of 1.1 g COD gVSS−1 d−1, a COD and sulfate removal of 80 and 35%, respectively, was obtained. In the baffled reactor, staging of the sulfidogenic VFA degradation occurred. Propionate and butyrate were mainly degraded in the first compartment. Their degradation was incomplete, resulting in elevated acetate concentrations in Compartment I. In the second and third compartment of the baffled reactor, a net degradation of acetate took place. Acetate was the sole substrate present in Compartment III and residual acetate concentrations of about 600 mg/l were present in the effluent at the highest specific sludge loading applied. In the three compartments, development of sludges with different maximum specific acetate degrading activities was observed. The maximal specific activity with acetate was three times higher in sludge growing in Compartment III as compared to sludge growing in Compartment I. Batch experiments showed that the addition of about 500 mg/l nitrate (COD/nitrate ratio of 2.5) increased the specific acetate degradation rate by 300%. Nitrate was denitrified (pH optimum of 7.0–8.0) with a temporary accumulation of the intermediate nitrite. When 2.3 g/l nitrate (COD/nitrate ratio of 0.5) was dosed in compartment III of the baffled reactor, the effluent acetate concentration could be decreased from 600 to 100 mg/l.