Kinetic characterization of hydrogen sulfide inhibition of suspended anammox biomass from a membrane bioreactor

The inhibition effects of hydrogen sulfide (H2S) on anammox-enriched biomass from a laboratory membrane reactor were analyzed in a series of batch respirometric experiments. The determined half maximal inhibitory concentration (IC50) was 4.67 mg H2S-S L-1 at the constant pH = 7.0 and the total sulfide concentration varying between 1 and 15 mg TS-S L-1. In another test, the IC50 was found to be 4.25 mg H2S-S L-1 under a constant total sulfide concentration of 11 mg TS-S L-1 and the pH varying from 7.0 to 7.9. The decrease in the anammox activity was due to the pH-dependent un-ionized form of H2S. The results were simulated using Activated Sludge Model No. 1, extended with the two-step nitrification and anammox processes. The inhibition effect of pH and H2S concentration was determined using the Hultman pH function and inhibition switching function, respectively. The maximum specific growth rate of anammox bacteria at the optimum pH (pH(opt) = 7.2) was 0.075 d(-1) (at T = 35 degrees C), whereas the inhibition coefficient of H2S, (K-i,K-H2s) was 4.9 mg H2S-S L-1. The experimental and simulation results confirmed that even low H2S concentrations have a strong inhibitory effect on the anammox process.