The toxicity effects of ammonia on anaerobic digestion of organic fraction of municipal solid waste

In this research the inhibitory effects of ammonia on the AD of synthetic OFMSW were examined under different total ammonia nitrogen (TAN) concentrations of 2,500, 5,000, 7,500, and 10,000 mg/L and at pH levels of 7.5, 8.0 and 8.5 using a factorial experimental design and statistical analysis. Reduction in Cumulative Biogas Production (CBP) at a TAN concentration of 2,500 mg/L was close to 10% for all 3 pH levels. For a TAN concentration of 10,000 mg/L the percent reduction in CBP was over 80% for all 3 pH levels showing significant inhibition due to ammonia with neglible influence due to change in pH. However, pH played a more significant role for TAN concentrations between the above two levels, as at a TAN concentartion of 7,500 mg/L, the percent reduction in CBP increased from 42.2% at a pH of 7.5 to 76.5% at a pH of 8.5. Regression analysis was used to estimate CBP and % reduction (%R) in CBP using a quadratic equation with pH and TAN as independant variables (R2 = 0.95 and 0.94). Methane produciton per g of COD removed was obsereved to vary from 264 mL CH4/g CODd for control reactors at pH 7.5 down to 1 mL CH4/g CODd for the reactor at pH of 8.5 which contained 10,000 mg/L TAN. Results of gradual ammonia loading also showed that mesophilic bacteria could be adapted to a TAN concentration of up to 5,000 mg/L at pH 7.5 through gradual TAN loading. At 10,000 mg/L TAN CBP in reactors with gradual TAN loading was more than 1.9-3 times the CBP in reactors with abrupt TAN addition.