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 10,000mg/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 2,500mg/L was close to 10% for all 3 pH levels. For a TAN concentration of 10,000mg/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 7,500mg/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 264mL CH4/g CODd for control reactors at pH 7.5 down to 1mL CH4/g CODd for the reactor at pH of 8.5 which contained 10,000mg/L TAN. Results of gradual ammonia loading also showed that mesophilic bacteria could be adapted to a TAN concentration of up to 5,000mg/L at pH 7.5 through gradual TAN loading. At 10,000mg/L TAN CBP in reactors with gradual TAN loading was more than 1.9-3 times the CBP in reactors with abrupt TAN addition.
Keywords: Anaerobic digestion (AD); Organic fraction of municipal solid waste (OFMSW); Total ammonia nitrogen (TAN).
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