The Effects of Ammonia on Anaerobic Digestion of the Organic Fraction of Municipal Solid Wastes

Abstract

The effect of ammonia on anaerobic digestion of the organic fraction of municipal solid wastes (OFMSW) was investigated in this study. This study involved two sets of experiments. First set involved the investigation of ammonia toxicity on AD of synthetic OFMSW only (SW), at three different phases and pH 7.5, 8.0 and 8.5. Phase 1 was the Batch Methane Potential (BMP) phase. BMP tests were conducted under ammonia concentration of 2500 mg/L, 5000 mg/L, 7500 mg/L and 10,000 mg/L and at pH 7.5, 8.0, and 8.5, using 500 mL Kimax® glass bottles. The total working volume of the mixture was 300 mL comprising 120 mL of mesophilic anaerobically digested inoculums, 30 g of OFMSW, various TAN concentrations ranging from 2,500, 5,000, 7,500 to 10,000 mg/L, and equal portions of buffer in form of NaHCO3 and KHCO3. The second phase of the experiment examined whether the tolerance of the bacteria to high ammonia concentration would improve by acclimating the microbes to high ammonia concentrations, through gradual TAN loading. TAN concentration was increased gradually at pH 7.5, 8.0 and 8.5 weekly. The third phase of the experiment was Semi-continuous batch phase. This phase examined the possibility of reducing the inhibitory effect of ammonia on AD, batch reactors at pH values of 8.0 and 8.5 containing initial TAN concentrations of 7500 mg/L and 10,000 mg/L. 3 g of the digestate containing high ammonia concentration(s) was replaced with fresh substrate at every 4 days, 7 days and 15 days. The second set of experiment involved study of the effects of ammonia on anaerobic digestion of OFMSW with real landfill leachate (SW+L). Phase 1 was BMP in which the effect of ammonia was examined at TAN concentrations of 7,500 and 10,000 mg/L. The phase 2 of the (SW+L) gradual TAN TAN loading. The possibility of adapting mesophilic bacteria to high ammonia concentration was examined. The results of the study confirmed that ammonia is toxic to AD, at high concentrations. Biogas production reduced with increase in TAN concentration. Reduction in Cumulative Biogas Production (CBP) compared with control reactors was as much as 43 %, 64 % and 77 % in reactors containing 7500 mg/L TAN at pH 7.5, pH 8.0 and pH 8.5. CBP reduced to 80-85 % in reactors containing 10,000 mg/L TAN across the pH examined. Also, replacing 3g of digestate containing high TAN concentrations of 7500 mg/L and 10,000 mg/L with 3 g fresh substrate improved the activity of the mesophilic bacteria as seen in the surges in biogas production when fresh substrate was injected into the reactors. Similar results were obtained on effect of ammonia on AD of OFMSW mixed with real landfill leachate to simulate an anaerobic bioreactor landfill. CBP reduced as the TAN concentration increased. Compared with control reactors, reactors containing 7500 mg/L TAN at pH 8.0 and pH 8.5 had 61 % and 80 % reduction in CBP. Likewise, reactors containing 10,000 mg/L TAN at pH 8.0 and pH 8.5 had 68 % and 85 % reduction in CBP, compared with control reactors. Study confirmed that pH influenced the toxicity and composition of Total Ammonia Nitrogen (TAN). At high pH (i.e. 8.5), FAN component of TAN was about 26 % and was inhibitory to the methanogens. Results also showed that mesophilic bacteria could be adapted to a TAN concentration of about 5000 mg/L at pH 7.5 through gradual TAN loading. Similar results were obtained on effect of ammonia on AD of OFMSW mixed with real landfill leachate to simulate an anaerobic bioreactor landfill. CBP reduced as the TAN concentration increased. Compared with control reactors, reactors containing 7500 mg/L TAN at pH 8.0 and pH 8.5 had 61 % and 80 % reduction in CBP. Likewise, reactors containing 10,000 mg/L TAN at pH 8.0 and pH 8.5 had 68 % and 85 % reduction in CBP, compared with control reactors. Study confirmed that pH influenced the toxicity and composition of Total Ammonia Nitrogen (TAN). At high pH (i.e. 8.5), FAN component of TAN was about 26 % and was inhibitory to the methanogens. Results also showed that mesophilic bacteria could be adapted to a TAN concentration of about 5000 mg/L at pH 7.5 through gradual TAN loading.