Treatment of Pig Manure Digestate Liquid and Study of Nitrous Oxide Emissions From Bioreactors
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Large amounts of pig manure containing high concentrations of nutrients are generated on pig farms. Anaerobic digestion is one of the sustainable technologies for pig manure management. However, most of the ammonium remains in the digestate liquid after anaerobic digestion of pig manure. This induces that the wastewater has a low ratio of readily biodegradable organic matter to nitrogen, which inhibits nitrogen removal when using conventional biological nitrogen removal processes and nitrous oxide (N2O) emission. Thus, the overall aim of this PhD research was to find a novel and efficient method for further treatment of the pig manure digestate liquid. During this research, a novel laboratory-scale intermittently aerated sequencing batch reactor (IASBR) system was constructed in the Environmental Engineering Laboratories at the National University of Ireland, Galway to treat pig manure digestate liquid for the purpose of organic matter removal, nitrogen removal and achievement of partial nitrification. In addition, the aerobic granular sludge sequencing batch reactors (SBRs), aerobic upflow biofilters, and chemical coagulation and struvite precipitation were also investigated for pig manure digestate liquid treatment. The specific objectives of this PhD research included: (1) to investigate pig manure digestate liquid treatment using IASBRs, aerobic granular sludge SBRs, aerobic upflow biofilters and chemical treatment, particularly for nitrogen removal; and (2) to study the characteristics of N2O emissions from the IASBRs and aerobic granular sludge SBRs. In the IASBRs, 75% - 90% of COD removal was achieved and the non-biodegradable COD in the effluent can be removed via chemical coagulation. The nitrogen removal efficiency and the nitrite accumulation efficiency were up to 76% and 80%, respectively. The intermittent aeration strategy, the aeration rate, readily biodegradable organic matter and denitrification were found to affect partial nitrification. In the aerobic granular sludge SBRs, the mean COD removal efficiencies were more than 99%. Under continuous aeration conditions, the highest nitrogen removal efficiency was 36% which confirmed that simultaneous nitrification and denitrification took place in aerobic granular sludge SBRs. In the upflow biofilters, the COD removal efficiency was up to 92%. The nitrogen removal efficiencies were 84% and 88% at two loading rates of 0.12 kg TN/ (m3¿ d) and 0.07 kg TN/ (m3¿ d), respectively. The attached biofilm had a more efficient nitrification and denitrification activity than that of suspended growth biomass in biofilters. Phosphorus removal efficiencies were 88%. In the chemical treatment study, more than 75% of COD and 85% of turbidity removals were achieved by coagulation when anion polyacrylamide and pH of 4.5 were adopted. In the struvite precipitation experiment, under optimal experimental conditions (pH= 10.5, and Mg2+: NH4+-N: PO43--P= 1.3: 1: 1.3), more than 96% NH4+-N was removed from the separated pig manure digestate liquid. The N2O emission from the IASBRs and the aerobic granular sludge SBRs were investigated using N2O microsensors. N2O emissions were 12.0% of the influent nitrogen loading rate in IASBRs. The aeration period and non-aeration period contributed to 91.7% and 8.3% of N2O emissions, respectively. The content of readily biodegradable organic matter in wastewater, dissolved oxygen, pH, nitrite concentrations and free nitrous acid affected the N2O emissions in the IASBRs. As for the N2O emission in aerobic granular sludge SBRs, the N2O emission was 2.2% - 8.2% of the influent nitrogen loading rate, decreased with the increase in the COD: N ratio and the aeration rate. The research results show that all the technologies examined were efficient in nutrient removal from the pig manure digestate liquid and can be used in practice. According to the experience obtained in this research, the best method for pig manure digestate liquid treatment is to combine the chemical treatment technology for nitrogen recovery and the biotechnologies.