Denitrification of high strength nitrified landfill leachate using raw and lightly composted commercial garden refuse (CGR) as carbon sources.
Zondi, Mzamoyendoda Samuel.
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Waste is commonly disposed in landfills, this result in the formation of leachate which needs to be treated to acceptable standards before being discharged to the environment. High concentrations of pollutants, particularly ammonia, in the landfill leachate are persistent even after the closure of the landfill and it requires ad hoc treatment. Treated leachate can still be characterized by high concentrations of nitrates, which exceeds the discharge standards. This phenomenon is observed in the Mariannhill landfill site in Durban, where leachate is nitrified in a Sequencing batch reactor and produces effluent with over 1000 mg/l (Trois et al, 2010a). Denitrification can be used to remove nitrate concentrations, this process occurs under anoxic conditions in the presence of an external carbon source. Denitrification treatment methods utilize chemicals such as methanol and ethanol as carbon sources, but the large scale application of these chemicals is often uneconomical. This research aims at identifying the cost effective treatment system for bio-denitrification that utilizes commercial garden refuse (CGR raw and lightly composted for 10 weeks “CGR 10”) as carbon sources. The feasibility checks for applying these substrates were based on the efficiency and kinetics of nitrate removal over a short and long-term period, thus providing the estimates for operational procedures. Initial characterization tests, batch and column tests were performed in the lab towards achieving the aim of this research. All batch tests achieved 100% of nitrate removal, but CGR raw was faster than CGR 10 with a time difference of 16% and 20% for batches at 100 and 500 mg/L, respectively. The significant difference in the kinetic removal efficiency was observed in batch tests at 2000 mg/L, where CGR raw was about 18 times faster than CGR 10 and about 2 times faster than that of CGR raw at 500 mg/L. Thus, the kinetics of nitrate removal in CGR raw at 2000 mg/L was suspected to be due to chemical reaction other than biological reaction. In the second set of batch tests the kinetics of nitrate removal for CGR raw was about 3 times that of CGR 10. The column tests, which were operated as continuous flow reactor did not achieve full denitrification due to high flow rate applied. First set of column tests (columns A) used previously used substrates to treat synthetic nitrate solution (500 and 2000 mg/L). Second set of column tests (columns B) used fresh substrates to treat pre-treated landfill leachate with nitrate concentration of about 2000 mg/L. CGR 10 achieved better removal efficiency than CGR raw when treating synthetic solution. Whereas, CGR raw achieved better nitrate removal when treating pre-treated landfill leachate. Decrease in flow rate improved the removal efficiency of the substrates. Dilution of nitrified leachate to about 500 mg/L could improve the efficiency of the substrates.