Denitrification of high strength landfill leachate using garden refuse compost and pine bark.
Landfill leachate, a toxic by-product formed through the decomposition of organic matter, is harmful to the environment and human health. After nitrification, the concentration of nitrate in discharged leachate may still present a potential threat to the environment. Further denitrification is required to reduce the high concentrations of nitrates in the nitrified effluents to below discharge limits. The eThekwini Municipality is currently nitrifying leachate from the Mariannhill Landfill site in a Sequencing Batch Reactor plant. After closure of the landfill (expected in 2012) the effluents from the plant will not comply with discharge limits, requiring an ad-hoc treatment. Denitrification, the conversion of nitrates to nitrogen gas, occurs in the presence of a carbon source in an anaerobic environment. Expensive methods are currently employed worldwide; however these tend not to be a viable solution for developing countries. This investigation aims at identifying an efficient, cost effective, feasible alternative to expensive easily biodegradable carbonaceous materials such as methanol, which promotes the use of natural organic sources such as pine bark and garden refuse. These organic substrates contain relatively high amounts of carbon and are readily available in the major eThekwini landfills. The suitability of these substrates as carbon sources for denitrification, were assessed using characterisation tests, small scale batch tests and larger scale columns. The preliminary stage of the research was to comprehensively characterise the substrates through conventional testing done on both the solid substrates and their eluates. The batch tests were conducted at 3 nitrate concentration levels: 100, 500 and 2000 mg/ℓ. A synthetic nitrate solution was used to simulate the treated Mariannhill Landfill site leachate. Substrates selected for large scale experiments in columns were, the fresh pine bark, the fresh Commercial Garden Refuse (CGR) and immature Commercial Garden Refuse (CGR) compost. Two nitrate concentrations (500 and 2000 mg/ℓ) at two different flow rates were used for the column campaign. Finally durability tests were conducted on previously used substrates of pine bark and immature compost to determine the period for which the substrates could be used as a means for denitrification before replacement was necessary. The characterisation tests indicated that the fresh materials had higher carbon to nitrogen ratios than the composted substrates. The CGR RAW substrate had the highest carbon to nitrogen ratio of 90.19 and although the ph value of 5.45 falls just outside the optimum range for denitrification of 6 – 8, it was expected that this would be the best performing substrate. All the batch tests showed positive results, with regard to achieving full denitrification with a 100% removal occurring in 5 of the 6 substrates, at all the different nitrate concentrations. The only substrate not to achieve full denitrification was the pine bark. The best performing substrate was the CGR RAW which achieved full denitrification at the highest nitrate concentration of 2000 mg/ℓ between 9 – 12 days. The column tests reflected promising results at C˳= 500 mg/ℓ during experiment 1, with all 3 achieving full denitrification. Once again the CGR RAW substrate columns reflected the best results. The column at 500 mg/ℓ displayed a HRT of 8.06 days was required whereas the higher concentration of 2000 mg/ℓ required a HRT of 8.40 days. During experiment 2, the CGR RAW substrate column at 500 mg/ℓ was the only one to achieve 100% nitrate removal. A HRT time required for full denitrification is less than 3.54 days. Further studies need to be done at different flow rates and concentrations to ensure that the reactor is robust and flexible to deal with the change in quality of the leachates during the life of the landfill. Lower concentrations need to be investigated to determine whether the substrates are suitable for all ranges of nitrates and leachates. The use of a combination of substrates as well as different levels of maturity is also required to determine the ideal material for their implementation in a full-scale reactor in the future. Larger scale reactors and different reactor configurations need to be investigated.