Denitrification of leachate using domestic waste at different levels of stability : simulations in batch test.

Abstract

Disposing of waste on land has been a method practiced by many countries because it is relatively inexpensive. This has led to the fast increase of landfilling option which is also due to increase of waste generation, resulting in the increase in the urgency of investigating cheap measures of treating wastewater (leachate) that is generated from landfills prior to its discharge to the environment. After the application of the process of nitrification using Sequencing Batch Reactor (SBR) such as is applied at Mariannhill landfill site, Durban, the treated leachate still contains high level of nitrate ranging from 500 – 2000 mg/ℓ, which greatly exceeds the discharge limit of 12 mg/ℓ. Ex-situ bio-denitrification has been used widely around the world in various technological applications (SBRs, anaerobic trickling filters, etc.) that generally employ expensive chemicals. Hence the need to investigate the removal of nitrates using in-situ biodenitrification processes using readily available carbon sources such as fresh commercial garden refuse (CGRraw) and composted commercial garden refuse (CGR10). Both carbon sources were mixed with waste that had been treated for 8 weeks (Cell 1) and 16 weeks (Cell 2). The aim of this study is to determine the viability of pre-treated general waste at different degrees of stability (carbon contents) as carbon sources for in-situ bio-denitrification in landfills. The focus was mainly on determining the suitability, the kinetics and the performance of the different substrate. The suitability of the substrates to perform denitrification was assessed based on the carbon content and carbon to nitrogen ratio in the substrate. On establishing suitability, the kinetic rate of denitrification was assessed for each substrate. The kinetics analysis was based on the time taken for full denitrification to occur and the concentration of the byproducts of the denitrification process such as Ammonia. Characterization tests were performed to determine the suitability of the substrates to be used as carbon sources for denitrification. In situ denitrification processes were simulated at smaller scale in the laboratory using anaerobic batch reactors, with biologically treated leachate and seeded Treated leachate from the Sequencing Batch Reactor. Batch tests were conducted at a nitrate concentration level of 500 mg/ℓ. The combination of 8 weeks treated waste with Fresh Commercial Garden Refuse (Cell 1 + CGRraw) and with Commercial Garden Refuse (Cell 1 + CGR10), respectively, provided the most suitable substrates for denitrification as they contained the highest carbon content as well as relatively high carbon to nitrogen ratio (C:N) . Although the 16 weeks treated waste together mixed with Commercial Garden Refuse (Cell 2 + CGR10) had the lowest C:N ratio, this could be due to a lack of homogeneity within the sample. The results of the batch tests confirms that 8 weeks treated waste (Cell 1) and 16 weeks treated waste (Cell 2) substrates were both too stable and contained too little carbon to attain full denitrification. In addition to the inability to attain full denitrification, Cell 2 leached out nitrate of approximately 500 mg/ℓ NO3-N back into the batch. The batch test results showed that the cells substrates augmented with CGRraw and CGR10 achieved positive results as full denitrification was achieved within a maximum of 7 days for Cell 1 and 14 days for Cell 2.