Sizing Leachate Dams and Treatment Infrastructure in South African Landfills

Abstract

Adequate leachate management through leachate collection, storage, and treatment is vital for preventing groundwater and surface water pollution during landfilling. The storage of leachate using lined pollution control dams, referred to as leachate dams, before treatment is common in landfills. However, leachate dams are at a high risk of leakage and spillage if they are not designed and sized appropriately. Furthermore, the size of a leachate dam directly informs the sizing of the leachate treatment processes that follow it by establishing a required average treatment rate. The purpose of this research is to provide a framework for the adequate design and sizing of leachate dams and leachate treatment infrastructure in South Africa, by investigating the regulations for the design of leachate dams and leachate treatment; investigating applicable processes for treating leachate; and analysing the suitability of hydrological models and methods to estimate leachate discharge and leachate evaporation for leachate dam size water balance modelling. A thorough literature review was conducted to study: (1) the regulatory requirements for leachate dam design and leachate treatment; and (2) the different methods and processes used for leachate treatment. Analyses were conducted to verify whether the Hydrologic Evaluation of Landfill Performance model (HELP model) is suitable for estimating leachate generation and discharge; predicted leachate discharge values computed by the HELP model using climatic records of an existing landfill were compared to leachate discharge values observed and recorded at the landfill. Furthermore, a method for estimating leachate evaporation was derived in this research; the method’s suitability was analysed by comparing its predicted leachate evaporation values, computed using climatic and leachate composition records at a landfill, to the actual leachate evaporation values observed and recorded at the landfill. To prevent excessive leachate seepage into groundwater, South African regulations mandate that leachate dams be designed with a liner system that is adequate for the waste disposed in the landfill. Moreover, the regulations mandate that leachate dams must be designed with a storage capacity large enough that they are not likely to spill more than once every 50 years. Finally, there is a requirement for leachate to be adequately treated for permanent containment or discharge into the environment; the regulations provide minimum requirements for treated effluent’s quality, as well as its allowable temperature and discharge rate if it is to be discharged into the environment. Studies on leachate treatment systems discuss the use of physical and chemical processes, such as flotation and precipitation; biological processes, such as activated sludge processes; and advanced treatment processes, such as reverse osmosis. The studies recommend the use of reverse osmosis as a final leachate treatment step, with a biological treatment and/or chemical treatment step preceding it. The percent error between the observed average daily leachate discharge rate and the average daily leachate discharge rate predicted by the HELP model was found to be 9.780 %. This proves this research’s hypothesis of 10 % was correct. However, this was achieved through the use of an apparent saturated hydraulic conductivity that was significantly higher than what is reported in existing literature for landfilled waste. This variance is considered to be attributable to the methods used in the existing literature for determining saturated hydraulic conductivity; the methods do not take into account large impenetrable pieces of waste, such as drums and containers, creating large flow paths. The percent error between the observed monthly leachate evaporation and the monthly evaporation predicted by the derived leachate evaporation method was found to be 7.91 %. This slightly surpassed the research’s hypothesis of 10 %. The derived method also predicted monthly leachate evaporation more accurately in the wet season than in the dry season. Both the HELP model and derived leachate evaporation method were found to be suitable for use in leachate dam sizing to ensure that leachate dams fulfil South Africa’s regulatory requirements on leakage and spillage. Further research is required using data recorded over longer periods in order to further verify the HELP model’s suitability. Given the significant variability in the chemical and biological composition of leachate, further research is also required using leachates with different compositions in order to further verify the suitability of the derived leachate evaporation method.