The role and significance of re-injection treatment for landfill leachate

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The Role and Significance of Reinjecting Landfill Leachate – Authors: He Houbo, Xu Dimin. Abstract: Based on a review of relevant domestic and international literature as well as experimental results, this paper explores the theoretical basis of leachate re injection technology, as well as the role and significance of such re injection, and offers some suggestions. Keywords landfill, infiltration, leachate, re-injection 1 Introduction Common methods for the treatment and disposal of municipal waste include landfilling, composting, and incineration. Landfilling is widely used due to its mature technology, low treatment costs, and ease of management. But this brings up the issue of leachate treatment. Treatment methods for landfill leachate include biological, physicochemical, and land treatment methods. Biological treatment methods include aerobic treatment, anaerobic treatment, and anaerobic-aerobic treatment. The physical-chemical methods mainly include chemical coagulation and sedimentation, electrolytic oxidation, activated carbon adsorption, density separation, chemical oxidation, chemical reduction, membrane dialysis, stripping, wet oxidation, and various other methods. Compared with biological methods, physicochemical methods are less affected by water quality and quantity, resulting in stable effluent quality; they are particularly effective in treating landfill leachate with low BOD/COD levels that is difficult to treat biologically. Due to the high treatment costs of physicochemical methods, they are generally used for pretreatment or advanced treatment of leachate. Land treatment of leachate includes various systems such as the slow infiltration system (SR), rapid infiltration system (RI), surface runoff (OF), wetland system (WL), underground infiltration land treatment system (UG), and artificial rapid infiltration treatment system (ARI). Land treatment primarily removes suspended particles and dissolved components from the leachate through processes such as filtration by soil particles, ion exchange adsorption, and precipitation. The organic matter and nitrogen in the leachate are transformed through the action of microorganisms in the soil, and the volume of the leachate is reduced through evaporation. The land-based methods currently used for leachate treatment are mainly recharging and constructed wetlands. Due to its advantages such as low investment, low operating costs, strong resistance to shock loads, and ease of operation, the recharging method has broad application prospects. This paper explores the technology for reusing leachate through aspects such as the theoretical basis for reinfusion, its functions, and its significance. 2 The theoretical basis for the treatment of leachate through re-injection: Robert K. Ham and others, in their simulated studies on waste degradation in landfills, discovered that the waste within the landfill layers had the ability to treat leachate, which led to increased attention and further development of the technology for re-injecting landfill leachate. Leachate re-injection is a method that involves using appropriate techniques to reinject the leachate collected from the bottom of the landfill back into it, from above or beneath the cover layer. This approach utilizes the biodegradation processes as well as the physicochemical adsorption, chelation, and ion exchange mechanisms present in the waste layers and cover soil within the landfill to treat the leachate. Abroad (such as in the UK), the re-injection method for treating landfill leachate has been applied to a certain extent ； In China, extensive research has also been conducted on the use of reinfusion methods for treating leachate from landfills, and a certain understanding of the mechanisms involved in this process exists. Xu Dimin and others conducted a detailed study of the factors affecting the reinfusion of landfill leachate; they found that adding a certain proportion of fine sand to the sub-clay used in the experiments improved the water permeability and air permeability of the covering soil layer. When the inflow load was between 6.6 and 115 g/(m2·day), a COD removal rate of around 98% could be achieved after two months of operation. The study also explained the principles underlying the reinfusion method for treating landfill leachate, as well as the process flow and technical parameters involved. Meanwhile, microscopic examination of the landfill simulation columns showed that after recharging, the organic residues in the landfill layer decreased, and microaggregates appeared; these aggregates had smooth edges, well-developed pores, and an increased number of microorganisms. In addition to the existing fungi and E. coli, a certain amount of protozoa, rotifers, worms, etc., were present. There are a considerable number of bacterial aggregates on the surface of the waste; these protozoa feed on bacteria, which helps to renew the biofilm and thus prevents pore blockage. Through the combined action of these organisms, the purification process can proceed smoothly. In the reclamation system, the waste layer formed by landfilling acts as a semi-natural anaerobic filter bed. The leachate is reinjected into the waste pile, and water purification is achieved through biological degradation, physicochemical adsorption, chelation, ion exchange, and filtration by the covering soil layer and the waste layer. Additionally, the amount of leachate can be reduced through evaporation from the soil surface and transpiration by the surface vegetation. 3 The functions and significance of leachate re-injection 3.1 Accelerating the stabilization process of landfills By re-injecting leachate into landfills, the moisture content of the waste increases, which in turn boosts the rate and extent of waste degradation, thus accelerating the stabilization process of the landfills. Timothy D. Baldwin, Jeffrey Stinson, Robert K. Ham, and others conducted on-site comparative experiments at three landfills representing different climates, and found that due to climate factors, the landfill in Palm Beach, Florida, had a higher moisture content in its waste; as a result, the degradation rate and degree of this waste were also higher. Through multivariate analysis, they concluded that moisture content is one of the most critical factors affecting the degradation of MSW (municipal solid waste) in landfills. Townsend et al. found that the stabilization rate of landfills is influenced by various environmental factors, among which moisture content is the most important. At higher levels of moisture content, microorganisms in the landfill become more active, and the settlement rate at sites where leachate is re-injected is 1.5 times that of sites where no re-injection occurs. In their experiments on the re-injection of leachate at landfills, Miller and colleagues found that such re-injection increases the number of microorganisms in the waste layer of the landfill, brings back the nutrients present in the leachate into the waste pile, and creates an environment within the landfill that is more conducive to waste degradation – characterized by increased humidity, a lower redox potential, and reduced concentrations of VFA and heavy metal ions. This accelerates the rate of waste degradation, increases the methane production rate in the landfill, as well as its settlement rate and overall degree of settlement, thereby speeding up the stabilization process of the landfill. 3.2 Reducing the volume of leachate and improving its water quality – Reinjecting leachate can help reduce its amount. Due to the changes in the physical properties of the surface soil in the landfill under recharging conditions, the water balance of this surface soil is affected; this increases its moisture level and alters the evaporation conditions, thereby impacting the evaporation rate and the degree of water saturation. With proper control of recharging conditions, soil moisture can be brought to a saturated state. If the water content in the topsoil exceeds the water holding capacity of the soil, the soil evaporation surface becomes a saturated evaporation surface; in this case, soil evaporation is similar to evaporation from a water surface, and thus the evaporation rate of the soil can reach that of water surface evaporation. However, due to the different heat capacities of soil and water, as well as the larger surface area of the soil surface, the evaporation capacity of the soil is greater than that of water surface evaporation under the same meteorological conditions. Furthermore, when there is vegetation in the topsoil of landfills, the recharged water can be intercepted by the plants and ultimately consumed through plant transpiration. Plant transpiration is also a major mechanism for reducing water volume; of the water absorbed by plants, less than a small portion remains within the plant itself, while over 90% of it is released through transpiration. Through experimental studies on the water balance mechanism of landfill leachate, Sun Yue found that the degree of soil moisture saturation has a significant impact on evaporation rates. As the amount of water recharged increases and soil moisture becomes sufficient, soil evaporation is determined by the evaporation capacity; the ratio of soil evaporation to infiltration is approximately 0.98. After the recharging rate reached 18 mm/d, the soil moisture content was high, and the evaporation rate at this time could reach up to 8.82 mm/d. In addition to reducing water volume, re-injection of leachate can also achieve the purpose of purifying its quality. The reinjection of leachate increases the moisture content of the waste in landfills, which facilitates the growth of biological communities within the waste. This transforms the landfill into a biological filter bed, allowing the organic substances present in the leachate flowing through the waste layer to be degraded by microorganisms (mostly anaerobic ones) into methane ； Reinjection of leachate can promote the reduction of SO42-- to H2S, and H2S reacts with heavy metal ions in the leachate to form sulfide precipitates ； Under recharging conditions, the leachate can quickly become neutral or slightly alkaline, which facilitates the formation of hydroxide precipitates from the heavy metal ions contained in it ； At the same time, the high-molecular-weight humic organic compounds produced during the degradation of waste can form stable chelates with heavy metal ions. Therefore, the re-injection of leachate can degrade the organic matter and heavy metal ions in it, thereby purifying the quality of the leachate. 3.3 The significance of leachate re-injection The significance of leachate re-injection lies in its role in controlling and treating pollutants throughout the entire landfill (including MSW and wastewater). Traditional design and operation methods for sanitary landfills involve collecting and storing solid waste, and installing drainage systems on top of the waste pile to reduce the amount of precipitation that seeps in. Although this treatment method reduces the amount of leachate produced, it slows down the degradation rate of the biodegradable components in MSW. The adoption of leachate re-injection technology can transform landfills from traditional storage sites into biological treatment systems for MSW. It can be seen that the re-injection of leachate is significant not only in reducing the pollution load of the leachate itself, but also in playing an important role in the control and treatment of pollutants throughout the entire landfill. Treating mineralized waste through leachate re-injection allows for the removal of pollutants such as COD from the leachate, and it is also possible to reduce the volume of the leachate through evaporation and transpiration. In contrast, for non-mineralized waste, especially that recently landfilled, the degradation of pollutants in the leachate resulting from its re-injection may not be significant. However, when considering both the pollution load in the solid waste and that in the leachate, re-injecting leachate accelerates the rate of degradation of the total pollution load in the landfill and speeds up the stabilization process of the landfill. Therefore, the positive benefits of re-injecting leachate in such cases cannot be dismissed based solely on the efficiency of removing pollutants from the leachate. Reinjection of leachate can significantly increase the moisture content of waste, accelerate its degradation rate, and speed up the stabilization process of landfills ； Reduce the time during which landfills have an impact on the surrounding environment ； Reduce monitoring and management costs for landfills after they are closed ； Increase the possibility of reusing land at landfills. In summary, compared with physical, chemical, and biochemical methods, the recharging method can better adapt to changes in the quality and quantity of leachate; it is a method that requires less investment, has lower operating costs, and can accelerate the stabilization of municipal landfills. 4 Recommendations 4.1 The issue of treating municipal solid waste leachate is receiving increasing attention. Leachate re-injection technology holds great promise for application due to its advantages such as low investment costs, low operating expenses, strong resilience to fluctuations in water quality and volume, and the ability to accelerate the stabilization of landfills. 4.2 The role of leachate re-injection technology is not limited to degrading pollutants in the leachate; therefore, research should focus on the management and control of pollutants throughout the entire landfill. The application of leachate re-injection should be considered during the design and construction of landfills. 4.3 More large-scale or full-scale experimental studies should be conducted. In engineering practice, leachate re-injection is influenced by factors such as waste composition, climatic and meteorological conditions, hydrogeological conditions, waste landfilling methods, changes in the waste due to re-injection, leachate composition, re-injection methods, and re-injection frequency. There are few factors that can be easily controlled or adjusted, and leachate re-injection is an unstable process that evolves over time; therefore, it is recommended that, where possible, as many pilot and full-scale experimental studies as feasible should be conducted, taking advantage of existing research findings. 4.4 The efficiency of leachate re-injection treatment in dealing with leachate is limited; therefore, research should be actively carried out on subsequent treatment technologies that are both technically feasible and economically viable, in order to meet the requirements for the compliant discharge of leachate. At the same time, the odor problem caused by the re-injection of leachate cannot be ignored. 5 References 1 Robert K Ham and Todd J Bookter. Decomposition of Solid Waste in Test Lysimeters. Journal of the Environment Engineering Division, 1982, 108(6): 147–1170. 2 Xu Dimin et al. Research on the Reuse of Leachate from Landfills. Journal of Tongji University, 1995, 32: 119–128. 3 Timothy D Baldwin, Jeffrey Stinson, Robert K Ham. Decomposition of Materials Buried Within Sanitary Landfills. Journal of Environmental Engineering, 1998, 153(11): 1193–1202. 4 Townsend G T, Miller W L, Earle J F K. Acceleration of Landfill Stabilization Using Leachate Recycling. Journal of Environmental Engineering, 1996, 122(4): 263–266. 5 Miller W L, Townsend T G. Leachate Recycling and the Enhancement of Biological Decomposition at Municipal Solid Waste Landfills, 2nd Annual Research Symposium