The growing number of municipal solid waste (MSW) landfills could risk leachate pollution in most countries, especially groundwater resources. Leachate is the highly contaminated liquid generated from anaerobic reactions and infiltrating rainwater in MSW landfills. A very high concentration of organic compounds poses a serious hazard to public health and ecosystems when untreated leachate releases into the surrounding environment and groundwater. Because of the adverse effects on ecosystems and the environment, leachate treatment gets a lot of attention. Therefore, an individual and integrated treatment technique are required to ensure public health safety and establish a sustainable leachate treatment technology. The conventional treatment methods are insufficient to fulfill the regulatory discharge requirements. The fundamental challenges associated with leachate treatment are its toxic components of humic/fulvic acids, heavy metals, and NH3-N. Most of the recent leachate treatment technologies have used a series of biological, physical, and chemical processes. However, none of the current treatment technologies is stand alone. Most integrated technologies are expensive, inefficient, difficult to operate, unsustainable, and energy-consuming. Therefore, there is an urgent need to develop new leachate treatment technologies that are eco-friendly, sustainable, easy to operate, low cost, low energy consuming, less sensitive, and more adaptive to the varying contaminants. In addition, the newly developed technology should be able to recover the organics, nitrogen, phosphorous, inorganic-xenobiotics, and energy from leachate. Hence, the literature survey includes groundwater pollution from open dumping and non-engineered landfills. Furthermore, it evaluates the strengths and shortcomings of various leachate treatment technologies currently available. Therefore, this doctoral dissertation aims to (i) characterize the physical, chemical, and biological properties of MSW landfill leachate collected from Al-Multaqa (MLL) and Barka landfill (BLL), Muscat, Sultanate of Oman; and (ii) explore the possibility of applying locally available Palygorskite clay as an individual adsorbent media to treat the leachate and determine it kinetics and isotheral behavior; (iii) integrate clay adsorption with fenton/H2O2 oxidation process to enhance the COD and NH3-N removal; and (iv) integrate clay adsorption with the bioelectrochemical system (BES) to enhance the COD and NH3-N removal; (v) evaluate the performance of the individual and integrated systems (clay-adsorption, fenton/H2O2 oxidation followed by adsorption, and BES followed by adsorption) to achieve the optimum COD and NH3-N removal. During leachate characterization, ninety-two (92) leachate parameters were determined and finally estimated, the leachate pollution index (LPI) to observe the overall environmental impacts. The study also determined the effect of rainfall on the leachate pollution parameters and LPI values. The results showed a significantly high concentration of COD (28000 - 33600 mg/L) and NH3-N (4173 - 4655 mg/L) in the raw leachate. Rainfall event lowers the concentration of COD (2400-2805 mg/L) and NH3-N (562-663 mg/L) in the raw leachate up to 83% and 87%. After rainfall, the normal LPI value 18 - 29% declined to 14 - 28%. The estimated LPI values were higher than similar studies in the literature. The results were attributed due to unsegregated MSW dumping into the landfills. Therefore, LPI monitoring is a good indication that estimates the requirement of leachate treatment. During individual clay-adsorption treatment, Palygorskite clay was used in four formats, i.e., raw crushed clay (RCC), 75µ sieved clay (75SC), 75µ sieved calcined clay (75SCC), and 75µ sieved and washed clay (75SWC). The 75SC showed the highest removal of COD (43.84%, equivalent to 35.56 mg/g) with the adsorbent dose of 180 g/L leachate at 9 hours HRT, while the RCC achieved the maximum NH3-N adsorption (19.39%, equivalent to 1.1 mg/g) at 9-hour HRT. The experimental data fit the well-known Freundlich isotherm for both the COD and NH3-N removal with R 2 ≈ 0.987 and 0.985, respectively. However, the individual clay adsorption did not show good COD and NH3-N removal performances. Therefore, a Fenton oxidation (FO) was added as a pretreatment step before clay adsorption to enhance the COD and NH3-N removal. The results of the integrated system were discussed in the subsequent paragraphs. In the oxidation-adsorption integrated treatment, four doses of Fenton reagents, i.e., 55.6 g, 83.4 g, 166.8 g, 278 g of reagents per Liter leachate (equivalent to 10, 15, 30, and 50 mM leachates solution), were investigated. Each reagent mixed leachate solution was kept for five different HRT as 2, 4, 6, 12, and 24 hrs. The results revealed that the 15 mM reagent dose in the leachate at 6-12 hours HRT produced the best oxidation performance with COD (35.78%) and NH3-N (35.52%). After the 15 mM FO, the 180 g 75SWC clay per Litre of leachate was applied as a post-treatment. The oxidation-adsorption integrated system achieved 59.48% COD and 69.73% NH3-N removal. The integrated system enhanced the removal efficiencies for COD and NH3- N compared to the individual clay adsorption. However, the integration of FO did not significantly improve the COD and NH3-N removal due to utilizing raw leachate without any dilution. Therefore, an integration of BES as a pretreatment step with clay adsorption was investigated to observe the removal performances using leachate mixed wastewater. In the integrated BES-adsorption study, the BES operated with two different external resistances (BES operated with 1 Ω was called BES-1, and BES operated with 100 Ω was called BES-100) utilizing a 1:4 ratio of leachate to wastewater. BES-1 showed 28% COD and 36% NH3-N removal individually, while the BES-100 showed 78.26% COD and 10% NH3-N removal. Applying 6 g 75SWC adsorption achieved an overall removal of ~98% COD and ~80% of NH3-N, which demonstrated an efficient treatment of leachate mixed WW. Such a high removal was achieved due to leachate dilution and BES addition. Therefore, integration of BES with locally available clay adsorption for leachate treatment could be considered the best treatment option in terms of system efficiency and operating cost. Moreover, the application of BES needs no extra energy and chemicals, which will make the leachate treatment system sustainable and eco-friendly. The raw leachate treated by individual clay adsorption showed less COD and NH3-N removal efficiency. Therefore, it is considered a less suitable leachate treatment method. However, the method was sustainable due to clay material's low-cost, nonhazardous, and reusable nature. Integrating Fenton-oxidation or BES as a pretreatment of clay adsorption enhanced the COD and NH3-N removal efficient compared to the individual clay adsorption. However, the integration of FO was less efficient and unsustainable due to the requirement of extra oxidizing chemicals. While the integration of BES was efficient and sustainable due to no need for extra energy and chemicals. Therefore, the integrated system was recommended to achieve enhanced leachate treatment in future research.