Use of Biochar and Zeolite for Landfill Leachate Treatment: Experimental Studies and Reuse Potential Assessment

Leachate is a highly colored liquid waste product of landfills that contains high concentrations of ammonia nitrogen (NH4+-N), refractory organic matter, and heavy metals. It is commonly disposed to local publicly owned treatment works (POTWs). With these potent characteristics, it interferes with the physical, biological, and chemical treatment processes within POTWs. Sequencing batch biofilm reactors (SBBR) and hybrid subsurface flow constructed wetlands (CW) have been used in the past to treat landfill leachate, but their design and performance can still be optimized. Addition of low-cost adsorbent materials to SBBRs and CWs, such as biochar and zeolite, have the potential to enhance leachate treatment by adsorbing NH4+-N and organic matter, respectively. This increases their retention in the treatment system and reduces toxicity levels to microorganisms. Further treatment of landfill leachate with advanced treatment processes comprised of ultrafiltration (UF) and reverse osmosis (RO) creates a variety of water reuse opportunities, such as non-food agricultural reuse and industrial reuse. This research evaluated two different objectives regarding landfill leachate treatment. The first study investigated the treatment of high-strength landfill leachate from a Florida landfill in a bench-scale adsorbent-amended SBBR under varying hydraulic retention time (HRT) conditions and compared the performance to a previous SBBR study that used lower strength landfill leachate. The SBBR contained lightweight expanded clay aggregate (LECA), clinoptilolite, which is a natural zeolite mineral, and biochar. The reactor was operated in a three-stage sequence: 1) rapid fill, 2) 3.5-day low aerobic react, and 3) rapid drain. The initial HRT was set at 18.9 days to match a similar total ammonia nitrogen (TAN) loading rate that the SBBR was known to be able to handle. The HRT was subsequently reduced to 14 days after 12 cycles and further reduced to 10.5 days after 8 cycles. TAN, nitrate (NO3--N), nitrite (NO2--N), total nitrogen (TN), soluble chemical oxygen demand (sCOD), 5-day biochemical oxygen demand (BOD5), and ultraviolet absorbance at 254 nm (UV254) and 456 nm (UV456) were measured to compare the removal performance across the varying HRT conditions. High total inorganic nitrogen (TIN) removal rate of 82.9, 109, and 122 mg/L-day were observed at HRTs of 18.9, 14, and 10.5 days, respectively. Due to low influent BOD5/TN and calculated high free ammonia concentrations, the high TIN removals were most likely due to mechanisms such as simultaneous nitrification-denitrification, shortcut nitrogen removal and partial nitritation/anammox. High sCOD removal rates of 168, 217, and 223 mg/L-day were observed at HRTs of 18.9, 14, and 10.5 days, respectively. The minimal difference between the 14-day and 10.5-day HRT conditions indicates that biochar was reaching its maximum adsorptive capacity. Decreasing color removal was observed throughout the study and desorption in the 456 nm was observed in the 10.5-day HRT. This provides further evidence that the biochar had met its maximum adsorptive capacity for organic matter, especially for those reflective at the 456 nm wavelength. An adsorption study confirmed that biochar had reached its maximum adsorptive capacity for sCOD. However, zeolite’s bioregenerative capability for TAN did not decrease with the SBBR media after two years of usage compared to the fresh media. In comparison with the previous SBBR study done by Gao (2020), the SBBR achieved higher TIN and sCOD removal rates with the Orange County landfill leachate than with the Hillsborough County landfill leachate. However, low color removals were observed by the end of the study. The results indicate that additional biochar should be added to the SBBR periodically to maintain high sCOD and color removal. The second study evaluated the post-treatment feasibility of CW-treated landfill leachate by UF and RO to meet reuse requirements. Four feed stream samples were collected from the Hillsborough County Southeast Landfill in Lithia, FL: 1) raw landfill leachate, 2) activated sludge (AS) treated landfill leachate, 3) gravel CW effluent (G-CW), and 4) gravel-zeolite-biochar CW (GZB-CW) effluent. It was deemed that CW effluent could not meet water reuse standards due to high electrical conductivity levels, therefore UF and RO is recommended as a post-treatment step. Model simulations were done with DuPont’s Water Application Value Engine software, which allows integration of UF and RO into a single software. A common UF design configuration was designed for the four feed streams comprised of 3 online trains and 1 offline train with 6 modules each. A common RO design configuration was designed for the four feed streams, consisting of a first stage with 2 pressure vessels with 6 elements each and a second stage with 2 pressure vessels with 3 elements each. An optimized GZB-CW UF design configuration was also designed, consisting of 4 online trains and 1 offline train with 4 modules each. The optimized design reduced the total number of UF modules due to the lower solids content of the GZB-CW effluent compared to raw landfill leachate and AS treated landfill leachate. The optimized GZB-CW RO design configuration was comprised of 2 pressure vessels, with 8 elements each for both stages, to maximize permeate water recovery. An equivalent annual worth analysis was developed on a 20-year design life at an assumed interest rate of 5% for seven different alternatives, including the four feed streams to UF-RO, optimized GZB-CW to UF-RO, raw landfill leachate to direct disposal and GZB-CW effluent to direct disposal. The optimized GZB-CW to UF-RO alternative was found to be 63% less costly than the raw landfill leachate to direct disposal alternative