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Description

Landfill leachate is a complex wastewater that contains organics, water, and nutrients that can be recovered as valuable resources. This project has investigated leachate treatment and recovery of potential resources from landfill leachate using innovative methods such as forward osmosis (FO), bioelectrochemical systems (BES), and advanced oxidation. The specific objectives were to recover or remove nitrogen, recover water, and study membrane fouling during leachate treatment using incorporated BES and FO systems. In addition, the project has also investigated the recovery of humic acid (HA) from leachate and the synergistic incorporation of FO, HA recovery, and Fenton’s oxidation for enhanced leachate treatment. The removal of leachate UV quenching substances (humic, fulvic, and hydrophilic acids) using a BES and a chemical oxidant with a focus on energy production and cost efficiency were studied.

Direct electricity can be produced from leachate using a microbial fuel cell (MFC) that works on the anaerobic degradation of organics in the anode chamber. The MFC fed with leachate produced a net energy of 0.123 kWh m-3 at a hydraulic retention time (HRT) of 20 days with periodical anode recirculation (1 hour in every 24 hours), and a 39 Ω external resistance (equal to the internal resistance of the MFC). The MFC system achieved 53 – 64% removal of COD. The results implicate that the MFC technology could be an energy-efficient method for leachate treatment, but the incomplete treatment will require the incorporation of other treatment methods.

Nitrogen in the form of ammonia can be removed and/or recovered in a BES through electricity driven movement to the cathode chamber followed by air stripping at high pH (>9). Applying this principle, an MFC removed 50 – 75% ammonia from a leachate. Similarly, 66% of the ammonia was recovered from a raw leachate fed in the anode chamber of a microbial electrolysis cell (MEC). The recovered ammonia was used as a draw solute in an FO unit for water recovery from the MEC treated leachate. An integrated microbial desalination cell (MDC) and FO system demonstrated 11 – 64% ammonia recovery, affected by current generation and HRT in the desalination chamber. Removal/recovery of ammonia from leachate will reduce the need for nitrogen treatment, although applications of the recovered ammonia warrants further investigation.

High-quality water can be recovered from leachate by using FO. In the MDC-FO system, 51.5% water was recovered from a raw leachate, and the recovery increased to 83.5% when the concentrated FO brine was desalinated in the MDC and then recirculated back into the FO unit. In an MEC-FO system with 2-M NH4HCO3 draw solution, water recovery from the MEC anode effluent was 51%, higher than that from the raw leachate because of the lower conductivity of the MEC anode effluent. The operational factors such as recirculation rates and draw solute concentrations could strongly affect energy consumption of FO during water recovery. Energy consumption increased with a higher recirculation rate and decreased with a higher draw concentration. Osmotic backwashing appeared to be effective for removing foulants in the beginning, while in long-term operation chemical cleaning could become essential for severe fouling.

Humic acids (HA), a key group of organic contaminants in leachate, can be recovered by simple adjustment of pH to < 2. HA can be used as a fertilizer, soil stabilizer, and a reagent in chemical reactions. HA recovery increased from 1.86 to 2.45 g L-1 at pH 2 after FO concentration, mainly because of the replacement of O in the HA structure by other inorganics (i.e., Cl, Na, K) with higher molecular weight. Fenton’s oxidation was incorporated with HA recovery as the optimum pH for Fenton’s oxidation is 2-4. With both FO and HA recovery, the reagent requirements for Fenton’s oxidation was reduced, for example, 25.2% for H2SO4, 34.6% for NaOH, and 35.1% for both FeSO4 7H2O and H2O2, compared to the Fenton’s treatment of raw leachate alone. The HA recovery also decreased sludge generation from Fenton’s oxidation by 29.1% because of the lower organics concentration and associated lower reagent doses. The multiple benefits (recovery of valuable HA, decrease in the reagent requirements, and sludge production) of this approach have potential to address the leachate brine discharge issue faced by the industry.

Humic acid and other recalcitrant leachate organics (i.e., fulvic acid (FA) and hydrophilic acid (HPI)), which are collectively called leachate UV quenching substances (UVQS) can escape biological treatment processes, lower the UV transmittance of waste streams due to their UV- quenching properties, and interfere with the associated disinfection efficacy. To address this, several treatment approaches have been investigated for UV absorbance reduction from leachates. In the MFC, nearly 50% UV absorbance was reduced at a 40-day HRT. The corresponding decrease of total organic carbon (TOC) was 75.3%, including the reduction of HA (95.2%), FA (77.2%), and HPI (67.5%) as TOC. Granular activated carbon in combination with the MFC reduced 92.9% of the UV absorbance and 89.7% of the UVQS as TOC. HA recovery prior to Fenton’s oxidation has improved the UV absorbance and UVQS removal performance from landfill leachate and up to ~93% UV absorbance and ~80% UVQS (as TOC) removal were achieved. In contrast, the oxidation with 0.2-M sodium percarbonate (SPC, Na2CO3 1.5 H2O2) reduced the organics concentration by only 15.1-15.6 % and UV absorbance by 7.9-43.4 % in 24 hours. The maximum COD reduction was 5.3 kg m-3 (with 0.05-M SPC), resulting in a treatment cost of $0.8 per kg of COD removed. These results indicate that MFC treatment of leachate is promising for UVQS removal. Fenton’s oxidation is probably the best treatment to remove leachate organics; our study has also demonstrated that the recovery of HA from leachate prior to Fenton’s can complement the UV absorbance reduction performance. In terms of cost, the newly investigated oxidant, sodium percarbonate, is the cheapest, however is limited in efficacy.

In conclusion, this project has demonstrated the feasibility of BES-based treatment of landfill leachate with recovery of several potential resources at lab scales. The BES treatment can reduce leachate organics concentration, UV absorbance, recover ammonia, and achieve water recovery in combination with FO. The comparative small volume of landfill leachate than municipal wastewater and varied composition may create a good need for BES based treatment of landfill leachate. Although BES treatment of leachate is promising, it will require a significant amount of work prior to its possible applications in practice. The treatment efficiency can be further enhanced by improving electrochemical reactions via optimized catalysts and/or externally applied voltage. Resource like HA in leachate should get more attention and further efforts can focus on purification of the recovered products. FO application to leachate treatment must consider the choice of an appropriate draw solute, which should require minimal effort for regeneration.

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