Impact of sulfate ion addition on electrochemical oxidation of anaerobically treated landfill leachate using boron-doped diamond anode

Simultaneous removal of organic and nitrogenous compounds in mature landfill leachate by a hybrid electro-oxidation-dialysis (EOD) system

Electrochemical process has been widely applied to eliminate recalcitrant contaminants (i.e., organic and nitrogenous compounds) in landfill leachate. This study aimed to evaluate the performance of a hybrid electro-oxidation-dialysis (EOD) system to minimize organic and nitrogenous compounds through a synergistic process of electrochemical oxidation (EO) and electrodialysis (ED) as well as the dissolved organic matter was characterized in terms of fluorescent component and molecular weight distribution. The EOD was carried out using boron-doped diamond (BDD) and Pt alternately. The results have shown that pH adjustment to acidic conditions is beneficial to EO. At optimal pH (pH 4), BDD-based EO is superior to removing COD and NH 4 + up to around 56% and 64%, respectively. During EOD process, the lower current density at 20.83 mA cm-2 is preferred for the recovery of nitrogenous ions (i.e. NH 4 + and NO 3 - ), especially for BDD-EOD. In addition, the dominant humic acid-like (HAL) and soluble microbial products-like (SMPL) substances in the mature leachate are mostly degraded to smaller molecules from 105 Da to 103 Da in both EOD processes. Overall, BDD-EOD favours indirect oxidation and has a higher energy consumption efficiency than Pt-EOD induced by direct oxidation for simultaneous removal of organic and nitrogenous compounds. BDD-EOD requires a lower total operation cost of around $2.33/m3 compared to Pt-EOD. It is concluded that the hybrid BDD-EOD process is technically feasible as a powerful pre-treatment approach to mature landfill leachate for refractory organics degradation and nitrogenous nutrients recovery.

Containment of sulfate in leachate as gypsum (CaSO4·2H2O) mineral formation in bio-cemented sand via enzyme-induced carbonate precipitation

Enzymatically induced carbonate precipitation (EICP) using urea hydrolysis is a well-known bio-cementation process that not only promotes the precipitation of calcium carbonate (CaCO₃) but can provide excess calcium cations for further reaction depending on the substrate constituents and reaction stage. This study presents the EICP recipe to contain sulfate ions in landfill leachate sufficiently using remaining calcium cations and a series of tests were conducted to validate its ability to retain sulfates. The reaction rate for 1 M CaCl₂ and 1.5 M urea was identified by controlling the purified urease content and the curing time of the EICP process. The results showed that 0.3 g/L of purified urease produced 46% CaCO₃ and reduced sulfate ions by 77% after 3 days of curing. The shear stiffness in EICP-treated sand was enhanced 13 times by CaCO₃ precipitation followed by 1.12 times increment due to subsequent precipitation of gypsum (CaSO₄·2H₂O) crystals implying sulfate containment. A cost-efficient EICP treatment using soybean crude urease instead of lab-grade purified urease exhibited lower sulfate removal efficiency (i.e., 18%) with only nominal formation of gypsum in the EICP-treated sand. The addition of gypsum powder was effective in increasing sulfate removal by 40% when soybean crude urease was used for EICP.

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L^-1 resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L^-1 at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L^-1. The nitrogen degradation mechanism proposed assumes a series-parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃^- was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater.

Electrochemical oxidation of pretreated landfill leachate nanofiltration concentrate in terms of pollutants removal and formation of by-products

This study compares the efficiencies of active (Ti/TiO₂-RuO₂-IrO₂ (TIR)) and inactive (Ni/Boron Doped Diamond (BDD)) anodes in terms of pollutant treatment and by-product formation in pretreated (chemical coagulation) landfill leachate nanofiltration membrane concentrate (PLNC). PLNC has high chemical oxygen demand (COD:4900 mg/L), total organic carbon (TOC: 1874 mg/L), total Kjeldahl nitrogen (TKN: 520 mg/L), ammonium nitrogen (NH₃-N: 21.35 mg/L), chloride (5700 mg/L) and sulfate (9000 mg/L - due to coagulant type). The parameters of COD, TOC, NH₃-N, TKN, free and combined chlorine species, halogenated organic compounds (HOCs), adsorbable organic halogens (AOX), and nitrate at different current density (J: 111-555 A/m²) and initial pH (pH_i:3.5-7) were compared for both anodes. The removal efficiencies at the optimum conditions (pH_i 5.5, 333 A/m² and 8 h) were obtained as 86.4% COD, 77.4% TOC, 93.4% TKN, 94.4% NH₃-N with BDD and 34.3% COD, 27.3% TOC, 93.7% TKN, 97.4% NH₃-N with TIR. According to gas chromatography-mass spectrometry (GC-MS) results obtained under optimum conditions, haloalkane/alkene, halonitroalkane, halonitrile, haloketone, haloalcohols, haloacids, haloaldehydes, haloamines/amides on both electrodes were detected as species of HOCs. In addition, the highest nitrate concentration was observed at the TIR anode, while the highest AOX concentration was observed at the BDD anode.

Effect of electrolyte composition on electrochemical oxidation: Active sulfate formation, benzotriazole degradation, and chlorinated by-products distribution

Electrochemical oxidation is an effective technique for treating persistent organic pollutants, which are hardly removed in conventional wastewater treatment plants. Sulfate and chloride salts commonly used and present in natural wastewater influence the electrochemical degradation process. In this study, the effect of electrolyte composition on the active sulfate species (SO₄^●⁻ and S₂O₈²⁻) formation, benzotriazole degradation-a model organic compound, and chlorinated by-products distribution have been investigated while using a boron-doped diamond (BDD) anode. Different Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratios with constant conductivity of 10 mS/cm were used in the experiments and applied anode potential was kept constant at 4.3 V vs. Ag/AgCl. The electrogenerated SO₄^●⁻ and S₂O₈²⁻ formation were faster in 10:1 and 2:1 Na₂SO₄:NaNO₃ ratios than in the 1:0 ratio. The ^●OH-mediated SO₄^●⁻ production has prevailed in 10:1 and 2:1 ratios. However, ^●OH-mediated SO₄^●⁻ production has hindered the 1:0 ratio due to excess chemisorption of SO₄²⁻ on the BDD anode. Similarly, the faster benzotriazole degradation, mineralization, and lowest energy consumption were achieved in the 10:1 Na₂SO₄:NaNO₃ and Na₂SO₄:NaCl ratio. Besides, chlorinated organic by-product concentration (AOX) was lower in the 10:1 Na₂SO₄:NaCl ratio but increased with the increasing chloride ratio in the electrolyte. LC-MS analysis shows that several chlorinated organic transformation products were produced in 0:1 to 2:1 ratio, which was not found in the 10:1 Na₂SO₄:NaCl ratio. A comparatively higher amount of ClO₄⁻ was formed in the 10:1 ratio than in 2:1 to 0:1 ratio. This ClO₄⁻ formation train evidence the effective ^●OH generation in a sulfate-enriched condition because the ClO₄⁻ formation is positively correlated to ^●OH concentration. Overall results show that sulfate-enriched electrolyte compositions are beneficial for electrochemical oxidation of biorecalcitrant organic pollutants.

Application of boron-doped diamond, Ti/IrO2, and Ti/Pt anodes for the electrochemical oxidation of landfill leachate biologically pretreated by moving bed biofilm reactor

Conventional biological treatments used in most Indonesian landfill sites are mostly ineffective in treating stabilized landfill leachates to meet the standard regulation. Thus, a combination of biological and electrochemical process is offered to successfully treat leachates containing a high concentration of organic and nitrogenous compounds. In this study, a moving bed biofilm reactor (MBBR) was applied prior to electrochemical oxidation by using boron-doped diamond (BDD), Ti/IrO₂, and Ti/Pt anodes with applied current of 350, 400 and 450 mA. The objectives were to investigate the effect of anode type and the applied current on the removal of organics as well as total nitrogen from the MBBR-treated leachate with electrochemical oxidation. The optimum removal of chemical oxygen demand (COD) observed on the Ti/Pt anode was 78% by applying 400 mA, with an estimated energy of 56.7 Wh g L^-1. In the case of Ti/IrO₂ and BDD anodes, the optimum removal of COD was 76 and 85% with an energy consumption of 58.9 and 36.9 Wh g L^-1, respectively, both achieved at 350 mA. Although all anodes showed less-satisfactory performances for total nitrogen reduction, around 46-95% removal of nitrogenous compounds was achieved by MBBR, with their partial conversion to nitrates.