Assessment of Micropollutants removal, By-products formation, and Effluent toxicity during Electrochemical wastewater treatment

Abstract

Electrochemical advanced oxidation processes (EAOPs) have been extensively used for water and wastewater treatment to improve water quality, remove micropollutants (e.g. pharmaceuticals, pesticides), and attain disinfection. EAOPs systems are versatile, operational at ambient conditions with low energy cost, easy to operate, and compact making them ideal for decentralized applications (i.e., household and small-community use). Despite the increasing field deployments for industrial and decentralized wastewater treatment, the issues regarding micropollutants elimination, by-products formation, and effluent toxicity persist. Toxic halogenated by-products could be generated from the EAOPs treatment of saline/brackish wastewater, while micropollutants that remain undetected using conventional targeted analysis may be unknowingly introduced to the environment. Unfortunately, these concerns have been largely overlooked in spite of their importance towards a sustainable EAOPs operation. Therefore, this study was conducted to shed some light on these pressing issues. Herein, the EAOPs treatment of municipal/livestock wastewater and source-separated urine were evaluated. Key operational parameters were identified and optimized to strike a balance between pollution control, by-products formation, and effluent toxicity. The results of this study were divided into three sections. (1) Increasing contamination of groundwater by heavy metals could potentially hamper the basic sanitation based on a septic system in developing countries. Therefore, this paper evaluated wastewater treatment by electrochemical activation of hydrogen peroxide (EAHP), persulfate (EAP), and free chlorine (EAFC) to simultaneously eliminate aqueous organic matter and heavy metals. Sacrificial iron anodes under galvanostatic regime activated the batch-injected oxidants under uncontrolled pH, to avoid the extra cost of control in decentralized processes (e.g., household use) to treat domestic wastewater. Response surface methodology (RSM) was used to determine the optimized conditions for EAP ([persulfate]0 = 25 mM, 24.4 mA·cm−2), EAFC ([free chlorine]0 = 35.5 mM, 44.4 mA·cm−2), and EAHP ([H2O2]0 = 91.1 mM, 45 mA·cm−2) towards total organic carbon (TOC) removal. Treatment of real wastewater under optimum conditions significantly reduced chemical oxygen demand (COD) and TOC in all treatments, complying with lenient effluent standards as well as the added benefit of complete As(V) and Cr(VI) removal. Although EAP and EAFC provided superior removal of TOC (70−75%) and COD (73−100%) within 3 h, respectively, effluent toxicity and operation cost (76−85 USD·m−3) were relatively high. EAHP was the best available option to secure non-toxic effluent with the least cost (63 USD·m−3). (2) Non-target screening (NTS) assessment can unravel the fates of micropollutants and transformation products (MTPs) during wastewater treatment. In this study, galvanostatic iron anodes were used to drive EAHP, EAP, and EAFC, for industrial-scale treatment of municipal and livestock wastewater with a focus on MTPs and effluent toxicity. Response surface methodology determined the optimized conditions for EAP (persulfate dose = 0.12 mmol·min−1, 26.5 mA·cm−2), EAFC (free chlorine dose = 0.29 mmol·min−1, 37.4 mA·cm−2), and EAHP (H2O2 dose = 0.20 mmol·min−1, 45 mA·cm−2) towards TOC removal. Probe-compound degradation revealed that HO•, SO4•− and FeIVO2+ species were simultaneously generated in EAP, whereas HO• and FeIVO2+ were the principal oxidants in EAHP and EAFC, respectively. The NTS using liquid/gas chromatography coupled with high-resolution mass spectrometry monitored the generation or removal of MTPs including compounds that have not been reported previously. The speciation of oxidants shifted by halide ions (Cl−, Br−) in real wastewater samples significantly affected the mineralization efficiency and by-product formation. The formation of halogenated by-products in EAFC and EAP substantially increased the effluent toxicity, whereas EAHP provided non-toxic effluent and the highest mineralization efficiency (75 − 80%) to be nominated as the best strategy. (3) Nitrogenous disinfection by-products (N-DBPs) are orders of magnitude more toxic than regulated carbonaceous DBPs (C-DBPs) such as trihalomethanes (THMs) and haloacetic acid (HAAs). Ironically, no studies on N-DBPs formation nor the factors affecting their generations during urine electrolysis have been reported so far. In this study, authentic urine samples were electrolyzed using boron-doped diamond (BDD) and TiO2/IrO2 anodes under various operational conditions (current density and [Cl−]:[NH3] ratio) and urine chemistry (fresh vs digested urine, pH). In addition to THMs and chlorinated HAAs, targeted by-products analysis showed that N-DBPs such as chlorinated haloacetonitriles (HANs), halonitromethanes (HNMs), and haloacetamides (HAcAms) as well as unregulated C-DBPs, chloral hydrate (CH), and chlorinated haloketones (HKs), were generated during urine electrolysis on both anodes. Meanwhile, the non-targeted analysis revealed the formations of by-products that were structurally similar to targeted DBPs and their intermediates. DBPs evolution on BDD anodes was associated with strong and early formations followed by removal, whereas DBPs accumulated on TiO2/IrO2 anodes (up to 60 mg·L−1) with sustained electrolysis. Urine digestion pre-treatment decreased the maximum [DBPs] by a factor of 3.4 and 2.4 on BDD and TiO2/IrO2 anodes, respectively. DBPs formation showed greater sensitivity to changes in [Cl−]:[NH3] ratio than current density, while unaffected by pH. Although the total [DBPs] drastically increased with continuous electrolysis beyond the breakpoint (i.e., complete NH3 removal), it ensured the degradation of highly toxic HANs and HAcAms, including several non-targeted by-products, resulting in decreased DBPs-associated toxicity. Nevertheless, the electrochemically-treated effluent remained toxic necessitating post-treatment before discharge.