Toxic Byproduct Formation during Electrochemical Treatment of Latrine Wastewater

Abstract

Electrochemical systems are an attractive option for onsite latrine wastewater treatment due to their high efficiency and small footprint. While concerns remain over formation of toxic byproducts during treatment, rigorous studies examining byproduct formation are lacking. Experiments treating authentic latrine wastewater over variable treatment times, current densities, chloride concentrations, and anode materials were conducted to characterize byproducts and identify conditions that minimize their formation. Production of inorganic byproducts (chlorate and perchlorate) and indicator organic byproducts (haloacetic acids and trihalomethanes) during electrolysis dramatically exceeded recommendations for drinking water after one treatment cycle (∼10–30 000 times), raising concerns for contamination of downstream water supplies. Stopping the reaction after ammonium was removed (i.e., the chlorination breakpoint) was a promising method to minimize byproduct formation without compromising disinfection and nutrient removal. Though treatment was accelerated at increased chloride concentrations and current densities, byproduct concentrations remained similar near the breakpoint. On TiO2/IrO2 anodes, haloacetic acids (up to ∼50 μM) and chlorate (up to ∼2 μM) were of most concern. Although boron-doped diamond anodes mineralized haloacetic acids after formation, high production rates of chlorate and perchlorate (up to ∼4 and 25 μM) made them inferior to TiO2/IrO2 anodes in terms of toxic byproduct formation. Organic byproduct formation was similar during chemical chlorination and electrolysis of wastewater, suggesting that organic byproducts are formed by similar pathways in both cases (i.e., reactions with chloramines and free chlorine).

Nature, Mechanisms and Reactivity of Electrogenerated Reactive Species at Thin‐Film Boron‐Doped Diamond (BDD) Electrodes During Electrochemical Wastewater Treatment

“…[2,13,17] This behavior is attributed to the formation of highly refractory organochlorinated byproducts which are very difficult to mineralize. [70] Another drawback of the presence of Cl À ions in wastewater during electrooxidation is the high risk of production of highly toxic chlorinated amines, [111,112] trihalomethanes (THMs) and haloacetic acids (HAAs) [102,[113][114][115][116][117] as well as hazardous perchlorate from the oxidation of organic pollutants by electrogenerated RCS. [70,88] However, at higher Cl À concentrations, lower mineralization rate is always observed due to the formation of larger quantity of recalcitrant organochlorinated byproducts that are highly resistant to the oxidation by electrogenerated BDD( * OH).…”

“…[70,88] However, at higher Cl À concentrations, lower mineralization rate is always observed due to the formation of larger quantity of recalcitrant organochlorinated byproducts that are highly resistant to the oxidation by electrogenerated BDD( * OH). [106,[121][122][123] Radjenovic group [106,116,124,125] have carried out extensive works on identification and quantification of generated halogenated byproducts in different media during the electrooxidation of electrodialyzed/non-electrodialyzed ROC using different electrode materials. [118][119][120] Indeed, several studies have reported the formation and accumulation of total adsorbed organic chloride and bromide especially during the treatment of ROC.…”

“…[106,[121][122][123] Radjenovic group [106,116,124,125] have carried out extensive works on identification and quantification of generated halogenated byproducts in different media during the electrooxidation of electrodialyzed/non-electrodialyzed ROC using different electrode materials. However, other authors [13,116,121] have shown that BDD anode mineralizes more effectively the generated HAAs and THMs compared to other electrode during the oxidation of ROC. Unfortunately, significantly higher formation of total HAAs and THMs was observed with BDD anode (62.4 and 9.1 μM for HAAs and THMs, respectively, at Q = 5.6 Ah L À 1 ) compared to Ti/SnO 2 -Sb (34.2 and 8.9 μM for HAAs and THMs, respectively) and Ti/Pt-IrO 2 (27.4 and 5.6 μM for HAAs and THMs, respectively) at similar charge passed.…”

“…[70,88] However, at higher Cl À concentrations, lower mineralization rate is always observed due to the formation of larger quantity of recalcitrant organochlorinated byproducts that are highly resistant to the oxidation by electrogenerated BDD( * OH). [70] Another drawback of the presence of Cl À ions in wastewater during electrooxidation is the high risk of production of highly toxic chlorinated amines, [111,112] trihalomethanes (THMs) and haloacetic acids (HAAs) [102,[113][114][115][116][117] as well as hazardous perchlorate from the oxidation of organic pollutants by electrogenerated RCS. [118][119][120] Indeed, several studies have reported the formation and accumulation of total adsorbed organic chloride and bromide especially during the treatment of ROC.…”

“…[118][119][120] Indeed, several studies have reported the formation and accumulation of total adsorbed organic chloride and bromide especially during the treatment of ROC. [106,[121][122][123] Radjenovic group [106,116,124,125] have carried out extensive works on identification and quantification of generated halogenated byproducts in different media during the electrooxidation of electrodialyzed/non-electrodialyzed ROC using different electrode materials. For electrodialyzed ROC, It was reported that the formation of THMs and HAAs increases with increasing the Cl À concentration, regardless of electrode material used during the electrooxidation of electrodialyzed ROC in 0.05 M of NaCl, NaNO 3 or Na 2 SO 4 .…”