Toxic Byproduct Formation during Electrochemical Treatment of Latrine Wastewater.

Justin T Jasper,Michael R Hoffmann,Yang Yang

doi:10.1021/acs.est.7b01002

Justin T Jasper, Michael R Hoffmann + Show 1 more

Open Access

https://doi.org/10.1021/acs.est.7b01002

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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).

Highlights

IntroductionOnsite electrochemical systems show promise for providing wastewater treatment to the billions of people lacking access to adequate wastewater treatment,[1] and these systems are currently being commercialized for application in both rural communities (e.g., rural schools in South Africa) and urban communities (e.g., apartment buildings in India)
Onsite electrochemical systems show promise for providing wastewater treatment to the billions of people lacking access to adequate wastewater treatment,[1] and these systems are currently being commercialized for application in both rural communities and urban communities
A combination of reactive chlorine species and direct oxidation provide reduction of chemical oxygen demand (COD)[4] and transformation of trace organic contaminants within 4 h (3.5− 4.5 V applied cell potential)[5−7] with rates enhanced at elevated chloride concentrations

Summary

Introduction

Onsite electrochemical systems show promise for providing wastewater treatment to the billions of people lacking access to adequate wastewater treatment,[1] and these systems are currently being commercialized for application in both rural communities (e.g., rural schools in South Africa) and urban communities (e.g., apartment buildings in India). Electrochemical systems can be powered by solar energy and do not require external water inputs, as treated water can be recycled for flushing.[2] in addition to being recycled within the system, once storage tanks are full, treated water is discharged to the environment due to system users’ urine input. Ensuring a high level of wastewater treatment is critical to protecting the receiving environment as well as human health if discharged water reaches drinking water sources or system users come in contact with recycled flushing water. Electrochemical treatment systems have been shown to provide effective treatment of latrine wastewater. Ammonium removal occurs via breakpoint chlorination,[8] and phosphorus can be precipitated as hydroxyapatite.[9]

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