Electrochemical Transformation of Trace Organic Contaminants in Latrine Wastewater

Abstract

Solar-powered electrochemical systems have shown promise for onsite wastewater treatment in regions where basic infrastructure for conventional wastewater treatment is not available. To assess the applicability of these systems for trace organic contaminant treatment, test compound electrolysis rate constants were measured in authentic latrine wastewater using mixed-metal oxide anodes coupled with stainless steel cathodes. Complete removal of ranitidine and cimetidine was achieved within 30 min of electrolysis at an applied potential of 3.5 V (0.7 A L^(–1)). Removal of acetaminophen, ciprofloxacin, trimethoprim, propranolol, and carbamazepine (>80%) was achieved within 3 h of electrolysis. Oxidation of ranitidine, cimetidine, and ciprofloxacin was primarily attributed to reaction with NH_2Cl. Transformation of trimethoprim, propranolol, and carbamazepine was attributed to direct electron transfer and to reactions with surface-bound reactive chlorine species. Relative contributions of aqueous phase ·OH, ·Cl, ·Cl_2–, HOCl/OCl–, and Cl_2 were determined to be negligible based on measured second-order reaction rate constants, probe compound reaction rates, and experiments in buffered Cl– solutions. Electrical energy per order of removal (E_(EO)) increased with increasing applied potentials and current densities. Test compound removal was most efficient at elevated Cl– concentrations present when treated wastewater is recycled for use as flushing water (i.e., ∼ 75 mM Cl–; E_(EO) = 0.2–6.9 kWh log^(–1) m^(–3)). Identified halogenated and oxygenated electrolysis products typically underwent further transformations to unidentifiable products within the 3 h treatment cycle. Identifiable halogenated byproduct formation and accumulation was minimized during electrolysis of wastewater containing 75 mM Cl^–.