Kinetics And Benefits Of Employing UV Light For The Treatment Of Aqueous Ammonia In Wastewater

Abstract

Nitrogen compounds, such as aqueous ammonia, are a widespread problem in the wastewater industry as they are toxic to numerous aquatic life, cause eutrophication, and contribute to various environmental concerns. Environment Canada has mandated new wastewater regulations, limiting un-ionized ammonia discharge to 1.25 mg/L, expressed as nitrogen. This study provides insight into methods for removing nitrogen compounds, specifically aqueous ammonia, from wastewater. Two wastewater treatment technologies were compared: Ultra Violet light and an electrochemical process. These treatments were evaluated individually, as well as in combination, to determine potential synergistic effects. The UV light experiments were conducted with a collimated beam apparatus and evaluated the effects of pH, UV fluence, initial concentration of nitrogen, and different chloride concentrations on nitrogen removal. Electrochemical experiments, using a Ti/RuO2 anode with a Ti cathode, evaluated the effects of pH and current density, at two different chloride concentrations on treatment efficacy. The photoelectrochemical process was evaluated in parallel with the electrochemical process to evaluate synergistic effects. All of the treatment technologies were used to treat secondary wastewater effluent obtained from a municipal wastewater plant, Mill Cove Water Pollution Control Centre, Bedford, Nova Scotia. Results from UV experiments indicated that pH had an impact on removal efficiency. At pH=8.0, no significant removal of total nitrogen was observed, whereas 47.6% and 59.5% removals were observed at pH=10.0 and pH=12.0, respectively. The degradation of total nitrogen was attributed to the removal of aqueous ammonia from the system. Different chloride concentrations did statistically affect removal efficiency. Increased initial contaminant concentration resulted in higher removal efficiencies, which suggests the underlying removal mechanism is proportional to the amount of contaminant present. In order to develop a kinetic model for the degradation of nitrogen compounds, the presence of hydroxyl radicals was confirmed via the degradation of para-chlorobenzoic acid. Results from the electrochemical experiments demonstrated that both initial pH, and current density had an effect on treatment efficiency. At both chloride concentrations, the system with the highest current density (15 mA/cm² at [Cl-]=530 mg/L and 40 mA/cm² at [Cl-]=5300 mg/L) at pH=9.7, resulted in the highest efficiency compared to pH=5.7. The production of DBPs (TTHMs and THAAs) was monitored when electrochemically treating municipal wastewater. Experiments conducted with both a UV light and an electrochemical cell resulted in no synergistic treatment benefits. However, lower levels of chlorine residual were observed in the photoelectrochemical process, which may result in lower DBPs. The photoelectrochemical treatment of municipal wastewater resulted in lower levels of THAAs compared to the electrochemical process.