Selective abatement and recovery of nutrients from wastewaters using electrochemical technologies

Abstract

Enormous amounts of freshwater and high-quality fertilizers are required to expand agricultural activity in support of the growing global population and required improvement in living standards. Wastewater contains a large amount of nutrients which, if discharged to rivers and lakes, may result in severe eutrophication with this phenomenon leading to the depletion of dissolved oxygen, deterioration of water quality and acute/ chronic death of aquatic creatures. Therefore, removal and recovery of these nutrients from wastewater will bring significant economic and environmental benefits. Electrochemical technologies such as flow-electrode capacitive deionization (FCDI) and electrochemical advanced oxidation processes (EAOPs) have been adopted for removal/recovery nutrients from wastewaters in view of their advantages over other alternatives (e.g., minimal requirement for auxiliary chemicals, amenability to automation, versatility and potential to be powered by renewable energy from wind and solar sources).

The first section of this thesis focuses on investigation of the electrochemical processes (involving both Faradaic and non-Faradaic reactions) in typical electrochemical systems with particular attention given to processes operating in capacitive deionisation (CDI) cells. We provide a detailed insight into both the positive and negative effects of these processes. Guidelines and strategies that could be used to reduce or eliminate the negative side effects of Faradaic reactions are presented and approaches to best utilizing Faradaic reactions in a positive manner (e.g., nutrients removal/recovery) are described. The latter section of this thesis focusses on the application of these electrochemical processes to selectively sequester/recover nutrients. Using an EAOP cell, ammonia can be efficiently converted into harmless nitrogen gas with concentrations of by-products (i.e., chlorate, nitrate and chloramine) well below WHO guidelines, with the assistance of in situ electro-generated active chlorine whereas nitrate can be abated using an FCDI cell, with non-electrostatic adsorption of nitrate to the carbon particles initially playing a vital role. An innovative capacitive membrane ammonia stripping (CapAmm) system, which is composed of a FCDI cell and a membrane contactor (hollow-fibre and flat-sheet), has been developed and found to be a promising alternative for ammonia removal/recovery from both dilute and high-strength wastewaters. As for the recovery end-products, further studies described in this thesis reveal that the transformation of ammonia into ammonia solution is the optimal choose in view of its high market value. Of particular interest is the low voltage used in CapAmm system for nutrients recovery, rendering its suitability for integration with photovoltaic power supply technology that can significantly reduce energy cost.

One major limitation of these approaches is the poor electrical conductivity of the carbon flow electrodes (several orders of magnitude lower than conventional static electrodes) with this restricting the nutrient removal/recovery efficiency, increasing the internal resistance and, ultimately, resulting in higher energy consumption. In addition, pumping of the flow carbon particles consumes energy, which must be taken into account in the overall energy budget of the technology. Scale-up from small bench size units to full-sized modules will also obviously be required. It is envisaged that CapAmm cells could be stacked together to provide the required throughput but challenges will need to be overcome, particularly with regard to optimizing flow electrode composition (i.e., electrolyte, carbon loading and active electrode materials), flow cell design (i.e., channel geometry and current collector materials) and operating strategy (i.e., slurry flow rate, charging/discharging time control).