Municipal wastewater treatment in microbial electrolysis cells

Abstract

Wastewater treatment often demands high infrastructure and usually uses a great amount of energy. Nowadays, there is growing consensus that organic components in the wastewater are a resource instead of an increasing problem. Bioelectrochemical Systems are emerging devices that could be developed into a new generation of sustainable wastewater treatment plants since they are able to treat wastewater while recovering part of the energy investment as hydrogen or electricity. The main objective of this thesis is to study the feasibility of different MECs (Microbial Electrolysis Cell) configurations for organic matter removal and hydrogen production with low energy consumption during domestic wastewaters treatment. The process scale-up from milliliters to liters of working volume of a microbial electrolysis cell (MEC) was demonstrated. Initially, a 50 mL MEC was operated on synthetic wastewater at different organic loads. It was concluded that process scale-up might be best accomplished using a “reactor-in series” concept. Consequently, 855 mL and 10 L MECs were built and operated on domestic wastewater. The three compartments 855 mL MEC, achieved a COD removal of 5.7 g-COD LR -1 d -1 and a hydrogen production of 1.0-2.6 L LR -1 d -1 by optimizing applied voltage and hydraulic retention time. Furthermore, a two 5 L MECs in series showed a COD removal rate of 0.5 g LR -1 d -1 , a removal efficiency of 60-76% and an energy consumption of 0.9 Wh per g of COD removed. Furthermore, the effect of the organic loading rate, hydraulic retention time, applied voltage and the configuration of a semi-pilot modular microbial electrolysis cell (MEC) on the energy consumption and COD removal during domestic (dWW) wastewater treatment was studied. The MEC reactor consisted of twin tubular units hydraulically connected in series and was Abstract xiv able to reduce up to 85% of the chemical oxygen demand (COD) concentration of the influent dWW at a relatively low energy consumption 0.3-1.1Wh per g of COD removed. Overall, the results identified both an organic loading rate (OLR) threshold that makes the use of MECs for dWW treatment feasible in terms of energy consumption and COD removal efficiency and an OLR threshold that justifies the operation of two MECs in series to provide the required degree of COD removal. Hydrogen production was limited by the reduced amounts of organic matter fed into the reactor, the poor performance of the cathode, and COD consumption by non electrogenic microorganisms. The presence of COD consuming microorganism that do not contribute to electrogenic metabolism severely affected the MEC performance. Finally, the carbon footprint of a domestic wastewater treatment plant (dWWTP) with an integrated MEC and its comparison with a conventional wastewater treatment plant was estimated. The dWWTP with an MEC technology was found to generate lower emissions than a traditional dWWTP, moreover, it would avoid a large amount of emissions if the energy contained in the hydrogen could be either consumed in the plant or injected into the grid.