ENERGY RECOVERY FROM WASTEWATER USING BIOELECTROCHEMICAL SYSTEMS DRIVEN BY REVERSE ELECTRODIALYSIS CELL

Abstract

ABSTRACT

Energy recovery from wastewater using bioelectrochemical systems driven by reverse electrodialysis cell

Syarif Hidayat Department of Civil and Environmental Engineering Hanyang University Graduate School

In the last decades, due to the intensified concern of environment and energy, the development of environmentally friendly energy sources is rapidly becoming an uppermost priority. So that, nowadays many countries try to develop technology which can produce the energy from either renewable or abundant material resources. The combination of bioelectrochemical systems (BESs) and reverse electrodialysis cells (RED) such as microbial reverse-electrodialysis electrolysis cell (MREC) and microbial reverse-electrodialysis cell (MRC) has been proposed to be used as an alternative method to produce active energy from either relatively abundant or renewable materials. These technologies can extract the energy stored in abundant materials such as an organic matters contained in wastewater and Gibbs free energy contained in salinity differences of seawater and river water into hydrogen gas and direct electricity. In this study, the factors affecting the reactor performance were studied. First, the effect of the presence of multivalent ions including Mg2+ and SO42- in the feed stack solutions on MREC performance were investigated. The experiments were conducted with the presence 10% of multivalent ions (MgCl and MgSO4) in the feed stack solutions. The maximum cell current decreased when a multivalent ions present in the feed stack solution, resulted in decreasing the hydrogen produced, hydrogen production rate, and yield. The reducing effect on reactor performance seemed to have been caused by the uphill transport phenomena, sulfate ion inhibition effects, and the precipitation of Mg(OH)2 on the cathode electrode. In the membrane stack, Mg+2 and SO42- ions were against the gradient concentrations (uphill transport) because the electromotive forces of Na+ and Cl- were higher than that of Mg+2 and SO42- which lead to decrease in cell current. The SO42- ions were transported from HC solutions through the anion exchange membrane into the anode chamber. Since the pH of the anolyte tends to decrease to lower than seven, soluble H2S would be predominant within the anolyte thus inhibited microorganisms activity. Additionally, the precipitation occurs on the cathode electrode since the pH of catholyte increases up to 12. Precipitation on the cathode surface generated an increased internal resistance and an increase in energy needed for the water-splitting process which leads to reducing the cell currents, hydrogen production, and rates.

Second, a continuous flow microbial reverse-electrodialysis electrolysis cell (MREC) was operated under non-buffered substrate with various flow rates of catholyte effluent into anode chamber to investigate its effects on the hydrogen gas production. Phosphate buffers solutions (PBS) is critical to reactor performance in bioelectrochemical systems. PBS provides the anolyte to be high conductivity and buffering capacity and leads to high current density by reducing ohmic resistance and maintaining neutral pH. However, utilization of high concentration of PBS in wastewater treatment leads to high operational cost and also contributes to environmental problems. In this research, catholyte effluent from MREC reactor is proposed as an alternative to PBS since it has high pH (>11) and conductivity (~50 mS cm-1). The addition of catholyte effluent into anode chamber increases the buffering capacity and conductivity of the anolyte, which may lead to improved reactor performance. The catholyte effluent was supplied continuously along with anolyte influent into anode chamber with the various flow rate and normalized into the anolyte influent salt concentration (0.10 M, 0.17 M, and 0.23 M). The increasing anolyte influent salt concentration to 0.23 M resulted in improved hydrogen gas production, Coulombic recovery, yield, and hydrogen production rate to 25 ± 1.4 mL, 83 ± 5%, 1.49 ± 0.15 mole –H2 mole-1-COD, 0.91 ± 0.03 m3-H2 m-3-Van day-1, respectively. These improvements were attributed to the neutral pH rather than increase in anolyte conductivity as there was no significant improvement in the reactor performance when the NaCl was directly added to the reactor. These results show that addition of catholyte effluent into the anode chamber improved the MREC performance.

Lastly, the different number of membrane pairs were applied in MRC reactor to determined the optimum membrane pairs for power production. The reactor performance was evaluated by measuring current production, power density, ion flux efficiency, COD removal, Coulombic efficiency, and energy efficiency during experiment. Additionally, internal resistances in anode electrode and stack were investigated by polarization curve. The reactor performance improved with the addition of a number of membrane cell pairs. However, Adding more membrane pairs increasing the total internal resistance resulted in decreased in an increase power density and maximum cell potential. The optimum membrane pairs was determined to be three based on power density and energy efficiency of MRC reactor. Further, the results show that MRC have better performance as compared to the individual systems, creating new technologies that can be used to extract energy from seawater and river water.