Energy coverage of ataköy-ambarlı municipal wastewater treatment plants by salinity gradient power

Abstract

The present study investigated the reverse electrodialysis (RED) energy generation potential of treated wastewaters from Ataköy and Ambarlı wastewater treatment plant (WWTP) when discharged at the Marmara Sea. The good quality of the WWTP effluents and their suitable conductivity (around 1 mS.cm⁻¹) resulted in a power density around 0.5 W.m^-2 when they are operated together with the Marmara Sea in a lab-scale RED system. This led to the generation of an approximative daily gross energy of 18.6 MW h and 16 MW h with Ataköy and Ambarlı WWTP, respectively. Considering optimal RED conditions able to reach a power density of 2.5 W.m^-2, Ataköy WWTP can generate net energy of 39.9 MW h.day which represents 25.1 % of the total energy demand of the plant. Similarly, the Ambarlı WWTP can cover 25 % of the plant energy demand by generating daily net energy of 36.8 MW h. Improving the characteristics of the ions exchange membranes (IEMs) by introducing highly selective and cost-effective membranes is part of the necessary solution to increase the power output of the process.

Introduction

The chemical potential difference between water bodies releases energy when two solutions with different concentrations in salinity mix; this is known as salinity gradient power (SGP) [1] or salinity gradient energy (SGE). Although not well known, it is renewable and clean energy suitable for coastal countries and can contribute to increasing the many renewable energy sources already available to participate in accelerating the fossil fuel mitigation policy adopted by many nations in the last century. In the principle of SGP, when a freshwater stream flows toward the sea (or high saline water), and discharge into it, there is an increase of the entropy of the system and a dissipation of a chemical potential gradient when the two solutions of different concentration irreversibly mix one to another, and can be converted into energy through specific systems performing a controlled mixing process [[2], [3], [4]]. The reversal of some desalination processes can convert the potential energy from the salinity gradient into electricity giving rise to new technologies such as RED and Pressure Retarded Osmosis (PRO) which are the most frequently studied membrane-based technologies to harvest the SGP [2].

Pattle developed and proposed the first concept of electrical energy extraction from the mixing of fresh and saltwater in 1954 [5] and the technique he described is similar to the process known today as RED. It was not until 1977 that the global potential of SGP was estimated by Wick and Schmitt to be approximately 2.6 TW [6]. This value is near to the estimated 2.8 TW values today [7]. In contrary to PRO where water is transported across a semi-permeable membrane, the energy extracted by RED occurs by the transport of ions through IEMs, negatively charged cation exchange membranes (CEMs) and positively charged anion exchange membranes (AEMs), during the mixing of two solutions with different salinity [8]. RED utilizes a flow system between electrodes and alternating CEMs and AEMs [9], where the transport of dissolved salt ions through the stack of alternating IEMs creates an electrical potential capable of generating electricity [7], [10]. The ions (Na⁺ and Cl⁻) in the concentrated solution compartments migrate across the membranes toward the diluted solution compartments; Na⁺ migrates through the CEMs toward the cathode while Cl⁻ migrates through AEMs toward the anode. Since the membranes are semipermeable, they allow anions to pass through the AEMs and cations through CEMs, by migrating in opposite directions from the concentrated saline solution to the diluted solution, but theoretically block the transport of co-ions and water [1], [7]. When an external load is applied to the electrodes, the migration of ions at opposite directions creates an ionic current at the electrodes which are converted to electrical energy [9], [11], [12].

Although sea waters and rivers are great candidates for SGP generation, recently some attention has been devoted to alternative feed solutions such as wastewaters and brines from desalination plants. To reach a quality standard to be reused, the treatment of municipal wastewater require energy which can make the process expensive. When treated wastewater is used in a RED stack, the effluent solution can still be reused for many purposes [13]. Istanbul is a metropolitan with over 15 million inhabitants making it one of the world's most populated and largest cities. According to the report published by ISKI (Istanbul Water and Sewerage Administration) in 2018, Istanbul disposes of many advanced biological treatment plants. Some of the WWTPs integrated membrane bioreactors (MBR), while ultrafiltration (UF) and reverse osmosis (RO) membrane processes are used as advanced treatment processes to purify part of the effluent from biological processes, resulting in high-quality effluent mainly reused in the watering of gardens. A total of 900,465 kW h.day⁻¹ electrical energy is consumed in the WWTP in Istanbul, while up to 336,417 kW h.day⁻¹ electrical energy is produced from anaerobic digestion of sludge [14]. The effluents from Istanbul WWTPs are discharged in the Marmara Sea. These effluents are low in conductivity (around 1.1–1.4 mS.cm⁻¹) and can be considered as low saline solutions and used to generate SGP during the mixing of these effluents with the Marmara Sea.

Up to now, a few investigations have been conducted in the field of SGP extraction by the RED process using real feed solutions. Most of the studies are focused on lab-scales fed with synthetic solutions. Real feed solutions are more complex and the presence of divalent ions in sea waters, dissolved matters and suspended solids found in natural water make the process difficult and require high engineering skills to be optimized. However, targeting different feed solutions such as brines concentrates from different desalination processes and wastewater treated effluent can diversify the potential of RED. When natural feed streams are used in RED without adequate anti-fouling treatments, a mixture of fouling can take place on the membranes mainly composed of remnants of diatoms, clay minerals, organic fouling and scaling [15]. Avci et al. compared natural feed streams salinity gradient power harvesting by RED with synthetic equivalent ionic strength and reported approximately 2/3 loss in power density. They attributed this result to increased membrane resistance, reduced open-circuit voltage (OCV) and the occurrence of uphill transport for Ca²⁺, Mg^2+, and SO₄²⁻ in the RED stack operated with natural feed streams [16]. Many researchers reported up to 50 % decrease in the power density within the first-hour of RED operation with natural feed streams [15], [17], [18]. Electrochemical impedance spectroscopy demonstrated that the decrease of the system performance is prevalently due to the significant increase of CEM resistance due to the presence of negatively charged organic matters in natural water bodies. As a membrane process, membrane fouling and concentration polarization could be some major effects with real feed solutions. In addition to antifouling strategies to clean and recover the membrane and the RED stack performance, pretreatment of the feed solutions, membrane modification to improve its properties and the fouling resistance [19] and profiled membranes [15] are alternatives.

Using Ataköy and Ambarlı WWTP as models, for being among the biggest WWTP of the city of Istanbul, the present study investigated the RED energy generation potential with municipal wastewaters treated by advanced biological treatment, MBR and UF processes together with the Marmara Sea. The energy potential will be determined and simulation with optimal conditions will help determine the net RED energy output in Ataköy and Amabarlı WWTP.