Electrodialysis with bipolar membranes for ammonia recovery in wastewater

Abstract

Municipal and industrial wastewater contains a significant amount of dissolved nitrogen. This is the results of the organic protein degradation and of the large employment of nitrogen (N), usually in the form of ammonia (NH₃), in the industry. Currently, nitrogen is removed in wastewater treatment plant (WWTP) by means of biological treatments. Globally, the most applied treatment consists in the combination of nitrification and denitrification. This technique is characterized by a high energy demand (44.3 MJ per Kg_NH4⁺ removed)  which accounts for 70% of the total energy consumption in the WWTP. A less energetically intensive alternative treatment is the Anammox process. However, this also presents significant limitations, especially when dealing with extremely polluted stream and in the flexibility of operation.
An alternative to reduce the energy cost of nitrogen removal by valorizing the content of residual streams is explored by the N2kWh project. The novel concept underlying the N2kWh project aims to use NH₃ from waste streams as a fuel source for a solid oxide fuel cell (SOFC). In this way, the energy cost for biological N-removal processes is cut, and, ideally, energy can be even produced. WWTP digested reject water was recognized as potential N-source for energy recovery as a result of its relatively high total ammonia nitrogen (TAN) concentrations (up to 1.5 g − N ⋅ L^-1). A (selective) concentration step and pH regulation are needed in order to convert the TAN in the reject water, mostly present in form of ammonium bicarbonate (NH₄HCO₃), into NH₃ gas which can be stripped and directly fed in theSOFC. Previous studies proved electrodialysis (ED) to be the most energetically efficient technology for the removal and concentration of ammonium bicarbonate. However, due to the high alkalinity in the obtained concentrate, a large amount of chemicals is required for the pH regulation. An interesting opportunity to avoid this chemical addition is the employment of a bipolar membrane electrodialysis (BPMED). Through this unique technology, it is possible to simultaneously achieve the concentration of TAN, as well as the regulation of pH without chemicals.
The main objective of this work was to evaluate the potential of BPMED for the recovery of NH₃ in the boundaries of the N2kWh project. The energy performance of BPMED was compared with the alternative use of conventional ED plus sodium hydroxide for pH control. In order to obtain valuable information on the operation, three setups were employed: regular ED, BPMED and BPMED coupled with two membrane vacuum stripping devices (VMS). For this purpose, a synthetic solution obtained by dissolving 6.6 g ⋅ L^-1 NH₄HCO₃ salt in demiwater was used to simulate the digested reject water. Experiments were designed to characterize the BPMED operation and clarify which processes influence the performance.
Experimental laboratory results showed that, in terms of energy consumption for the NH₄⁺ removal, the BPMED used more energy compared to the ED. The removal of 90% of the initial NH₄⁺ is achieved by using an average of 13.2±0.1 MJ ⋅ Kg_NH4^-1. This value was 3.3 times higher than that achieved with regular ED. This discrepancy was explained by the extra-elements in the stack and the water dissociation process, responsible for the higher voltage measured during BPMED operation. The energy consumption was proven to be higher also as a consequence of the lower current efficiency for salt transport (73% compared to 95% of conventional ED). This was due to the more severe undesired diffusion processes, mainly gas diffusion and hydroxide leakage from the alkaline stream to the diluate, taking place within the BPMED stack. However, the energy consumption for ammonium removal in BPMED was still more than three times lower than the energy consumed by nitrification-denitrification and comparable to what used by anammox. 
The implementation of the membrane vacuum stripping modules, in series with the BPMED, only slightly increased the overall current efficiency (up to 3.4 % current efficiency gain). The benefit in current efficiency was less significant for higher concentrations and pH in the alkaline stream. Besides that, the tested technology tandem (BPMED+VMSs) was capable of stripping NH₃ and, consequently, reduce the gas diffusion over the stack. The energy consumed for the production of NH₃ and for the ED operation to reach a pre-defined pH and TAN concentration was estimated to be equal to 82.55 Wℎ per L of concentrate. This value was substantially higher than what consumed by BPMED (48.7 Wℎ) to achieve the same result in the alkaline stream.
To conclude, the studied BPMED system was demonstrated to be superior in terms of energy consumption for the recovery of NH₃ gas when compared to the combination of ED and chemical addition. BPMED showed also crucial advantages for the environment, design and safety of the treatment facility. The energy consumption for the removal of nitrogen with BPMED was proven to be lower than what used in the competitive biological technology. Finally, neither the technology nor the operation methods employed were specifically designed/optimized for this application. This made the use of BPMED for the N2kWh purpose even more interesting and promising.