Minimal Bipolar Membrane Cell Configuration for Scaling Up Ammonium Recovery

Abstract

Electrochemical systems for total ammonium nitrogen (TAN) recovery are a promising alternative compared with conventional nitrogen-removal technologies. To make them competitive, we propose a new minimal stackable configuration using cell pairs with only bipolar membranes and cation-exchange membranes. The tested bipolar electrodialysis (BP-ED) stack included six cell pairs of feed and concentrate compartments. Critical operational parameters, such as current density and the ratio between applied current to nitrogen loading (load ratio), were varied to investigate the performance of the system using synthetic wastewater with a high nitrogen content as an influent (NH 4 + ≈ 1.75 g L –1 ). High TAN removal (>70%) was achieved for a load ratio higher than 1. At current densities of 150 A m –2 and a load ratio of 1.2, a TAN transport rate of 1145.1±14.1 g N m –2 d –1 and a TAN-removal efficiency of 80% were observed. As the TAN removal was almost constant at different current densities, the BP-ED stack performed at a high TAN transport rate (819.1 g N m –2 d –1 ) while consuming the lowest energy (18.3 kJ g N –1 ) at a load ratio of 1.2 and 100 A m –2 . The TAN transport rate, TAN removal, and energy input achieved by the minimal BP-ED stack demonstrated a promising new cell configuration for upscaling.

Ammonia Recovery from Domestic Wastewater Using a Proton-Mediated Redox Couple

“…10,11,19−21 Bipolar membrane ED processes offer an energy-efficient means for ammonia recovery (1.3−5.1 kWh kg −1 N) owing to low voltages needed for splitting water. 31,32 Note, however, that the NH 4 + −N concentration in synthetic wastewater (14.3−111.1 mM NH 4 + −N) was higher than that tested in our work (5 mM NH 4 + −N), which could have resulted in a higher energy consumption than 3.2 ± 0.1 kWh kg −1 N (separation) when applied to low-strength wastewater. In addition, prohibitively expensive bipolar membranes would add economic burden on the overall cell construction.…”

“…This facilitated the recovery of ammonia by membrane stripping. The voltage required to achieve an efficient separation was less than 0.5 V (0.47 ± 0.01 V at 9.6 A m –2 ), which was lower than that required for electrolysis of water in ED (1.2–4.0 V at current densities between 7 and 50 A m –2 ). ,, The use of inexpensive redox couple and carbon-based electrode provides additional benefit over ED-based approaches that use costly metal electrodes to achieve electrolysis of water. ,,− Bipolar membrane ED processes offer an energy-efficient means for ammonia recovery (1.3–5.1 kWh kg –1 N) owing to low voltages needed for splitting water. , Note, however, that the NH 4 + –N concentration in synthetic wastewater (14.3–111.1 mM NH 4 + –N) was higher than that tested in our work (5 mM NH 4 + –N), which could have resulted in a higher energy consumption than 3.2 ± 0.1 kWh kg –1 N (separation) when applied to low-strength wastewater. In addition, prohibitively expensive bipolar membranes would add economic burden on the overall cell construction.…”

“…In addition, the need for regenerating flow electrode to avoid the accumulation of salt would lead to frequent interruptions of the operation. Recently, several studies have explored the use of bipolar membrane in ED for ammonia recovery from wastewater. − Dissociation of water occurring at the bipolar membrane produces proton and hydroxide at a relatively low theoretical voltage of 0.83 V compared to that of electrolysis of water (1.23 V), but membrane costs remain a challenge for large-scale applications …”

Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems

“… 15 This is mainly associated with poor selectivity of membranes and the necessity to recirculate solutions to reach a high effluent concentration. 26 The diffusion of neutrally charged species like NH 3 , H 3 PO 4 , and CO 2 (H 2 CO 3 ) cannot be solved with the same strategy of using asymmetric BPMs like in other applications, 63 as charge selectivity is invalid in this case. 15 Although the concentration of ionic species can be larger than that of the neutral species (especially for CO 2 which has mediocre solubility in aqueous solutions), the unhindered crossover of neutrally charged species via diffusion can still exceed the crossover of ionic species at low current densities (up to 40 mA cm –2 for the example of 0.5 M phosphoric acid).…”

“…In order to maintain charge neutrality and to supply the required ion current, the depletion of ions (proton and hydroxide) triggers further water dissociation, in line with Le Chatelier’s principle. The reverse bias has been demonstrated in acid/base production, water electrolysis, CO 2 electrolysis, and resource recovery via pH swing. , The ion transport mechanism in reverse bias has been well studied, in particular for extreme pH gradients (i.e., pH 0 vs pH 14) and unbuffered (initially neutral) solutions. , …”