Nutrient-energy-water recovery from synthetic sidestream centrate using a microbial electrolysis cell - forward osmosis hybrid system

Recovery of nutrient, water, and energy from high-strength sidestream centrate offers benefits such as reusable resource, minimized discharge and cost-savings in mainstream treatment. Herein, a microbial electrolysis cell - forward osmosis (MEC-FO) hybrid system has been investigated for integrated nutrient-energy-water (NEW) recovery with emphasis on quantified mass balance and energy evaluation. In a closed-loop mode, the hybrid system achieved recovery of 54.2 ± 1.9% of water (70.4 ± 2.4 mL), 99.7 ± 13.0% of net ammonium nitrogen (8.99 ± 0.75 mmol, with extended N₂ stripping), and 79.5 ± 0.5% of phosphorus (as struvite, 0.16 ± 0.01 mmol). Ammonium loss primarily from reverse solute flux was fully compensated by the reclaimed ammonium under 6-h extended N₂ stripping to achieve self-sustained FO process. The generated hydrogen gas could potentially cover up to 28.7 ± 1.5% of total energy input, rendering a specific energy consumption rate of 1.73 ± 0.08 kWh m⁻³ treated centrate, 0.57 ± 0.04 kWh kg⁻¹ COD, 1.10 ± 0.05 kWh kg⁻¹ removed NH₄⁺-N, 1.17 ± 0.06 kWh kg⁻¹ recovered NH₄⁺-N, or 5.75 ± 0.54 kWh kg⁻¹ struvite. Recycling of excess Mg²⁺ reduced its dosage to 0.08 kg Mg²⁺ kg⁻¹ struvite. These results have demonstrated the successful synergy between MEC and FO to achieve multi-resource recovery, and encouraged further investigation to address the challenges such as enhancing hydrogen production, reducing nutrient loss, and optimizing MEC-FO coordination towards an energy-efficient NEW recovery process.