Recovery of ammonia and phosphate from aquaculture effluent using a reactive vacuum membrane distillation-vacuum membrane distillation/nanofiltration hybrid system

The primary obstacle to extracting nutrients from aquaculture effluent lies in the fact that they are present only in minuscule amounts, rendering the conventional recovery methods inefficient and economically unfeasible. To address the issue, a two-stage membrane system, reactive vacuum membrane distillation-vacuum membrane distillation/nanofiltration (RVMD-VMD/NF) was developed in this work to concentrate ammonia and phosphate simultaneously from low-nutrient fresh aquaculture effluent. RVMD was effective to recover ammonia to a higher concentration factor (59-fold) at 40 °C. The temperature was then increased to 70 °C to further concentrate phosphate using the VMD system, and the results were compared to the pressure-driven nanofiltration (NF) system. VMD demonstrated an excellent capability to concentrate phosphate without any loss to the recovered water and achieved a comparable concentrating effect based on theoretical volume reduction, while NF only achieved half of the targeted value. Considering low-nutrient sources for treatment, RVMD followed by the VMD is a better approach in concentrating both ammonia and phosphate than the RVMD-NF system but RVMD-VMD required a slightly higher total cost (RM 1.68/L) and energy usage (0.53 kWh/L) compared to RVMD-NF of RM 1.66/L and 0.53 kWh/L. The application of the RVMD-VMD system to the fresh aquaculture effluent resulted in a remarkable 77-fold of recovered ammonia (49.33–3790 mg/L) with a separation factor (SF) of 554 and up to 6 factors of phosphate (16.67–92.52 mg/L). Besides, clean water was produced as a by-product. This innovative system provides a sustainable and effective solution for managing aquaculture effluent, with the potential to achieve zero-waste discharge.