Study of the operational parameters in the hollow fibre liquid-liquid membrane contactors process for ammonia valorisation as liquid fertiliser

Abstract

The transformation of wastewater treatment plants (WWTPs) to resources recovery plants (nutrients and energy) is forcing the development of specific technologies for selective recovery. Ion-exchange and hollow fibre liquid-liquid membrane contactors (HF-LLMCs) appeared as promising technologies to reduce operation costs, at ambient temperature, on the N-recovery from main-streams of urban WWTPs. A concentrated ammonia stream (4.5 g/L), recovered from treated wastewater using zeolites and NaOH, was used to test different operational parameters on the performance of the HF-LLMC process for ammonia recovery. These tested parameters were HF-LLMC position (horizontal and vertical), HF-LLMC feed and acid streams inputs (shell and lumen), type of acid stripping solution (0.4 M H3PO4 and 0.4 M HNO3), membrane drying, flow rate for each stream (263–770 mL/min), number of steps (1 and 2) and number of membrane contactors (1 and 2 in series). The maximum ammonia recovery (>95%) was achieved by a one-step configuration using two vertical membrane contactors in series; using the shell side for feed stream and the lumen side for the acid stripping solution (HNO3); at 450 mL/min and 770 mL/min flow rates for feed and acid stripping solutions, respectively. The liquid fertiliser composition obtained was around 4.6% N (from NH4+). Additionally, the ammonia treated stream, rich in NaOH, had residual levels of NH3 (about 200 mg/L) that could be reused for the regeneration of the zeolites. This optimized HF-LLMC process allowed to recover ammonia from a tertiary treatment solution of an urban WWTP and at the same time to produce ammonium salts that could be used as liquid fertilisers in irrigation applications. Therefore, this technology promotes the circular economy concept for nutrient recovery in urban WWTPs.