Development of Polyelectrolyte Multilayer Membranes for Ammonium Recovery from Anaerobic Digestate

This research aims to develop a membrane separation process to treat anerobic digestate for the recovery of liquid ammonia, which can be used as fertilizer. Industrial manufacturing of ammonia is an energy-intensive process and accounts for a significant release of greenhouse gases. Anaerobic digestate (AD), which contain organic carbons (COD), ammonium, and phosphate, represents a promising resource of nutrients which can be recovered for use as fertilizers.

Membrane separation process can be an effective way to treat such a wastewater feed, provided membranes with tunable organics/nitrogen selectivities can be designed. Our work focusses on developing such a tunable platform using polyelectrolyte multilayer membranes (PEM) by the surface modification of a loose nanofiltration (NF) membrane (NF 270) using layer-by-layer (LbL) deposition methods. The LbL method involves the sequential deposition of alternately charged polyelectrolytes on a given subtsrate for a desired number of cycles. The central hypothesis of this work hinges on the fact the LbL strategy would provide a precise way to control both the water flux across the membranes as well as the selectivity and therefore show better permeability/selectivity trade-offs compared to commercial membranes. Two polyelectrolytes were used for the surface modification: poly (diallyl dimethyl ammonium chloride) (PDAC) and poly (styrene sulfonate) (PSS). The performance of those PEM membranes were compared against a commercially available NF-90 membrane,which is a “tight” NF membrane. Our initial experiments suggest that in comparison with the commercial nanofiltration/reverse osmosis membranes, these PEM membranes exhibited higher rejection of organic carbon with an equivalent phosphate removal. Furthermore, PEM membranes’ low ammonia rejection and high water flux with respect to commercial RO membranes make them more feasible to use against AD for ammonium recovery. Beyond the current focus, we aim to establish these PEM membranes as an effective platform for useful resource recovery from varied wastewater sources.