Ammoniacal nitrogen concentration by osmotically assisted reverse osmosis

Abstract

Ammoniacal nitrogen is a commonly encountered nitrogenous pollutant produced by industrial processes present in wastewater. In this work, model wastewater containing ammoniacal nitrogen (in the form of NH4Cl) was used as a feed solution in the demonstration of osmotically assisted reverse osmosis (OARO) using a multi-stage hollow fiber membrane module setup. The OARO process aimed to dewater the model wastewater and effectively concentrate the ammoniacal nitrogen in solution for easier recovery. Split feed OARO, wherein the feed stream is split either into the concentrated retentate or refluxed back into the permeate side flowing counter-current with the feed, was conducted and the influence of several operational parameters, such as applied pressure, feed flow rate, reflux ratio, on the concentration process was investigated. Also, the properties of the feed solution, i.e. initial feed concentration and presence of other ionic species, such as NaCl, were also tested. Higher concentration efficiency was observed at the following conditions: higher applied pressure, lower flow rate, and higher reflux ratio. The use of low concentrated feed solutions might be useful to decrease the applied pressure and the occurrence of internal concentration polarization was also avoided. The results in this work show that OARO could be a practical membrane-based process for enrichment and recovery of ammoniacal nitrogen from wastewater to further realize a circular economy, in which waste materials such as ammoniacal nitrogen are recovered and reused to produce valuable products.