Effect of fouling on performance of pressure retarded osmosis (PRO) and forward osmosis (FO)

Abstract

A layered model is developed to describe mass transport through fouled membranes in pressure retarded osmosis (PRO) and forward osmosis (FO) processes. This resistance-in-series model accounts for salt and water transport through the active layer, support layer, external boundary layers, and the cake layer formed by foulants. Closed-form algebraic expressions for the water flux, salt flux, salt concentration profile, and overall transport coefficients are presented for FO membranes in both membrane orientations. Organic fouling experiments using alginate are used to validate the model and observe the effects of feed salinity, cross-flow velocity, and membrane orientation on foulant accumulation rates. Increasing feed salinity and cross flow velocity both lead to a decrease in foulant accumulation in PRO orientation. Under identical operating conditions, foulant accumulation rates are comparable in FO and PRO modes. The model is then evaluated to show how foulant accumulation affects FO system performance in both membrane orientations. The model is also used to elucidate the effect of fouling on power production in PRO: fouling affects the optimal PRO operating pressure, and a 0.5 mm-thick cake layer can reduce power production by 50%. The mathematical expressions developed serve as a simple tool to predict the performance of fouled membranes in both PRO and FO processes.