Impact of reverse nutrient diffusion on membrane biofouling in fertilizer-drawn forward osmosis

Biofouling in fertilizer-drawn forward osmosis (FDFO) for water reuse was investigated by spiking pure bacteria species Pseudomonas aeruginosa PAO1+GFP and using three different fertilizers KNO3, KCl and KH2PO4 as draw solutions. The performance of FO process for treating synthetic wastewater was assessed and their influence on the membrane fouling and in particular biofouling was evaluated relative to the type of different fertilizers used and their rates of reverse diffusion. FO performances using KNO3 as draw solute exhibited severer flux decline (63%) than when using KCl (45%) and KH2PO4 (30%). Membrane autopsy indicated that the mass of organic foulants and biomass on fouled membrane surface using KNO3 as draw solute (947.5mg/m2 biopolymers, 72µm biofilm thickness and 53.3mg/m2 adenosine triphosphate) were significantly higher than that using KCl (450mg/m2 biopolymers, 33µm biofilm thickness and 28.2mg/m2 ATP) and KH2PO4 (440mg/m2 biopolymers, 35µm biofilm thickness and 33.5mg/m2 ATP). This higher flux decline is likely related to the higher reverse diffusion of KNO3 (19.8g/m2/h) than KCl (5.1g/m2/h) and KH2PO4 (3.7g/m2/h). The reverse diffused potassium could promote the organics and bacterial adhesion on FO membrane via charge screening effect and compression of electrical double layer. Moreover, reverse diffused nitrate provided increased N:P nutrient ratio was favorable for the bacteria to grow on the feed side of the FO membrane.