Performance, fouling and cleaning of a thin film composite hollow fiber membrane during fertiliser-drawn forward osmosis process for micro-polluted water

摘要

The fertiliser-drawn forward osmosis (FDFO) process uses fertilizers as draw solutes to extract fresh water from polluted water. The diluted draw solutions after the FDFO process can be directly used for irrigation, which addresses the challenges of irrigation water shortage and eliminates the demand of draw solute recovery. In this work, a thin film composite (TFC) hollow fiber membrane has been developed for the FDFO process. The effects of solutes, feed temperature and foulants (alginate, octanoic acid and gypsum) in the feed solution on its performance were investigated. The membrane showed more severe fouling in the active layer facing draw solution (AL-DS) orientation as more foulants and scalants tended to attach on the porous substrate surface and penetrate into the substrate pores. Hydraulic flushing using DI water, NaOH and HCl solutions was conducted to evaluate the reversibility of the fouled membrane after FDFO operation in the AL-DS orientation. It was found that the 1 M NaOH solution was not effective in removing the alginate fouling layer as alginate tends to penetrate into the membrane pores under alkaline conditions. Octanoic acid-fouled membrane could achieve almost 100% recovery after flushing with 0.1 M NaOH. The octanoic acid can be dissociated into ions under alkaline conditions and thus easily scoured off from the membrane surface. The gypsum-scaled membrane could obtain a 78.2% water flux recovery after 0.2 wt% HCl flushing. Nevertheless, the water flux dropped rapidly in the second run due to the residual crystals on the membrane surface and within the membrane pores.