Abstract
Fouling is an inevitable phenomenon with most of the water treatment systems. Similar to RO, NF and other membrane-based systems, fouling also seriously affects the performance of low-cost forward-osmosis (FO) systems and disturbs the overall efficiency of these systems, and various cleaning practices have been evaluated to restore their designed performances. This study evaluates the performance of various physical and chemical cleaning techniques for hollow fibre forward-osmosis (HFFO) membrane. HFFO membrane was subjected to various fouling conditions using different brackish groundwater qualities and model organic foulants such as alginate, humic acid and bovine serum albumin. Results indicated that physical cleaning affects differently the flux restoration according to the type of foulants (i.e. inorganic or organic) and the crossflow rates play an important role in membrane cleaning in both membrane orientation. The higher cross flow Re values at any particular area seem important for the cleaning. With hydraulic flushing, the flux performances of HFFO were recovered fully when operated in AL-FS orientation, as high shear force helps to detach all scaling layers from the surface; however, the lower shear force did not fully restore the flux for the FS membrane in AL-DS orientation. Chemical cleaning was planned for the fouled HFFO membrane, and HCl and NaOH were used in various combination sequences. It was found that HCl did not clean the membrane used for AL-DS orientation for combined fouling. HCl cleaning (at pH 2) was found to be more effective for removing inorganic scale, whereas NaOH cleaning (at pH 11) for a similar period successfully restored the flux for all the membranes used for FS with inorganic and/or organic foulants. ethylenediamine tetra acetic acid (EDTA) was also evaluated for its cleaning performances and it was found that compared to NaOH, EDTA cleaning (1mM concentration at pH 11) showed superior results in terms of membrane cleaning, as it helped to successfully restore the membrane flux in a very short time.
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