Optimization of air/water cleaning (AWC) in spiral wound elements

Air/water cleaning (AWC) is effective in reducing problems with biofouling and particulate fouling of spiral wound membrane elements. One hour daily AWC was found to be useful, however, leading to a given downtime of NF/RO installations. This work deals with optimization of the AWC process to reduce the downtime of the installations using AWC. Reference, daily AWC and weekly AWC measurements were compared in two pilot runs using spiral wound membrane elements. AWC on spacer-filled channels with flat sheet membranes were carried out using a membrane fouling simulator (MFS) for visual observations, determining the amount of air during AWC and to determine the biomass removal on biofouled flat sheet membranes. Daily AWC was found to be more effective than weekly AWC, expressed as the increase in normalized pressure drop (NPD) of respectively 5% and 144% after 21 days. Weekly AWC was effective in reducing the high increase in NPD down to 20% at day 21. From visual observations of the MFS it was found that air bubbles were broken up to the size of the squares of the feed spacer. The amount of air in a vertically positioned MFS correlated to the set air/water ratios higher than four. Biomass from biofouled membrane sheets in the MFS by AWC was removed to 83%. A further optimization of AWC in spiral wound elements will be carried out using the MFS set-up.