Purification and regeneration of diluted caustic and acidic washing solutions by membrane filtration

The aim of the work was to purify diluted caustic and acidic streams from dairy cleaning-in-place processes (CIP). Nanofiltration of diluted dairy washing solutions, i.e. sodium hydroxide and nitric acid solutions, was studied with spiral wound modules. In addition, re-concentration of purified caustic and acidic washing solutions by reverse osmosis was studied. Nanofiltration reduced the COD content of the caustic washing solution by 76–90% depending on the concentration ratio used. The alkaline/acid content of the nanofiltration permeate was similar to the feed. By reverse osmosis treatment alkaline NF permeates were concentrated from 0.2 to 0.7% NaOH. Concentration of nitric acid did not succeed. The membranes retained their initial pure water fluxes when treated with the conventional CIP procedure after the filtration run.