Second use for old reverse osmosis membranes: wastewater treatment

This paper describes some tests made with old seawater reverse osmosis (RO) membranes, modified to be reused as filtration membranes. These modified membranes were then used to reduce suspended solids in the tertiary treatment of municipal wastewater. The purpose of the tests was to establish the feasibility of recovering old (discarded) membranes in order to reduce industrial solid waste, as well as reducing costs in new filtration membranes. The active layer in the old RO elements was skinned off by recirculation of a potassium permanganate solution, which drastically reduced the salt rejection properties of the membranes, down to 2%. The elements so conditioned were then used as membrane filters inside pressure vessels, which had also been used in RO plants and recovered for a new purpose. The pilot plant was designed to reduce both suspended and dissolved solids from wastewater secondary effluent, with a production capacity designed for 6 m(3)/h. The reused membranes proved capable of reducing suspended solids (93.9% average reduction in turbidity), and the overall separation efficiency reached 97%. The RO unit reduced 98.5% conductivity at a 60% recovery ratio when operating at 32 bar. The filtration membranes developed a high pressure drop, and must be cleaned quite often, both by flushing at high recirculation ratios and by chemical cleaning. The cleaning procedures must be optimised and reduced to a minimum.