Principles to use reverse osmosis for drinking water supply and reduce concentrate disposal

Abstract

Possibilities to reduce reverse osmosis concentrate flow and operational costs were investigated. The existing solutions are described using chemical softening and evaporation techniques. The presented article is devoted to investigation of possibilities to remove calcium carbonate from concentrate due to calcium deposition on “seed” crystals. A new technique to reduce concentrate flow is presented that consists of operation of membrane unit in circulation mode whereby concentrate is circulated through a “seed” reactor. In reactor calcium and carbonate ions deposit on the “seed” crystals thus reducing scaling hazard and concentrate TDS value. Water treatment flow diagram is presented and technical parameters of membrane facilities are determined that enables us to design the whole membrane process. Product flow on each membrane stage is determined to calculate the required amounts of membrane modules on each stage; scaling rates and calcium carbonate deposition rates are evaluated; caustic consumption is determined. Experimental technique is proposed and described to determine scaling rates in membrane modules and the amount of hardness removed from concentrate after treatment by the “seed” and the minimal volume of concentrate that could be reached. Relationships are presented to determine operational costs (energy and chemicals consumption), scaling rates in membrane modules on the first and second stages and total hardness removal during concentrate recirculation through the “seed” reactor on the third stage.