Modelling of a low-pressure reverse osmosis system with concentrate recirculation to obtain high recovery levels

A big problem for the desalination plants situated far from the sea is the elimination of the concentrated brines. The volume reduction of the final concentrate could allow an economic transport and a high recovery level. In order to solve this problem, a reverse osmosis system with recirculation (RRO) is proposed. In this system, a part of the final concentrate is recirculated in order to obtain high concentration ratios at low applied pressure. The recirculated stream flows in the permeate side in counter-current reducing the osmotic pressure difference, which is the driving force for solute transport. In this way, a low pressure is needed to establish the water flux. Formally, the RRO system is similar to a continuous distillation or a liquid-liquid extraction, and the flow recirculation ratio, defined as the relation between the flows of product and recirculated stream, is the main operative parameter. In this work a model of the system is used to evaluate the efficiency and to determine the situations where it can be preferable compared to other configurations or unit operations.

The proposed model can be useful to study the efficiency of an specific process where the high osmotic pressure is a critical factor for separation. For a more real approach, optimal reflux ratio, energy consumption, adequate velocities to avoid scaling, and the reverse osmosis attached system must be considered all together. In a real system, methods to obtain high homogenisation in the permeate side should be employed to come closer to the ideal conditions studied [3]. For a particular case of reconcentration of a brine stream of 35 kg•m!3, a recovery of 60% was obtained including a system to concentrate the permeate up to the feed concentration. Theoretically it is possible to obtain a high recovery with a system like that for saline waters allowing the transport of the concentrated brines to the sea, but the economy of the process will depend on energy and equipment expenses.