How Does It Work?
Traditional Reverse Osmosis
Reverse osmosis membranes require high crossflow, a continuous flow parallel to the external surface of the membranes, which prevents the accumulation of rejected salts by sweeping them away. Since only a small portion of the water flow is purified and most of it is sweeping the membranes, the water efficiency, or recovery, of each individual membrane is very low.
Traditional reverse osmosis is designed in a sequential multi-membrane, multi-stage configuration, in which each membrane in the series is making a small contribution towards the overall water recovery.
Balancing the crossflow and flux (of permeate) across multiple membranes and stages, is not trivial, often becoming impossible as the number of stages increase or as water conditions vary. The challenge is to drive sufficient crossflow to the tail membranes without fouling the head membranes by exceeding their flow and flux limitation.
Every stage, which is comprised of multiple sequential membranes, can only achieve 50% recovery. It takes 2 stages to achieve 75% recovery, but despite of the considerable 25% waste, this is the standard configuration of most industrial and municipal reverse osmosis systems. Adding stages can increase recovery, but it increases system and operational complexity, greatly reduces flexibility and increases risk of failure.
A stream of pressurized concentrate is constantly discharged from the tail membranes, wasting water and energy, and exposing those membranes to permanent high concentration conditions that causes scaling when saturation limits are approached.
Simple or dead-end filtration is the most common and efficient filtration technique. Water is driven through the filter leaving only the dirt behind, with equal feed and permeate flow rates during normal operation mode, and with periodic filter flushes or replacements. Exceedingly high recoveries can be achieved as water is only wasted during the flushes.
This efficient and flexible technique could not be applied to reverse osmosis due to the lack of crossflow which is essential for the operation of reverse osmosis membranes.
Closed Circuit Reverse Osmosis
Desalitech has invented an elegant way of operating crossflow reverse osmosis membranes in a highly efficient and flexible simple filtration device. Like any simple filtration device, our CCRO systems feature equal feed and permeate flow rates during normal operation mode. At a software based set point, the system automatically flushes out all the concentrate, and then returns to its normal operation mode. The flush is triggered by the CCRO operating software, based on any combination of flow, concentration, pressure and additional set points. During the concentrate flush step, the system continues to be fed and to generate permeate, while concentrate is pushed out of the system in one sweep.
Crossflow for standard reverse osmosis membranes is attained with a single stage of parallel membrane housings and a low pressure circulation pump that generates optimal cross flow conditions. Concentrate is recirculated to the membrane feed and recovery increases with each concentration cycle. Recovery is achieved in time with recirculation and not in space with multiple membrane stages in series.
This innovative reverse osmosis technology features a step change in reliability, flexibility and efficiency over traditional reverse osmosis.
The diagram to the right illustrates the CCRO process.
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