More from less.
By investing in good
Investing in good RO
process and system designs.
Reverse Osmosis is a maturing and proven technology to remove dissolved solids and most contaminants for many industrial applications and for seawater desalination to potable water. But the optimisation of a RO system and its effective operation is not without challenges – from technical aspects to economical considerations. While we may be spoilt with choices of advanced RO membranes, energy-saving devices and cleverly designed parts, components and instrument, we shall start by investing in good RO process and system designs, first.
Process designs and configurations.
Different arrangements. Different Output.
There are several applied and proven ways to configure the arrangement of RO vessels, subject to considerations for recovery ratios, permeate’s qualities, space constraints, product water storage and facilities, cost factors etc. The common staging designs and configurations of RO vessels include 1- and 2-stage, single- and two-pass (or multi-pass), permeate blending and permeate recirculation processes.
RO membrane elements and pressure vessels.
One lives within another. Compatibility essential.
Right selection of RO membrane is essential. Standard industrial RO membranes are commonly available in Ø4”, Ø6” or Ø8” of varying lengths and performance features. Selecting RO vessels from different manufacturers that can house the chosen membrane compatibly requires intimate knowledge of the products. The total number of membrane elements determined would affect the decisions on the number of pressure vessels and the ideal process and configuration in a RO system. RO membrane elements are arranged in series from one end to other end of a vessel. As the filtration efficiency decreases from the first element at the feed-end to the last element within the same vessel, it is essential to determine the ideal number of elements to be housed in each vessel. The combinations could vary from one element in one vessel to 6 or 8 elements in a vessel.
1-stage vs. 2-stage processes.
Recover less. Recover more.
A RO system can be configured with a 1-stage or 2-stage separation process (or multi-stage beyond 2-stage). These process configurations are frequently used when the demand for permeate’s qualities is not the greatest concern. In a 1stage process, the feed water passes through the RO elements in the pressure vessel(s) only once to attain the permeate needed, and the concentrate is discharged (or, as an option, is recirculated to the feed water line to recover more permeate).
Whereas in a 2-stage process, to increase the recovery through a RO system, the concentrate from the 1ststage process becomes the feed water to another stage of RO elements to produce more permeate, combined with the permeate from the 1st stage arrangement. Subject to feed water’s qualities, while 1-stage RO process’s recovery rate may be up to 50%, a 2-stage process may increase the recovery rate to 75-80%.
In a 2-stage (or other multi-stage) process, where technically and economically viable, the pressure vessels containing RO elements can be staged in a 2:1 array configuration to increase the recovery rate further. In a 2:1 array arrangement, which is typically used, the combined concentrate of the first two RO pressure vessels becomes the feed water to the next one vessel, before the final concentrate from this vessel is discharged from the RO system. This helps increase the recovery to typically exceed 90%.
Single-pass vs. double-pass processes.
Where higher permeate’s qualities matter.
A single-pass RO works just like a 1-stage process where the feed is filtered only once through the RO elements within a single pressure vessel. So the permeate’s qualities are generally the same as that of a 1-stage RO process.
But where higher permeate’s qualities are desired, a double-pass RO process is usually recommended. In this case, the permeate from the first pass process becomes the feed water to the second pass, producing much higher permeate’s qualities eventually because the feed water essentially goes through “two RO systems”, though both passes are configured in one RO system. Where feasible and multiple RO vessels are involved, a double-pass RO system may be configured in 2:1 array ratio.
Permeate blending process.
Remineralising the permeate.
This process is used when a certain salinity of permeate is required, especially for drinking water. Generally, post-RO water’s mineral composition is significantly changed and partially reconstituted to form finished water that may cause corrosion in water distribution piping systems (e.g. water with low alkalinity is corrosive to iron pipes). Water that contains little or no hardness would also be considered unhealthy for potable use. One of the remineralisation techniques is by blending a small quantity of the source water into the permeate stream, before storage and distribution, though this process may be limited by the excessive presence of undesired minerals/contaminants.
Permeate recirculation process.
Stabilising feed pressure and permeate’s qualities.
In a circumstance where the feed water’s temperature may vary greatly (e.g. changes of temperature due to climatic seasons), the feed pressure into a RO system may have to vary to keep the permeate flow constant. But the change in feed pressure may cause instability in permeate’s quality. To prevent this, part of the permeate can be recirculated to the high-pressure pump upfront to keep the feed pressure constant, when then permeate flow is higher than the expected output value. By keeping the feed pressure constant, the permeate quality can be ket relatively constant.