subject: Improved Recovery Reduces Water And Effluent Costs [print this page] Reverse Osmosis (RO) is an excellent broad spectrum treatment process, removing virtually all the types of impurity present in water particulates, colloids, organics, hardness salts, alkalinity, other cations/anions, bacteria and viruses. The RO membrane also acts as a very fine filter removing 99% of suspended and colloidal solids, bacteria and organic molecules with molecular weights in excess of about 200Da (Daltons). This makes the process particularly attractive for applications where treated water not only has to be low in TDS but also of high clarity and free from bacteria, such as that used for soft drinks and pharmaceuticals manufacturing. Older technologies, utilising chemical or ion exchange processes, cannot offer this breadth of purity or product security.
During reverse osmosis, feedwater is pumped past the input side of a semi permeable membrane under pressure in cross-flow fashion and reverses the natural osmotic tendency of the solution, hence the term "reverse osmosis". This action produces two streams:
a) purified water, known as permeate
and
b) highly-concentrated dissolved solids, known as concentrate or reject.
Feedwater passes through the membrane as permeate and the rest exits the membrane as a concentrate that contains most of the salts, organics, and essentially all particulates. One of the perceived weaknesses of the RO process, however, is this wastage of water typically 25% of the feedwater is used to wash impurities to drain on a continuous basis and this represents a significant operating cost.
This problem has been addressed by utilising Concentrate Recovery RO Water systems to take the water Recovery from the primary unit and extract a proportion for recycling, thereby increasing overall recovery. This approach can also increase the capacity of existing installations, producing more purified water without increasing raw water demand.
By utilising smaller RO water systems to achieve this, the main large RO is at less risk from fouling or scaling and the chemical/hydraulic conditions can be separately adjusted to optimise performance. This may involve intermediate pretreatment stages to cope with the higher impurity loads.
Typically, the RO Water permeate from the Concentrate Recovery RO unit can be added to that from the main system, returned to the raw water inlet, or used for boiler feed applications.
A Concentrate Recovery RO water system enables users to cut water consumption by around 20% and effluent discharge volumes by up to 50%, lowering costs on both counts and significantly reducing their water footprint helping to meet sustainability targets. Payback on investment in such a plant can be achieved in less than two years.
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