Feed water for DM plant

A. DM normally means demineralizing water. It means vastly different things to different people, from water softening, to distillation, to ultra filtration to reverse osmosis to deionization. So what do you mean?

A. The feedwater quality can have a dramatic effect on the performance of a DM (Demineralized Water) system. Typically you designed for worse case and adjust the gallons (m3) throughout to insure that the capacity of the ion exchange resin is not consumed. The regeneration of a DM system, assuming is is ion exchange, is not 100%. Therefore changes in influent water quality can have an effect. The ion exchange system could be either cation/anion separate beds or mixed beds.
As for softened water as influent? Yes, that would reduce calcium and magnesium loading on the resin. But ion exchange is usually anion capacity limited, therefore the softener would not help that end of the problem.
DM water is typically used for some steps of the regeneration. Knowing what you want to accomplish with recycling the water would help to answer your question. This response assumes using ion exchange resin. As stated by other responders, DM water can be a combination of RO or other technology. It would depend on the conductivity (or resistivity)and specific concentrations in the effluent to be produced by the system

A. Hi Bharat. It's not necessary but it may be highly advisable :-)

RO removes salts and contaminants via 'sort of' a filtering process through a membrane. Applying pressure to the contaminated water pushes a portion of the water through the membrane and leaves a portion behind. The portion that passed through the membrane is relatively clean, and the portion that did not pass through is now left with all of the contaminants and is usually discarded. It tends to be a relatively cost-effective way of purifying water and is used in such applications as desalination.

DM can remove more contaminants than RO and generate purer water, which can be necessary, but it can be relatively expensive because you consume acid to remove all the cations and caustic to remove all the anions.

So the case can certainly exist where purer water is required than RO can generate, so you need DM, but the feedwater is contaminated enough that it makes economic sense to do the initial removal of contaminants with RO before the water subsequently passes to the DM system.

Regards,

A. If the TDS of the feed water is > 500 mg/L, then an RO before the DM system is cost effective and desirable in most cases. Also, if you are intending to make ultra-pure water, then an RO will make this easier by removing most of the non-ionic or weakly ionic solutes before demin.

A. Typically, an RO will reduce DM chemical consumption by up to 95%.