[sunshine windows]

Hi all,

Could somebody please explain exactly what the RO and DI systems are used for. I realise they are both used in the purifying process but some people on this site say they don't need to bother with an RO system and just use resin?

Anyone that has an indepth knowledge of how the purification of water is achieved. I would be grateful if you could explain it to me.

Sunshine

[Tim Morton]

Hi, I'm no expert on these things, but i'll share what i know.
DI = De-Ionization, the water passes through a vessel which contains a special chemical compound that purifies and De-Ionises the water.
Ro = Reverse Osmosis. The water passes through some filters and through a membrane that extracts the impurities from the water. The impurities are then carried away in waste water.
Some people use just the DI as their tap TDS is quite low, others tds is quite high and so they use a RO as well, they do this because the DI uses resin that can be quite expensive and you go through a lot if your tds is high. Therefore you would use a RO to reduce the tds down a lot.
Ro systems are more expesive to set up than DI, but their running costs are lower than just DI IF your tds is high.

Hope this helps, someone else who knows more will be along soon to explain better.

Tim

[carl clearview]

Sunshine,

Di is a term used to describe a process called deionisation. This basically means removing ions (dissolved mineral particles) from a water source. Di resin for large water treatment plants are usually in separate tanks, one is full of cat-ion resin and one full of an-ion resin. In window cleaning and other applications where huge quantities of water purification are not required, the resin is mixed. Hence the term mixed bed resin. For a number of years, this form of water purification was the norm in industry because the resin could be regenerated. However, this is a complex process that requires the use of harsh chemicals such as acids. The reason the resin needs to be regenerated is that as the ions in the water flow over the resin beads, the ions attach themselves to their opposites; much in the same way a magnet attracts its opposites. The resin becomes exhausted and then needs to be changed or regenerated.
Di systems have some advantages, not only are the very efficient when they purify water, they fill at the speed of your tap pressure and waste no water at all. The down side to Di purification is the cost. Unless you are living in a very soft water area and i mean an area where the TDS (Total Dissolved Solids) reading is low, around 30-50 max they are expensive to run.

R/O is a term used to describe a reverse osmosis filter. This form of filtration is far cheaper to run because it self flushes. Water flows across a semi-permeable membrane, the water is under pressure and the smaller water molecules pass through the membrane. The larger particles of dissolved solids (minerals) continue through the filter housing and are flushed to drain. These systems are now in very wide use within large industrial water purification plants and have replaced Di as the preferred form of filtration. The down side of these systems is that they do not recover 100% of the input water. Reverse osmosis is a very reasonable form of water purification, although it is important to buy the best R/O unit you can afford as this will reduce problems caused by insufficient flow rates and early filter changes.

Sorry for the essay!

Regards,
Carl.
 

[sunshine windows]

Great replies guys. That has definately explained things for me. 

I have another quick question though. 

Can the waste water that you are producing from the RO unit be saved instead of drained away. Then could this water be repassed through the system so that you're not wasting as much water.

Sunshine

[Tim Morton]

You can try recycling the waste through the RO again, but this would clog up your RO membranes real quick. The best option is to pipe the waste water around your garden to water it.
Some of the better RO systems produce less waste, something to look for in a RO setup.

Tim

[carl clearview]

Sunshine,
There is no set recovery for a reverse osmosis unit. Each filter can be set for a particular application (by the supplier for example). For instance, if you were filtering sea water, a far higher hydrostatic (pump pressure) is required to overcome the osmosis effect and thus reverse it. The more mineral content (high TDS) in the water, the lower the recovery rate. In a desalination (sea water purification) plant, the reverse osmosis filter would be set at around 20% recovery on the permeate (filtered water), with 80% of the input water as concentrate (waste water).
Because we are only filtering tap water with a maximum allowable drinking TDS of 500, we can increase the recovery rate of the reverse osmosis filter to 75% recovery with only 25% waste. Any greater concentrate percentage would be a waste. The reason that most of under the sink R/O units have a much higher rejection rate than this is because their designed application is filtering 200 or 300ltrs a day of drinking water and the high rejection rate enables the filter to last longer for this small amount of purification, remember for drinking water applications speed of fill is not important.
We aim to set our 4040 membranes at the recommended 25% rejection rate. If you wanted to recover the concentrate from the R/O unit, you would simply have to replace the 4040 membrane 25% quicker than you would normally. A 4040 membrane has a life expectancy (for 600ltrs a day in 300TDS feed water) of approximately 4-5 years. These filters cost around £300 to replace.
Recovering the concentrate is a simple matter of connecting the concentrate flow to the input flow of the reverse osmosis membrane housing. Of course you could pass this water through a separate resin vessel but running costs would be high.

Best Regards
Carl