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Reverse Osmosis System Questions

What doesTDS mean?

TDS stands for Total Dissolved Solids. This is how we measure the level of impurities in your water. Typically we use a hand held or inline TDS meter which will give you a figure in PPM (parts per million) or micro Siemens per cm² (µS/cm²). The meter works by measuring the conductivity of the water - the lower the figure the purer the water.

Purity can also be measured in resistance in which case you'll get a figure in mega ohms - in this case the higher the reading the purer the water. This type of measurement is most common in laboratories and is hardly ever used in the window cleaning industry.

What TDS reading should I expect after the RO membrane?

This very much depends on the quality of the water you are putting into the membrane and the pressure you are applying. Water chemistry and temperature also play their part. As a general rule of thumb you can expect 90% rejection if your inlet pressure is 40 psi. If your pressure is better you'll get a lower TDS if it is poorer you'll get a higher TDS.

For example if your inlet water TDS is 300 ppm and your water pressure is 40 psi you can expect a tds around 30 ppm. We should point out that in practice you can often get better than this even at lower pressures however this gives you a realistic idea of what you can expect.

If you're using a pump at say 80 psi pressure you can usually expect anything from 95 to 98% rejection depending on the flow rate of the pump you are using and the membrane.

What is the difference between rejection and recovery?

Rejection is the amount of impurities rejected from the water and is given as a rejection percentage. If your water has an inlet TDS of 200 ppm and the membrane reduces this to 10 ppm then your rejection rate is 95%. The membrane has rejected 95% of the impurities in your water.

Recovery is the term we give to the amount of water you are able to make as pure water compared to how much goes to drain. With a single membrane you might end up with 25% of your water going to product (also called permeate or pure water) and the other 75% going to waste - in this case we would say you have 25% recovery.

There is no or little flow from my membrane

Membrane flow rate is dependent on the pressure and flow to the membrane. If your pressure and/or flow reduce then this has a knock on effect for your membrane.

The first place you should check is your pre-filters. As your filters catch and trap debris it clogs up and reduces pressure to your membrane so you should first try changing your filters. If you are unable to change your filters or want to make sure your filters are the issue before installing new ones then simply remove your filters and run the RO system with empty filter housings. If you find that the membrane instantly improves then the problem is your filters (note: you should not run your system with no filters for prolonged periods as you will damage your membranes).

If removing the filters made no difference then you should look at your system for possible causes of pressure drop. Check there are no kinks in your hose, that your tap fitting is not faulty, that there is nothing stopping the water going to drain - basically any physical impediment to flow. If you are using a water softener you should try bypassing the softener as there may be a problem here (eg clogged bed if not regenerated often enough).

Make sure there has been no major drop to your mains pressure - there could be a water mains burst in your area for example that could reduce pressure.

Once you have eliminated all other factors then we have to look to the membrane itself. The membrane will stop producing water when its surface area is blocked. This can be with scale (calcium carbonate), biological fouling, or if using a private supply it could even clog with iron deposits or peat. In any case the membrane will need to be replaced. You should check especially for signs of biological fouling (you may physically see growths but also a slimy or soapy feel to the membrane can be an indicator) as you will need to disinfect your kit before installing a new membrane to ensure the new membrane doesn't become contaminated.

If scale is the problem you may be able to see some crystalline deposits on the end of the membrane when you remove it but not always as the scale build up may be inside the membrane. If you allow it to dry out then crystals may develop but a better way of checking is to weigh the dry membrane. A new dry HF4-4040 for example will weigh 4 Kg when new (dry weight) but if it becomes clogged with lime scale then it will weigh more.

Knowing what caused your membrane to fail helps you to avoid the same thing happening again.

Membrane flow has reduced

Again filters should be your first stop but it could also be a drop in temperature or a drop in your mains feed pressure. Make all the same checks as discussed in "There is no flow...", if nothing improves monitor the situation - it may just be a temporarily glitch in your supply.

You could also try adjusting your throttle (waste water valve) closing it a little will increase pressure and may increase flow, however if you do this ensure that it doesn't cause the TDS reading to go up - if it does then readjust the throttle until the TDS comes back down.

Membrane TDS has gone up

This again is linked to pressure so go through all the check for low flow first so make sure you're getting the best pressure you can.

Next thing to try is adjusting your throttle (waste water valve) - you may be over or under recovering. What this means is that you may have the valve too closed and that is driving up your pressure but also forcing the membrane to push through more dissolved solids - opening the valve reduces pressure a little but will bring the TDS down as the membrane is able to work properly.

Sometimes you may have the throttle open a little too much in which case you're not getting enough back pressure that again causes the tds to go up so by closing the valve the pressure increases and the TDS drops. It's a matter of trial and error to find the "sweet spot" where you get the best possible TDS. The best method is to start with the valve fully open then try different increments to find the best position - usually takes only a few minutes.

If pressure is fine and the recovery (waste valve) is set right (usually about 1:3 or 1:4 product: waste ratio) then we need to look at the membrane.

If the membrane is unacceptably high then it must be replaced as there will be physical damage to the membrane for the TDS to rise. Causes can be frost damage, biological fouling but most commonly from over recovering - this is where you have been running the membrane with too little water to drain which causes the pores of the membrane to stretch and allow more impurities through.

Why is the TDS very high then goes down after a minute or so?

This is due to a phenomenon called salt diffusion.

When you r membrane is doing nothing over a period of hours the salt from the waste (or dirty) side of the membrane moves across the membrane to the pure side until both sides of the membrane are of equal concentration. The effect of this is that when you first start running your membrane this high TDS water comes through the product. Within a minute or two the TDS goes back to normal.

If you flush your membrane (fully open the waste valve) for about a minute most of the high TDS water is flushed away so the TDS comes down faster to normal levels.

My flow rate is slowing down but my TDS improving

This is a classic sign of scaling. TDS remains the same or actually improves as the surface area decreases due to deposits of calcium carbonate. You can stop things getting worse by using a water softener however you cannot reverse the effects of scale on the membrane, or at least not economically. On large scale commercial systems citric acid or other descaler might be used but this is difficult to attempt on small scale RO systems.

Should I use a water softener?

Yes if you have hard water there are many advantages to using an automatic water softener.

Removing calcium carbonate from the water stops your membrane from scaling up. As it removes the calcium carbonate it replaces it with sodium which the membrane can more readily reject so this improves the product water tds. HOWEVER a water softener also reduces your pressure by at least 1/2 bar so if your pressure is on the low side the pressure drop can negate a lot of the advantages of using a water softener. Also if you do not regenerate the water softener often enough firstly it stops having any effect on your water chemistry, it also causes a greater pressure drop as the softener clogs with debris. If left long enough without a regeneration bacteria can start to grow in your resin bed which will eventually contaminate your membrane.

Manual water softeners should be avoided as they are rarely regenerated often enough and the method of regeneration is not fully effective in removing all the deposits from the resin making it less and effective at removing hardness each time it is regenerated. A manual softener obviously has the advantages of being cheaper and not requiring electricity but regenerating takes about an hour and has to be done every fill (depending on size and water hardness) which is quite a time commitment for most window cleaners and even taking care to use enough salt go through slow rinse, fast rinse etc. you will struggle to replicate the efficient regeneration cycles of an automatic water softener.

In conclusion use a water softener only if you have hard water, have good pressure and have an automatic water softener.

How often should I regnerate my water softener

It depends on the hardness of your water and how many litres of resin are in your water softener. If you know these details it is a relatively straight forward equation:

(litres of resin x 50)/ppm calcoum carbonate.

For example if your water softener holds 10 litres of resin and your hardness is 300 ppm calcium carbonate...

(10 x 50)/300 = 1.67m3 or 1670 litres between regenerations.

If you have a 500 litre tank and put 2/3rds of your water to drain you will use 1500 litres of water every fill of your tank so to ensure your water stays soft you'll need to regenerate after every fill (or before every fill if you prefer).

How much salt is needed per regeneration?

If you have an austomatic softener the softener will take care of thsi for you, if you are manually regenerating you should be using 120 grams of salt per litre of resin so if you have a 10 litre softener you will need 1.2 kg of salt per regeneration.

How often should I be changing my filters?

This is very much a how long is a piece of string type question. It depends!

It depends on how much water you pass through your filters, how much sediment is in your water, the concentration of chlorine in your supply to name but a few factors. Your inlet pressure plays big part also - if your inlet pressure is high your filters can become more clogged before dropping the pressure enough that you have to change them. If your pressure is very low a small drop in pressure may stop your membranes from working so only partially clogged filters will need changing.

In terms of chlorine removal filters are given a figure by the manufacturers however this figure is somewhat misleading. For example an EP-10 filter is rated at only 22,700 filters, but this is to reduce 99% of chlorine at a chlorine ppm of 2 which is about 4 times higher than we typically see so immediately you can multiply this by 4. Then when we reach this capacity the filter doesn't stop removing chlorine it just becomes less effective at removing chlorine, even at 90% removal the filter is doing a good enough job to protect your membrane so you can see how it becomes difficult to put a figure on how long one will last. Usually it will clog up with sediment before it reaches its capacity for removing chlorine.

Our best cover all advice is to change your filters every 8 weeks or sooner if the pressure drop becomes too great - if you have 20" filters then every 4 months unless they clog sooner. The more often you change them the better protection you are affording your membrane so changing them every 4 weeks is even better.

My TDS starts off okay but part way through a filll starts to increase

If you get a good TDS part of the time it is unlikely a problem with the membrane itself - when they fail they fail completely rather than intermittently failing. The most obvious answer to this one is that your mains pressure drops during production, for example at tea time the water usage by your neighbours goes up which brings the pressure down or perhaps the washing machine is in use.

We have had an example of this happening where the customer has monitored the pressure throughout and the pressure remained quite stable but about half way into his fill the tds inexplicably went up every night. After extensive questioning we came to the conclusion that it was to do with his softener.
He was filling a 1000 litre tank. To make 1000 litres he was putting a total of 3000 litres through his softener, however as his softener was quite small after 1600 litres had passed through, the softener no longer had an effect on his water as it had reached capacity. It wasn't changing the pressure however he now had a high amount of calcium carbonate in his water rather than sodium. As RO membranes find it harder to reject calcium carbonate when the amount of hardness was increased his TDS also went up.

The problem was initially solved by doing a second regeneration half way through his fill and later solved by doubling the size of his softener resin vessel.

In conclusion if the membrane TDS goes up during a fill you need to look at the path of water to the membrane rather than the membrane itself to find your solution.

How long should my resin last?

We have a simple equation which will give you a "ball park" figure for how long your resin will last:

(Litres of resin x *25)/ppm of water
*25 is the exchange coefficient of the resin so remains constant
EG, If you have 25 litres of resin with a TDS of 20 ppm...

(25 x 25)/20 = 31.25 tons of water or 31,250 litres

This is only a guide as many things can vary this figure, for example for the first minute or two your membrane will kick out very high TDS which will eat away at your resin faster until it comes back down to normal. Also during production your pressure can fluctuate and in turn your outlet TDS fluctuate a little and finally it depends on your water chemistry - different minerals have a different molecule size which effects the capacity of the resin, HOWEVER the equation is usually around the right figure and is a good indicator of the life you can expect.

Note if your are using nuclear grade resin this lasts about 20% less then general grade resin.

Is there a correct way round to install a membrane?

YES! Take a look at our section "How to install a membrane"

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