Window cleaning equipment   01757 702487

 

 

Advice page. 

What is all this about reverse osmosis?

Why do I need a pump flow controller?

 
Back to RO systems  Reverse Osmosis Information for window cleaning

First of all, what is reverse osmosis?  I am not a scientist or whatever so I will explain it as I understand it.

Most systems are assembled as follows

2 or 3 pre filters then the membrane and then possibly a de ionisation filter. The membrane is the heart of the system so I shall start there.

A membrane will balance anything from one side to the other such as when your lungs pass oxygen into your bloodstream because the O2 is higher in your lungs than your bloodstream. The O2 will balance out.

Similarly the membrane in the RO system will pass water molecules from the inlet side to the pure water side.. You are aware that if climbing somewhere such as Mount Everest climbers suffer O2 starvation because the pressure gets less so therefore the pressure of O2 in your bloodstream gets less. We need pressure to cause the good stuff to pass through the membrane. Most membranes need more than the pressure in your mains water to work at their optimum so need a pump to boost pressure. Most membranes need high pressure though there are low pressure membranes on the market but these generally cost more.

Booster pumps. If you don't have very high water pressure you will need a booster pump otherwise your membrane is working the same as your lungs would at the top of Everest. Your system is using water without producing much pure. Most RO membranes need around 50psi upwards to work at their optimum. The performance of the booster pump needs to be compatible with your membrane or membranes although any pump is generally better than none.

Membrane capacity. A 100gpd membrane has a capacity to produce 100 US gallons per day. NOTE. US gallons, not UK gallons although you will need pressure to get anywhere near this. This is the makers rating but will depend on condition and pressure

Warning. If you have a booster pump fitted the transformer will run warm if not switched off regularly. That is why a lot of ebay listings for booster pumps have small print at the bottom stating they are not suitable for window cleaning or continuous running. They are OK for window cleaning if you turn them off periodically to allow the transformer to cool down. I have mine running with a timer,  90mins on and 30 mins off, so, straight away your 100gpd is down to around 75gpd due to being switched of for 25% of the time. 75 USGPD  is 284 litres per day. 

Hopefully you now understand how the membrane part of the system works, so what about the rest of it.

1st pre filter The first stage of nearly all systems is a polypropylene fibre sediment filter, usually of 5 microns, though others specifications are available. This filter designation is usually written down as pp5  meaning polypropylene 5 microns. The sediments taken out are particles of rust. sand, silt etc.

2nd pre filter- Granulated activated carbon filter (GAC) Removes Free Chlorine, Fluoride, Odour, Organic Contaminants, Pesticides and Chemicals.

3rd pre filter -A Carbon Block Filter. Removes Trihalomethanes, Cysts. Pesticides, Volatile Organic Chemicals and 99.95% of Giardia and Cryptosporidium 

4th stage 1 or more membranes.

Finally for window cleaning you usually need a DI (deionisation filter), often called a polisher. Once the pure water leaves the membrane the TDS (total dissolved solids) reading will be very low but not always low enough for window cleaning. Depending on the quality of your mains water it could be anywhere from around zero up to around 25. The DI filter will bring it down to zero.

The DI filter takes a DI resin which has a life and will only take out a set amount of impurities. If you are fortunate enough to live in a very soft water area you can often get by with just a DI filter and not need a RO system at all, but if you live in a hard water area the TDS needs to be brought as low as possible before going through the DI. Of all the replaceable filter mediums the DI resin is the most expensive hence the RO system to bring the TDS to a low level before the DI.

How big a DI filter do you need?

How good at maths are you?

There are quite a few variables such as pressure, condition of membranes, how hard your water is etc. so these can only be approximate figures, though you should be within 10%.

Most reverse osmosis systems reduce the tds to around 5% of mains so if your main water has a reading of 300 you could expect 15 from the ro. If your mains is 100 you could expect around 5. Then comes the di

A litre of resin will take out a tds of approx 20000 so if you have a ro output of 10 then divide 20000 by 10 = 2000 so 2 thousand litres of water per litre of resin.

My system has a mains input of 300 and an ro output of 15. I have a di with .75 of a litre. Divide 20000 by 15 gives 1333 litres of water so my di should give .75 of that which is 1000. I actually get just over 1000 before the tds starts to rise so I know my system is up to scratch.

So to summarise

Divide your mains tds by 20     

Divide 20000 by the answer and that is how many litres of pure you can expect per litre of resin.

For those who like a formula - Litres of pure water  per litre of resin is 20000 divided by (mains tds/20)

My system is 300 mains, divide by 20 = 15.

20000 divided by 15 = 1333 litres of pure water per litre of resin

I hope my working out is correct. If not or if you disagree with my figures please let me know.

I am now selling bespoke systems with the starting point being as follows:-

1st stage 10" pp5

second stage 10" carbon filter

3rd stage  100gpd membrane

4th stage 10" DI with refillable resin cartridge.

This then can be added to from a large list of options and I will build the system to suit you.

So what options do you need.

Unless you have a very high mains pressure you will a booster pump. The pump fitted will depend on the size and amount of membranes so the extras should be ordered at the same time, as the restrictors also need to be balanced with the pump and membranes.

If you are on a meter then I would suggest a second membrane being fed from the waste of the first membrane. This can almost double your pure without increasing your water consumption though the tds reading of the second membrane output may be slightly higher.

Membrane options available are 

1 x 100

1 x 200

1 x 300 Though I find in practice due to restrictions in piping etc it is nearer 260

2 x 100  in series or parallel

3 x 100 in series or parallel

2 x 200 . 

2 x 300. Not done any testing on this yet but I would expect around 550 depending on restrictor etc.

If you have a water meter I would suggest the membranes need piping up in series but without a meter then either will do, depending on how eco friendly you are.

If you fancy a bit of fine tuning yourself I would suggest an adjustable restrictor. 

Restrictors. Why they are there and how the size can affect your systems output.

There needs to be some restriction in the waste outlet to maintain pressure in the system. Without this all your filtered water would go straight out of the waste and not be forced through the membrane. 

Flush. A manual flush is usually a loop that bypasses the restrictor with a valve  closing off the bypass. To flush the system the valve is opened allowing all the pressure to flush straight through the bypass thus cleaning the membrane.

If you increase the pressure in the system then more water will pass through the membrane thus improving the pure to waste ratio, but, only up to a point. Eventually if the pressure is raised too much then dissolved solids are forced through and the TDS will rise, plus obviously the pressure cannot be raised too much or seals will start to leak, but if fitting extra membranes it gives the owner the chance to do a bit of fine tuning to optimise the systems output. Certainly worth doing if you are on a meter.  

 

So, which system do you need

First choice is how much water do you need. 100gpd membranes are the most common and if you start with a 100gpd system this can easily have extra membranes added later to make a 200 or 300 system.

If you need more than 300gpd you need to consider a system with larger membranes

Second consideration in choosing your system depends on your mains TDS reading. If you live in a soft water area with a mains tds of around 100 then a typical ro output would be around 5 which you may be happy with. Many window cleaners aim at anything below 10.

If your mains tds is higher than 200 then you really need to consider a system with a di or as some window cleaners do, use a none DI RO system and run the water through a DI filter as you use it. In a soft water area many window cleaners would use DI alone and not bother with a RO at all. But due to the high cost of resin this is only worthwhile if you are fortunate to live in a very soft water area.

 

Why use a pump controller?

You can run a pump on the pressure switch and operate without a controller.

This may work for you but it does not work for everybody for a number of reasons:

1. In running the pump until it hits the pressure switch you are running the pump as hard as it can possibly work. This means that the pressure in the system will be at its absolute maximum. This puts strain on the hoses, on the fittings and on the pump. It therefore increases the chances of split hoses, leaking joints and increases wear on your pump, shortening its life.

2. As the pressure switch cuts out the pump is at maximum, working as hard as it possibly can and therefore drawing maximum current from the battery. Hence as the switch opens it is under great strain. The high current and the fact that the pump is an inductive load (with stored energy) means that the pressure switch contacts are likely to arc and eventually they will fail.

3. Without a controller the pump will not stop until it hits the pressure switch. With a controller with dead end detection properly calibrated, the pump will be stopped a long time before this high pressure and high pump load is reached. This means that because the pump is turned off earlier you are saving battery energy, thus increasing both the battery life and the amount of time you can work on a battery charge.

4. Without any type of flow constrictor the water will be at full flow. Good for putting out fires, but not so good for cleaning windows. The aim should be to work with only sufficient water to get a good clean. This is a benefit because the aim is to conserve the water in order to increase the amount of work possible from a tank. You also save time because you don't have to make as much pure water and you save resin as well. Working with large amounts of water can also cause a hazard for those around you, especially in cold conditions.

 

A controller will help you work more efficiently - cover more work in the same time - create the opportunity to do extra work each day - reduce how often you are changing RO and DI filters so saving you money.

You will also extend the life of your batteries and pumps and protect the whole of your system from the risk of split or blown connections/hoses.

In short a controller will save you money in the long run and allow you to earn more in the short term

 

 

If you have anything to add to the above that will help other window cleaners, or need to ask a question just drop me a line. to sales@extend2wash.co.uk  website http://www.johnhewitt.com or http://www.extend2wash.com 

 

How to select a RODI  http://www.youtube.com/watch?v=1QKJbO4UBJw&feature=related

http://www.youtube.com/watch?v=WtCqynlmHRc&feature=related

 

 

Back to controllers
Warranty and distance selling regulations
 
 
 
 
 
 
 
 
 
 
 
 

RO systems

100gpd to 400gpd
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
tel. Selby 01757 70248