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Pure Water is a term used in the window cleaning industry. The process is when ordinary tap water is filtered through a purification system, to remove all the natural minerals. In addition to Hydrogen & Oxygen (H20), tap water also contains minerals. For drinking purposes this isn’t a problem, however when it comes to window cleaning the likes of calcium, magnesium and other minerals remain, when the water evaporates away. This can leave hard water stains and unsightly spotting. You’ll notice these build up on any glass surrounding showers or taps at home, or even after washing your car.

Pure Water

Pure Water is is often described as ‘deionised water’ due to the type of filtration used. After deionising, pure water dries to leave a completely spot free finish, as opposed to general tap water that dries to leave minerals on the windows, which is the reason why hard water stains and unsightly white spotting occur.

When applied to Window Cleaning, using pure water through a water fed pole and brush means that there is no need to squeegee the water from the window. Wet the glass, scrub it, rinse it and leave it! That's all you need to do, you walk away and leave the glass wet and over a short time it dries to a spot free finish

Testing Water Purity:

You can test your tap water with a TDS Meter which measures electrical conductivity and produces a value indicating the level of dissolved solids (minerals) in the water. For window cleaning purposes, a value of 000ppm (parts per million) of dissolved solids is considered pure and will produce a streak and spot free finish on the glass. It’s reasonable to effectively clean windows with readings of 0-7ppm, though this is just a rule of thumb, you may be able to clean with readings higher than this depending on what specifically is in your water. Trial and error will help you determine where the line is drawn.

R.O. (Reverse Osmosis):

Reverse Osmosis (RO) is a process whereby water is forced at high pressure through a semi-permeable membrane. It is the reverse of a natural process found within the human body – Osmosis. This process naturally moves water from a low ion concentration to a higher ion concentration across a semi permeable membrane, within the body, this process is used to take water into individual cells.

Reverse Osmosis is effectively the opposite; it applies high pressure to water with a high mineral content, pushing this through a semi-permeable membrane. Much like a coffee filter, the membrane catches and removes most of the mineral content, meaning the water on the other side is considerably ‘cleaner’.

The reverse osmosis process usually removes 85-95% of the total dissolved solids within the water. However, what many don’t realize is that to make the water completely pure and suitable for Window Cleaning you still require the Deionising vessel (DI), this removes the final 5-15% to completely purify the water. When you hear people use the term ‘RO system’ within the Window Cleaning industry, assume that they actually mean RO/DI System, as both processes are necessary to bring water right down to a pure 0ppm.

A full RO/DI System comprises of four stages. The first two stages of the process are pre-filters, these are carbon and sediment filters. These do very little in terms of purification; the main function of these pre-filters is to protect the membrane by removing very specific contaminants. Membranes can be a little delicate and so the role of the sediment filter is to trap larger particles that may cause damage. The carbon filter then removes further chemicals that may contaminate the membrane, such as chlorine – maximising the life of the membrane.

The RO membrane itself is the third stage, the membrane is responsible for the majority of the purification and is the primary component in the system, it is also the most expensive to replace. It’s usually simple to pick out the membrane as the housing is likely to be considerably bigger than the other components.

The Final stage is the DI Vessel. This removes any of the remaining minerals from the water (usually 5-15% approximately) leaving the product water completely pure and ready to clean with!

D.I. (Di-Ionising):

A DI Vessel is necessary to fully purify water as the final stage of an RO/DI system, but it is also possible to use DI filtration as a standalone filtration unit. Deionisation (DI) filters go by many names. You may have heard any of the following; they all effectively refer to the same thing.

Deionising Vessel, DI Vessel, Resin Vessel, DI Filter, Resin Filter, Polishing filter, Ion Exchange filter, Resin Beads. DI vessels contain a deionising resin and function by exchanging positive hydrogen and negative hydroxyl molecules for the positive and negative contaminant molecules in water. Positive chemicals such as calcium, exchange places with the hydrogen molecules and the same happens for negative chemicals such as iodine.

Eventually, all of the positive and negative molecules are ‘spent’ and the DI resin must be changed. Compared with an RO/ DI system, filtering DI only is very simple and requires almost no know-how. DI filters also allow you to filter on demand meaning you can work and purify simultaneously.

In soft water areas, this is the preferred filtration option due to the simplicity and the fact that usually this style of filtration system is cheaper to purchase. In soft water areas the demand on the filtration unit is low and therefore the running costs are low also. I’ll go into the benefits of each a little further in the next section.

RO or DI?:

The first thing to know before considering each is the purity of your input water (i.e the purity of your tap water), and roughly how many litres per day you will need. A simple TDS Meter can be used to determine, in parts per million, if the water in your area is hard, soft or somewhere in-between.

The reason you will need to know this is that the hardness of your water is going to be a major determining factor of your running costs. Let’s take the DI option first, as mentioned above – resin is ‘spent’ as it purifies water and the more purification it needs to do, the faster the resin is spent and needs replacing. Using resin to purify water from 350ppm right down to 0 in a DI only system will be spent far more quickly than as the final stage to an RO/DI system where it may need to purify from 35 down to 0. For this reason, it is usually advised that DI only filtration is only suitable if you have soft water, or are using low volumes of water each day.

On the other hand, the RO/DI system does have its own consumable components; and all four stages will need replacing at different stages. However the comparative costs are far lower than a single stage DI only system. The pre-filters need changing regularly, every month if the water is 300ppm upwards and every quarter if the water is 100ppm. These filters can both be changed for a few pounds each. The membrane is by far the most expensive component but this will last anything from 12 months to a few years, depending on the water quality and usage.

Generally speaking, the running costs of an RO/DI system are lower, however the cost to buy an RO/DI system is much higher, be it a static system, a van mounted system or anything in between. In a hard water area, the additional cost upfront to purchase the goods will quickly be recouped in lower running costs, however in a soft water area where running costs are low anyway, it may be much longer before you see the return, which is where the DI only option comes into play.

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