Types of Filtration Explained - RO/DI or DI Only?

Types of Filtration Explained - RO/DI or DI Only?

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When looking for a pure water system, at some point most window cleaners will come across this question;


What is the difference between an RO and a DI Only System, and which do I need?


In the simplest terms, RO and DI purification systems exist in many industries as a means to reduce the mineral content of water. When applied to Window Cleaning, the water will need all of the minerals stripped away for it to be suitable, as some minerals have a tendency to leave ugly white spots on the windows when the water evaporates away. When water is purified to a point that it has a mineral content of 0ppm (parts per million), it is considered to be pure.

You may hear that you can clean with a water quality of up to 10ppm and some Window Cleaners boast cleaning right up to 20 or 30ppm with perfect results and this may be the case. The truth is that not all minerals dry to leave ugly marks on the windows, the issue is that this can change from one area to the next and it’s not really feasible to selectively remove the minerals that dry to leave white marks, meaning it is a lot more simple (and cost effective) to simply remove them all and guarantee perfect results. 

Let’s break down the two schools of thought regarding water purification systems within Window Cleaning to understand them a little further and help clarify which may best suit your circumstances.


What is an ‘RO System’?

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! I’ll go into a little more detail on how the DI Filter works in the next section.


What is a ‘DI Only’ System?

As noted above, 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.


What are the Benefits of each?

So now that we’ve covered what each system is and how it works, there are a few things to consider before deciding which best suits your circumstances.

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 (Found HERE) can be used to determine, in parts per million, if the water in your area is hard, soft or somewhere in-between. In the UK there are huge differences between the soft water of Scotland/ Wales (anywhere from 30-100ppm) to the hard water of Bristol or London (300-450ppm). The map below gives you an idea of what sort of water to expect.

U.K. Water Hardness Estimates, based on Location

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 in.



So now you have an idea of the type of filtration methods, you can make an educated decision on what may suit your business. The next question is which type of filtration system to go for – Static, Portable or Van Mounted Systems.


—> Static, Van Mounted or Portable Window Cleaning Systems

You May Also Be Interested In: 

–> Should I Switch from Traditional To Pure Water Cleaning

–> Filtration Components – Explained!

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