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Pure Water Filtration Components Explained

Water filtration for Window Cleaning can be a bit of a minefield. If you're hear, then hopefully you've gone some way to understanding the basics of water purification - don't worry if not, we've got a helpful guide that covers a broad overview of water purification and how it's used in Window Cleaning, check out our Introduction to Pure Water Cleaning.

Within each purification system there are different stages of filtration, and understanding each will allow you to get the most out of your system.

The two primary types of filtration within the Pure Water Window Cleaning world are RO/DI Filtration & DI Only Filtration.

An RO/DI system is a four stage system, alternatively a DI only filtration system features just one single stage. Below is a breakdown of the components found in each:


Stages 1 and 2:  Pre-Filters

Pre-Filters are used in RO/DI Systems, but are not required for DI Only filtration


What are Pre-Filters?

Pre-filters are a combination of two filter types used immediately before a reverse osmosis membrane. They vary in size, type and quantity depending on who has made the system and what the system is manufactured for, though the reason for them is to filter specific things from the water.

They will not reduce the purity of the water a great deal, though you will see a slight reduction on a TDS meter. The primary reason for using pre-filters is to remove from the water anything that can quickly contaminate a membrane - the primary (and most expensive) component in an RO/DI System. 


Why Are They Important?

Pre filtration in a RO/DI system is important due to the delicate nature of reverse osmosis (RO) membranes. To understand what pre filtration is required for your system, you have to take into account the type of water being provided to the system (i.e tap water, rainwater, bore-hole water etc).

Different sources of water will each have different qualities when it comes to mineral content and this is where selecting the right pre-filters is important to give the right protection for the RO membrane.

In some circumstances it can be beneficial to use a water softener before the water hits the sediment Filter. Take a look at our article: Do I Need A Water Softener 


Types of Pre-Filter


    As previously mentioned, water softening filters are only necessary in certain circumstances. The job of the water softener is to remove calcium, magnesium and other minerals from the water prior to going into your system. The process will not only extend the life of your membranes, but also extend the life of any booster pumps you may have fitted to your system. Simply put, if you can reduce the high level of calcium, then you can also reduce the build up of lime scale.

    Water softeners use a softening resin to treat the water and can be regenerated using salt water.



      Sediment pre filters are solely there to protect the carbon filter and RO membrane from physical particles in the water. The filter is a tightly wound white cartridge which is inexpensive and simple to replace. As a rule of thumb, these should be disposed of after around 15,000-20,000 litres of use.

      If not changed frequently enough these filters can affect the flow of water to your membrane resulting in a loss of production and, if not used at all, can quickly have a dramatic effect on the life of your membrane. Generally speaking it is recommended to change these often, as they are inexpensive to buy and simple to change.

      If you don't track your water usage, it may simpler to make a habit of changing these every 4-6 weeks based on a full time windows cleaners average water usage. 



        These filters are used for removing chlorine, volatile organic compounds, taste and odour. The main benefit of a carbon filter in a Window Cleaning setup is to protect the membrane from chlorine. Chlorine damage to a membrane can happen fairly easily and is irreversible. RO membranes damaged by chlorine will experience poor quality production water and will need replacing more quickly. Simply put, if you neglect to change this regularly you’ll be getting a new membrane far earlier than you should.The good news is that much like the sediment filter – these are inexpensive and very simple to change.

        You may sometimes find GAC (Granular activated carbon) filters in RO systems; this is a type of carbon filter mainly used in domestic RO Systems. Generally speaking, the more common type of carbon filter is a ‘carbon block filter’

        We recommend changing this along with your sediment filter every 4-6 weeks, based on a full time windows cleaners average water usage. 


        Stage 3: Membranes

        Membranes are the primary component of an RO/DI System, but not necessary for DI only Systems


        The Reverse Osmosis (RO) membrane is responsible for most of the purification in any RO/DI System. It is also the largest and most expensive of the filtration stages. The RO membrane will perform 90-95% of the purification when operating optimally, though this will decrease as the membrane ages, or if the water going in has low pressure or a low flow rate.

        RO membranes are semi-permeable and when used under pressure, can remove many types of molecules from ordinary tap water. This includes bacteria. RO Systems have a number of industrial applications, and are even used for filtering drinking water. 

        Window Cleaning - Pure Water Filtration

        The RO membrane is the most expensive component in the purification process, but also lasts the longest. A membrane should last from 12-18 months if maintained correctly, though in some soft water areas some even get years at a time between membrane changes. As the membrane begins to age you’ll notice two things begin to happen, and knowing at which point to change the membrane is something not well understood.


        The two indicators that a membrane may need replacing are:
        • Reduced Production Water Volume
        • Reduced Production Water Quality

        As the membrane gets older, the production time will increase, the output water purity readings will increase and sometimes the ratio of pure to waste water will increase (40:60 or 50:50 rations are most commonly seen in Window Cleaning Systems). Regular maintenance (via flushing the membrane) can help to increase the membranes lifespan, and a longer flush can help if these problems begin to happen.

        A decrease in the quality of the water will also mean that the final stage in the filtration process, the Deionising (DI) Resin, has to do additional work, and as a consumable item – you’ll end up spending more than necessary replacing resin. 

        To illustrate, a fully functional membrane will perform 90-95% of the purification, so if the input water (normally tap water) TDS is 100 ppm, then you should expect it to be leaving the membrane and entering the DI vessel between 5-10 ppm. You can check this with a TDS Meter

        As the membrane ages, you may find that it now leaves the membrane at 20 ppm, which will mean that the resin inside the DI Vessel (Stage 4 in the RO/DI System - see below) is having to do twice the work to purify the water to zero, and therefore it will be significantly less time between resin changes. At approximately £80 per bag, this is going to quickly add up. 


        Stage 4: Deionising Resin

        The final stage of an RO/DI System, and the only stage for DI Only Filtration

        A Deionising (DI) Vessel is a controlled container that channels water through a Mixed Bed Resin and is an essential component of a Pure Water System.

        DI vessel is commonly the final process in an RO/DI System, and is the only stage capable of purifying the water right down to 0 parts per million (ppm) or completely pure. This is also the simplest of the stages to maintain as it doesn’t require flushing and will simply stop outputting 0 ppm when the resin is spent.

        Most people choose to clean up to 7-10 ppm before changing their resin,any higher and you’ll begin to see unsightly spotting as the minerals dry on the glass.

        You will often see DI Only Systems in soft water areas, this simply means the DI Resin is used as a single stage purification system, to purify tap water right down to pure (without Pre-Filters or Membranes).

        DI Resin is capable of purifying any level of water hardness right down to 0 ppm, the issue is that the higher the tap water readings – the quicker the resin is spent. So in hard water areas the DI Vessel may need changing weekly. Of the 4 stages outlined above, DI Resin comes with the highest running costs. As such, in hard water areas, where the resin will need changing more often, it is often preferable to run a full 4 Stage RO/DI System to reduce running costs. 

        You can find DI Resin Here


        Where To Go Next

        Now that you know how an RO/DI and DI Only system works, which type is best for me

        RO/DI or DI Only - Types of Filtration Explained

        Choosing Static, Portable or Van Mounted Systems

        Could I Benefit from Water Softener?

        Back To The Complete Guide Start Page