Pure Water for Window Cleaning: Spot-Free Glass and Building Faces

The team cleans the whole face of the office block in the morning. When the work is done the glass looks spotless; but towards noon, when the sun hits it, white marks appear on the surface. The client complains, the team goes back up the same face and the result is no different. In that cycle the problem is not the cleaning method or the staff — it is the water used. Pure water for window cleaning was developed to solve exactly that problem.

In this guide we take up how water spotting forms, why wiping does not remove it, and how rinsing with low-conductivity water changes the result. We also explain in the language of the business which water quality is needed, why softening alone is not enough, and how the system is sized.

Why Do Windows Stay Marked When They Dry?

When water evaporates on a surface, the minerals dissolved in it stay on the glass and leave a white mark as it dries. That mark is not a cleaning error but a result of the content of the water. Treated water with a reduced dissolved solids load leaves no such residue behind when it dries.

The mechanism is simple and entirely physical. As a drop of water on the glass evaporates, only the water molecules go into the air; the calcium, magnesium and other dissolved components in it stay on the surface.

However carefully the cleaning is done, therefore, if the water used for rinsing contains minerals the result is the same. Once drying is complete the mark becomes visible.

The mineral content of water varies from area to area. For a technical framework on the idea of hardness, the US Geological Survey’s water hardness resource can be consulted; the resource sets out that the dissolved minerals in water occur naturally and that the amounts differ by area.

That finding is the starting point of the pure water for window cleaning approach: the problem is not on the surface but in the water applied to it.

How Does Water Spotting Form?

The drop attaches to the surface and begins to evaporate. As the water falls, the mineral concentration in it rises; when the drop has dried completely, the minerals stay on the surface as a ring or a mark. Because sun and wind accelerate evaporation, the mark forms more clearly.

Picturing the process makes it clearer:

What that difference means for the business is clear: in the first scenario the team has to go back to the same surface, in the second the job is finished in one pass.

There are also conditions that make a mark more visible. Faces in direct sun, windy weather and hot surfaces all accelerate evaporation, and that leaves a denser mineral mark.

Surfaces other than glass get their share of the effect too. The same mineral residue builds up on aluminium and composite cladding, on stainless railings and on frame profiles. A pure water for window cleaning application is therefore an approach covering the whole face of the building.

Why Are Detergent and Wiping Not Enough on Their Own?

Wiping spreads the water on the surface but does not remove the minerals in it. When the thin film of water left behind dries, the mark forms again. On upper floors and wide faces it is also practically impossible to wipe every surface; that makes the problem permanent.

The logic of the traditional method is this: take the water off the surface as fast and as completely as possible. That approach works; but it depends on two conditions — being able to reach the surface and being able to take off all the water.

In practice those two conditions are often not met:

That table shows why the problem has to be solved not with the method but with the input. Wiping cannot stop the mineral staying on the surface; it only spreads it.

The same logic applies in a different sector. The source of the marks left after drying in car washing is the same; we took the subject up separately in our car wash water treatment article. The focus of this piece is glass and building faces.

Figure 1: How a water mark forms on a glass surface

How Is Pure Water Used in Window Cleaning?

The treated water is applied to the surface through a telescopic brush pole system. The brush releases the dirt mechanically while the running water rinses the surface. The surface is then left to dry on its own; because the dissolved solids load in the water has been reduced, there is no need for a wiping step.

The logic of the application differs from the traditional method on one basic point: the aim here is not to take the water off the surface but to rinse with water that leaves no mark behind.

The process runs in these steps:

1. The Water Is Treated

   Mains water is passed through the system and its dissolved solids load reduced. The outlet quality directly determines the result of the application.

2. The Surface Is Brushed

   The dirt on the surface is released mechanically by the brush at the end of the telescopic pole. The water goes on flowing through the brush at the same time.

3. It Is Rinsed with Plenty of Water

   The released dirt and residue are carried off the surface with treated water. The rinse has to be thorough.

4. The Surface Dries on Its Own

   The water left behind evaporates and, containing no minerals, leaves no mark. There is no need for a wiping step.

The fourth step is the clearest difference of the method. The team can move on to the next section without drying the surface, and that markedly shortens the workflow on wide faces.

On the system side the installation can be made in two ways. In a fixed installation the treatment unit sits inside the site and is fed from the line. In a mobile installation the unit is carried on a vehicle; that approach is preferred by teams working at different sites.

What is decisive in both builds is the same: the quality of the outlet water. Pure water for window cleaning The result of the application depends directly on that quality.

Which Water Quality Is Needed?

The aim in the application is for the conductivity of the water to be as low as possible. Conductivity is the indicator of the dissolved ion load in water and is directly related to whether it leaves a mark. Reverse osmosis and deionisation are therefore used together in most builds.

Conductivity is the basic parameter followed in this application. When the value rises there are more dissolved ions in the water, and that raises the risk of a mark after drying.

It would not be right to give a numerical target here; the quality needed is established from the profile of the inlet water and the expectations of the application. We took up what the measurement tells you and what it does not in our dissolved solids and conductivity article .

Two stages generally work together to reach the quality target:

We took up how the membrane works and what it separates in our how reverse osmosis works article. We explained the technical side of deionisation deionised water article in detail; we are not repeating the technology here.

The equipment used in the application is built around those two stages too. For the main treatment, high-capacity reverse osmosis systemsare used, and for the final stage, deionised pure water systems .

On the resin side, mixed bed products are used; when the time to renew comes, the bed is refreshed with mixed bed pure water resin .

Is Softening Enough?

No. Softening exchanges the calcium and magnesium ions that give hardness for sodium; but the dissolved solids load remains in the water. That water still leaves a mark when it dries. A spot-free result calls for the conductivity to be lowered, and that is a different stage.

This is the most common misdirection in the application. The “there is a limescale problem, let us fit a softener” approach is right for protecting equipment; but it is not enough for the aim of leaving no mark on a surface.

The difference between the three approaches can be summed up as follows:

The first row of that table is the summary of this section. Softening does not meet the aim of this application; but it is not left out either — as the next section shows, it takes on the job of protecting the membrane.

We took up how softening works and which criteria it is chosen on softening selection guide .

The Advantage on Upper Floors and Building Faces

Because the work can be done from the ground with a telescopic pole, the need for scaffolding and platforms can be reduced on some surfaces. Because the wiping step disappears, the time taken on wide faces is shorter. Conditions for working at height are always established in line with the relevant health and safety regulations and an expert assessment.

Measured language is needed in this section. The method can reduce the need for access on some surfaces; but it cannot be claimed to offer a solution valid for every building face.

The effects of the application on site are these:

The limits of the method should be stated too. A telescopic pole gives access up to a certain height; beyond that, different solutions are assessed. Extra work may also be needed on surfaces with heavy contamination or sticky residue.

Figure 2: Rinsing a building face with treated water

The Effect on Chemical Use

When working with treated water, the need for detergent can be reduced in many applications; part of the dirt is released by mechanical brushing and plenty of rinsing. No chemical residue is left on the surface either, because the rinse water leaves no mark behind.

The size of that effect varies with how dirty the surface is and with the application; it would not be right to promise a percentage here.

What the change on the chemical side means for the business comes under two headings. The first is the change in consumables; the second is that no residue is left on the surface after rinsing.

The second heading matters particularly. In the traditional method, detergent that is not rinsed off enough can leave a separate mark on the surface, adding a problem on top of the mineral mark. Rinsing thoroughly with treated water removes that risk.

There is a simplification on the team’s side too: when the amount of chemical carried falls, site logistics get lighter. That is a practical gain for teams working over wide areas.

Which products are used on which surface is outside the scope of this article; that assessment should be made in line with the manufacturer’s information for the products you use. What is taken up here is only the water side of the pure water for window cleaning application.

How Is the System Sized?

In sizing, the surface area cleaned each day, the number of teams working, whether use is mobile or fixed, the storage tank required and the hardness and dissolved solids load of the inlet water are assessed together. The final capacity is engineered from the water analysis.

Sizing in this application depends on two variables: how much water will be consumed and how much load the inlet water carries. The second is often overlooked; yet a system of the same capacity performs differently on different inlet water profiles.

The headings to assess are these:

1. Have the Inlet Water Analysed

   The hardness and dissolved solids load directly determine both the stage build of the system and the consumables it uses.

2. Work Out the Daily Surface Area

   How much building face is cleaned on an average day is the basic input for the amount of water needed.

3. Establish the Number of Teams

   How many teams work at the same time? Simultaneous use defines the peak demand.

4. Decide Between Mobile and Fixed

   A vehicle-mounted build is assessed for teams working at different sites, a fixed installation for those working at a single location.

5. Plan the Storage Requirement

   The tank capacity is established to balance the rates of production and consumption; on site work that is a critical heading.

6. Confirm the Infrastructure

   The equipment layout, the power supply, the drain connection and the filling point are settled in advance.

The fifth step is decisive for site teams in particular. The system produces continuously; but the team consumes heavily on site. The tank capacity balances those two rates.

We took up the general logic of a capacity calculation and what to watch for at business scale in our industrial-type system selection guide . The final capacity is always engineered from the inlet water analysis and the daily surface area.

Maintenance and Continuity of Operation

The continuity of the system depends on three headings: renewing the pre-filtration on time, monitoring membrane performance and following the resin bed. Where the inlet water is hard, a pre-softening stage protects the membrane and makes consumable use predictable.

In this application maintenance directly affects the quality of the output. When the resin tires the conductivity rises and marks start to appear on surfaces; that means doing the job again.

The headings to follow are these:

• Outlet conductivity: The most direct indicator of the application; it should be monitored regularly.
• Pre-filtration: The sediment and carbon stages are renewed when saturated; they protect the membrane.
• The state of the membrane: Changes in the production rate and the outlet quality are monitored.
• The resin bed: It is renewed when it has completed its capacity; the outlet conductivity shows that.
• Pre-softening: On hard inlet water it reduces the scaling load on the membrane.
• Following the salt: Where there is pre-softening, the salt level is checked regularly.

The fifth item is the most decisive economically. Where pre-softening is not used on hard inlet water, the membrane tires faster and the resin bed saturates far sooner. Used for that purpose, industrial softening systemskeeps consumable use under control.

We took up the structure of the membrane and the mechanism of ageing membrane filter article, and the working principle of resin and the factors determining its life in our resin article .

Water Point Su Arıtma Teknolojileri operates from Esenyurt, İstanbul under its founder Ali Genç, with more than 20 years of experience in the sector.

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