TDS is short for “Total Dissolved Solids”. It is a reading that we as window cleaners, take from the water we use to clean windows, when using a pure water cleaning system. It is not a factor in ‘traditional window cleaning‘, just pure water window cleaning, also called water fed pole cleaning / reach and wash etc.
The TDS reading tells us of the number of particles of solid matter such as very small particles of metals, calcium and other minerals and salts etc that are held in suspension, or dissolved in the water. These ‘dissolved’ solids are very small – less than two micrometers in size and undetectable by eye. They stay in suspension in normal conditions and don’t sink, but stay dissolved as part of the fluid.
Wikipedia describes TDS as “Total Dissolved Solids (often abbreviated TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in: molecular, ionized or micro-granular (colloidal sol) suspended form.”
This shows you that a TDS reading can also be taken from other liquids, but as a window cleaner, I am interested in the TDS of water that I am going to use for cleaning windows.
How Is TDS in Water Measured?
TDS is measured using an electronic device which analyses the water using electrical conductivity, then tells us how many Parts Per Million (PPM) are dissolved solids. This way, we can get exact details of what level of dissolved solids we have in our water.
Why Is TDS Important To Window Cleaners?
Have you ever washed an outside window, then rinsed it off with fresh tap water, maybe from a garden hose or just a bucket and cloth? When it dries on the glass, you end up with spots or dirty marks all over the glass. Your nice clean window that you spent so much time and work on are no longer clean.
How did this happen when you used fresh clean water? These marks are the solids that were dissolved in the water and didn’t run off the glass, but stayed in suspension until the water dried, and they were left on the window. Windows are great at showing up any dirty marks – especially on sunny days.
Maybe if you rinsed off your windows with bottled water afterwards? No, you may get an even worse result! Bottler water, sometimes called ‘mineral water’ contains a lot on TDS in the form of minerals. You could get lots and lots of white spots all over your nice windows by rinsing off with bottled water.
You can see why TDS is important to a professional window cleaner. You wouldn’t want to pay someone to leave your windows with spots or streaks every time them came to ‘clean’ your windows would you?
What TDS Level is Good for Window Cleaning?
The lower the TDS, the better. In general we want zero ppm, or very close to it in the water we use for cleaning windows. In my opinion, anything between 0 – 10 ppm is good for cleaning windows (for my local tap water). It is not reasonable to expect to be able to get that level of purity from normal tap water, so we must have a way of removing these dissolved solids before cleaning windows with the water.
How Do You Get Rid Of These Dissolved Solids From The Water?
There are a number of ways, and again, I am just interested in how a professional window cleaner deals with the problem. We have two main methods of removing these dissolved solids – Reverse Osmosis and De-ionisation.
Basically, water is put through an extremely fine sieve with holes so small that the majority of these dissolved solids can’t go through. The much purer water is collected at the other side and now has a far lower TDS reading.
De-ionisation, also sometimes called Demineralisation, is a chemical process rather than a physical one like reverse osmosis. Water is passed through a special substance called an “ion exchange resin”. A chemical process takes place which ‘exchanges’ the dissolved minerals with hydrogen ions and hydroxide ions. These new ions then combine to form water. Since most dissolved solids in tap water are salts (the things that cause spotting on windows), this process is very efficient and creates a high level of water purity similar to that of distilled water.
There are other methods used to lower the TDS such as distillation and carbon filtration, but these are not generally used by window cleaners. Well, not quite true… carbon filters are often used as first stage filtration when the primary method is reverse osmosis.
It is generally thought that deionised water is the best for our purposes as window cleaners. Though each of these methods above has it’s positives and negatives. Often the choice of which purification method a window cleaner will use, comes down to how much dissolved solids are in the water source. Generally, if there is a high TDS in the original source water then it is more cost-effective method to use the reverse osmosis process followed by finishing or ‘polishing’ as it is sometimes called, the water by de-ionisation. Treating only with de-ionisation is only cost-effective if the source water TDS is low enough.
Just when you thought that a window cleaner just needed a bucket of soapy water, a cloth and a squeegee! Well, actually many still do just that – I still see them working in the same areas that I do. But, the modern window cleaner keeping with updated methods, and looking to give his or her customers something a little bit more, has these things to consider as well.