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Water Treatment

Water is the most important consumable in your wash. It can be easy to overlook as you just turn the tap and there it is. It is the base of all your soaps and waxes. It is the medium in which the dirt is trapped and removed from the surface of the car. It is what you use to rinse the dirt off the car and in its spot free form is what guarantees a perfect blemish free finish to the surface of the vehicle.

The main forms of water treatment encountered on most washes are softening, de-chlorination and reverse osmosis (RO). Many washes also have a reclaim system but that is a topic for another day.

All fresh water entering the site should first be softened by passing through a water softener. Water softening essentially is the removal of calcium carbonate and other mineral ions such as magnesium which are the primary component of water hardness (measured in grains per gallon). The main reasons for the removal of these are to improve the effectiveness of your soaps and to prevent scaling in your equipment, especially on the membranes of your RO system, which greatly reduces their capacity and they are expensive to replace. In hard water situations, the mineral salts in the water react with the soap to form an insoluble precipitate we call soap scum. This is difficult to rinse away and also effectively reduces the amount of soap available to do the cleaning. If your water is too hard you will be using more soap to achieve the same clean you get with softer water or just not cleaning as well as you should which leads to unhappy customers. Therefore, softening your water ensures reduced soap costs and increased efficiency as well as more happy customers and repeat business.

Water softeners on your car wash work by a process of ion exchange. Basically the positively charged mineral ions are attracted to the negatively charged resin beads in the softener. Eventually the resin beads will have absorbed all the mineral ions they are capable of and will have to be regenerated by flushing them with a brine solution. During this process the sodium ions in the brine solution replace the mineral ions attached to the resin beads and the softener is ready to process another batch of water.

All washes should have at hand a water hardness test kit and should regularly check the hardness of the water before and after the softeners. Typically you are looking for less than 1 gr/gal after the filter. (Are they?) If your water is too hard, ask yourself WHY? Is the softener working correctly? Does it regenerate frequently enough? Are you actually using salt? Is the brine tank topped up?

The following diagrams show water flow through your softener during various stages of the cycle.

This softened water is typically what is used in your soap and wax applications and for general rinsing.

The next stage in the treatment of this water is the removal of as many other foreign bodies as possible for the spot free function. Typically the first stage in this further treatment is dechlorination by means of a carbon filter. The carbon filter works in a similar manner to the softener. As the water passes through the activated carbon media the negatively charged chlorine is attracted to the positively charged carbon. Carbon filters need to be regularly back flushed (generally on a timer set to come on at night when the wash is quiet) to “fluff up” the carbon media and prevent the water from finding the path of least resistance through the filter and forming channels. This back washing also presents fresh surface area of carbon to the water for chlorine absorption. If properly serviced and maintained, one pound of activated carbon provides a surface area of about 100 acres on which to absorb chlorine and other volatile organic compounds. Carbon filters are not effective at removing the minerals that are removed by the softener and the back washing process does not remove the attached chlorine it simply turns the carbon to expose fresh surface area for further absorption.

It is important to remove this chlorine from the water before the Reverse Osmosis (RO) system. The RO membranes are very sensitive to corrosion by chlorine.

All sites should have on hand a chlorine test kit to check that the carbon filter is effectively removing chlorine from the water, thereby prolonging the life and efficiency of the RO membranes and improving the quality of your spot free water.

The next stage is the 25–1 micron filter between the carbon filter and the RO membranes. It is used to remove the worst of the remaining particles before the water enters the RO system. This filter will have a pressure gauge either side and should be changed every six months or when you see a pressure drop of 10 psi across the filter, whichever comes first. All sites should carry a spare filter ready to change. In extreme cases, a very heavily clogged filter here could trip the low water pressure switch on the system.


25 – 1 Micron filter housing on RO system showing pressure gauges either side

The final stage is Reverse Osmosis. Reverse osmosis is the finest level of water filtration available. To understand Reverse osmosis lets first look at osmosis.

Osmosis occurs when pure water flows from a dilute solution through a membrane into a higher concentrated solution. It is a fundamental law of nature that this system will try to reach a state of equilibrium. That is the same concentration on each side of the membrane.

This is very simplified but basically osmosis is the movement of solution through a semipermeable membrane from an area of low concentration to an area of high concentration as the fluids seek a state of equilibrium.  A very basic analogy could be putting a raisin in some water. Leave it for a while and it swells up. There is a greater concentration of sugars inside the raisin so water passes through the skin into the raisin in an attempt to equalise the concentrations. Eventually the pressure of the raisins skin prevents it from absorbing any more water.

Remove this swollen raisin from the water and it will remain swollen even though the water could pass through the skin it doesn’t as it is attracted to the area of high concentration. To get this water from the raisin we need to squeeze it. To apply pressure. This application of pressure to force the water from the area of high to the area of low concentration is what happens in reverse osmosis.


In your car wash pressure is applied to water passing over the RO membranes this forces some of the water through the membranes leaving behind all dissolved salts and inorganic molecules which are the cause of “spotting”. The water that passes through the membranes is carried to your spot free holding tank. The water that does not pass through but flows over the membranes is the reject.

Your RO system will have two flow gauges on the outflows. One showing the waste water or reject and one showing the make of spot free water. Rejecting water is actually an important part of the process as it is this reject water constantly flushing over the membranes that removes the impurities from the system preventing them from clogging the membranes.

You can check the quality of your spot free water with a TDS (total dissolved solids) meter. TDS is measured in NTU. To Know if your RO system is operating correctly you should collect your water sample from the inlet to the RO storage tank. The stored water in the tank itself may be contaminated, for example if the lid has been left off, so may not always give a true reading of what is actually leaving the RO membranes. Obviously you should also check the tank water as this is what is being applied to vehicles.

Ideally for a spot free finish you should be looking for a TDS reading of 0 -30 NTU. 

For any further questions on Water Treatment and Water Treatment Products please contact Prowash's friendly staff on 03 8340 3200


Date Published: 5 June 2017