Water-chemical mode (the chemistry) of the car wash

Ivan Tikhonov

The article provides author’s reasoning about the organization of the chemistry of a touchfree car wash.

First, you need to understand what water quality is really required for the technical needs of a car wash. What impurities in the water can be the cause of poor-quality car washing and how does the chemical composition of water affect the effectiveness of the use of car detergent compositions?

It is known that when the water dries after washing, white stains can remain on the surface of a car. This happens if the water contains a large amount of calcium carbonate. Let’s look at the mechanism of the formation of this plaque.

Salts in water are initially in a dissolved state. Thus, if we observe sodium chloride in the solid state as well-known common salt, then in dissolved form, sodium cations and chloride anions are contained in water. In order for the cation to combine with the anion and precipitate from the water in the solid state, it is necessary to supersaturate the solution of this salt.

The case with calcium carbonate is somewhat different compared to other salts. The solubility of calcium carbonate is determined by the carbon dioxide balance. The more hydrogen cation (acid) is in the water, the higher the concentration of calcium carbonate can be in the water. But in a dissolved state in water, it will not predominantly be calcium carbonate, but calcium bicarbonate. The dissolution of calcium carbonate as a sedimentary rock and its conversion into a dissolved form of calcium bicarbonate occurs due to the fact that carbon dioxide enters the water. Carbon dioxide reacts with water and forms carbonic acid (H₂CO₃), which dissociates into a hydrogen cation (H⁺) and a bicarbonate anion (HCO₃^–).

СО₂ + Н₂О = Н₂СО₃ = Н⁺ + НСО₃^–

The hydrogen cation dissolves calcium carbonate to form calcium bicarbonate. In this case, one bicarbonate anion forms from the hydrolysis of carbon dioxide, and the second from calcium carbonate.

СаСО₃ + Н⁺ + НСО₃^– = Са(НСО₃)₂

This process is reversible. When carbon dioxide is removed from the water, solid calcium carbonate begins to form. A certain concentration of carbon dioxide in water corresponds to a certain concentration of calcium bicarbonate. This is called the carbon dioxide balance of water. When this balance is in equilibrium, the saturation index (Langelier) of such water is zero.

If external conditions change and carbon dioxide begins to be removed from the water, then, accordingly, solid calcium carbonate begins to form.

In a touchless car wash, carbon dioxide is intensively removed by spraying water. Accordingly, conditions are created for precipitation of calcium carbonate. This circumstance becomes critical in the warm or even hot season in the sun when the aerated water on the surface of the car dries quickly. In this case, all the calcium carbonate precipitates on the surface of the paintwork and leaves white stains. For fresh water (salt content up to 1000 mg/l (ppm)), the remaining salts contained in it will not give such an unpleasant effect. Here is an example from my own experience. Water with a hardness of 6.0 mg-eq/l and an alkalinity (HCO₃^–) of 4.5 mg-eq/l gives a stable white coating when washing a car in the sun, even without using a contactless technology.

The quality of washing can be negatively affected by the content of organic matter in the water. As a rule, almost all organic matter in the water of surface sources is represented by humic acids. Their concentration is characterized by such an indicator as oxidizability. For drinking water, oxidizability should be no more than 5.0 mgO₂/l. To do this, water from a surface source is subjected to coagulation. The higher the oxidizability is, the greater the likelihood of a poor-quality car wash due to the increased presence of humic acids in it.

If the car wash uses drinking water from the central water supply, then as a rule it has low oxidizability and the carbon dioxide balance is shifted towards the dissolution of calcium carbonate. Thus, there is more carbon dioxide in the water than dissolved calcium bicarbonate. Water is corrosive. This is due to the fact that in the process of coagulation in water, aluminum salts are hydrolyzed in the first stage with the formation of either sulfuric or hydrochloric acid (depending on the coagulant – aluminum chloride or sulfate) and part of the bicarbonates passes into carbon dioxide. At the same time, hydrochloric or sulfuric acids formed as a result of hydrolysis turn into calcium salts, replacing bicarbonate, which turned into carbon dioxide.

As a result, coagulated water tends to corrode equipment with hydrogen depolarization, since there is now a little more carbon dioxide in the water than it is required to maintain the carbon dioxide balance of the existing amount of calcium.

Therefore, when using such water in a car wash, with moderate water hardness, the likelihood of white plaque formation is high only in summer. At the same time, there is no possibility of a plaque of humus of various compositions.

If you use water from a mineralized artesian well, the probability of calcium carbonate deposits is extremely high, but in artesian water, due to the lack of contact with atmospheric air, there is no organic matter. Water from a surface source that has not been coagulated and clarified can cause both mineral and organic deposits on the car paint surface.

There is one more point. How does the chemical composition of water affect the preparation of the cleaning solution? I was only able to find general recommendations on this subject. Thus, for hard water it is necessary to use shampoos with complexing agents by analogy with high-quality washing powder. When complexing agents are used, hardness salts are included in the complexone and thereby increase their solubility, but still have the possibility of precipitation only at higher concentrations. Therefore, the composition of shampoos should include phosphates. Then calcium will be removed from the water in the form of the formation of solid hydroxyapatite, which should be alarming in the case of using non-contact pressure washer. Even without being an expert in this matter, it is clear that these additives are a rather controversial decision and are rather required as a necessity in the case of using hard water for washing. This is almost the same as in water treatment for steam boilers. Complexons are not a replacement for the main water treatment, but are used only at the stage of adjusting already prepared water.

As a result, based on the presented reasoning, several recommendations can be made on the development and maintenance of the chemistry of contactless car washes.

1. The source water must be subjected to Na – cationization. As a result, the water will be softened. Instead of bivalent and trivalent cations, it will contain only monovalent sodium cation. Sodium salts will not leave a coating or sediment on the surface of the car after washing, even under conditions of intensive removal of carbon dioxide and rapid drying of water. At least for water with a total salt content of not more than 1000 mg/l (fresh).
2. To remove organic acids, it is convenient to use reverse osmosis water desalination systems (reverse osmosis). To avoid rapid clogging of reverse osmosis membranes with hardness salts, water should be softened before reverse osmosis.
3. If the water has not previously been subjected to coagulation and clarification, it is necessary to install filters with sand and active carbon before the water softening system. This will extend the life of the cation exchanger and reverse osmosis membranes. It may be necessary to dose sodium hypochlorite into the stream before a filter has been loaded with quartz sand or equivalent.

Let’s look at each item individually.

The standard scheme of Na – cationization is convenient if it is used in car washes, because it does not require wastewater treatment. Softened water is used at the car wash and it is drained into the sewer after the settling tanks. Water is also drained into the sump during the regeneration of the cation exchanger. As a result, there is a natural dilution of salt-concentrated regeneration effluents from the softening unit. In the case that the salinity of the source water is less than 500 mg/l, the salinity of the wastewater after mixing the regeneration effluents and washing effluents will be no more than 1000 mg/l that complies with regulatory requirements. Depending on the composition of the source water, this ratio may vary, but for most surface waters with moderate hardness this ratio will be maintained. For source water with a hardness of 4 mg-eq/l, the specific consumption of table salt, which is spent on obtaining 1 cubic meter of softened water, will be about 0.5 kg, or 500 grams/m³ (500 mg/l) (in the case of proper management of the softening process and low sodium content in the source water). Thus, the salinity of the mixed effluent will be twice the salinity of the source water. In this case, wastewater treatment will not be required, which makes the use of Na – cationic water softening a very convenient process for obtaining softened water in car washes.

Control over the process of Na – cation exchange water softening can be simply and quickly carried out using the methodology described in the articles:

«Ion exchange water softening. How do I find out if the installation functions properly?» – https://tiwater.info/en/ion-exchange-water-softening-how-do-i-find-out-if-the-installation-functions-properly/

«The method of controlling the process of ion exchange water softening» – https://tiwater.info/en/the-method-of-controlling-the-process-of-ion-exchange-water-softening/

The use of this method of control does not require special training of operators. For this control method, a patent for invention has been obtained – No. 2744346 “Method of controlling the operation of the Na-water cationization plant” dated September 15, 2020. This control method has been successfully implemented in several steam boilers and one contactless car wash.

It is important to note that in the absence of control over water softening, with an insufficient number of regenerations of the softener, the Na – cationization unit starts operating in the Ca, Mg – cationization mode. Thus, the hardness salts accumulated on the cation exchanger and in the absence of regeneration, the process of the exchange sodium ions of the source water and Ca, Mg occurs. As a result, the hardness of the water after such a “softening” installation becomes even greater than the original hardness. For example, when the hardness of the source water is 4.0 mg-eq/l, the hardness of “softened” water with insufficient regeneration reaches 6.5 mg-eq/l with a certain frequency. In this case, it is generally better to work without softening. Therefore, the quality control of the softening unit is the main condition for a successful water chemistry of a car wash, as well as other water consumers.

To completely remove organic matter from water, as well as to reduce the salinity of water, it is convenient to use systems of reverse osmosis water desalination. For these purposes, it is sufficient to use the lowest selective reverse osmosis membranes. In any case, a decrease in the salinity of the filtrate after reverse osmosis in relation to the source water will indicate the complete removal of organic matter from the water. Even the smallest organic acids are much larger than individual ions. Moreover, the use of highly selective membranes will result in a reverse osmosis filtrate with a pH value lower than 6.0 (for most hydrocarbon type surface waters). At the same time, car shampoo manufacturers allow the use of water with a pH range of 6.0 to 9.0.

The use of low selectivity membranes will require less pressure to carry out the process of reverse osmosis water separation. You can use relatively cheap membranes, analogues of membranes from well-known companies. It is only necessary to take into account that the pH range of the washing solution of analogue membranes should be 2.0-11.0 (and not 1.0-12.0 for expensive membranes of well-known manufacturers). This suggests that cheap membranes will degrade after chemical osmosis washing much faster than expensive membranes. This requires high-quality pre-treatment of water before reverse osmosis. In the case of a car wash, this requirement must be met in any case (Coagulation-clarification, softening).

In this case, we can say that the quality control of the reverse osmosis filtrate should be carried out by the electrical conductivity of the filtrate and source water. The reduced value of the electrical conductivity (salinity) of the filtrate in relation to the source water already indicates the high-quality work of osmosis in this case. At the same time, it is important to make sure that there is no admixture, which can also occur inside the body of reverse osmosis membranes. In my opinion, for the purposes of a car wash, there is no need to talk about a certain high selectivity of reverse osmosis plants (96-99%), because the filtrate (permeate) for washing purposes is not required as a distillate, or as feed water for high-pressure steam boilers. But only if water softening is carried out before osmosis.

Summing up: the use of Na – cationic softening of previously prepared (clarified) water, followed by reverse osmosis desalination, makes it possible to obtain prepared water for high-quality non-contact car washing. At the same time, the resulting wastewater from the water treatment system will not require additional treatment before being discharged into the sewer. In any case (whether there is water treatment or not), coagulation and settling of waste water after car washing will be required.

It is extremely interesting to use the circulation system of water consumption. This will be the subject of a separate article.