Water-chemical mode of a car wash. Recycling water supply system.

Ivan Tikhonov

The article discusses the system of circulating water supply of a car wash. The main stages of water treatment and wastewater treatment are described. It considers the possibilities of the configuration of the recycling water supply system in order to obtain maximum efficiency of operation of the car wash complex under the condition of constant high quality of the prepared water and almost complete absence of wastewater.

The rational use of water at a car wash is, perhaps, one of the main conditions for its effective operation. At the same time the quality of the prepared water should be of a high level. The resulting wastewater will require disposal. For the conditions of the water use at a car wash, it is possible to organize a relatively simple and efficient scheme of recycling water supply system. Wastewater after the treatment can be used as source water for the car wash water treatment system. At the same time, the wastewater treatment system and the water treatment system will be able to complement each other effectively.

In the organization of the recycling water supply system, it is necessary to use simple, understandable, the most common technological processes.

The water quality required for car washes is discussed in the article [1]. The article shows that the greatest efficiency of washing is achieved when using softened water together with partially desalinated reverse osmotic water. As a result of the use of such water at the car wash station wastewater is generated, which belongs to the category of industrial wastewater. In such waters the COD index (chemical oxygen consumption) significantly exceeds the BOD index (biochemical oxygen consumption). The wastewater of car washes, first of all, a large amount of suspended solids and surfactants (surfactants) will containe. The presence of petroleum products, oils, humic substances, alkali, etc. is possible. Therefore, physico-chemical, rather than biological methods of purification are used for the treatment of industrial wastewater, due to the complexity of oxidation of the pollutants present in the water with oxygen from the air.

Figure 1 shows a schematic diagram of the circulating water supply of a car wash. Wastewater from the car wash station enters the primary tank. In the primary settling tank, natural precipitation of substances suspended in water occurs. First, sand and silt are deposited in the primary settlement tank. After the primary settlement tank, the water is sent to the coagulation stage. Three reagents are dosed into the water before the clarifier: sodium hypochlorite (H), coagulant (C) and flocculant (F). Dosing of the reagents will allow the destruction (oxidation) of stable organic compounds and their coagulation. As a result, sludge is formed in the clarifier, which settles. Clarified water is sent to the secondary sedimentation tank. The final deposition of coagulated contaminants contained in the water takes place in the secondary settling tank. After the secondary tank, the water can either be discharged or reused.

Figure 1 Scheme of recycling water supply of a car wash

It is necessary to achieve such a quality of treated wastewater after the secondary tank, which, in the case of discharge into the water supply source, will not have a negative impact on natural water sources. Therefore, the water after the secondary tank can be used as the source water for the car wash water treatment system. This water must be subjected to additional purification, which will consist in sequential filtration of water through a quartz sand filter and an activated carbon filter. This will eliminate the ingress of suspended and organic substances remaining after the secondary settling tank into the purified water and remove an excessive amount of sodium hypochlorite on the activated carbon. Then the prepared water is sent to the Na – cationite softening plant. Softened water is collected in the Na-water tank. From the tank, softened water is supplied under high pressure to the washing station and mixed with the auto shampoo. From the same tank, the softened water enters the reverse osmotic desalination (RO) unit. Partially desalinated water (filtrate) enters the tank and it is fed into the washing station. The wastewater after washing enters the primary tank. Then the cycle of water treatment and wastewater treatment is repeated.

Wastewater will form in the sand and carbon filter. The composition of such wastewater allows it to be sent directly to the primary settling tank. This wastewater will be re-subjected to coagulation and clarification and will be reused in the water treatment system. The concentrate after the reverse osmosis installation can also be sent directly to the primary tank. Since the filtrate after the washing station is also drained into the primary tank, and it will dissolve the concentrate, the increase in the salinity of the recycled water will not occur.

Wastewater after the installation of Na – cation will require separate disposal [2] and it cannot be drained into the primary settling tank. Nevertheless, the wastewater consumption after Na – cation unit will be at least 10 times less than when using it in a direct water supply system.

As a result of the operation of such a recycling water supply system, only the wastewater of the softening plant (in a minimum amount) will be formed, as well as periodically removed sludge with a humidity of 80-90% from the primary settling tank and the clarifier.

To compensate for water losses, it is necessary to recharge the recycling water supply system with source water. Recharge must be performed before the water treatment system (sand filter). For most natural (fresh) waters (especially water from household drinking water supply), the composition of the water treatment of the recycling water supply system will ensure the necessary quality of the prepared water. If the make-up water source contains specific pollutants, then it is necessary to provide dosing systems for coagulant, flocculant and sodium hypochlorite before the sand filter.

With a competent organization of the functioning of the circulating water supply system, the make-up water consumption will be no more than 10% of the total water consumption by the washing station.

For example, if the consumption of prepared water at the washing station is 35 tons/day and the total content of coarse, suspended and organic substances is a significant amount of 100 g/liter, then the amount of sand, silt and sludge formed will be: 35000 * 100 = 3,500 kg. If we proceed from the sludge humidity of 85%, we get: 3500 * 0.85 = 2975 kg of water.

It turns out that the loss of water with sludge will be about 3 tons. Accordingly, the recharge consumption will be equal to 3 tons /day, which corresponds to less than 10% of the total water consumption of the car wash. Thus, the water Na – cation unit will consume 10 times less tableted salt than in the case of direct water supply, because only the make-up water will be softened. Of course, we can talk about a small amount of calcium bicarbonate, which will be formed in wastewater as a result of washing. But this will only happen in the case of acid dissolution of possible limestone contaminants present on cars. The only water source with a slight acid reaction is reverse osmosis filtrate. But based on the conditions of its use (final washing), its direct contact with possible limestone particles is actually excluded. In the primary tank, the filtrate will already be mixed with slightly alkaline (as a result of aeration during the process of contactless washing) softened water, as well as alkali auto shampoos, so the dissolution of limestone particles will not occur. Rather, on the contrary, the small amount of hardness will be precipitated in the primary settling tank or clarifier by alkaline effluents.

All technological installations of the water treatment system are automated and allow the process to be carried out completely in automatic mode. Only a rare periodic loading of tableted salt into a salt-dissolving tank will be required. The organization of the operation of the clarifier presents a certain complexity. The complexity is due to the uneven composition of incoming wastewater. This process can be automated (to determine and dose the optimal quantity of reagents in automatic mode) using water quality control sensors, such as sensors of ORP, pH, electrical conductivity.

It can be said that the use of the recycling water supply system in accordance with the proposed scheme will practically stop the discharge of wastewater. It will only be necessary to remove the sludge with a certain amount of water. In the worst case, the recharge consumption will be no more than 10% of the total water consumption of the washing station. The consumption of tableted salt will decrease at least tenfold.

List of referents:

  1. Ivan Tikhonov. Water-chemical mode (the chemistry) of the car wash
  2. Ivan Tikhonov. Waste water from the Na ion exchange plant. Composition. Disposal method. https://tiwater.info/waste-water-from-the-na-ion-exchange-plant-composition-disposal-method/