CHOOSING A SEPARATOR FOR A CAR WASH

Choosing a separator for your car wash

As environmental awareness is growing in our country, we are becoming more aware of the threat posed by the discharge of polluted wastewater into the environment. More attention is being paid to the condition of rainwater and sanitary sewage, discharged to sewers, natural receivers, surface waters and underground waters.

Legal requirements for separators

According to the Decree of the Minister of Infrastructure of 20 July 2002, we are obliged to treat rainwater and process wastewater discharged, inter alia, from polluted city centres, petrol stations or car washes. According to the Act, in wastewater discharged into natural reservoirs and urban sewers, the content of suspended solids at the outlet should not exceed 100mg/l and the content of oil-derived substances should not exceed 15 mg/l. In car washes, it is not possible to obtain wastewater meeting these parameters. It is necessary to use oil separators to treat the wastewater before it is discharged into the sewer system. Fully retentive separators are dedicated to car washes because they treat the entire flow that can be discharged into the sewer system. Full retention units are used in areas where there is a high risk of contamination.

How does an oil/water separator function?

An oil-derived substances separator is a sanitary installation device designed to purify water of hydrocarbon contaminants. The separators treat process wastewater that comes, for example, from car washes, workshops and vehicle storage areas. In the oil-derived substances separator, the incoming wastewater is cleaned of oil-derived substances and solids (e.g. sand, earth, gravel, etc.). The first stage of wastewater treatment consists of separating the solids using a settling tank (so-called sand trap) by gravitational sedimentation. Separators with an enlarged settling tank are dedicated to car washes due to the larger amount of mineral deposits (sand, stones, etc.). In the second phase, the wastewater flows into the separation chamber where, with the help of special filters, the separation and subsequent collection of oil particles takes place. In this phase, the contaminated water flows through a special coalescing or lamella filter, which is designed to facilitate the flow of the oil-derived substances to the surface, where the separated particles aggregate to form a layer of oil-derived substances. In the third stage, i.e. the final treatment phase, the wastewater flows out of the unit via an outlet into the sewer. The outlet is equipped with an automatic float closure, which closes the tank when it overflows with accumulated oil substances.

Car wash - waste water discharge process

In car washes, highly contaminated wastewater with petroleum substances such as petrol, oil or grease cannot be discharged directly into the sewer system. Oil separators are used to protect sewers and bodies of water from the oil in the wastewater. They retain the environmentally hazardous substances contained in the discharged wastewater, and the treated water can be further discharged into the sewer system or into a sump. In car washes, the technological flow is forced by pressure washers, the sum of whose flows will allow us to design a separator with an appropriate flow. It is also worth noting whether the car wash will be roofed, otherwise additional calculations should be made for the open area collecting rainwater. The drainage line for the washing stand must first have a drainage grating, then a sand trap and a Class I full-flow petroleum separator. A flow rate of 1.5 l/s is assumed for one car wash station (of course, it is better to check what capacity the pressure washer has).

Car washes also use systems to return the treated water. A closed-loop system (known as 'ZOW') in carwash facilities treats the wastewater to such an extent that the reclaimed water can be used repeatedly. Closed-cycle treatment uses well-known technology, namely:

• sedimentation and de-oiling by gravity in a horizontal flow settling tank - the first treatment stage,
• sedimentation and chemical-assisted deoiling (flocculation) in a multi-stream package - second stage of treatment,
• filtration and sorption on an adsorption bed filter (granular diatomite) - third treatment stage.

In European countries, closed water circuits are increasingly used in car washes. In Poland, this type of installation is usually lacking. One reason for this is the relatively low price of water and the small sewage charges (if any). In addition, the high investment cost means that installing ZOW systems is currently an expensive undertaking for the carwash owner.

Oil separator types and types

Many separator solutions are currently available on the market. The Polish standard PN-EN 858:2005 divides separators into two categories, class I and class II. The separators are made of polyethylene, which affects the life of the installation and ease of maintenance, and some are made of reinforced concrete. For car washes, full retention separators are installed, which means full flow through the separation chamber, while for car parks and outdoor areas, a hydraulic bypass separator can be used due to the higher flows caused by heavy rain (increased long-term precipitation). We also divide the solutions by the method of filtration (coalescing cartridge, lamella cartridge). The following is an elaboration of the issues:

- Class I separator - designed to achieve an oil concentration of less than 5mg/l at the outlet under standard test conditions. Class I includes coalescing and lamella separators.

- Class II separator - designed to achieve an oil concentration below 100mg/l in the effluent under standard test conditions. Class II includes separators using mainly mechanical separation (sedimentation and flotation).

- Separator with settling tank - in this unit, an additional sedimentation process takes place for larger mineral impurities, i.e.: sand, silt, stones. The settling tank has the additional task of protecting the installation from clogging. Separators with an enlarged settling tank are dedicated to car washes, they catch large amounts of mud and sand, thus guaranteeing safe operation of the device.

- Separators with a pumping chamber - are used when it is not possible to discharge treated wastewater by gravity. The pumps pump the treated wastewater to the place where it is discharged into the gravity sewer.

- Separator with coalescing cartridge - the primary physical process in this separator is flotation assisted by coalescence, i.e. the coming together of small oil particles dispersed in water to form larger clusters, allowing for more efficient flotation of contaminants. Depending on the type of separator, coalescence takes place on an appropriately selected filter type.

- Laminate cartridge separator - the contaminant separation process takes place through the use of multi-stream technology by passing the wastewater through a specially constructed filter cabinet consisting of removable lamella sections. During the flow, fine oil particles carried by the liquid are deposited on the undersides of the lamella plates. After reaching a certain size, the separated oil particles rise upwards along the underside of the plate due to buoyancy forces, forming an oil film on the surface.

- Separator with bypass - additionally equipped with a bypass with a flow higher than the nominal flow. The bypass is a safety solution in situations of excessive waste. The separator is most often used to treat rainwater at car parks or vehicle dismantling yards.

The separators can be fitted with additional accessories, such as alarms and monitoring sensors. The proposed alarm systems guarantee environmental safety, provide monitoring comfort and reduce operating costs. The petroleum substance alarm system has the ability to connect up to max. The petroleum-derived substances alarm system has the option of connecting up to three sensors located inside the separator, making it possible to monitor overfilling, sludge levels and the thickness of the dirt layer. The alarm device provides acoustic notification of emergency situations. In addition, it is equipped with LEDs to indicate the current operating status of the separator.

Selection of oil/water separators for car washes - calculations

The selection of a separator for a car wash is carried out by calculating the washer flows which will create the maximum water flow in accordance with PN-EN 858:2005.

General formula for selecting separators according to PN_EN 858:2005

NS=(Qr + fx ∙ Qs) ∙ fd

where:
NS - flow volume,
Qr - nominal rainwater flow in l/s,
Qs - maximum process waste water flow in l/s,
Fd - density factor (we usually assume the value of 1),
fx - separation impediment factor (we usually assume the value of 1).

Calculation of process waste water volume (e.g. car washes and flow rates generated by high-pressure washers)

QS=QS1 + QS2 + QS3 +...

where:
QS1 - effluent from the tapping points (Table 1.),
QS2 - effluent from car washes and/or self-service wash bays (check the washer flow),
QS3 - effluent from high-pressure washers and cleaning units.

Table 1 Volume of produced effluent from point of use depending on valve diameter

Qs2 AND Qs3 - Car washes - 3l/s should be taken for each car wash and 1l/s of effluent for each additional station, unless car wash flows are much higher.

Calculation example

The following is an example of separator selection for a self-service three-station car wash:

- Nominal rainwater effluent flow: 0l/s,
- Max process effluent flow: 6l/s,
- Density factor: 1,
- Separation impediment factor: 1.

Ns= (0+6+1)-1=7

Selection summary

For a self-service three-station car wash, the instantaneous flow will be 7l/s. The separator for this car wash should have an enlarged settling chamber and a flow of not less than 7l/s. Too low a flow rate makes it impossible to achieve the correct process wastewater treatment parameters, while too high a flow rate is not economically viable.

Maintenance

According to the Decree of the Ministry of Environmental Protection of 24.12.97, Journal of Laws No. 162, item 1135, oil-derived substances are classified as hazardous substances. Maintenance and servicing of oil/water separators must be carried out by a qualified company which is authorised to transport and handle hazardous waste. Maintenance of oil/water separators consists of periodic emptying of retained waste and cleaning of the separator interior. The periodicity of cleaning depends on the rate of waste accumulation (at least once a year). Cleaning should be carried out when the tank is half full of sludge or 4/5 of its maximum oil holding capacity. After cleaning, the separator should be filled with water and the float valve should be set to the operating position.

At present, there is a wide range of different oil/water separators on the market. Before purchasing a separator, an engineer should be consulted to select the right separator for your needs. Only a correctly selected and operated separator will properly fulfil its function and protect the environment for many years.