The NETCAR company uses a cleaning system that has a very high water consumption and produces a large quantity of waste water (with a high overall pollutant load).

Although these problems are very common in many enterprises and industrial sectors, this exercise focuses on one specific case:

A facility for cleaning tanker trucks that transport chemical products (here referred to chemical tanker truck wash facilities).
International regulations covering the transportation and loading and unloading of chemicals require the inside of tankers to be totally cleaned out once they have unloaded the chemicals being transported to eliminate all chemical remains.

Water consumption in chemical tanker truck wash facilities is very high, as is the amount of waste water that is produced (practically the same as consumption). Pollutant load is also very high. This means a large investment by the NETCAR company in an on-site treatment plant.

Tanker lorries that come to the facilities also generally have the outside and underside of the vehicle cleaned as well.

The activities of the NETCAR company are undergoing expansion and there is a big increase in the number of tankers being cleaned, which means that the treatment plant cannot comply with set limits for the dumping parameters of waste water due to the excess volume of water being treated.

Tanker truck inside wash units

This has a dual structure with a series of connections that fits onto the tanker man-holes by way of a plate; a set of satellite nozzles underneath the plate spins at high speed from the water under high pressure.

Following a preliminary hose-down and a visual check of the inside of the tank, a set of very high pressure pumps (100 to 200 bar) injects water through pipes to each plate and set of satellite nozzles that cleans the inside walls of the tank. The water that is injected at high pressure may contain detergent and also be hot (the water is heated in a boiler with a gas-oil burner).

There are two cleaning units which means that two tankers can be cleaned at the same time, while two more are being prepared to be washed.

Vaporisation is also carried out in the internal wash unit area. This uses steam from a boiler with a gas-oil burner.

Dirty wash water falls to the ground and is collected in channels leading to the waste water treatment plant.

The preliminary hose-down consumes 20% of the volume of water used in inside cleaning.

Tanker truck outside body wash unit

This is a tanker truck outside body wash facility that is especially for extra-long tankers and is similar (although larger) to regular car washes.

Together with the cleaning brushes and sections for drenching, detergent wash and end rinse, the facility has a water decalcifying unit for the end rinse. High pressure hoses are installed in the truck wash.

Dirty wash water falls to the ground and collects in a separating sump. From here, it is pumped to the waste water treatment plant, mixed together with the water from the inside wash.

Waste water treatment plant

Waste water from both the inside and outside washes is collected in a separating sump.

From here, it is pumped to a degreasing plant (which works by flotation) by a bubble cavitation system. The grease and hydrocarbons are dragged to the surface by the air bubbles to form a creamy mass that is collected by cable-operated blades that carry the grease to a container, where an authorised waste manager collects the waste.

The degreased water gathers in a large vented homogenisation tank with a 16-hour waste water production capacity.

The water is pumped from the homogenisation tank at a constant flow to the following sections of the treatment plant.

The first section involves a physiochemical treatment where a coagulation process takes place, followed by flocculation. An automatic pH control is installed (generally involving the use of caustic soda) to ensure the system operates correctly (most of the waste water is acidic).

The water from the physiochemical treatment is pumped to a separation sump. The clarified water overflows and flows by gravity feed to the biological reactor. The sludge collected (by time delay) from the bottom of the separation sump is pumped to a sludge thickener.

The biological reactor has fine bubble membrane diffusers with an outside blower system. The most important part of the waste water treatment process takes place in the reactor. The mixture of biological sludge and purified water goes into a separating tank. The separated water goes into a sump and can then be dumped directly into the sewerage system or can be passed through a filtration system. The separated sludge at the bottom of the separation sump is recycled to the biological reactor and part of it (more or less depending on the biological control of the active sludge in the reactor) is drained off and processed in the sludge thickener (the same sludge thickener as in the physiochemical process).

The final filtration system consists of a semi-automatic silex and anthracite filter, which is only used where necessary. The final filtration process is not necessary when the biological treatment is working correctly and the biological sludge separates out well.

Excess water from the sludge thickener returns by gravity feed to the homogenisation tank. Thickened sludge (more from the physiochemical process than the biological treatment) are mixed with lime wash (prepared in a tank with agitator) and are pumped to the dehydration system.

Dehydration of the sludge is carried out using a filter press, in which the sludge is passed through rack-mounted gauzes under high pressure. The pressure exerted by the plates that the gauzes are attached to press dries and dehydrates the sludge (now filter cake), which falls into a container where an authorised waste manager can collect it.
Water drained from the filter press is pumped to the homogenisation tank.

The diagram below shows how the water is purified:

The pollutant load of the waste water produced in tanker truck inside washing is highly variable, depending on the products that have been transported in the tanks and also how much they have been emptied (i.e. the amount of product that remains inside the tanker when it reaches the inside wash unit).

For this reason, a very large homogenisation tank is necessary (in order to homogenise as much as possible the quality of the waste water to be treated in the treatment plant). It also underlines the importance of the physiochemical system, which serves to pretreat and protect the following biological treatment.

The established limits set by the local municipal authorities for the parameters of the pollutant load dumped into the sewerage system are relatively low and the quality of the treated water that is dumped is acceptable when the treatment plant is working correctly.

Waste water produced in the tanker truck outside body wash facility contains practically nothing more than earth and dirt, together with a little grease, although the pollutant load is very low (except for it being very cloudy as a result of suspended matter).