Industrial wastewater 3

Biological effluent purification of petrol stations

1. General.
Many petrol stations have toilet facilities for their customers. The pollution level of the resulting effluent generally mounts up to several tenths of pollution units (P.E.). This level of pollution is too high for the effluent to be evacuated into the soil or into the groundwater without extensive treatment. In many situations where no adequate sewerage is available a small-scale purification plant should be installed, which does not only performs the mechanical but also the biological purification of the effluent. Generally the staff working at the petrol station will not have the adequate expertise with regard to this process.
Therefore, the purification equipment will have to meet the high demands of impeccable operational safety. The "BIO-CLEAN "equipment meets these criteria as found in the practical case study now completed.
This practical study was performed by one of the biggest environmental engineering company in the Netherlands, and focused on several types of small-scale purification plants varying between 5 and 750 P.E. The equipment of the "BIO-CLEAN" units were in this study. Apart from the above mentioned advantages, the application of biorotors also proved to be financially beneficial ( including supervision and maintenance).
The use of "BIO-CLEAN" biorotors offers the adequate solution at an economical price.
Hereinafter the working of the units will be discussed on the basis of the data available on petrol stations along the motorways in outlying areas.
2. Composition of the effluent.
The calculations are based on the following assumption for raw effluent: Supply avg. 5 m3/day. Evacuation between 06h00 and 23h00 plus peak-evacuation.

B.O.D. 5 - content

270 mg/ltr


660 mg/ltr


320 mg/ltr

P. Tot.

20 mg/ltr


170 mg/ltr

This is based on the data of an average petrol station by a national study of a big engineering company. For stations with annex restaurant and to select the adequate purification unit, buffer possibilities and installation capacity is to be calculated according to the water consumption.
3a. Buffer tank.
The concrete buffer tank consists of four compartments, separated from one another by specially designed partitions allowing solid parts to settle and floating film ( including oil) to be retained in the tank. Given a total content of 25 and 31m3, the patented operation method allows for 20 to 25 m3 respectively to be used as a variable buffer to cope with large differences of supply, and without causing turbulence in the first compartment where most of the materials are settling. The settled sludge will be mineralized by means of an anaerobic decomposition process into an odorless end produce which can be marketed as manure. The buffer can absorb the supply of effluent of several days. The performance of the buffer tank for the biologically decomposable components B.O.D., C.O.D. and N-Kj is around 30%.
3b. Biorotor.
An immersion pump is attached in the last compartment of the buffer tank, which is driven by a floating switch of the biorotor. The effluent is pumped into a dose measuring cistern preceding the biorotor. By means of specially designed dose measuring bins, mounted on the rotating axis, a constant and equal quantity of effluent is dosed in the first rotor compartment where the larger portion of B.O.D. is decomposed by micro-organisms attached to the gasket in the rotating basket. Within a short time they absorb the residue and transform it into new bacteriological material. Not all B.O.D.'s are decomposition will take place in the second stage again lowering the B.O.D. pollution level. After the B.O.D. have disappeared another process, called nitrification, takes place in the third rotor. Nitrating bacteria transform nitrogen into nitrite and subsequently into nitrate. This process takes place in the fourth rotor.

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