Deballasting water

Depending on the size of the oil tanker, deballasting water accounts for 25 to 33% of the vessel s effective capacity. It is estimated that the amount of HC contained represents about 0.4% of the transported product, i.e. a potential concentration of 1% of HC in the water. 

If the water is drawn off and the oil gathered in the buffer tank with the right planning and execution in the terminal, the water for treatment after the storage tank has the following characteristics  

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In deballasting water, the COD is chiefly due to HC but measurement is made more difficult by the strong presence of Cl". 

These figures should be compared to French requirements (see Section 2.1)on discharge volumes (excluding rainwater and deballasting water). 

Except for rainwater and deballasting water, the amount must be lower than ... 

Deballasting water which has stayed a few days in its tank. 

Tire safety role prevails with relatively nonoily but cold deballasting water. To cope with the odd, unexpected slug of oil, simple oil-removing wells offering a residence time of 12 to 20 min at a limited horizontal flow rate of 200 m-h can be built (what is termed the API Special-purpose Separator). 

All waste water except for deballasting water is collected in a single network and undergoes complete physicochemical and biological treatment (see Fig. 61). 

Deballasting water, very low flow rate, after storage in a 2000 tank. 

Some oil industry water is very saline (petrochemical plants, deballasting). It may be necessary either to dilute the sample or to rinse the cake more generously with distilled or demineralized water. 

Buffer storage tank for high hut sporadic flow rates, such as horn storms or oil tanker deballasting. If no economically feasible facility can be built to deal with them directly, they must be stored in closed (deballasting) or open gravity-flow basins (storms). The basins must then allow the water to be drawn off in volumes spread out over several hours or days so it can be dealt with in the purification facility. 

Oil separation therefore usually works without reagents. It is designed to equalize residual HC levels at the physicochemical purification inlet by reducing peak HC arrivals, whereas before it often used to be the only purification step and had to be sized up considerably. Depending on how fine the existing emulsions are, the resulting levels of insoluble HC may in fact vary from 20 to 150-200 mgd (in petrochemical plants). In principle, they can not be anticipated or calculated. A certain approach can, however, be acquired in the case of simple effluents (deballasting, produced water). 

Residual concentrations of 20 to 50 mg " are put forward when the separators treat so-called "easy" effluents such as deballasting or produced water. It generally constitutes unstable mechanical emulsions that are more readily treatable chan some refinery WW, or petrochemical WW in particular. It should also be pointed out that the potential efficiency is seldom given in conelation with the HC measurement method (see page 80). The main point to remember is how well these separators work on a lot of hot process water and produced water and that they are suitable for low flow rates. 

Two French refineries have successfully applied two quite different schemes. In both cases, deballasting and desalting water was excluded (see Chapter 6). 

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