To prevent pollution by noxious liquid substances from chemical tankers, effluent from tank washing operation shall be discharged to a reception facility at least until the concentration of the substance in the effluent has fallen to the residual concentration specified for that substance. At the tank washing, the concentration of the residue in the washing water discharged from the tank is manily affected by the amount of residue and the amount of remaining water in tank during the tank washing.
The residues in the tank are the residue in the tank well of the tank's sucton point and the residue which clinges to the tank wall surfaces. Now, by using stripping pipe or straw pipe, the residue in the tank well is decreased to about one litre or so much. So the most of residue is almost clingage residue on the tank wall surfaces. It is important to estimate the amount of the clingage residue on the tank wall surface. It depends on the viscosity of the liquid. The viscosity is affected much by the temperature. Then the amount of the clingage residue is examined through a year by using a model tank.
Concerning the remaining water quantity in the tank during washing, it can by decreased by using a small diameter nozzle washing machine. Therefore the washing effect by the small diameter nozzle washing machine is studied.

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