21. (1) No ballast or rubbish, and no other thing likely to form a bank or shoal or to be detrimental to navigation, shall, without lawful excuse, be cast or thrown into any such port or into or upon any place on shore from which the same is liable to be washed into any such port, either by ordinary or high tides, or by storms or land-floods and no oil or water mixed with oil shall be discharged in or into any such port, to which any rules made under clause (ee) of sub-section (1) of section 6 apply, otherwise than in accordance with such rules. (2) Any person who by himself or another so casts or throws any ballast or rubbish or any such other thing or so discharges any oil or water mixed with oil, and the master of any vessel from which the same is so cast, thrown or discharge, shall be punishable with fine which may extend to five hundred taka, and shall pay any reasonable expenses which may be incurred in removing the same. (3) If, after receiving notice from the conservator of the port to desist from so casting or throwing any ballast or rubbish or such other thing or from so discharging any oil or water mixed with oil, any master continues so to cast, throw or discharge the same, he shall also be liable to simple imprisonment for a term which may extend to two months. (4) Nothing in this section applies to any case in which the ballast or rubbish or such other thing is cast or thrown into, or the oil or water mixed with oil is discharged in or into, any such port with the consent in writing of the conservator, or within any limits within which such act may be authorized by the Government.