A treatment vessel for separating contaminants from a base liquid is provided. The vessel comprises a first cylindrical portion, a frusto-conical portion, a second cylindrical portion and a conical portion, the conical portion having a bottom outlet arranged to discharge heavy contaminants from the vessel. Over time, the heavy contaminants settle on the sides of the conical and frusto-conical portions. In order to remove the settled contaminants, the vessel is provided with at least one clarified liquid inlet disposed at or proximate to the frusto-conical portion. The clarified liquid is sourced from either the treatment vessel itself, or another treatment vessel in the same system. The clarified liquid is reintroduced into the vessel in a manner so as to cause a downward circular flow to scour out the frusto-conical portion and the conical portion so as to assist in removal of heavy contaminants that settle on the sides thereof.