After delivery, who is responsible for cleaning the tanker out?

Some places require once its cleaned/purged to have seals in place on the tank from tankwash before they will load it...

 

I remember delivering chemicals to a Brunswick bowling equipment factory. My dispatcher reminded me to rinse the hose and pump before leaving the consignee. The procedure was to pour pails of solvent into the tank trailer and pump the solvent mix into drums provided by the consignee. Of course, those drums were already full. What a mess. I still think of that location as a super-fund site if the plant ever closes.