Always keep your black tank valve closed, even when hooked up to a sewer connection. It is critical to have a decent amount of liquid in the black tank when you open the valve to empty it out. If you make the mistake of keeping the valve open, you will develop the dreaded poop pyramid that has plagued many new RVers. The pyramid forms because all the liquid waste drains out, leaving the solids behind. With no liquids and chemicals sloshing around in the tank to dissolve the solids, clogs and smells will develop. Only open the black tank valve when emptying the black tank into a ground sewer connection. Then immediately place fresh water and chemicals into the black tank.