Apropos has a 20 gallon stainless steel holding tank (black water tank). An electric toilet flushes into it, and a drain at the bottom leads to a macerator pump for emptying it. A port at the top side is for a vent. Ventilation is very important for holding tanks and is needed for turning anaerobic bacteria (smelly) into aerobic bacteria (not smelly). A 3/8″ hose connects to the existing vent port and runs about 20 feet to a thru hull fitting at the bow (port side). This location was chosen to be at the highest part of the hull to keep sea water from going in. The problem was that the vent size was too small, which limits the amount of air reaching the tank.

The improvement involved adding another vent with a larger port and a larger hose. This should more than double the amount of oxygen reaching the holding tank. An additional task was to add an 8″ port to the top of the tank, useful for inspections and cleaning. I also removed the tank monitor that’s mounted on the top of the tank and gave it a good cleaning.

I used a hole saw to make the hole for the vent, and a jigsaw to cut the hole for the inspection port. Drilling and cutting into stainless steel requires special carbide tipped drill bits and jigsaw blades and a lot of patience (it took over an hour to cut the hole for the 8″ port). I ran the 5/8″ vent hose to the same location as the original vent, except I put the thru hull fitting on the starboard side so now the tank is vented to both sides of the boat.