It’s probably not something you want to think much about, but much more than often (and we thought!), we get asked how exactly an RV toilet works. Sure, it seems pretty self-explanatory (you go, you push the lever thingy, you forget about it!), but the science behind the typical RV toilet is pretty interesting. Okay, maybe “interesting” is a stretch. But it’s at least good to know you your RV’s systems work.

Your toilet at home
A home plumbing system is made up of three basic components: a water supply system, appliances (primarily, a toilet) and fixtures, and a drain system. Obviously, pressurized fresh — or potable — water enters your home through the main water supply line connected to a municipal water company or to your well if a local water supply is not available. An American average family of four, by the way, uses about 400 gallons of water per day (on all sorts of water needs — not just the toilet!).

Once in your home, the water supply tends to split off and a pile runs to the hot water heater (some homes, like larger RVs, have on-demand water heating systems). From the hot water heater, a hot water line parallels the cold water supply line to showers, bathtubs, clothes washers, dishwashers, and sinks. Toilet and outside faucets are examples of fixtures that only require a cold water supply. The water supply to faucets and appliances are controlled with faucets and valves.

Wastewater — any water used within the home that is not consumed — enters the drain system to be removed from your home. Wastewater must first flow through a trap, a U-shaped pile that holds standing water and prevents sewer gases from entering your home. If not used for a lengthy period of time, these traps can become dry and sewer odors can enter your home. You’ll know by the distinctive smell.

The drain system within your home works entirely by gravity. Wastewater flows downhill through a series of large diameter pipes. These drain pipes connect to a vent system that brings fresh air to the drain pipes, preventing suction that would either stop or slow the free flow of wastewater. Vent pipes exit your home through one or more roof vents. You must take care to ensure that these roof vents don’t become clogged.

All wastewater flows to the main waste and vent stack. The main stack curves to become a sewer line that exits the house near the foundation. In a municipal system, this sewer line runs to a collector sewer line in the street or in the rear of your home’s lot. Where sewer service is not available, the lines run to a septic system, which is, essentially, a mini wastewater treatment facility.

Your home’s toilet is a rather ingenious contraption. When the handle is pushed, the lift lever raises a rubber seal — called a flapper or tank ball — in the water holding tank. Water in the tank dishes down through the flush valve opening in the bottom of the holding tank into the toilet bowl. Wastewater in the bowl is forced through the trap into the main drain.

When the toilet tank is empty, the flapper seals the holding tank and a water supply valve, called the ballcock, refills the holding tank. The ballcock is controlled by a float ball that rides on the surface of the water. When the holding tank is full, the float ball automatically shuts off the ballcock, and thus the flow of water into the toilet’s holding tank.

Your RV’s toilet
The toilet plumbing system in your RV has some similarities the one in your home, but also some differences. Your RV also has a water supply system, either from a shore (RV park or site) supply or your RV’s fresh water holding tank. Shore-supply water is typically pressurized (the park or resort gets it from a local municipality or their own well) while your RV has a special pump to pressurize the water system if its onboard fresh water holding tank supply is used. In this case, the pump will run to “charge up” a certain pressure in your RV’s water supply system and then run again when that pressure is depleted.

Like your home, your RV has a water heater to provide hot water, although many larger and newer RVs have on-demand systems that heat water on an as-needed basis. Your RV’s toilets only have a cold water supply line, like those in your house.

Wastewater in an RV is treated completely differently than in your house. For one thing, almost every RV has holding tanks, one black for anything that goes through your toilet(s) and one grey for everything else. Some larger RVs have more than one black and/or grey holding tanks. The most we have seen has been five — two black tanks and three grey tanks (that was a loooooong cleaning job!).

Your RV’s holding tanks are simply large, rectangular boxes that are designed for one thing: to hold waste and wastewater until it is time to empty them. They are not intended to be any sort of septic system so there is no need to add enzymes, caustic chemicals or anything else that will break down waste. In fact, doing so will only result in your holding tank — especially your black one, where most RV owners put enzymes and chemicals — containing a thick, sticky slurry of waste sludge. That’s what is getting on your holding tank’s sensors, sticking to the sides and top of your holding tanks, and generally slowing down the entire system, if not actually promoting the possibility of clogs and waste mounds. Bad stuff those enzymes and chemicals.

Almost no RV toilet has a trap. In fact, most have a straight shot into the black water holding tank, although some — especially fifth wheel trailers — can have some bends in the pipe between the toilet and holding tank. These can be potential problems as things like wads of toilet paper and especially large “deposits” of waste can get stuck in the pipes. It’s time to call the professionals in those cases!

Most RV toilets have a simple lever to one side (almost always the right) where fully pressing it will result in a small amount of water jets out, mostly to clean the bowl. Some RV toilets also have a sprayer nozzle nearby for rinsing. So it’s gravity and a small amount of water that aids in flushing, instead of the volume of water that sits in the bowl of a home toilet.

As you know, waste in the holding tanks — whether the black tank or grey — should be emptied regularly, but always when the holding tanks are three-fourths to completely full. In fact, we recommend to our customers that they fill up all of their holding tanks with fresh water (a large bucket works well here) before each dumping. That way, there is extra water in each tank (always a good thing) and a completely full tank will empty with enough down pressure to create a swirly tornado-like whirlpool of water that will (pretty) thoroughly clean out the holding tank.