Guide to RV Wastewater: Gray vs. Black Water

First: the basics of RV waste water

It’s completely normal for you to simply flush the toilet or let water go down the sink and think of the process as complete. However, the act of flushing the toilet or draining a sink is just the beginning when you are talking about waste water in an RV. 

The waste plumbing system of your RV is usually divided into two separate systems: the black system for solid waste from toilets, and the gray system for liquid waste from sinks and showers. The black system consists of a 3-inch black PVC pipe that runs from the toilet to the holding tank. In many RVs and other travel trailers, the length of the pipe is kept very short, such that the toilet flange is glued to a threaded fitting attached directly to the tank, to minimize the opportunity for failure. These tanks are usually vented through the roof. 

RV toilets range significantly, from plastic economy models to full china-bowl electric-macerator flush models. The flush valve has two positions, one to fill the bowl with water and the second to flush it. This is accomplished with either a hand elver or a foot pedal, and once flushed, a sliding blade or ball valve will open to empty the contents of the toilet down the pipe. The toilet flange is similar to that of a fixed structure, but the seal is made of rubber instead of wax, so if the toilet is ever removed, you’ll likely need to replace the rubber seal. 

Gray systems are PVC pipes that are smaller in diameter, usually 1.5 inches, routed from each water fixture - sink, shower, dishwasher, and/or washing machine - to at least one gray holding tank. Each gray holding tank has a device to prevent gas backflow, and they are vented through smaller vents sometimes hidden in the walls or cabinets to reduce the number of roof fixtures. 

Anti-siphon trap vent devices (ASTVD) may be attached near the draining appliance to a vertical pipe. These have a neoprene or rubber flap on the inside of them that opens when there is a vacuum, though higher-end models may have spring-loaded ASTVDs. ASTVDs are usually easy to care for, but if you notice a gray water odor problem (oh yes, they happen!), it usually means they need maintenance or replacement. 

Not black or white, but somewhat reusable gray water

Gray water refers to water that generates from daily activities like washing dishes and hands, doing laundry, and taking showers in your RV. Water that runs off from the sink, shower, washing machine, and dishwasher would all fall under the banner of “gray water.” Although it’s relatively less hazardous than black water, because it does not contain human waste, gray water is still nonpotable water and can carry bacteria and other contaminants, so it should not be consumed. 

Gray water composition can vary pretty significantly, depending on what you do in your RV. Grease, food particles, soaps, detergents, and traces of other personal care products are not uncommon. Since it’s “half dirty,” you may be able to reuse it for non-potable water uses, like flushing toilets, watering plants, or cleaning the outside of your RV. 

Some RVs have more than one gray holding tank, not just for additional storage, but to accommodate the plumbing slope required by the NFPA 1192 building code. Since RVs may only have 3 or 4 inches from the top of the floor to the top of the tanks, there is sometimes not enough vertical drop to meet this requirement of at least ⅛ inch per foot. One solution is to add additional tanks to get around the chassis and add slope. Clever, huh?

Managing and disposing of gray water

To manage your gray water, start by using soap that is environmentally friendly, and try to avoid disposing of harmful substances or chemicals down the drain. This can reduce the harmful components in your gray water, which is especially important if you plan to reuse it. You should also keep a separate, designated garden-hose exclusively for using gray water, as you don’t want it to contaminate your other water supply with gray water. 

It may be useful to consider installing a gray water tank or recycling system in your RV to reuse water for non-potable purposes. These devices can reduce your overall water consumption and help you make better use of your water supply. 

Black water: possibly hazardous and definitely not reusable

Black water comes from RV toilets and sometimes from kitchen sink disposals. It’s usually significantly more unsanitary than gray water: if you don’t take proper care to prevent contamination and resultant health risks, black water can make you or others ill. Unlike gray water, which can be reused, black water should not be reused, as it contains human waste and may be hazardous. 

Managing and disposing of black water

As you might imagine, it’s essential for both personal hygiene and the environment to treat and store black water properly to minimize its potential effect. RVs with bathrooms are equipped with black water tanks that can store this waste until you can safely dispose of it at a designated dumping station. 

Black tanks can collect solids and start to smell terrible, quickly. You’ll want to use plenty of water to keep the system flushed out as well as toilet treatments, which allow the tank and nature to do their work of breaking down the waste. If you don’t use enough water in your black tank, solids can pile up in the bottom of the tank and accumulate until you aren’t actually able to complete a tank flushing, which is both a gross and expensive RV repair. 

To prevent this, make sure the tank is flushed and then add a few gallons of water along with your toilet treatment of choice before anyone uses the bathroom in your RV. It may be tempting to opt for a homemade chemical treatment, but remember that mixing the wrong chemicals in your black water tank can damage the system and create hazardous gasses, which can further damage the system or cause illness to those in proximity. Any time solids are introduced to the system, make sure to add plenty of water, especially before dumping. 

You may also consider adding a macerator, which is designed to pump effluent uphill to a degree and can attach directly to the sewer-hose connection. These are increasingly popular in RVs, as they help solids break down more quickly and move things through the system, preventing clogs and associated repair costs. 

In short, as long as you handle your waste water appropriately and frequently, it should not be an issue for your RV travels. Remember to never drink either black or gray water, never reuse your black water, and do your best to minimize the impact of your gray water to make the most of your RV experience. 

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