RV black tank: how to dump it, and how to treat it

The two tanks, and what each one holds

• There are two waste tanks. Water from the sinks and the shower goes into the grey water tank. Toilet waste goes into the black water tank. They are separate tanks with separate drain valves, and this guide is about the black one.
• The black tank is the one that can smell and the one that can block. The grey tank holds soapy water and food particles. The black tank holds sewage and toilet paper, and it is the tank every problem in this guide belongs to.
• Some RVs have one drain valve, some have two. With one valve, the grey and black tanks release at the same time. With two, you release the black tank first and then the grey tank, so the grey water rinses the outlet and the sewer hose. There are labels on the outside of the RV showing where the drains are.

How much water goes in, and why water is the whole system

• Put water in before the tank sees any waste. Add up to 5 gallons of water to an empty black tank to help break down solids. You can pour it in through the toilet, or hold the flush pedal down. Five gallons is the limit for this first fill.
• A smaller amount goes in with the treatment. One routine is to release one to two quarts of water into the bowl, add the treatment, then flush and let at least 2 gallons of water run into the tank.
• Keep water in the bowl itself. Keep at least 2 inches of water in the toilet bowl. It lubricates the ball valve seal and acts as a vapour barrier between the bowl and the tank, so tank air does not come back up.
• Why water matters more than anything else. A black tank is designed to hold waste suspended, so that it can flow out. Low water is the biggest single cause of buildup on the tank walls. Exposed waste sticks to the surfaces instead of staying suspended, and that is what coats the sensors and starts a mound under the toilet.

The dump procedure, step by step

Do this at an approved dump station or a full hookup sewer connection. The order matters: black first, then grey.

1. Ensure the RV is level for easier drainage.
2. Remove the waste pipe cap and attach the sewer hose to the RV waste pipe.
3. Secure the other end of the sewer hose at the approved dump location, and make sure it is seated there.
4. Open the black tank dump valve, and close it when the black tank is empty.
5. Open the grey tank dump valve, and close it when the grey tank is empty. The grey water follows the black water down the same hose and rinses it.
6. Make sure the hose has drained completely into the sewer.
7. Remove and store the sewer hose, then reattach the waste pipe cap.

Dumping the black tank first and the grey second is what lets the grey water cleanse the outlet and the sewer hose. Some coaches have only one valve, which releases both tanks at once.

If the coach has a macerator pump instead of a gravity drain, the same order applies: empty the black tank first, then the grey, so the grey water rinses the pump and the hose. Watch the hose while the pump runs and do not leave the unit unattended.

Why the black valve is never left open at a full hookup

At a full hookup site the sewer connection is right there, and it is tempting to leave the black valve open so the tank drains as you use it. Do not do it. If the valve is left open, the liquid drains away and the solids are left behind with nothing to carry them out, and that is how a blockage forms in the tank. Keeping the valve closed prevents debris from accumulating.

Never leave the black tank drain open continuously when connected to a campground sewer system, and never leave the drain valves open at all times while camping, especially in cold weather. The black valve stays shut between dumps.

When to dump: let it fill, and dump before you travel

• Let the tank fill to about two thirds before you dump it. A fuller tank empties with more force, and that force is what carries the solids out. Emptying a tank that is only part full moves the liquid and can leave the solids where they are.
• Jayco's own trigger is a little higher. Its manual says that when the tank is near three quarters full, it should be flushed. The point is the same either way: dump a full tank, not a quarter full one.
• Dumping too early works against you. If the tank is dumped before it reaches about two thirds, solids may not evacuate completely, and what is left behind becomes the foundation for the next buildup.
• Dump before you travel. Dump the grey and black tanks before travelling to avoid exceeding weight limits and to avoid potential damage to suspension components.
• The tank weight is not in the cargo figure. The weight of the tank contents is not factored into the RV cargo carrying capacity, so a full black tank is weight no cargo calculation accounts for. Travelling with full tanks can push the coach past its tire ratings, its axle rating or its gross weight rating, and it affects how the coach handles.
• If you cannot dump, slow down. If you are dry camping and cannot empty the tanks straight away, reduce your speed until you reach a dumping station.

The tank flush, if your coach has one

Some coaches have a black tank flush. The inlet is usually on the utility centre panel, labelled "Tank Flush", or on the side or back of the RV with a caution label beside it. Inside, it connects to a sprayer in the black tank, and a check valve in the line prevents backflow.

• Flush the black tank every time you dump it. The sprayer knocks debris off the tank walls and keeps the sprayer openings from clogging.
• Dump the black tank first and leave the black tank drain valve open. Then connect a garden hose from a water supply to the black tank flush inlet.
• Turn the water on and flush until the water running out of the black tank drain valve is clear, not discoloured or cloudy.
• Turn the water off, disconnect the hose, close the black tank drain valve, and refit the waste pipe cap.
• Never connect a hose to the flush with the drain valve closed, and never leave a hose connected to the flush when it is not in use.
• Use a separate hose. Never use the same hose for the black tank flush that you use to fill the fresh water tank.

Chemicals: what the makers say, and what never goes in

• Use a black tank treatment, and use it after dumping. Add the treatment to prevent odours and break down waste, and follow the dose on the bottle or packet.
• Enzyme and bacteria treatments do one job; chemical treatments do another. Enzyme treatments are blends of enzymes and beneficial bacteria that break waste and toilet paper into smaller particles. They are non-toxic, naturally occurring and biodegradable. They are not deodorants, and they rely on fragrance to mask odour. Chemical treatments act faster, but the residue they leave behind kills enzymes.
• Residue from one kind stops the other from working. Remnants of a chemical-based treatment will kill the enzymes and reduce the performance of an enzyme product. If a chemical treatment has been used in the tank before, the tank has to be cleaned before an enzyme treatment will work properly.
• Dose it early. You can dose the tank before you leave on a trip to give the enzymes time to activate. They can take up to 24 hours to reach full effect.
• What never goes in. Anything that does not dissolve. Feminine hygiene products, paper towels and moist towelettes will damage a macerator and void the warranty. Flush only organic human waste and toilet tissue.
• Household toilet paper. Use only RV-grade single-ply toilet paper. Ordinary paper that resists breaking down clogs the tank and feeds the buildup.
• Undiluted chlorine or caustic chemicals such as bleach or drain opener. They damage the seals in the toilet and the dump valves.
• Automotive antifreeze and caustic chemicals such as laundry detergent. They can damage the plastic and rubber parts of the plumbing system.
• Formaldehyde-based products are a health hazard, irritating to the eyes, nose and throat, and toxic if ingested. They are best avoided.

The pyramid, and how to avoid it

A pyramid, sometimes called a mound, forms when solids drop into the tank without enough water to disperse them. Instead of mixing and breaking apart, they stack up directly under the toilet. Over time the mound dries and spreads out along the tank walls and around the sensors.

• What it causes: false sensor readings, odour, and a tank that does not dump completely.
• What feeds it: too little water per flush, and a black valve left open so the liquid drains away and leaves the solids behind.
• How to avoid it: use enough water every time, keep the black valve closed, dump when the tank is mostly full, and add water after every dump.

Sensors, and why they read wrong

• How they work. Most black tank sensors are conductivity probes mounted through the tank wall at set heights, typically one third, two thirds, and full. When liquid touches a probe it completes a circuit and the monitor panel lights up.
• Why they read full on an empty tank. When waste or paper residue coats a probe, it can falsely complete that circuit even when the tank is empty. Sensor problems are usually buildup, not broken electronics.
• The classic fault. A panel that always reads two thirds full usually has residue or paper fibres bridging the two thirds probe. There is usually no electronic reset; cleaning the buildup off the probes restores the reading.
• What cleans them. Fill the tank at least halfway with water, add a treatment that targets paper, let it sit, drive the coach if you can so the motion helps loosen the residue, and dump when the tank is mostly full. The black tank flush helps as well.
• You can also look. To check the black tank by eye, press the toilet foot pedal down fully and shine a flashlight through the waste gate opening.
• The monitor panel. It shows the fresh, grey and black tank levels and the battery. Press one switch at a time and the level lights come on for that tank. Touch screen models show the level as a percentage from 0 to full.

Winter: drain and protect the tank

• Winterize before the first freeze. Winterize at the end of the season, or whenever the coach will be exposed to temperatures at or below freezing. Damage from freezing is not covered by warranty.
• Empty the waste tanks. Make sure all waste tanks are empty, then pour RV antifreeze into the toilet and down into the black water tank.
• Use only RV antifreeze. Automotive antifreeze and windshield washer antifreeze are toxic and must never go into the fresh water system. Antifreeze must never enter the water heater, the water filter, the refrigerator or the fresh water tank.
• If the coach has a macerator, run antifreeze through it. With the tanks empty, pour RV antifreeze into the black tank and run the pump until antifreeze discharges from the nozzle, then switch the pump off.
• Tank heaters, if fitted. The heater pads cycle on at 44 degrees Fahrenheit and off at 64. Turn them on when there is liquid in the tanks and the outside temperature is 35 or colder. Turn them off when the tanks are empty, when you are dumping the tanks, when the fresh water tank and lines are empty or being drained for storage, and when the coach is connected to city sewer with the gate valves open.
• Winterize the black tank flush, if fitted. Flush and empty the tank first. Then blow the line out with a compressor set to no more than 30 PSI, with the black tank drain valve open and the waste pipe cap off, for 30 to 60 seconds. Disconnect, close the drain valve, and refit the cap.

Sources

• Jayco Towable Owner's Manual, 2027, for the two waste tanks and what each holds, the water amounts and the 2 inch bowl level, the seven step dumping procedure and levelling, the two thirds and three quarters dump trigger, the warning never to leave the black drain open at a campground sewer, the dump before travel and tank weight notes, the tank flush steps and its warnings, the treatment and single ply toilet paper and bleach advice, the monitor panel, and the winterizing steps.
• Thetford, How to Prepare Your RV Waste Tank for Enzyme Treatments, for enzymes and bacteria breaking down waste and paper, treatments not being deodorants, chemical residue killing enzymes, cleaning the tank first, and dosing up to 24 hours ahead.
• Thetford, SANICON Turbo Owner's Manual, for emptying the black water tank first so the grey water cleanses the system, not leaving the unit unattended, and flushing only organic human waste and toilet tissue.
• Walex Products, Complete Guide to RV Tank Sensors, for how the conductivity probes work and why they misread, mounding and pyramid formation, the two thirds dump rule, and adding water after every dump.
• Jayco Embark 2023, Black Grey Water Holding Tanks, for the full hookup two thirds rule, the caution never to leave the black tank drain open, and the automotive antifreeze and caustic chemical warning.