It’s the eternal question all RVers seem to face. So why is deciding which tank additive such a hard decision? You may be surprised by what we think.

As RVers, no one likes to bring up the topic of waste. Except us! Odd as it may sound, we’ve become a bit of “experts” in the subject of poop and a lot of our customers seem to actually like to talk about it.

Time after time, we get questions about tank additives. Are they worth it? Which one to use? How often should we use them? Liquid, powders or pods? What about the homemade stuff? Does the GEO method really work?

Today, we’re going to give you three reasons why RV holding tank additives are a bad idea. Period.

And then we’ll give you three things you should do instead that will make all of your tank additive questions go away.

So why do we use tank additives? The number one reason is probably that we are told we should. Your dealer probably recommended it and may have even given you samples when you bought your RV. Various product suppliers run advertisements and even have booths with products at rallies. And we keep asking each other, “Hey, what tank additive do you use?”

Well, we’re here to say, “Stop it!” Here are three reasons why:

Potential health risks

Some tank additives feature formaldehyde, what the EPS says is “a probable carcinogen; moderately toxic to humans.” OSHA adds that formaldehyde causes known health hazards when touched, swallowed, or breathed. In 2004, the National Cancer Institute reports the International Agency for Research on Cancer (IARC) reevaluated the existing data that formaldehyde is a probable human carcinogen and reclassified it as a known human carcinogen

So, formaldehyde is bad stuff.

Other additives have enzymes or live bacteria which break down the waste in holding tanks. These are not inherently toxic or especially harmful, but the chemicals added to these additives for preservation, color, or odor control are potentially dangerous if ingested or even smelled.

Harmful to the environment

We tend to dump our RV’s holding tanks and forget about whatever comes out of them when we’re done, right? Any “problems” become someone else’s — the RV park, the local wastewater processing plant, whatever.

But using tank additives is very likely causing environmental hazards. These chemicals and their by-products may pass through onsite wastewater treatment systems, flowing to the soil, groundwater, and possibly nearby surface waters. They may also corrode treatment system parts, creating a safety hazard.

When chemicals, such as formaldehyde, are added to septic systems, they can cause bacteria in the system to die. When this happens, the septic system cannot treat waste adequately. Solids that are allowed to pass from the septic tank, due to inadequate or incomplete treatment, may clog the RV park’s or local wastewater processing plant’s leach field.

Formaldehyde does not control odors by killing the microorganisms that generate them, as is popularly believed. Instead, it reacts chemically with odor-causing compounds to render them less odorous. However, the organic strength of most formaldehyde-based deodorizers is so high that the resulting mixture in a holding tank is fifteen to twenty times stronger than typical domestic wastewater, even after several days residence in the tank.

Furthermore, these treatment systems may send inadequately or incompletely treated sewage to the surface, threatening the health of people or pets who come into contact with it. Or it may percolate to groundwater, where the chemicals and untreated wastewater could contaminate nearby drinking water wells, rivers, and streams.

When we dump our RV’s tanks, very little “carriage” water is used, as compared to typical domestic sanitary waste streams. Holding tank wastewater is far more concentrated than typical domestic wastewater. Highly concentrated holding tank wastes, containing variable amounts and types of chemical additives, make the proper design of treatment facilities difficult.

You’re only causing more problems

Many RVers think adding chemicals or enzymes to their holding tanks to break down whatever is in there is a good thing. After all, if the resulting mixture is more liquid than solid, then theoretically the tank should be easier to empty.

But what you’re actually doing is helping those solids to break down into a sticky, pancake batter-like mess. And that thick slurry is getting stuck on your holding tank’s surfaces, making your tank sensors fouled up, and generally making a big mess.

By using tank additives, you’re actually making the “health” of your holding tanks much, much worse.

So, since additives have potential health risks, are harmful to the environment and are causing problems within your holding tanks, what can you do instead? Easy. Here are three things to do immediately:

Stop using tank additives

Your tanks don’t need them. Instead of adding chemicals and enzymes to your holding tanks so the solids are broken down and odors masked, simply empty your tanks more often.

Instead of waiting until your holding tanks are absolutely full, empty them every other day. After all, your holding tanks are just that — tanks that hold something (in this case, waste and water). They are not septic systems and were never intended to hold, break down, and process waste.

Use more water

Unless you’re boondocking or have restricted water availability, you doubtless have access to unlimited water and unlimited ability to dump your tanks.

So use more water when you’re using your toilet. It’s a good idea to follow every “number two” with a full bowl of water. And we all know to fill the toilet bowl at least halfway before using it.

When you’re dumping your tanks, add more water to them by either using your black tank flush system or by directly dumping water into the toilet bowl while keeping the stopper valve open. If your tank sensors aren’t reading properly (hey, we know a company that can help you with that!), watch or listen down the toilet bowl to see or hear the liquid in the tank as it approaches the bottom of the toilet.

Remember, a tank that is 90%-100% will generate the “tornadic” action (a cleaning whirlpool) that will help pull solids out of your holding tanks.

Finally, when you are dumping your black tank, keep your onboard flush system engaged. The spray will help to keep things moving along and adding more water to the whole process is overall a good thing.

Flush it, baby

Almost every modern RV or motorcoach has some sort of onboard, in-tank flushing system in their black wastewater holding tank. Most times, it’s a stationary or spinning nozzle that sprays out water (through a reduction fitting) at 100-150 PSI.

It’s a great system to add water to the tank and “wash down” the tank’s surfaces, but it’s not perfect. Over time, the nozzles can become corroded or plugged and the amount of water and pressure they emit will diminish. Still, they are better than nothing and should be used as instructed (as well as we recommend in the point, above, while emptying your black tank).

A better solution is to use an external backflush valve, like the Valterra Flush King. This is a specially designed valve with a gate on one end, a connection to your RV’s main sewer pipe on the other, and a fitting for a garden-type hose in between.

The process with this is, after you have emptied your tanks (you can — and should — use it on both black and grey tanks), you close the gate valve leading to your long sewer hose (“stinky slinky,” as we say in the business), open the tank pull for one tank at a time (keeping the other closed), and then open the garden hose knob, letting the holding tank flood full of water.

At most RV park’s water supply pressure (40-60 PSI), the typical holding tank will fill in four to seven minutes (depends on the water pressure and size of your tank).

When the tank is full (either by reading your tank sensor or looking/listening down the toilet, shower/tub, galley sink, etc.), you turn the water off and open the gate valve to the sewer hose. The resulting rush of water — that “tornadic action” of swirling water — will pull out debris from your holding tank and waste system.

We recommend using a backflush process like this about once every two or three tank empties and to back flush the black tank two to four times each time and the grey tank(s) once or twice.


So we’ve given you three reasons why using tank additives is a bad idea and three things you can do to make your holding tanks work better, smell nicer, and be better for you and the environment.

Now, no more questions about tank additives. Just kidding, we get them all the time!