Proper Care and Feeding of the Waste Water Tanks

There are many hotly debated issues in the RV world, but this is probably one of the most heated. It seems that every product on the market claims to do things that no other product can, and each has it's following. I will try to address the problems of your tanks here and also to explain what we do and why. Like most things in life, this is not a simple thing with only one correct answer.

The wet compartment of our Cruise Master motorhome.

Most RVs have two waste water holding tanks, but a few have three. The reason is that waste water comes in two types. The term "black water" means the mix of water and human waste that comes from the toilet. It is kept in a separate tank because the solids and the paper that go into it must be dealt with in a different way than other waste and because some areas still allow the dumping of gray water on the ground. The term gray water is all other waste water from your RV. This tank(or tanks) collects the drainage from the sink(s), shower and the kitchen. A very few RVs have a garbage disposal but they are still rare and I have no experience with one. In a few RVs there are two gray water tanks to save on long runs of drain plumbing. There are also a few cases where the bathroom sink may drain into the black tank. As a rule most owners will put much more water into the gray water tank than into the black tank. For that reason, the RV industry is beginning to put larger gray tanks than black, but the many still use two tanks of the same size. If I were designing an RV, I would make the gray tank about twice the size of the black tank. Just what size you need will depend upon the number of people using the RV, the size of the fresh water tank and the ways you conserve water. It is not possible to predict just what each owner will need because the water use is so very different. We used to travel in a motorhome that had only 40 gallons of water and we we could make that last for five days. Today we have 80 gallons of water and we generally can make that last about five days. We just don't conserve as much as we once did. As a rule, the gray water tank should be 2/3 the size of the fresh water tank since most of your used water ends up there and black 1/3. Usually the total of the waste tanks will be about the same as the fresh water supply and that seems to be a pretty good design.
Toilet Paper
This is a hot topic in the RV world and it can be a cost issue if you always use the RV type of paper. The cost is much greater and the need is frequently debated. Like most full-time RVers, we have come to a point that we no longer go to the expense of buying the RV products. We have found that if you always buy TP that is marked as "septic sase" then it will be fine. There are melt tests that many advocate using to put a tissue into a jar of water and see how long it takes to dissolve.  This test is OK, but it really isn't the whole story since in the tank there is either bacteria or chemicals that speed the process. It is interesting, but really doesn't prove much. Scott tissue(septic safe) is by far the most popular choice in the full-time community and it is what we use. The reason is that it is readily available and it is one of the least expensive. Another thing is that single ply papers do break down most quickly.
Among the most debated RV issues is the use of chemicals in the tanks. Most owners seldom use any treatment in the gray water, even though there are chemicals that are suggested for both tanks and others that make one for each tank. There are no solids in the gray water but it does have things like soap and grease in it. There will be some build-up on the walls of the tank and that probably is not inhibited by use of most chemical products. For the black water, I would suspect that the vast majority of RV owners do use one of the products that are on the market. Products that contain formaldehyde are discouraged because of damage to the septic systems of cities and RV parks as well as the environment. We avoid them because we do not know just what is true and there are plenty of alternatives. Another type of product uses an alcohol base. It works by killing the natural bacteria, just as  formaldehyde does. It then liquefies the solids by chemical action and perfumes the results to keep odors down. The third type of product is bacteria based and it is the one that we choose when we use anything. There are two types of naturally occurring bacteria in waste, aerobic and anaerobic. What makes the septic systems work is the aerobic bacteria and the odor is created by the anaerobic bacteria. The bacterial products work by enhancing the growth of aerobic and destroying the anaerobic bacteria. They do not inhibit the natural process, but speed it and they also add some odor control. They are clearly not harmful to septic systems.
What we do
Like most full-time RV folks, we started out using various products. We soon moved to the bacterial products and we have continued to do that for more than ten years now. We have found that we very seldom need to add anything to our tanks as long as we are generous with our use of flushing water. If the weather is really hot, we do add a small amount of a bacterial product, whatever is least expensive. But we do that very seldom and never in cool weather. We always allow the black tank to reach at least 1/2 full before we dump the tank and if we dump early we put in water to bring it to that level. That is done to allow the rush of liquid from the tank to carry out any remaining solids that have not become liquid. When we sit still, we leave the gray water dump valve open, closing it for a few days to fill up about once each month to flush it out. A family might find that some additive is needed for the black tank because they may fill it too quickly to allow the natural process to complete the liquefying process but foe a couple it will usually be at least a week between dumpings of the black tank which is enough time for the natural process to do the job. Use generous amounts of water in the black tank no matter what product you choose to use, or not use. Water is the universal solvent and it is critical to the process.If you use enough water the other issues become much less important.
Cleaning Tanks
Keeping the waste tanks clean is another issue of debate. There are many different theories of what is best, and since I have not used all of them, I will only comment on what we have found to work. In a seminar by a Thetford representative, it was stated that the very best cleaning agent for the inside of waste tanks is the product Trisodium phosphate, otherwise known as TSP. It can be found in home supply stores in the wallpaper and paint department or from Amazon. We have found that a pretty good substitute for that is a good dose of laundry detergent. Do not use dish detergent because it will suds and you may have bubbles that take some time to go away. We try and use TSP about once each year and laundry detergent every few months. What we do is to mix enough for a strong solution of either product for about 1/3 the volume of your waste tank into a gallon or two of water and then empty that into each waste tank. Next we fill the tank to the 1/3 or so level and then travel to our destination. It is even permissible to use the waste tank if need be, one or two times in route. Once we arrive, we dump the tanks as soon as we get there, and then use as normal again. I do this for the gray tank as well since we have found that very often the gray water will actually smell worse than does the black. My chemist daughter-in-law tells me that it is because the gray does not have the natural bacteria of the black tank and thus tends to be anaerobic.
I am also a believer in the use of tank flushing systems, especially for the black tank. There are several quality flushing products which can be installed into the tanks but it is best if done while the tanks are new and clean. If you do not have one of these, there is also a wand which can be used on the end of a water hose to spray through the open toilet valve that is very effective and some also use flush devices that mount on the sewer hose connection, but I have found those to be the choice of last resort. I have one, but remain unimpressed.

Old wives tales and RV folk lore
The RV world is just as filled with folk lore and myths as is any other part of society. Some of the most common ones relate to the care and use of waste tanks. It used to be quite common for folks to suggest the use of Pine Sol but in time it got around that such products would damage the seals in the system and it has since pretty much gone away. Probably the currently most popular one is what is called the "Geo Method." It is likely popular mostly due to the effort of it's inventor who spends great effort in promoting it. It says to add Calgon© water softener, laundry detergent and liquid chlorine bleach to your tanks. Having looked at it pretty closely I can only say that the inventor lists no real credentials for his knowledge and I have discussed it with an environmental chemist (my daughter-in-law), a waste water treatment operator who visited some of the RV forums and with an engineer who is senior partner in a company that designs and installs waste water and septic systems for large companies, cities and larger home developments. All three had grave doubts about this "system." If you visit enough RV gatherings you are bound to meet folks who use one or more of these home brews and who will insist that they do work better than anything else!
I suggest you consider this before you choose what treatment you will use. Visualize what it means if one day you open the valve to dump your black water tank and nothing comes out! I suppose that you know about using a "snake" or similar tool to open up a plugged drain. But when you do that, you are above the point of the plug and when it opens the water goes down the drain, away from you. Now consider the problem in your black tank. Since the tank is between your toilet and the drain lines, it is very unlikely that you can get this device to go down into the drain line to through the toilet. That means that to clear the problem you must remove the drain hose from your sewer connection to get access to it. Next, open the offending dump valve. Now you need to run the device up the drain line toward the full tank. Consider just what is going to happen to announce that you have succeeded in clearing the line. Everything will come out with a rush and it is going to be difficult to miss! Many RV shops will not work on this type of problem so................. I think that this might be what is called a "learning experience!"
After many years of RV experience there is one thing that I can tell anyone with confidence about waste tanks. If you use water generously enough in you waste tanks you will probably never experience plugged drains and as long as all parts of the plumbing are working properly you will have little odor problem. Waste tanks are amazingly forgiving if you use enough water.
If you have the bathroom exhaust fan running when you flush that will usually draw sewer gas from the tank into the RV and a leaking plumbing vent on the sink drain can bring in just a bad an odor from the gray tank.

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