This is Part 6 of Dave Solberg’s “Everything you need to know about RVing” series. Today, Dave looks at RV sewage systems and tells you just about everything you need to know about them. This is useful whether you’re going on your first or 101st RV trip.
An understanding of your RV’s drainage and sewage system is important for regulating smells, proper waste treatment, and disposal.
There are two waste water holding tanks in most RVs: the black tank, which is for sewage from the toilet, and the gray water tank, which is cleaner waste from the shower and sinks. However, drain water from all sinks does not always go to the gray water tank. Depending on the RV’s floor plan and the location of the sink, some will drain into the black water tank.
Measuring tank levels
These tanks have a level monitoring system that shows the level inside on a monitor panel. For years, probes were inserted into the side of the tanks and as the fluid level would rise, the fluid would create a closed circuit, which means 12-volt power crosses over to the designated probe and lights up the 1/3, 2/3, and full lights.
A challenge that almost every RVer faces at some point is inaccurate monitor panel readings. When the tank is emptied, sludge, toilet paper, and other deposits can still cling to the side of the tank and create continuity or a closed circuit and show false readings. It is important to use the proper treatment, periodic maintenance, and proper dump procedure. More on that later.
Emptying the RV’s sewage system tanks
The tanks are emptied by connecting a dump hose to the bayonet at the valves in a service compartment or underneath in trailers. The larger pipe (3”) is the black tank and the smaller (1.5”) is the gray water valve.
Most RVs come with a very basic or cheap dump hose that will be a headache when dumping the black and gray water tanks. I recommend getting a reinforced hose that has a clear elbow with a universal adapter at the end. My favorite is the Thetford Titan as it has an uncrushable hose, and heavy-duty connections on each end. You can find a kit here on Amazon.
You might also need an extension as sometimes it is not possible to get close enough to the dump station for a 15’ hose.
Tip: Keep the valves closed even if you have a dump station at your campsite. Smells and sewer flies can come from the underground system into your holding tanks and inside the rig. You will want the waste to accumulate and allow the treatment to digest the waste. Leaving the black water tank valve open allows the fluid to drain but not the solids, which creates an effect called pyramiding. It is recommended to let the waste get up over half full.
Proper dumping
Most first-time owners simply hook up the hose and pull the lever. You will hear advice such as, “Dump the black water tank first, then open the gray water valve as it is cleaner water and will clean the hose”. However, if you want to keep the tank and probes clean, you will need to take a little extra time with the black tank.
Dump the black tank first and wait until there is nothing coming through the clear portion of the adapter. Fill the black water tank back up to at least 2/3 full. A black water flush valve is the best option for this as it is mounted to the side of the tank. Hooking up a garden hose not only helps fill the tank but cleans the sides of the tank and probes. If your unit does not have one as a standard feature, you can add an aftermarket valve if you have access to the side of the tank. It is a female 3/4” connection either in the service compartment or on the side of the rig like this model on a 5th wheel.
Be careful to monitor the level of the tank as you do not want to overfill the black tank and create a backflow at the toilet!
If you do not have a black tank flush valve, you can fill the tank and clean the sides using a flush wand through the toilet. It’s not as handy and you will need to get a hose into the rig, which can be done through the bathroom window. You can find this on Amazon here.
Another option is to use the Valterra Hydroflush, that is in the below photo. It is a clear 45-degree anti-siphon adapter with a 3/4” connection for a hose. Get one on Amazon here.
You will be amazed at how dirty the waste water is at the second dumping. I even recommend filling a third time if time permits (dump station etiquette is important if there are other RVers waiting).
After dumping the black tank two or three times, open the gray water valve to drain that tank. This only requires a single dump.
Toilet paper, tank treatments, and more for RV sewage systems
Next, you want to use the proper treatment and even toilet paper for the black water tank (sewage). An enzyme-based treatment will create an aerobic digestion of the waste and toilet paper, which will reduce the rotten egg smell, break down the waste faster, and reduce improper monitor panel readings. Formaldehyde-based products and common household cleaners such as pine oil-based products will create anaerobic digestion and are not designed for proper sanitation treatment even though they do mask the smell initially.
My recommendation is the Thetford AquaMax product, as I have used it and various earlier products for the past 30 years. You can find it on Amazon here.
Unlike the sewer system in your home, toilet paper in an RV will sit in the black water tank. It needs to dissolve quickly so it will not cling to the side of the tank and create a false reading of the monitor panel. Not all TP is good for use in an RV, so we conducted a test of the top five brands. Once again, Thetford came out on top, dissolving the fastest of all the brands tested. You can find it on Amazon here.
This is a little more expensive than others because it is 2-ply, so it is soft and thick!
The spade valves of your black and gray water system have rubber seals that can dry up and crack when left dry for a period of time, such as when your RV is in storage or sitting unused. It is a good idea to lubricate them occasionally with Thetford Drain Valve Lubricant by simply dumping some of the product down the toilet and shower. You can find it on Amazon here.
I like spraying the valve rods with CRC Dry Lube Silicone during the dump process just to keep them lubricated.
When dumping your black and gray water tanks, it is recommended that you use thick nitrile gloves, and disinfectant wipes, and to keep everything in a plastic container away from any fresh water hoses and connections.
I also like to use a spray bottle of bleach and water mixture to sanitize everything. Use 1/8 cup bleach to 1 gallon of water and use it to spray the connections and hands, and dump the rest down the hose when finished.
Tank Blaster can be used periodically to clean the black water tank and Thetford’s Grey Water Tank Odor Control Cleaner and Sanitizer helps get rid of soap scum and hard water odors.
Tip: A cap of Calgon in the black and gray holding tank occasionally will help soften the hard water and make cleanup easier. Also, spray CRC lubrication on the dump valve rods to help keep them clean and lubricated.
Did you miss the first 5 installments of this series? Here they are.
- Part 1: Tips to make your first RV trip more enjoyable: Pre-trip preparation
- Part 2: The ABCs of RV terminology: Every word and term you should know as an RVer
- Part 3: Powering your RV: Essential 120-volt electricity tips and gadgets for every RVer
- Part 4: Understanding everything about 12-volt house batteries
- Part 5: Your RV’s fresh water system: What you should know
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.
Read more from Dave here.
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Some additional comment: There is a big difference between dumping an RV that has been stationary (e.g., seasonal stay) vs. frequently on the road. That difference is tank agitation. Read the fine print. Most additives recommend agitation. I’m not an engineer, but sure wish my black tank on my seasonal rig came with an attached ultrasonic transducer like the little jewelry cleaning gadgets. About every 5-6th dump, I have to use a flex shaft wand with a high pressure spinning head to actually clean the sensors.
Second comment: my built-in black tank flush does just that and nothing more. It is about as effective at rinsing the tank as holding down the flush pedal. I installed a spray head in our other rig’s black tank where I had easy access to the tank and it does a much better job of getting sticky solids down to the drain.
I’d like to add one more tip (instead of flushing the black tank first, as described above):
After connecting the stinky slinky, fully open the gray valve for a couple of seconds, then close it. THEN, open the black valve and proceed as described above.
The reason for this is that doing so “pressure tests” the system for leaks and end-to-end integrity; if there’s a weak spot it’s only gray water to clean up.
(Don’t ask me how I learned this technique. 🙂 )
You beat me to the draw! That’s exactly what I’ve been doing~! All you need is ONE loose connection or burst and you’ll be sorry!
From a Plumber- I do the same thing for the same reasons. And I know my s..t.
👍😅 That’s appropriate, Jesse. I wonder if there are any folks in your profession that use that as their slogan.🤔 Probably. Have a good evening/night. 😀 –Diane at RVtravel.com
Haha, “You’re #2 is our #1!”
I think I’ve seen that one before, Chris.🤣 Have a good evening/night. 😀 –Diane
We’ve been RVing 60+ years, and the only time we ever use any type of holding tank treatment is when we get it free at RV shows. We are full-timers that travel, usually never staying in one location more than 1–2 weeks at a time. Personally, we never use anything to ‘breakdown’ the solid waste in the blackwater holding tank, we simply use extra water. We do not boondock or dry camp for more than a day or two, open the fullway termination valves only when actively dumping. We have had no issues with odors and as for the sensors, about 95% of the time we just do not use them. The breaking down of biological solids cause gases and odors, RVs have holding tanks not septic tanks. Just an opinion
Based on a previous RVtravel poll, the majority do not use rv-specific toilet paper.
We don’t either. But, we decided long ago not to put TP down the toilet. We have a small, foot-operated covered trash can from Costco. Every so often, we just pull the plastic bag out and dispose of it.
Thank you, Dave! 🙂 Thank you for the toilet paper suggestion! We typically buy Scotts RV two-ply toilet paper in bulk from Amazon. I did mistakenly get single-ply once. 🙁 We are still working through that mistake. Have a great week, safe travels, and safe stays! 🙂