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Most RVers take their waste water tanks for granted, dump some chemical down the black water tanks and dump them when they get full.  Although there isn’t a lot of maintenance required, there are things you need to know to keep the sensors reading correctly and some tips to reduce the awful smell that can sometimes be associated with them.

First a little bit about them. Most of the black and gray water tanks are made of rotocast plastic to produce a stronger seamless design.  The black water tank is intended for mainly toilet water and the gray water tank for shower water.  When it comes to waste water from the sinks in the kitchen and bathrooms, it all depends on the layout of the sinks and which tank is closer to drain to.  This sometimes causes a problem when a kitchen sink is dumped into a small black water tank as it fills up faster and needs to be dumped more often.

Since the black water tank holds sewage, it needs a treatment or chemical to start the breakdown process and reduce smells.

It is important to use a treatment that has enzymes and utilizes an aerobic digestion process.  Chemicals like bleach, pine scented cleaners and other household products do not start the digestion process and create anaerobic digestion and gassing which could contain methane and other harmful gasses.  Also make sure the treatment product you use is formaldehyde and bonopol free.  There are several brands on the market such as the Thetford line that comes in either liquid or dissolving pouch. 

Another important product is to use an RV recommended toilet paper that will dissolve fast in liquid otherwise the paper will cling to the side of the tank and could cause false readings of the level sensors.  Check out the RV Repair Club Members video we did here: Selecting The Best Toilet Paper For Your RVs Waste System

By using the correct treatment and paper, the digestion and decomposition starts and will not only make for an easier to clean tank, but is also environmentally friendly and recommended by sanitary sewer treatment plants.

The gray water tank does not need a chemical or treatment every time since it’s mostly cleaner shower and sink water with maybe some soap or cooking grease.  However it can also get stale and create a smell that should be treated or cleaned periodically.  You can use normal household bleach with a 1 cup bleach to 20 gallon water solution and let it slosh around some or one of the products designed by OEM suppliers.  The Chlorine in bleach can cause damage to rubber seals if left in the tank and dump valves for extended periods of time so it’s best to rinse them out immediately if you use bleach.

Proper Dumping

This may seem trivial, however it is important to let the black and gray water tank get over ½ full before dumping and you should never leave the valves open when at a campground that has  a dump station at your site.  Leaving the valve closed not only keeps the smells from the underground sewage system out of your rig, but it also allows the tank to accumulate liquid that helps dissolve solids and allows the treatment to work.  If the valve is left open for the black water tank we get a situation called “Pyramiding” that eventually turns hard and that’s a far as I need to go with that!  In the gray water tank, we need the soaps and grease to continue to dissolve with the liquid and leave when dumped otherwise it can coat the side of the tank and cause improper monitor panel readings from the probes.

 When it’s time to dump, dump the black water tank first and let it drain until there is no sound of effluent running.  It’s a good idea to get a dump hose that has either a clear or translucent elbow to see if there is flow.

Once the tank seems drained, fill the black water tank at least 2/3 full with clean water.  The best way to do this is by hooking up a hose to a black water flush valve that will spray pressurized water around the tank and clean off the sides as well as the probes.  If you do not have a black water flush valve, you can hook a hose up to a flush wand and stick it down the toilet.  Make sure the hose is only used for this procedure.  Flush the black water tank several times until the liquid coming through the clear valve is clean.  You will be surprised how many times this takes!  After that, dump the gray water tank.


As stated earlier, there isn’t much maintenance required if you use the right chemicals and dump properly.  However since the dump valves are a push and pull type with rubber seals, it’s a good idea to dump some valve seal conditioner down the tanks to keep them lubricated and protect against chemicals that can deteriorate the seal. 

Valterra is the main supplier of dump valves and they recommend using a silicone lubricant spray on the posts of the handles but not WD40 as it will disperse the lubricant in the valve.  If you have a cable valve, the handle is on one side of the vehicle and the valve on the other.  Check to make sure the cable is tight and operating freely. 

The biggest issue with waste water tanks is typically inaccurate monitor panel level readings. Most manufacturers use probes that go through the tank with an energized probe at the lower level on one side and three probes up the side across from it.  As the liquid rises, it arcs across to create a closed circuit which allows the current to pass through and goes to the monitor panel.

Effluent, toilet paper, and even hard water deposits such as calcium and lime can coat the side of the tank and make the same closed circuit which means an inaccurate level reading.  As described above, using the correct toilet paper and pretreatment can help.  However if you are experiencing an inaccurate reading it would be a good idea to super clean the tank with Thetford’s Tank Blaster product which has been quite a success.

About the author: Dave Solberg: Managing Editor, RV Repair Club

For the last 25 years, Dave has conducted RV maintenance and safety seminars, developed dealer and owner training programs, written RV safety and handyman articles, authored an RV handbook reference guide and logged over 100,000 miles on the road in an RV.

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