How To Empty Your RV Holding Tank - Camping World
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The majority of travel trailers, fifth wheels and motorhomes have onboard holding tanks.  These tanks collect the sink/shower drain water and the sewage waste from the toilet(s).  These are called the grey water tank and the black tank, respectively.  Generally the two holding tanks volume totals about the same gallons as the fresh water reservoir capacity.  The grey equals about 60% of the quantity and the black 40%.  So, if an RV’s fresh water capacity was 80 gallons, the holding tanks would be about 48 gallons of grey and 32 gallons of black.

These tanks must be drained regularly as they become full.  This is accomplished at a RV dump site or a sewage connection in a full service camp site.  This includes both holding tanks as even the grey water must be disposed of in the same manner.

Let's look at the procedure of dumping the tanks at a designated RV dump site.

Step 1: Install the sewage drain hose, being certain to check that all fittings are secure. 

Step 2: Open the black tank gate valve.  This is the "T" handle that is on the side where the discharge piping is much larger. Always remember that the black tank must be drained first and preferably when there is grey water present in the grey tank.

The black tank, on many RV’s, drains better when they are closer to full as opposed to only a partially quantity.  The greater volume tends to increase the turbulence as it exits which helps in better tank evacuation.  Therefore, you may find yourself dumping the black every second time you dispose of the grey water.  But, much of the dumping needs will be determined by your actual specific application.

Step 3: Once the black tank has drained completely it is best, if possible, to flush the tank with clean water.  This cleans the tank walls and washes away debris that may collect in the corners where drain turbulence is low. 

Tank flushing can be done by connecting a hose from a water supply tap to the appropriate input port, if your RV is so equipped.   If your unit lacks the built-in flushing circuit, you can install a simple "Back Flush" rinse adaptor.   Use a Flush King, or similar product, available at your local RV parts supplier.  This can not only be used to flush the black tank, but will also work great to clean the grey too. 

Keeping the tanks clear of any build up over time is very important.  The tank level sensor can only work efficiently if the walls are relatively clean.  Additionally, sewage odors can be minimized or eliminated if both the waste tanks are cleaned in this manner regularly.

The Flush King consists of a separate 3" gate valve and a 45 degree clear elbow with a standard hose input port.   The clear elbow allows the waste water to be viewed as it is draining and being flushed.  You should flush the tanks until you observe clean non-opaque water flowing out the drain hose.   So, if your RV is equipped with internal tank flushing, I would highly recommend the purchase of a clear plastic elbow so that you can also view the condition of the draining water.

Step 4: Once you have emptied and flushed the black tank, now the grey water must be evacuated.  With the black tank valve now closed, open the grey’s drain gate valve.  The grey is controlled by the gate valve at the smaller discharge piping.  Either way, they both flow into the large 3" discharge sewage hose. 

Emptying the grey water last, as well as the tank flushing, helps to clean the large 3" sewage hose of the more harsh waste and associated odors.   The grey tank should also be flushed from time to time, but it is not necessary to carry out this at every dump event.

There will be instances after evacuating your tanks that you may not be able to flush even the black tank, i.e. a rustic basic dump site that lacks fresh water service for flushing purposes.  This is okay, as emptying the grey always last will insure a relatively clean utility hose.

How to empty rv holding tank

Now let's look at the set up when you are hooked to a fully serviced site.

Step 1: The black valve should be in the closed position while the grey is open.  This will allow immediate drainage of shower and sink water, just like home!

Step 2: The black just needs to be monitored.  When the black water is about three quarter full, the grey water valve should be closed. 

Step 3: After the grey has collected some volume of water from showering and basic daily usage, perhaps the following day, the black can be released and drained. 

Step 4: Once all has been flushed the black can once again be closed and the grey opened.  Again the hose will be rinsed by the grey water discharge.

What if your black tank has never been really flushed out and now the level sensor is not working, or at least very inaccurate?  Well, here are some heavy duty cleaning tips.

**TIP: Normally, I would recommend emptying your holding tanks entirely prior to hitting the road to travel.  This helps to reduce the vehicle’s weight, which in turn aids in fuel economy.  However, this procedure requires travelling with at least a half full black tank. 

With at least a half full black sewage tank, get ready to hit the road.  Buy two or three of bags of ice and pour them directly into the RV toilet and assure they are all flushed down into the holding tank.  Now get driving, braking, turning, and accelerating a little bit more aggressively than your normal behaviour.  The floating ice in the holding tank will scour the walls and cause a mechanical cleaning that is capable of removing build up that may have formed over time.

An additional tip to keep your black tank clean is to pour a couple of ounces of household dish soap into the toilet every now and then.  Once flushed into the holding tank, this aids in removing build up as well as keeping it clean and providing a less adherent surface to support such growth.

There are other options available today that change the way the RV sanitation systems operate.  Macerators have now become common place as an option on many popular motorhomes.  Thetford's Sani-Con, for one, can not only reduce the viscosity of what was considered  normal black tank waste, but can allow it to be disposed of via a 1" hose, even up hill.  

So, what exactly are the main advantage of this feature?  Well, there are many!  Firstly, you have the ability to be able to empty your black tank, and grey for that matter, in a household toilet providing it is within the reach of the discharge hose.   Unlike pretty well all RV sewage systems, the San-Con system does not rely on gravity to move the liquid.  It pushes it and can propel its output above its own elevation.  It also boasts the easiest and cleanest operation available.  Having personally used this product, I can vouch for much of the manufacturer’s claims.

So, unlike in the game of poker, when it comes to RV holding tanks, a flush beats a full house. 


Posted: July 20, 2015

Tags: Tips & Tricks  

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