Mod #85: Grey Water Potty Flush

Mod #85: Grey Water Potty Flush

Submitted on: 07/22/09

     Category: interior, plumbing
Mod Rating: 12345

(33 ratings)

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Mod Description:

This modification was designed and completed by Professor Randy T. Agee, D.Ed., April 2009, on a Cedar Creek 34SATS fifth wheel trailer. This article carries a copyright. Permission must be obtained from the author before duplication or reposting. Contact

A fellow campin’ buddy and his wife had joined Nancy and I around the glowing embers of a campfire. We were toasting some marshmallows as I detailed the specifics of the modification described below. When I was finished with my description, I commented that in reality the modification was totally overkill as the same thing could easily be accomplished with a 2 gallon bucket from the Dollar Store.

David’s reply to my statement was novel, “You do it that way because you can”.

Mod Difficulty:

He was right. Many of the modifications I make to our RVs are done simply because I can. And why not? I enjoy dreaming up, designing and implementing these modifications. Of course, I am always careful not to do anything that will compromise safety or the structural integrity of the RV.

One of our favorite camping spots is at Big Meadows on the Skyline Drive in Virginia. The mountains are absolutely magnificent; the deer so tame they walk right up to your campsite and an occasional black bear may ramble down a nearby path. We generally make the trip up to the mountain at least twice a year.

Like many federal campgrounds, Big Meadows does not offer any hookups at the RV site. The campground’s dump station and potable water supply for RVs is almost a mile away. Thus, you must contain and pump your own water and deal with your waste by using the appropriate RV holding tanks.

Our fifth wheel camper has a 62 gallon fresh water tank while the bath, galley and toilet holding tanks are 40 gallons each, for a total of 120 gallons of waste water storage.

Depending on the length of our stay, we often have the need to refill the fresh water tank and remove waste water from the bath tank. The galley and toilet holding tanks are more than adequate in capacity for storing waste water for at least a week. Conservation of both our fresh water and bath tank waste water space is important if we wish to avoid time consuming refills and dumps using portable containers. Of course one way to accomplish this is simply not to shower. But, in our camper that is not going to happen. The wife likes clean bodies on our sheets and I do not like sleeping on the couch.

I got to thinking about this before we made our spring trek to the mountains and came up with an idea that evolved into a RV plumbing modification that I believed would save both fresh water and grey bath holding tank space.

What bothered me was how we used fresh water to flush the toilet. In my way of thinking this was wasteful. Why do we need fresh water when we have a tank full of slightly used bath water? If I could devise a simple method of routing bath tank grey water to the toilet I would not only save fresh water but I would reroute some of the bath tank water to the black tank, which really has excess capacity when dry camping.

The plan involved installing a cut-off valve so that the fresh water supply could be completely isolated from the potty’s water flush line. Water from the bath tank would then be pumped to the potty for flushing. A switch next to the potty would allow us to turn the pump off at night or when we reverted back to a fresh water flush system. This switch could easily be located next to the pump rather than on the wall. I just liked having the switch accessible.

I had an unused Shurflo 12 Volt DC diaphragm pump that I had swapped out for a more efficient microprocessor controlled variable flow pump a few months earlier. I mounted this pump on the utility wall in the basement of the camper (photo #1) and ran wiring for an on/off switch up through the floor of the potty area so it resides next to the potty (photo #2). Similar pumps are available from a number of suppliers. Northern Tool is one source that has pumps in the $50 range. You do not need a high flow pump. One of the smaller 40 psi pumps that provide 1 gallon per minute should work fine.

The next task was to cut into the water line to the potty and insert a cut off valve so that it would be totally isolated from the fresh water system when turned to the off position. On the potty side of the cut-off valve I added a “T” into the line that is connected to the output of the Shurflo pump with a brass 1/2″ NPT pipe fitting to 3/4” garden hose fitting (photo #3). This fitting was found at Lowe’s and includes its own cut off valve so that there is no backflow when I return to using the fresh water system. Since the water lines are PEX, you will need special fittings or the tool for crimping the collars as I described in my Water Accumulator submission. Actually, the accumulator I showed there is now being used in this potty water flush system.

The intake of the Shurflo pump has a male fitting to connect to a 10 foot length of regular garden hose. The other end of the hose is connected to the water hose size adapter on the end of the main dump outlet (photo #4). If your RV has the solid cap it can be easily exchanged for one with the garden hose connection. Most any RV parts and accessory store carries the caps with the garden hose outlet. The bath water tank dump valve is left open so water is present at the outlet.

It worked great! Yes, it may be overkill, but if you enjoy dreaming up and making mods to your RV this one is pretty cool. If you don’t like doing unnecessary mods, get yourself a bucket.

ModMyRV recommends these parts for this mod:
SHURflo 2088-422-444 Classic 2.8 GPM Pump
SHURflo In-Line Filter - 1/2″ Barb Input x 1/2″FPT Output
Valterra Products, Inc. F02-3106BK Black Bulk EZ Coupler Valve Cap with Handle

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32 Comments For This Mod

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  1. Bob Vaughn

    We used to camp at Big Meadows on the B R parkway every winter…..
    I would be concerned about the soap scum clogging up the diaphragm in the water pump. plus slivers of soap that tend to go down the drain……Otherwise it sounds like a neat way to get rid of bath water……..It may even clean your black water tank in the process….

  2. ModMyRV

    That could be a problem over time. I would add a pump strainer on the inlet side to protect the pump. This would catch the big pieces but likely would have to be cleaned on occasion.

  3. Professor95

    I do have a screen strainer on the input to the pump. It is one of those strainers with a rubber washer attached that is used on washing machine lines. If fits inside the female end of the garden hose fitting. I put it there primarily to catch hair that may be in the bath water. Iapologize for neglectecting to include this info when I put the mod article together - I totally forget about that piece. You know what they say about getting old…. Your memory is the second thing to go. I don’t remember what they said the first thing was.

    Soap slivers would not be a problem in our system as we only use liquid body wash soaps.

    - Randy Agee (mod author)

  4. ModMyRV

    Thanks for the clarification professor95. I like washer screen idea better than the pump strainer. It’s both cheaper and easier to get to for cleaning. And good point on the soap slivers. I too use liquid soap. Why? Because I did the shower dispenser mod:

    Now what was I saying?

  5. DumOleBob

    This is about as clever a Mod as I’ve seen! We only boondock and recycling the gray water is a terrific idea. My concern is there is so little space in our 23′ rig I don’t know where the hardware might fit. BUT, I will think on it!.

    Anyway, Mr. Professor - YOU DA MAN!!!

  6. wclogger1

    Great idea, I have just done mine but used a 3 way Sea Tech valve & one way check valve to prevent backflow. I also installed a 3″ wye between my grey drain valve & my extra termination valve for pump inlet. Otherwise,it’s terrific.
    Rgds, Bob

  7. Robert

    There is absolutely no way I would ever implement this modification. After using the pump and gray water to flush the toilet, the line is filled with gray water. Then if the valve is switched to use fresh water, it means that the line has gray water with fresh water right behind it. Even with all the backflow prevention valves in the world, I wouldn’t want to take the chance of contaminating my fresh water tank. Gray water contains numerous bacteria that simply do not belong in the fresh water tank. Things like backflow prevention valves do not work perfectly.

    [Offensive content deleted per forum rules by admin on 09-07-09]

  8. DP

    Nice mod Doc.. The water line terminates at the potty either way and has more of a contamination potential in my opinion that the fresh water connected directly to the toilet. whoops, that last sentence came out like most of my projects. Where does this extra “,” go? :-)

  9. Professor95

    To Robert,

    Sorry, my degree is not in English. Yes, the “I” should have been a “me”. Thanks for the correction.

    Your comments about having fresh water behind gray water when switching back to a normal system are interesting. Apparently you are of the opinion that the contaminated gray water might flow backwards into a pressurized line or perhaps the bacteria will somehow migrate back into your fresh water tank.

    After thinking about your concerns, I will have to agree that there is a remote possibility of cross contamination if the water source was switched and the operator did not make an effort to flush any remaining gray water out of the line to the potty. Allowing a system to rest under this condition would not be a good idea. Simply remembering to properly flush the water lines by running fresh water into the potty should remove any chance of cross contamination.

    The inclusion of a check valve into the line before the diverter valve is an excellent idea. I totally agree that this inexpensive device should be part of the modification.


  10. Lloyd BC

    This looks like a great idea, but I would delete the fresh water option (our toilets don’t require water to flush), tap into the ‘grey’ line before the valve, with a shut off to enable filter cleaning, thus eliminating all cross-contamination concerns. As I need to replace my ‘grey’ valve, I may just install the pick-up connection now. You can always use a bucket of fresh until you build some ‘grey’ stock. There are ‘Home’ systems being marketed world-wide that do just this. Let’s face it, water shortages world-wide are making us be a little more careful with wastage. A possible added advantage would be the soapy flush water helping to keep the bowl clean.

  11. Lloyd BC

    I’m seriously looking at this as my next mod, But as I said before, I’m going to eliminate the ‘Fresh’ option. We have a ‘79 Layton 22′. I think I have found the perfect spot for the ‘grey’ pump. Below the fridge and furnace where there used to be a useless space, I have installed a Xantrex XD40 3-stage charger/converter. Behind and left of this is an empty space to the rear of the fender well. All of the water supply lines to the bathroom, including a double ‘T’ with lines to the shower, toilet, basin, are at the front of this space, and the grey discharge line is just below and rear. I’m having seepage problems with the existing grey valve, so I’m going to place a new valve between it and the main discharge, and a tap-off fitting between the existing and new valves, to supply the grey pump. All I will have to do is open the old valve while keeping the new one closed (of course the two valves will provide reasonable isolation of grey from black weater). The grey pump control switch will mount just below the fridge, alongside the furnace. As for fittings required to provide the tap-in: (1) 1-1/2″ ABS san.T, (1)1-1/2″ABS X 1/2″ FIPT reducing bushing, (1)PVC 1/2″MIPT x male garden hose adapter (irrigation dept).
    By the way, replacing the old converter with the Xantrex, in its new location, has allowed me to gain a ‘pots and pans’ cupboard below the stove, where the old converter lived (and died).

  12. Don (Sneaks)

    I too would be worried about cross contamination and would pass on this mod even with a check valve. But I would consider building a separate grey water transfer system simply because like you, my grey water tank fills long before the black tank. Using fresh water for toilet flush doesn’t really impact the amount of total fresh water carried. I’m for eliminating even the remotest chance of cross contamination. That’s why cities mandate anti-siphon valves in home sprinkler systems, a far cleaner system than our grey water semi cesspool.

  13. Professor95

    Many concerns have been expressed on cross contamination using the gray water flush modification. I cannot unequivocally say that this could not occur. The risks involved are dependent upon potential failure of a valve(s) and, of course, operator error.

    I have re-examined my original install and have added a check valve to the system as a secondary fail-safe device. I am comfortable with the mod on my coach - primarily due to the piping arrangement and the remoteness of the PEX supplying the flush water to the potty.

    The toilet in our RV uses considerably more water than those in previous units we have owned. The previous style foot operated Thetford plastic models could be flushed with little more than a teaspoon of water. Unfortunately, the porcelain Sea Land toilet in our CC does not allow for frugal water conservation. Without changing the valving on the Sea Land, considerable fresh water is used due to the auto bowl refill feature.

    Any modification made to a particular model RV may not be practical or possible on a differing model. Each mod has to be evaluated for the environment in which it will be used. This mod should not be done if you are unsure of or uncomfortable with issues related to cross contamination in your particular application. The old gray water bucket by the potty trick still works well!

  14. Lloyd BC

    Sorry to hear that your ‘Thundermug’ is such a water guzzler, and sure glad that neither myself nor my buddy have that brand. We ‘Boondock’ 99.9% of the time. I don’t often have a water problem (40gal fresh, 30 gal. grey and black), but my buddy, with only 15 gal fresh, has to refill quite often. That’s why both he and I have 12V transfer pumps and carry water jugs.

  15. Mark Cowperthwaite

    We live in our RV full time, a family of 5, soon to be 6. We have a 33′ Nomad Scout. We are currently rigged up(and it seems to be a pattern) that we have water and power available, but no septic dump. We currently haul a tank a week up across the neighbors lawn, about 500′ and you can’t use the ball attachment as the lawn is too unstable. I would be inclined to use this mod, but not bother with the fresh water switch, just use the grey water, thus no backflow issues. Great ideas, I am hungry for more.

  16. Mike Miller

    I too would be concerned with the connection of grey and fresh water systems. (But I like the term: ‘Thundermug’!)

    I you have a low water using version of the ‘Thundermug’, you could run the grey water to a hand sprayer that is next to the ‘Thundermug.’ Just spray down the bowel to flush. The soap in the grey water should help the black tank handle waste.

    Another change to consider would be to tap the side of the grey tank a few inches from the bottom. Solids and the like should stay in the tank. The hair filter is a good idea.

    Mike Miller
    Hillsboro, Or
    1978 GMC Birchaven
    1973 GMC 26 foot.

  17. Lloyd BC

    My feeling is that you want to make the mod as simple as possible, and tapping into the side of the tank adds complexity. Using a short section of garden hose (or an old washing machine water hose) with its screw attachment, along with the washing machine screen, should take care of any hair or solids. Also, if you tap into the side of the grey tank, you have to wait longer to build up sufficient water for flushing.

  18. GC

    I implemented grey water recycling in my coach May 2008 and have enjoyed almost double the days without hookups ever since. I find it better balances my grey and black tanks so they become full at about the same time with fresh water left over (I like to be sure I have some fresh water left as a safety). I used a purchased system from USI-RV which gave me all the parts to tap into the side of my grey tank, filters, tubing, etc. Installation took a little while but it looks very professional and integrated into the coach. The install instructions were very easy and complete.

  19. GC

    Oh, sorry the web site is

  20. ehenke

    I appreciate all the advise. I plan on using only the grey water tank to recycle grey water for flushing. Way I see it, I already put a little diluted water softener down the commode to help keep the smells out and assist with keeping the black water tank clean.

  21. Dube2

    I dry camp at the So Cal desert nearly every time I use my 42′ toyhauler. I carry 136gal fresh, 100 gal grey and 50 black and we still run out or get close to running out of water. it will last 5-6 days with an occasional shower for me and the kids and the wife every day. Anyway, been wanting to do this MOD for several years now. In my own opinion, the cross contamination is a slight posibility, but that’s like anything else. can you count how many times you shook another mans hand and wondered if he washed his hands often. sick feeling, but its a part of life these daz. Like one persone said, we’re talking about a water line thats connected direcctly to a toilet for crying out loud.

    The two pressious things we need to watch out for in the hot desert is our Fresh water and Cold Beer and not in any particular order.

    My plan is to tap directly into the Shower grey tank, run a 1/2″ line to the pump inlet, t-connect to the hose feeding the toilet, check valve on both the fresh and grey water supply also a ball valve on the fresh water side. my pump will be controlled from my existng bathroom light switch. very simple and it will work just fine. I’m looking for a mico sized check valve, any ideas? can’t tell in photo #3 what that brass fitting is? the only one I can find so far is a 1/2″ PVC Check Valve at the local hardware. it’s huge, in an RV kinda way.

  22. Portagie68

    Did you leave the other switch hooked up. I moved my switch also and getting ready to hook the other one by the sink back up. According to what I have been taught it will work.

  23. twal

    Think I will install a second pump to pump grey watet to the toilet. If I cut the line going to the toilet, put a ball valve in then a check valve then a tee to grey water pump also with a ball valve and then connect to toilet, it should give me the option of flushing with fresh or grey. It should also allow me to winterize the grey pump and line.

  24. Namchief

    This is a great mod… for those who are worried about cross contamination I understand. I plan on disconnecting the freshwater completely and only using grey water. Oh I ran out of grey water????? turn on the bathroom faucet or shower, most of can do one or the other while still sitting. Professor Thanks for the great Mod!!!!

  25. Namchief

    This is a great mod… for those who are worried about cross contamination I understand. I plan on disconnecting the freshwater completely and only using grey water. Oh I ran out of grey water????? turn on the bathroom faucet or shower, most of can do one or the other while still sitting. Professor Thanks for the great Mod!!!! I also plan on not using your method of getting the grey water but taping into the tank so it would not be something to be setup it is 24/7 mod,,,, thanks again

  26. Chr$

    I’m gonna do this mod, but disconnect the fresh water altogether. All it takes is gravity and an open valve in the fresh water system to suck grey water into the fresh water system.

    May I make an additional suggestion, for everyone here. You mounted the pump with the motor below the pump head. I recommend that the pump always be mounted with the motor up above the pump head. Then if (when, actually) the pump head leaks, it won’t ruin the motor. I did this with my freshwater pump in my old motor home since the motor was rusted from a leaking pump head. Saves a lot of headaches. Seems NONE of the manufacturers do this. I also mounted it in a plastic box with a drain. If it does leak, it will not rot the floor.

  27. Lloyd BC

    Well, never did get around to doing the mod before buying NEW TT.
    Lo, and behold, the first thing we had problems with was the water pump. 2 things wrong: 1- The pump was mounted vertical, motor end down (contrary to pump installation directions), and failed within 4 months (replaced under warranty). 2- The pump was mounted on the interior panel (1/8″ plywood) of the wall separating the master bed support structure from the front cargo storage, and sounded like someone beating on the wall with a 2 lb. hammer!
    Well, when I (not the dealer) put the new pump in, I mounted it horizontal (with the seepage drain down) inside a foam lined plastic tub, which is screwed to struts inside the above mentioned wall. Silence! Water line vibration in the rear of the TT is now louder than the pump and no problems with pump failure in almost 2 years, so I can think about proceeding with this mod.
    Oh, in the meantime, I have added electronic ignition to the stovetop burners (not oven though), thanks to Mr. Bar-b-q.

  28. Lloyd BC

    Well, I’m getting closer to getting the mod done, but I’ve been thinking about some of the concerns of other writers re: cross contamination. Here’s an idea that might alleviate all concerns.
    Instead of connecting the two supplies together with a collection of valves and other hardware, why not simply place a shut-off valve in the original fresh water supply to stop flow to the toilet. Then add a hand held spray (similar to the ‘Thetford Water Saver Spray’ option, which to me looks like a standard pull-out kitchen sink sprayer).The hand spray would be connected to the pressurized gray water supply only. No interconnection, no cross contamination worries!
    Further, by going with the addition of water softener and laundry detergent to the gray water (per the GEO holding tank treatment link: I think you’ll find the ‘Bowl’ stays much cleaner (Harder wash-down action with spray and less cling with softener).

  29. Lloyd BC

    Well, here I am again! Started using the GEO method on the holding tanks, and the ‘Black’ water monitor is working great! The only time it goes wild, is when you flush (It momentarily shows ‘FULL’ , then drops back to correct reading in a minute or so). Probably due to the monitor probes being less than 4″ to the side of the toilet drop, and when you flush, the probes are showered with conductive waste. I do however, have a problem with the grey tank monitor, and there’s no way to do a good flush. As for a good wash-down in grey tank, I’m looking at replacing the first 90 bend (from tank) in grey outlet plumbing with a ‘Sani-T’ (main connections in line with tank outlet). I will then build a permanent flush wand from 1/2″ copper pipe (holes placed so they blast at sensors, which are less than 1 foot away) My reasons for using copper pipe: I have everything required except ABS fittings, the loss in Cross-Section Area is 15%, and there are no glue transition problems. If I go to 1/2″ PVC pipe I will lose 30% CSA, have nothing in stock, and may face gluing problems
    Using a 1/2″ MIPT- 1/2″ copper adapter and a 1/2″ FIPT- 1-1/2″ ABS reducing adapter, I can connect the ‘Flusher’ to a short length of garden hose (permanently installed on trailer), which I will connect to regular garden hose for a good flush (of the sensors at least).
    Your thoughts?

  30. Lloyd BC

    Well, here’s something interesting: Today I did a test run of the two spare pumps I have (Flojet Quiet Quad-about 10 years old, and a 25 year old [at least] ShurFlo 205-103{still working fine, BTW}. My test was to see which pump would provide the stronger spray pattern from a standard kitchen spray nozzle. Now you would think the pump with the higher GPM rating would be the winner, wouldn’t you? (especially when it’s also at least 10 years newer) The ShurFlo is rated 3GPM @ 0 psi, the FloJet 3.2GPM @ 0 psi. Note: all the plumbing and electrical supplies are identical, as is the ‘Lift’ distance (~1.5ft). Both pumps were running wide open, 0 psi, so pressure switch setting did not enter into the equation.

    Guess what, the ancient Shur-Flo performed better (not by much, but it did shoot the spray a bit farther). I guess maybe the older pump was rated a bit conservatively, where the newer one was rated at its very best.

  31. Daniel

    My unit came with a toilet hand wand. Maybe just plumb that directly to the grey, and leave everything else the same to avoid the cross contamination issue. Mind with the existing toilet plumbed directly to the fresh, perhaps that risk already exists. conventional toilets get their isolation with an air drop in the tank area that probably doesn’t exist in the rv toilet. The wand with a disconnect for the head would make an easy point to cross pump the shower grey to the black should the need arise.

  32. AlexT

    I’m only planning to buy a trailer, but I’m surprised that re-use of grey water is not by design. Just an idea. Why can’t it be simplified by using only grey water for toilet flushing? Fresh water hose to the toilet should be rerouted to deliver fresh water to the grey water tank. Of course it would be on only (controlled by a switch) if there is not enough grey water in the tank. So you press a pedal, grey water will be used. If there is not enough grey water, pull a switch and flush again: a portion a fresh water will go to the grey tank and pumped for a flush from there. Bottom line: there is no chance for fresh water contamination.

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