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Mod #63: Macerator Pump

Posted By ModMyRV On December 31, 2008 @ 8:23 am In featured, plumbing | 14 Comments

Macerator pumps provide a partial answer to RV sewage disposal, chopping waste into a thin slurry and pumping it out through a garden hose to a proper disposal. And that’s all there is to it. They’re not magic and they’re not maintenance free. They can, though, eliminate the most onerous part of the dumping chore if used properly.

Mod Difficulty:

Most RVers use macerators in a temporary fashion and only when a full hookup is unavailable. In doing so, they miss out on some of the real benefits of a macerator. Temporary hookups involve adapting a macerator to an RV sewage outlet and attaching/reattaching it just as you would a “slinky” sewer hose. While this allows pumping sewage long distances and into otherwise unavailable receptacles, like septic tanks, plumbing cleanouts, sewers, or toilets, it also requires handling something even more clumsy (and prone to spilling) than a regular slinky. Storage and electrical connections (macerators demand hefty wiring) can also be a problem in this case. (See later for pointers on using a “temporarily” connected macerator.

Many RVers have found that installing a macerator in a semi-permanent fashion is a far better way. Semi-permanence means that it can be used without attaching and disconnecting, it can be bypassed with valves, and can be easily removed for maintenance. Carefully installed, a macerator can be used full-time with only a flick of the wrist and nary a drop of spillage.

Because of the variety of plumbing layouts in RVs, it’s impossible to show a specific arrangement of fittings. The drawing shown in the image above (Figure 1) shows a typical RV waste tank and valve arrangement modified to accept a semi-permanently mounted macerator. If you’re fortunate, you’ll have just such a straightforward, squared-off arrangement. Some RVs have lines that come together at angles or other odd-ball configurations. In these cases, it’s a matter of planning the layout to use angled fittings, squeezing things in the space available, and using branches from the main line rather than the simple in-between arrangement shown.

A–The macerator. Shown upright
A - Macerator pump shown upright
, it may be flat or angled as convenient. Make sure you allow enough room to unscrew and remove it for maintenance. Shown is a macerator with 1½” male pipe thread fitting option. It’s much better than the 3″ slip-on version in some cases since it can be installed in smaller areas.

BThe 3″ black water line cut
B - 3″ black water line cut
with a 3″ tee inserted. The outlet is reduced to 1½” to save space and allow using smaller and cheaper valves and fittings. Use an “offset” 3″x 1½” bushing. Usually available in RVstores, seldom in plumbing stores. Position the outlet hole at the lowest point in the line. Don’t use a ready-made reduced tee. The side outlet in these will be centered and allow crud to accumulate in the line.

CA 1½” slider valve
C - A 1½” slider valve
connected to the bushing. Again, valve may be at any angle convenient. Use cheap plastic/nylon valves in the “Grentec” or “Valterra” style. Metal valves like “Thetford” cost more, get corroded and stick. (The idea here is to have a dumper that doesn’t require your playing with poop.) The valve can be glued in place. If it ever leaks, it’ll usually be the flat part, held together with nuts and bolts, that can be removed and replaced if there’s just a bit of slack in the line.

DA 1½” slider valve
D - Another tee (1½”), optional
. The system will work without it, but if you want to avoid problems, you’ll use it. The side outlet of this tee will point up. The easiest size for the outlet, ready-made or “bushed” is ¾” (see E). This tee and (E) allow venting the pump when both valves are closed and attachment of a fresh-water “flush” line to the pump. Venting allows you to run the pump for several seconds after the sewage valves are closed and extract the last bit of stuff that might otherwise dribble. A flush line will be appreciated the first time you have to remove the pump for maintenance. NOTE: In an RV where existing valves are located under the middle of the rig and are operated by very long rods, this valve is a necessity. Those long lines can hold gallons of sewage.

E–(Optional) Soft vinyl tubing
E - Soft vinyl tubing
of a convenient length. Run it upwards, generally, until above the top level of black and gray tanks, and you’ll have a “sight” gauge. A garden-hose stop valve at the top allows connecting a cleaning hose and shutting off the line (to prevent spilling if your tanks get overfilled).

FAnother tee, with side outlet
F - Another tee, with side outlet
, to which the pump is attached. Ideally, it’ll have a 1½” female-screwed fitting to which you can attach the male fitting on the pump. You can, of course, adapt the 3″ slip fitting on a pump with various plumbing fittings and clamps. Ask a good plumbing store person for “no hub-style” fittings (neoprene flexible fittings with clamps attached). These come in an enormous variety and are used by plumbers to fit new parts into existing lines. The pump attaches to (F), but it weighs about 7 lbs. Support it. Most macerators have a good mounting plate. Insert a piece of dense foam or similar between the pump mount and whatever you attach it to for shock-absorbing flexibility as you bounce down the road. (Computer “mouse pads” are cheap, very dense but still thin foam and pieces of them are perfect for this and many other RV uses.)

GA valve as at (C)
G - A valve as at (C)
, but from the gray water side.

HA tee similar to (B)
H - A tee similar to (B)
, but usually 1½” to start with in most RVs. If 2″, you can reduce as with (B).

IThe original gray valve
I - The original gray valve
. Reconnect it to (H) so the system can still be dumped in the old-fashioned way if needed.

JThe original main valve
J - The original main valve
. Reconnect it as with (I). Depending on space used for the project, the pipe from (I) to (J) and from (B) to (J) may have to be adjusted. This can be done with simple couplings.

KThe outlet from the macerator
K - The outlet from the macerator
. Depending on which model, it may have a ¾” male garden hose thread or a 1″ barb/clamp fitting. In either case, connect a clear poly hose to it and run the hose to a convenient point where you can attach a valve and a quick-connect fitting to the garden hose(s) that empty the system. The manufacturer recommends 1″ hose, but 3/4″ or 5/8″ works fine. The end of the poly part and valve should be where easy to reach but as close to the pump as possible. Why poly/clear? So you don’t get surprised when disconnecting.

As shown, a macerator may be used with no more than slipping on a garden hose “quick connect,” opening valves, and switching on the power. There’s more to it, though, in assembling all this.

Make a detailed drawing first. Holding up a large piece of cardboard and marking existing fittings makes it easier since you can work with a full size plan and measure accurately. If you’ve a rig with a sealed bottom, good luck.

When you first begin the installation, you’ll have dumped and flushed the lines thoroughly (or that first cut of the three-inch line would have been a tad messy). Once you get it all together, for your first test, fill the tanks part-way with fresh water. Connect a garden hose and run it to a dumpo. Close (I) and (J). Open (C) or (G) and the valve from (K)
Close (I) and (J). Open (C) or (G) and the valve from (K)
. Turn on the pump. If there are leaks, you want to find them now, not when the tanks are full of nasty stuff. After it runs a bit, close the valve from one tank and open the one from the other. With both valves closed, run the pump a little bit. Hear the difference in sound when it sucks air? Open the vent at (E) and do it again a bit. Stop and look at clear hose portions. Open (C) and (G)
Open (C) and (G)
and fool around some more. Fiddle around like this several times and you’ll see how you can control valves, pump and vent without spilling.

That’s almost all there is to it except for things like dropping the hose with the pump running and similar tricks that will provide an enormous amount of entertainment for your neighbors.

TipHose: an old fresh-water hose will work, but it’s hard to coil up. Use a top-quality, 5/8-inch “Gates” brand flexible-when-cold hose. 50′ cut to about 10′ for use with a hookup (depends on where connection is) and remainder for longer runs.
TipQuick-connects. Shop around, but stick with a major brand so you won’t find yourself with obsolete stuff you can’t match. (Use the “Nelson” brand of quick-connects.)


TipHose Valves: Simple plastic hose stops were oringinally used for this mod. Cheap and they work, but they clog and leak easily. ¾-inch or 1-inch plastic ball valves are now used with an adapter at each end to fit either barbed/clamped fittings (as with poly hose) or garden hose thread (as applicable). Adapters in any size are available at good (not necessarily big) hardware stores. The valves have a “Teflon” ball that won’t clog or leak (and seldom break) for about $5 if you shop around.
TipPower: Macerators draw up to 14 amps. That’s a lot! Fortunately, they don’t run very long, emptying 30 gallons in about 6 minutes. They do need a full supply of 12 volts, so don’t scrimp on cheesy wire or connections. Use #12 wire and a good switch rated to carry about 20 amps DC (a skimpy switch will drastically reduce power going to the motor, like squeezing a water hose). Mount switch near the valves and poly “see-through” hoses so you can manipulate everything quickly and easily.
TipPVC/ABS Fittings: You can use either or a mixture of both (and will probably use a mix). Make sure you get multi-purpose cement (read the label) that will join PVC/ABS/CPVC fittings. Also, since close quarters are required, get slow-set cement (ask the clerk) that will allow you extra time to manipulate/adjust fitting when you work in tight spaces.
TipPAINT IT! When done with the job, paint the thing (along with all your existing pipe and fittings) because sunlight (UV) deteriorates plastic (and that’s why most RVers have busted fittings so often). Even if you decide you’re not interested in a macerator, do yourself a favor and paint all the plastic pipe and fittings under your RV anyhow — it’s cheap insurance.
TipMore on Fittings. You’ll probably have to use an RV store for the offset bushings mentioned. All others are best bought at a good building supply store or plumbing store. They have odd angles and other good stuff (like the flex couplings and flex bends and tees) that will surprise you. The basic fittings are cheap. Don’t hesitate to make your own. Cut a chunk off of one and epoxy it (if it’s a loose fit) to a chunk from another instead of just gluing it. Get creative. You can remake plastic fittings into about anything.

Note: Portions of this text are from an article by phred Tinseth. The original article in it’s entirety can be viewed here: http://www.phrannie.org/macerator.html

ModMyRV recommends these parts for this mod:
Flojet 18555-000 Waste Water Pump
Wayne Water Sump Pump Discharge Hose Kit (66023-WYN1)
Macerator Discharge Hose and Adapter

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