[Macerator Mounting]
The Black water tank on the GMC is only 4 inches deep.   What makes the black tank on the GMC work is liquid and to keep from plugging up the toilet, it is necessary to have liquid in the tank.  To have a separate gray and black water tank may  not be desirable.

When using the toilet after dumping the tank, use lots of water.   Remember , if you use the coach water the black tank will not get full until your fresh water is empty.  They are almost the same size.  You need water in the black tank to keep it from plugging up the toilet.

Don't worry if your Macerator does not completely empty the tank.  You will need about 3 gallons of liquid to have the black tank work.   Only dump the tank when it is full..  It will mix better when it is full than if it is empty.

If you see white toilet paper in the bottom of the toilet dump a gallon of water into the toilet to flush away the solids.  Use a bucket to dump in the water, you need a lot of volume of  liquid to flush away the solids and the pump will not do the job.  When the Black tank is empty, there is not enough liquid to clear away the solids from under the toilet.  Use lots of water when flushing after you have just dumped the Black tank.

Dump the Black tank only when it is full.  The Black tank needs lots of Liquid to work well.  You will know when the Black tank is full when you run out of water in the fresh water tank... They are almost the same size.

Dump the Black tank when you arrive home or at  the new camp site, not when you are leaving.  Driving down the road combines the solids and the liquid in the Black tank and will help the tank come clean when you dump.  Dumping after a long time sitting, will tend to leave solids on the bottom of the tank.


Draining the Tank  considerations.

This drawing tries to show that the Black Tank is 6.25  inches high at the rear and 3.5 inches at the front.  The Macerator inlet (shown by the small circle) is 1.5 inches in diameter.  The drain  pipe is at the bottom of the slope of the tank and using the original drain configuration all the material will drain out.

If a Macerator is installed (shown by the small circle) in the middle of the drain  as most pipe connections tend to do, about 2 .5 inches of material will remain in the tank.  They tell me this is about 3 gallons that will remain in the tank.


This drawing shows the suction entrance to the pump is the small notch in the inner circle of the impeller chamber.  Therefore,  the discharge pipe of the pump must be at the top to draw down the maxim liquid level.   This is not a big deal except the factory orientation of the pump  (with the mounting feet down) is upside down for GMC mounting.  The usual and easy way to mount the pump is with the rubber mounting feet up to the bottom of the coach.  This mounting almost defeats the eccentric mounting we buy to position our pumps at the bottom of the drain pipe.

The answer to this problem is to rotate the pump head so that the discharge pipe is up when the mounting feet are up.  The problem is that two of the bolts holding the pump chamber to the motor should not be removed because they are glued in with loctight and will twist off.  So the only way to rotate this head is to remove the 4 nuts and then the impeller and now the nead can be removed and rotated.  The result is your new $100 pump is now in pieces and putting it back together is a little tricky since you should not over torque any of the mounting bolts.

If offset fittings are used, the material remaining will go down to 1.5 inches with the factory orientation and about .5 inches if the head is rotated,  which will still leave some material to settle in the bottom of the tank.

If the connection to the Macerator is lowered more and more, the level will go down.  The tank will not be pumped dry unless the Macerator connection extends to  below the bottom of the tank.

The good news is, the Macerator never, never wants to run dry.  The manufacture  recommends that you position the discharge line so that there is always liquid resting on the impeller.  Some users have even drilled and installed a zerk fitting so the pump can be lubricated when the unit will not be used for a long time.  This will keep the impeller from sticking and running dry.  Here is some good information on the Macerator.  Go to the section on maintenance.



It is clear from the above print, that no matter what size inlet your Macerator is fitted with, the chopper blade and the  inlet to the pump is 1.5 inches.  Even though the pump is self priming, it will effectively stop pumping when the liquid level gets below the notch in the pump chamber.  You can estimate the location of this water level notch by noting the solid boss on the outside of the pump head.  This boss is located at the height of the pump inlet notch.

There is no advantage in having the 3 inch flat face Macerator over the Hose Barb one shown above.  The front of the pump is a 1.5 entrance hole to the chopper blade which is the same as the Hose Barb.