Tuesday, April 28, 2015

Composting Toilet: Part 2

--Blogpost written by Bob

Back at the end of March 2014, we decided to replace our conventional marine toilet with a composting toilet.  (I suppose if you want to be technical, it is actually a desiccating toilet because it removes moisture, rather than true composting, simply because there is not enough elapsed time for composting.)  At that time, we were pretty much set on a Nature's Head but I wanted to investigate the Air Head a little closer before purchasing the toilet.  After a more thorough evaluation, we decided on the Air Head, primarily because of the angled back version which was more suitable for our installation.

I was glad to get rid of the old conventional marine toilet, the holding tank, and the
odors emanating from the discharge hoses.  It always seemed unnatural to me
to mix seawater (and all it's little critters) with a week's worth of human wastes
and store it on a boat which is closed up most of the
week in the hot summer heat.

There is some interesting terminology that is used in relation to the composting toilet. The toilet includes a "liquids container" and a "solids container"--these terms are self explanatory.  When using the composting toilet, one makes a "deposit" or "donation"--this is somewhat self explanatory too.  The collection of the composted solids after a month or more of usage is called a "harvest".

Removal of all Old Hoses & Fittings

While it was a dirty and smelly job, I was glad to get rid of all the old discharge hoses, the siphon loop, and the y-valve.  ( I had already removed the holding tank and installed the auxiliary diesel tank in its place.).  Because of the confined space, it took about 3 hours to remove the marine toilet, all the old hoses, and fittings.

It's hard to believe all the volume that is consumed by hoses and fitting used in a
conventional marine toilet and this load didn't include the holding tank.

In this photograph the old conventional marine toilet was removed and the new
composting toilet had not yet  been installed.  It looks like a lot of space is available.

After the old toilet was removed, Maggie thoroughly cleaned the head cabinet and the area surrounding the toilet--this took a couple hours to finally get rid of the odor.  When the cleaning was complete, I reinstalled the large shelf in the head cabinet--I refinished this shelf over the winter, removing the holes for the old discharge hoses.  We gained a lot of storage space in this cabinet as result of the conversion.

We gained a lot of space in this cabinet (located to the right of the toilet) as result of the 
conversion to a composting toilet. The hole in the upper center of this image is for the 
new toilet's air exhaust hose.(The kinked shower sump pump discharge hose in the 
upper right of this image was changed to using a plastic elbow to eliminate the kinking.)

Preliminary Fit Up of Toilet

Before hooking up the toilet, we simply sat it in position on the shelf in the hull liner where the conventional marine toilet was previously mounted.  Our first impression was that it was quite tall and we would actually needed a stool to sit on it comfortably.  Because it was so much taller than our old toilet, we hit our head on the underside of the deck as well.  It seems to me that the composting toilet could have been made shorter but we are going to learn to live with it since many others are using the same design.

Another thing that became obvious is that we needed to extend the supporting shelf by about 3 inches to support the liquids container on the front.  I decided to fabricate a shelf extender from some pine boards I had in my home workshop.  When I test fit the shelf extender on the boat, I realized that I had to shape it with a belt sander because the riser and the shelf were not a 90-degree angles.  This took about an extra hour to get it to fit correctly.  I fastened the shelf extender using two 1/4-inch stainless steel lag screws.

This photograph shows the shelf extender in place before the toilet was mounted.
After this photograph was taken I removed the shelf extender for varnishing.

Mounting the Toilet

With the shelf extender in place and the toilet sitting in position, I carefully marked the locations of the toilet supports on the liner shelf.  I removed the toilet from the area and removed the support feet from the toilet, marked the mounting hole locations, and drilled them out per the instructions.  

Following the instructions exactly, I first marked the outside surfaces of the toilet's support
feet while fastened to the toilet and before mounting.  Then, I removed the toilet
and the feet and marked the hole locations.

Air Handling

The continuous movement of air is what makes a composting toilet work (particularly in humid environments).  Air is drawn into the toilet through a screened opening, into the solid waste compartment, and then it flows out through the exhaust hose until it leaves the boat.  The exhaust fan is located at the exit point from the boat.  (I ordered 20 feet of air hose with the composting toilet since I was unsure how much length I would need--this turned out to be a smart move.)

The most convenient place to exhaust the air from the toilet was outside the cockpit coaming just forward of the trailing edge of the dodger window.  I purchased a (SeaDog-brand stainless steel) cowl vent to protect the fan (located just inside the coaming) and to direct the exhaust downward toward the deck.  I had to cut the cowl vent's length so that it would fit the available space in the coaming and locate it such that the opening was directed straight downward.

The cut down cowl vent has an open exit area of 6.3 square inches and accounting for a screen with 60% open area, the open area is reduced to 3.8 square inches--the flow area of the 2-inch diameter inlet hose is 3.1 square inches.  So, the cut down cowl vent with a stainless steel (9-mesh) screen will not restrict air flow. 

I fastened a 9-mesh stainless steel screen into the cut down cowl vent using two pop rivets.
The screen is intended to prevent wasps and spiders from building nests in the cowl
vent and blocking the egress of air from the new toilet.

I used the right-angle fan discharge mount supplied by AirHead.  I designed an adaptor plate constructed from 1/2-inch thick Starboard that was to be mounted inside the cockpit coaming.  The right angle fan housing mounts on this adaptor plate on the inside while the cowl vent mounts to it through the fiberglass coaming on the outside.  I installed eleven stainless steel threaded inserts (two different bolt patterns) to receive the 10-24 screws used to mount the fan housing on the inside and the cowl vent on the outside.

The custom adaptor plate allows the right angle exhaust with the enclosed fan to be mounted
on the inside the cockpit coaming while the cowl vent is mounted outside the cockpit
coaming.  I used stainless steel inserts to receive all the mounting screws.

I used computer-drawn templates to define the pattern of holes in the coaming.  I drilled the top center hole in the pattern from the inside.  Then, I applied the adhesive-backed template to the coaming on the outside and drilled all six holes through the coaming.

The hose opening for the right angle exhaust mount actually faces aft and the exhaust 
hose makes a generous U-turn just prior to the exhaust.  The fan is very 
quiet--you can barely tell it is running.

Because of its out-of-the-way location, to replace the fan in the future, I have to remove the cowl vent on the outside which releases the adaptor plate--it will be a two-person job!  The small fan is not going to last forever and I plan to carry a spare or two while cruising.  The fan will run continuously (24/7/365) and will draw about 5 watts of power--this is a negligible draw on our solar panels.

Adding the Composting Medium

The medium to be added in the solid container comes in the form of a 650 gram brick (about the size and color of an actual brick).  We put the coco coir brick in a bucket with 2 quarts of fresh water and kept breaking it up.  We allowed it to sit overnight and, by the morning, it absorbed all the water and was ready to be put into the solids container.

The coco coir brick and two quarts of water turned into about 3/4 of a bucket
of composting medium, ready to be put into the solids container.


We are now a "zero discharge" boat and, not only environmentally responsible but, we also save money in holding tank pump out costs.

The installation was completed within two weekends, with some additional work done
in the evenings during the week.  Note the holes from the old hoses on the lower
right side of this image--they will be addressed in our next blogpost.

The cut down cowl vent is inconspicuous and works perfectly so far.  I have some
concerns about the possibility of a wave hitting the deck in rough seas, getting up
inside the cowl vent and shorting out the fan but we'll see how it goes.

We used 12.5 feet of air hose in our installation--5  feet of hose is the length normally included with the toilet and I ordered 20 feet.  The total cost of this project was about $1200, not including eliminating the discharge seacock and thru-hull fitting (no longer needed) during my next haul out at the end of May.

For a boat owner contemplating a similar do-it-yourself installation, the composting toilet installation is clearly a two-weekend job.  Because of the small spaces in which to work, the fact that a boat's structure involves angles and curves, and a few little unexpected glitches along the way, no boat project ever gets accomplished as quickly as one would expect.

Our next blogpost will be about covering up the old holes used for discharge hoses for the old marine toilet.  I will make a follow up blogpost on our new composting toilet after we've used it for a couple of months.

Thanks for following our blog!


  1. We had great luck with the airhead in our old boat. Our new lf38 still has a traditional head but I expect I'll be returning to the airhead in the next couple of seasons.

  2. Why wait a couple of seasons? Thanks for your comment!