Composting Marine Heads

Sun-Mar's Ecolet Mobile and the Air Head are pondered upon for a season. The Ecolet is more of a fire-and-forget vehicle, but demands more power and space. The Air Head requires you to commune more closely with nature.

[Editor’s note: This is an archive article maintained in our files to provide background information on previous and existing portable “composting” head designs and to offer support for our most recent report on composting heads for boats, also called “desiccating heads,” which will appear in the July 2021 issue.]

The safe disposal of human waste in the waters of the world is a major problem today. A steady increase in the number of boats and a growing awareness of how easy it is to damage a fragile environment have made it less feasible to simply chuck the waste overboard-laws against this kind of discharge are getting tighter with each passing year.

marine compost toilets

This has left boaters with three alternatives: head offshore to a point where it can be legally dumped, store it until it can be pumped out to a shore-based treatment facility, or pre-treat it to a point where it’s safe to dump it in a legal discharge area.

Storing waste is at best a nuisance and at worst a mess. Pump-out stations aren’t always available when you need them, and storing a tank-anywhere from 3 to 50 gallons-of untreated sewage is a constant annoyance and worry. Any leak, whether in the tank, hoses, or valves, can convert your boat to a stinking mess. Disposal, even if a pumpout station is available, can also be messy. And pre-treatment of sewage presents its own set of problems.

The Coast Guard recognizes three classes of installed marine toilets. Portable toilets don’t count, since they’re not installed. Type III Marine Sanitation Devices (or MSDs-the USCG’s love of acronyms is exceeded only by its distaste for non-obscure terminology) are holding tanks, pure and simple. To quote the Federal Register: “Type III MSDs are commonly called holding tanks because the sewage flushed from the head is deposited into a tank containing deodorizers and other chemicals. The contents of the holding tank are stored until it can be properly disposed of at a shoreside pumpout facility (Type III MSDs can be equipped with a discharge option, usually called a Y-valve. This allows the boater to direct the sewage from the head either into the holding tank or directly overboard. Discharging the contents directly overboard is legal only outside the U.S. territorial waters which is 3 or more miles from shore.)”

The “deodorizers and other chemicals” serve only to stop bacterial action in the holding tank and mask the odors of the stored sewage. They don’t really address the problem of getting rid of the mess; they merely make life a bit more bearable during the period prior to pumpout. And they may well interfere with on-land sewage treatment, if the concentration of boaters at a given locality is high enough.

A holding tank doesn’t reduce the volume of the sewage-if you put 20 gallons into a holding tank, there’ll be 20 gallons to deal with at pumpout time.

Type I and II MSDs are the other types that are encountered on pleasure boats. (Type IIs are for boats greater than 65 feet in length; we’ll just refer to Type I here.)

Type I MSDs are flow-through systems: sewage is macerated and disinfected, and then pumped overboard. (This type of system is represented by units like the familiar Lectra-San from Raritan Engineering.) Again quoting, “The effluent produced must not have a fecal coliform count of greater than 1,000 per 100 milliliters, and have no visible floating solids.”

If local regulations permit dumping of effluent that meets that standard, a Type I is a good bet, providing you have the electrical power to run one. More and more localities, though, are instituting “No Discharge” regulations, and the use of a Type I MSD is becoming more and more proscribed. (A House bill by Congressman Jim Saxton is an effort to reverse that trend-see the sidebar on page 5.)

Composting Heads
Composting heads offer several big advantages for sailors: They require no through-hull fittings, no plumbing, and no separate holding tank. They have few moving parts. The solids are reduced fairly quickly, and take up less space than the mixed sewage, urine, and flush-water of a holding tank. On the negative side, the toilets themselves take up more space in the head compartment, they require a through-deck fitting and vent, and they need a constant supply of electricity (in very small to moderate amounts) to perform at their best.

Composting heads are essentially Type III MSDs, but with an important difference. Rather than simply storing sewage, composters separate the solid waste from the liquid portion, and convert the solid portion-the one that presents environmental problems-into an easy-to-handle, safe, non-odorous humus. The liquid waste is either stored or evaporated.

Composting of solid waste is a natural process. It’s what happens to dead leaves on a forest floor, or to fallen trees. In the process of composting, or aerobic decomposition, oxygen-using bacteria feed on the organic matter. They consume carbon, nitrogen, phosphorus, and other nutrients, releasing carbon dioxide. This conversion of carbon to carbon dioxide produces a good deal of heat, warming the organic mass, and thus speeding up the reaction.

Composting is a process of aerobic digestion. Anaerobic digestion, which occurs when sewage is not supplied with sufficient oxygen, is a malodorous process, while aerobic digestion is odor-free. Composting MSDs require a lot of oxygen, and are usually equipped with fans to insure that there’s an adequate supply. The sewage is mixed with a fibrous organic material such as peat moss, which serves to maintain a spongy mass that allows oxygen to penetrate. It also helps absorb excess liquid and supplies carbon to help maintain a good carbon/nitrogen balance.

A compost pile operating properly will heat up to about 140 to 160 F within a few days. Temperatures in this range will destroy disease-causing bacteria and protozoa (one-celled organisms.)

Temperatures will remain high for several days, after which it becomes necessary to mix the pile to provide additional aeration. Composting action can slow down drastically in cold weather, so a typical composting MSD will have an electric heater to make sure that the composing continues in the proper temperature range.

When the composting action is complete, what’s left is a black odor-free powder that’s free of dangerous bacterial contamination. It’s safe and not unpleasant to handle, and can be stored in the MSD itself, or in plastic bags or any other container. The humus, however, cannot legally be dumped overboard within US territorial waters-it must be brought ashore and disposed of on land. It makes a fine fertilizer for your flowerbeds, but maybe not for your vegetable garden.

Urine, which presents much less of an environmental and health hazard than does solid waste, is a major problem with composting toilets. It can’t be composted, and the boater using a composting MSD has only two choices: store it, or evaporate it. Stored urine, while not particularly hazardous, does develop a strong ammoniacal stink. Storage within the MSD itself is limited, so stowing it until landfall usually means transferring it to sealed plastic jugs, which add to the space requirements of a larger-than-usual head.

Evaporation works, but requires a lot of electrical power, which is often not available except from a shore supply. Urine, though it’s virtually sterile, still is considered raw sewage, and can’t be legally dumped within three miles of shore. Apparently you can’t be fined for peeing overboard (unless they get you for indecent exposure) or for peeing while swimming, but you can’t pee into a container and then pour it overboard.

Air Head and Ecolet Mobile
Early this past summer, we set up two composting toilets approved by the Coast Guard for marine use-the Air Head, which many have seen advertised in sailing magazines and at boatshows, and the standard version of the Ecolet Mobile, from Sun-Mar Corporation, probably the biggest maker of composting toilet systems in the world.

marine compost toilets

The Air Head consists of only a few parts, none mysterious. There’s an upper unit with standard toilet marine seat assembly and a lever-operated trap door; a lower unit below the trap door to catch and compost solids, a liquids jug, a plastic “shroud” to hide the jug, and a vent hose with a small fan in the end to extract moisture from the solids tank. Inside the solids tank there’s a stainless stirring bar, activated from the outside by a stainless crank handle. There are stainless brackets to hold the whole unit down. These are designed to be spread out sideways so that the tank can be removed for emptying. The main toilet elements are made of high-density polyethylene and are of excellent quality. All up, the empty unit weighs 18.5 lbs.

The Sun-Mar Ecolet is a somewhat more elaborate device. It has three chambers: a composting drum located under the standard-size toilet seat; a compost finishing drawer underneath the drum; and a lower evaporating chamber for liquids. The compost finishing drawer accepts partially composted material from the composting drum, and allows it to complete the composting operation without any introduction of raw sewage. There’s a crank handle for rotating the composting drum and a drum lock arrangement for transferring material from the drum to the finishing drawer. The finishing drawer slides out to simplify emptying. There’s a 3″ diameter vent with a built-in 12-volt 4-watt fan.

To evaporate liquids the Ecolet is equipped with a 110-volt heating element that runs at 120 watts. (A 12-volt, 120-watt heater is an option.) The heating element should be run “if and when power is available.” Otherwise, unevaporated liquid needs to be let out through a 1″ drain and stored in a holding tank, or transferred to some other container for disposal.

The Ecolet is tall, with the seat 29″ from the floor, so that a folding footrest 10-3/4″ from the floor is provided. It’s solidly constructed of fiberglass and stainless, and weighs in (empty) at 40 pounds.

Before putting the Air Head into use, we tried to see if we could make it leak water from the urine tank. We mounted the head on a board, as per instructions (and had to shim it forward 1/2″ in order to get the spring-topped rubber gasket/stopper to make hard contact with mouth of liquid bottle). We then rolled the board back and forth across a barbell to 20-25 degrees either side, violently, to get a sloshing effect, with the bottle filled with water to the refill line. We were able to get one or two drops to appear outside the bottle mouth, but nothing more. Then we tilted the whole arrangement to 45 and sloshed hard. No leak on one tack; a few drops on the other. With the whole assembly held at 90 (full knockdown) for about 15 seconds, about 3 tablespoons of liquid dribbled out.

We also tried some pitching and hobbyhorsing movements, approximately 20 fore and aft. This had no noticeable effect.

We didn’t subject the Ecolet to the same tilt tests, simply because there’s no way for an appreciable amount of liquid to leak out unless the toilet is tipped beyond 90, at which point we would have bigger worries.

We set up the Air Head in the PS office bathroom, tossed in a couple of small bags of peat moss for the start-up, per the instructions, and got to work (two men on the job).

We tested the Ecolet at a boat club in western Connecticut. Our test area consisted of a very small room-all right, a closet-and our testers were two stalwart volunteers from the club (again, both male).

Into the Ecolet drum we added 1-3/4 gallons of peat moss mix and a quart of Sun-Mar Microbe Mix (topsoil, we’re told, would also work), sprayed “Compost Quick” into the drum and evaporating chamber, and plugged in the fan and the heater. Testers were instructed to add one cupful of peat mix after each bowel movement, and to rotate the drum 4-6 turns every third day.

The Ecolet is rated by Sun-Mar for one adult in continuous/residential use, or three adults or a family of four including small children, for weekend or vacation use. The Air Head is rated for a month of everyday use by a couple, or roughly five months of vacation use by a couple. The bottom-line solids capacity for a five-month season (which allows the contents time to break down) is about 80 uses.

We used both toilets for four months.

The Air Head Experience
The main idea of the AirHead, and the feature that separates it from the EcoLet and other composting toilets, is that it separates urine from solid sewage via the design of the upper part of the toilet. Liquids go down drain holes in the forward part of the bowl, and solids are dropped through a trap door into the main tank (solids tank). The liquid tank is emptied out, either ashore, legally offshore, or into a holding tank via an optional hose connection. The contents of the solids tank are mostly water anyway; they gradually break down and settle, with the liquid evaporating and being withdrawn through a vent hose by a fan, and out through a deck vent.

In dropping solid waste, you have a choice of going with the tank closed and using a coffee filter to make the catch, after which you open the hatch and drop filter and contents into the tank; or going filterless and making the drop straight through the open bomb-bay door. This method is simpler and makes for less paper and faster composting. After use, you shut the bomb-bay door and turn the stainless-steel hand crank half a turn or so to help mix and settle the contents.

The separation scheme works fine for anyone sitting down. Men standing up have to aim for the holes-no big deal in a bathroom or calm water, more of a challenge in a seaway.

For optimum composting, the moisture in the solids tank should be kept at a certain level, not too soggy and not too dry. Too wet, and things get smelly; too dry and composting slows and it becomes hard to turn the crank. We found that it was no problem to balance the moisture. We never added water, but we occasionally added small amounts of peat moss to help drying.

For the first few days, the Air Head was quite aromatic, but then it took on that not unpleasant smell of peat and humus familiar to those who have used well-established outhouses. Within a week after that there was virtually no smell at all, except for a short time after use.

We hooked the extraction fan (a muffin fan that runs, according to our measurement, at .049 amps) to a 12-volt power supply and ran it constantly. When we were in the office we ran the bathroom’s regular AC fan vent, so that we were essentially venting into a vent. When we were away from the office we left the Airhead’s muffin fan on, but turned off the bathroom vent. After the composting process was well established, there was very little smell day to day; after a weekend with no one in the office there was a slight musty smell in the bathroom.

marine compost toilets

We kept no strict account of pee-only sessions after noting that the urine tank holds a gallon safely-around 8-12 pees. It’s easy to empty the tank on an unmoving platform, but would be somewhat trickier with the head compartment in motion.

In four months, two men made a total of 61 solid deposits, 26 in month one, 20 in month two, 8 in month three, and 7 in month four. These concentrations had more to do with our summer office habits than our gastrointestinal tracts.

We think the manufacturer’s estimate of 80 uses is about right-our 61 uses, when settled, occupied about two-thirds of the solids tank. However, ultimate capacity assumes adequate evaporation and break-down time between deposits. It would be easy to imagine a family of four, or two couples, pressing the limits of the tank on a week-long cruise, if the Air Head was the only place to go and it could not be emptied out.

The way to avoid over-piling is to pay attention to the cranking and the addition of peat. We found, during heavier periods of use, that the only way to do this is to open the trap door and watch the distribution of material as you crank. You want to leave things level for the next person, then toss in a bit of peat to help the drying and reduce the smell of the new addition.

We had a mild infestation of sewer flies (aka moth flies, drain flies, filter flies) that lasted about 10 days. These are very small, fragile, mothlike flies that look and behave a lot like fruit flies. We’ve all seen them in our houses. According to an Ohio State entomology department fact sheet online, they tend to hatch in garbage, stagnant water, sewage disposal beds, rain barrels, etc. They live about two weeks. They’re not much of a nuisance, but we didn’t like them in the Air Head, so we sprayed ’em, turned over the compost, and sprayed again. Then we reversed the muffin fan in the vent line and put a shot of spray down through there. That seemed to do the trick. If the aerosol insecticide interfered with the composting process, we couldn’t tell.

After four months of use, the Airhead weighed 33.5 pounds (10 lbs. top section, 23.5 lbs. solids tank). This would indicate that our 61 deposits reduced themselves to a mere 15-pound mass.

We did have some fit-and-finish quibbles. First, the gasket/stopper for the liquid tank needs to be strongly seated in the bottle mouth. It’s held down on the lip of the bottle by a spring. A better arrangement, in our view, would be to have a captive, gasketed cap on the upper unit that would screw down on the neck of the bottle and pull the lip tight against the gasket. This would leave a small space under the bottle that would need to be shimmed, but it would be unlikely to leak in any seaway, and would be easy to engage and disengage.

The “modesty shroud” over the urine tank is a pain, requiring the removal of two knob-screws (and their painstaking reinsertion after emptying the tank). We ended up leaving the shroud off entirely and taping a piece of paper around the front of the transparent blue plastic urine bottle, in case any of our infrequent visitors might be offended by the sight of the seemingly green contents of the bottle. We would suggest a translucent bottle with a simple Velcro retainer strap, and no shroud at all. Ideally the bottle would show the level of the urine, but in no particular color.

The stainless bails that hold the Air Head to the structural surface in the head compartment are convenient, and we were surprised at how tenacious they were during our leak tests. Even so, we would probably use a different hold-down system between the head platform and the lower-unit carrying handles-maybe turnbuckles, maybe lashings.

The Ecolet Experience
By the time we shut down operations on the Ecolet, our testers had urinated 159 times and defecated 78 times (1.6 times per day and 0.8 times per day per person, respectively.) Neither reported any odor in the room (one of our intrepid engineers climbed up to the roof and reported a very faint odor from the stack for the first day or so and nothing discernible after that. The test was, in a word, uneventful.

Our testers had no real complaints about the Ecolet, except for the slightly messy necessity of adding peat moss with each bowel movement. Peatmoss is light and fluffy, and a bit difficult to handle without some spillage; we did our testing on land, and we’d expect that the task would be trickier in a seaway. Cranking the drum over every three days presented no problem, except for remembering when to crank. A calendar in the head would help in this respect. We tried turning off the fan for a week with no adverse results. It’s likely, though, that the eight-foot stack we used was enough to provide a pretty good natural draft.

Urine proved to be no problem, as we kept the evaporative heaters connected for the duration of the test, and evaporation was complete. If sufficient power hadn’t been available, urine would have to be stored, as with the Air Head.

Anything that can simplify matters of the head compartment-holes in the boat, plumbing, storage space for waste, pump-out hassles, expense-is devoutly to be pursued, and both the Ecolet and the Air Head are worth consideration.

The Ecolet will only fit on a boat with a good-sized head compartment. While its footprint is actually only marginally bigger than that of a standard toilet and plumbing fittings, it takes up a lot of cubic feet and airspace. And it needs plenty of electrical power to work at its best.

The Air Head is more suitable for boats with smaller head compartments and less electrical capacity. It will also require a smaller hole to be drilled through the deck for the vent.

The Ecolet is in some ways more “civilized” than the AirHead, in that its size, capacity, and mechanisms are designed to manage waste without much help or attention from you. The Air Head is smaller, a bit simpler, and demands very little electricity. The only trade-off we can find, however, will be a show-stopper for some people: To a certain extent you have to commune with your own waste, and that of everyone else who uses the head. Yes, you can get it to a point where it hardly smells, but you will be inspecting it, pouring it, stirring it, and generally living close to it. In our opinion, it’s no worse than having it sloshing around in a holding tank, awaiting a trip to the pump-out station, but many would argue that waste out of sight is waste out of mind.

Sun-Mar Ecolet dealer prices range from $800 to $1,000; the Air Head costs $770.

Also With This Article
Click here to view “The Saxton Bill.”

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at


  1. I’ve been using the AirHead when on our boat for nearly 20 years. I plumbed the urine directly into a 5gal holding tank. Besides our poo and peat moss we put our kitchen compost in as well. With two of us living aboard we empty both about every 3 weeks. The solids are double bagged and put in a dumpster. One thing you didn’t mention is that if the AirHead it tilted back the pee may not flow into the pee section. I’ve mounted it on a hinged platform so I can adjust the angle forward when we are on long port tacks.