June 1998

Volume 7, Number 6

Article Contents

Phoenix Composting Toilet Sets New Standard

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Phoenix Composting Toilet Sets New Standard


The Phoenix Composting Toilet has one to three shafts with tines for aeration. Compost is removed at the bottom.

Source: Advanced Composting Systems

For years, Clivus Multrum has been the de facto standard for composting toilets in North America. Clivus was introduced in North America from Sweden during the 1970s by Abby Rockefeller. Almost single-handedly, the Clivus established composting toilets as a viable alternative to conventional flush toilets. While Clivus remains an active player in the composting toilet field, the product’s dominance in the field could be threatened by the growing popularity of a challenger: the Phoenix Composting Toilet from Advanced Composting Systems of Whitefish, Montana.

Glenn Nelson founded Advanced Composting Systems in the mid-1980s. Prior to that he was a licensed manufacturer of Clivus toilets (one of two in the U.S.) and played a key role in improving the Clivus design.

Through working on the Clivus design, he gained a great deal of insight into the workings of composting toilets—then, with the Phoenix, he designed a very different product.

Advantages of composting toilets

Composting toilets allow human wastes to be converted into nutrient-rich compost, which can be used to fertilize plants—though most experts recommend against fertilizing food crops with composted human waste. Composting toilets save a lot of water by eliminating the use of water for toilet flushing (or at least eliminating most of the water). This reduces extraction pressures on aquifers or surface waters and (with systems connected to municipal sewage treatment plants) reduces the energy and environmental costs of treating wastewater. Compared with standard on-site wastewater disposal systems (septic tank and leach field), a composting toilet reduces nutrient loading of the aquifer and lower-elevation surface waters fed by groundwater.

How the Phoenix
toilet works


Phoenix composter with three aeration shafts (crank attached to middle one).

Source: Advanced Composting Systems

The residential-scale toilets come in three sizes, rated for two people, four people, or eight people. These have either two or three modular compartments. The larger models are taller but have the same footprint as smaller models. On initial set-up, the composting chamber is “charged” with wood shavings, peat moss, and water to provide an environment that will be conducive to biological decomposition. During use, waste enters the top either through one or more 12”-diameter (300 mm) chutes from waterless toilets located directly above the tank, through 4”-diameter (100 mm) sloped pipes from micro-flush toilets not positioned directly above the tank, or through 112”-diameter (38 mm) hoses from vacuum-flush toilets located below the top of the tank. A separate shaft from the kitchen can also be used to deliver compostable kitchen waste to the composting tank. During normal usage, a bulking agent (pine shavings are recommended) is regularly added to the composting toilet to assist in the decomposition process.

Waste gradually builds up in the tank and decomposes through the action of aerobic bacteria. A shaft with tines on it is manually rotated with a crank to periodically mix the decomposing waste, ensuring adequate oxygenation.

In the smallest model, there is only one mixing shaft; on the larger models there are either two or three. The manufacturer recommends turning the crank weekly, but one user EBN spoke with turns it daily—partly for the exercise (it takes significant effort, he said). Decomposed waste that has been there the longest is at the bottom of the tank, where it can be periodically removed—typically once a year. Optimal decomposition occurs when the insulated tank is in a space that stays at about 65°F (18.3°C).

With a properly operating family-sized composting toilet, less than a cubic yard of composted waste is removed annually. This is because most of the mass of the human waste is converted into carbon dioxide and water vapor (the primary products of decomposition), which are volatile gases. In public facilities, the amount of compost removed can be far greater (see below).

An electric fan and vent pipe are located at the top of the tank to pull fresh air into the tank through specially designed air baffles in the sides and to remove gases and odors. This vent usually extends up through the roof, and the five-watt fan operates continually, using about 45 kWh per year. Sustainable building expert Jorg Ostrowski, of Calgary, Alberta added a small light by the fan that attracts fruit flies (often an unwanted member of the compost ecosystem), which are then ejected from the composter. Both fan and light can easily be operated with a photovoltaic panel and battery.

Liquids are separated from solids through a drain at the bottom of the tank and either resprayed over the pile or removed. With vacuum-flush and micro-flush toilets, excess liquid may need to be removed; however, with waterless toilets, liquids are more commonly recirculated back into the pile using the manual respray system supplied with the unit. Depending on the conditions, it is sometimes necessary to add additional water to keep the pile moist enough for optimal decomposition.

Comparisons with Clivus Multrum

Those readers familiar with the Clivus Multrum composting toilet know that this system has a large tank (significantly longer than that of the Phoenix, though not as tall) with a sloped bottom. The idea is that waste entering the tank at the uphill end will gradually slump downward as it decomposes. The problem, according to Nelson, is that decomposed waste at the bottom of the pile in a Clivus doesn’t slide along as it is supposed to. Recently added waste often flows over the top of the older waste. The result is that waste that has not been adequately decomposed may get removed during cleanout. That’s not good from a health and safety standpoint.

Nelson knows this because he has done extensive testing of both Clivus and Phoenix toilets. Every six months he puts a layer of dated tags, such as color-coded marbles, in his composting toilets (there must be a joke in here somewhere about “anal retentive,” but we’re not going to touch it). With the Clivus he found that some of these would never come out. “People work with just one-half or one-third of the compost pile [with the Clivus],” he told EBN. With the Phoenix design, on the other hand, Nelson’s marked tags stay right on track—first-in, first-out.

Another problem Nelson had with the Clivus was pooling of liquid waste at the bottom of the tank, often becoming anaerobic (a smelly type of decomposition that occurs when insufficient oxygen is available). This led to his liquid-separation system in the Phoenix that allows manual respraying of the liquids onto the top of the composting pile. Alternately, the liquid can be drained away—either into a conventional (downsized) septic system or a peat filtration bed.

Barry Greess, director of operations at Clivus, admitted that some of the compost remains in the rear pocket of the tank, but said this isn’t a problem. He told EBN that the company is making continual improvements to their line of composting toilets and welcomes the more active role Phoenix is playing in the industry. “If there are more companies out there talking up human waste recycling, it’s a better thing all the way around,” he said. “The more the merrier.”

User experience

Jorg Ostrowski, of Calgary, Alberta installed a Phoenix Composting Toilet in 1994 and is, so far, very happy with its operation. He has not yet begun removing composted waste, because the toilet typically takes several years to build up enough compost that some needs to be removed. Ostrowski and his family use the toilet as a household composter, adding kitchen waste, weeds from the garden, even dust from the vacuum cleaner—“anything biodegradable.”

At Grand Canyon National Park, the Phoenix Composting Toilet is quite popular, with eight toilets currently in use and several more going in. The Indian Garden facility at the bottom of the Canyon has had several Clivus composting toilets in use for about 15 years and one Phoenix toilet for eight or nine years. “I like the Phoenix much better than the Clivus toilets,” Indian Garden manager Larry Simkins told EBN. “It’s much easier to remove the compost from the Phoenix,” he said, which is an important issue given the high usage the toilets get. In 1995, the last year for which they have usage records (automatic counters on toilet stalls), the Phoenix averaged 100 uses per day, with peak usage significantly higher. The park removes four to six 55-gallon drums of compost from the Indian Garden Phoenix toilet each year, flying it out by helicopter. Simkins also feels that the Phoenix does a better job at composting the waste—probably because of the aeration that occurs when the tines are rotated. Since the park first began using the Phoenix, all new composting toilets installed have been Phoenix models, according to Simkins.


The Phoenix Composting Toilet is manufactured out of rotationally molded polyethylene. The polyethylene used in the interior baffles is post-industrial recycled, but the plastic for the tank itself is made from virgin resin. Nelson has tried to incorporate more recycled-content plastic, but it just hasn’t been cost-effective, he told EBN.

Be aware that the Phoenix toilet is not cheap. The list price ranges from $3,500 for the smallest residential model to about $6,000 for the largest unit (designed for a public facility). In suitable soils, this can equal the cost of putting in a septic tank and leach field, but it is generally less expensive than a conventional in-ground system on poor soils. And of course there is no water use, which can save money over time. We don’t attach a value to keeping groundwater uncontaminated, so our economic analyses don’t factor that in. All products are shipped directly from the manufacturing facility in Whitefish, Montana, though there are several “sales associates” throughout the U.S. and Canada. To date the company has produced and sold just over 300 toilets.

For more information:

Glenn Nelson

Advanced Composting Systems

195 Meadows Road

Whitefish, MT 59937

406/862-3854, 406/862-3855 (fax)


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June 1, 1998