A Low Cost Site-Built Composting Toilet System

R.L. Crosby, Biorealis Systems, Inc

There is tremendous demand in rural Alaska for safe, reliable, affordable sanitation systems able to provide the convenience of flush toilets in locations where water supplies and wastewater disposal options are limited. Aerobic composting toilets offer one viable solution. Historically, however, composting toilets have earned a rather dubious reputation in rural Alaska, with at least as many reported failures as successes. 

Existing units generally fall into two categories: units sized for small families and/or weekend cabin use, costing about $1,000-$1,500 (e.g.
Sun-Mar , BioLet, Envirolet, etc.); and larger, full capacity units sized for larger families and full-time residential use (e.g. Clivus Multrum, Phoenix, AlasCan, etc.). These units typically start at about $4,000, and go up from there, with installed costs running as high as $20,000. Neither option provides a satisfactory solution to the general problem. The smaller units tend to be inadequate for full time residential use and require more attention, while the larger units are generally priced beyond the reach of those who need them the most. 


Following is a description of a simple, large capacity composting toilet system developed by Biorealis Systems, which can be owner-built for less than the cost of typical smaller units, yet provide performance and features not found on systems costing far more. The complete system, including a flush toilet, plywood enclosure, two composter modules, piping and ductwork, can be built for less than $2,000.  Features include: 

  • Batch feed: There is no contact between finished compost and fresh waste. All material removed from the unit is fully aged and can be handled safely.
  • Modular: Modules can be added or removed as desired to accommodate any requirement, from single family, to multi-family, to institutional.
  • Low maintenance: No moving parts to fail. Only non-corroding materials in contact with wastes. Annual maintenance requires removing, emptying and replacing a plastic drum - which can be done off site. Removed material is completely aged, light, odor-free humus. Tank can easily be handled by one person, and will fit through a 30" doorway.
  • Simple Design: Can be locally built, using commonly available materials. Does not require specialized tools or skills to build, lends itself to local self-help solutions, creation of local jobs, small business opportunities.
  • Low energy use: Aeration/evaporation system uses exhaust air from existing household ventilation system (If none exists, one can/should be added, which would also address other health issues - i.e. indoor air quality). No other energy input required.


Total system components include: 

  1. A waste delivery system
  2. An airtight plywood enclosure
  3. Composter modules
  4. Heat recovery ventilation (HRV) system, and 
  5. A method of dealing with or disposing of excess liquid.

Waste Delivery System Wastes can be delivered to the composter in a variety of ways. Depending on specific application requirements, priority may be given to: first cost, operating cost, maceration, reliability, fail-safe operation for unsophisticated users, or end user acceptance.

Basically, systems can be either gravity feed or pumped feed. Gravity feed is simple and inexpensive, but is limited to applications where composter can be located directly below toilet. Pumped feed systems add cost and complexity, but allow locating the toilet at a greater distance from the composter.  Options include: 
  • Indoor outhouse"Indoor Outhouse": Composter located directly below waste chute. Simplest, lowest cost, no moving parts, odor-free, least water use, can more easily accomodate kitchen wastes in addition to toilet wastes.

  • Gravity feed toilet: Composter located directly below low flush marine or RV toilet (Sealand Traveler Model 910, Approx cost: $200). Minimum horizontal distance = 45 deg slope on 3" waste pipe. Toilet must be able to handle flushing small amounts of dry sphagnum moss, and should be fitted with spray nozzle option to facilitate rinsing the bowl. 

  • Sealand toiletManual pumped waste system (low flush toilet, small holding tank, manual diaphragm pump, Approx cost: $400). Toilet empties by gravity into small tank under the floor. Pump evacuates holding tank through1-1/2" hose. Pump can be mounted under floor, handle through floor, behind, or adjacent to toilet. Installation diagram.

    Usage: Pump the handle 5-6 strokes just before flushing, to develop a vacuum in the holding tank. When the toilet is flushed, the vacuum will rapidly evacuate the bowl and tank, macerating wastes in the process. 

  • 12 VDC electric pumped waste system (low flush toilet, holding tank, 12VDC macerator pump, 12V power supply, battery, wiring, controls: Approx cost: $600-700) Operates similar to manual system described above, but with pushbutton.

  • 120 VAC electric pumped waste system: (Marine toilet, package sewage lift station: e.g. Zoeller "Quik Jon" holding tank, sewage pump & controls: Approx cost: $1,000).
  The manual pumped waste system would appear to offer the best compromise of features, best suited to typical rural Alaska applications for those who absolutely must have a water flush toilet (in spite of the added complication. See discussion of disposal options & costs below).  It combines a flush toilet system, and the design flexibility of a pumped waste system with the relatively low cost, reliability and simplicity of a non-electric system. This system could also be used as a backup for either electric system described above.

Enclosure The composter enclosure serves two functions. It is an exhaust plenum - a component of the household ventilation system - and it encloses the composter modules. It should be built from moisture resistant materials (i.e. exterior grade plywood painted with a durable, waterproof, washable coating), and all joints, duct and pipe penetrations caulked and sealed airtight. 

The enclosure must be sized to hold the maximum expected number of modules, and provide adequate maintenance access. It can be built into a corner of the house (or mechanical room or garage), or be a separate insulated structure located adjacent to the house. Wherever possible, an interior installation is preferable, eliminating the complication of building and maintaining reliable temperature control in a second, insulated structure, additional penetrations through the heated envelope, connecting insulated ductwork and piping between the two, and associated potential freeze-up problems. 

Composter Module(s): Two or more composter modules are required for a typical installation. The actual number will depend on expected usage. Multiple modules allow compost in one (or more) modules to age as a batch, while only the currently active module is being filled with fresh material.
  Composter drumA module consists of a perforated 55 gallon plastic drum centered inside of a 24" dia x 48" high, open topped plastic tank. The bottom of the inner drum is supported 12" - 14" above the bottom of the tank, on legs which also act as spacers, centering it inside the tank. The space below the drum provides a sump to hold excess liquid ("compost tea"). A 1-1/2" bulkhead fitting is installed in the tank wall just below the bottom of the inner drum, maintaining the liquid level just below the bottom of the exhaust duct opening, and allowing excess liquid to overflow to disposal (see discussion of disposal options, below).
  DrumtopThe inner drum diameter is nominally 2" smaller than the outer tank diameter, leaving a 1" air space between the two. A 6" plastic duct (polyethylene pipe) extends from a few inches above the top of the tank down through the bottom of the inner drum, open to the space below, terminating 1-2" above the standing liquid level. The top of this duct is connected to the exhaust fan so that air from the surrounding enclosure is drawn into the open top of the active module, down past the perforated inner drum to the bottom of the tank, into the bottom of the exhaust duct and out. The idea is to maximize evaporative surface area and air velocity past the wetted surfaces.

In the pumped waste configurations described above, wastes from the holding tank are discharged into the top of one module - the "active module". A manual diverter valve on the piping system selects between active and inactive modules. In the gravity configurations, some means must be provided for physically moving heavy, full drums under the chute. Where space permits, a "lazy susan" rotating turntable holding three drums is an excellent solution. For this configuration, a minimum inside dimension of 56" x 56" is required. Bearings, casters and hardware are available from any number of industrial equipment sources. Material cost per module is about $300.

Individual exhaust air inlets (e.g. bathroom, laundry & kitchen exhaust grilles) are ducted to the enclosure with backdraft dampers on each duct. The HRV exhaust fan draws household exhaust air in through these inlets, through the plenum, through the active composter module, and discharges it to a roof vent fitted with a venturi type extractor cap.

It is assumed that the house has, or will have installed a compatible heat recovery ventilation (HRV) unit. Lacking that, a small, low-power exhaust fan, and/or tall stack fitted with an extractor cap can be used. The final choice of ventilation system will be site-specific, depending on a variety of factors, including the cost of fuel, cost (and reliability) of electricity, average and design ambient temperatures and humidity, available alternative energy options (i.e. solar, wind), choice of waste delivery system, and available liquid disposal options. 

Heat Recovery Ventilation System Careful consideration must be given to compatibility with, and proper maintenance of the HRV unit. It must be designed so that the supply air side of the heat exchanger core is always at positive pressure relative to the exhaust side, must have no cross contamination from exhaust air side to supply air side, and it must have an easily and thoroughly cleanable core, filters, and condensate sump. (We have successfully used the composter with a heat pipe-based HRV of our own design, and with a vanEE 2000) 

Condensate flow rate will be higher than normal, and will be contaminated. Proper consideration must be given to dealing with it. An ideal solution is for the drain hose to drip condensate onto the aging compost in an inactive module.

Disposal Options The quantity of liquid generated in the composter is primarily a function of the delivery system. The lowest water use (reasonably priced) flush toilet we have found still requires about 1-1/2 pints per flush. At this rate, four people, each using the toilet 4-5 times per day, will put more water into the composter than can be evaporated under normal conditions - and that is not even considering the moisture content of the waste material itself, or times when the HRV is operating at reduced capacity (i.e. during periods of high humidity or extreme cold when the HRV is in defrost mode). 

In our experience, the HRV can normally evaporate about 2-4 gallons/day, and will generally handle all moisture produced by 3-4 people in a waterless system (i.e open chute directly above the composter), but it will not evaporate all moisture produced by any of the flush toilet systems described above, or large families whose members remain at home most of the time. This is an important point to consider when choosing a delivery system. The "Indoor Outhouse" option described above offers some distinct advantages.
  PondEffluent treatment/disposal options include: secondary treatment unit, holding tank, outhouse, small drain field, drywell, "constructed wetlands" ditch planted with aquatics, etc. The ultimate selection depends on the expected quantity of effluent and on a variety of site-specific requirements (i.e. permeability and temperature of soils, groundwater level, availability of further treatment options, pumping and/or hauling equipment, etc.). 

In our experimental house, we have demonstrated that it is quite feasible to purify all household wastewater to drinking water quality. Once this has been reliably accomplished, the decision of what to do with it is largely a matter of personal preference.  Photos show indoor artificial wetland and pond, components of the system. 

Preparation, Usage

Fill one drum with loose hay, spray a little water over it to moisten, and add earthworms (say, 1,000 redworms, lumbricus rubellus, or eisenia foetida; $20 mail order). Set handle on diverter valve to discharge wastes to this module, connect flex duct from HRV to top of 6" plastic duct. Turn on HRV. The hay provides bedding and carbon source, the worms homogenize and aerate the mass.

  • Turn on HRV, use toilet as normal. It will take about 6-8 months for a family of four to fill one drum.
  • Keep a decorative crock or other container with dry sphagnum moss (or other carbon source - e.g. sawdust, shredded leaves, grass clippings, etc.) and a scoop in it, next to the toilet. Every few usages, toss a handful into the toilet before flushing.
  • When the handle is pumped and the toilet is flushed, macerated wastes are discharged over the top of the hay. Liquid percolates down through the hay, excess liquid drains out through the perforations in the drum, to collect in the sump at the bottom of the tank. The continuous air circulation flowing past the perforated inner drum and across the surface of the water in the sump will evaporate 2-4 gallons/day (depending on air velocity, relative humidity, temperature). Liquid collecting in the sump may be recirculated back over the top of the compost pile, or overflow to {disposal option}.
  • When the first module is almost filled, prepare a second module as above, set handle on Y-valve to pump wastes to the second tank, and move flex duct to the other module. (For a more sophisticated setup, provide a Y-fitting on ductwork with a splitter damper. Then, just flip the damper position, similar to diverter valve, on each changeover.) The first tank is allowed to age as a batch. Earthworms keep homogenizing and aerating the compost in both tanks. Experience has shown that, when nutrients in the aged tank become depleted (i.e. when fully composted), earthworms will migrate to the active tank.
  • Empty fully aged compost from first module, and refill with hay to prepare for next usage. Compost can be used as a valuable soil amendment (placed around trees, shrubbery, ornamentals). Additional tanks/modules can be provided to accomodate larger systems or other multi-family installations.

Summary There continues to be a tremendous demand in rural Alaska for safe, reliable, affordable sanitation systems. Though composting toilets would appear to offer one very viable solution with distinct advantages, they have not been widely adopted in rural Alaska, for various reasons, ranging from inadequate capacity to high cost. 

The system described here is designed to address some of the known problems that have plagued previous installations. It is relatively simple, easily maintained, and can be locally built from off-the-shelf materials. The batch design allows safe handling of fully aged material only. Modularity provides a cost effective way to accommodate any size installation or changing requirements. On-site assembly reduces freight costs and lends itself to local self-help solutions and/or "cottage industry" opportunities. Perhaps most importantly, if the system is locally built, it will be well understood, there will be a greater sense of "ownership", and consequently there is a greater likelihood that, if and when problems do arise, they will be corrected rather than just abandoning it, as has typically been the case in the past.


In addition to product information, these sites also provide some excellent information on composting in general - design criteria, maintenance and installation details and information. 

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