Converting "Waste" into Nutrients - Treating Household Organic Waste

by Marc Rosenbaum, P.E.

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June 6, 1998

For South Mountain Company and Island Cohousing, West Tisbury, MA

Overview
"Waste" Streams Created in the Home
Humanure Treatment Options
Composting
Review of Composting Toilets Commercially Available in the U.S.
Graywater Treatment Options

1.0 Overview

This report looks at how to best handle human biological waste, graywater, and food waste. The premise is that the current systems in predominant usage are both causing pollution and squandering potential nutrients. The focus is on composting toilets as a solution which is applicable at scales ranging from single family residential to cohousing developments to medium size commercial/institutional buildings. Sources for information presented include: Sherwood Reed, PE, a designer of constructed wetlands, and an expert on on-site disposal systems; vendors Tad Montgomery, David Del Porto, Bill Wall, Don Mills, Glenn Nelson, Allen White, Fraser Sneddon; Environmental Building News; writer Carol Steinfeld; Gap Mountain Permaculture Mouldering Privy Technical Bulletin; The Humanure Handbook, by J. C. Jenkins; various references and users. The report was commissioned by South Mountain Co., Inc. and Island Cohousing.. Its purpose is to assess the various manufacturers of composting toilets and decide which would be appropriate as potential suppliers for the 16 homes and Common House at Island Cohousing.

2.0 "Waste" Streams Created in the Home

Humans persist in eating food (for the sake of completeness we will include food-like substances such as Pop-Tarts or Cheez Whiz, etc., in this category) and drinking liquids, and as a result produce fecal material and urine (for which Joe Jenkins' term "humanure" will be used in this report to begin to break down the association with the word "waste") and food scraps. Typical production per person is 0.3 - 0.6 pounds per day of fecal material and about 2 pints of urine. Food scraps vary significantly. If composting is used to treat humanure, my opinion is that food scraps are best composted separately, which reduces the likelihood of flies in the compost toilet tank. This is not universally accepted in the composting industry. Food composting can be done outdoors in a simple bin, mixed with garden refuse, grass clippings, etc. More sophisticated composters exist for scales somewhat larger than single-family houses, and they usually include means to aerate and sometimes mix the pile. This report will assume that food scraps are separately composted.

The other stream produced is graywater - water from sinks, bathing facilities, and washing machines. Both humanure and graywater can be sources of pollution. Both have similar levels of biochemical oxygen demand (BOD), which is an indication of organic matter present, based on the demand for oxygen necessary to decompose it. Putting a high BOD waste stream into surface water can de-oxygenate the water, upsetting the ecological balance necessary for fish and other aquatic organisms. When directed to a leach field, it can cause an anaerobic biomat, leading to premature system failure. Graywater also has similar levels of total suspended solids (TSS), which also can clog leach fields. Phosphorus levels may actually be higher in graywater (I assume due to soaps/detergents, which are user-controlled), while nitrogen is much higher in humanure, and is mostly in urine. Both phosphorus and nitrogen (in the form of nitrate) are significant pollutants when they reach groundwater or surface water, but are also potent fertilizers when they are returned to active soil layers. So the key is getting the nutrients to the correct location! Conventional septic tanks/soil absorption systems do a poor job of removing the nutrients from the waste stream, thereby risking pollution and wasting nutrients.

3.0 Humanure Treatment Options

There are a number of alternatives for treating humanure - small packaged sewage treatment plants, solar aquatic waste treatment (Living Machines is one product), constructed wetlands, sand filters, and composting systems. All but the composting systems first mix humanure with water. There are several disadvantages to these systems:

A substantial increase in water use in the buildings, used as flush water for toilets. In areas of limited water supply, this may have a significant impact. In almost all cases, energy has been used to collect or pump the water, and in some cases to store, filter and/or purify the water used. This is true even in systems where rainwater or recycled graywater is used as flushwater, since the storage capacity must be provided, as well as filters and energy for pumping.
Increased system complexity, due to pumps, valves, controls, filters, heaters, etc., which are often part of the system and must be maintained.
Substantial energy use, depending on the system type. Energy may be used for pumping, mixing, blowers/aeration, or heating.
The nutrient value is not available for agricultural purposes, and in some cases, is not removed from the waste stream (e.g., constructed wetlands do not remove nitrates unless substantially oversized or supplemented with aeration.)
Other disadvantages may include: the need for an enclosure for machinery or aquatic systems and quantity of relatively flat land area needed to accommodate the system.
In contrast to the above, composting toilets are contained within the building, have few moving parts, use little power (one model uses a 12 watt fan only), conserve water, keep nutrients out of the groundwater and permit their use as fertilizer. Different products vary in their complexity, power use, and ease of use, but the better products should be acceptable on all three counts.

4.0 Composting

Composting occurs because organisms, predominantly aerobic bacteria, break down organic material and convert it to forms usable by plants. The bacteria function best when they are in a warm (75-100°F, or more), moist (45-70% moisture content), dark, aerated environment, working on material which has a proper balance of carbon to nitrogen (30:1). Commercially available composting toilets are designed to create this environment, with varying degrees of success. They are typically ventilated with a fan, to exhaust odors and to aerate the pile. They sometimes have built-in heaters, so that a minimum pile temperature of 65 °F can be maintained. Composting activity doubles for every 10 °C (18 °F) increase in temperature, so maintaining this minimum temperature is important for satisfactory results and it is advantageous to increase pile temperature whenever possible. Users are asked to add carbonaceous material, such as wood shavings or rotted sawdust (hardwood best), garden scraps, popcorn (!), finely chopped straw or hay, etc., optimally at each use, but at least weekly, to feed the microbes and add carbon (humanure has a carbon:nitrogen ratio of 5-10:1, and urine 0.8:1).

Composting can occur at different temperature levels. Mesophillic bacteria work at the lower temperatures typically found in composting toilets, up to about 100 °F. Thermophillic bacteria work in the 95-160 °F range. The temperatures are important for killing pathogens in humanure, rendering it safe for soil application as fertilizer. Pathogens carried in humanure, such as viruses, worms (especially eggs), amoebas, bacteria, and protozoa, can be transferred to soil or groundwater by improperly functioning treatment systems. Septic tanks do not remove these pathogens. What kills pathogens is a combination of time and temperature - all pathogens are rendered harmless if kept at 50 °C (122 °F) for one day. Lower temperatures take a longer time. Roundworm eggs can survive in soil for several years. Therefore, composting toilets should feature either long retention times and/or high temperatures. High temperatures, especially without supplemental heat, are difficult to achieve. This is because of several reasons - compost tanks are usually in spaces which are at lower than room comfort temperature; they are ventilated to provide odor control and oxygen (the odor control aspect requires substantially more air flow than aeration does); and water is evaporated by both the ventilation airflow and the biological heat generated by composting, taking about 9000 BTU of heat per gallon of liquid evaporated, thereby cooling the pile. This makes it unlikely that the end product removed from a composting toilet can be guaranteed to be pathogen-free, although the likelihood of pathogen presence is low. This is why vendors advise handling finished compost with gloves, and burying it around fruit trees as opposed to digging it into garden soil.

Joe Jenkins, author of The Humanure Handbook, has been composting his family's humanure for 15 years or more. He composts in an outdoor compost pile, moving a 5 gallon pail which is located below the toilet seat indoors out to the pile every few days. After each use of the toilet, rotted hardwood sawdust is added to cover the addition. Garden waste and food scraps are added to the compost pile, not to the toilet pail. Every time the pail is dumped onto the compost pile, it is covered by straw, hay, or weeds. The compost bin has two compartments, and they are used alternatively. Jenkins cautions against turning the pile, which he says just interrupts the composting organisms and lowers the pile temperature. When one bin is full, it sits for about a year before being applied to his garden. He monitors the temperature of the pile with thermometers, and regularly achieves thermophillic temperatures, even though the pile is outdoors (rural Pennsylvania). The pile freezes in the winter, composting stops, and then it begins again in the spring as temperatures rise. His combination of thermophillic temperatures and reasonable retention time makes him confident enough to use the compost on his garden. Although what he is doing would probably give regulators fits, it is surely much safer than the new practice of dumping sewage sludge on agricultural land. And no method of handling humanure is less expensive.

With proper design, thermophillic composting could probably be achieved in commercial composting toilets. Locating the pile indoors is better than outdoors, because tighter control can be achieved on moisture and temperature. Ventilation air should probably be reduced, the tank should be insulated, and attention should be paid to adding carbonaceous material after each use. At the least, it seems that composters should be installed on top of an inch or two of rigid insulation if located in a basement space.

Like commercial composters, site-built composters, such as the Gap Mountain Permaculture Mouldering Privy, are not expected to achieve thermophillic temperatures; therefore retention time is designed to be in excess of one year.

5.0 Review of Composting Toilets Commercially Available in the U.S.

Discussion with some of the people mentioned above convinced me that the small, self-contained composters, in which the entire unit sits in the bathroom, are inadequate to handle full-time use in a home, and really are only suited for intermittent use. This left the following list of vendors (not guaranteed to be exhaustive):

AlasCan, Inc., 3400 International Way, Fairbanks, AL, 99701 - 907 452-5257. Essentially a house-sized waste treatment system, combining graywater and blackwater.
Bio-Sun Systems, Inc., RR#2, Box 134A, Millerton, PA, 16936 - 717 537-2200, 717 537-6200 fax - contact Allen White. Two models, both suited for high use areas.
Clivus Multrum, Inc., 21 Canal Street, Lawrence, MA 01840-1801 - 800 962-8447, 508 794-8289 fax - contact Don Mills. Various models. New England Distributor: Clivus New England, PO Box 127, North Andover, MA 01845, (978) 794 - 9400, 9444 (FAX), contact: Bill Wall.
Ecotech, 152 Commonwealth Avenue, Concord, MA 01742-2968 - 978 369-9440, 978 369-2484 fax - contact David Del Porto. Carousel.
Advanced Composting Systems, 195 Meadows Road, Whitefish, MT 59937 - 406 862-3854, 406 862-3855 fax - contact Glenn Nelson. Phoenix. New England Distributor: Tad Montgomery, PO Box C-3, Montague, MA 01351, (413) 367-0068.
Sun-Mar Corp., 5035 N. Service Road C2, Burlington, Ontario, L7L 5V2, Canada - 800-461-2461, 905 332-1314, 905 332-1315 fax - contact Fraser Sneddon. Various models, from compact to separate tank. US Address: Sun-Mar Corp., 600 Main St. Tonawanda, NY 14150.
5.1 AlasCan

Designed for use in permafrost regions, the AlasCan is a self-contained system which treats humanure, graywater, and food scraps (delivered via a garbage disposal.) It is the largest unit of all the products designed for home use. It uses ultra-low flush toilets to transport waste. It is quite automated, using blowers, pumps, and mechanical agitators to support the composting process. System cost is $9-10,000.

This system seems energy intensive and mechanically intensive. Its initial cost is high, and presumably shipping from Alaska would be costly. Although it appears to be effective and well engineered, its cost and complexity eliminates it from consideration for this project.

5.2 Bio-Sun

This company makes large tank systems made of recycled HDPE. There are two models, the WRS 500P (65,700 annual uses), which is 101 inches long, 51 inches wide, and 46 inches tall, and the WRS 1000P, 101 x 52 x 71. They also make a kit which can be retrofitted into an existing house. Their preference in new construction, however, is to have the builder cast a room into the foundation wall, say 12 feet square, and they will provide the working parts to make a composter out of it. The concrete is waterproofed with liquid neoprene. Bio-Sun's angle on composting is that other systems do not provide enough air or enough surface area of the pile, so their system uses a grid of PVC pipes and two blowers, one of which sends air to the underside of the pile (which is a one foot thick bed of peat moss and topsoil to begin). This blower is low flow/high pressure, and the exhaust blower is high flow low pressure. Together, they use close to 3000 kWh/year, plus the energy to heat the make-up air! The idea is that energy replaces maintenance. These systems evaporate all the liquid, there is no leachate, and there is a 96% volumetric reduction of the waste. In a residential setting, they probably will go for decades without needing emptying!

The exhaust flow is hundreds of CFM, which would cause a serious depressurization problem in most homes. Bio-Sun was unaware of combustion backdrafting as an issue.

Because of the energy intensivity and the potential for backdrafting, I don't recommend this approach, although I wish other companies were using recycled materials for their containers.

5.3 Clivus Multrum

Clivus is the pioneer in composting toilets, due primarily to the tireless dedication of Abby Rockefeller, the owner of Clivus Multrum USA, to this technology. Some of the other manufacturers were formerly associated with Clivus. Clivus Multrums are inclined vault composters, which means that the fresh waste is deposited at the upper end of a sloped tank, and as composting occurs, the composted material moves glacially down the incline to the access hatch, where compost can be removed. Liquid moves through the pile and is collected at the bottom of the unit, where it is pumped to a holding tank if it becomes excessive. Keeping the solids out of the liquid prevents the pile from going anaerobic and ceasing to compost.

Clivus makes small models for vacation cottages (M1 and M2) but doesn't recommend them for year-round home use. They accommodate only one toilet chute. Clivus makes a Model 12 which is plenty large enough for a home (or two - it is rated at 22,500 uses annually.) The M12 is just slightly larger, at 30,000 uses annually. Both are 104 inches long and about 60 inches high - the Model 12 is 47 inches wide, and the M12 is 62 inches wide.

Both use in-line centrifugal blowers to ventilate. The Model 12 uses a 93 watt fan, and the M12 uses a 43 watt fan. This is a big improvement over older Clivus models. Either fan can have its flow rate reduced with an electronic speed control, but my own tests show that not much power is saved at lower flow rates, as the motor efficiency drops. The smaller fan translates to an exhaust flow rate of 50-70 CFM (my guess) depending on installation, which is reasonable for a whole-house exhaust-only ventilation system. Clivus does not supply or recommend heaters, but the tank needs to be in a 65 °F environment. There is no purpose-built way to supply heat to the tank. Some of the make-up air comes through a vent panel on the composter, and some comes down the chutes of waterless toilets. The air does not appear to be as well-directed to promote aeration of the pile as in some other units.

Leachate is pumped automatically to an auxiliary storage tank, which then can be pumped to be used as liquid fertilizer or taken to a sewage treatment plant for disposal. Most of the nitrogen is in this liquid. Quantities generated will depend on the usage, whether the toilets are waterless or use some water for flushing (Clivus recommends against water flushing), and (I suspect) how much additional carbonaceous material is added. It seems that some manufacturers aren't pushing users to add material regularly, in contrast to what Joe Jenkins says in his book. This is likely more based on considerations of user behavior rather than what is optimal for composting.

Clivus recommends their automated pile moistener system, which is designed to keep the pile from drying out (which halts the composting process.) This is controlled simply by a timer, which can be adjusted to the needs of the particular installation. One owner I spoke with had start-up problems first with excess fluid needing to be pumped frequently, then with the pile growing rapidly due to drying out, after the service person from Clivus New England (Clivus' New England dealer, located in North Andover, MA - 978 794-9400) turned the watering frequency down to minimize fluid build-up. When the pile dries out, composting stops, so it grows more quickly. At this point, the watering rate appears OK to the user. A technical improvement, which I believe Clivus offers on their larger units, is a watering system which is controlled by sensing the pile moisture content directly. This should reduce the risk of over-or-under-watering greatly.

Maintenance includes periodic addition of a carbonaceous material (Clivus NE recommended once per month to the user I spoke with) and monthly raking of the pile to keep it from growing too high and to mix in the additive. This is an unpleasant process, due to both the physical layout of the unit (long and low) and the fact that the rake must be cleaned afterwards. Clivus has been prototyping a unit with built-in rake, but that is not currently available. The built-in rake will preclude one potential advantage of the M12, which is that it is large enough physically to bring three or four 14 inch diameter waterless toilet chutes to the tank, enabling it to potentially serve a duplex. Clivus likes to see the chutes located as far as possible to the rear of the tank, however, so even though this is possible, they don't recommend it.

Issues with the Clivus include: disposal of the leachate; potential for insect infestation (currently an issue for the above-mentioned user); the maintenance required, the difficulty of adding heat directly to the composter, thereby requiring the unit to be in a 65 °F space; the potential for creating what one author calls "shit-crete" - excess compaction of the pile; potential mixing of fresh and composted material during maintenance. Clivus' rate of technical innovation is slower than might be expected from the oldest firm in the industry, but their market is only about 10% residential, so these users have not been the drivers of their design efforts.

The automatic leachate pumping is an advantage.

Clivus Multrums have been widely used, and the company has shown staying power in this relatively new and still tiny industry, and they should be included as one of the potential suppliers for this project.

5.4 Carousel

The Carousel is manufactured in Norway and in Massachusetts. It is circular, about 52 inches in diameter. The large unit is about 5 feet high, and will serve 5-8 people in year round use, according to the vendor (the printed specs rate it at 4 adults full time use, which they calculate at 5,800 uses annually, assuming a 65 °F environment.) The medium unit is a bit over two feet high, serving half as many users. The medium unit can be upgraded to a large with some amount of effort, although the vendor didn't know the cost of doing this off hand, and hadn't done it before. Unlike all the other commercial systems, the Carousel is a form of batch composter. The inner container is divided into four quarter arc segments, and is rotated periodically when the chamber currently being used is full. While the next three chambers are being used, the first chamber is slowly continuing to compost. It is only emptied when it is time to use it again, typically at least 18 months after it was rotated out of active use. This design ensures no mixing of fresh and composted material. The smaller size of each container provides a higher surface-to-volume ratio for the pile, which makes aeration easier, so compaction is less likely to occur.

The unit is ventilated by a 43 watt in-line centrifugal fan, which is large enough to be used as whole house ventilation. Make-up air enters either down the dry toilet chute (they use 8 inch diameter) or through a 4 inch diameter opening at the top of the tank. This opening feeds the air distribution piping located at the bottom of the unit, and offers an opportunity to supply the pile with air which is warmer than basement temperature, at a good location for aeration. The vendor even suggested that they had fed this opening with filtered dryer exhaust. One could easily imagine ducting air from the warmest part of a home to this opening, such as from the heat rejection coils of a refrigerator, or from around the heating unit.

The Carousel has a hose connection for leachate with a length of transparent tubing to act as an inspection device for ascertaining leachate levels. There is no pump for the leachate, so it must be monitored and the tank must be located so it can be drained by gravity into a container for use or disposal. They recommend putting the tank on a stand of some type to facilitate both leachate drainage and compost removal. The vendor notes that the area below the carousel rotating axis must have a solid concrete pad, because that is where most of the weight is carried. They actually recommend Foamglas as the rest of the insulation (a foamed glass product) because of its superior load carrying ability, although there are styrofoams with similar capacity available. One of the ideas proposed by the vendor (which makes a lot of sense to me in any of these systems) is to run a loop of radiant heat tubing below the composter tank, as a way to add heat if needed. Good idea, and cheap to set up initially.

The Carousel can be purchased with a heater which is electric, and has the characteristic of adjusting its input to the sensed temperature of the tank. It appears as though this heater could be retrofitted if needed, as it is on the tank exterior. I prefer the solution noted above, as it would be much cheaper to run (although more costly perhaps to install with the controls - some thought required here about how to put this small loop in parallel with a heating zone, so no additional controls would be needed.)

The unit can take up to three of the 8 inch diameter chutes they use. (This is substantially smaller than the chutes used by some other products, but the vendor justifies this on the basis of there being over 35,000 units in use in Norway, and the fact that larger chutes don't prevent the skidmark problem.) It would be quite a challenge to design three bathrooms so that this would actually work in practice!

The vendor offers two different waterless toilets, both made by Vera. The first is a fairly traditional waterless stool, and the second has a rotating cup which seals off the chute. The user rotates the cup 180 degrees with a lever before use, then rotates it back to drop the waste into the chute. A push button allows rinsing of the cup as needed. This stool is costly, but may overcome the aesthetic issues some people have about using a dry toilet, and it is much less costly than the Nepon foam toilet (list of ~$650 vs. $2000), which has also been used with some composters. It seems clear that the trade-off has to be more maintenance - it's hard to beat a basic waterless toilet for low maintenance. The vendor recommends adding a bulking agent every tenth use.

Issues with the Carousel include: disposal of the leachate, including the inconvenience of having to manually drain it (it can be piped to a holding tank if levels can be maintained, or an external pump could be mounted;) and potential for insect infestation. Its advantages include: non-mixing of fresh and composted material; no maintenance of the pile is required; heat can be added via the air stream, which appears to be delivered to a useful location in the composter; and, in small housing units, the potential exists to start with the medium unit and upgrade it if needed to a larger unit.

Carousel would be appropriate for use in this project.

5.5 Phoenix

The Phoenix is a "big box" composter, like the Clivus. It has a footprint of 39 inches by 61 inches long, and comes in three sizes, ranging from 55 to 73 to 92 inches tall. The middle size unit (Model 200) should be large enough for most families, as it is rated at 11,000 annual uses at 65 °F. It can be upgraded to the larger unit by adding a center section. Unlike the Clivus, there is no inclined vault. The units have horizontal shafts which have tines arrayed radially around the shaft, which both halt the downward progression of the waste and allow for mixing if needed. The unit is filled at start-up with wood shavings, which prevent the raw waste from falling to the bottom. It is recommended that bulking agent is added with each use. The less frequently bulking agent is added, the more mixing becomes important. Mixing is accomplished by using a large wrench (provided) to turn the upper shaft. It is done from in front of the unit, and no contact with the pile is necessary, as the unit remains closed.

The Phoenix can be set up with a waterless toilet (preferred) or an ultra-low flush unit such as the Sealand. There will definitely be leachate to be drained if a flush toilet is used. The Phoenix, like the Carousel, does not have a pump, and must be drained manually. The manufacturer claims that users with the dry toilets often don't have leachate at all. This may be because the Phoenix has an air flow system that brings air in over the leachate and then past the pile through a series of baffles, which should help promote evaporation. It also has a manual hand pump mounted on the unit which enables the user to pump leachate from the bottom holding area to the top of the pile. This leachate recirculation prevents the pile from drying out on top, inoculates the fresh material with microorganisms, and promotes evaporation. They recommend the pumping be done once a week.

The Phoenix has the same need for being in a warm environment as the other composters. No heater is available as an option. It has been designed so that make-up air can be introduced at the front of the unit through a connection that can be ducted to a source of heat. In their remote applications, the company uses simple solar collectors to generate warm air to the units. In a home, any source of air warmer than the basement would help the composting process. Alone amongst the products offered, the Phoenix has a tank which has a wall which has about 5/8 inch of foamed polyethylene (PE) inside 1/4 inch of solid PE, providing some insulation for the unit.

The Phoenix is the most conserving of electricity. In the remote applications, it uses a 4 watt DC fan. Applications with AC power use 12 watts to power the same fan. This pulls about 20-25 CFM through the unit. This is enough for aeration, but is minimal for bathroom exhaust and whole house ventilation. In certain cases, the Phoenix would be more susceptible to backdrafting the composting vent stack than units with higher airflow. The manufacturer has made larger fans available where needed. It would be easy to specify a larger fan, or to retrofit one if needed.

Issues with the Phoenix include: disposal of the leachate, if it is generated, including the inconvenience of having to manually drain it (it can be piped to a holding tank if levels can be maintained, or an external pump could be mounted;) and potential for insect infestation. Although it appears that there exists the possibility of mixing of fresh and composted material, the manufacturer claims that the bulking agent and the tines prevent that from happening. Its advantages include: mixing of the pile is accomplished from outside the unit; heat can be added via the air stream, which appears to be delivered to a useful location in the composter; and, in small housing units, the potential exists to start with the medium unit and upgrade it if needed to a larger unit. It is also the most electrically efficient unit.

Phoenix would be appropriate for use in this project.

5.6 Sun-Mar

Sun-Mar is best known for their small, self-contained composters which are located within the toilet room. They make larger units, the largest of which is the Centrex Plus A/F. This unit is rated at 5 adults full time use. It is 70 inches long by 27 inches wide by 32 inches high. The unit contains a horizontal rotating drum with two sections in series. The waste is deposited into the first drum, which as it fills up, spills over into the second drum, which as it fill ups, spills over into the collection drawer. The drum is designed to be manually rotated two or three times a week to mix the piles. Bulking agent is added twice a week.

Leachate drains to two holding trays below, which maximize the surface area available for evaporation. An fan drawing about 30 watts exhausts the composter, drawing about half of its air through the composter, and half recirculating within the unit to enhance evaporation. There are also two electric heaters which are thermostatically controlled to maintain favorable temperatures in the unit. It appears to be easy to connect a source of heated air to the unit.

There is a 1 inch drain connection for leachate, and no internal pump is provided. Sun-Mar makes all of their models in a non-electric version for homes without power, so this connection is required on the non-electric models, and they say it is optional on the powered A/F model. This is because the intention is to evaporate the leachate, which should be possible in a dry toilet set-up, since 500 watts of heater is available.

A distinct disadvantage of the Sun-Mar is that it can accommodate only one dry toilet (10 inch diameter chute.) Additional toilets must be capable of moving waste laterally through a four inch pipe.

Issues with the Centrex Plus A/F include: disposal of the leachate, if it is generated, including the inconvenience of having to manually drain it (it can be piped to a holding tank if levels can be maintained, or an external pump could be mounted;) potential for insect infestation; potentially high electric use; restriction of one dry toilet connection. Its advantages include: mixing of the pile is accomplished from outside the unit; heat can be added via the air stream, which appears to be delivered to a useful location in the composter; fresh and composted material are separated; and it is quite a bit less expensive than the other units, listing at $1649.

The limitations of the Sun-Mar preclude its use in this project.

5.7 Composting Toilet Recommendations

As part of the work to prepare this report I spoke to Carol Steinfeld, a writer who has collaborated with one of the vendors (Carousel) to write a book about users' experience with composting toilets. Based on her interviews, she leans towards the Carousel, the Phoenix, or the AlasCan - these units have the most satisfied users. I lean somewhat towards preferring the Phoenix, and Carol Steinfeld probably would lean towards the Carousel. I like the potential that the Phoenix has for producing no leachate. However, I also appreciate the guaranteed residence time that the four compartment Carousel offers. I expect that issuing the RFP and reviewing the response will make the choice between Phoenix, Clivus Multrum, and Carousel clearer, since price and service will also be important factors.

I recommend that the issue of temperature be taken seriously. Putting a loop of Pex tubing below the composter seems like a great way to use a low cost heat source to add heat if required. I also recommend that the users be educated to add wood shavings or other approved bulking agents after each use, to promote aeration and a better carbon-to-nitrogen ratio. It would be good to encourage automatic leachate pumping for those manufacturers who do not currently supply it.

6.0 Graywater Treatment Options

The solids in graywater can be more easily decomposed than those in blackwater, so systems can be simple. If stored untreated in a tank, the graywater can easily go anaerobic and smell. Since it may contain some pathogenic organisms, the best strategy is to get graywater quickly to the biologically active soil layer, delivered below grade so as to eliminate the possibility of surface ponding and pathogenic contact with people or animals. In the biologically active zone of the soil decomposition occurs most quickly.

In 1994, California took the lead in permitting graywater to be used for sub-surface irrigation. Graywater from kitchen sinks and garbage disposals is not permitted. Typical systems use a surge tank (to accept graywater from high rate drains, such as a bathtub or clothes washer) and a dosing pump which sends the graywater out to a mini-leachfield or system of sub-surface irrigation tubing. There are no sand filters, only some kind of coarse pre-filter to remove hair, lint, and other materials which will likely clog the distribution system. (An excellent resource on these systems is Robert Kourik's Gray Water Use in the Landscape, a short how-to booklet.)

Graywater systems designed to accompany composting toilets must handle all of the graywater, including what comes from the kitchen. The first step is reducing graywater quantity, by specifying low flow shower heads, faucets, low water use horizontal axis clothes washers, and water efficient dishwashers. Garbage disposals should not be used - compost food waste instead. Grease traps on kitchen sinks will reduce maintenance on components downstream in the system. Toxics should be kept out of the wastewater - use non-chlorine bleaches, phosphate and boron-free cleaning agents, and don't put other chemicals down the drain. The backwash from water softeners, if added to the graywater treatment system, should be potassium-based rather than sodium-based.

Systems designed to treat all of the graywater typically include a prefilter (often pantyhose!), a sand filter, in which BOD5 and TSS are reduced, and a dosing tank with a pump, which sends the pre-treated graywater out to the soil treatment beds. Clivus has been doing bi-level treatment beds, sending the effluent out at about 6 inches depth in the warmer months, and 18 inches depth in the colder months.

In essence, the pre-treatment system does a similar job as a septic tank, and the treatment beds are a variation on a leachfield. More attention is given to choosing plants which respond well to the conditions created by the graywater.

Two other options for pre-treating graywater are constructed wetlands and sand filters. Both are built outdoors. A constructed wetland uses wetland plants planted in coarse crushed stone to remove BOD5 and TSS from the graywater. The water level is designed to be below the surface of the stone, keeping it away from contact with people and animals. Using a constructed wetland allows significant downsizing of the leachfield if the regulatory agency allows it.

A sand filter provides filtration of the solids, and provides a medium for bacteria to break down the organics in the graywater. As in the case of the constructed wetland, sand filters allow significant downsizing of the leach field.

In cases where soil percolation is poor or non-existent, and/or high water table makes a leach field impossible, a self-contained, lined system can be built which will evaporate all of the graywater. In northern climates, freezing potential typically requires that these systems be housed in a greenhouse. When they are uncovered, they are susceptible in any climate to being overwhelmed by significant rain storms.

My recommendation is that the least cost system be chosen for graywater. If possible, heat should be recovered in each home with a GFX. In most states, the use of composting toilets will permit a reduction in the leach field required to treat the graywater. This reduced septic tank/leach field system, if the soil conditions are satisfactory, is likely to be the least cost solution, and the easiest to get permitted.

- Marc Rosenbaum