I. Description

With this type of system, the residential plumbing system is separated, with the blackwater (toilet wastewater) portion of the waste stream treated in a composting toilet. The greywater portion of the waste stream (wastewater from all non-toilet plumbing fixtures) is collected in the building's plumbing system and conveyed outside by gravity to a buried septic tank (See fact sheet for Septic Tanks) Effluent from the septic tank is then conveyed either by gravity flow or pumping to either final treatment and disposal in a subsurface soil disposal system, or is disinfected prior to surface disposal.

Composting toilets aerobically decompose toilet wastes and produce a material which (if sufficient pathogen reduction has occurred and if metals and other regulated parameters are sufficiently low in concentration) may be used as a beneficial soil amendment.

II. Common Modifications

Kitchen wastes may either be composted and combined with the composting toilet product, composted with the toilet wastes, or combined with the greywater for pretreatment in the septic tank. This decision may depend upon local regulatory requirements, and whether it is desirable to use surface or subsurface final onsite disposal for the greywater portion of the waste stream. Depending upon site conditions and the type of disposal system used for the greywater portion of the waste stream, if kitchen wastes are included in the greywater it may be necessary to modify the design to provide for greater pretreatment prior to final soil disposal.

III. Technology Status

Many homes in Central Texas and elsewhere have separated grey and blackwater systems. Composting toilets are a relatively well-developed, and fairly commonly used technology as compared with other alternative wastewater technologies.

IV. Applications

The lower volume of wastewater produced from separated greywater systems (as compared with combined grey and blackwater flows), and the decreased organic loading to the system allows for a reduction in area required for subsurface onsite disposal.

The use of composting toilets allows for a significant reduction in water consumption, and may reduce nitrogen and pathogen loading to a subsurface disposal system.

V. Limitations

In order to ensure that the composting process is properly maintained in the composting toilet, the waste material must remain adequately mixed, with the moisture content in an acceptable range. Odors can result from anaerobic conditions if too much moisture accumulates in the unit. The fairly regular attention required for composting toilet units that receive much usage may be objectionable to some home owners. Retrofitting an existing home with a composting toilet unit may require structural modifications and be fairly costly. Electrical power is required for most composting toilet units.

In 1993, a new (revised) set of federal rules was promulgated which regulates the wastewater residuals from composting toilets. These rules (40 CER, Part 503 at the federal level, and Chapter 312 in Texas) also regulate the treatment and disposal of septage and municipal wastewater sludge. In order for the residuals from a composting toilet unit to be used as a soil amendment at a residence, the federal rules require that testing be conducted to demonstrate adequate pathogen reduction. In addition, it must be demonstrated that certain metals concentrations are below an acceptable level. Testing for these parameters on a regular basis can be fairly costly. The State of Texas has not yet developed recommendations for addressing these issues for individual systems which will comply with the requirements of the "503" rules.

VI. Typical Equipment/Number of Manufacturers

Composting toilet units are available from a number of manufacturers and suppliers in the U.S. Septic tanks for greywater pretreatment are available from several local sources.

VII. Performance

For typical residential greywater, primary treatment by septic tanks can provide for approximately 40 to 60% removal of both BOD5 and TSS if the tank is sized for a detention time of about 3 to 4 days (See Septic Tanks fact sheet) . Some organic nitrogen removal occurs through the solids removal process.

Several commercially available composting toilet units have undergone NSF testing to ensure that they function properly for a particular design usage. For the composting process to occur properly, there must be an appropriate available carbon to nitrogen ratio in the composting mixture, as well as suitable moisture content and temperature. Sufficient mixing must occur to maintain aerobic conditions.

VIII. Residuals Generation

U.S. EPA literature generally recommends that septic tanks be pumped at least once every 2 to 5 years (See Septic Tanks fact sheet)

For NSF-tested composting toilet units serving 4 to 6 persons on a continuous basis for approximately six months, composted product must be removed from the unit about 1 to 4 times annually.

IX. Overall Reliability

Properly constructed and installed septic tanks serving greywater systems can have service lives in excess of 20 years and be considered highly reliable.

Reports on the results of NSF testing for various composting toilet units are available from:

X. Operation and Maintenance Requirements

Septic tanks should be pumped at an average frequency of 2 to 5 years, in order to prevent too much sludge and solids build-up. The pumping frequency for septic tanks receiving greywater should be less than for those receiving combined wastes.

Residuals (composted product) from composting toilet units must be periodically removed, and disposed of in accordance with state and federal rules (40 CFR - Part 503) . A source of available organic carbon and/or bulking agent, such as peat moss or saw dust, may need to be added to the composting unit to help maintain appropriate C:N ratios, moisture content, and aerobic conditions. The moisture and temperature of the compost must be properly maintained. Adequate mixing must occur, whether provided manually or automatically (by a mechanical device in the unit)

XI. Potential Environmental Impacts

A thorough site evaluation must be conducted in order to determine the appropriate level of pretreatment prior to final onsite subsurface disposal of the greywater portion of the waste stream. Septic tanks can provide for only primary levels of treatment.

Odors may result from composting toilet units if anaerobic conditions occur. The composted product must be tested for adequate pathogen reduction, and to ensure that concentrations of metals and other regulated parameters are below accepted limits.

XII. Energy Consumption

Septic tanks do not require power consumption. Wastewater drains by gravity from the house to the tank.

Energy consumption for composting toilet units is estimated at 2 to 3 KWH per day per unit, with each serving an average of two persons on a regular basis.

XIII. Costs

Current rates for pumping septic tanks are from about $150 to $200 for tanks less than 2,000 gallons. If a tank is pumped once every 3-1/2 years, the cost for maintaining the tank will be about $50 per year, with a pump and haul cost of $175. Costs for maintaining composting toilet units are based upon the composted product being transported to a facility which is permitted to receive municipal sludge and septage.

Costs summary:

Capital:	Septic Tank (1,000 gallons) installed,	$2,000
	Composting Toilet Units (2, each with a daily design capacity of 2 persons),	$5,000
O&M:	Septic tank pumped once every 3-1/2 yrs.,	$4.17/month
	Residuals removed from composting toilet units 3 times annually (@ $100 by licensed transporter), on the average,	$25/month
	Estimated maintenance/repair/replacement costs for composting toilet units (assuming that a $300 repair is required for each of the units ever five years, and the units are replaced, @ $2,000 after 10 years),	$31.75/month
	Composting toilet energy use (estimated at 1,825 KWH/year),	$12.17/month
20-year NPW	(not including design and permitting costs),	$15,738.22

XIV. Aesthetic Considerations

Septic tanks are buried below grade, and usually do not have significant visual impacts on the site.

Composting toilets may not be desirable to some residents, for varying reasons. The composted residuals must be periodically removed from these units. Occasional mixing and/or carbon addition may be necessary, depending upon the design and usage of the units.

XV. State and Local Acceptance

Composting toilet units are currently in use in Central Texas and other parts of the State. However, the Texas Natural Resources Conservation Commission (TNRCC) has not yet developed recommendations or guidelines for handling the composted product from residential composting toilet units for compliance with the state and federal sludge/septage management rules. The overall acceptability of this onsite treatment technology will likely depend on the development of those guidelines.

XVI. References

1. The National Sanitation Foundation (NSF), 3475 Plymouth Road, Ann Arbor, Michigan; Reports on the performance of composting toilet units.

2. U.S. EPA, "Onsite Wastewater Treatment and Disposal Systems Design Manual", EPA/625/l-80-012, October 1980.