Barrel Composting Toilet System:
Capacity Calculations and System Sizing

(Updated 12/15/2016)


Toilet Capacity Calculations:

Note: Capacity calculations for composting toilets are the responsibility of the toilet designer or manufacturer.  NSF Standard 41, the testing standard by which composting toilets are certified in the United States, does not establish capacity standards.  In regard to toilet capacity, NSF certification indicates only that the toilet meets the capacity specified by the manufacturer.  

The Barrel Composting Toilet is a batch type system in which fresh fecal matter is segregated from older fecal matter during the primary composting process.  The design is based on 55-gallon high-density polyethylene barrels, which serve as composting chambers.  One barrel (the “active” barrel) is used until it is effectively full (per calculations below).  The contents of that barrel are then allowed to compost for a minimum of 4 months, during which time no fresh material is added.  During this 4 month composting period, the barrel is known as an “aging” barrel.    

The number of aging barrels is dependent on the number of people per day using the toilet. 

The following calculations are based on trade accepted practices and 6 years of continuous daily use of this design by the designer.    

Input volume per person per day.  Industry accepted capacity calculations are based on  ~ 0.25 gal. per adult per day total volume of combined feces, toilet paper and dry carbonaceous material.  Barrel Composting Toilet capacity calculations are based on ~ 0.5 gal. per adult per day total volume of combined feces, toilet paper, incidental urine  and dry carbonaceous material. 

The difference between industry accepted volume calculations and those recommended for the Barrel Composting Toilet are due to two factors:

The first is that the Barrel Composting Toilet is a zero discharge design in which all leachate is contained within the barrel composting chambers (except urine, which is diverted to a subsurface leaching chamber).  Urine incidentally excreted during defecation is contained within  the composting chamber and must be factored into the volume calculations. 

The second factor is the greater quantity of dry, carbonaceous material used in this design (all feces and toilet paper are completely covered with dry carbonaceous material after each use).  This practice insures an effective carbon/nitrogen balance and also increases social acceptance of the technology.  Long experience strongly suggests greater user acceptance if users do not have to look at the previous users' feces, as is common in some commercial composting toilet designs.    

Total capacity of one standard high-density polyethylene barrel =  55 gal.

Effective capacity of one standard polyethylene barrel, allowing 9” of vertical clearance between top of barrel and top of compost = 41 gal.

Effective capacity in days per barrel per adult = 82 days (41 gallons per barrel ÷ 0.5 gal. per person per day)

Six years of daily use of this design by two adults demonstrates that the capacity of the active barrel is approximately 50% greater than the above calculation indicates.  This is due to the reduction of compost volume caused by the decomposition and drying of the compost.  This volume reduction occurs even in outdoor sites during winter months, assuming a 50 degree Fahrenheit average temperature during the 4 coldest months, or a heated indoor site in colder climates. 

Design capacity is calculated from effective capacity as follows:

Design capacity = effective capacity plus 50%.  Thus, 82 days per barrel per adult + 50% of 82 = 123 days (4 months) per adult per barrel in full-time use.   

System sizing is based on the design capacity calculations.  The number of 55-gallon barrels required = the number of adults using the toilet on a daily basis + 1.

1 adult requires a 2 barrel system (1 active barrel + 1 aging barrel)
2 adults require a 3 barrel system (1 active barrel + 2 aging barrels)
3 adults require a 4 barrel system (1 active barrel + 3 aging barrels)
4 adults require a 5 barrel system (1 active barrel + 4 aging barrels)


Effective Leaching Chamber Absorption Area Calculations for Urine Diversion System:
(Per Arizona Department of Environmental Quality Water Pollution Rules, Title 18, Chapter 9, A310-D)

A permitted composting toilet system in Arizona that incorporates a urine diversion system is required to drain that urine to an existing permitted wastewater system (municipal sewer or septic system), or to an underground disposal system.  The permitting authority at the state or county level should be consulted, as the size and design of such a disposal system may be somewhat confusing to an applicant unfamiliar with the rules. 

For those interested, the Arizona rules for subsurface (underground) waste water (their term, not mine) disposal systems are based on 3 interrelated variables.

1) The characteristics of the liquid being drained (ranging from pure water to liquid contaminated with organic matter, suspended solids, oils, etc.). 

2) The volume of liquid (in this case urine, which varies from about 0.21 - 0.53 gallons per person per day). 

3) Adjusted soil absorption rate (SARa), which is a measure of how quickly the native soil in which the disposal system is installed can absorb liquid with particular characteristics, as noted in variable 1 above.  This varies from around 0.23 gallons per square foot per day for urine draining into clay soils up to about 1.20 gallons per square foot per day for urine draining into sandy soils.  Thus, in highly absorptive sandy soils the size of the disposal system can be considerably smaller than would be the case in clay soils.    


5 gallon bucket infiltration chamber calculations (described on our Urine Diversion System page): 

Adjusted SAR (SARa):

--SAR of 0.20 gallons per square foot per day was assumed, based the least absorptive soil type allowable (per R-18-9-A310-D-2-ii without a site-specific SAR): 

--SARa of 0.23 gallons per square foot per day was calculated (per R-18-9-A312-D-3)

This was based on the following characteristics of urine:

--Total suspended solids (TSS)  = 0 milligrams per liter (Composting Toilet Demonstration Feasibility Study, Volume 2: Appendices, Table A6)

--5 day biological oxygen demand (BOD5) = 6250 milligrams per liter (Composting Toilet Demonstration Feasibility Study, Volume 2: Appendices, Table A6)

Design flow:

--Urine volume was calculated at 0.37 gallons per person per day, based on the median volume according to the U.S. Department of Health. (Medline Plus website ( U.S. Department of Health, National Institutes of Health, U.S. National Library of Medicine: "The normal range for 24-hour urine volume is 800 to 2000 milliliters per day (with a normal fluid intake of about 2 liters per day).")

Effective Chamber Absorption Area of Leaching Chamber made from 5 Gallon Bucket:

A typical round HDPE 5-gallon bucket = 0.875 foot diameter = 0.6 square feet of soil absorption area

--0.6 square feet of soil absorption area x 1.8 (per R-18-9-E302-C-4-a) = an effective chamber absorption area of 1.08 square feet


Assuming an SARa of 0.23 square feet per gallon per day as noted above, half of a 5 gallon bucket with an effective chamber absorption area of 1.08 square feet would service up to 4 adults in full time use.  For more adults, more bucket leaching chambers can be installed in series. 


ADEQ Onsite Wastewater Rules R-18-9

Composting Toilet Demonstration Feasibility Study, Volume 2: Appendices, Table A6

U.S. Department of Health, National Institutes of Health, U.S. National Library of Medicine (

 Barrel Composting Toilet Page