Barrel Composting Toilet System:
Calculating Effective Leaching Chamber Absorption Area for Subsurface Urine Disposal

(Per Arizona Department of Environmental Quality Water Pollution Rules, Title 18, Chapter 9)

 

The Barrel Composting Toilet System utilizes a urine diversion system for the disposal of urine to a subsurface leaching chamber.  The following calculations are for determining the effective leaching chamber absorption area for subsurface disposal of urine in accordance with ADEQ Onsite Wastewater rules as noted below. 

In an effort to cover the majority of soil types likely to be encountered in Barrel Composting Toilet installations, these calculations assume the least absorbent soil types allowable without a site specific soil absorption rate (SAR) (per R-18-9-A310-D-2-ii).    

Additionally, the Barrel Composting Toilet System advocates constructing leaching chambers from the inverted bottom half of a standard HDPE 5 gallon round bucket.  Bucket chamber characteristics are noted below.

Soil characteristics:

SAR = 0.20 gallons per square foot per day

Urine characteristics:

--Volume per person per day = 1.2 liters (0.26 gallons) (Composting Toilet Demonstration Feasibility Study, Volume 2: Appendices, Table A6)

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

--BOD5 = 7500 milligrams per 1.2 liters = 6250 milligrams per liter (Composting Toilet Demonstration Feasibility Study, Volume 2: Appendices, Table A6)

Leaching chamber characteristics:

--Absorption area of bucket leaching chamber = 0.6 square feet (0.875 ft. bucket diameter = 0.6 square feet). 

Adjusted soil absorption rate (SARa) and effective chamber absorption area:

--SARa) = 2.15 gallons per square foot per day (per R-18-9-A312-D-3).

--Effective chamber absorption area = 0.6 square feet x 1.8 = 1.08 square feet (per R-18-9-E302-C-4-a).

Conclusion:

Even in the least absorbent soil types allowable without a site-specific SAR, a bucket leaching chamber is adequately sized for up to 9 people in full-time use.  As additional insurance against over-loading of soil, the design specifies 1 leaching chamber for each barrel in the system.  The chamber serving a given barrel is therefore unused during the 4 month minimum aging process for that barrel. 

 

References:

Arizona Department of Environmental Quality, Onsite Wastewater Rules R-18-9 http://www.azsos.gov/public_services/title_18/18-09.pdf

Composting Toilet Demonstration Feasibility Study, Volume 2: Appendices, Table A6 http://www.ghd.com/PDF/ToiletFeasibilityVolume2_Appendices.pdf