Composting Toilet Cover Materials:
Absorbency Testing

Updated 11/01/2013

 
Cover materials differ in their ability to absorb liquid.  Moisture management in composting toilets is partly dependent on the absorbency of the cover material used.  This is particularly true for composting toilets that do not utilize a leachate drain.  (The Barrel Composting Toilet System and the Bucket-to-Barrel Composting Toilet System are this type of design).  A more absorbent cover material can be used to help dry out compost that is too wet.  Conversely, a less absorbent material may be appropriate for a toilet that is too dry (although it is usually easier simply to add more liquid to the compost). 

As a guide to the absorbency of common cover materials, the following testing was done.  The test is simple and easily replicable by anyone wanting to compare the absorbency of various cover materials.  In the near future we hope to test other commonly used cover materials such as shredded paper, chopped leaves, etc.  The results will be posted here.     

 
Dry samples of 4 different cover materials were tested, including conifer sawdust, conifer wood shavings, finely chopped straw and horse manure (sifted through 1/2" hardware cloth).   

Each material was tested by putting a 1 cup sample into a 1 quart jar, then adding 1 cup of water.  A lid was put on the jar and the jar was shaken for about 30 seconds.  The jars were then left to sit undisturbed with lids on.  After 24 hours, the contents of each jar was emptied into a fine-mesh kitchen strainer and the volume of water was passed through the strainer for 2 minutes and then measured. 

Here are the test results, arranged from most absorbent to least absorbent:                               

Material
(1 cup sample with 1 cup water added)

Volume of water passing through strainer
after 24 hours

Percent of water absorbed
 after 24 hours

    

Sawdust (conifer)

1/4 cup

75%

    

Horse manure (finely screened)

1/3 cup

66%

    

Wood shavings (conifer)

2/3 cup

33%

    

Straw (finely chopped)

2/3 cup

33%

    

.

Samples of cover material with water added

24 hours later, water from cover material sample is drained through strainer into measuring cup.

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