What not to compost?

by Peppy
(Cape Town)

Hello Caroline,

thank you for your very informative article about the subject of compost toilets. I always enjoy the topics you cover on Greenidiom.com.

I've got a question. I recently read that one should not compost sawdust from CCA pressure treated lumber because it is apparently saturated with cancer-causing chemicals such as chromated copper arsenate. This material is apparently available to the consumers in many parts of the world. Is this true? If so how can I make sure to use only sawdust that is non contaminated with this kind of poison?

I would love to start my own compost toilet but this subject of poisonous sawdust scares me a little.

I am looking forward to hear from you and hope you can help me.

Kind regards

Peppy Peppinski

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Mar 02, 2018
your concern about sawdust is justified
by: Caroline

Hi there Peppie

This is a complex question, but your concern is justified. According to Joe Jenkins' book we should avoid sawdust from sawmills where wood is treated with poisons and is also dried, and use sawdust from lumber operations, which is still moist and has not yet been treated. This not only avoids the wood preserving poisons, but using damp sawdust that has already started to decompose a little is more effective at reacting with the humanure to accomplish odor reduction. The lumber sawdust is also coarser, facilitating aerobic decomposition in the pile. However he did live in a forested area with lumber operations.

The composting process seems to denature a lot of poisons, and binds some heavy metals, but I'm not sure of these ones you mentioned: chrome and copper. As for the metalloid non metal, arsenic, it is a deadly poison to humans. You don't want that in your vegetable garden. Unless you trust the FDA completely and this article dismissing the dangers convinces you: https://www.unce.unr.edu/publications/files/ho/2009/fs0909.pdf.

The purpose of sawdust in the compost is to absorb the liquid, and supply carbon rich material to create the ideal carbon nitrogen ratio for decomposition, and odor removal.

Sawdust is not the only candidate. You can add the following materials bearing in mind that the relationship I give is in the pile not the bucket. Since the pile is mainly straw, you will need less carbon material in the bucket.

Based on my calculations, you would need twice as much sawdust and or newspaper as humanure in a relationship 2:1. Use 3:1 for straw and 4:1 for garden leaves. You can also sprinkle compost in the bucket, or what I call fines, degraded broken up leaves that are very fine. I get mine off the street, sweep it up from the gutters. The fines have enormous capacity for absorbing liquid. Use whatever appropriate plant material you have available. Permaculture intelligence would actually suggest using something that comes off your property if possible, rather than importing it from somewhere else using a vehicle that burns fossil fuels, but, use your discretion and creativity and enjoy the satisfaction of closing another cycle and reducing your destructive footprint as a human being.

Please stay in touch and tell me what experiences you have using different composting materials. Finding the cover material best for you is THE practical issue with Humanure composting, the rest is easy. Read Joe Jenkins or other literature and make sure what you are doing is safe. Heat, decomposition processes and time are all pathogen reducing factors.

Good luck.

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