What do we know about odorous gases from composting toilets (or from composting in general)?

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  • Marijn Zandee
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

Dear Elisabeth,

If you want to be correct, you probably have to say that the ventilation is there to ensure that the degradation process in the toilet is predominantly aerobic.

I guess composting toilets are usually not UD, so it seems likely that most of the ammonia comes from spots/pockets/puddles of urine. Maybe also some from the anaerobic processes, but that should be a very small amount.

The H2S, from the anaerobic process should also be tiny amount if the toilet works well, but we smell it at very, very low concentrations.

Methane is indeed odorless.

Regards

Marijn
Marijn Zandee

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  • joeturner
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

Ah, that's true, it is the Sulphate gases and ammonia which are particularly smelly.

That said, I think it is unlikely that a composting toilet is going to be aerobic, or at least fully aerobic. The production of the gases will presumably depend on the size of the latrine and the depth of the faeces in it etc.

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  • Elisabeth
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What do we know about odorous gases from composting toilets (or from composting in general)?

Recently a question came up in Wikipedia whiled editing the article on "composting toilets": en.wikipedia.org/wiki/Composting_toilet

The sentence stated:

"a ventilation unit to provide air to ensure aerobic conditions, to allow water to evaporate and to vent odorous gases."


Another editor added:

"a ventilation unit to provide air to ensure aerobic conditions, to allow water to evaporate and to vent odorous gases, such as methane."


I then deleted the "such as methane" because I thought it would give people the impression that they are dealing with an anerobic process here. Normally the toilets should be aerobic and produce no methane.

However, then Joe pointed out to me in 3 e-mails:

All forms of composting produce some methane because it is impossible to produce totally aerobic conditions outside of a wind tunnel! A composting toilet is always going to produce it.

I suspect a composting toilet is impoorly aerobic and is producing methane

This looks useful www.theseus.fi/handle/10024/44240


So now I changed the sentence to:

a ventilation unit to provide air to ensure aerobic conditions, to allow water to evaporate and to vent odorous gases, such as hydrogen sulfide, ammonia and methane


Is that valid? Should we say something about how much anaerobic conditions might prevail even in a "good" composting toilet? Is methane really odorous? According to Wikipedia it is not:

At room temperature and standard pressure, methane is a colorless, odorless gas


Therefore, it doesn't really make sense to list methane in a list of odorous gases from composting toilets as it has no odour? Only H2S and ammonia do?


Regards,
Elisabeth


P.S.

The abstract from above mentioned thesis which Joe found is:

A Study of Gas Emissions from Dry Toilets
Tsang, Pui Ki
Tampereen ammattikorkeakoulu
2012


Composting the human excreta in a dry-toilet is a widely applied procedure for stabilizing its organic matter content. A proper water balance in compost is an essential element in maintaining the microbial activity in the compost-mixture, and the moisture content of the compost can be significantly decreased by too intensive ventilation, which affects directly the composting performance. . The inevitable gas emission from the dry toilet, which indirectly reflects the composting process, is another important concern. Also, due to evaporation and aeration of the composting tank, precious nutrients, mainly ammonia-N, can be lost.

The main aim of the study was to understand the composting process from the gas emissions of the excreta. Two dry toilet models were used: Naturum, where urine is separated from the faecal matter, and the Dual-layer dry toilet, which is a mixed composter. The monitoring time of three months was divided into four periods, regarding the adjustments of air ventilation and moisture content. The scope of the work was to evaluate the CO2, O2, H2S, NH3 and CH4 emissions, temperature and relative humidity from the composting, and also to optimize the air exchange and consequently the composting process.

From the results it can be concluded that the moisture content of the compost was successfully improved by reducing air ventilation. In the Dual-layer dry toilet, all gas emissions followed the same pattern as the moisture content. However, the moisture content difference was rather small; therefore, a final conclusion about the relationship between the moisture content of compost and the emission rate of gases cannot be drawn. In addition, the composting performance of the Naturum toilet tended to be better than in the Dual-layer dry toilet because of a higher moisture content and smaller composting scale. The input faecal nitrogen loss from the Naturum was also smaller than in the Dual-layer dry toilet. The Dual-layer dry toilet had a problem with increasing compaction of anaerobic volume in the middle part, which affects the composting performance.

The cumulative emission of the gases in the 3-month monitoring time was calculated. For the Dual-layer dry toilet, the cumulative emissions in the four periods were 2.9g±28mg for H2S, 2508g±20g for CH4, and in the last three periods 12g±193mg for NH3. In Naturum, only the H2S emissions could be quantified, being about 418±148mg over the 3-months. The recommended parameters for both toilets were a moisture content of 35-40%, and an air ventilation rate of 5L/s for the Dual-layer dry toilet and 2-3L/s for the Naturum respectively.

Dr. Elisabeth von Muench
Freelance consultant on environmental and climate projects
Located in Ulm, Germany
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