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

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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.


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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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  • 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

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

Marijn Zandee wrote: Methane is indeed odorless.


Nit picking here: pure methane yes, but biogas often smells quite a bit due to other gasses included, and commercially available methane usually has a smelly compound added for security reasons.

A bit OT: are there any studies under which conditions UDDTs turn smelly? My experience is that they do when overloaded (too many people using them in too short time), which of course makes sense due to lacking time to dry. But has there been studies recommending number of users per time relative to air humidity and temperature (and dry material added)?
It seems to me that overloaded UDDTs is one of the main reason why people stop using them.

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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)?

I wonder if the conditions are particularly different in a UDDT to a sewer with regard to gas: en.wikipedia.org/wiki/Sewer_gas

OK, of course there industrial effluents in a sewer, but the smell is, I think, mostly from organic gases. I also don't know if the mixing of methane with ammonia etc makes a bad smell worse - although I can confirm that the smell associated with industrial composting of faeces is high, particularly when the heap is turned - and you tend to feel it at the back of the throat.

Exactly the conditions where the smell becomes unbearable in a UDDT or from a latrine I don't know, but would assume it is to do with the amount of available oxygen with the faeces.
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  • christoph
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

Hi,
would like to contribute from my understanding:

a) I think it is important not to mix thinking about UDDT and composting toilets - partly similar but processes are different.

b) Definitely Methane is odorless and stays like that even mixed with other gases. The other gases might be smelly but never methane.

c) I think it is worthless to be discussing if in a normal well composting toilet (not composting heap as this relates to composting toilets with different organic, protein etc loading) are some tiny spots anaerobic. As Marjin pointed out - predominantly aerobic describes it well as that is the important aspect.

d)

particularly when the heap is turned - and you tend to feel it at the back of the throat.

This is typical for NH3, as by turning large amounts of NH3 are freed – and is probably due to the much higher protein concentrations. In my knowledge not the case in composting toilets.

e) I heard sometimes about complains of smell from the ventilation of composting toilets – maybe somebody with more experience can add if this is normal or part of a not so well maintained or overloaded toilet.

Regards
Christoph
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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)?

It may well be true that dehydration and urine diversion has an important impact on gas release and odors.

But in terms of a composting toilet specifically, I agree with Geoffrey Hill, Susan Baldwin and Björn Vinnerås when they state:

Numerous composting toilet studies indicate a failure to produce sanitized material let alone stable and mature compost low in foreign matter as defined above due to a variety of causes including: poor design, overuse, insufficient maintenance, low temperatures, anaerobic conditions, and excessive urine.


I also agree with their conclusion that if thermophyllic temperatures are not reached in composting toilets, they can hardly be said to be composting at all.

www.sciencedirect.com/science/article/pii/S0301479713000169

I also agree with others who suggest that the amount of anaerobic conditions in a composting toilet will depend on the moisture content, which may be hard to control in any given circumstance - for example Miguel Angel Lopez Zavalaa and Naoyuki Funamizum suggest low dry matter contents produce both aerobic and anaerobic conditions in a composting toilet.

www.tandfonline.com/doi/abs/10.1080/1065...0702242#.VRa1dWMRc6I

The measured gas releases reported in the above doctoral thesis indicate that the study dry toilets were imperfectly aerobic.

As Chirjiv Anand and Defne Apul state:

Adequate aeration is necessary to maintain aerobic conditions for composting. Lack of oxygen in the pile can cause anaerobic conditions which leads to odor issues and lowers the rate of composting.


www.sciencedirect.com/science/article/pii/S0956053X13004923

Indoor composting systems have limited air, by necessity because it in an enclosed space. In my view this means that it is highly likely that there are considerable amounts of a composting toilet which are actually anaerobic. Furthermore, even industrial composting systems with forced oxygen lose ammonia and methane, suggesting these too have considerable amounts of anaerobic conditions in the heap.

Controlling the moisture content, temperature, C/N ratio, size of storage space, type of carbon material, surface area etc will all impact on the amount of oxygen in the compost and therefore the overall aerobic status, but I still agree with Hill et al. that the notion of aerobic composting toilets is essentially a misnomer. With actually measuring the gas releases, I cannot see how one could tell whether the anaerobic sites are overall large or small.

When and where the odors become a problem is almost entirely related to local conditions and management. It might also be true that reports of people failing to use installed facilities due to the smell reflect cultural expectations rather than large amounts of odor/gas released by them.
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  • pkjha
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

Dear Elizabeth and others

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


Ventilation unit is not for providing air to waste. It is only for exit of gases.Inside compost pit there is always a bit higher pressure of gases than atmospheric pressure and air can't flow against the pressure gradient. Air flow will always be from pit to atmosphere through vent pipe.
Anaerobic condition can't be ruled out in any pit toilet/ composting toilet or septic tank. Odor is due to hydrogen sulphide and ammonia, methane is odorless.
Aerobic condition can be provided only through forced aeration in composting toilets. It has never been reported so far. However, for waste water treatment there are technologies based on forced aeration. In such case only carbon dioxide gas is produced, that is odorless.
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  • F H Mughal
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

Composting is an aerobic process. Aerobic processes yield water and carbon dioxide. Odors from sanitation systems are due to hydrogen sulfide and ammonia gases. Methane is not a odorous gas.

Arno: That article: "Understanding the Generation and Chemistry of Odours," was informative. Thank you

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

Hi everyone,

Just a couple of comments on this important subject of smell.

Why do we not see more on-going research on the gases actually being emitted from different toilets? Can't air samples be analyzed somewhat easily in gas chromatographs (if researchers have access to these)? What about the university projects funded by the Gates Foundation?

On this front, I would encourage researchers to look at the effect of using physically, chemically and biologically complex soil/"finished biosolids" as cover material in UDDTs, since they may absorb/consume these gases before they get out.

Best wishes,
Chris Canaday

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Puyo, Pastaza, Ecuador, South America
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  • markanday
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

It is mainly H2S ( Hydrogen Sulphide) which emanates during the dehydration process of excreta confined in the vault. Ventilation just facilitates the safe passage of this gas to higher up in the atmosphere or else it would give very smelly in th toilet enclosure as well as the premises. .
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  • canaday
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Re: What do we know about odorous gases from composting toilets (or from composting in general)?

Hi Markanday,

Thanks for your comment.
Can we create conditions in which H2S does not get produced?
Can ''finished biosolids'', used as cover material, absorb or consume H2S?
Welcome to the Forum. Please tell us more about yourself.

(Joe, that eThekwini Municipality mentions health risks from extracting biosolids from properly used UDDTs is apparently due to psychology and social norms, not documented cases of people getting sick. So many people, including you, have the mindset that ''once feces, always feces, thus the world human population is gradually converting all of the non-feces in the biosphere into feces.'' The biosolids that come out of a properly used UDDT are not feces any more, based on microbiology or any other variable you choose. They had been feces for a while and before that they were other things, including soil if we turn the clock back far enough. There may be some Ascaris eggs, but we can fine tune things to control them also, as I have done here in Ecuador.)

Best wishes,
Chris Canaday

Conservation Biologist and EcoSan Promoter
Omaere Ethnobotanical Park
Puyo, Pastaza, Ecuador, South America
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