Rapid composting of human waste in cold temperatures
One of our programmes is investigating a way of rapidly composting human faeces which can then be used as crop fertiliser. Unfortunately there are harsh winters which can affect this composting during the northern Winter (i.e. approx Nov-March). The methods that we were looking at trying can only get temperatures during this time up to 30 degrees - perfect for pathogen multiplication, but not for killing of pathogens.
Temperatures in this region can range during the year between -27 to +30 degrees.
Does anyone know of any methods that can be used in cold climates to allow composting to happen quickly but effectively?
Hi John I have asked a variety of people for some input on your question here and am awaiting some responses. Oxfam do a lot of research on the subject of composting but I'm not aware that they focus on anything in cold climates.
In theory you should be able to raise the compost to a higher temperature, more like 55oC, but this will be dependent on your system design and is likely to take more time than usual (maybe several weeks). As you know the composting process is an exothermic reaction, but is dependent on the 'fuel' composition, moisture content and aeration. The size of the compost heap or pit will also affect the process - you'll need to go back to your heat transfer lectures looking at heat loss relative to surface area compared to heat gain by the mass of the compost volume.
There is an excellent book about composting written by Roger Haug (The Practical Handbook of Compost Engineering) but unfortunately it doesn't seem to be available in download-able format.
Please send through some details of your system (size, orientation, is it aerated or stirred, is it insulated, above ground or below, etc) and I will see if I can find some more specific advice.
All the best Harriette
If you are looking at large volumes, your first option would be to consider burying the composting unit below the frost line thereby insulating the unit and reducing heat loss. A second option would be to provide an energy input to maintain temperatures. This could potentially be achieved by maintaining an anaerobic digester & using the methane produced to provide heat to the digester, potentially using a heating coil within the unit. Another alternative is, as Harriet suggests, to incorporate Fourier's law into your design and construction). All of the above obviously come at a cost, both in terms of process management and capital costs. Unfortunately there is also a trade off in your objective as operating a digester to produce methane reduces the quality of the solids produced as a fertiliser. You would also need to provide a source of digestible carbon (e.g. food wastes), and potentially animal manure ( cattle or poultry) to your input stream to maintain optimal conditions and an effective microbial community within the digester. You also need to consider disposal of the effluent from the digester, which would be highly polluting within the aquatic environment. Potentially this effluent could be used directly as a liquid fertiliser, following settlement and/or screening. The likelihood is that the effluent would need to be treated aerobically, to reduce Biological Oxygen Demand (BOD) & Chemical Oxygen demand (COD) before discharge. An alternative to an aerobic digester would be to construct an artificial wetland/reed bed to remove nutrients, BOD and COD from the effluent. These however require management and a large land surface. See http://americanbiogascouncil.org/pdf/... for an outline. For household volumes you could use dry toilets, see https://wikiwater.fr/a10-ecosan-ecolo... as a starting point. These could provide a solution to the sanitation issue, but not your composting temperature. You could review the WHO guidelines on the safe reuse of human faeces and gray water in agriculture to determine if it is feasible to provide sufficient retention time within your system to allow safe reuse as a fertilizer. See http://www.who.int/water_sanitation_h....
Hope this helps.
Hi John, I have been in touch with Roger Haug, the author of The Practical Handbook of Compost Engineering, who has offered the following advice:
There are two potential problems in cold weather composting. One is thermodynamic and one is kinetic. Let’s start with the thermodynamic issue. While you don’t see it, the major mass moving through a composting system usually is air. When the air is cold it takes a great deal of energy to warm it. Further, when you warm it up it then carries more water vapor and the composting material must also supply the heat of evaporation. So the substrate needs to contain a high level of biodegradable material so that sufficient heat is released. Human faeces is highly biodegradable so the question is what other amendments are they adding to the mix. They are probably adding other amendments to produce a friable mixture that allows air flow. I have no idea what they’re using. In general, hardwood sawdust is more biodegradable than softwood sawdust. I once had a composting facility that switched to hardwood sawdust to improve their energy balance.
If the starting substrates are reasonably degradable, then we need to look at controlling the air flow through the composting mixture. Obviously, less air flow means less heat removed with the off gases. Then we need to control loses to the surroundings. If this is a low-tech windrow system, then larger windrows will be better than smaller ones.
Now let’s talk about kinetics. I once visited a biosolids composting facility in New England. It was winter and the piles would not heat up. It appeared that the starting mixture was energy “rich”, but the piles just sat there. Temperatures were near freezing and the starting substrates were all near freezing. Mind you, the biosolids were anaerobically digested at 95 0F but were allowed to sit around and lose all that heat. I reminded them that that is why we put perishables in a refrigerator…to decrease their rates of reaction. Their problem was that the kinetics were so slow at 0 deg F that nothing would get started. If the substrates retained their temperature, the kinetics might be fast enough to “light the biological fire”. If all else fails, temporarily heating the incoming air might be enough to get things started.
There is a type of composter that is particularly well suited for cold climates. There are several names: box composter, tunnel composter, Herhof Box. Picture a square shaped tunnel, perhaps 8-ft by 8-ft and say 100-ft long. The length is variable depending on the amount of composter material. It has a floor, two walls, and a top. One end is closed, while the other has doors that can be opened and closed. All sides are insulated. With the doors open, a front end loader loads a batch of composting substrate into the tunnel, filling it to about 80% of its height. The doors are closed and air begins to be ... (more)
Hello. Thanks very much for the responses so far. The information I am getting from our field teams is still not a lot, but what I can say is as follows:
- Normally added to the initially substrates (i.e. the human waste) are vegetable residues, tree leaves and branches. Any other potential additions, such as rice hays and corn stalks, are kept for animal food.
- They are looking at introducing windrows with black plastic sheets to protect from the rain and snow, and with PVC poles for aeration. I have asked the team for some more detailed designs on these that I can share with you.
- A biogas digester has been implemented by another agency, but fresh and semi-processed sludges were combined due to the agricultural demand. Nevertheless, it is very difficult to reproduce this type of approach at a smaller, household level, due to a lack of resources.
- The team are now researching the tunnel composter as has been mentioned, and would be interested in introducing a similar concept. However, the main issue again here is looking how this can be somehow replicated at a simpler, household level (and if this is even possible at a household level, or whether this approach does require slightly larger volumes than would be possible at a household level).
I'll add more information as soon as I have it.
Thanks very much, John
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