[Stoves] Composting at 70 deg C ?

[ajheggie at gmail.com]

On Monday 30 May 2011 03:00:04 rajan_jiby at dataone.in wrote:

>
> Dear Frank,
>
> Pasteurization of Milk is carried out around 70 deg C ( hope I remember
> correctly ), when most of the bacteria are destroyed ( some bacteria
> must be surviving ).

Yes but all the pathogenic ones should be gone and the souring ones 
reduced to low levels which they take a long time to recover from in a 
sealed container.
>
> My question is : Can composting happen naturally at 70 deg C ( ie.
> without any external heating to assist ) ?

Definitely; in UK legislation requires us to deal with food waste 
containing animal products by heating it to 70C ( following breakdown in 
swill sterilisation feeding this waste to animals is now banned to break 
the pathogen cycle, principally following Foot and Mouth Disease).

There are a number of ways of doing this and the output from these systems 
is generally spread on the land.

Because of the problem from vermin and pathogens all need to take place 
inside buildings whereas green waste composting without animal by 
products can take place in the open in windrows which are regularly 
turned to allow air to oxygenate the pile.

In vessel composting of waste combined with animal by product waste seems 
to be becoming the norm but I have serious reservations that it is 
properly meeting the legal requirements, UK has a derogation from the EU 
standard which allows lower temperatures for longer duration. The general 
rule is that every part of the heap must reach 70C for at least one hour. 
The thermophylic microbes that compost the sugary and fatty parts of the 
biomass raise the temperature of the compost to 70C by oxidising these 
high energy volatile fractions of the waste. This plus the following 
maturation period where mesophylic bugs continue to break down the less 
available parts of the biomass reduce the biological oxygen demand such 
that it becomes benign for application as compost. All the energy in the 
waste is lost and the biomass is essentially returned to water and CO2 
within a few years.

A division of the vegetation management company I work for has a more 
refined version of this for food waste only ( i.e. discarded food with no 
woody fraction) This is macerated into a soup and fed into a succession 
of sealed, well insulated, cylindrical silos which are agitated and fed 
with compressed air. Their temperature rises rapidly to 70 C and they 
stay in the first vessel at this temperature for a few hours. Because 
this is a technically refined and homogeneous method it is better 
monitored than ordinary in vessel methods and guarantees all the liquid 
reaches 70C using a smaller proportion of the energy in the food. Because 
of this the BOD of the liquid is too high to apply straight to land ( you 
would get spectacular growths of mould on the soil and oxygen would be 
robbed from the soil as well as runoff being a problem)  maturing in 
another large vessel reduces the BOD.Again there is no energy benefit and 
the process consumes significant amounts of electricity..

The method preferred by government is to recover energy from the volatile 
solids in the food waste by digestion in anaerobic conditions to produce 
biogas, a mixture of methane and CO2, which can be refined and fed into 
the natural gas grid, burned for process heat or in an engine. A D Karve 
has posted much on this subject. These methanogenic bacteria are 
mesophylic and work best at around blood temperature ( they have evolved 
to live in bovine rumen) and, as much of the energy remains in the gas, 
the vessel often needs external heating, to keep them working optimally. 
Even though the temperatures are lower the system neutralises most 
pathogens and there are restrictions on which land can be dosed with the 
digestate to guarantee any pathogens don't survive into the food chain. 
At the industrial level anaerobic digestion for methane to power is 
fairly capital intensive and UK government offers a large incentive (Feed 
in Tarriff) to encourage the investment in capital. Unfortunately as 
there is a large gate fee (about £50/tonne) for this waste it has paid 
operators to invest in the much cheaper in vessel systems and probably 
fudge the temperature monitoring in unfavourable conditions. The 
dewatered sludge from anaerobic digestion has proved to be an interesting 
feedstock for biochar but I have lost the cite. An anaerobic digester 
with biochar production should give both energy and environmental 
benefit.
>
> If external heat is applied, what could be the maximum arrived
> temperature range for the compost pile ?

I don't think any bugs living on these biomass substrates will survive 
much higher than 70C although there are some that feed on iron salts and 
sulphur that exist at higher temperatures in volcanic regions i.e. 
undersea or in geysers.
AJH