Re: DIG-L: a note on "philosophical" vs. "technical" discussi

["Les. Gornall" <ae198@dial.pipex.com>]

>Subject:     Re: DIG-L: a note on "philosophical" vs. "technical"   discussi
>Sent:        27/2/20 1:21
>Received:    22/2/0 14:56
>From:        doelle, doelle@ozemail.com.au


>Of course, liquid and solid fuel from anaerobic digesters is a much safer 
>biofertiliser than straight manure. Organic farming using manure is 
>criminal and should not be allowed. It is an enormous potential health 
>hazatrds.

Dear Dr Doelle and the group, 

According to your statement Dr., all the Northern Ireland farmers I know 
should be locked up or fined!   The biological arguement for 
pasteurisation or hygenisation by AD is not so clear as you make out. The 
problem appears to me to be in the details of the ratios.

Take ecoli for example.  A manure might contain 5 log ecoli per 100ml, a 
mesophilic digester may kill 3 log as long as the digester is not fed 
continously and well stirred (as with a good CSTR) since in a well 
stirred CSTR with a 20 day RT it is axiomatic that 5% of the fresh input 
is expelled with the digested output.  The infectivity level of 
ecoli(157) is I believe 25 colony forming units in 100 mls  or just over 
1 log.  The output from the digester is at least 2 log and may contain 
3-4 log  - which is quite enough to kill children working with the 
material.

So why digest for pathogen control.

 In Denmark the best scientists and vets have approved of AD with various 
caveats.

1.  If the digestion of wastes is 'hygenic' then all the farms that 
receive the waste may be regarded for veterinary purposes as separate 
farms.

2.  If the digestion of wastes is not 'hygenic' then the farms that 
receive the waste are regarded as one farm and therefore if one farm 
supplying waste to the digester is infected with an outbreak of say 
salmonella, then all the farms are quarrantined.

3. 'Hygenic' digestion is clearly defined as a digestion without short 
circuiting in the digester for a minium of 8 hours at a minimum 
temperatue of 53 degrees C. That is to say the minimum temperature must 
never fall below the set limit. Graphing recorders are required to prove 
the point.

So far (a decade) this has worked well in practice and has enabled many 
Centralised AD units to be built and operated economically.

In the UK:

The best recent summary of the AD industry in recent years has been 
written by 27 AD companies and consultants co-ordinated by the UK's 
environment council.  The issue of pathogen control by digesters has been 
very carefully researched. The pathogen kill conclusions appear in 
appendix 7 of the "Good Practice Guidelines - Anaerobic Digestion of farm 
and food processing residues - the development of a sustainable 
industry."  Published by ETSU, Harwell.

APPENDIX 7  -------snip 

PARASITES
Anaerobic digestion at 35 degrees C (known as mesophilic digestion used 
by the vast majority of farm digesters), has been shown to reduce the 
numbers of viable lavae and eggs of some parasitic worms to undetectable 
levels after one week although there is generally poor control of 
'Ascaris', 'Tacnia' (sic), and 'cryptosporidium'.  However the oocysts (a 
type of egg) of these worms, which can infect any animal ingesting them, 
remain viable after 50 days at 35 deg.C.  Digestion at 55 deg. C 
significantly reduces the number of oocysts, but many viable units still 
persist, even after digestion for extended periods of time.  Mesophilic 
anaerobic digestion will not significantly reduce the infectivity of 
slurry contaminated with parasites.

PATHOGENIC BACTERIA
Experiments with pig slurry have shown that mesophilic digestion for four 
days will destroy 90% of a population of Salmonella bacteria, although 
low levels of the bacteria will persist even after 70 days digestion.  
This persistence may be due to poor mixing within the digester, allowing 
pockets of bacteria to survive in a crust on the surface.  Properly 
managed, thermophilic digestion can reduce bacterial levels by an even 
greater factor, but again low levels may persist due to poor mixing.  
Mesophilic anaerobic digestion will significantly reduce the levels of 
pathogenic bacteria, but will not eliminate them completely from the 
waste and so the COSSH [ Control of Subtances Hazardous to Health 
regulations 1994 -LKG] assessment will determine the measures required to 
control the potential damage to human health. 

VIRUSES
Research findings have shown that mesophilic anaerobic digestion reduces 
virus numbers in sewage samples, but viable virus units may still be 
found to be present, even after extended periods of digestion.

-----snip


There is much work to be done in this important area - I am collecting 
papers on the subject and would be grateful for any references from 
readers.





Best Regards




Les. Gornall
Director
Practically Green 
Environmental Services
Solar House
Magherafelt
BT45 6HW
Northern Ireland
+44 1648  32615 Tel./Fax

http://www.practicallygreen.com

Homepage of "Practically Green" and "Practically Green News"

DIGESTION List Sponsors, Archive and Information
http://www.crest.org/renewables/digestion-list-archive
http://www.crest.org/renewables/biomass-info/
Beginners Tour of Biogas
http://WWW.roseworthy.adelaide.edu.au/~pharris/biogas/beginners