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Dear All:
To justify mesophilic over thermophilic digestion
because of its claimed better process stability and beneficial nutrient yield
represents
incorrect thinking. The principal purpose of
treatment is to maximize the removal of pollutants by conversion to
solids. It goes without
saying that the less the resulting solids the
greater the efficiency of treatment. Biosolids and liquid effluents from
mesophilic
treatment can hardly be thought of as a useful
source of nutrients as the nutrient concentrations are very low and both the
solids
and liquids still contain pathogens.
While traditional single-phase themophilic
treatment is indeed difficult if not impossible to control,
two-phase thermophilic digestion can be very easy to
control. At this higher temperature (60 degrees C) pathogens are virtually
eliminated from both the treated solids and liquids. The solids
remaining are perhaps 5-10% of those left over after mesophilic
treatment. These few solids can, however, still be safely
used as a soil conditioner or amendment, not a fertilizer. For those still
interested
in fertilizer values, the liquid effluent from thermophilic digestion contains far greater levels of ammonia-nitrogen than mesophilic
treatment
as this specific nutrient is generated during anaerobic digestion. As such the liquid
effluent from thermophilic digestion is a splendid
candidate for
irrigation water so long as the ammonia-nitrogen
nutrient uptake of the crop or grassland is equal to or exceeds the application
loading to prevent nitrate contamination of
groundwater.
It is true that thermophilic treatment requires
greater heating than mesophilic treatment. Two-phase thermophilic
digestion generates
twice the quantity of methane as mesophilic because
of its ability to achieve twice the level of volatile solids reduction.
This greater
gas production easily produces the additional
energy required to heat the digester. Additionally, there should be
no credible concern
over the higher
operating temperature because of the ease of capturing the vast majority of
the exit heat through the use of heat exchangers.
In short, mesophilic digestion represents obsolete
technology compared with the far greater efficiencies and associated
pathogenic
destruction resulting from two-phase
thermophilic digestion. As an example, a two-phase thermophilic digester utilizing
fixed growth
bacteria can successfully treat at least ten (10) times the organic loading of a traditional high
rate single phase mesophilic digester.
The good news is that most mesophilic digesters can generally be converted or upgraded to this
type of a treatment dynamo. The true
potential of anaerobic digestion if far greater than most of us realized a few short years
ago. Eliminating thermophilic digestion is but
a technology step backwards regardless of how its
justification is formulated. Regards-Chuck Steiner, WaterSmart
Environmental, Inc.
----- Original Message -----
Sent: Sunday, February 13, 2000 4:55
AM
Subject: Re: DIG-L: Anaerobic
digestion
Dear Lisa,
In a cold climate like Europe I would never
recommend a thermophilic digester, particularly if you require the
by-products.
As I mentioned in an earlier message, mesophilic digesters
reduce pathogens significantly and as you know, also the BOD by approx. 60%.
Thus you have enough nutrients left in the solid and liquid effluents to
compost or use it as an excellent resource for aquaculture, eg algae and fish.
Also the biogas requirement to keep your temperature all year round at 35-37C
is far lower and you can sell the rest or use the rest of the energy for other
purposes.
In the case of a thermophilic digestion, it has been claimed
that the BOD can be reduced by 80% +, pathogens are of course further reduced,
biogas yield is higher, BUT you require a much higher amount of biogas for
heating your digester. Your by-product opportunities are significantly
reduced, since the solid and liquid effluents are much poorer in
nutrients.
Both types have their place. I prefer the mesophilic one
because the economics appear to be much better, IF you believe in an
integrated biosystem. Thermophilic digesters are good if you do not believe in
an integrated biosystem.
I hope this helps.
Best regards
Horst
Doelle
At 11:51 12/02/00 +0000, you wrote:
>Dear
all,
>
>My name is Lisa Pritchard and I currently work for a
company called
>GreenfinchLtd based in Shropshire, UK. Our demonstration
plant is a 20m3
>anaerobic digester which treats 5-7 tonnes of
source-separated household
>kitchen waste (biowaste) each week from
approximately 1500 households.
>
>One of our objectives is to
recycle and reuse of by-products of the AD
>process. This does not
include agricultural land application. As you
>know regulations are
getting tighter so we do not want to rely on land
>application to reuse
the nutrients effectively. Hence we are reusing our
>by-products through
various processes including aquaculture
and
>vermiculture.
>
>Our digester is mesophilc. Do you see
any clear advantages for running our
>digester at thermophilic
temperatures, bearing in mind our climate and
>need to reuse all
by-products?
>
>Thankyou and look forward to any
replies,
>
>Lisa Pritchard
>
>
>
>*** greenfinch@burford.co.uk using SmartMail v4.00.15 ***
>--------------------------------------------------
>
>
>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
>
>
Horst
W.Doelle, D.Sc., D.Sc. [h.c.]
Chairman, IOBB
Director,
MIRCEN-Biotechnology
FAX: +617-38783230
Email:
doelle@ozemail.com.au 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
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