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The Renertech Biogas Process.
Wastes to Energy
in a Clean Coffee Environment.
Ken C
Calvert.
Jan C von Enden.
January 2003
www.venden.de
The Renertech process for making biogas from coffee
waste waters was originally developed in in the
In a matter of hours rather than days.
The Coffee production industry is, in general, not noted for its care of the
environment.
Our
key word is a generated one, ‘CLEMEG’: Clean, Lean, Mean & Green.
The underlying philosophy for developing this integrated system was to use only naturally
available substances, and use them in
the most economical way to allow for
its use in remote underdeveloped areas with a minimum
of infrastructure
and supply.. .
The basis of the process was the discovery that in starting
from the traditional two
stage UASB process a new second stage could be inserted, whereby a surplus of ground raw limestone or marble chips
could be used to automatically buffer a SCVFA solution,
(short chain volatile fatty acid), largely
acetate, at pH 6.1. At the
same time, over a period of more than five years, a mix of psychrophylic
methanogenic bacteria were isolated from coffee
soils, to which had
been added the gut
contents of many
cold blooded species of fish, reptiles and insects.
As
a result of prolonged enrichment techniques, we
now have a septic
strain of anaerobes whichwill gas freely
on only coffee waste waters at low
pH and at ambient temperatures. This anaerobic sludge now constitutes a valuable resource for the global coffeeindustry. It is the intention
of the discoverers not to patent this process in any way and to freely disclose
any new developments that come to hand, so that the coffeeindustry of the third world
may benefit. However,
for those companies and institutions who want to short circuit the
5 year development period, supplies of the Renertech Sludge can be made available under a licensing agreement.
three
stage UASB process, also provides the potential for reduction of carbon dioxide
in the output gas by taking out half of the CO2 at the fermentation stage.
The traditionally understood reaction for production
of biogas starts from acetic
acid and produces equal amounts of methane and carbon dioxide.
CH3 COOH = CH4 + CO2
However, neutralizing the acid first with raw limestone produces a molecule of carbon dioxide in the
first stage, which can be got rid of
before the effluent enters the biogas digester.
2CH3COOH +
CaCO3 = Ca(CH3COO)2 + CO2 + H2O
Then, reacting only the acetate ions, produces only one molecule of free carbon dioxide as against two of methane. This makes for a raw biogas with a
much higher energy
level. While this is of little gain
to anyone aiming at a stripped natural
gas, for
low cost ‘Village’
or ‘Institutional’ level operations
with straight biogas ‘per
se’, this is a considerable advantage.
Ca(CH3COO)2 + H2O = 2CH4
+
CO2 + CaCO3
However, as the solubility product’s
of other calcium salts,
principally phosphates
and
a calcium/magnesium complex called ‘Struvite’, are much less by several
orders of magnitude, the above carbonate
reaction never gets enough calcium ions left over to allow it to go to completion.
What can be said however, is that the readiness of the high levels
of calcium ions in the reaction to precipitate in one form or another, does encourage the formation of relatively heavy granules which
allow for a much faster rate of effluent flow through the digester
without losing active
material. This would encourage us to
promote the EGSB process
over the UASB, but formal trials have not yet been carried out.
The practical outcome nevertheless is that the biogas coming off a
‘Renertech Process’ digester, is much richer
in methane than a typical
UASB reaction. This has
allowed it
to be fed directly into a diesel dual fuel engine without the necessity to
strip the remaining carbon
dioxide from it first. The wet gas
is simply passed through a bed of metallic iron, to wit a drum full of bashed up rusty tin
cans, to strip out the sulphides and reduce the moisture levels. This iron sulphide
process is completely self regenerating and very simple. Once one digester is working in a new area,
the high output of granular sludge seed material, due to the struvite
precipitation process, makes
start up of further digesters
only a matter of days in
stead of weeks or months.
To achieve a 6-8 hour process turn
around
of waste water within the coffee industry, it is
necessary
to concentrate the processing waste waters by intensive recycling. Every
six to eight
hours a
new tank or silo should be used to store the pulpage,(freshly pulped beans), and a fresh batch of water
is used
to restart the process. For
the next six hours that
water, plus all
the makeup water required, is drained out through the
bottom of the tank of pulpage and recycled back to the
machines and the levels of sugars and enzymes allowed
to build up to the point where the water
is heavily coloured and almost soupy.
This means that all
of the pulpage, particularly that
at the bottom of the tank, has received the same
dose of concentrated pectolytic enzymes at
a temperature several degrees
above ambient, caused by the
recycling water system. What ever hour
before
The overall process includes a full environmentally
friendly clean up of wet factory waste
waters. The Khe
Sanh factory started off with a
very high water usage system using a
pair of Pinhalense DC3/6 pulpers and demucilators in
a semi washed process.
This
plant was converted into a fully washed process by recycling the factory water supply, pumping the demucilated
coffee up into a stainless steel silo
and allowing the washing and
pulping water to drain down through the
silo for up to six hours of pulping.
The
water from the demucilators was discharged directly
to the first stage fermenter or acid pond. Every six hours the pulping water was
changed and the coffee pumped to an alternate silo. The first silo was back flushed with water which was used to kick off the next shift of
pulping, the coffee was then left to soak under clean water. The discharged recycle water,
from
the previous six hours, was then also
sent
down to the acid pond, a long narrow concrete tank of approx
200 cubic metres,
sized to hold around one days
throughput of heavily
recycled wash water and mucilage from
pulping more than 100 tonnes of cherry. When each silo full of coffee
was given a further wash the
following morning, 8-10 hours after
pulping, only very clean fully fermented
and washed wet parchment was discharged.
As
well as the build up of sugars and pectolytic enzymes
in the recycling wash water, there was also as significant rise in temperature.
By the time that the dirty water has flowed down the full length
of the fermentation or acid pond ,
around 15-20 hours,
the pH has
dropped to 3.8, and all the mucilage
has come out of solution and floats as a thick orange
scum which is allowed to build up on
the surface for several days and turn into a thick black crust which can be raked off periodically and deposited with the screened pulp solids for
composting. At the far end of the acid pond there will be a clear middle layer of yellow acid
water under the mucilage and
over the settled solids. This is then pumped
on to the next stage of neutralisation. The rate of ‘acetification’ or fermentation to acid can
be speeded up considerably by bleeding
off a small percentage
of this acid water and mixing
it back into the intake of the acid pond to create
a ‘feedback’ process..
Use
was made of an old
25,000 litre steel tank
which was three quarters filled with
screened 2-5mm limestone chips. Acid wash water is pumped into the bottom of
the tank through a manifold
and up through about 1.5
to 2metres depth of chips, with a residence time of 1-2 hours.
Once again the surface is covered with a foam of
CO2 generated solids, mucilage
and a fine black material which is
considered to be condensed tannins and polyphenolic
materials which have proved in the past to
seriously restrict the efficiency
of the biogas sludge if they were not removed. Once again, the clear solution from over the limestone
and under the foam layer , now at a pH
of more than six can be drawn off and
used for the next stage and the floating layer periodically raked off and transferred to the pulp solids for compost.
It is import to have available facilities to flush this tank and stir up the
limestone bed sufficiently to
strip off the biological film from
the chips which will slowly choke off the flow
rate over a period of 2-3 weeks. The use of a wide diameter tank with an open top
would enable the froth and
polyphenolic scum to be removed more easily by raking off the floating material, just like
the acid pond.
At Khe Sanh, the major part
of the neutralized wash water is presently discharged into a constructed wet land made in three sections,
which will be described later. Because
of financial constraints, only a 5000 litre pilot scale UASB digester
is working at present. This
consists of a 3.5 metre high stainless steel tank. Over the
inlet manifold in the
bottom of the tank is a layer of more limestone chips about 350mms deep. Above that is the sludge layer which can be up to 1.5 metres deep when
inactive, but fluffs up and granulates to make a 2 metres plus deep
bed of activated sludge. This sludge will settle and remain quiescent for
up to 12 months at a time. However at the beginning of the next season it
will reactivate in about a week. The top portion of the tank contains the gas/solids/liquids separator about 500mms under the surface of the discharge
water. It is believed that the EGSB process, using a taller digester would
be a logical progression over the present
system, but this has not been tried
yet. It is planned to build
a new larger digester of ‘ferro-cemento’ materials
for the coming season, which will incorporate
these improvements. Trials are presently under way to give some practical
numbers as
to gas production against tonnes of cherry, rather than the theoretical and difficult to determine kgs. of
dried volatile solids etc. With
the great variation in effluent strengths, Cherry is the only real measure of inputs into
the system for practical evaluation.
The discharge effluent from the digester passes through a
small settlement tank, mainly to collect and recycle escaping sludge, and then flows by gravity to the afore mentioned wet lands. The first pond ,
because of the present heavy discharge
from the neutralizing tank as compared to the biogas digester, still carries a lot of BOD and is not a good environment at present for growing anything. Only a few reeds and rushes survive.
The second pond has been planted out with local varieties of hollow
stemmed reeds and rushes. These plants actively pump enough oxygen down to their roots to allow them to survive in a totally
anaerobic environment, and they are good
reducers of both BOD and COD. After the biogas digester, they constitute the
second line of
biological filtering. In colder climates much greater use would have to be made
of the
reeds and rushes because our third stage is relative only to tropical
climates. The tertiary filter pond is much deeper, 1.5 metres,
and is filled with floating water
hyacinth which, if the pond is big enough, should take out the majority of
the fertilizer salts, the nitrates, potassium, condensed tannins etc, and any
remaining phosphates. At our present
stage of development however, there is still too much BOD coming from the acetates being
discharged from the neutralising
tank, not to mention the unreacted
acid from the acid pond, to
allow the water hyacinth to thrive.
They are only
just surviving. When they
do require thinning out, there are several options available to utilize
the excess material, of which the easiest is to chop them up and add them to the composting mix! More biogas, animal feedstuffs and SCP are also possibilities The compost is used as fertilizer to return
to the coffee.
Although this process has been developed
specifically for the coffee industry, it has also kept abreast of developments in
the olive processing and the red wine industry.
It is believed that
it could be adapted to any
fruit or agricultural products
industry which has problems with COD anthocyanins and
high levels of fruit sugars in large volumes of cold waste processing
waters.
Along with this attempt at setting up a fully sustainable coffee processing industry, the further use of Vetiver grass over a period of several years, to create a terraced coffee growing system, with no mechanical earthmoving required, could lead onto an environmentally friendly yet fully mechanized harvesting system which can convert much of the drudgery of excessive hand labour into much more pleasant working conditions and a renewed image of coffee as a progressive forward looking industry.