9 June 2010Page 1 of 5 BIODIGESTER FACT SHEET The digestion
process When people eat food, the food passes through their
digestive system.The food material that the body does not use is
expelled from the body in the form of urine, feces and gas.A
critical part of digestion in animals is done by microbes.Some of
the microbes in digestive systems and in nature produce a gas,
methane, as a waste product. What is a biodigester? A biodigester
uses bacteria to break down organic matter and capture methane
released by the bacteria in a process called anaerobic
fermentation.Anaerobic means the micro-organisms digest the food in
the absence of oxygen.There are several kinds of microbes at
work.Some break the food into simpler molecules of sugars and
acids.Other microbes, which find oxygen toxic, are able to break
down the simple organic matter to form gases, including the
burnable gas methane.Methane, the main chemical in natural gas, is
trapped and can then be burned for heating and electricity.The
leftover organic solid waste can be used as fertilizer, a soil
supplement or further composted. The most common type of digester
is called a wet biodigester because they are used to digest
liquefied manure on farms around the world.Many people use wet
digesters to produce methane from manure and then they use the
methane to power their stoves and other appliances (Meynell,
1978).Compostable matter as well as slaughterhouse waste can be
placed in a wet biodigester.However, all matter that goes into the
digester must be pulverized in order to become water
soluble.According to B. Xuan An et al. (1997) 15 liters of water
are needed for every kilogram of manure.Even more water is needed
for other material because manure already has high water content.
Wet biodigesters need to be kept in a warm environment.If they are
in a cold environment they need to be heated, which means more of
the energy generated from the digester needs to go to its own
heating.According to BioFerm Energy Systems, wet biodigesters use
10%-30% of the energy they generate in order to operate.Additional
energy is also needed to treat waste water emerging from the
system. What is a Dry Fermentation Biodigester? A dry fermentation
biodigester also uses microorganisms to decompose organic matter in
an oxygen-free environment (anaerobic digestion).However, with dry
biodigestion, little or no water is added; moisture from the
feedstock is often enough to support the process.In the digestion
process, organic waste is loaded into enclosed areas called
fermenters.Each batch of waste stays in an individual, oxygen-free
fermenter for a 28-day cycle.Small amounts of liquid drain from the
feedstock.This water containing dissolved organic matter and
microbes, is called percolate.Throughout the 28-day cycle, the
percolate is sprayed onto the feedstock daily.The percolate is
allowed to drain through the waste, then is collected and
reused.The percolate is recycled during the 28-day cycle and is
used for future batches as well.Each fermenter also has an in-floor
heating system that is set at a constant temperature of 9 June
2010Page 2 of 5 100.4F.At this high temperature, the waste is able
to decompose at a faster rate.Of the biogas produced by the
micro-organisms, 45-70% is methane (BioFerm Energy Systems,
2009).The biogas is collected in a holding cell where it is then
transferred to a generator where heat and power are produced.Below
is a diagram that depicts the route of the organic waste: *Pictures
taken from BioFerm *An animated version of the BioFerm process can
be seen at http://www.biofermenergy.com/us/ Compost to fermentation
chamber Biogas goes to generator where heat and power are produced.
Biogas is produced and collected. 9 June 2010Page 3 of 5 What is
the anaerobic digestion process? The anaerobic digestion process
allows waste to be turned into biogas.The ideal waste stream
contains a balance of essential elements (C, N, P, S), digestible
sugars, pH, and water content (not too wet). Some of these factors
can be balanced by chemical additions, but that would increase the
cost of operating the biodigester.Some plant materials contain
chemicals that inhibit microbial growth, and these should be
minimized. According to BioFerm there are four steps in the
Anaerobic digestion process: Hydrolysis, Acidogenesis/Fermentation,
Acetogenesis, and Methanogenesis.Different types of bacteria
complete each of these steps: hydrolytic bacteria, acidogenic
bacteria, acetogenic bacteria and methanogenic bacteria.Hydrolysis
is when bacteria break down fats, carbohydrates and proteins by
removing water.The result of hydrolysis is fatty acids, sugars and
amino acids.In Acidogenesis/Fermentation acidogenic (acid forming)
bacteria convert the fatty acids, sugars and amino acids into
carbonic acids and alcohols as well as hydrogen, carbon dioxide and
ammonia.Acetogenesis occurs when anaerobic bacteria, called
acetogenic bacteria, convert organic carbonic acids and alcohols as
well as hydrogen, carbon dioxide and ammonia into acetic acid,
carbon dioxide, and hydrogen.Methanogenesis uses bacteria called
methanogens to convert hydrogen and carbon dioxide to methane and
water.Methanogens convert acetic acid and hydrogen to carbon
dioxide and methane.The very simplified conversion is C6H12O6 3CO2
+ 3CH4. Below is a diagram of the anaerobic digestion process: Are
there drawbacks for a dry-fermentation biodigester? With the dry
fermentation biodigester there are many benefits, but there are
some drawbacks as well.The main reason these biodigesters are not
as common in the USA as wet biodigesters is that they do not
accommodate manure, which is a plentiful and problematic waste
product in this country.Especially in Wisconsin, which has the most
manure-fueled wet digestion facilities in the country.Secondly food
waste, which is a much more prevalent biodigester fuel in urban
areas, is rarely collected in North America.In Wisconsin, food
waste and food-soiled compostable material (paper, cardboard) make
up about one-quarter of landfilled waste, the largest component
suitable for diversion. All digesters need to be heated to take
full advantage of decomposition, but dry digestion requires less
energy because the water in wet digesters takes more energy to
heat.according to BioFerm dry fermenters use only 5% of the energy
generated to power itself. 9 June 2010Page 4 of 5 All digesters
produce odors, but facilities can be enclosed and built with air
filtration systems that convert odor chemicals to odorless forms
(again using microbes to do the work). Dry biodigesters do not
produce wastewater because the compostable material does not need
to be water soluble or made into sludge in order for the
micro-organisms to break it down and release methane. What will the
dry-fermentation biodigester do for UW Oshkosh? Waste
Management:The dry-fermentation biodigester will reduce the amount
of solid waste UW-Oshkosh sends to the landfill.Food and yard waste
will be diverted to the biodigester.However, the biodigester will
need a minimum of 6,000 tons of feedstock per year, which is about
20-times more than the university will produce.When the plant
opens, most of the feedstock will come from food manufacturer
waste, farm waste, and the City of Oshkosh Yard Waste site adjacent
to the plant.A small fraction will come from community food waste
originating at schools restaurants and grocers. Long-range plans
are to steadily increase the input of community food and yard
waste, potentially supporting municipal organic material collection
currently operating in most of Europe and Japan as well as over 90
communities in North America (Yepson 2009).Products:The
biogas-fired generators will produce electricity and heat.The
electricity will be fed to the grid.The heat will be piped as hot
water to the Campus Services Center, next door, and run along heat
exchangers for building.The generators will be large enough to use
biogas piped in from the Oshkosh Wastewater Treatment Facility
located across the street.The electricity production will be nearly
3 million kilowatt-hours, about 10% of the campus electricity
use.The heat generation could be up to 9% of campus heating needs,
but much of the heat may be sold to neighbors as there is only one
campus building in the vicinity.The compost by-product can be sold
as a soil amendment or given to farmers or other businesses that
provide the biodigester with yard and food waste. Research and
Education:The Aquatic Research Laboratory is in the process of
being remodeled so that it can also support experiments using
miniature biodigesters.Since our biodigester is the first of its
kind in the nation, University faculty and students will have the
opportunity to test waste for potential future biodigester
investors.The University will be able to use the lab to estimate
the amount of methane microorganisms release, the types of microbes
that work best, and the chemical makeup of the digested solids.The
lab will also has a classroom, teaching lab, conference rooms and
offices to support courses, workshops and visiting scientists as
part of a Renewable Energy Institute. QUICK FACTS: Generator
Capacity: 450 kW Feedstock Estimate:6,000 tons organic matter per
year Biogas from wastewater treatment plant:35% addition to
biodigester production Net Electricity Generation
estimate:3,000,000 kWh per year Net Heat Generation: 120,000 Therms
per year (= 3,400,000 kWh per year) 9 June 2010Page 5 of 5
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