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Microsoft Word - Community_Biogas_Gell_Nov_11_08.docReview of Small Scale, Community Biogas in the Industrialized World
Kealan Gell
Abstract
Although there is a wealth of large scale anaerobic digestion (AD) in rich
countries and small scale AD in poor countries, small scale is uncommon in
rich countries. It is considered uneconomical for energy or fertilizer
production, and the landfill and sewage infrastructure and associated
regulations can reduce the incentive further. Numerous studies have shown
negative cost benefit analysis for community scale digesters. Still, one may
expect to find AD technology on a small scale, similarly to household and
community composting, home brewing of alcohol, bio-diesel production and
composting toilets. As cheap energy becomes scarcer, local waste treatment
and fertilizer and energy production may become more attractive.
Decentralized anaerobic digestion is considered the most sustainable organic
waste and wastewater treatment option for society. Already many community
groups in the UK are interested in AD on a small scale. This paper reviews
working, planned and attempted (very) small scale anaerobic digestion in the
developed world, and provides an economic assessment such technology.
We find that although traditional, financial assessment of such small units is
often unfavourable in the industrialized world, an integrated, non traditional
economic assessment shows much potential for such units. Possible
opportunities and paths are suggested for the community sector to utilize such
technology.
*****
This report has been produced to provide members of the Community
Composting Network (CCN) with a background on small and decentralized
anaerobic digestion projects and opportunities to use this technology. The
CCN is a network of non-for-profit organizations engaged in community
composting, primarily in the United Kingdom.
The author is now producing an MSc thesis on this topic at Wageningen
University in the Netherlands and continues to collaborate with the CCN.
Outline
• Introduction
o Basics of Design and Process
o Scales of Anaerobic Digesters
• What has been done?
o Common, big projects in the industrialized world
o Common, small projects in the developing world
o Uncommon, small projects in the industrialized world
• Economic assessment of small projects
o Traditional economics
o Non-traditional economics
• Potential for the community sector
o Opportunities for adoption of small scale AD
o Barriers to wider adoption of small scale AD
o Current level of interest from communities in the UK
• Conclusion
• References
Introduction
Anaerobic digestion is a natural process in which biomass is consumed by
micro-organisms in the absence of oxygen. This process can produce energy
in the form of biogas, and safe, stable fertilizer in the form of digestate. The
chemistry and biology of the process has been explained in detail (Fry and
Merrill 1973; House 2006; Wikipedia 2008) and can be aided by mixing,
heating and inoculating the mixture, inside of a sealed tank.
Anaerobic digestion has been exploited to produce biogas and fertilizer for
hundreds, perhaps thousands of years by the Chinese, Assyrians and
Persians. In the 19th century, biogas from sewage was used to light streets in
Exeter, England and in the late 20th century biogas was used to fuel hundreds
of vehicles across Sweden and millions of homes’ cookers in India, as
reviewed by (Tietjen 1975; Harris 2008; Sjoholm 2008).
The benefits of anaerobic digestion have been widely published. Both life
cycle assessment and cost benefit analysis have found anaerobic digestion to
be the best method of dealing with household waste, (Edelmann, Baier et al.
2005; Hogg, Gibbs et al. 2007), and it is also an optimal technology for
greenhouse gas flux (Frigon and Guiot 2005) and integrated treatment of both
sewage and kitchen waste (Zeeman, Kujawa et al. 2008).
Incentives in the United Kingdom, such as the Landfill Directive, greenhouse
gas credits and Renewable Obligation Credits for renewable energy improve
the prospects of AD, but the economics behind the current rush to install
digesters around the UK are still risky, depending on feedstock (Schwager
2008).
Economies of scale mean that in larger facilities operating costs can be
reduced, per unit, to the point that, in the current economic framework, very
large AD facilities can be profitable whereas small ones are not. If energy and
transportation prices continue to rise and the demand for local waste
treatment, energy, and fertilizer increases, this framework may change.
Regardless, decentralized waste treatment and reuse has many advantages
(van Lier and Lettinga 1999), is widely considered to be the most sustainable
form of waste treatment, and can provide community self sufficiency and
benefits to society far preferable to large scale, centralized treatment (Lettinga
2008).
This paper examines the prospects of AD on a small scale, for organizations
such as the members of the UK Community Composting Network. Common
AD projects are briefly presented, then several case studies of less common
community scale systems in developed nations are analysed in detail. An
economic assessment of small scale AD is presented, and, finally, the
prospects for this technology in the non-profit sector are considered, including
a survey of current interest.
Basics of Design and Process
During anaerobic digestion carbon in the feedstock undergoes two transitions:
from biomass to acids (acidogenisis) and from acids to gas (methanogenisis).
Sometimes these processes are separated into two tanks, and the first stage
can be used independently, to produce acids useful for compost. Further
details on process kinetics can be obtained in your favourite textbook covering
anaerobic digestion, such as (Metcalf & Eddy, Tchobanoglous et al. 2002).
One important distinction is between very low solids (less than 2%) in a
sludge blanket reactor, traditional wet digestion (solids content below 12%),
and dry, high solids (25% or higher solids) digestion that is more complex and
more recently exploited. Common designs include a floating lid or a balloon
membrane to hold biogas, above a plug flow or completely mixed clay-brick,
steel or concrete tank or a vertical cylinder where dilute water flows upwards
and loses solids to a sludge blanket. Substrate may be pasteurized to reduce
pathogens. A worldwide review of current process designs in the
industrialized world has been published by Juniper Inc. (Schwager 2008) as
well as various other publications covering units in developing countries
(Buren 1998; Wellinger 1999; House 2006).
Biogas is roughly 60% methane and 39% carbon dioxide, with small amounts
of water vapour, hydrogen sulphide, and ammonia gas. It may be burned as
is, or purified to over 99% methane, at which point it becomes identical to
natural gas, and can be used accordingly (to produce light, heat, electricity,
plastics, chemicals, transport fuel, etc).
Digestate from wet digesters is a low solids, nutrient rich liquid. All nitrogen,
phosphorous, potassium and trace nutrients from the feed stock are retained
in the digestate, making it suitable as a fertilizer.
Scales of Anaerobic Digesters
Anaerobic digestion (AD) can take place on any scale. Measurements of
scale often refer to:
• Input of biomass, tonnes per annum (tpa) or per day
• Size of the tank, volume in cubic meters
• Energy produced, power in MW or kW
In an efficient, completely mixed digester, biomass remains in the reactor for
between 17 and 50 days (retention time) or longer in less efficient systems,
where efficiency depends largely on temperature and microbial aptness. All
else being equal, tank size is directly proportional to the input of biomass.
Energy production depends on the energy content of the feedstock, ranging
from manure at 1700 kWh/ tonne total solids to fat at 9400 kWh/ tonne of total
solids, and is also proportional to input of biomass.
Millions of very small (less than 50 tpa), wet, farm digesters exist in
developing countries such as China, whereas thousands of much bigger (over
1000 tpa) digesters exist in developed countries. For context, most
organizations engaged in community composting in the UK process less than
50 tpa.
Loading a digester with 50 tpa translates into around 130 kg of biomass per
day, which would likely require considerable labour. In England, the annual
production of food waste is around 250 kg per person per year (Chesshire
2008), and faecal production per person is approximately 250mL per person
per day (Zeeman, Kujawa et al. 2008), or 90 kg per year. The volume needed
for a single person’s food waste and faeces in an efficient system can range
from 50L to 1000L (Zeeman, Sanders et al. 2000).
Common big projects in the industrialized world
Biogas projects in the developed world are generally large. In Germany the
average installed facility increased from 50 kW in 1999 to 330 kW in 2002
(Wellinger 2005). Most European digesters are either On-Farm, digesting
manure and/or agricultural biomass, or Centralized Digestion of sewage, food,
industrial, and/or municipal waste (Nichols 2004).
In the UK, common scales are estimated below (Chesshire 2008; Scurlock
2008), costing hundreds of thousands, or millions of euros.
Annual input, m3 Power production, MW
On Farm 1000-2000 0.10-1.0
Centralized 10,000 0.50-10.0
On Farm systems usually involve treating large quantities of manure, and
require less planning and regulation but are also less likely to make money
from gate fees and large scale power generation. In Scotland a number of On
Farm digesters have been financed by government in order to reduce nitrate
pollution from manure, however, despite large farm size, farmers often do not
have the capital to build such large digesters.
Centralized systems involve treating a combination of industrial waste,
sewage sludge and municipal food waste. A review of Municipal Solid Waste
(MSW) digestion (RISE-AT 1998) provides a number of examples and finds
very large scale digestion (100,000 tpa) to be most economical. Germany
and Denmark continue to lead the world in this field, while England now has
just seven municipal food waste digester sites that have passed regulations
for food waste treatment (listed below), while many others are planned around
the country and more than ten UK based companies advertise engineering
expertise in such technology in a recent Compost Association AD Report,
2005. A recent review of anaerobic facility providers rates hundreds of
processes and services worldwide, indicating a wide range of expertise and
track records (Schwager 2008).
Municipal Food Waste Digester Sites Approved by ABPR in the UK,
downloaded from Defra’s Website, August 2008:
The trend in biogas usage in most of the industrialized world is towards
pipeline quality methane or vehicle fuel whereas in the UK it is towards
electricity generation due to new government incentives (Schwager 2008).
Regulatory requirements for biogas projects in the UK include attaining a
Waste Management License, clearance from Animal By-Products
Regulations, and local county approval. Standards are currently being
developed for agricultural use of digestate in the UK, that allow for
classification of the digestate as a quality product rather than waste. Other
legislation such as the EU Waste Prevention and Recycling Thematic
Strategy, The Landfill Directive and the UK Energy White Paper provide
concrete support to biogas projects (Holliday 2005).
Common small projects in the developing world
The biogas industry in the developing world is much less centralized and
regulated than in the industrialized world. The most common facilities are
small farm digesters running primarily on manure, which provide local cooking
energy, particularly in China and India (Wellinger 2005). Small scale
anaerobic digestion has resulted in a reduction of ecological stress in India’s
remote villages (Agoramoorthy and Hsu 2008), as well as clean cooking fuel
and safe waste treatment and fertilizer production for the population.
Outlines of the design and operation of small digesters have been published
widely, for example (Fry and Merrill 1973; Buren 1998; Wellinger 1999;
Matthews 2004; House 2006). Micro-scale digester designs include the
common underground clay and brick pit, the oil barrel with a floating or sealed
lid and the ditch or tunnel with a rubber membrane to capture methane.
Government programs have had widespread success installing small farm
digesters, as well as contributory efforts by local movements (Feliciano dos
Santos 2008) and western aid (PACE 2008), LeAF, WSUP, GTZ.
In some parts of rural India, the government may subsidize 80% of the capital
costs of building a digester, as long as a contract is made which forces the
owner to use the digester for at least twelve years (Agoramoorthy and Hsu
2008). Construction and use of small digesters has been common for several
decades in India and the benefits are better understood than in most
industrialized countries.
Uncommon small projects in the industrialized world
In some instances, small scale digestion results from the mainstream large
scale digesters shrinking in size or from companies testing substrates for
large scale application. In others instances, small biogas reactors have
appeared as independent projects by communities or farms. Various scales
of sewage sludge digesters such as septic tanks are already widespread, and
could be integrated with community projects digesting food or other biomass
waste.
In addition, digesters designed for particular industry effluents are already
operating in the industrialized world, and these projects have lead the way in
decentralized treatment. Combining industrial waste with agricultural and
domestic waste may lead to improved treatment for all parties, for example in
Wisconsin, USA, dairy farm manure is mixed with truckloads of waste ice
cream, creating a locally produced, easily digested, high energy substrate.
Dry Fermentation
Dry fermentation has been used since the 1940s and due to recent
advancements in Germany and The Netherlands (ten Brummeler 2000), has
become more widespread on both the large and small scale and perhaps
attractive for ecological sanitation, as reviewed by (Köttner 2002). Garage-
box type digesters can be loaded by dumping feed stock on the ground and
closing a sealing door. They require no mixing and very little liquid, but
require careful gas handling to avoid explosion. Variations including plastic
membranes or steel containers which process less than 2000 tpa, often below
300 tpa on farms. These systems are becoming more common in France and
Germany (Lukehurst 2008).
Farm Digesters
Decentralized composting and biogas production has increased recently in
Austria, with 93% of plants processing under 5,000 tpa and producing
electricity at under 100 kW; over 202 plants process under 500 tpa, versus 10
plants that process over 10,000 tpa (Amlinger 2005). This decentralization
has been partly a result of local farms providing organic waste treatment for
local authorities, and higher government premiums for electricity produced by
small biogas plants than large ones. The Austrian government has also
interpreted the EU Animal By-Products Regulations in such a way that
restrictions on farms are much less severe than in other EU countries.
Bioplex, a UK company has developed a small, mobile digester called the
Portagester. It is commercially available to a minimum size of 50-150 tonnes
per year for around £10,000. This model is not designed to produce methane,
rather to process material through the first stage of anaerobic digestion,
fermenting biomass into solids for composting and liquor, and pasteurising
material to pass UK animal by-products regulations. Bioplex also offers a
second stage digester for the liquor, which does produce biogas.
At Growing Power Inc. in Milwaukee, USA, a high solids food waste digester
has been built within an urban community food production project, in
collaboration with Wisconsin State University. It produces acids for
composting from various imported waste food and on-site farm waste. The
facility was financed by a research grant; however the digester is in use as
part of a non-profit, community operation, integrated with the heating
greenhouses and other operations on an urban farm.
Residential and Household
Two successful decentralized projects in Germany have been used as best
practice examples by Ecosan GTZ. Both projects involve the digestion of
sewage on site, one on a farm in Bessenbach, where the substrate is 5110
m3 per year of cattle manure and sewage from 14 full time people, 280 full
time cattle as well as up to 260 people served in the restaurant (GTZ 2005a).
The total investment for the digester system was around 200,000 euros, while
annual operational income and savings from energy and fertilizer production
add up to 50,000 per year. The other GTZ best practice example was
installed in a housing development in Lubeck, where approximately 4.8 L/day
of black water and food waste are produced, per capita, by approximately 360
people in 117 apartments. This project cost 600,000 euros for the ecological
sanitation whereas the entire eco-housing complex cost 2 million euros (GTZ
2005b).
Otter Rotters, a member of CCN in the UK, has created an AD project in
Southern England for several hundred households. The facility was approved
by the Animal By-Product Regulations and secured funding for capital costs,
however the operations are not economical and have, for the moment ceased.
A planned local government food waste collection project in the same region
has proposed to take the food waste elsewhere. The facility at Otter Rotters
was originally designed by the British company Bioplex but was changed and
redesigned significantly by the Otter Rotters team. The facility did not
produce biogas, rather it pasteurized food waste under anaerobic conditions.
The solids from digestate, which still contained in-tact solids was then
composted and used in horticulture, on site, and the digestate liquids were
kept in anaerobic chamber where biogas was recovered, and water was
reused in the process. Bioplex is currently installing a similar design at a
prison in the UK.
Three Japanese systems classified as “infrastructure free,” treating 20
households’ wastewater and organic wastes are argued to be cost effective.
The bottom-up, decentralization approach of involving citizens and
municipalities in city planning suits the current trends in demographics in
Japan. An assessment was made of 30-year life-cycle cost performance of
three current systems for wastewater treatment for a small community in
Japan (Anilir, Nelson et al. 2008). Quoting from Anilir, “These systems are;
wastewater gardens with biogas production, an anaerobic digester gas
system integrated with fuel cell technology and a heat and power unit (CHP)
combined with a biogas-producing reed bed system, all of which treat
wastewater and result in useful end products, closing the life cycle with low
maintenance, a lower environmental load, and two to four times smaller
development cost than centralized options in both rural and urban
communities.”
A Dutch system for the on-site digestion of food waste and black water for 32
houses in Sneek, the Netherlands uses a more complex treatment method to
produce biogas. An Upflow Anaerobic Sludge Blanket Reactor (UASB) is
used to concentrate the organic fraction of wastewater and produce biogas at
much lower temperatures, with lower concentrations of carbon, in smaller
reactors with better removal efficiency than is possible otherwise. Magnesium
is added to the effluent from the UASB to precipitate Struvite and recover
valuable phosphorous, and finally a nitrogen removal process is employed to
release nitrogen gas straight from ammonia through a new process known as
Anoxic ammonium oxidation (Anammox) or Completely Autotrophic Nitrogen
removal Over Nitrite (CANON) reviewed by (Sliekers, Third et al. 2003). An
overall energy balance including consumption from vacuum transport from
toilet and kitchen waste grinders as well as savings from sewer, sewage
treatment plants and drinking water yields a 200 MJ per person per year
advantage over centralized treatment systems. This includes the realistic 131
MJ per person per year of electricity produced by biogas but does not include
the total 374 MJ/person /year energy value of the biogas that may be
recovered up to 90% efficiency in the form of heat. It was noted that food
waste approximately doubles biogas yield from wastewater digesters
(Zeeman, Kujawa et al. 2008).
In Berching, Germany, the Huber Headquarters also employs a small biogas
digester and complete reuse system for water, nutrients and organic matter
produced by its several hundred employees on site. Huber is a company
producing various products and engineering for wastewater treatment, and
often uses membranes in its systems. It offers a very small, on site treatment
unit for industrialized countries and also builds systems for developing
countries (Huber 2008).
An small residential AD system has been demonstrated on a large household
in the mountains of Italy in collaboration with the University of Padova, Italy.
The reactor is a mesophillic completely mixed, two cubic meter digester,
heated with hot water from a wood burning stove and a solar heater,
eventually will be heated with biogas. The system has worked well, except
that heat energy input is not recovered as biogas energy. A weeper system
irrigates a small area of native plant species, which successfully
phytoremediate grey water, separated urine, and the effluent from the
digester, originally black water and food waste.
Two tanks of 1.5 m3 were used in food waste digestion trials which led to the
development of a 5,000 tpa facility treating food and garden waste in England.
One of the reactors was maintained at 56 C and the other at 36 C. The
performance of the warmer (thermo-phillic) digester was slightly better,
although both digesters produced approximately 0.4m3 methane per kilogram
of volatile solids and averaged at 3.1m3 per day but reached a maximum of
30m3 per day??? (Banks, Chesshire et al. 2008)!
Past projects
In the 1990’s, three household scale (1.2m3) Up-flow Anaerobic Sludge
Blanket (UASB) reactors were operated for two years in various different, rural
areas in the Netherlands. These reactors treated grey water and black water
or only black water, and achieved the production of 300L of biogas per day
with 60 to 70% methane content and an average efficiency of removal of 60-
72% COD and BOD removal respectively (Bogte, Breure et al. 1993).
Temperature was found to be a very important factor, with significant increase
in treatment efficiency above 12 C.
In the 1970’s, Jean Pain of France built himself a large, household digester,
which was heated by situating the digester tank inside of a large pile of
compost, in a batch type reaction. Chipped brush wood was used to produce
humus for on site horticulture as well as biogas to produce electricity, heat
and vehicle fuel. The reactor was a 30 ft pile of brush and wood chips,
layered with pipes to heat water for his home. He left behind an institute in
Belgium and a book, available on-line.
Living Machine Projects such as Ocean Arcs in Vermont, USA often operate
anaerobic digesters as the first stage of treatment in the living machine
wastewater treatment process. Another living machine was built north of
Doncaster, but released the methane without flaring it for aesthetic reasons.
Unfortunately both of these operations are no longer in operation, due to
financial problems.
John Fry was a pioneer of small scale biogas in South Africa, England, and
the USA. He welded oil drums together and wrote a book on small systems
for producing biogas (Fry and Merrill 1973). During World War Two, biogas
use in Europe, particularly England and Germany blossomed, due to the
shortage of oil. Huge biogas balloons were strapped to the top of busses to
store fuel.
Planned projects
• Bradford Organics Composting Service (CCN members) currently
developing plans for food waste AD
• London CRN, (CCN member) is developing a small anaerobic digester
under a parking lot in the London area.
• WyeCycle (CCN member) is considering introducing small scale 1m3
plastic tank digesters from India, from a company called Sintex.
• Fairfields Waste Management (CCN member) is developing an AD
program for 20,000 tonnes/ year
• Low Impact Living Project, UK (CCN member) has hosted a course on
how to run cookers on small scale biogas. They do not operate one
regularly but do plan to in the future.
• Llanidloes, Wales, feasibility study for a community project showed
economics to be very unfavourable (Holliday 2005)
• Several other small, on site are operating around the UK, at CCN
member sites and farms, but the author is waiting for details.
• There is emerging interest from greenhouse hydroponics operators, as
there is a significant amount of green waste (often landfilled), which
could be digested on site to provide heat and boost CO2 levels in the
greenhouse.
Research and Pilot Projects
There is also experimentation and research on small scale AD through
various institutions.
Pilot projects for big AD installations are designed to be just the same as their
full size counterparts, only smaller. Greenfinch, a UK company, has built
many small fibreglass digesters to test various substrates, some integrated
with aquaculture, and some may be rented to test substrate, however
generally these plants are not operated or distributed outside research and
pilot projects. As a novelty, Greenfinch manager Michael Chesshire operates
a digester run on whey and grass at his home, where the biogas is used for
cooking. North American projects include two large mobile digesters to
demonstrate the technology in Colorado (Schellenbach, Turnacliff et al. 1977)
and Quebec (Electrigaz 2006).
Researchers are finding decentralized, anaerobic solutions to wastewater
(Elmitwalli, Sayed et al. 2003; Abegglen, Ospelt et al. 2008) as well as
combined wastewater and kitchen waste (Zeeman, Sanders et al. 2000; Han,
Shin et al. 2002; Zeeman, Kujawa et al. 2008).
Completeness of Projects
***please comment on completeness***
One relevant project in Wales, one in England, and one in Italy have been
discovered since last writing.
Economics of small projects in industrialized and developing countries
Traditional economic assessment
This section considers the traditional monetary analysis of a four cubic meter
biogas digester for a rural community in an industrialized country versus in a
developing country.
It is clearly economical for the rural Indian family, as demonstrated by the
millions of installations each year, whereas it is not financially economical in
England, as demonstrated by the lack of projects. An assessment of Capital
Costs follows the Operating Costs and Benefits.
Operating Costs and Benefits of a four cubic meter digester
United Kingdom
(Estimated by Author)
India (Agoramoorthy
and Hsu 2008)
Waste treatment
- all human and
animal faecal matter
and food scraps for
several people and
animals
None directly to user
Waste is disposed of
through existing sewer/
septic tank and landfill
infrastructures, often
provided without fees by
government.
Significant benefits
Carries worth of
avoiding health
problems and other
disposal costs - reduces
clinic visits by ½. Small
herds of livestock
horticulture provide a
consistent feedstock
Energy production
- 400 to 3200 Litres of
biogas per day
Benefit
Biogas produced may
provide up to 12kWh
(£1.20) per day, £438 per
year and makes small
contribution to community
energy consumption.
Not as clean or convenient
as natural gas, requires
compression, equipment,
etc
Significant benefit
4 hours of cooking fuel
per day (enough for
cooking) and savings of:
£ 30.0/ year on wood +
less deforestation
£ 18.0/ year on
kerosene in rural India
Biogas is cleaner than
firewood fuel and
associated labour, and
provides enough energy
for cooking
Fertilizer production
- 40 to 160 Litres per
day of NPK rich liquid
Small benefit
Nutrient rich effluent water
most often has a negative
value. For fertilization,
cheaper, more convenient
petroleum derived nutrient
rich liquid is produced
Significant benefit
Better (more nutritious,
safer, easier to handle)
than fresh manure for
horticultural cropping.
£11.3/ year saved on
fertilizer purchase
Environmental health Not significant Benefit
- avoidance of
pollution such as
water contamination
and resource
depletion through a
closed loop human life
support system
Environmental health does
not directly impact the
public. Local water and
soil quality are trivial to
quality of life. Soon, GHG
emissions may command
a price.
Crucial for those who
depend directly on their
local environment for
food, water, shelter and
materials.
Maintenance and
Operational Labour
-starting, loading, and
operating the digester
Operator Salary
Very significant cost
Training and time required
for loading, monitoring
process and health and
safety, etc. Difficult to
justify in neo-classical
economics
1/4 time: £4,000 per year
£16,492/ year poverty line
in England (13 million
below poverty line in
England)
Cost
Digester requires
valuable buckets of
water and human
labour; however these
are well worth the
benefits. Training also
required
1/4 time: £10 per year
£41/ year poverty line in
India (300 million below
poverty line in India)
Annual Net Income
Benefit /Cost Ratio
Negative £5562
0.11
Benefits: £438,
Costs: £4000
Positive £49
5.9
Benefits: £59,
Costs: £10
***Is the cost of labour accurate???***
At a larger scale, in the industrialized world, various other benefits may be
reaped, such as the sale of heat and electricity and gate fees for imported
waste materials. Still, on the scale of 250 tonnes per year (considered small
scale in UK) a recent feasibility study in Wales highlighted the many benefits
of community, proximity and environmental health but also the high cost in
conventional economic terms. The
following two tables were taken
directly from this report reproduced
without permission from (Holliday
2005).
Capital Costs for a one cubic meter digester
United Kingdom
(Estimates by Author)
Rural India
(Agoramoorthy and Hsu
2008)
Digester Tank
£300 for plastic or steel
tank with insulation, and
3” plumbing, fittings,
valves
£125 total construction
costs. Government
subsidies may cover
80% of this for some
households, given
agreement of long term
use (becomes £25 for
user)
Mixer, Heater,
Pasteurizer,
Automated Controls
£1000 predicted, after
design, experimentation
and development
£0. None needed, low
efficiency system with
manual mixing and
warm climate, included
in labour
Input tank,
Effluent tank,
Gas storage
£500 for membrane or
floating lid and two other
tanks
£0. Built into ground,
gas storage on digester,
included in digester tank
cost
Integration into existing
infrastructure
£3000 Re-plumbing and
replacement of toilets,
or solids separation
from blackwater, system
for collection and short
distance transportation
of solid and liquid food
wastes (20 households)
£500 Safe biogas
handling and burning
equipment for cooking
indoors
£0. No existing sewage
or garbage
infrastructure,
£2? Pipe and primitive
cooking element to burn
biogas, may be included
with government
subsidized system
The one cubic meter digester in the UK would require a more complex reactor
system, due to the colder climate and the higher public and regulatory
expectations from facilities.
Non-traditional economic assessment
This includes benefits and costs incurred by wider society and includes non-
financial benefits for the community.
Operating Costs and Benefits of a one cubic meter digester
United Kingdom/ Industrialized Country
Waste treatment
Avoids point source pollution from landfill and
sewage effluent
- all human and
animal faecal matter
and food scraps for
several people and
animals
Avoids cost of sewage infrastructure, 6.34 GWh/year
(1% of national energy expense) to treat sewage in
England and Wales (WaterUK 2006).
Energy producing instead of energy consuming
Minerals and organic material are re-used
Avoidance of huge costs of centralized infrastructure
Energy production
- 400 to 3200 Litres of
biogas per day
12kWh per day provides enough energy for cooking,
or some heating and hot water
Overall energy balance of decentralized anaerobic
treatment brings benefits of 200MJ/person/year
compared to conventional system
Energy is completely carbon neutral, requires no
transportation or complex external infrastructure for
production (compared to hydro, wind, solar)
Energy production is not vulnerable to external
factors and will not be sensitive to price volatility
Fertilizer production
- 40 to 160 Litres per
day of NPK rich liquid
Very useful for local horticulture, requires no
transportation or external infrastructure for food, fuel
and fibre production on site
Nutrient overloading in surface water is a major
threat to ecosystems worldwide (MEA 2005), that
digestate use can minimize
Environmental health
- avoidance of
pollution such as
water contamination
and resource
depletion through a
closed loop human life
support system
As we move to a more locally based and sustainable
society, environmental health will directly impact the
public
With locally produced fertilizers and energy, city
farms and community gardens can drastically
improve the quality of community spaces.
GHG emissions are minimized with this option of
organic waste management.
Widespread adoption will lead to overall reduction of
resource depletion and therefore an improved future
Maintenance and
Operational Labour
-starting, loading, and
operating the digester
Training and time required for loading, monitoring
process and health and safety, etc.
No odour or strenuous physical work, as with home
composting.
Creates an opportunity for familiarity with process
engineering – which can be educational and useful.
Creates an opportunity for community participation
partnership, and resilience through re-use of
resources
Creates usefully trained personnel
Potential for the community sector
Barriers to wider adoption of small scale AD
“Generally the short-term economic interests of well-established structures
comprise the major bottleneck for making progress on the route to sustainable
and robust public sanitation. However, regarding the enormous social and
economic benefits of the decentralized sanitation and resource recovery-
concept, natural mineralization-based treatment systems irrevocably will be
substitute, for the nowadays applied, highly centralised sanitation concepts
with their complex and expensive treatment methods.” (Lettinga 2008)
The traditional, financial economics of AD facilities are by far the biggest
barrier to development. Nobody can make money by operating small AD
units. Even with subsidies and renewable energy incentives, the cost benefit
or net present value of even large digesters are often unfavourable (Schwager
2008).
Other risks and challenges include potential health hazards from pathogens,
biogas explosions, and odour. Regulations, policy and licensing fees can also
be barriers to community systems and in 2008, the lack of commercially
available products and the extent of technological design and development
required is another barrier.
To date, the author has not found commercial distributors of small scale,
residential or community on-site biogas and fertilizer producing digesters in
the industrialized world. Perhaps if a simple, affordable miniature digester
were commercially available, home-owners and community projects,
especially those engaged in gardening and farming would consider installation
of such a unit, instead of a traditional septic tank or compost heap, for
example.
??Please comment on this! Do replicable, commercially available units exist
on the market?
A final, and related barrier is to adapt the infrastructure in the industrialized
world to take advantage of small scale AD technology. In addition to low flush
toilets and food waste grinders, in order to recover a value from the biogas
energy and nutrient rich liquid, the unit must be well integrated. For example,
if the heating of the AD reactor, pasteurizing of the substrate, and heating of
hot water in a well insulated building or greenhouse were combined, more
value could be recovered than isolating the reactor in its own building.
Various uses of heat must be considered, including local industry, food
production, heating, drying, cooking, etc. Also, irrigation of on site
horticulture, aquaculture or some form of plant production or phytoremediation
may recover a value from the effluent liquid and further reduce the polluting
effect of liquid discharge into the environment.
Opportunities for adoption of small scale AD
Because of the current financial environment, adoption of decentralized AD
units depends on citizens and groups having a strong desire for their services,
and/or an understanding of their wider benefits to society. In this context, the
third (non-profit) sector has a major advantage. The third sector is based on
providing benefits to society and is comprised of groups and projects not
designed to make money, rather to act in the public interest.
Similarly to community composting, small and decentralized AD can be
an attractive solution to meet landfill, sewage and greenhouse gas reductions,
and provide numerous other benefits to society.
The Community Composting Network is situated ideally to disseminate
information on suitable decentralized AD for the UK, for the sector to realize
the benefits. The next steps to follow from this report are to create a
commercially available product guide, an operational manual, and plans for a
pilot scheme and a training program. Given the lack of commercial products,
the product guide may need to include many individual components and
design specifications.
With publicly available information and commercially available
equipment, an individual may create a small anaerobic digestion unit.
The current level of interest from communities in the UK
CCN surveyed membership for their level of interest and knowledge in
decentralized AD in August 2008, and all results were very positive, four
members indicating they would like to form a steering committee for CCN’s
work in this field and would like to run pilot projects, if possible.
Conclusion
Although decentralized anaerobic digestion is not widespread in the
industrialized world, diverse examples indicate that it can bring significant
benefits to society. Like many other third sector activities, small and
community AD units will not bring financial profits to the owner, according to
traditional economic assessment, mainly due to the cost of labour for
operation. However, if the goal of a community is to be self sufficient, create
fertilizer and energy and avoid pollution, a small AD unit can be very
profitable.
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