Memorandum submitted by the Sustainable
Organic Resources Partnership Expert Advisory Group (Waste 53)
ANAEROBIC DIGESTION & CLIMATE CHANGE
SUSTAINABLE RECYCLING OF ORGANIC RESOURCES
WITH RENEWABLE ENERGY PRODUCTION
A SORP EAG PAPER
August 2007
INTRODUCTION
Anaerobic biological technologies have been
used by mankind for many centuries, initially for food and beverage
production and more recently in the water industry for treating
sewage sludge and generating renewable energy. Our need to tackle
climate change will demand widespread adoption of sustainable
solutions and this will favour biological anaerobic technologies.
These processes offer high energy yield, high treatment rates
and low residuals production; important benefits increasing popularity
of digestion in the future.
The treatment of organic materials involves the breakdown
of large composite structures, initially into complex molecules
of proteins, fats and carbohydrates and eventually to smaller
molecules to allow the reuse of essential carbon, phosphorus and
nitrogen for the growth of new plant and animal life. The biological
pathways which facilitate this can be aerobic or anaerobic. Both
occur naturally but aerobic pathways are rate limited by the transfer
of oxygen to the organic material, and whilst man in his ingenuity
has been successful in overcoming this by using energy intensive
aeration processes, these contribute further to climate change.
In nature, concentrated organic materials resulting
from decomposition of plant and animal life undergo recycling
by the anaerobic route and it is this pathway which is used and
intensified in anaerobic processes such as Anaerobic Digestion
(AD). This is the preferred technology used by the water industry
for stabilising sewage sludge and providing beneficial safe sustainable
recycling of biosolids (ie treated sewage sludge) to the environment.
There are many other strong putrescible organic wastes
and these have traditionally been buried in landfills or spread
on land, generally without prior treatment. The Landfill Directive,
and EU diversion targets mean that by 2015 some 65% of putrescibles
wastes must be diverted from landfill and treated. AD offers an
environmentally friendly, energy producing, fully proven and resilient
alternative to landfilling of putrescible organics.
In the UK the total amount of "waste"
organics is about 40 million dry tonnes p.a.[152],
[153],
[154],
[155]
(see Table below) and if all of this was treated using AD it could
in theory provide about 10% of the total UK electrical energy
requirements. Diversion of organics from landfill has a further
significant beneficial impact on global warming by eliminating
uncontrolled emissions of methane from the decomposition of the
waste and converting them to renewable energy as heat and electricity.
| Organic Waste Energy Sources
| Production | Tonnes oil
|
| Dry tonnes p.a
(million)
| equiv.
(million) |
| Sewage Sludge | 1.4 | 0.2
|
| Livestock Manure | 15 | 2.2
|
| Commercial Food | 16 | 4.5
|
| Domestic Food | 7.5 | 2.1
|
| TOTAL | 40 | 9
|
| |
|
In the UK over the last decade, there has been significant
technical advancement and financial investment in advanced anaerobic
digestion to increase utilisation of existing assets and safeguard
the beneficial recycling route for sewage sludge to land.
Advanced digestion involves pre-treatment before digestion
to increase hydrolysis and acidification of the waste. This provides
a substantial efficiency gain in conversion of organics to gas
and renewable energy along with increased throughput, a higher
quality, pathogen free, "compost" like end-product and
reduced operating costs. Much of the AD expertise derives from
this experience in the water industry sector and despite differences
in the input "waste", is transferable to other "waste"
sectors. Thus existing knowledge allows fast-tracking of process
application to other "wastes" such as municipal household
(organic fraction), domestic and commercial food waste, and livestock
manures.
ANAEROBIC DIGESTION
BIOCHEMISTRY
Anaerobic digestion involves the breaking down of complex
organic materials, typically consisting of carbohydrates, proteins
and fats, into simpler compounds, in the absence of oxygen. Hydrolysis
reactions bring about the solubilisation of solid particles and
these are then broken down into smaller molecules of simple sugars,
amino acids and fatty acids. The process continues until all degradable
material is converted to acetic acid or hydrogen and ultimately
to biogas (a mixture of methane and carbon dioxide). Other essential
nutrient elements such as nitrogen and phosphate are liberated
from the original complex material largely as simple inorganic
salts.
The process can take place at ambient temperatures and in a single
reactor, however, optimisation is achieved by using higher temperatures
(35-55°C) and multiple reactors. The residue from digestion
is known as the digestate. This is a stable, nutrient rich humus
which is used sustainably to improve soil quality and fertility
with the addition of valuable stable organic matter and nutrients.
FOOD WASTE
Each year in the UK about 7 million dry tonnes of food waste
is produced as part of the manufacturing processes and there is
a further 9 million tonnes of "out of date" or damaged
food waste from supermarkets. Currently most of this is disposed
of in landfill sites. This waste has a significantly higher energy
value than the organic residues currently treated by AD (ie sewage
sludge and livestock manure) and is generally available uncontaminated
in large amounts, providing an ideal feedstock for AD treatment
and recycling to farmland.
MUNICIPAL WASTE
About one third of Municipal household waste is organics
which includes kitchen waste, garden waste and disposables such
as nappies as well as paper and cardboard. Not all of these are
biodegradable but kitchen waste is and it rapidly becomes putrid
and smells. It is ideal for anaerobic digestion whereas garden
waste is best composted, and paper can be more directly recovered.
Current collection practice focuses on source sorting of clean
waste (paper, bottles, cans, garden waste) whilst food waste is
mixed in with the non-recyclables. In modern high density housing
this can cause odour nuisance and health concerns amongst residents.
The unsorted non-recyclables are then collected by the municipality
and the organic fraction can then be separated out and treated.
This process is generally known as Mechanical Biological Treatment
(MBT) and the Biological stage is typically AD or composting.
Perhaps a better method is to separate the kitchen waste at source.
When coupled with frequent collection of these organics using
biodegradable plastic bags, odour and health worries are no longer
a concern and this waste provides an ideal feed for AD. There
are a small but increasing number of kitchen "waste"
sorting schemes across the UK serving both composting and AD technologies.
AGRICULTURE
Agriculture is responsible for 6.5% of the UK's GHG's (Greenhouse
Gas) emissions[156].
The emissions of interest for AD, are those resulting from the
decomposition of organic matter in livestock manure. The methane
emitted from liquid manure management systems can be captured
and used as a renewable energy source. In the UK this is about
700,000 tonnes p.a. of carbon equivalent.
AD is used in the European Union particularly in Germany, Austria
and Denmark, to allow capture of this methane from livestock manures
and by recycling the nutrients in the digestate back to the land,
the use of inorganic fertilisers can be substantially reduced.
In Germany the plan is for 10,000 AD plants by 2010 with
an installed electrical capacity of 3,000 MW capable of supplying
about 5% of Germany's electrical requirements. These agricultural
based plants in Germany have benefited from the stable "Feed-in
Tariff" arrangements (equivalent to 4 renewable obligation
credits—ROCs) and a flexible feed approach allowing manure,
energy crops and food waste all to be used.
There is much interest in the UK farming industry in AD.
However, uncertainty about long term ROC values, increases the
risk for any investor. More secure payback of capital can be achieved
by co-digesting commercial food waste (which arrives with a gate
fee) along with the manure. A number of such schemes are now operating
in the UK.
ENERGY CROPS
There is much interest currently in using sustainable energy
crops for production of bio-ethanol and bio-diesel. Whilst AD
provides an excellent solution for treating putrescible organics,
it is also possible to directly digest energy crops such as sugar
beet, wheat grain and maize (before they become food and waste)
for energy production.
AD should generally provide a more efficient conversion from crop
to usable fuel than other technologies[157].
For example in producing bio-ethanol from sugar beet, the energy
required for running the process is equivalent to 46% of the energy
produced. By comparison such "parasitic" energy use
is only 18% in producing biogas energy by AD from sugar beet.
AD produces biogas which is a premium fuel, ideal for electrical
power generation and for heating using combined heat and power
plants (CHP). The CO2 present in biogas can also be removed and
the methane fed into the gas grid or compressed and used as a
vehicle fuel. The latter is being trialled in Sweden, and despite
the parasitic energy use increasing it is still a very competitive
22%.
In comparing efficiencies of various processes for renewable
energy production, clearly capital as well as running costs must
be optimised. Whilst it is difficult at present to obtain reliable
data for the above new applications, what is clear is the simplicity
and resilience of the biological AD process using low temperature
and simple robust tanks. This should ensure that over the long
term, the total cost for an AD renewable energy installation,
remains highly competitive.
BARRIERS TO
AD GROWTH IN
THE UK
All sustainable energy and waste technologies are inevitably
more expensive than present practices. Without incentives, market
economics will not provide the investments needed to deal with
climate change.
There are two main drivers for AD; gate fees for accepting the
waste and enhanced prices (ROCs) for the renewable energy generated.
In the longer term, viability will depend upon how investors view
the risk-reward profile with a crucial factor in the analysis
being investor confidence in the value and long term stability
of ROCs.
The present ROCs regime has worked well and provided an incentive
for AD particularly where existing digestion assets exist. The
UK water industry is a good example and output of renewable energy
from AD has increased threefold between 2002 and 2006.
Current generating capacity in the water sector is 40MWe
and sector data shows plans to expand this to 115MWe by 2010 with
an ultimate potential of 186MWe. The discussion proposal to reduce
the ROC incentive to 25% of its current value will curtail energy
based investment in this sector. This will also have a knock-on
effect to other sectors since the water sector is the cradle of
innovation in AD technologies in the UK and supports a world class
UK based supply chain. Growth and knowledge transfer to new applications
in the UK will be best achieved by building on a stable base in
the water sector.
The current definition of a waste within the Waste Framework
Directive is also a barrier in that it limits recycling of prime
quality organics material, eg "food waste" and this
also limits the co-digestion of different wastes. The EA and WRAP
have adopted an innovative approach to overcoming this, namely
the development of treated product quality protocols along the
lines of BS PAS100 for compost. This approach should greatly assist
the application of AD technology in other sectors.
The scale of any sustainable AD development is ultimately
constrained by the balance between nutrients supplied and the
crop needs on the surrounding land to utilise the recycled nutrients,
especially nitrogen in the digestate. This is overcome by dewatering
the digestate and treating the liquors.
FORWARD VIEW
Anaerobic Digestion (AD) is the ideal process for treating
"waste" generated from all stages of the food chain
including the farm, abattoir, manufacturer, distributor, commercial
and home food preparation to the resultant sewage sludge. Food
"waste" has a high energy value and in capturing this,
AD contributes to UK electricity needs whilst simultaneously facilitating
recycling of essential nutrients and humus to the soil. AD can
also use crops as well as "wastes" to generate renewable
energy, especially electricity, and in a more efficient process,
with lower parasitic energy requirement, than that used for bio-ethanol
and bio-diesel production.
In practice there will be a variety of AD plants and whilst some
will be dedicated to particular applications such as sewage sludge
and household waste others will take a variety of feedstocks including
livestock manures, food waste and energy crops. The new WRAP/EA
quality protocols planned for the UK should revise the legislation
and provide a level playing field for recovery of all biodegradable
organics including those currently classed as wastes. In the longer
term AD's great strength as a natural biological process, able
to convert putrescible organics into renewable energy and control
odour, will become increasingly valued.
Sustainable Organic Resources Partnership Expert Advisory Group
December 2007
152
Sewage sludge data from e-digest of environmental statistics,
published February 2006. Back
153
Livestock manure data from Biomass task force report, 2006. Back
154
Commercial food waste data from Encycle News, November 2006. Back
155
Domestic food waste from DEFRA annual waste arisings survey 2003
and OU Household waste survey 2004. Back
156
Agriculture emission from NFU and Methane to Markets Partnership. Back
157
Energy crop conversion data from Dr Andrew Salter, Southampton
University, personal conversation. Back
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