Biomass and Bioenergy, Vol. 9, Nos 1-5. pp. 365-376.

1995
Pergamon Elsevier Science Ltd
0961~9534(95)00103-A Printed in Great Britain
0961-9534/95 $9.50 + 0.00

ANAEROBIC DIGESTION OF MUNICIPAL SOLID WASTE: A MODERN

WASTE DISPOSAL OPTION ON THE VERGE OF BREAKTHROUGH

K BRABER

NOVEM B.V.
P.O. Box 8242,3503 RE UTRECHT, The Netherlands

ABSTRACT

This paper is the synthesis of three years of work in the field of anaerobic digestion (AD), waste
management issues in general and many plant visits to operational digestion installations, carried out in
the framework of the International Energy Agency’s Bioenergy Agreement, Annex XI’s Anaerobic
Digestion of Municipal Solid Waste (MSW) activity.

The paper presents an overview of the current status and deployment of AD-technology for MSW,
systems available on the market, opportunities and barriers relating to this technology when competing
with alternative ways for the management of MSW (landfill, incineration, aerobic composting),
including its advantages (renewable energy source a.o.) and limitations.

Furthermore, AD is placed in the context of end product and energy markets and in the context of
recycling and environmental aspects in general.

KEY WORDS

Anaerobic digestion, municipal solid waste, waste disposal/management, renewable energy, biogas,
composting, recycling, digestate, compost

BACKGROUND OF ANAEROBIC DIGESTION

Anaerobic digestion (AD) itself is not a new thing. The natural biological decomposition
processes at the core of AD have been harnessed in plants all over the world. For over 100 years
AD has been used to treat sewage sludge and over the past three decades much experience has
been gained on AD of other liquid wastes such as farm wastes and high strength industrial waste
waters.

For a high solid waste such as the organic fraction of MSW experience is less extensive. The
first large scale investigation of AD/MSW was “RefCom” in the USA, which was initiated in
1978 and closed in the mid 1980s. The last 10 years have seen a variety of pilot scale plant
investigations in Europe, with progress towards commercialisation over the past live years. As
can be seen from the IEA-brochure’ approximately 15 plants are now in full-scale operation
(November, 1993) and almost 20 are under construction or being planned.

.w 9-m-x 365
366 K. BRABER

Currently, biological treatment methods such as cornposting and AD offer the only route for
recycling organic matter and nutrients from the organic fraction of MSW. Cornposting
represents an energy consuming process (around 30 - 35 kWh, is consumed per tonne of waste
input) and is generally considered to be commercial. Anaerobic digestion is a net energy produ-
cing process (100 - 150 kWh, per tonne of input waste) but its commercialisation is not yet fully
demonstrated. Nevertheless, AD is increasingly recognised as having a place in modern waste
treatment. With all modern waste treatment technologies there are problems relating to the
complexity of waste which need to be addressed. AD/MSW is no exception; but there are
successful plants currently in operation which demonstrate that problems associated with MSW
are surmountable.

DIGESTION IN MSW MANAGEMENT

Composition of MSW

MSW is an inhomogeneous waste, which may be divided into a number of sub fractions:

?? Digestible organic fraction. Biogenic organic matter which is readily degradable i.e. kitchen
wastes, grass cuttings etc.
?? Combustible fraction. Slow digestible and indigestible organic matter i.e. wood, paper,
cardboard, plastics and other synthetics etc.
?? Inert fraction. Stones, sand, glass, metals, bones etc.

In general about 50% of MSW consists of organic matter.

Current disposal methods for MSW

In most countries MSW is collected as a mixed stream and disposed of, typically, by landfill or
incineration. These disposal methods do not take advantage of the properties of the organic
fraction or its potential for cornposting and recycling nutrients and organic matter. Generally
only a small part of the MSW is recycled (waste paper, bottles etc.) usually excluding the
digestible fraction.

In recent years both source separation and recycling have attracted increasing attention.
Consequently, separate fractions of MSW are becoming available for more advanced treatment
prior to disposal or recycling. Table 1 shows how AD can be integrated into general MSW
management. It shows the various fractions in MSW and treatment options.

Feedstocks for AD-technology

Source separation

In a number of participating countries source separation of MSW is encouraged. This includes

separation of the organic fraction (also known as “green waste” or “biowaste”).
 Anaerobic digestion of municipal solid waste 367
Table 1 Fractions in MSW and treatment options.

Organic fraction Paper fraction Plastic fraction

Source Separation . AI) @jr.,energy ad refuse * recycling ?? recycling
?? cornposting c digestion incineration
Mechanical * AD forenergy and retie m digestion ?? incineration

Separation ?? incineration 0 recycling

‘4 cornposting
* landBl1
No Separation 4 incineration 0 incineration 0 incineration
(integral MSW) . landfill . landfill . landfill
Mixed with other codigestion codigestion NA
waste

Metals Inerts: stones, Various Other

sand non-organics wastes
manure etc.
Source Separation ?? recycling 0 recycling 0 incineration codigestion
landfill
Mechanical Separation 4 recycling . landfill 0 incineration NA
0 recycling . landfill
No Separation ??landfill . landfill ?? incineration NA
(integral MSW) . through 0 (incineration . landfill
incinerator: as bottomash)
0 recycling
Mixed with other NA NA NA codigestion

Experience has shown that this method of separation provides the best quality product in terms
of heavy metal and plastic contamination despite heavily relying on the co-operation of
householders.

Where source separation has been widely introduced the results are encouraging. Experience in
Germany, Denmark, Switzerland and the Netherlands indicate between 50 and 70% of the total
organic fraction of MSW is successfully collected. The quality of the organic fraction collected
varies from reasonable up to very good, enabling the production of an acceptable compost.

The composition of the organic waste is important in determining which treatment method is
most appropriate. Yard (or garden) waste contains much lignocellulose material, which does not
readily degrade under anaerobic conditions. On the other hand material such as kitchen waste is
too wet and lacking in structure for aerobic cornposting. In general: if the material collected is
predominantly kitchen waste anaerobic digestion is the most appropriate treatment method and
if yard waste dominates, aerobic composting is preferred.

The inclusion of paper in the organic feedstock for anaerobic digestion is controversial in
relation to recycling. Certainly it will digest and experience in Denmark indicates that many
forms of paper improve biogas yield.
368 K. BRABER
Mechanical separation

Mechanical separation provides the only route to obtaining an organic fraction of waste where
source separation is not available. The fraction is more contaminated than that obtained through
source separation with consequences for digestate quality: experience shows contamination
affecting the environmental quality (heavy metals) and acceptability (plastic content) of the final
digestate end product. In many countries compost derived from mechanical separation will not
meet the standards required for useful application as soil conditioner. In this case the major
benefits (economic, energy and environmental) relate to the production of biogas, when used as
a source of renewable energy. Volume reduction for subsequent disposal may be regarded as the
major incentive.

Opportunities and barriers to AD/MSW

Opportunities/Advantages

Energy benefits

?? Net production of energy, providing the biogas is used.

?? Reduced CO2 emissions, by displacement of fossil fuels.
?? Potential to treat the “wet” fraction of MSW which is less amenable to incineration.
?? Potential for the economics to benefit from (future) higher energy prices and/or buy back
tarrifs.

Environmental benefits

?? Environmental benign waste treatment.

?? Recycling benefit, providing the compost is used.
?? Reduction of odours.
?? Lower land requirement compared to aerobic cornposting.
?? Potential to treat the organic fraction of MSW in countries considering banning landfilling of
waste.
?? Potential salt reduction in compost, compared to aerobic composting.

Waste treatment benefits

?? Volume reduction of the waste for subsequent disposal.

?? Potential for codisposal with other organic waste streams (e.g. industrial wastes such as food
processing waste and agricultural wastes such as manure).

Barriers/Disadvantages

?? Cost: this is a major barrier, as AD is (slightly) more expensive than composting in many
cases.
?? AD/MSW does not treat whole waste, just a fraction of it.
?? The use of AD to treat MSW is a novel application. Information on economic and practical
issues is not widely disseminated.
 Anaerobic digestion of municipal solid waste 369
?? There are persistent materials handling problems, particularly related to plastics (especially
occurring with mechanical separation).
?? Waste water may need to be treated before disposal.

Implementation of MSW digestion

The decision to implement AD of MSW rather than alternative technologies depends on a

number of factors:

?? Site specific circumstances.

?? Outlet possibilities for the energy produced.
?? Energy prices and taxes.

?? Energy purchase tan-ifs.

0 Costs of alternatives/ taxes on alternatives.

?? Policy e.g. renewable energy and recycling policies,

?? Land prices.

?? Market for compostidigestate.

?? Status of different technologies.

Although at the moment alternative technologies e.g. cornposting may appear cheaper energy
prices, levies on waste disposal and land prices can radically alter this situation.

Influence of policy and waste treatment prices

The IEA participating nations represent a wide range of local policies, economic environments
and energy price scenarios. With this background it has been possible to examine the influence
of different policy in different countries on the implementation of AD/MSW.

In some countries policy makers influence the implementation directly by introducing

compulsory source separation of the organic fraction of MSW (e.g. NL) or indirectly by
increasing the costs of incineration and landfilling (e.g. Denmark) and by encouraging higher
purchase tarrifs for renewable energy (e.g. the CO2 tax and energy tax in Denmark and the non
fossil fuel obligation in the UK).

Codigestion

MSW is not the only waste stream suitable for AD. In many cases it may be an advantage
(economical and regarding energy production) to codigest the waste with another waste stream
(e.g. industrial waste such as food processing waste of agricultural waste such as manure). This
has been examined in detail in the IEA Activity in Annex X (AD of Biomass). It is important to
stress that MSW represents a stable income stream for such a codigestion plant (in terms of the
gate fees) and will thus be an important influence on the economics.

THE TECHNOLOGY

The main steps in the treatment of MSW using AD are:

370 K. BRAFJER

?? pretreatment
?? anaerobic digestion
?? post-treatment.

Several plant concepts exist. In addition there are numerous pilot and research plants in
existence. To get an overview only the most important plant concepts in operation or in
construction at full-scale are considered, see table below.

Pretreatment

Before anaerobic treatment it is necessary to pretreat the MSW in order to increase the
digestibility and ensure an end product that can be applied on farmland and gardens.
Pretreatment processes are described below. In some cases the process steps as described below
serve more than one purpose. For example, in wet systems separation and particle size reduction
can take place more or less simultaneously or in one piece of equipment. As a general rule
handling in a biogas plant is far more easy when MSW is source separated.

Sorting

The quality of MSW as received will determine whether further mechanical or even handsorting
may be required. Indeed, even source separated organic MSW will require some further
separation to remove wrongly sorted materials such as plastics, metals and larger sized
materials.

In some cases the source sorted organics are delivered in bags (the use of plastic bags should
however be discouraged). In these cases a bag opener may be necessary. Separation is performed
either before or after digestion, but usually before.

The separation is performed either under wet or dry conditions. Wet separation is most suitable
for wet digestion systems, whereas dry separation is most appropriate for dry concepts.

Wet separation is usually performed in a pulper. The pulper not only provides separation, but
also results in particle size reduction and even ‘suspension’ of the softer parts of the organic
matter.

For dry separation rotating drum systems are widely applied.

Particle size reduction

Size reduction is necessary to render the organic matter to an appropriate and more
homogeneous size distribution. The size reduction is performed using screw cutting, milling,
drumming, pulping, shredding, etc.

Digestion

Organic matter decomposes mainly in two ways depending on the availability of oxygen. In an
atmosphere without oxygen (anaerobic conditions) organic matter will decompose and produce
 Anaerobic digestion of municipal solid waste 371
AD PLANT CONCEPTS TREATING MSW ON FULL SCALE

VALORGA AVECON
1 phase 1 phase
mesophilic mesophilic
dry digestion wet digestion
mixed by gas injection mixed by gas injection
._-________-_--_-----_--_ .~~-~~-_------_-_________
15 - 20 days retention time 15 - 20 days retention time
1988 Amiens (France) 1990 Vaasa (Finland)
55,000 ton/year mechanical separation 2 1,000 ton/year mechanical separation
DRANCO KOMPOGAS
1 phase 1 phase
thermophilic thermophilic
dry digestion dry digestion
recirculating digested material
.-----------------------_ mechanical mixed
.------------------------
18 - 2 1 days retention time 13 - 25 days retention time
1992 Brecht (Belgium) 1992 Rtimlang (Switzerland)
15,000 ton/year source separated 10,000 ton/year source separated ihand
sorted
BTA PAQUES
2 phase three stages
mesophilic mesophilic
wet digestion wet digestion
anaerobic filter
mechanically mixed
.-_------------_--------- .------------------------
4-6 days retention time < 10 days retention time
199 1 Helsingor, Denmark Breda (Netherlands)
20,000 tonne/year source separated Fruit and vegetable waste
BIOCEL DSD - CTA
1 phase, batch 2 phase
mesophilic mesophilic
dry digestion wet digestion
percolator
.------------------------ CO digestion
.------------------------
2 - 3 weeks retention 15 days retention time
1993 Flevoland (Netherlands) 1987 Plauen (Germany)
25,000 ton/year (?) ton/year
ITALBA CODIGESTION
1 phase 1 phase
mesophilic thermophilic
wet digestion wet digestion
_-_--_------_--~----_--~-
.------------------------
10 days retention time 15 days retention time
Bellaria (Italy) 1988 HerningNegger (Denmark)
4,000 ton/year 1,000 ton/year
ANM
2 phase
mesophilic
wet digestion (1 - 3%)
anaerobic filter
combination of aerobic/anaerobic
digestion
_-_-_------------------~-
4 days retention time
Oldenburg (Germany)
3,000 ton/year source separated
372 K.BRABER
biogas: a mixture of methane (CH,) and carbon dioxide (CO*). In the presence of oxygen
(aerobic) no useful biogas will be produced.

Anaerobic digestion occurs by itself in nature where the right physical conditions are created
like the bottom of lakes, in swamps, peat bogs and in landfills.

However, by controlling (in plant) conditions such as temperature, humidity, microbial activity
and waste properties, the process may be stimulated and accelerated. The anaerobic digestion
takes place tnrough the action of a consortium of four different types of microorganisms:
hydrolytic, fermentative, acetogenic and methanogenic.

The gas yield depends mainly on factors such as digestability of the organic matter, the
efficiency of the adapted bacteria culture, the retention time in the digester and the digestion
temperature. In thermophilic digestion (50-55°C) the rate of digestion is more rapid than at
mesophilic digestion (3540°C). Furthermore, the gas yield is increased by increased retention
time.

Anaerobic digestion may be performed in various ways. Currently, the leading concepts are:

?? dry continuous systems

?? dry batch systems
0 wet continuous systems
?? co-digestion (mainly using liquid manure or sewage sludge).

In general, these systems perform digestion in one stage. Except for the wet continuous systems,
these are often operated in two stage plants.

Each concept has its own characteristics, advantages and disadvantages. On the next pages a
summarising overview is presented of the systems and their suppliers.

Dry continuous system

The process involves a continuously fed treatment system with a dry matter content of 20 - 40%.
It can either be completely mixed or function with plug flow.

Dry batch system

Batch feed (completely feeding of the digestion unit which is then sealed so it is gas tight and
left for 2 - 3 weeks prior to complete emptying). During the closure period percolate is
recirculated to stimulate mixing and digestion.

Wet continuous systems

The systems can be divided into conventional slurry systems and anaerobic filters. Conventional
slurry systems usually function in codigestion of animal manure, MSW and other wastes at a dry
matter content at about 10%.

Anaerobic filters usually comprise two or multi-phase systems where the hydrolysis and
acidification occurs in the first reactor and methane fermentation in the second reactor.

High concentrations of methanogens are retained in trapping devices onto which the bacteria can
adhere and not be flushed out. All solids must be removed to avoid clogging of filters.
 Anaerobicdigestionof municipalsolid waste 373
Mixing

The digestion procedures can be divided into plugflow, completely mixing or anaerobic filters
(see above). The mixing can be done either mechanically or by gas injection.

?? Plugflow
The biomass flows in a way that prevents fresh biomass from being mixed with partially
digested biomass.

?? Completely mixed
Fresh biomass is pumped at intervals directly into and out of the reactor and thereby
immediately mixed with the biomass undergoing digestion.

Post-treatment

After digestion the material usually needs some kind of refining before it can be used for
horticulture or agriculture.

If the MSW is treated wet the material can be spread directly on the farmland as slurry or it can
be separated in a solid and liquid fraction. The solid fraction can be matured to compost for
about 2 - 4 weeks and the liquid fraction can be spread on farmland or treated in a wastewater
plant.

If the MSW is treated in a dry process the digested material is usually dewatered and matured to
compost. The liquid effluent can be used for mixing and inoculation of the incoming MSW, but
there will usually be effluent in excess that has to be spread on farmland or cleaned in a
wastewater plant.

ENERGY ASPECTS

Energy potential of MS W

Typically 1 tonne of organic MSW produce in the range of 100 - 200 m3 of biogas. The main
components of biogas are methane (CH,) and carbon dioxide (CO,). A typical composition is
given below:

Energy content 20-25 MJ/m’

Methane (CH4) vol.% 55-70%
Carbon dioxide (CO,) vol.% 30-45%
Hydrogen sulphur (HS) 200-4000 ppm

Gas production from anaerobic digestion of MSW largely depends on the composition of the
feed stock. A high content of slowly digestible matter like yard waste reduces the potential. The
digestion system itself can also influence on the potential.
374 K. BRABER

Energy efficiency

To determine the true meaning of anaerobic digestion for energy generation one has to assess the
energy balance of the whole system from input to output including the utilisation of biogas.

The pretreatment, posttreatment and other utilities of a MSW treatment plant consume their own
share of energy. Generally, a plant consumes 20 - 40% of the energy content in the produced
biogas for the process (heating, mixing etc.).

After substraction of the autoconsumption a net electricity production of 100-l 50 kWh per tonne
of OF-MSW is found. Compare this to composting which usually needs a net electricity
consumption of 30-35 kWh per tonne input.

Utilisation of biogas

Three basic options can be distinguished for the utilisation of biogas:

?? production of electricity: generally, gas engines or gas turbines are applied, sometimes
making use of the residual heat (cogeneration). Typically a gas engine has an electric
efficiency of 33%.
?? production of heat in a boiler: generally implying the sale to an energy consumer or for
district heating,
?? upgrading of the biogas to distribution grid quality.

Energy policy and prices

The sale of energy is an important aspect of anaerobic digestion as most AD plants will convert
biogas to electricity and heat. The purchase prices for electricity and heat differs a lot in the
various countries. Some countries have introduced higher electricity tariffs for producers of
renewable energy, like the UK with the Non Fossil Fuel Obligation. High energy prices
stimulate anaerobic digestion.

END PRODUCTS AND MARKETS

Beside biogas, AD produces solid and liquid by-products, which can have a fertiliser value. The
amount, quality and type of product depends mainly on the quality of MSW feedstock and
digestion principle.

Products from dry digestion

The main product of a dry process is a digestate which can be matured to a compost product. An
excess of liquids can be spread as a slurry on farmland or treated in a wastewater plant.
Spreading on farmland is the best solution in regard to recycling of the nutrients.
 Anaerobic digestion of municipal solid waste 375
Products from wet digestion

Digestate from a wet digestion can be spread directly as a slurry on farmland or it can be
separated in a liquid and a solid fraction. The solid fraction can again be matured to a compost-
like product.

The market for compost

This differs and depends on factors such as:

?? quality of the produced compost, with respect to environmental aspects and the biological
characteristics of the compost: plant nurturing value, void of weed seeds and viruses,
?? public acceptance of compost as a soil improver,
?? amount of available local resources of peat,
?? polices to the exploitation of peat and the destruction of land areas,
0 prices for peat,

Compost quality criteria

Compost made from MSW has to comply with quality standards that meet consumer and market
requirements. The following criteria are important to ensure the product marketability:

?? it must not present any health hazards,

?? it must be largely free of impurities (plastic, glass, metals, vital weed seeds and viruses),
?? the level of heavy metals and other toxic substances must meet requirements,
?? the product must give a visually attractive overall impression.

The composted/digested material has to be refined to produce high quality compost. Source
separation is preferred to meet the demands for the quality of the final compost product.

ECONOMICS

From the previous sections it will be clear that it is very difficult or even almost impossible to
draw general conclusions on the economics of AD in all of the participating countries. The
boundary conditions and influencing factors determining the economics vary too much to do so.
Therefore, to give some idea of economics reference is made to a detailed study in the
Netherlands by Haskoning2, which has been undertaken to compare anaerobic digestion with
aerobic cornposting.

From this it is clear that the treatment costs for both aerobic and anaerobic processes are varying
widely depending upon capacity. It also shows that -for the Dutch situation- at this moment
anaerobic digestion in specialised MSW digesters is somewhat more expensive than the closest
alternative of aerobic cornposting. However, one has to take into account that the low energy
prices in the Netherlands do not discourage cornposting and that the low selling price for
electricity supplied to the grid do not particularly encourage AD. According to the Danish
experiences codigestion of OF-MSW with manure could be significantly cheaper than AD in
specialised plants, provided that other (legal) conditions and/or policy are not prohibitive. The
gap (AD approximately US $ lo-20 per tonne more expensive as cornposting) that has been
376 K.BRABER
illustrated in the Netherlands can be largely influenced by energy prices and electricity purchase
tariffs: where both are increasing anaerobic digestion becomes rapidly competitive or even
cheaper than composting. Hence, choosing for AD now is choosing for a more sustainable
future.

ACKNOWLEDGEMENTS

Dr. Patricia Lawson, ETSU, UK; Mr. Henrik Bjarke Msller, B.Sc., Herning Municipalities,
Denmark; Mr. Snrren Tafdrup, M.Sc., The Danish Energy Agency, Denmark; Dr. Arthur
Wellinger, InfoEnergie, Switzerland; Prof. David Chynoweth, University of Florida, USA; Mr.
Dieter Schlag, LfU, Germany; Valorga, France; Organic Waste Systems, Dranco, Belgium;
Kompogas, Btihler, Switzerland; Paques Solid Waste Systems, The Netherlands; Heidemij
Realisatie, Biocel, The Netherlands; BTA, Germany; Helsingor Municipal Works, Denmark;
Samenwerkingsverband Midden-Brabant, The Netherlands; and all other contributors not
mentioned here by name.

REFERENCES

1. IEA-brochure: Biogas from Municipal Solid Waste: overview of systems and markets for
anaerobic digestion of MSW; IEA Bioenergy Agreement; Task XI; Activity 4; July
1994; NOVEM, Netherlands Agency for Energy and the Environment, UTRECHT, The
Netherlands. 1994
2. Conversietechnieken voor GFT-a&al: ontwikkelingen in 1992; Haskoning, Royal Dutch
Consulting Engineers and Architects, Nijmegen, The Netherlands. 1993