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Desk top study on digestate enhancement and treatment
Enhancement and treatment of
digestates from anaerobic digestion
A review of enhancement techniques, processing options and novel
digestate products
Project code: OMK006 - 002 Research date: Feb 2012 May 2012
Date: Nov 2012
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Enhancement and treatment of digestates from anaerobic
digestion
Executive Summary
Background Anaerobic digestion (AD) is a well established
process for the treatment of organic wastes and the generation of
renewable energy. Historically the digestate produced from the
process has been applied to land as a fertiliser or soil
conditioner. However with a planned increase in the number and
capacity of AD plants to treat a variety of organic waste streams
in the UK, digestate enhancement technologies are gaining more
attention. Digestate enhancement technologies could be assessed by
an AD operator looking to provide any of the following options for
an AD plant:
increase the value of digestates;
secure use of digestates;
create new markets for digestate products; and
decrease the operating costs (OPEX) of the facility.
Objectives This study aims to identify digestate enhancement
technologies and techniques, in order to raise awareness of them
within the UK waste sector. The study has considered well
established techniques, as well as novel or emerging processes
currently under development. The project has reviewed technologies
applicable to all digestates produced from the anaerobic digestion
of a variety of feedstocks, whether or not they are compliant with
PAS 110 or the Anaerobic Digestion Quality Protocol (ADQP). A key
objective of the study is to raise awareness in the UK waste sector
to the opportunities and challenges of digestate enhancement. The
output of the study supports the delivery of a number of actions
contained in the AD Strategy and Action Plan (June 2011) and the
delivery of Scotlands Zero Waste Plan. Methodology Data has been
collected from an extensive desk based literature search, direct
contact with technology providers, relevant industry focus groups,
academic research, conference papers, policy documents, relevant
industry texts and manufacturers literature and legislation. A
web-based search was also undertaken. This information has been
used to construct technical data sheets for each technology
considered which form an appendix to the report. In addition a
number of examples of applications of novel technologies have been
included in the report.
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Enhancement and treatment of digestates from anaerobic
digestion
Findings This study has found that there are a wide range of
technologies available for digestate enhancement. Technologies are
available to create a range of novel digestate products such as
concentrated or balanced fertilisers, which have the potential to
be marketed as products. However, no single technology has been
found to be relevant for all applications so a range of solutions
will be required to accommodate the increasing volumes of digestate
generated within the UK. From an EU waste sector perspective, there
are clearly similar challenges and goals to the UK waste sector;
but AD investment in Europe has been driven by a series of
different drivers and supported via different energy subsidy
regimes. EU funded support of the research and development of
digestate products and markets has assisted EU member states in
making investment decisions over the last ten years. Research and
development continues with a focus on the development of enhanced
products. UK markets for waste derived digestates are immature.
There is existing competition in land based markets, not least with
conventional inorganic fertilisers. However, in the future as
natural phosphorous resources decrease and the cost of inorganic
fertilisers increase, farmers will look to find alternative and
potentially cheaper sources of nutrients for their crops. The key
challenge in the short term will be to manage increasing quantities
of digestates seeking markets and secure outlets. Operational
experiences should be sought from the EU, where systems have been
installed and digestate products created to satisfy outlet
demand.
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Enhancement and treatment of digestates from anaerobic
digestion
Contents
1.0 Introduction
.................................................................................................
1
1.1 Objectives
.................................................................................................
2 1.2 Aims
.........................................................................................................
2
2.0 Methodology
.................................................................................................
3 3.0 Digestate
Enhancement................................................................................
4
3.1 What is Digestate?
.....................................................................................
4 3.2 Why Use Enhancement Techniques?
............................................................ 4
4.0 Digestate
Enhancement................................................................................
6 4.1 Pre-Digestion Enhancement Techniques
....................................................... 6
4.1.1 Thermal
Hydrolysis...........................................................................
6 4.1.2 Autoclave Systems
...........................................................................
6 4.1.3 Enzymic Liquefaction
........................................................................
7 4.1.4 In-Vessel Cleaning Systems
..............................................................
7
4.2 Post-Digestion Enhancement Techniques
..................................................... 9 4.2.1
Physical Enhancement
Techniques................................................... 12
4.2.2 Thermal Enhancement Techniques
.................................................. 14 4.2.3
Biological Enhancement Techniques
................................................ 16 4.2.4 Chemical
Enhancement Techniques
................................................. 20
5.0 Digestate Enhancement Systems
............................................................... 23
5.1 Digestate Treatment Systems
....................................................................
23 5.2 Digestate Enhancement Systems
............................................................... 26
5.3 Current Barriers to Enhancement Systems
.................................................. 27 5.4
Technology Example: Barkip Biogas Facility
................................................ 28
5.4.1 Background
...................................................................................
28 5.4.2 Process Description
........................................................................
29
5.5 Technology Example: Lee Moor EFW
......................................................... 30 5.5.1
Background
...................................................................................
30 5.5.2 Process Description
........................................................................
31
5.6 Technology Example: MINORGA Bio fertiliser, Norway
.............................. 32 5.6.1 Background
...................................................................................
32 5.6.2 Process Description
........................................................................
33
6.0 European Perspective
.................................................................................
35 6.1 Background
.............................................................................................
35 6.2 Survey Data
.............................................................................................
35 6.3 Transport Optimisation
.............................................................................
36 6.4 Summary of EU Waste
Sector....................................................................
37
7.0 Summary
....................................................................................................
38
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Enhancement and treatment of digestates from anaerobic
digestion
Glossary
AD Anaerobic digestion. Process of controlled decomposition of
organic matter under
anaerobic conditions.
Aerobic Molecular oxygen available.
Anaerobic No oxygen source..
Anaerobic Digestion
Quality Protocol (ADQP)
End of waste criteria for the production and use of quality
outputs from anaerobic
digestion of source segregated biodegradable wastes.
Anoxic No available source of molecular oxygen.
Auto thermal Condition at which an exothermic reaction is self
sustaining and no additional energy is
required from an external source.
Bio methane Methane generated by anaerobic digestion.
Biogas Gas generated by an anaerobic digestion process.
Typically composed of 60% methane
and 40% carbon dioxide.
BOD Biological oxygen demand. Defined as the amount of oxygen
required by aerobic
bacteria to oxidise the organic matter within the sample.
CAPEX Capital expenditure.
CH4 Methane.
CHP Combined heat and power. Cogeneration of heat and power from
combustion of a
fuel(gas).
COD Chemical oxygen demand. Defined as the amount of oxygen
required to chemically
oxidise the organic matter within the sample.
Digestate, Fibre Fibrous fraction of material derived by
separating the coarse fibres from the whole
digestate.
Digestate, Liquor Liquid fraction of material remaining after
separating coarse fibres from whole
digestate.
Digestate, Whole Material resulting from an anaerobic digestion
process that has not undergone post-
digestion separation.
Dry solids (ds) Measure of solids content within the digestate.
Defined as the % of mass remaining
after drying at 105C.
Evapotranspiration The combined effect of evaporation and plant
transpiration (normal water loss to the
atmosphere from plants).
H2S Hydrogen sulphide.
MBR Membrane bioreactor. The combination of a membrane process
with a suspended
growth bioreactor.
MBT Mechanical Biological Treatment: waste processing facility
that combines a mechanical
sorting facility with a form of biological treatment such as
composting or anaerobic
digestion. MBT plants are typically designed to process mixed
wastes and as such are
not capable of achieving PAS 110 or PAS 100.
Moisture content Measure of water content within the digestate.
Defined as the % of mass lost after
drying at 105C.
NH3 Ammonia.
OPEX Operating expenditure.
Pasteurisation Process step during which the number of
pathogenic bacteria, viruses and other harmful
organisms in material are significantly reduced or eliminated by
heating the material to
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Enhancement and treatment of digestates from anaerobic
digestion
a critical temperature for a specified period of time.
PAS 100 Publically Available Specification that controls the
quality input to compost and the
process is managed and operated to generate composts that
protects the environment
and meets market needs.
PAS 110 Publically Available Specification that controls the
quality input to anaerobic digestion
and the process is managed and operated to generate digestate
that protects the
environment and meets market needs.
Polyelectrolyte High molecular weight organic polymer used to
assist flocculation in solid liquid
separation.
RHI Renewable Heat Incentive. Financial incentive for the use of
renewable heat.
RO Reverse Osmosis: A membrane filtration technology that
utilises a selective reverse
osmosis membrane to retain molecules and ions while allowing the
solvent, ions and
small soluble molecules to permeate through.
ROC Renewable Obligation Certificate. The main financial support
mechanism for large
renewable electricity projects in the UK.
Sanitisation Biological process that eradicate or reduce
pathogens to acceptably low, sanitary levels.
Syngas Abbreviation of synthesis gas. Gas mixture that comprises
of carbon monoxide, carbon
dioxide and hydrogen produced by the gasification of a carbon
containing fuel.
UF Ultra Filtration: A membrane filtration technology that
utilises a selective ultrafiltration
membrane to retain soluble macromolecules and larger
contaminants while allowing the
solvent, ions and small soluble molecules to permeate
through.
Acknowledgements
The authors would like to thank the various organisations who
provided information and advice on digestate enhancement systems.
Particular thanks to Tim Evans (Tim Evans Environmental Ltd), Nigel
Horan (Aqua Enviro), Paul Bardos, Claire King and Ursula Kepp (r3
Environmental Ltd), Steve Wooler (HRS), Oddvar Tornes (IVAR IKS),
Christian Toll (AeroThermal), Mike Weaver(Pyreg), Tobias
Finsterwalder (FIMTEC GmbH) and Ian Crummack (DONG Energy).
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Enhancement and treatment of digestates from anaerobic digestion
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1.0 Introduction
The use of anaerobic digestion (AD) to recover value from
organic wastes within the UK is emerging as an important treatment
system and is forecast to increase significantly. There are
currently 233 AD plants operating within the UK with a capacity to
treat 5.4 million tonnes of material:
51 AD facilities operating on waste including food waste.
36 AD facilities operating on farm waste.
146 AD facilities operating on sludge generated by waste water
treatment works.
There are currently planning applications to develop a further
222 facilities in the UK (1WRAP held data, Nov 2012) which will
provide for significant increase in processing capacity. AD
converts organic matter into biogas, a source of renewable energy,
and a nutrient rich organic fraction known as digestate. Biogas can
be used to generate electricity and heat to power the process.
Excess power can be sold to the National Grid and excess heat can
also be utilised, if the right infrastructure exists. The most
commonly used digestion system is wet mesophilic digestion
operating between 25C and 40C; the digestate produced from this
process is an organic slurry, rich in nutrients such as nitrogen
and phosphorus. Other less common systems include dry digestion,
which uses a feedstock with very high dry solids content and
thermophilic digestion, which operates at higher temperatures (50C
- 60C). Currently the majority of AD facilities recycle the
digestate to local agricultural land as an organic fertiliser
(Fuchs et al., 2010). However the window for land application is
limited to agricultural and crop requirements (Orr, 2011), and for
large capacity AD plants, a substantial area of land is required to
provide a secure and suitable market for the digestate. If
application to agricultural land is not feasible, due to transport
distances, legislative requirements or other restrictions,
digestate can be used for land reclamation. This is particularly
relevant for digestates from mechanical biological treatment (MBT)
applications, as the use of digestates derived from mixed waste
materials is currently restricted to use on land restoration
projects only. As the use of AD increases the demand for
agricultural land will also increase, potentially requiring plants
to transport digestate further in search of suitable land. This is
important for the increasing number of centralised AD facilities
operating in urban areas. Digestate must therefore be carefully
managed to ensure it is utilised as a resource and maximum benefit
is achieved whilst avoiding excessive transportation costs.
1 Whilst the data is accurate to the best of WRAPs knowledge,
WRAP offers no warranty and accepts no liability relating to the
completeness or accuracy of the information contained within.
Information is compiled by various parties and recipients should
make their own independent enquiries before relying on the
information contained within the document.
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Enhancement and treatment of digestates from anaerobic digestion
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1.1 Objectives
WRAP has acknowledged the need to raise awareness of digestate
enhancement technologies to secure land application or develop
novel products which is the key objective of this study and is a
key action in the AD Action Plan. The study focuses on digestate
produced from anaerobic digestion processes that cover:
both non-waste and waste-based digestates, whether or not they
are compliant with
PAS 110 or the Anaerobic Digestion Quality Protocol (ADQP);
mixed-waste digestates as the output from Mechanical Biological
Treatment (MBT);
amended sludge digestates or co-digestates, derived from
feedstocks including
sewage sludge; and
sludge digestates (biosolids), solely derived from sewage
sludge.
1.2 Aims
The aim of this study is to identify technology and techniques
for the enhancement of digestate from straightforward dewatering to
the development of novel products. The project has reviewed
technologies and enhancement techniques applicable to all
digestates produced from the anaerobic digestion of a variety of
both waste and non-waste feedstocks, whether or not they are
compliant with PAS 110 or the Anaerobic Digestion Quality Protocol
(ADQP). The output of the study is to support the delivery of a
number of actions contained in the AD Strategy and Action Plan
(June 2011) and the delivery of Scotlands Zero Waste Plan. It is
vital that digestate enhancement is seen in a holistic way as part
of an overall materials processing and re-use system. It is
important that the overall case for sustainable wastes and resource
management is not negated by inappropriate digestate management
choices, for example:
the use of downstream processing to treat digestate that
consumes more energy
than is likely to be generated by the AD facility;
the generation of large volumes of effluent for treatment that
create an unacceptable
overall carbon or water footprint for the AD facility;
the transmission of harmful impacts to soil and groundwater that
are substantially
greater than using alternative materials such as composts or
conventional fertilisers;
the reduction in carbon benefit of systems that generate high
greenhouse gas (GHG)
emissions, for example the atmospheric release of methane (CH4 )
or nitrous oxide
(N2O);
the inappropriate development of AD facilities that have
negative impacts on the
public perception and economic viability of digestion as an
effective waste
management and energy recovery option; and
high capital and operating costs that limit the financial
viability of AD and increase its
reliance on public subsidy.
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Enhancement and treatment of digestates from anaerobic digestion
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2.0 Methodology
In order to identify and assess possible digestate enhancement
techniques a detailed desk based literature search has been
undertaken. Sources for the literature search included technical
reports, academic research, conference papers, policy documents,
relevant industry texts, manufacturers literature and legislation.
A web-based search was also undertaken. In addition to the
literature search, information was requested from a number of
anaerobic digestion organisations and interest groups within the UK
and EU. Over 30 organisations were approached to participate and
provide information for this study. A full list of organisations
contacted can be found in Appendix 1. Unfortunately a number of the
organisations contacted were unable to participate and provide
information for this study, partly due to commercial reasons or
perceived conflict of interest. Data obtained from the research was
compiled and used to construct technical data sheets for each
enhancement technique identified, which can be found in Appendix 2.
The aim of these data sheets is to provide a brief description of
the operating principle of the technology/technique, operating
conditions and associated benefits, challenges and opportunities.
In addition to developing technical datasheets, a number of example
enhancement projects have been included in the report. These focus
on the application of emerging technologies which either
significantly enhance digestate quality or support the development
of novel digestate products.
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3.0 Digestate Enhancement
3.1 What is Digestate?
Digestate refers to the material produced by the process of
anaerobically digesting biodegradable materials. Digestate consists
of a mix of microbial biomass (produced by the digestion process)
and undigested material. The volume of digestate produced will be
approximately the same as the feedstock volume, although the mass
will typically be reduced by approximately 15%. Digestate contains
all the nitrogen, phosphorus and potassium present in the original
feedstock and as a consequence has value as an organic fertiliser.
Typical nutrient values for digestate are given below, however the
actual nutrient content will be highly dependent on the type of
feedstock processed (Chambers, 2011).
Nitrogen: 2.3 - 4.2 kg/tonne.
Phosphorous: 0.2 - 1.5 kg/tonne.
Potassium: 1.3 - 5.2 kg/tonne.
(NNFCC, 2012)
Consideration must be given to the relationship between the
quality of the feedstock and the quality of the digestate. The
digestate will contain all material that has not biodegraded and
converted into biogas within the process, therefore any
contaminants in the feedstock will remain in the digestate. A good
quality, well prepared feedstock will therefore produce a good
quality digestate compared with poor quality feedstock which will
produce a poor quality digestate.
3.2 Why Use Enhancement Techniques?
The majority of digestate produced in the UK is spread to
agricultural land as fertiliser, either as whole digestate or as a
separated fibre (Fuchs et al., 2010). Although this is a good use
of the nutrients within the digestate, the value of the digestate
to the producer is low (Horan, 2012). Once the costs of
transportation and spreading are taken into account the digestate
value can be close to zero, and may even be a cost to the producer
(Lewens, 2011). The application of nitrogen in organic materials to
agricultural land is regulated by the European Nitrates Directive
(91/676/EEC.) (Fuchs et al., 2010). As a consequence the spreading
of digestate to land is controlled (based on nitrogen content) and
dependent on location and crop demand. This can result in digestate
being transported greater distances to find suitable land-based
markets and avoid over application; this will increase transport
and operational costs. Furthermore land application is only
appropriate during the growing season, requiring digestate to be
stored for significant months of the year. More information on NVZs
can be found on Defras website,
http://www.defra.gov.uk/food-farm/land-manage/nitrates-watercourses/nitrates/.
If the number of operating AD facilities increases, as currently
predicted, local competition for land based markets will also
increase, with a consequential impact on transportation and
spreading costs.
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Enhancement and treatment of digestates from anaerobic digestion
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The key aims of digestate enhancement techniques are to:
increase the value of the digestate;
create new markets for digestate products;
reduce the dependence on land application;
ensure more secure and sustainable outlets for digestate
products; and
potentially reduce the operating cost of the facility.
Consideration has been given in this study to enhancement
techniques and technologies that can be applied at three key
stages:
pre-digestion;
within the digestion process ( i.e. in-vessel); and
post-digestion.
Each system considered is aimed at supporting the objective of
enhancing the quality of the digestate or providing potential to
develop new digestate products.
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4.0 Digestate Enhancement
4.1 Pre-Digestion Enhancement Techniques
Pre-treatment systems employed upstream of anaerobic digestion
can be used to enhance the digestion process, and as a consequence
digestate quality. There are a number of techniques available to
pre-treat the feedstock and improve the availability of organic
constituents to enhance the digestion process. In addition the
removal of contaminants and debris from the feedstock to the
digestion process is key to maintain digestate quality and in the
extreme secure stable operation of the digestion process. These
systems are discussed below.
4.1.1 Thermal Hydrolysis
The thermal hydrolysis process (THP) is a high-pressure,
high-temperature steam pre-treatment application for anaerobic
digestion feedstocks. The feedstock is heated and pressurised by
steam within a reaction tank before being rapidly depressurised
(flashed). This results in the breakdown of cell structure within
the biomass; as the organic matter is presented to the digester in
a broken-down condition, the digestion process is more effective
resulting in increased gas production and improved digestate
quality. To ensure the process is thermally and economically
efficient the system requires a dewatered feedstock at between
15-16% dry solids. As a consequence dewatering systems are an
important pre-treatment stage. Details of dewatering systems are
provided in Section 3.4.1. As the thermal hydrolysis process
utilises a dewatered feedstock increased digester loading is
achieved and therefore gas production is increased. The quality of
the digestate is improved as the hydrolysed digestate is
pasteurised, easier to dewater and achieve higher dry solids
product, resulting in a product that is easier to store, handle and
transport (CAMBI, 2011, Veolia, 2008). The process has widespread
waste water treatment applications operating on sewage sludge. The
process is being developed for organic and food waste applications
in Europe, particularly Norway.
4.1.2 Autoclave Systems
An autoclave can be used to pre-treat digester feedstocks in a
similar manner to thermal hydrolysis. The autoclave is a pressure
vessel that steam treats its contents at a constant temperature and
pressure, serving to pasteurise, clean and break-down organic
matter within the feedstock. As the organic matter is presented to
the digester in a broken-down condition the digestion process is
more effective resulting in increased gas production and improved
digestate quality. After processing inorganic material and
contaminants can be easily removed via mechanical separation,
providing a clean, pasteurised, organic rich feedstock for
anaerobic digestion (AeroThermal Group, 2008).
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4.1.3 Enzymic Liquefaction
An enzymic liquefaction system has been developed by DONG Energy
A/S. for use on mixed waste streams prior to digestion. Called the
REnescience process, the system comprises three process stages to
breakdown and separate the organic matter from within the feedstock
prior to digestion. Stage one is a non-pressurised thermal
treatment utilising either hot water or steam, which opens the
feedstock to make it accessible to enzymes. In the second stage,
enzymes are added to liquefy and further breakdown the cell
structure of the feedstock. The prepared feedstock is then digested
in the third stage of treatment. Following digestion the component
fractions are separated such that an organic rich liquid for
land-based application can be easily separated from inorganic
material and physical contaminants. The system appears to be suited
to the pre-treatment of mixed waste streams (i.e. MBT). A pilot
plant of the REnescience process is currently operational in
Denmark where a range of waste materials, including municipal solid
waste, source segregated food waste and sewage sludge has been
processed. A schematic diagram of the REnescience Enzymic
Liquefaction Process is shown in Figure 1.
Figure 1. Schematic of REnescience Enzymic Liquefaction
Process
4.1.4 In-Vessel Cleaning Systems The inclusion of in vessel
cleaning systems as an enhancement technique may not initially
appear appropriate. However, detailed consideration must be given
to the nature of the waste materials being feed into the digestion
process i.e. waste containing varying quantities of:
plastic
timber
fibres (both natural and man-made textiles),
grit/sand/soil
metal fragments
solid fruit residues(pips/stones/stalks/peel)
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Enhancement and treatment of digestates from anaerobic digestion
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Whilst the digestion process itself utilises significant mixing
and agitation, the digestion vessel will act as a repository of all
feedstocks. Heavy materials will tend to settle and lighter
materials will tend to float to the top of the vessel and become
entrained within a scum and foam layer. In-vessel cleaning systems
can be used to good effect to remove contaminants from the
digester, improving both digestate quality and preventing the
build-up of inerts. In the extreme, hydraulic retention time in the
digestion vessel can be severely reduced if these inerts are not
removed. This can lead to impairment of the digestion performance
and eventually potential process instability. Floating material can
become dislodged, adversely affecting the quality of digestate, and
in the extreme placing at risk the security of the land-based
outlet and/or PAS 110 accreditation. Proprietary systems have been
developed to overcome these problems with in-vessel cleaning
techniques. Grit and heavy solids material accumulating at the
bottom of the digester vessel can be directed by a rotating scraper
system to the edge of the digester where it is removed and
separated from the digestate. The separated digestate is returned
to the digestion process. The separated grit/solids can be used as
an aggregate amendment for construction or potential land
remediation. However the land remediation operation will require a
permit (Finsterwalder Umwelttechnik GmbH & Co. KG, 2012). A
typical in-vessel system is shown in Figure 2. Floating material,
such as plastics and rags can also be removed by a rotating
skimmer. Material is forced to the edge of the digester where it is
removed and separated from any entrained digestate. The separated
digestate is returned to the digestion process and the separated
solids disposed to landfill.
Figure 2. Typical scraper system installed within a digester
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4.2 Post-Digestion Enhancement Techniques
The enhancement techniques identified by the research undertaken
in this report are summarised in Table 1 below. Techniques have
been divided into categories based on the type of process employed
i.e. physical, chemical or biological. Where multiple technologies
are available for the same enhancement principle (i.e. drying)
these have been divided into sub categories.
Table 1. Digestate Enhancement Techniques
The listing of enhancement techniques in Table 1 does not
contain all possible treatment types, and it is not an endorsement
of those presented. However, the listing serves to illustrate
potential options and provide information obtained in this
study.
Physical Thermal
Thickening (Belt) Drying (Rotary Drying)
Thickening (Centrifuge) Drying (Belt drier)
Dewatering (Belt press) Drying (J-Vap)
Dewatering (Centrifuge) Drying (Solar)
Dewatering (Hydrocell) Evaporation (scraped surface heat
exchangers)
Dewatering (Bucher press) Conversion (Incineration)
Dewatering (Electrokinetics) Conversion (Gasification)
Purification (Ultrafiltration and Reverse Osmosis) Conversion
(Wet air oxidation)
Conversion (Pyrolysis)
Biological Chemical
Composting Struvite precipitation
Reed Beds Ammonia recovery (Stripping + Scrubbing)
Biological Oxidation Ammonia recovery (Membrane Contactor)
Biofuel Production (Algae) Ammonia recovery (Ion Exchange)
Biofuel Production (liquor as process water) Acidification
Biofuel Production (hydrolysis of fibre to Bioethanol) Alkaline
Stabilisation
Microbial Fuel Cell
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Enhancement and treatment of digestates from anaerobic digestion
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The following sections of the report provide a brief description
of each of the treatment principles and how they may be employed.
Further information on each of the techniques can be found in the
Technical Data Sheets included in Appendix 2. Each Technical Data
Sheet provides a schematic process flow diagram for the techniques,
as well as a brief process description. The aim of the Technical
Data Sheets is to provide an overview of the principles and
objectives of each technology, as well as an indication of any
particular challenges that may need to be considered in
implementing the system. The flow sheet presented in Figure 3
provides an overview of the digestate enhancement techniques and
how these can be combined into viable treatment systems. This is
not an extensive list of treatment possibilities but highlights the
principles available. The dependencies of some technologies on
pre-treatments are also captured within the overview. For example,
if thermal drying is to be employed the flowchart indicates that
dewatering is likely to be required as a pre-treatment. Dewatering
will produce a liquor stream which must also be treated, by
membrane purification for example. Depending on local site
conditions and requirements, the number of techniques required and
the complexity of the treatment processes can vary considerably.
This is discussed in more detail in Section 5. Given the
dependencies between the technologies, digestate enhancement system
design must be approached holistically. The available outlet must
also be considered along with the demand for digestate products.
For example, if nutrient recovery is to be employed a market for
the recovered products must be secured. Once the desired outputs
have been established a choice of process/technology can be made.
It is likely that a number of different technologies will be
available for selection and at this stage a detailed cost benefit
analysis will be required in order to determine the preferred
solution on a site specific basis.
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Enhancement and treatment of digestates from anaerobic digestion
11
Figure 3. Overview of Digestate Enhancement and Treatment
Techniques
Digestate
Land
Application Acidification
Land
Application
Drying
Land
Application
Dewatering
Fibre
Drying
Land
application
Enzymatic
hydrolysis
(Biofuel)
Composting
Incineration Land
application Gasification
Reed Beds
Land
Application
Nutrient
recovery Algae Production
Direct
discharge to
watercourse
Discharge to
watercourse /
further
treatment
Microbial Fuel
Cell
Evaporation
Land
Application
Energy Recovery Energy Recovery
Ash Disposal* Residual carbon
disposal**
Energy Recovery
Wet Air
Oxidation
Energy Recovery
Further
treatment
Concentrated
Fertiliser
Land
Application
Concentrated
Fertiliser Biofuel
Discharge to
watercourse /
further
treatment Nutrient addition
Balanced fertiliser
Land
application
Purification
(UF + RO )
Notes
*Ash recovery and product development required
**Residual carbon product development required
Biological
Oxidation
Liquor
Alkaline
stabilisation
Pyrolysis
Char disposal
Land
Application
Land
Application
Land
Application
Enhancement / treatment process Products / benefits
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Enhancement and treatment of digestates from anaerobic digestion
12
4.2.1 Physical Enhancement Techniques
Physical enhancement techniques can be used to separate the
solid and liquid fractions of the digestate. The separated liquid
fraction is termed digestate liquor and the separated solid
fraction is referred to as digestate fibre. This simple first step
enables the separated fractions to be treated individually,
providing a wider range of subsequent treatment options. The
physical techniques considered in the study can be broadly split
into three categories - thickening, dewatering and purification.
These physical techniques are well established in waste water
treatment; thickening and dewatering applications are the
conventional approach to reducing the volume of digestate for
subsequent storage, treatment processes or transport off site. The
application of these physical techniques may be considered a
natural progression into AD facilities and have potential for
retro-fitting to existing plant.
Thickening
Thickening is a term used to describe the partial separation of
the solid and liquid fractions to achieve a digestate of 5 10 % dry
solids and a separated liquor. At this solids concentration the
digestate is a thick liquid. Thickening is typically employed as an
initial pre-treatment stage to reduce the volume of the digestate
for subsequent storage. Increasing the solids concentration not
only reduces the volume but can also improve downstream processing
in terms of throughput capacity and associated electrical and
chemical consumptions. Often polyelectrolyte can be added to
digestate to improve coagulation and increase the overall solids
capture and operability of the thickening system (Evans, 2008).
Increasing solids capture is important to ensure the separated
liquor does not impose a high biological treatment demand on waste
water treatment systems.
Dewatering
Dewatering is a term used to describe the separation of the
solid and liquid fractions of digestate to achieve a separated
fibre content typically greater than 18% dry solids and a separated
liquor. When whole digestate is dewatered, 80% of the mass is
removed in the liquor fraction, leaving a dewatered cake of
approximately 20%. The ammonium and potassium will be partitioned
into the liquor whilst the phosphorus will be largely retained in
the dewatered cake. (Fuchs et al., 2010). Dewatering is often
employed as a first step in digestate processing. The digestate
fibre is a semi-solid cake which is easier to store. This combined
with reduced volume greatly simplifies handling and reduces
subsequent transport costs. Dewatering is also an important
treatment technique to improve the feasibility of land application.
However as the nutrient content will be lower than in the original
whole digestate, nutrient content will need to be considered in
securing land-based outlets. Dewatering digestate and reducing the
water content also enables a number of other technologies, such as
energy recovery, to be economically employed (see Figure 3).
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Enhancement and treatment of digestates from anaerobic digestion
13
The liquor generated by the dewatering process will contain high
levels of ammonium and potassium. Subject to site specific
requirements it is likely that this liquor will require a form of
treatment before it can be discharged to the public sewer. It may
be possible to recycle a fraction of the liquor for feed
processing, where the liquor can be used to dilute the feedstock to
an acceptable solids concentration. However, the remainder of the
liquor will require treatment, involving the removal of nutrients
from the liquor, by either recovery or oxidation, to enable the
liquor to be discharged. As with thickening, polyelectrolyte can be
added to digestate to improve coagulation and increase the overall
solids capture and operability of the dewatering system. Increasing
solids capture is very important to ensure the separated liquor
contains a minimal quantity of digestate solids to limit the
biological treatment demand. A typical sample of dewatered
digestate is shown in Figure 4.
Figure 4. Dewatered Digestate
Purification (Ultra Filtration and Reverse Osmosis)
Physical purification uses a membrane as a physical barrier
which acts as a molecular sieve retaining contaminants, yet
allowing water to permeate through. Subject to specific membrane
selection, the permeable membrane separates contaminants from the
digestate, at a molecular level; this produces a permeate stream
potentially suitable for direct discharge to watercourse, and a
concentrate which can be applied as a fertiliser (Chiumenti et
al.). Depending on the type of membrane selected, different
contaminants will be retained on the membranes. Ultra filtration
(UF) membranes are capable of retaining soluble macromolecules and
larger particles; reverse osmosis (RO) membranes are capable of
retaining small molecules and ions. Due to the small pore size of
the membranes (
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Enhancement and treatment of digestates from anaerobic digestion
14
4.2.2 Thermal Enhancement Techniques
Thermal techniques use thermal energy (heat) to either remove
water from the digestate to increase solids and nutrient
concentrations or to recover energy from the digestate (such as via
combustion).
Thermal Drying
Thermal drying can be used to significantly reduce the remaining
water within the digestate to produce a product of up to 98% ds
(SEVAR, 2012, Siemens, 2011). As the thermal energy required to dry
the digestate is directly proportional to the moisture content of
the feedstock, dewatering is typically employed as a pre-treatment
technique prior to thermal drying. The thermally dried product has
a greatly reduced volume and, as it is a dry solid material, can be
easily handled, stored and transported. If required the dry product
can be pelletised to suit product use and aid both storage and
transportation. The dried product has a number of potential uses
but is normally applied to land as an organic fertiliser or used as
fuel for energy recovery. At the elevated temperatures utilised
within the thermal drying process, ammonia will come out of the
solution and be contained within the evaporated fraction. This will
need to be condensed to form condensate, a high strength liquor
which will require treatment prior to discharge. It is possible for
condensate treatment to be combined with treatment of dewatering
liquors. In the case of solar drying no pre-treatment is required
(Degremont, 2012, Thermo - Systems, Veolia, 2006b), although it may
be beneficial; also no condensate treatment is required. Digestate
is fed into a ventilated greenhouse where water is evaporated by
thermal energy derived from the sun. The digestate is continuously
turned to ensure consistent product quality. If an integrated
system is developed, waste heat from a combined heat and power
(CHP) system can also be utilised to supplement solar energy via
underfloor hot water piping systems. A typical sample of dried
digestate is shown in Figure 5.
Figure 5. Dried Digestate
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Enhancement and treatment of digestates from anaerobic digestion
15
Digestate Concentration (Evaporation)
To concentrate digestate or increase dry solids content,
evaporation can be applied. Evaporation utilises thermal energy
(heat) to release the moisture within the digestate and increase
both nutrient and solids concentration. Unlike the drying
techniques discussed above, evaporation aims to retain the
nutrients and a proportion of the moisture contained within the
digestate. Evaporation is typically utilised for liquor or whole
digestate treatment. The final solids concentration will be
dependent on the desired product, but concentrations of up to 20%
ds can be achieved. As with thermal drying, high temperatures will
cause ammonia to be released. This can be overcome by decreasing
the pH of the digestate, typically with acid dosing, prior to
evaporation (HRS Heat Exchangers, 2010). This approach allows the
digestate liquor to be converted into a concentrated
fertiliser.
Incineration
Incineration provides an alternative use of digestate to
land-based application. Digestate is combusted in order to achieve
destruction of organic matter (Perry, 1997). If the moisture
content within the digestate is sufficiently low and the
incinerator efficiency is high, the process can become auto thermal
(the process generates sufficient heat to allow combustion to
continue without the need for an external heat source or additional
fuel) and energy recovery can be achieved (Envirotherm GMBH,
Veolia, 2006a, Tchobanoglous et al., 2004). Autothermal operation
will typically require a dry solids content of >40%. The
incineration process is best suited to digestates with a high
calorific value or where land-based application is not financially
viable or practical. Ash from the process can be recovered and used
as a construction material for roads or for concrete production.
Phosphorus can also be recovered from the ash by acid leaching.
Gasification
Within the gasification process, the oxygen supply is limited to
enable partial combustion of organic matter within the feed in
order to produce a synthesis gas (syngas). Syngas is a mixture of
mainly carbon monoxide and hydrogen, which can be burnt to produce
energy (Perry, 1997). As with incineration, for the process to
operate efficiently, the feed digestate must have a low moisture
content and ideally be in a dry pelletised form (i.e. the product
of a thermal drying process). Gasification provides another
alternative use of digestate to land-based application. However the
technology has yet to be fully developed for this application
(Evans, 2008).
Wet Air Oxidation (WAO)
In the wet air oxidation (WAO) process organic material is
oxidised within the liquid phase, rather than in the gaseous phase,
in contrast to other combustion processes. WAO is achieved at
elevated temperatures and high pressure to prevent evaporation.
These conditions also enable chemical oxidation of mineral
components within the feedstock (Chauzy et al., 2010). The products
from WAO are a mineral sludge, a liquid effluent and off gasses
(Siemens, 2006).
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Enhancement and treatment of digestates from anaerobic digestion
16
Full scale plants are operational in Europe - the largest
installation is Brussels WWTW which treats digested sewage sludge
from a population of 1.1 million. Heat recovery is possible under
the right conditions making the process auto thermal (Veolia Water,
2010). The feed for the process is whole digestate, meaning that no
pre-treatment is necessary to reduce the moisture content of the
feedstock compared with other thermal destruction technologies.
However, post-treatment of the by-products, mineral sludge and
liquor, may be required.
Pyrolysis
Pyrolysis processes heat the digestate in an oxygen free
atmosphere breaking down organics within the feedstock into char
and syngas. The syngas typically contains mainly hydrogen, methane
and carbon monoxide (Perry, 1997). For the pyrolysis process to
operate efficiently the feed digestate must have a low moisture
content, and similar to gasification, often requires digestate in a
dry pelletised form. The pyrolysis process reduces the mass of the
digestate by 70%, significantly reducing transport costs. The char
produced by the process can be used as a soil amendment or as a
partial replacement for peat in growing media production; both of
these applications are undertaken in accordance with appropriate
regulatory controls (PYREG, 2011). Pyrolysis process technology has
been proven for this application however it is not yet well
established.
4.2.3 Biological Enhancement Techniques
Biological enhancement techniques use naturally occurring
micro-organisms to convert organic matter within the digestate in
order to stabilise the digestate, reduce the organic load or
produce novel products such as biofuels. Composting
The composting process aerobically breaks down organic matter in
the digestate, resulting in the conversion of ammonia to nitrate
which is more stable, and a highly mobile nitrogen source for
plants (Tchobanoglous et al., 2004, Botheju, 2010). Temperatures
within the compost process can reach 70C or more due to the
intensity of microbial activity, hence pasteurisation can be
achieved. However the ability to achieve pasteurisation will be
dependent on the composting process and the associated process
control. If physically suitable, the digestate can either be
composted on its own or it must be co-composted with a range of
standard composting feedstocks, such as wood chip and green waste.
As an additive to standard composting the digestate provides a
source of nitrogen, phosphorus, magnesium and iron, as well as
moisture. The standard composting feedstocks provide a bulking
agent, improve the carbon (C):nitrogen (N) ratio and consistency of
the final product (Evans, 2008). Co-composting is therefore
beneficial for both waste streams. Compost quality is and its
subsequent use is regulated by PAS100. Provided the required
controls are in place, digestate from source segregated waste can
be used as a compost feedstock in compliance with PAS 100.
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Enhancement and treatment of digestates from anaerobic digestion
17
Reed beds
Digestate reed beds can be used to dewater, sanitise and
mineralise the digestate over a long period of time, typically
10-15 years (Nielsen and Willoughby, 2005). Whole digestate is fed
into a sealed basin containing a bed of reeds. The digestate is
treated by bacteria within the root systems of the reeds and
evapotranspiration drives off water, typically dewatering the
digestate to 30-40% dry solids. Liquor collected from the basins
can be recycled as process water or used for irrigation. At the end
of the treatment period the beds are dug out and the digestate
applied to land (ARM Biosolids, 2012, Blumberg, 2012). The area
required for treatment is dependent on the type of digestate but
typical loading rates are between 20 and 60 kg dry solids/m/yr. A
typical view of digestate reed beds is shown in Figure 6.
Figure 6. Digestate Reed Bed
Biological Oxidation
Biological oxidation can be used to reduce the loading of
biological oxygen demand (BOD) and ammonia in the digestate. The
process is most commonly used to treat the digestate liquor prior
to discharge either to sewer or watercourse, however it can also be
used as a pre-treatment stage or used to treat the whole digestate
(wet composting). Typically the digestate is aerated in the
presence of bacteria which oxidise the BOD and ammonia. The
treatment of liquors in this manner is well proven but can have
high operating costs. The process produces a biological sludge as a
by-product which can be returned as a feedstock to the digester.
Examples of these processes include membrane bioreactors (MBR),
sequencing batch reactors (SBR), moving bed bioreactors (MBBR) and
the SHARON process.
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Enhancement and treatment of digestates from anaerobic digestion
18
Biofuel Production
Biomass within the digestate has the potential to be utilised as
a feedstock for biofuel production. Several possible techniques are
currently being developed. Digestate liquor can be used as a
feedstock for the production of algae which in turn can be
converted to biofuel (Iyovo, 2010, Uttleu). Water separated from
the algae can be used as
process water or for irrigation; waste algal biomass can be used
as a digestion feedstock.
This process is currently operational at pilot scale in the
Netherlands (Algae Food & Fuel,
2009).
A typical view of algal bioreactors is shown in Figure 7.
Figure 7. Bioreactors for the production of Algae from digestate
liquors
The digestate fibre can also be converted into biofuel by a
process of hydrolysis and biological fermentation (Yue, 2010).
Ethanol yields from the process are reported to be comparable to
some traditional energy crops (Teater, 2011). This process is only
currently operational at laboratory scale. It has also been shown
that freshwater and nutrients used for bio-ethanol production from
traditional energy crops can be replaced with dewatered liquor
(Gao, 2010). Using digestate liquor in this manner has been shown
to significantly increase ethanol yields.
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Enhancement and treatment of digestates from anaerobic digestion
19
Microbial Fuel Cell
Microbial fuel cells (MFC) are a novel application of fuel cells
that has potential to produce
bioelectricity from the biological oxidation of organic matter.
The process utilizes the ability
of particular microorganisms to transfer electrons directly to
an anode during respiration
(Aelterman, 2006). The reactions take place under anaerobic
conditions. This process is
only currently operational at laboratory and pilot scales.
Laboratory trials have shown the
process to be capable of removing 3.99kg COD/md (Peixoto,
2012).
A schematic of a microbial fuel cell is shown in Figure 8.
Figure 8. Schematic of a typical microbial fuel cell.
(Zeng et al., 2010)
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Enhancement and treatment of digestates from anaerobic digestion
20
4.2.4 Chemical Enhancement Techniques
Chemical enhancement techniques utilise chemical reactions and
equilibria to recover nutrients from the digestate or modify its
properties.
Struvite Precipitation
Struvite is the name commonly used for the chemical compound
magnesium ammonium phosphate which can be used as an inorganic
fertiliser (Evans, 2009). Under the correct conditions struvite can
be precipitated, allowing ammonium and phosphorus to be extracted
from the digestate. pH adjustment and magnesium ion addition are
usually required (Nawa). Struvite is recovered as a solid material,
well suited for export for use as either a fertiliser or as a base
feedstock for fertiliser production. Phosphorus is a finite global
resource and as a consequence struvite recovery is likely to become
more important in the future (Driver, 1998). This process will not
normally remove all of the ammonium from the digestate, as there
are insufficient quantities of phosphorus present in the
digestate.
Figure 9. Struvite products. Precipitated struvite crystals
(left) and granular struvite pellets produced in a fluidised bed
reactor (right)
Ammonia Recovery
Ammonia, in the form of ammonium, can be recovered from the
digestate for use as a concentrated fertiliser or a chemical
feedstock. A number of different techniques are commercially
available (Maurer et al., 2001). The efficiency of all of these
techniques can be improved by increasing the temperature and the pH
of the digestate (Gutin, 2011). If waste heat from a combined heat
and power (CHP) system is used to increase the temperature of the
process, financial support from the Renewable Heat Incentive (RHI)
can potentially be claimed.
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Enhancement and treatment of digestates from anaerobic digestion
21
Ammonia can be stripped from the digestate by contacting with
air or steam. Ammonium can then be recovered by scrubbing the
stripping gas in a second column (Colsen International, ngeles De
la Rubia et al., 2010). Depending on the scrubbing solution used,
ammonium can be recovered in a number of forms including ammonium
sulphate and ammonium nitrate, both of which have value as
inorganic fertilisers (Evans, 2009). Membrane contactors can also
be used to recover the ammonia (Liqui-Cel, 2009). Digestate and
sulphuric acid are fed, counter currently, on opposite sides of a
microporous hydrophobic membrane. Gaseous ammonia is removed across
the air filled pores of the membrane where it reacts with the
sulphuric acid to produce ammonium sulphate.
Figure 10. Schematic of a membrane contactor (Liqui-Cel,
2009).
Ion exchange processes recover ammonium by adsorption. Digestate
is fed into a packed bed of adsorbent where the ammonium is
selectively adsorbed by ion exchange. Once saturated the column is
taken off-line and regenerated, recovering the ammonium. The form
of the recovered ammonium is dependent on the regenerating solution
used (Maurer et al., 2001). A wide range of adsorbents are
available including zeolites, clays and resins (Cooney et al.,
1999).
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Enhancement and treatment of digestates from anaerobic digestion
22
Acidification
Sulphuric or other acids can be added to the whole digestate
prior to land application to decrease the pH and shift the
ammonium/ammonia equilibrium towards ammonium. This reduces
nitrogen loss from the digestate once applied to land. Careful
consideration must be given to the soil type of the land bank as
application of acidic digestate will not always be acceptable
(Ministry of Economic Affairs Agriculture and Innovation of the
Netherlands, 2010, Frandsen et al., 2011).
Alkaline Stabilisation
Alkaline stabilisation raises the pH of the digestate in order
to achieve pathogen kill, neutralise odours (typically hydrogen
sulphide) and prevent the digestate from becoming septic. However
increasing the pH can cause ammonia to be released causing odour
issues. Lime is typically used for the alkali stabilisation
(Tchobanoglous et al., 2004). This technique is commonly used to
treat dewatered sewage sludges.
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Enhancement and treatment of digestates from anaerobic digestion
23
5.0 Digestate Enhancement Systems
5.1 Digestate Treatment Systems
The number of enhancement techniques employed, and the
complexity of the treatment system, will be highly dependent on the
available land-based outlets, potential markets and the desired
digestate products. Treatment systems are highly site specific and
no single system will be optimal for all sites. For small sites,
with readily available local agricultural land, it may be possible
to spread the whole digestate to land, providing it is compliant
with the relevant legislation (or end of waste status via PAS110
and the ADQP) and satisfies the seasonal spreading requirements.
Under these conditions there may be no argument for any digestate
enhancement. However, as the distance to the land-based outlet
increases further enhancement may be required, such as dewatering
or drying to optimise storage, handling and reduce transport costs.
Investing in such enhancement technologies must also consider the
implication of associated by-products as significant volumes of
liquor will be produced, either as filtrate/centrate or condensate
which will require treatment prior to discharge to sewer or
watercourse. Where no land-based outlet is available thermal
conversion may be the only economic option, potentially requiring
dewatering, drying and associated liquor treatment. In recent years
there has been increased focus on creating marketable products from
digestate. Possible methods for achieving this include nutrient
recovery and the addition of nutrients to create a more balanced
fertiliser. Two possible techniques for enhancement are highlighted
in the examples included in Section 5.4 and 5.6. The Scottish and
Southern Energy (SSE) energy from waste plant at Barkip utilises
HRS scraped surface heat exchangers to recover nutrients from the
digestate. The IVAR IKS biofertiliser plant produces an organic
fertiliser (MINORGA) from food waste and sewage sludge in Norway
following digestion and thermal drying. Technologies are also
emerging to create novel products from digestate such as biofuels.
These technologies are still at an early stage of development but
have the potential to provide interesting possibilities in the
future. The following diagrams provide examples of an integrated
treatment system designed to recover nutrients from digestate,
recycle dewatered digestate fibre products to land and convert
digestate fibre to other product forms. These systems will not be
applicable to all plants but the diagrams aim to highlight how
technologies can be combined and the need to integrate the systems
to create a wide range of digestate products. A schematic of a
potential integrated digestate enhancement system for the liquor
stream is shown in Figure 11 and the fibre stream is shown in
Figure 12.
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Enhancement and treatment of digestates from anaerobic digestion
24
Figure 11. Flow Diagram of Potential Digestate Enhancement
System (liquor)
Digester Dewatering Digestate
Liquor
Membrane Bio
Reactor
Magnesium Hydroxide
Struvite
Precipitation
Ammonia Stripping
Magnesium Ammonium Phosphate
(Struvite)
Sulphuric Acid Ammonium
sulphate
Effluent
Water reuse
Heat
Air
Biogas CHP
Power
Sludge
Feed
Potential products or resource recoveries
Process Stages
Whole
Digestate
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Enhancement and treatment of digestates from anaerobic digestion
25
Figure 12. Flow Diagram of Potential Digestate Enhancement
Systems (Fibre)
Digester Dewatering
Whole Digestate
Digestate Fibre
Heat
Biogas CHP
Power
Feed Liquor to treatment (Figure 11)
Thermal Drying
Incineration Gasification
Ash recycling / disposal
Boiler
Steam Turbine
Heat
Alkaline Stabilisation
Land Based
Outlets
Syngas to CHP/ Gas turbine
Composting
Lime
Pyrolysis Biochar recycling
Power Power
Potential products or resource recoveries
Process Stages Enhancement
Option gate
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Enhancement and treatment of digestates from anaerobic digestion
26
5.2 Digestate Enhancement Systems
The following describes the combination of treatment
technologies/techniques that have the potential to generate a
number of complementary digestate products see Figure 12. Digestate
is first dewatered to separate the liquid and solid fractions. The
dewatered fibre can then be applied directly to land or utilised
for energy recovery. The first stage of the liquor treatment
process presented in Figure 11 is aerobic reduction of chemical
oxygen demand (COD) within a membrane bioreactor (MBR). The process
is configured such that no ammonia is oxidised. Sludge generated by
the MBR is recycled back to the digester. Effluent from the MBR is
dosed with magnesium hydroxide to increase the pH and magnesium ion
concentration, enabling struvite precipitation. Heat from the CHP
is also used to improve struvite removal and the use of heat in
this manner may be eligible for financial support from the
Renewable Heat Incentive (RHI). Struvite is precipitated and
extracted for use as an organic fertiliser. As equimolar amounts of
ammonia and phosphate are used in struvite production, and
digestate is relatively rich in ammonia, there remain significant
quantities of ammonium within the digestate liquor. In order to
recover this ammonium the liquor is fed into an ammonia stripper;
as the pH and temperature of the digestate have already been
increased the conditions are more suitable for the stripping
process. In addition, the risk of fouling within the ammonia
stripping column is greatly reduced as the COD has already been
removed by the MBR. The stripping process recovers the ammonia as
ammonium sulphate. Sulphuric acid and heat are used within the
process; again this application of heat from the CHP may also be
eligible for financial support from the Renewable Heat Incentive
(RHI). The treated liquor from the process can be reused as process
water, used for irrigation or discharged to sewer. Alternatively,
an additional treatment stage can be added in the form of reverse
osmosis (RO) to produce higher quality process water or enable
direct discharge to a watercourse. Combining the individual process
units into the treatment system above provides an integrated and
holistic treatment process capable of producing both solid and
liquid fertiliser products from digestate. Waste heat from the CHP
may also be utilised in order to generate additional income from
the RHI and the effluent is suitable for reuse within the process.
However this system incorporates several complex subsystems that
require careful integration and operation.
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Enhancement and treatment of digestates from anaerobic digestion
27
5.3 Current Barriers to Enhancement Systems
The use of digestate enhancement technologies/techniques faces a
number of barriers in the UK, preventing their widespread adoption.
The most significant barrier is the current cost of installation,
as well as the operational costs associated with the technologies.
This barrier is directly linked to the cost of alternative disposal
arrangements such as landfill or energy recovery facilities.
However, the relatively low value of digestate products and the
associated cost of developing outlets or markets for these products
is also a significant barrier. It is imperative that the
installation of a single digestate enhancement system does not
frustrate, or in the extreme negate, subsequent treatment process
additions. However the cost of installing a suite of totally
integrated enhancement systems, as described in Section 5.1 and
5.2, presents a significant financial challenge that the sector is
unlikely to be able to fund at this stage. A key step to overcoming
these barriers is to raise awareness of the waste sector to
enhancement technologies and techniques, to reduce costs and to
emphasise the financial benefits of implementing enhancement
systems to secure potential income from digestate. Whilst this
research exercise has identified a range of potential techniques
and methods for treating and enhancing digestates in the UK these
options will not be adopted until the business models exist that
ensure that the financial investment is worthwhile. There is a need
to raise awareness of potential improvements to digestates and
ensure that the industry is aware that there may be advantages in
developing flexible sites where changes can be adopted as new
technologies become available.
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Enhancement and treatment of digestates from anaerobic digestion
28
5.4 Technology Example: Barkip Biogas Facility
5.4.1 Background
HRS Heat Exchangers have been contracted by Scottish and
Southern Energy (SSE) to install their Unicus scraped surface
evaporators for digestate liquor treatment at the Barkip anaerobic
digestion plant. Barkip biogas facility is the largest anaerobic
digestion plant in Scotland. The site, located in a former landfill
site in North Ayrshire, will process up to 75,000 tonnes of waste
food, manure and organic effluent sludges. The plant is the first
of its kind to incorporate the heat exchanger technology developed
by HRS. Scraped surface heat exchangers use heat generated from the
process to concentrate the liquid fraction of the digestate into a
nutrient-rich fertiliser.
Scraped surface evaporator plants are designed to overcome
fouling issues associated with the evaporation of organic
digestate. The interior surface of the heat exchanger tubes is
constantly cleaned by internal scrapers to reduce fouling and
increase heat transfer efficiency. Although this is the first time
the technology will be utilised for digestate processing, the heat
exchangers are well proven for other applications, most relevant
being the concentration of pig manures.
Key Facts
Technology Supplier: Xergi, HRS Heat Exchangers.
Client: Zebec Energy on behalf of Scottish and Southern
Electric.
Throughput: 75,000 tpa.
Feedstock: Food waste, animal manure, organic sludges.
Technologies employed: Two stage Thermophilic Anaerobic
Digestion, centrifuge, scraped surface heat exchangers.
Project stage: Operational, PAS110 certified.
Capital cost: 1.3 million (evaporation plant only).
Electrical generation: 2.5MW.
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Enhancement and treatment of digestates from anaerobic digestion
29
Figure 13. Scraped Surface Heat Exchanger Cut Away
5.4.2 Process Description Digestate from the process is fed into
a centrifuge in order to separate the liquor and fibre fractions.
The digestate liquor is then pre-treated with acid prior to
evaporation within the scraped surface heat exchangers to prevent
ammonia loss within the evaporator. The volume of acid dosed is
dependent on the digestate and the desired retention. Within the
evaporator the liquor is concentrated to approximately 20% Dry
Solids (DS). The evaporator operates under vacuum at temperatures
between 50C and 70C. Heat required for the process is provided by
the combined heat and power plant (CHP). This application of heat
may be eligible for financial support under the Renewable Heat
Incentive (RHI). The evaporators at Barkip are capable of treating
10,800 kg/h of digestate liquor and producing 1,565 kg/h of
concentrate. The concentrate can then be mixed with the separated
digestate fibre to produce a nutrient rich solid fertiliser for
export. Distilled condensate for the process is recycled as process
water with any excess discharged to public sewer. For the Barkip
application the heat exchanger tubes have been constructed from
Duplex steel due to the high chloride content within the feedstock.
Key Benefits
Nitrogen is retained within the digestate, improving
fertiliser
potential.
Concentrated fertiliser reduces transport costs.
Additional income from Renewable Heat Incentive (RHI).
High levels of N/P/K retained within the digestate.
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5.5 Technology Example: Lee Moor EFW
5.5.1 Background
British engineering company AeroThermal Group Ltd has been
granted planning permission to develop a sustainable waste and
resource treatment facility at the site of Imerys Minerals Ltd at
Lee Moor in South Devon. AeroThermals autoclave is a pressure
vessel that steam treats its contents at a constant temperature and
pressure, serving to sterilise, clean, and break-down organic and
lignin structures and reduce waste volume by approximately 60%. The
Lee Moor facility will utilise an autoclave to pre-treat the
digester feedstock. Pre-treatment by autoclave pasteurises, cleans
and breaks down organic matter and lignin structures within the
feedstock. This enables contaminants within the feedstock to be
removed more effectively, greatly enhancing biogas generation and
the quality of the digestate. Once operational the site will divert
58,000 tonnes of waste from landfill every year and generate 26
gigawatts of renewable electricity. Recyclable materials will also
be recovered from the waste stream and the stable digestate, a
by-product of the Advanced Anaerobic Digestion (AAD) process, will
be used to help restore parts of the adjoining Lee Moor China Clay
workings.
Figure 14. Autoclave Pressure Vessel
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5.5.2 Process Description Feed material is loaded into a pair of
autoclaves in weighed 10 tonne batches. The autoclaves are fed up
to 10 times per day, each at a maximum of 10 tonnes per batch via a
system of conveyors from the weighing hopper. The autoclaves
operate in an alternating batch mode: residual steam is recycled
from the duty unit that has completed its processing to the
autoclave that has been loaded and is waiting to start the cycle.
This procedure not only improves the steam utilisation efficiency
but also significantly reduces the release of steam to atmosphere.
Once loaded the duty autoclave is rotated. Flights within the
autoclave lift the feed material towards the top of the chamber
where it then falls back to the bottom of the vessel to create a
continuous mixed flow. Steam is then injected until the autoclave
internal pressure of 5.2 bar and a temperature of 160C is achieved.
These conditions are maintained for the duration of the treatment
process. After treatment the autoclave is returned to atmospheric
conditions, the bottom door is opened and the rotation of the
vessel is reversed. This allows the flights within the vessel to
act as a screw conveyor and force processed material out. After
processing, inorganic material and contaminants can be easily
removed via mechanical separation providing a clean, pasteurised,
organic rich feedstock for anaerobic digestion. Digestate from the
anaerobic digestion process is dewatered by a conventional
centrifuge. The digestate fibre is used for land restoration.
Liquors from the process are partially treated by dissolved air
filtration before being recycled for feed preparation. Excess waste
water which cannot be recycled will be treated by a membrane
bioreactor (MBR) to enable direct discharge to an adjacent
watercourse.
Key Facts
Technology Supplier: AeroThermal Group Ltd.
Throughput: 58,000 tpa.
Feedstock: Municipal waste.
Technologies employed: Autoclave, Anaerobic Digestion,
centrifuge dewatering.
Project stage: Pre construction.
Capital cost: 15 million.
Electricity generation: 3MW.
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5.6 Technology Example: MINORGA Bio fertiliser, Norway
5.6.1 Background
Research has been undertaken in Norway at Stavanger Regional
Wastewater Treatment Plant (RWTP) into developing a digestate-based
organic fertiliser with a consistency, particle size and nutrient
composition comparable to mineral fertilisers. Extensive field and
product trials have concluded with an organic product called
MINORGA. During the period 2007-11 extensive trials and field
experiments were undertaken into the development of an organic
fertiliser based on thermally dried digestate. The research has
been conducted at Stavanger Regional Wastewater Treatment Plant by
the plant operator IVAR IKS in conjunction with the HST Valuable
Waste Company. The concept of the fertiliser product is based on
supplementing the phosphorous within the thermally dried digestate
produced at Stavanger with the addition of nitrogen and potassium.
The resulting product, called MINORGA, is a granular organic
fertiliser with an N-P-K ratio of 10-2-5.
Figure 15. MINORGA Pellets
Key Benefits
Pasteurisation.
Waste resource recovery.
Waste minimisation.
Digestate recycling.
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33
The product is considered more environmentally friendly than
similar mineral fertilisers leading to less run-off and a prolonged
fertilising effect. In that context, the product also offers a
better balanced phosphorus supply (Tornes et al, 2010). Agronomic
trials showed no significant differences in nutrient uptake between
MINORGA and commercially available mineral fertilisers. Spreading
tests performed by a conventional farm spreader showed distribution
patterns very similar to that of commercially available mineral
fertilisers. 5.6.2 Process Description
The development to date has been based on the manufacture of an
organic fertiliser product from a mixture of digestate and mineral
compounds such as urea and potassium chloride. The facility to
produce the fertiliser product will be integrated into the existing
sewage sludge treatment process at Stavanger (RWTP). The fertiliser
facility comprises of storage silos for the addition of urea,
potassium chloride, meat bone meal or other mineral salts/high
quality organic by-products, a dosage and mixing system,
pelletising plant and a big bag loading and packaging system.
Thermally dried digestate produced by the Stavanger RWTP will be
directed to a batch dosing and mixing system before the mixture is
transported to the pelletising plant. The pellets will pass through
an air cooler followed by a sieve to ensure uniform product quality
before the product is stored in a product silo. Undersized pellets
will be recycled back to the pelletising system. The final product
will be packed in big bags containing 600 kg of MINORGA, the
registered name of the organic fertiliser. Pasteurisation of the
digestate is achieved within the thermal drying plant; the presence
of organic pollutants has been systematically investigated in
surveillance studies and found to be either absent or at negligible
and acceptably low levels. The process flow diagram for the system
is schematically presented below.
Key Facts
Technology Supplier: Graintec, Denmark.
Throughput: 10,000 tpa of MINORGA.
Feedstock: Sewage sludge and organic wastes including
domestic food waste and catering waste from hotels.
Technologies employed: Mesophilic anaerobic digestion,
thermal drying, nutrient addition, palletisation.
Project stage: Contract established.
Capital cost: NOK 40M (4.3M).
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Figure 16. MINORGA Process Schematic
The total investment costs are approximately NOK 40 million. To
reduce operating costs IVAR IKS continue to investigate alternative
sources to urea and potassium chloride including nitrogen and
potassium recovery from dewatering liquors generated at the
regional wastewater treatment plant.
Key Benefits
Pasteurisation.
Waste minimisation.
Waste resource recovery.
Low transport volume of dried and pelletised product.
Production of a marketable organic fertiliser.
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6.0 European Perspective
6.1 Background
Landfill rates for municipal waste have decreased steadily from
62% in 1995 to 37% in 2010 in the EU-27, with 38 % of municipal
waste recycled or composted/digested in 2010 compared to 17 % in
1995. EU and national policies targeting municipal waste have been
important drivers of this development (EEA 2012). The biomass
categories used as substrates (feedstock) for anaerobic digestion
in European biogas production are animal manures and slurries;
agricultural residues and by-products; digestible organic wastes
from food and agri industries; the organic fraction of municipal
and catering wastes; sewage sludge; and dedicated energy crops such
as maize, miscanthus, sorghum, and clover particularly in Austria
and Germany (Al Seadi et al. 2008). Digestate composition and
qualities are a function of input materials and process approach,
hence enhancement technologies need to be robust and capable of
dealing with a range of inputs in order to achieve significant
market penetration.
6.2 Survey Data
A 2011 survey of AD facilities across Europe (including the UK)
identified several thousand specific facilities (excluding waste
water treatment plants). Typically these are co digestion
facilities accepting a variety of different inputs, constructed by
a variety of different technology providers (Voss 2012). The survey
indicated that EU countries can be grouped based on the number of
AD facilities identified as follows:
>>1,000 facilities: Germany.
>>100 facilities: Austria, Belgium and Luxembourg
combined, the Netherlands.
~ 100 facilities: Denmark, Italy, Czech and Slovak republics
combined, UK.
< 100 facilities: Finland, France, Hungary, Poland, Portugal,
Spain, Sweden.
~ 10 facilities: Bulgaria, Greece, Latvia, Romania,
Slovenia.
The level of detail is variable but indicates that the majority
of these digester facilities accept biomass crops in Austria and
Germany; whereas facilities in the UK, Finland, France, Sweden, the
Netherlands focus on waste derived from agricultural or urban
sources. The number of digestion facilities reported in the survey
is higher than a study quoted by the Technical Report for
End-of-Waste Criteria (EC JRC 2011), which identified 166 anaerobic
digestion facilities for biowaste and municipal solid waste across
15 EU countries. The difference between the total number of AD
facilities is explained by the fact the End of Waste report did not
include farm based systems. Typically solid/liquid separation is a
precursor to any further product enhancement treatment, which is
very similar to digestate management practice for wastewater
treatment plants.
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6.3 Transport Optimisation
There is growing interest in the development of more readily
transportable products from digestion. Many European facilities
operate solid/liquid separation, to reduce transport costs for the
solids fraction, to facilitate storage (e.g. over the closed
season) and to increase the potential radius of digestate use. Some
facilities use evaporation to concentrate liquid fractions using
waste heat, again to increase the effective radius of use. This is
of particular interest in Germany where subsidies for electricity
production provide the incentive for AD facilities, where there is
no immediate CHP opportunity (Voss 2012). Hybrid systems utilising
both AD and composting systems are also in use. Composting of
digestate with additional feedstocks, is undertaken to reduce water
content and overcome stability and odour problems; in addition
hybrid systems have been found to improve materials handling and
improve product value (e.g. Swedish Gas Association 2008). The IEA
Bioenergy Task 37 group recommend that the solids fraction should
be stored without disturbance, or even composted, in order to avoid
methane emission (Lukehurst et al. 2010). The Netherlands view the
development of solutions to digestate use as important in enabling
the expansion of AD as a biogas resource, and has a range of
enhancement approaches under consideration, including the
extraction of substitute fertilisers from digestates (New Gas
Platform, Green Gas work group 2010). The availability of
opportunities for digestate use is a key factor in selecting the
location of AD facilities in the Netherlands (New Gas Platform,
Green Gas Work Group 2010, Energy Transition, New Gas Platform
2011). Example configurations include the following (IEA Bioenergy
Task 37 2012, Swedish Gas Association 2008): Boden plant, Sweden:
operating since 2003: thermophilic (55oC) co-digestion of
sewage
sludge (960 tonnes dry solids per year) and household food
wastes (1,200 tonnes per
year) producing biogas for transport vehicle use and waste heat
which is used for district
heating. 1,600 tonnes of digestate is produced per year, some of
which is used to
produce a soil conditioner. Digestate is de-watered by a
centrifuge plant to
approximately 30% dry solids. The dewatered digestate is stored
in silos and transported
by truck.
Helsingborg plant, Sweden: operating since 1997, feedstock:
household food wastes,
food industry wastes and pig manure, approximately 45,000 tonnes
per year. Digestate
is pumped to farm users via a 10 km pipeline (capacity 20,000
tonnes per year).
Karpalund plant, Sweden: operating since 1996, feedstock:
household food wastes,
manure, slaughterhouse waste, approximately 60,000 tonnes per
year. Digestate is
sieved to remove debris such as plastics and then dewatered
before storage and use.
Inwil plant, Switzerland, operating since 2008, based on a
thermophilic plug flow
digester treating source separated collection of municipal solid
waste and two mesophilic
continuously stirred tank reactor digesters treating mainly pig
manure. Industrial waste
is also accepted. The total waste volume treated is
approximately 60,000 tonnes per
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Enhancement and treatment of digestates from anaerobic digestion
37
year. After dewatering the solid output (13,000 tonnes per
annum) is matured under
aerobic conditions, and the liquid output is treated by
ultrafiltration and reverse osmosis
to obtain a concentrated liquid fertiliser (10,000 tonnes per
annum) and clean water.
The liquid fertiliser is transported to farmers. (Similar liquid
fraction treatment
configurations have been tested at pilot scale in Sweden -
Svenskt Gastekniskt Center
2010).
6.4 Summary of EU Waste Sector
There is widespread interest in the development of enhanced
products from digestates. The work of the International Energy
Authority (IEA Bioenergy Task 37) indicates research interests for
processing digestate into value added products are present in
Austria, Denmark, Finland, France, Germany, Netherlands, Norway and
Sweden. The exact nature of the research being undertaken is not
always specified in the IEA documents but is largely related to
chemical, physical and thermal processes for solid liquid
separation, and downstream conversion of solids or liquids into
fertiliser products such as struvite. Research in Germany includes
an investigation of the utilisation of CO2 and nutrients from
digestate for micro-algae production and hydrothermal gasification
of digestate for additional CO2 and CH4 production (IEA 2010, 2011,
2012). Over a number of years research has also been supported
under the EC Framework Research Programmes, including several
hundred projects related to anaerobic digestion through the
Cooperation, Ideas, People and Capacities sub-programmes. These can
be viewed using the CORDIS database at
http://cordis.europa.eu/home_en.html. A small proportion of these
projects are related to digestate enhancement. These include
investigations of: struvite recovery, sulphur recovery (for high
sulphur content wastes), ammonia stripping and recovery, ethanol
production, algal production, and thermal conversion of digestate
to energy. The EU funded Eco-Inn