digestates from anaerobic digestion a review of enhancement techniques and novel digestate...
DESCRIPTIONDigestate form AD
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
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.
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.
Enhancement and treatment of digestates from anaerobic digestion
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
Enhancement and treatment of digestates from anaerobic digestion
AD Anaerobic digestion. Process of controlled decomposition of organic matter under
Aerobic Molecular oxygen available.
Anaerobic No oxygen source..
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.
CHP Combined heat and power. Cogeneration of heat and power from combustion of a
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, Liquor Liquid fraction of material remaining after separating coarse fibres from whole
Digestate, Whole Material resulting from an anaerobic digestion process that has not undergone post-
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
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.
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
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
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.
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).
Enhancement and treatment of digestates from anaerobic digestion 1
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.
Enhancement and treatment of digestates from anaerobic digestion 2
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.
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
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.
Enhancement and treatment of digestates from anaerobic digestion 3
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.
Enhancement and treatment of digestates from anaerobic digestion 4
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.
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.
Enhancement and treatment of digestates from anaerobic digestion 5
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:
within the digestion process ( i.e. in-vessel); and
Each system considered is aimed at supporting the objective of enhancing the quality of the digestate or providing potential to develop new digestate products.
Enhancement and treatment of digestates from anaerobic digestion 6
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).
Enhancement and treatment of digestates from anaerobic digestion 7
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:
fibres (both natural and man-made textiles),
solid fruit residues(pips/stones/stalks/peel)
Enhancement and treatment of digestates from anaerobic digestion 8
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
Enhancement and treatment of digestates from anaerobic digestion 9
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.
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)
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
Enhancement and treatment of digestates from anaerobic digestion 10
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.
Enhancement and treatment of digestates from anaerobic digestion 11
Figure 3. Overview of Digestate Enhancement and Treatment Techniques
recovery Algae Production
Energy Recovery Energy Recovery
Ash Disposal* Residual carbon
treatment Nutrient addition
(UF + RO )
*Ash recovery and product development required
**Residual carbon product development required
Enhancement / treatment process Products / benefits
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 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 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).
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 (
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 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
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 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.
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).
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 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.
Enhancement and treatment of digestates from anaerobic digestion 17
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 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.
Enhancement and treatment of digestates from anaerobic digestion 18
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,
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.
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)
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 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, 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.
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).
Enhancement and treatment of digestates from anaerobic digestion 22
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 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.
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.
Enhancement and treatment of digestates from anaerobic digestion 24
Figure 11. Flow Diagram of Potential Digestate Enhancement System (liquor)
Digester Dewatering Digestate
Magnesium Ammonium Phosphate
Sulphuric Acid Ammonium
Potential products or resource recoveries
Enhancement and treatment of digestates from anaerobic digestion 25
Figure 12. Flow Diagram of Potential Digestate Enhancement Systems (Fibre)
Feed Liquor to treatment (Figure 11)
Ash recycling / disposal
Syngas to CHP/ Gas turbine
Pyrolysis Biochar recycling
Potential products or resource recoveries
Process Stages Enhancement
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.
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.
Enhancement and treatment of digestates from anaerobic digestion 28
5.4 Technology Example: Barkip Biogas Facility
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.
Technology Supplier: Xergi, HRS Heat Exchangers.
Client: Zebec Energy on behalf of Scottish and Southern
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.
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
Concentrated fertiliser reduces transport costs.
Additional income from Renewable Heat Incentive (RHI).
High levels of N/P/K retained within the digestate.
Enhancement and treatment of digestates from anaerobic digestion 30
5.5 Technology Example: Lee Moor EFW
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
Enhancement and treatment of digestates from anaerobic digestion 31
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.
Technology Supplier: AeroThermal Group Ltd.
Throughput: 58,000 tpa.
Feedstock: Municipal waste.
Technologies employed: Autoclave, Anaerobic Digestion,
Project stage: Pre construction.
Capital cost: 15 million.
Electricity generation: 3MW.
Enhancement and treatment of digestates from anaerobic digestion 32
5.6 Technology Example: MINORGA Bio fertiliser, Norway
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
Waste resource recovery.
Enhancement and treatment of digestates from anaerobic digestion 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.
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).
Enhancement and treatment of digestates from anaerobic digestion 34
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.
Waste resource recovery.
Low transport volume of dried and pelletised product.
Production of a marketable organic fertiliser.
Enhancement and treatment of digestates from anaerobic digestion 35
6.0 European Perspective
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.
Enhancement and treatment of digestates from anaerobic digestion 36
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
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
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
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