Aerobic and Anaerobic digestion waste projects

In previous articles of this publication, we consider: (i) the nature of waste projects; (ii) the key commercial, legal and policy issues arising in the context of Waste-to-Energy (WtE) projects (including using anaerobic digestion); (iii) the key policy settings for waste projects in some of the key Asia Pacific jurisdictions; and (iv) the size and shape of waste projects (other than WtE projects) generally. While we considered feedstock and its interface with waste projects in passing, we did not consider feedstock in detail. The quality of feedstock is critical to all waste projects, particularly those using aerobic and anaerobic digestion technologies.

Therefore, in this article we consider feedstock in detail, in particular in the context of aerobic digestion and anaerobic digestion technologies. This reflects the prevalence of the use of these technologies for waste projects, particularly in areas that are not heavily urbanised, and in areas that are urbanised or urbanising and seeking to reuse organic waste more effectively, in particular agricultural and farming and food waste. In this context, we consider the interface between councils and municipalities and policy development and implementation, and the broader role of policy across the entire waste industry. The purpose in doing so is to assist councils and municipalities (and other procuring entities) in framing assessment and consideration of aerobic digestion and anaerobic digestion technologies as possible options for waste projects, and, if viable as waste projects, to achieve the policy outcomes sought. In considering aerobic and anaerobic digestion technologies, councils and municipalities are looking to achieve outcomes higher up the Waste Management Hierarchy than are achieved using other waste projects.

It is often the case that councils and municipalities have to implement more challenging policy outcomes without access to the scale offered by the waste industry as a whole; typically, this is the case for councils and municipalities in jurisdictions with more developed waste collection systems that have duties that include the collection or disposal of waste, or both, and in particular the collection and disposal of household waste or municipal solid waste (MSW). Because of these duties (and the ability to raise local rates and taxes), these councils and municipalities provide key support of the policy outcome of diverting MSW from landfill which may be promoted by a landfill or waste levy. MSW contains a number of fractions, including a reusable fraction¹  and a recyclable fraction,²  and within each fraction there is organic and inorganic material which is non-digestible. For the reuseable and recyclable fractions in MSW to be reused or recycled it is necessary to separate the organic material from the inorganic and other non-digestible fractions. Separating at source, or between source and ultimate processing and treatment at a waste project, can be costly and may not achieve the required level of effectiveness to allow the digestion technology to achieve the performance contemplated and often provided for contractually.

There are opportunities for the private sector to work with councils and municipalities (and other entities procuring waste facilities or services) to achieve the necessary scale in terms of quantities of waste processing and treatment and, in so doing, increase the derivation of reuseable and recyclable fractions, and the production of reuseable and renewable resources from a diverse range of waste streams. In many jurisdictions, councils and municipalities are combining to achieve scale across their council and municipal areas. In the United States of America, these technologies have been used to achieve scale and, as a result, significant levels of reuse, recycling and diversion from landfill.

In the fourth article of this publication, Waste-to-Wealth: Have we reached a tipping point?, it is stated that, in the context of all waste projects, the "Four Cs" – Compatibility, Contamination, Composition (and Characteristics) and Capacity – needed to be considered and addressed. This may be said to be particularly the case in the context of waste projects using aerobic and anaerobic technologies. In many ways, this article expands in detail on the Four Cs described in the fourth article, providing a more detailed and nuanced explanation of the importance of the Four Cs.

In this article, we provide an overview of: (i) aerobic and anaerobic digestion technologies; (ii) the waste that may be used (and may not be used) in each process and treatment technology (feedstock); (iii) the key commercial and legal issues for councils and municipalities (and other entities) procuring, or procuring services from, waste projects using aerobic and anaerobic digestion technologies; and (iv) the opportunities from a policy perspective to maximise the use of waste streams to divert waste from landfill.

In the context of heat and energy production, the process of the putrefaction (or decomposition) of organic matter (biomass), produces gas from the putrefaction of organic material disposed of to landfill, which may be collected/gathered from landfill as landfill gas (LFG). Alternatively, if the biomass is subject to an anaerobic digestion technology designed to produce biogas, biogas will be produced.³  Both LFG and biogas contain methane (CH4).

As a result, LFG and biogas may be used as feedstock that is burned to produce heat or energy, or both. In addition, biogas may be used as a feedstock for biofuel production. LFG and biogas are each a renewable energy source.⁴ As noted below, the production of biogas using anaerobic technology is preferred because of its better environmental and public health outcomes. However, this preference is only likely to be realised if the policy framework provides disincentive to send waste to landfill in the form of a landfill or waste levy and an incentive to produce biogas in the form of favourable renewable energy pricing and the requirement that retailers and large users of energy source energy from renewable resources.

This article has been authored by legal practitioners who have worked on waste projects using aerobic and anaerobic technologies around the world. The observations made are derived from the authors' general experience of the issues that need to be considered and, ideally, addressed in procuring a successful aerobic or anaerobic waste project, alone or as part of a waste project using different technologies (for example, a Mechanical and Biological Treatment (MBT) waste project). The authors have experience in respect of a broad range of aerobic and anaerobic digestion technologies processing a variety of feedstock, including sewage, green waste,⁵  agricultural and farming waste (including animal litter/manure/slurry⁶  biomass from crop harvesting and early stage crop processing), by-products of animal slaughter, preparation and processing (including carcasses and intestines),⁷  Food Waste⁸ and MSW.⁹  This article does not, however, describe in detail how specific aerobic and anaerobic digestion technologies work (including, for example, dry and wet digesters), or the particular issues of which the council or municipality (or other procuring entity) needs to be aware in respect of particular technologies. Rather, we identify matters of which councils and municipalities (and other procuring entities) need to be aware.

Feedstock for aerobic and anaerobic digestion technologies

The following table details in general terms the categories of waste that may be used as feedstock for aerobic and anaerobic digestion:

As will be apparent from the table, the Food Cycle comprises production,¹³  harvesting, processing, sale and consumption of food. At each stage of the Food Cycle, waste is produced. It is important to understand the Food Cycle, and each point in the Food Cycle at which Food Waste arises.

It is noted that some crops are grown and harvested for the purposes of the production of heat and energy, but for the purpose of this article these crops are not considered to be waste.

In the context of possible co-digestion (considered below), it is worth considering which feedstock may be mixed with other feedstock, including use of existing anaerobic digestion facilities.

While in most circumstances it is necessary (or recommended) to pre-sort MSW, most feedstock benefits from some form of pre-sorting (and some feedstock also benefits from the addition of water, including that derived from effluent) including – in the case of aerobic digestion recyclables and inorganic organic material and in the case of aerobic digestion – pre-sorting to remove grit and sand, and non-digestible material (organic and inorganic). The effect of the pre-sort on the composition (and characteristics) of the feedstock needs to be considered. The authors have worked on waste projects with complex and virtually exhaustive pre-sort processes, which have – despite separation at source or pre-sorting prior to processing – resulted in a concentration of base metals (critically lead), which meant that the compost produced did not comply with the required standards for use and sale.

Overview

Key issues

If a council or municipality (or other procuring entity) is considering (or a number of councils or municipalities are considering collectively),¹⁴  using an aerobic or anaerobic digestion technology for a waste project or as part of a waste project, it is important for the council or municipality (or for each of them, or any other procuring entity) to consider the context in which it is (or they are) procuring the waste project. Therefore this article considers in detail the vital importance of understanding feedstock that is under the control of councils and municipalities (and any other procuring entity), being Controlled Waste Streams. Also, it is important to understand the waste streams arising within the area of the council or municipality (or councils or municipalities) which are not under the control of such council or municipality (or councils and municipalities), and which may be consolidated with Controlled Waste Streams to allow increased scale and thereby lower unit processing and treatment costs. For example, agricultural and farming waste (if any) and other Food Waste arising within the area. In this context, we consider the key feedstock that may arise and may offer opportunities for increased processing and treatment, and as such reuse.

Feedstock is only one element of decision-making for the purposes of procurement. Feedstock is inextricably linked to the size of the council or municipality (or councils or municipalities); the existing waste collection system; the existing population and projected population growth (including in each instance, its distribution and degree of urbanisation); existing infrastructure (including existing waste projects and, in certain circumstances, existing wastewater treatment facilities) and existing landfill capacity, and the remaining life (both design and regulatory) of that infrastructure and existing landfill; and other planned waste projects (including waste projects that may compete for feedstock, typically feedstock is not included within Controlled Waste Streams).Depending on the feedstock, it is important to understand the willingness of the population to separate fractions of the waste stream at source, including the Food Waste fraction¹⁵  from MSW,¹⁵ and the cost and effectiveness of doing so.

In addition, it is important to consider the extent of markets for compost (including their scale and geographical locations) in the case of an aerobic digestion waste project and other organic products, and the market for biogas and biofuels (in the case of an anaerobic digestion waste project), including any potential off-takers for heat and energy (in the case of biogas) and biofuel, including the councils or municipalities themselves (or any other procuring entity including the private sector).¹⁶  In this context, councils and municipalities (and any other procuring entity) should consider alternative energy and fuel supplies, from a net cost/revenue perspective: this will involve considering the projected short, medium and long term net cost/revenue position, including the net present value of avoided and deferred costs of new landfill, and in jurisdictions in which landfill is being or has been phased out (as in the European Union), other means of disposal.

These matters speak to the cost and revenue profiles of a waste project, and as such its affordability and sustainability (ie whether it is viable). In the context of the cost profile, experience shows that it is important to understand how to monitor and avoid contamination or, if it is not possible to avoid contamination, then to manage it (effectively, the system cost of avoiding or managing contamination, and how to decontaminate or pre-treat feedstock if required). The starting point for avoiding contamination of certain waste streams is likely to be the education of the residents/constituents of the council or municipality, and the provision of bins for separation at source of fractions within the waste stream. Education and the provision of bins have a cost. If the feedstock for a waste project is to include Food Waste separated at source from MSW, the assessment of the cost and effectiveness of separation at source is particularly important and that education regarded as a starting point only. There is a need for on-going assessment of the collection system and the effectiveness of separation at source. Any documentation under which any council or municipality agrees to take waste services from a waste project must not restrict the ability of the council or municipality to change any aspect of its collection system. This is a commercial and technical issue (including to respond to change in law) and is necessary to enable councils and municipalities to respond to social licence as it changes over time.

In all contexts, "social licence" must be understood ie, whether the activity enjoys the acceptance and trust of those affected. We do not offer any specific comments on social licence, other than to note that it is a function of the constituency of each council or municipality, and in some situations the wider community, and that social licence changes over time. As we noted in our fourth article in the Waste-to-Wealth Initiatives: Examining policy settings in Asia Pacific, culture is critical as it relates to implementation of any policy, including the development of any waste project: to obtain and to maintain social licence awareness of the cultural setting is key.

Aerobic and Anaerobic digestion

1. Aerobic digestion involves biological digestion of organic matter (in the presence of oxygen) to derive compost¹⁷ (and associated storage of carbon and nitrogen in the compost) and the "production" of heat, water (including by evaporation)¹⁸  and carbon dioxide (CO2). The use of aerobic digestion technologies allows pasteurisation of the organic matter processed and treated into compost, with the process of pasteurisation removing pathogens and ammonia. As noted below, to produce compost effectively aerobic digestion requires organic matter of a consistent form and type, and consistent aeration of the organic matter. The production of compost reduces the mass and volume of organic matter that might otherwise go to landfill (including as a result of the evaporation of water). In the context of councils and municipalities (and other procuring entities), being aware of this reduction in mass and volume allows an informed assessment (and measurement) of the real level of diversion from landfill. 
2. Anaerobic digestion involves biological digestion (in the absence of oxygen) to process and treat organic matter to derive energy (using anaerobic digestion). As noted below (and as noted in article 2: Waste-to-energy projects), anaerobic digestion requires a consistent form and type of organic matter (such as animal litter, manure or sewage, Food Waste or crop residue from harvesting and processing) held in a controlled environment to produce biogas (primarily, methane (CH4) and carbon dioxide (CO2)) and residue. As we explain below, while biogas can be used for some purposes without further processing, it tends to be processed further to remove CO2, hydrogen sulphide (H2S)) and water vapour. This process, and critically the net cost/revenue consequence of removal of H2S, is considered in further detail below.
3. Comparison of aerobic and anaerobic digestion: Both aerobic and anaerobic digestion technologies differ from other waste technologies. Aerobic digestion technologies may be regarded as the most straightforward of technologies, and are often combined with other technologies, such as mechanical treatment in a composting hall to pasteurise the organic material. Anaerobic digestion technologies contrast in particular with thermal or thermochemical treatment of organic and inorganic matter¹⁹  in that the non-gaseous residue from an anaerobic digestion technology is bio-solids and sludge, bottom ash or bio-char in the case of thermal or thermochemical treatments.
   
   In the context of waste projects generally, this contrast is critical. We have worked on a number of waste projects using aerobic (and anaerobic) digestion technologies that have used contaminated feedstock, with resulting contamination of compost that cannot be marketed and must be disposed of to landfill; or anaerobic digestion projects that have not achieved consistent use of feedstock, with resulting congestion and negative diversion rates (on a mass basis), because the processed and treated feedstock using a wet anaerobic digestion technology is heavier than the feedstock introduced. These dynamics will be the subject of article 6 of this publication.
   
   As a general statement, aerobic digestion takes place at a faster rate, and the capital costs of an aerobic digestion waste project are lower than an anaerobic digestion waste project. Depending on the technology used, as a general statement, the operating costs of some aerobic technologies are higher. We say "as a general statement", because the feedstock used for many anaerobic digestion technologies needs to be well understood and well-regulated operationally, particularly the composition and the characteristics of the biomass used as feedstock: the effectiveness of certain anaerobic technologies is affected by certain parts of the organic fraction. For example, grass (and wood) is a lignin with abrasive characteristics that is unlikely to be suitable as a feedstock for anaerobic digestion (unless its characteristics are overcome by mulching and pulping), and experience indicates that this may have an impact on effective operation and the cost of operation (including higher than anticipated wear affecting equipment and part life cycles, and higher personnel costs). Conversely, aerobic digestion technologies require "structured" feedstock, rather than "sloppy"/wet feedstock.

MSW as a feedstock

In a similar vein, it will be important to assess whether the nutrients in animal litter/manure/slurry-based feedstock may result in operational congestion or increased levels of wear on components or equipment (thereby reducing design life). For example, the mineral struvite can be derived from anaerobic digestion, but in our experience its presence can cause operational problems within the piping systems of some anaerobic digestion technologies. In addition, the use of MSW without any separation at source or other pre-sorting is unlikely to provide an appropriate feedstock. Separation at source or pre-sorting of the organic fraction in MSW may provide a feedstock appropriate for an anaerobic digestion technology. There are examples of separation at source of the organic fraction (typically, the Food Waste fraction) working effectively, as is the case in the United Kingdom and other European countries. MSW is not suitable as a feedstock for aerobic digestion.

Given that the organic fraction of MSW can be a relatively high percentage of its mass, effective separation at source is a key part of any waste collection system seeking to deliver the Food Waste fraction of MSW from households to waste projects. In the United Kingdom, while councils and municipalities continue to develop anaerobic digestion facilities for source-separated Food Waste, this is losing momentum because the collection costs and levels of contamination mean that, overall, operational costs are such that, in the circumstances, other technologies offer better value for money. Because there are no hard Food Waste recycling targets in the United Kingdom (although there are overall MSW recycling targets), it is often cheaper to send Food Waste retained within (rather than separated at source from within) MSW to WtE plants. This is informing an industry view held by some players in the United Kingdom waste sector that the calorific value of MSW will drop over the next ten years as more Food Waste is retained within MSW, and as the plastics fraction within MSW reduces.

Quantities of feedstock required to produce heat and energy

As a general statement, the following quantities of feedstock are required to produce the following heat and energy:

As a general statement, the calorific/gross heating value of natural gas (comprising predominantly methane or entirely methane) is in the range of 38.5 to 40 MJ/Kg, and will produce approximately 11 kWh of kinetic energy (power).

In comparison, the heating value of biogas (comprising methane and carbon dioxide, and as such heavier than natural gas) will produce approximately 5 kWh (50 per cent methane) to 9.67 kWh (97 per cent methane) depending on the proportion of methane and carbon dioxide.

As noted in the first column above, it is appropriate to assume that somewhere between 60 and 70 per cent of biogas produced from anaerobic digestion of the various feedstock will comprise methane, with the balance comprising carbon dioxide. In practice, the percentage of methane may be less, perhaps as low as 50 per cent.

^ The higher the methane content of the biogas, the greater the energy that can be derived from it, likewise the higher the methane content the higher the calorific/heating value of the feedstock.

Pure methane has a calorific/higher heating value of approximately 55MJ/kg.

Regulatory context

While this article focuses on practical commercial and legal matters (including in the context of any policy to encourage diversion from landfill, reuse of renewable resources, and the production of renewable energy) from a regulatory perspective, it is important to understand the nature of the feedstock that may be delivered to and used at any waste project, and the approvals required (and the likely terms of those approvals) in respect of each type of feedstock delivered. In jurisdictions with developed waste collection systems and industries, it is likely that specified kinds of waste may be processed and treated using particular technologies. For example, in the context of aerobic digestion for composting, green waste and organic waste are likely to be defined as feedstock that may be used, while food waste, general waste and regulated waste are likely to be defined as waste that may not be used. In contrast, in the context of anaerobic technologies Food Waste, green waste, organic waste and regulated waste are likely to be defined as waste that may be used for anaerobic digestion. In other jurisdictions, household waste or MSW is defined for the purposes of laws promoting the use of advanced thermal technologies.

In the context of any procurement, it is important to recognise the likely approvals required, and the time it is likely to take to obtain them, to orientate the private sector (and in some jurisdictions the public sector) in this way. In addition, considering the regulatory frame of reference at the procurement stage is critical in assessing the broader environmental and public health benefits which differing waste projects may achieve, and in this context compliance with obligations imposed on councils or municipalities. Furthermore, in some jurisdictions, there is often a strategic dimension to seeking approvals, and this is best assessed as part of the procurement stage (if not before).

In the context of a waste project using an anaerobic digestion technology to produce biogas to be burned to produce heat or energy, or both, or with further processing, biofuel, it is important to understand the extent to which the viability of the waste project is dependent on being able to realise revenue from the sale of renewable energy or biofuel as a direct or indirect result of policies, being policies that may be subject to change. In addition, in the context of the use of any compost derived from mixed organic waste, biogas produced from animal litter/manure/slurry or Food Waste (critically animal by-products or meat), it is likely that there will be particular regulatory oversight and requirements, including in respect of use of the residual sludge.

Interface between the Four Cs and feedstock: feedstock is fundamental to the design, life and health of any waste project²⁰

Feedstock is key for any waste project

Understanding the available feedstock is the starting point for the procurement of any waste project. The type of feedstock determines the technology used and the design of that technology, and the composition and characteristics of the feedstock determine the outputs from the waste project (products and residuals) and, as such, the economic sustainability of the waste project.

Feedstock is even more important for aerobic and anaerobic digestion waste projects

• In the context of any procurement of, or the provision of services from, a waste project using an aerobic or anaerobic digestion technology, understanding the composition of the organic matter available as feedstock (and the associated characteristics of that organic matter) is key, in many ways more so than for other waste projects using other technologies.
  
  Also key in assessing the outputs (products and residuals) and, as such, costs and revenue, is understanding the mass of the organic waste arising and available within the applicable area (or areas) or the quantity of organic matter which the procuring council and municipality (or other procuring entity) is/are prepared to commit to delivering, if any. Understanding these matters will allow the council or municipality to make a decision on whether to contract on a waste-arising basis (as is increasingly the case) or on a quantity basis.
• The Composition of the organic matter in the waste stream is critical to:
  
  (i) the Compatibility of each technology being considered to process and treat the organic matter available as feedstock for that technology: Is the composition of the organic material available as feedstock capable of being processed and treated by that technology? Are there any restrictions on the ability of, or impacts on the efficiency of, that technology to process and to treat the contemplated organic material?
  
  (ii) the Contamination levels within the organic matter available as feedstock for each technology being considered: is the anticipated contamination of the organic matter available as feedstock such that it will affect the efficiency of the technology, and, if so, how, and what can be done and at whose cost to avoid or to remove contamination, or to mitigate the effect of the contamination? In addition, is the contamination such that it may affect the ability to market any product produced?
  

Providing information about the Composition of the available organic matter as feedstock, and the required Capacity for each waste project, provides each proponent with a clear basis on which to respond to the procurement process being undertaken. In addition, requiring each proponent to provide details as to the requirements of, and the sensitivities of, the technology in respect of composition and contamination will improve the ability of the procurement team to make an informed decision as to the most appropriate technology, including the risk of choosing one technology over another.

In addition, if the procurement process being undertaken is to procure services, providing details of the likely waste arising over the period of time during which the services are to be provided (Term) and other sources of organic matter that may comprise compatible feedstock (within the applicable area or areas) will enable the proponent to provide pricing reflecting the size and scale of the waste project that may be developed, the capacity of that waste project, and the use to which the capacity may be put during the Term to maximise the use of that project and to minimise the cost of the service to the applicable council or municipality (or other procuring entity). From the point of view of the council or municipality (or other entity) procuring the provision of services from a waste project, it is for the service provider to take the risk in the composition of the feedstock, and to manage that risk. This is particularly the case in respect of aerobic and anaerobic digestion waste projects.

The importance of information

• Avoiding the mistakes of others: There are examples of waste projects in which councils and municipalities assumed a technology could process and treat certain waste streams without any need for consistency of feedstock or separation at source of the required organic fraction from the waste stream or the pre-processing or pre-sorting of feedstock, rather than requiring the procurement process to "prove up" and to demonstrate that the assumption was indeed correct. The subsequent issues presented by each of these examples could have been avoided by more rigorous procurement processes, including the provision of information by proponents demonstrating the ability of the waste project to source, accept, process and treat feedstock to achieve the outcomes sought from the procurement.
  
  Taken together, this information (and other associated information) will provide a clear basis for making a fully-informed procurement decision as to the technology, including any mitigation that may be required.
• What information needs to be available and understood? In the context of waste projects generally, feedstock is fundamental. However, in the context of aerobic and anaerobic waste project facilities it is the most important factor of all. It is difficult to overstate the importance of understanding:
  
  (i) the impact of the means of the collection of feedstock, including the effectiveness of any separation at source;
  
  (ii) the compositional range of organic matter that each waste project facility is able to process and to treat;
  
  (iii) the composition of organic matter available for use as feedstock in the applicable area or areas from which the organic matter will be sourced; and
  
  (iv) in respect of the material which the facility is able to process and to treat (as determined by paragraphs (ii) and (iii)), the possible circumstances that may affect its composition and, as a result, may impact on the performance of the waste project facility, and the consequences for the council or municipality and the waste project itself of differing levels of performance.
• Biogas from anaerobic digestion: While the proposition is that all organic matter can be processed and treated by anaerobic digestion, this proposition needs to be treated with the utmost caution. The proposition is not wrong of itself, but experience shows that efficient, and indeed optimal, operation of anaerobic facilities is achieved using feedstock that is consistent (e.g. animal litter/manure and sewage, macerated fruit and vegetables or other Food Waste) in its composition (and its characteristics). A number of examples exist of waste projects that have not recognised this. It is important to be aware that, if certain organic matter is to be processed and treated, it will require pre-treatment (including mulching and pulping) to avoid abrasion and congestion of the vessels used as anaerobic digesters.
  
  On average, depending on the circumstances and the technology used, biogas contains:
  
  — 50-80 per cent methane (CH4);
  — 20-50 per cent carbon dioxide (CO2); and
  — traces of other gases, including hydrogen sulphide (H2S, which is toxic) and nitrous oxide (which is a fundamental measure of air quality).
• Methane from biogas: Methane gas (CH4) has high energy potential, and, as such, can be used to produce energy. Equally important is the fact that CH4 has 21 times the global warming potential of carbon dioxide (CO2). In other words, one tonne of CH4 emitted to the atmosphere causes 21 times as much impact as one tonne of CO2 emitted to the atmosphere: on a per tonne basis, CH4 contributes 21 times as much to climate change as CO2. Rather than allowing CH4 emitted from the natural putrefaction process of organic matter in landfill to escape into the atmosphere, from an environmental and public health perspective it makes sense to capture CH4 in a controlled environment and to burn it. Combustion of CH4 transforms it into heat (and CO2). In burning CH4, councils and municipalities (and other procuring entities) may be said to be harvesting the energy potential of waste and, in so doing, reducing emissions to the atmosphere, and lessening the impact of waste (produced by human activity) on climate change.
• Feedstock and Residue: The feedstock for aerobic and anaerobic digestion is also known as biomass. While biomass may be regarded as an all-encompassing term, which includes organic matter derived from farming (including from harvesting) and gardening, food production and processing, and animal litter/manure/slurry and sewage, different types of organic matter have different composition and characteristics, and different technologies are suited (or better suited) to certain organic matter, while others are not. For regulatory (and likely social licence) reasons, some biomass cannot be used as feedstock for some technologies.
  
  In the United Kingdom and other European Union countries, some waste project developers specialise in Food Waste (sourcing from councils and municipalities, shopping centres, supermarkets and educational and correctional institutions) while others have chosen to specialise in the agricultural waste sector. As outlined below, Food Waste offers great potential as feedstock in jurisdictions generating Food Waste capable of being processed and treated, and may be regarded as a feedstock that remains the most prospective across many jurisdictions.
• Aerobic digestion: If an effective aerobic technology is used, the biomass, which the waste project facility is licensed to use as feedstock, will be pasteurised and stabilised to produce compost (containing carbon and nitrogen,²¹  and phosphates and sulphates).²²  Composting has value, both in itself and in the products derived from it. Composting reduces the mass and volume of the biomass processed and treated by 50 per cent and 80 per cent respectively, thereby avoiding use of airspace/void space at landfill. The compost produced has value in the nutrients contained within it (nitrogen, phosphorous, potassium and calcium), and as humus (given its humic fraction)²³  for use in agriculture or horticulture or organic matter for use in rehabilitation. However, there is a cost associated with the logistics of getting compost to market or to its point of use and in its use itself (including the cost of spreading large volumes of compost compared to relatively smaller volumes of its market substitute, fertiliser).
  
  In addition, the value of compost varies depending on the final uses to which it can be put, which in turn may be dependent on a policy. Aerobic digestion results in the production of carbon dioxide (CO2) and water. The chemical characteristics of the organic matter at the start of the aerobic digestion will tend to be the same as at the end (including nitrates, phosphates and sulphates), less CO2 and water. Also it is likely there will be contamination and residue arising from the processing and treatment of organic matter including, depending on the source of the feedstock, a form of run-off that will need to be contained and possibly treated.
  
  If a council or municipality (or other procuring entity) contracts for the processing and treatment of biomass supplied by it, its expectation will be that the waste project²⁴  will take the risk of the level of contamination in the biomass delivered (possibly not completely, but certainly to a specified percentage level) and that the waste project will take the risk in the market (critically, the market price) for compost produced and the cost of getting it to market, and the cost of diverting any organic material produced by the waste project that is not compost that can be sold in a market.
  
  Aerobic digestion facilities can be stand-alone facilities (for example, open window composting, but this takes up space, and may result (or some may say, always results) in odour issues) or form part of Mechanical Treatment (MT) and Mechanical and Biological Treatment (MBT) waste facilities (these facilities often have aerobic maturation halls at the back-end)²⁵
• Anaerobic digestion: Assuming an effective anaerobic technology is used, biogas²⁶  and bio-solids²⁷  (if the feedstock is animal litter/manure/slurry or sewage) and sludge (if another feedstock is used) will be produced. While the biogas produced by an anaerobic digestion facility may be used for cooking and for heating without further processing, the removal of carbon dioxide (CO2)²⁸  and hydrogen sulphide (H2S) (and water and water vapour) from the biogas allows that gas to be marketed as natural gas,²⁹  or what is sometimes referred to as bio-methane. In addition, the removal of H2S avoids corrosion of boilers and engines and ancillary systems³⁰  and will enable the use of biogas for heating and as biofuel (including using gas-fired engines, including Combined Heat and Power Plant engines). The cost of removal, and the effectiveness of the means of removal (usually scrubbing to make a liquid chemical residue for disposal or to make sulphur cake),³¹  will need to be demonstrated. Given that CO2, H2S and water vapour can be (and H2S and water will be) removed from biogas, procuring councils and municipalities (and other procuring entities) will want to understand the cost of removal and the revenue benefit of doing so, as will the waste project. If it is necessary to remove CO2 in order to market the biogas or to market biofuel, waste projects will have to demonstrate that this can be done, and the cost and effectiveness of doing so.³²
  
  In order to determine the quantity of biogas that will be produced from a particular feedstock (and, therefore, the likely revenue from the sale of biogas or biofuel, or savings from the use of biogas or biofuel), the composition and characteristics of that feedstock will need to be understood, and the biochemical methane potential (BMP) of the biogas determined. The enhanced processing of biogas is likely to increase its value, and in turn the revenue that the waste project is able to earn, if it performs as contemplated. The net income position of the waste project will depend on realising greater revenue modelled on the basis of the increased capital costs of the enhanced processing of biogas.
  
  While anaerobic digestion has been used to process animal litter/manure/slurry and sewage, anaerobic digestion technology is also used to process and treat other feedstock, including in some cases the organic fraction separated from MSW and green waste. In the United Kingdom, the use of anaerobic digestion may be split into three broad, but distinct, sub-sectors depending on the feedstock: (i) sewage anaerobic digestion undertaken by water companies; (ii) Food Waste anaerobic digestion undertaken by councils and municipalities; and (iii) agricultural, diary and abattoir anaerobic digestion undertaken by merchant waste projects.³³  In the context of some jurisdictions, it is possible that councils and municipalities³⁴ may take a role in each of these sub-sectors, possibly working with the private sector. 
• Contamination of feedstock: Depending on the nature of the feedstock and the technology, it is likely that contamination will be less critical for the purposes of processing and treating organic matter in an anaerobic environment, except for the risk of chemical contamination which might kill the micro-organisms³⁵ present in the anaerobic environment to digest the organic matter to produce biogas, ie to produce CH4 (and CO2). In addition to managing contamination of this kind, the waste project will be concerned to ensure that the composition (and characteristics) of the organic matter used as feedstock are compatible with the anaerobic technology, both to optimise biogas production and to minimise operational and capital cost issues that may arise from feedstock with certain chemical and physical characteristics.
  
  If a council or municipality (or other procuring entity) contracts for the processing and treatment of biomass supplied by it, its expectation will be that the waste project will take the risk in the level of contamination in the biomass delivered, and the project will take the risk in the market for biogas produced, and the cost of diverting any bio-solids or sludge³⁶ produced by the waste project. Effectively, the council or municipality (or other procuring entity) will not want to take performance risk in the waste project; rather it will want to make a procurement decision based on demonstrated performance in the circumstances in which the waste project is going to operate (including in its proposed location, recognising that ambient temperature in a location can influence the decisions taken).
  
  However, the more certainty a council or municipality is able to provide, and the more risk it is able to take in respect of feedstock specification or biogas and biofuel specification, or both, the lower the overall costs of the waste project for the council or the municipality (or other procuring entity).
• Co-digestion projects: Given that anaerobic digestion is a proven technology for biogas production (as demonstrated by existing facilities and infrastructure), there appears to be an increasing interest in the use of existing facilities to maximise capacity of those facilities and that infrastructure so as to increase biogas production through increased throughput of feedstock and through the use of what has been termed anaerobic co-digestion, being the use of different types of feedstock.
  
  The effective use of anaerobic co-digestion offers councils and municipalities with existing facilities the ability to increase biogas production by using existing facilities and infrastructure, and in so doing provide for the more effective processing and treatment of feedstock which might otherwise have to be disposed of to landfill (or possibly by other means). In addition, anaerobic co-digestion offers councils and municipalities (and other procuring entities) the opportunity to consider technologies that are able to co-digest and, as such, achieve greater size and scale than a technology that is not able to co-digest. In this context, in addition to the issue of consistency of the characteristics of the feedstock, the complementarity and compatibility of different feedstock needs to be understood in any procurement process.
  
  The use of animal litter/manure/slurry and other organic material may be regarded as offering reasonably prospective opportunities for co-digestion. If a council or municipality (or other procuring entity) contracts for provision services from a co-digestion project, it should expect that the waste project will take risk in each feedstock and the performance of the project, including the production of biogas and sludge or fertiliser (or if further processing is undertaken, compost).³⁷
• Key parameters: In the context of co-digestion projects it is particularly important to understand the parameters for each technology. The key parameters for effective aerobic composting include the porosity of the organic matter and as such the aeration and oxygenation (the level of oxygen),³⁸ the moisture level,³⁹ the relative proportion of carbon to nitrogen (C:N) in the feedstock, the pH value of the feedstock and the temperature. From this it is clear that aerobic digestion requires feedstock with "structure" (ie physical structure).
  
  Most aerobic and anaerobic technologies require consistent parameters, critically the heat, moisture and carbon to nitrogen (C:N) ratios,⁴⁰ with variations affecting the digestion process. The key parameters for aerobic digestion vary depending on the phase of composting.⁴¹ There are certain materials that are not suitable for composting, including biochar or coal ash, fish, meat, cooked food waste, roots and seeds, nappies, and most Recyclables (including glass, metals and plastics), although some papers may be used. Therefore, if MSW is being contemplated as a feedstock it is important to recognise the need for effective separation at source or other pre-sort elsewhere.
  
  The key parameters for effective anaerobic digestion will very much depend on the feedstock available and the technology chosen, but as a general statement the total solid content, temperature and retention times (tied to whether the anaerobic digestion takes place in mesophilic or thermophilic conditions) and pH values (which values will vary depending on the stage of digestion). Critically, anaerobic digestion involves a micro-biotic process by which organic material is converted into methane in an oxygen-free (ie anaerobic) digester.⁴² To convert organic material into methane (CH4) takes time. The bacteria must have contact with the organic material being digested (digestate): the better the contact, the greater and more effective the digestion. Because the digestate comprises bacteria, contamination of the organic matter in the feedstock or too high a pH value can affect the operation of the digester. The operating parameters of each anaerobic digestion technology are different, but consistency of feedstock (quality and quantity) is critical to the consistent production of biogas: to achieve consistency and to achieve optimal carbon to nitrogen (C:N) ratios⁴³ mixing of feedstock is often undertaken.
  
  Finally, the operating parameters of each anaerobic digestion technology will need to be understood in terms of the speed at which feedstock can be fed into the digester. Overloading a digester affects costs and revenue and can give rise to liability under contracts into which the waste project enters. Organic loading rate (OLR) is expressed in Chemical Oxygen Demand (COD)⁴⁴ or Volatile Solids (VS)⁴⁵ are means of measuring OLR.

Jurisdiction-by-jurisdiction

The powers and duties of councils and municipalities in particular jurisdictions varies: in some jurisdictions the powers and duties of councils and municipalities may be limited to the collection and disposal of waste, in others the powers and duties may extend to roads, water, wastewater (including sewage) and waste. In addition, policy makers and councils and municipalities in some jurisdictions have the advantage of having seen what has worked in other parts of their counties and other jurisdictions around the world.

The introduction of landfill levies/waste disposal levies may send pricing signals to councils/municipalities or waste producers required to pay them. Likewise, phasing out or scarcity of landfill. The introduction of container deposit/refund schemes is likely to reduce the volume of certain recyclable materials because it encourages effective pre-sort of those recyclables which are covered by the scheme.

The implications of continuing as is...

As noted above, in the absence of waste projects, both the organic and inorganic fractions in the waste stream will go to landfill. If organic matter goes to landfill, LFG is released from landfill as the organic matter, as putrescible waste,⁴⁶ decomposes over time.

As a general statement, if organic matter is left to decompose over time, the LFG released will typically comprise 50 per cent methane (CH4) and 50 per cent carbon dioxide (CO2) (with traces of nitrogen, oxygen, hydrogen and carbon monoxide (CO)), and organics such as ammonia (NH3) and sulphides. Some organics released are harmful to both the environment and to public health (and some are odorous and toxic, some both).

LFG may be captured, and flared, or captured and fired, to produce energy. The capture and firing of LFG is regarded as a better solution, and more environmentally friendly, than slow release given the global warming potential of methane (CH4). While capture is better than non-capture, avoiding production of LFG is a better outcome still because it captures a greater quantity of CH4 and does so in an environment that avoids (or minimises) the odour associated with the slow release of LFG. To achieve this, putrescible waste is diverted from landfill to a waste project.

Achieving increased diversion from landfill is not just a council and municipality issue, however. It is a broader policy issue in which all putrescible waste should be viewed as waste to be avoided. If however waste is not avoided, the waste that arises should be regarded as potential feedstock for waste projects and aerobic and anaerobic digestion technology solutions.

Also, in some jurisdictions and in some council and municipal areas, the use of land for landfill may have lost, or be in the process of losing, its social license, or be preventing the use of surrounding land for higher and better uses. This is often the case in jurisdictions and council and municipality areas with increasing populations and urbanisation.

Increased diversion of organic matter as putrescible waste from landfill

Balancing ideals and practicalities

As noted above, to increase the diversion of putrescible waste from landfill there needs to be a reduction in organic matter going to landfill. For this to be achieved, policy needs to encourage avoidance of organic matter waste. To the extent that organic matter waste is not avoided, and waste arises, policy needs to allow (and, in some cases, to facilitate) the development of waste projects to process and to treat such organic matter waste to achieve improved environmental and public health outcomes.

As we have noted in other articles in the Waste-to-Wealth Initiatives series, waste projects comprise WtE (mass burn; gasification; pyrolysis and plasma, and anaerobic digestion to derive biogas); FOGOs47 (the technology used depending on the food organic content); ORFs48 (tending to use aerobic digestion); MBTs (some using aerobic and anaerobic digestion), MTs and anaerobic digestion.

Given the wide range of waste projects, the Waste Management Hierarchy can be used as a frame of reference. However, it needs to be seen as a frame of reference for all in the community, not just councils and municipalities. In this context, having collected data to understand the feedstock, it is necessary for all stakeholders to be pragmatic, balancing the ideals with that which is achievable practically.

Affordability and value for money are fundamental to achievability, including by reference to other technologies and the outcomes that they can achieve. For example, FOGO projects represent an ideal outcome, but because their effectiveness depends on the composition of the waste that is placed in collection bins going to FOGO waste projects, they often work most effectively in communities receptive to education, and locations that do not experience higher temperatures.

FOGO projects may be said to go beyond the need for social license; rather they require a social contract with the community such that food and garden organics in the waste stream are separated at source effectively by the community before they are delivered to the FOGO project. This social contract has been achieved in smaller communities and, over time, in larger ones.

Leaving to one side the fundamental concepts of affordability and value for money, if councils and municipalities look to develop waste projects by reference to the Waste Management Hierarchy, the outcome sought is to maximise reuse. The key practical overlay to the application of the Waste Management Hierarchy is the level of contamination in the waste stream, and the ability to avoid or to mitigate the levels of contamination, before delivery to the waste project or, alternatively, leaving this for the waste project to manage. There is no "one-size-fits-all" approach to the management of feedstock separation and avoidance or management of contamination.

Understanding the feedstock available (composition and mass) and its source, and the cost of separating feedstock at source or at transfer stations, will inform decision-making, as will the level of contamination avoidance and management that needs to take place before the feedstock is delivered to the waste project.

If feedstock with manageable levels of contamination is delivered to waste projects using aerobic and anaerobic digestion technologies, this will allow reuse of organic matter through the production of compost and the production of biogas and with further processing of residue compost, in each case reducing putrescible waste going to landfill.

In the context of achieving higher levels of diversion from landfill and higher levels of reuse using aerobic and anaerobic digestion technologies, there are at present a number of waste streams that are not being accessed systematically to maximise diversion from landfill, and to maximise biogas production. In addition, as noted above, there are opportunities to use feedstock and technologies to maximise levels of biogas production. By achieving maximisation of diversion from landfill, councils and municipalities with landfills achieve longer lifecycle opportunities for those landfills, and possibly lower unit costs by utilising existing assets and infrastructure, and net income benefits from the production of biogas, through processing to produce natural gas or biofuels.

In addition, cooperation between councils and municipalities (and other procuring entities) may allow greater diversion from landfill and greater reuse by virtue of scaling up waste projects to process and treat organic matter and, in other instances, inorganic matter, which would otherwise go to landfill. Cooperation between councils and municipalities (and other procuring entities) offers a real opportunity to extend concepts of achievability by extending the concepts of affordability and, as such, value for money by scaling up waste projects to take more waste and to take a greater variety of waste streams.

Accessing feedstock

1. Other sources of feedstock: The Food Cycle comprises production, harvesting, processing, sale and consumption of food. At each stage of the Food Cycle, waste is produced. It is important to understand the Food Cycle, and each point in the Food Cycle at which waste arises (Food Waste). The greatest quantity of waste arises in the first four phases (production to sale) of the Food Cycle. Depending on the area of the world (and areas of each country), consumers are responsible for between 1.25 per cent and 38 per cent of the Food Waste from the Food Cycle.
   
   For the balance of this article, we focus on Food Waste, but in a future article we will analyse other possible feedstock for each waste project technology, including organic matter that may be processed and treated using aerobic and anaerobic digestion technologies, such as animal by-products (including carcasses and intestines) and biomass from harvesting of crops (including arable crops, such as maize and wheat).
   
   Consistent with the Waste Management Hierarchy, food waste is to be avoided (either not produced or consumed, either by humans or animals). If it is not possible to avoid food waste, the Waste Management Hierarchy contemplates the reuse of Food Waste. The use of Food Waste to produce heat or energy, or both, and possibly compost represents reuse. If councils and municipalities and other procuring entities work together to use technologies (and existing facilities in some instances) to process and to treat Food Waste with other feedstock, benefits of scale will be realisable if there is sufficient Food Waste available as feedstock.
   
   Certain organic matter in Food Waste provides better feedstock than other organic matter. For example, pre-processed fruit solid waste and vegetable solid waste provide optimal feedstock for anaerobic digestion. Post-processed, but pre-consumed, fruit solid waste and vegetable solid waste is less suitable because of possible packaging contamination, but may still be capable of use if contamination is managed effectively. Post-consumed Food Waste is less suitable still because of possible contamination with paper, metal cans, glass, plastics, containers and utensils, but does not mean that post-consumed food cannot be used as feedstock. It is fair to say that, if the composition and characteristics of the Food Waste are known, waste projects should be able to configure designs to allow processing and treatment. For example, the United Kingdom has anaerobic digestion facilities that accept and treat out-of-date packaged Food Waste comprising food and dairy products, with all packaging stripped mechanically before the Food Waste is treated anaerobically.
   
   Organic matter in Food Waste derived from source-separated organics (green bins or other source-separated organics (SSO)) is likely to be contaminated to some extent. The issue is to what extent and whether the contamination can be removed and, if so, the likely cost of doing so. In the absence of an effective separation at source system, organic matter in Food Waste that is derived from MSW will have levels of contamination that make it unsuitable as feedstock for anaerobic digestion. To derive the organic fraction from MSW requires a pre-sort and screening of the feedstock on delivery.
   
   For these purposes, the origin of Food Waste needs to be understood both in terms of consistency of composition and consistency of mass arising over time. As stated above, waste arises from production through to point-of-sale (essentially undertaken at a commercial level) and after the point-of-sale (essentially at a domestic production or domestic purchasing level). From a policy perspective and a commercial perspective, it is easier to send policy signals to (and to effect policy enforcement in respect of) producers, processors and retailers than it is to consumers. Effective separation at source of various waste streams, critically the organic fraction (and possibly Food Waste from garden waste green bins) from the inorganic fraction, and the recyclable fraction from each other fraction, means that the fractions separated as SSOs may be processed and treated so as to divert waste from landfill (or other Waste Management Hierarchy outcomes).
   
   In terms of the sources of Food Waste, hospitals, prisons, schools and universities, cafes, shopping centres, malls and supermarkets, food centres and restaurants, and food production facilities, are all likely to generate highly putrescible feedstock with a high BMP.
2. The importance of information, in particular about MSW: Given that councils and municipalities have a duty (and the power and revenue-raising ability) to collect MSW, and to dispose of it (for well-established and well-understood environmental and public health reasons) and given the likely level of the organic fraction within it, including Food Waste, thought and resources need to be applied to determine how to separate the fractions within MSW at source or elsewhere.
   
   In addition, the organic fraction (and Food Waste in particular) makes up a significant proportion of the mass of MSW arising within each council and municipal area, and, as such, its diversion from landfill will be of benefit. The issue for councils and municipalities is to assess the MSW arising within their areas and its composition, and in so doing to assess, realistically, how much of the organic fraction in that MSW may be separated at source, separated at a transfer station or separated at the ultimate processing and treatment destination, and at what cost. As noted above, in the United Kingdom, some councils and municipalities have found that the operational cost of separation at source and the levels of contamination of MSW have resulted in leaving the Food Waste organic fraction within MSW.
   
   Having this information available allows councils and municipalities to make decisions about the means by which MSW may be processed and treated, assuming for present purposes separation at source or elsewhere. In this context, it is important to understand the waste arising that may be collected and consolidated to provide further feedstock for each use identified, and that policy makers consider whether to incentivise or to dis-incentivise those generating waste to use one technology or not to use another technology so as to achieve the scale to support the preferred (and ideally optimal) technological outcome. In addition, while a wet anaerobic digestion technology may not be suited to the processing and treatment of MSW, there are however smaller scale dry technology plants that may be used.

Conclusion

As noted in article 4 of this publication: Have we reached a tipping point?, the changes introduced by the People's Republic of China at the end of 2017 have jolted the waste industry around the world. This jolt has prompted policy makers, councils and municipalities, producers of waste and the waste industry more generally to consider policy afresh, including funding and implementation decisions for waste avoidance and waste processing and treatment. In some jurisdictions this is an opportunity: given the maturity of industry participants who have developed and applied technologies around the world, and mature bases for decision-making, jurisdictions have a broad range of technologies to assess and to choose from. In jurisdictions with well-developed waste collection systems, there is an opportunity to develop waste processing and treatment facilities in a coordinated way, each project or coordinated collection of projects maximising the objectives of the Waste Management Hierarchy in a way that recognises local conditions and practicalities, with only waste that cannot be processed by a waste project at a higher level in the Hierarchy permitted to be processed by a project at a lower level in the Hierarchy.

With special thanks to Francesca Arciuli, Graduate, for her contribution to this article.

Previously published in Infraread, Issue 13: June 2019

1. As a general statement, reuse includes use or reuse of any material in the waste stream including as a result of processing or treatment, and as such the production and use of compost amounts to reuse from aerobic digestion, as does the production and use of fertiliser (most prevalently phosphorus (P) is recovered and reused, but also nitrogen (N) and potassium (K)) or compost from further processing to pasteurise the organic material in the sludge (which will contain NPK too). Anaerobic digestion is also able to produce reusable material from animal slaughter (and associated animal by-product production), principally stomach contents, bio-degradable organs and tissue, and waste water treatment, and food preparation, processing and treatment (most prevalently from waste water arising from preparation, processing and treatment).

2. As a general statement, recyclables include aluminium, glass, high density polyethylene (HDPE), liquid paper board (LBP), mixed plastics, polyethylene terephthalate (PET) and steel, and often paper and cardboard. Even if source separation is used, it is likely that material derived from MSW as feedstock for aerobic or anaerobic digestion will be contaminated with recyclables and other non-digestible materials.

3. Aerobic digestion technologies do not allow for the collection of biogas.

4. In many jurisdictions, the production of renewable energy entitles the owner of the waste project to some form of green certificate or right.

5. Green Waste is organic material from domestic "green" bins, which typically includes Garden Waste from domestic gardening activities and Green and Botanical Waste from activities of councils and municipalities, typically maintaining parks and gardens and lopping and topping of trees. Green Waste may be used as feedstock for FOGOs (Food Organics and Garden Organics Facilities) and ORFs (Organic Recovery Facilities), including those using aerobic digestion technologies. If to be used as feedstock for anaerobic digestion or in the context of co-digestion, the Green Waste is best mulched before processing and treatment.

6. The capture of biogas from animal litter/manure/slurry uses well-established anaerobic digestion technology. It is possible further to process the sludge derived after digestion.

7. Animal slaughter, processing and preparation produces animal by-products and waste, with the waste water produces a feedstock for anaerobic digestion to produce biogas.

8. Food Waste includes organic material derived agricultural and farming production (including all forms of agricultural and farming activities, including dairy, cereal crops, fruit, vegetables, etc), food processing and preparation, and sale and consumption.

9. MSW is municipal solid waste (as distinct from sewage or waste water).

10. Sugar cane production produces biomass (called bagasse) which is and has been used as feedstock for heat and energy facilities around the world for many years using thermal technologies, but which may be used as feedstock for aerobic digestion composting. For decades sugar cane growing regions (for example Queensland) have used bagasse and cane trash to satisfy heat and energy requirements. While beyond the scope of this article, it is estimated that roughly 50 per cent of all bagasse and cane trash is collected. Bagasse and cane trash are capable of being used to produce ethanol.

11. Rice husk/straw waste can be used in anaerobic digestion, including co-digestion. Anaerobic digestion technologies in rice growing regions of the world are increasingly being considered.

12. Lignin is an organic material that is non-digestible through anaerobic digestion.

13. Agriculture/Farming and food production generally produce methane – in some crops more than others, and in some livestock more than others.

14. There are many examples of councils and municipalities procuring waste projects collectively, with well understood and "tried-and-tested" governance structures and compliance procedures (including any necessary competition clearances or exemptions). Equally important is ensuring that private sector equity investors and debt providers understand the credit risk of contracting with councils and municipalities without the provision of state guarantees.

15. Food Waste extracted from households or businesses may include waste from the preparation of food, leftovers or food that has passed its best-by-date, sell-by-date or use-by-date.

16. For example, in the case of a waste project producing a product that can be used as a fertiliser (rather than as compost) the ability of farms to take the product will be key, as will the proximity of a sufficient number of farms to take the product (including from a regulatory perspective).

17. The production of compost requires taking the organic fraction (or more accurately part of the organic fraction) from the waste stream and processing to it produce organic matter in the (more) stable form of compost. The compost is derived from the process breaking down at a slower rate than the organic matter in the waste stream from which it is derived, and which may otherwise have been disposed of to landfill.

18. The production and loss of water vapour reduces the mass of the organic matter, thereby diverting it from landfill through natural processes.

19. Anaerobic, thermal and thermochemical technologies being technologies that can produce renewable energy.

20. The Four Cs are four key risks that must be considered in developing a waste project and which project sponsors will be concerned with, being: Compatibility, Contamination, Composition (and Characteristics) and Capacity.

21. Composting converts nitrogen in its unstable form as ammonia into a stable organic form: chemically, nitrogen in compost is stable and as such is released slowly.

22. Composting is undertaken in an aerobic environment in which the presence of oxygen (through aeration) allows microbial processes (involving actinomycetes, bacteria and fungi) to break down the less complex organic matter (including fats, proteins, starch, and sugar) and, at higher temperatures, to kill pathogens, leaving organic matter that will decompose in the environment in which it is located over time. From a technology perspective, it is important that the microbial stability is understood, given that it is fundamental to composting.

23. Humus is the principal organic matter component of soil, constituting approximately 65 to 75 per cent of the total organic matter in soil. Humus has the key role in the fertility of soils. As such the humic fraction of compost has value for both horticultural and agricultural use, and in some jurisdictions for restoration of native vegetation and forestry plantations, and road verges and central reservations on roads and highways, particularly for councils and municipalities with responsibility for roads and highways.

24. A waste project being the entity owning and operating the waste facility at which the organic matter is to be processed and treated.

25. In the United Kingdom aerobic digestion remains incomplete, but is less prevalent, and in some MT and MBT waste projects it is regarded as no longer economically sustainable, in particular because WtE plants provide a cheaper option.

26. By way of background, biogas is a term that is used to refer to gas that is produced by the biological breakdown of organic (bio-genic) matter in an oxygen-free environment. Biogas is produced from the anaerobic digestion of organic matter. It is derived from biomass, animal litter/manure/slurry and sewage/wastewater for the most part. Biogas comprises primarily methane (CH4) and carbon dioxide (CO2) (likely to be approximately 70 per cent CH4 and 30 per cent CO2). As LFG contains biogas, ammonia and sulphides, including H2S, will be present. Both CH4 and CO2 are greenhouse gases, CH4 contributing around 25 per cent to global warming, and CO2 around 70 per cent, although (and as noted in the body of this article) the global warming potential of CH4 is 21 times that of CO2.

27. Bio-solids and sludge will remain after anaerobic processing and treatment of animal litter/manure/slurry and sewage (and wastewater generally). It is necessary to dispose of bio-solids/sludge, and the cost of this disposal (transport and final disposal) needs to be considered in the choice of any anaerobic technology.
It is possible to process bio-solids/sludge further to produce compost in a controlled environment using thermophilic digestion (typically, at a temperature of around 55 degrees Celsius in an aerobic environment) or vermicomposting (using earthworms in an aerobic environment). As with anaerobic co-digestion (considered below) there appears to be increased interest in combining feedstock comprising bio-solids with other organics (with a higher carbon content) to produce a higher quality compost product within a shorter period of time using a combination of thermophilic digestion and vermicomposting.

28. The removal of CO2 increases the energy context of the biogas because there is more methane. CO2 may be removed in a number of ways, including water scrubbing, polyethylene glycol scrubbing, carbon molecular sieves, and membrane separation.

29. value of biogas and natural gas is a function of the percentage of methane. The percentage of methane is a function of the reservoir or seam from which it was taken in the case of natural gas or the organic material and its characteristics in the case of biogas. The calorific value of natural gas from a reservoir or a seam will be higher than that of biogas derived from waste: this is because the percentage of the natural gas that is methane is rarely less than 85 per cent by volume and is likely to contain heavier hydrocarbons.

30. H2S may be removed in a number of ways, including water scrubbing, NaOH scrubbing, activated carbon (converting H2S into sulphur and water), iron chloride dosing (to create iron sulphide salt), iron oxide (needs water) and air/oxygen dosing of biogas.

31. Sulphur cake is produced from the oxidation of hydrogen sulphide. Sulphur cake can be used in agricultural preparation.

32. In addition to the removal of CO2 and H2S it may be necessary to remove halogenated hydrocarbons to comply with the recommendations of engine manufacturers (typically, using activated carbon, through which CH4 is able to pass, as is N).

33. Merchant waste projects being projects that take risk in the quantity of waste delivered.

34. Given the role of councils and municipalities in respect of road, water, sewage/wastewater and waste.

35. The micro-organisms are able to survive in this anaerobic environment by sourcing oxygen from the organic material itself and from inorganic oxides present in the organic matter.

36. Bio-solids/sludge have to be processed and treated further and will need to be disposed of lawfully, with the key costs of disposal being the actual cost of disposal and the cost of transportation to the ultimate point of disposal.

37. Further processing will require the pasteurisation of the organic matter within the sludge and fixing the organic material or the mixing of the resulting organic material to create an organic material that is humic.

38. Aeration of the digestate is required, whether forced or passive.

39. With aeration the moisture level is critical for effective microbial digestion.

40. In many aerobic processes, wood (and possibly paper) provides carbon content and nitrogen content from Food Waste (and in a closed processing and treatment environment, possibly animal litter/manure/slurry, and less likely sewage).

41. There are four key phases of composting: (i) mesophilic; (ii) thermophilic; (iii) cooling; and (iv) maturing.

42. There are four stages of anaerobic digestion: (i) hydrolysis (involving hydrolytic bacteria); (ii) acidogenesis (or fermentation) (involving acidoengic bacteria); (iii) actogenesis (involving acetogenic bacteria); and (iv) methanogensis (involving methanobacterium and methananosarina): some authors refer to three main stages (hydrolysis, acidogenesis and methanogenesis), but our preference it so refer to four stages. At a high level and in broad terms, the first stage (hydrolysis) breaks down complex organic material (including cellulose) into soluble molecules, including amino and fatty acids and sugar (hydrolysis is of critical importance in digestate with a high organic content), and during this stage chemicals may be used to reduce digestion, the second stage decomposes carbohydrates in the absence of oxygen, converting the acids from hydrolysis into ammonia, and simple organic acids, carbon dioxide (CO2) and hydrogen (mimicking the way in which milk sours if left to sour), the third stage converts the organic acids into acetic (from amino acids), butyric and propionic acids, and ethanol, and the fourth stage converts acetate (through methanosarcina) and hydrogen and CO2 to methane (CH4) (through methanobacterium).

43. Optimal C:N ratios in anaerobic digesters are said to be between 20:1 and 30:1: a high C:N indicates an increased consumption of nitrogen and lower biogas production, whereas a low C:N indicates ammonia accumulation with higher pH values and, as such, the digestate can kill methanogenic bacteria.

44. COD is measured by mass: is a simple measure of water and waste water quality, critically the level of organic content of the water. Anaerobic digestion technologies alone cannot reach the required levels of COD to allow discharge.

45. VS is measured by volume: is a simple measure of volume before and after complete combustion of a cubic metre of feedstock. Essentially, how much of each cubic metre of feedstock is biodegradable (BVS) and how much is not is a refractory VS (RVS). A high VS with a low RVS means that the feedstock is suitable for anaerobic digestion. The level of volatile solid degradation indicates the level of biogas production.

46. Organic matter that decays as a result of putrefaction.

47. Food and Garden Organics facilities.

48. Organic Recovery Facilities.