The Waste Hierarchy was derived from the fact that cheap landfilling and incineration were still the main methods by which Municipal Solid Waste (MSW) was disposed of, at the end of the 1980s [16]. By 1990, there were 186 MSW incinerators in the EU, declining by 52% in 2007 due to the availability of large regional landfills. However, obligations about incineration before landfilling triggered a decline in material recovery from waste (e.g. metals) [17]. Environmental impacts of landfilling were also relevant for that EU waste policy, for instance, emissions of greenhouse gasses as methane. In 1995, landfills in the EU accounted for around 3% of total greenhouse gas emissions [18]. Such a high contribution was unacceptable; therefore, the management of waste required a change, with the aim of reducing greenhouse gas production through improved landfill diversion and consequently, it could be argued that the concern for gas emissions shaped waste management in the EU for the next twenty years. One of the key outcomes in the EU waste policy was the introduction of the landfill directive [19] equipped with strict requirements for landfills to prevent and reduce the adverse effects on the environment. This is also enhanced by the revised waste framework directive, which was established on the principle of minimizing disposal (reduce, reuse and recycle) and defined material recovery targets aiming to encourage recycling strategies.

The pretreatment of waste prior to landfill [20] but avoiding overcapacity of incineration and digesting, has been embedded in the EU directives, for instance the Landfill Directive, 1999 which limits the amount of biodegradable municipal waste going to landfills [19]. This favours expansion of waste management facilities, like AD technology, that have high priority according to the Waste Hierarchy. For the future, Europe 2020 strategy targets on climate change and energy [21] also prescribe prevention of greenhouse gasses [22]. All of these initiatives have paved ways for the EU policies aiming at material circulation in economies branded as a circular economy [23]. When it is broadly comprehended, this concept assumes less input of material resources and an increased share of renewable and recyclable materials, as well as reduced emissions, fewer material residuals and keeping the value of products in economies [24]. However, it will also be challenging to reduce the level of waste generation and align waste management objectives with those of the circular economy imultaneously. In practice, avoiding waste is the best waste management option; however, per capita municipal waste generation is still increasing in around one-third of all Member States. Moreover, two-thirds out of those waste prevention policies developed by the Member States focus on information and awareness raising, and most of these only emphasize change at the household level. Thus, for the EU expanding and strengthening of the existing policies are a key priority for reducing the environmental burden and reaching the overarching aims of a resource-efficient economy [25].

In the last few years, the demand for waste treatment facilities in the UK has increased considerably, which is likely due to pressure from central government and the EU to divert biodegradable waste from landfill [26]. The country consequently presented an increase of 8.2% waste treatment capacity from 2012 to 2014 [26] as an example. Out of the 209 million tonnes of waste treated in 2014, the used facilities amounted to 0.9% energy recovery, 43.6% recycling and all recoveries other than energy recovery, 10.4% backfilling, 3.6% incineration, 23.1% landfilling and 18.3% land treatment and release into water bodies.

The waste management regime has accordingly progressed from policies based on the control of waste disposal activities to embrace aims for waste prevention and recovery. This trend shapes growing attempts to harmonize waste policy objectives at both the EU and UK level [27]. The UK municipal waste definition has also been modified because some Local Authorities only report on waste collected from households as municipal waste, while others include commercial waste [28]. Likewise, the UK government included the biodegradable municipal waste (BMW) sent to landfill [28] , which amounted to 7.7 million tonnes (2016), representing 22% of the 1995 baseline value [26] , as an example.

These factors encouraged waste management facilities to change and prompted the increment of facilities like AD, which sets out five steps for dealing with waste in priority order according to article 4 of the revised EU Waste Framework Directive (Directive 2008/98/EC). For instance, from 2010 to 2015, the total electricity generated from Bioenergy saw a substantial 147% increase, out of which anaerobic digestion quantified 7.5%; while as the electricity that generated from landfill decreased by 3.2% [21]. The decreased electricity generated from landfills is possibly a warning of reducing landfilled biowaste within those specified years and could suggest 0.2 million tonnes of residual household waste has moved away from the landfill. As mentioned earlier, this could acknowledge the implementation of the EU legislation on waste plus the recycling targets for municipal and packaging waste and for reducing landfill [21].

The waste management system in GM has a long history and it has been developed significantly in the past 10 years, particularly after the decision was taken by Greater Manchester Waste Disposal Authority (GMWDA) to work towards Zero Waste [29]. GMWDA is responsible for managing the waste of nine of the ten local authority districts in GM, with one (Wigan) being managed by Wigan Waste Disposal Authority (WWDA) [30]. In April 2009, the local authorities within GMWDA entered a 25-year Private Finance Initiative (PFI) contract with Viridor-Laing to manage GM’s MSW, which accounts for 2.5% of the total waste in the UK. Likewise, Wigan has a 25-year contract with the FCC (UK) Limited waste management company to collect residual waste, Eco-ganix to collect mixed food and garden waste and Viridor to collect dry-recycling.

Both Waste Disposal Authorities (WDAs) are currently responsible for the MSW management in GM, agreeing to produce a Joint Waste Development Plan Document lasting until 2027 [31] with the tagline “Our aim is zero waste”. As a result of the joint and coordinated activities the authorities diverted 54% from landfill in 2013/14, an increase of approximately 13% from the 2008/9 figures [16]. This was achieved through the infrastructure financed by the PFI contract, which built advanced in-vessel composting, Mechanical Biological Treatment (MBT), and Material Recovery Facility (MRF) facilities, as well as upgrading the existing household waste recycling centres to allow a more user-friendly approach to recycling.

Based on the Greater Manchester Joint Waste Development Plan Document (2012), it is evident that the Authority does not have any intention to add more waste treatment facilities (like energy recovery, recycling and composting) untill 2027 [31]. Rather, it favours waste prevention, the top priority according to the Waste Hierarchy. Thus, the focus is avoiding an increase in waste by the restricting household waste collection services, for example, by either moving to a three weekly system or reducing the capacity of residual waste through providing households with a smaller bin. Through these mechanisms and other behaviour change activities, the GMWDA developed a plan to reduce household waste to 400 kg per household by 2025 [32] , of which biowaste will be around 160kg. Through those already existing waste facilities and listed prevention methods, the GMWDA aims to achieve the 2025 targets (e.g. 90% diversion from landfill).

However, the worst case should also be considered. Moreover, its impact on environment and quality drop while it is transported to the transfer loading station and other related emissions should be here highlighted. Thus, in order to achieve the contract targets, GM must further increase its efforts towards 90% diversion from landfill to align with European and UK targets. Decentralizing industrial processes may potentially be a route towards achieving this. As stated before, they can reduce the cost of waste mobility and pollution, mainly by removing the water content of the biowaste at its source level, simultaneously enhancing society’s awareness. This type of system demands a high social commitment, and thus generates links between people sharing the same purposes [33]. Ultimately, the concept that fundamentally believes waste is a resource, is not sustainable in the long term unless the producer (household) directly starts to benefit from it.

Figure 2 depicts that, there are currently 44 facilities across 23 sites, managing over 1.1 million tonnes of waste per year [34]. These are: the Educational Centres, which enhance the knowledge of the people towards recycling, Household Waste Recycling Centres, which are used to recycle and dispose of the household wastes safely, Mechanical Biological Treatment (MBT), which separates waste into several fractions such as light/heavy fractions, ferrous and non-ferrous metals, and organic materials; the In-Vessel Composting plant, which attaches the natural composting process, to recycle organic waste into quality compost products and for later use as a soil improver; the Thermal Recovery Facility, a

process by which energy is produced from burning waste at high temperatures; Transfer Loading Stations, which are used as a place for the local waste collection vehicles to deposit their waste cargo prior to loading into larger vehicles; Material Recovery Facilities sort commingled recycling like glass bottles and jars; finally a Thermal Power Station is a combined cycle gas turbine power station within Greater Manchester. The quantity in each box of Figure 2 shows the total number of each facility in GM.

There is an argument to say that in order for local collection authorities to contribute to landfill diversion further, a decentralized system of waste management is preferable. The decentralized treatment is supposed to improve local economies, create jobs, use local resources, deliver quality products, and create less hazardous waste as a product of the process [35]. It also carries other indirect advantages such as reduced fuel consumption, reduced contamination of groundwater, reduced air pollution and lower road maintenance cost [36]. Generally speaking, a decentralized waste management system is usually assumed to have more benefits for the environment than otherwise, and may not produce the unwanted waste as a result of complex industrial processes, thus enabling further landfill avoidance.

The main impediment to the decentralized system is a higher cost per waste unit. Another problem with decentralized systems is that MSW is viewed as a great burden to the local environment and ecosystem (smells, rodents, hygiene) and creates a challenge compared to the existing waste management paradigm that allows people to forget about their waste once it is collected by the local authority. Yet a change in this behavior could allow communities to manage their waste and eliminate fuel expenditure for collection rounds. This is especially pertinent for biowaste as it is dense compared to packaging waste and carries an extra fuel burden as a result of its water content. This proposed paradigm makes the participation of the community especially crucial because decentralized management as a principle devolves authority to the citizens, who in turn become more responsible for their own waste management system. This is a specific responsibility requiring an assignment of subordinates with the necessary skills to manage both the waste flows and the AD process itself. The type of biowaste treatment that is mostly used in western countries is a centralized waste management system where methods such as the industrial AD or composting are implemented to manage food and/or garden waste. The difference between the two types of treatment is summarized in Table 1.

Current waste infrastructure in the UK is partially designed to cope with the level of biowaste created in the UK, which stands at about 4 million tones [42] according to the UK Digest of Waste and Resource Statistics if one assumes that biowaste is the only food waste. This figure could double if garden waste is also considered. Even though composting and AD are commonly employed waste treatment facilities within the UK at the moment, a considerable amount of the biodegradable waste still is sent to landfill. Even not yet least due to the current landfill tax levy that is incurred on every tonne of waste sent to landfill. For example, in 2015, around 8 million tonnes of biodegradable waste, including biowaste, was landfilled [43]. Hence, the untreated biodegradable waste may pose a challenge to the UK waste industry if it isn’t effectively managed, and more interestingly, in each year, insinuating waste is increasing correspondingly. Therefore, the challenge will be trickier unless comparable measures are taken. The action that has been taken by Wales, where funding is offered for free back garden Composters as a method of decentralized waste management [44] , is exemplary for the rest of the UK since similar efforts may be required elsewhere if they are to have any noticeable effects on the overall waste and recycling targets.

Biowaste, as mentioned in section 2.2, has fallen at household level through the ‘Love Food Hate Waste’ campaigns launched in recent years within Greater Manchester. However, biowaste is still a significant proportion of the household waste bin [45]. As GMWDA figures indicate, 40% of waste collected at the kerbside is biowaste, but individual breakdowns of this biowaste waste are currently unobtainable. Nevertheless, the figures do indicate that GM produces more biowaste, than the national average. Based on figures from DEFRA (2017) the waste production per household was 407 kg in the year 2015 [42]. Since an average of biowaste at household level is around 40% of the mass production, thus, approximately 163 kg of biowaste is collected per person. Moreover, it is expected that there are around 2.3 people per household [46] , thus, roughly 375 kg is collected from an average household. It is estimated that 1000 kg of residual household waste produces 992 kWh [30] in energy from its biowaste; therefore, each household in GM could generate approximately 1 kWh per day or 3000 MWh per year in GMWDA.

The current waste management systems in GM have looked at addressing this issue, and based on Figure 3, MSW has declined by approximately 9.4% over six years (2005/6 to 2010/11). Furthermore, the waste infrastructure in GM is such that any biowaste that enters the residual waste stream is separated at an MBT plant and utilized in an AD process. This could be argued as an extremely effective way to manage a large amount of waste, but it does have the drawback of the requirement for transportation and the need for highly skilled personnel to manage the technology properly.

In addition to the previously mentioned drivers of AD development, the price of electricity generated from the combustion of biogas in a CHP engine has also played its own role in spreading the construction of AD plants across EU countries, which also has a direct relationship with the number of AD plants that are constructed in the Member States [58]. Hence, the nations that are more likely to implement AD are the ones that offer the greatest financial incentives for electricity generation. However, as mentioned in 2.1, the recently revised waste framework directive, as an example, also has its own input on expanding and maximizing the number of the AD systems across the member countries and attracting investors in the waste industry [23].

MSW production per person in the EU was 481 kg in 2013 [59] and there was also a trend of reduction by 0.8% from 2002-13 [59]. Therefore, a fair estimation of the EU’s MSW is 473 kg per person per year, of which 189 kg is potentially biowaste based upon previously mentioned figures. The population of the EU was estimated to be approximately 510 million in 2015 [60] , therefore, approximately 96 million tonnes of biowaste were produced in 2015. Thus, the AD plants that were built prior to 2015 could only process 9.4% of the total potential arising from biowaste. In addition, around 40% figure is an average value for biowasteas a percentage of the total MSW, but in reality, the value of biowaste can fluctuate from 18% to 60% [7] depending upon the local conditions. Based upon such estimations, the EU may have to manage up to 142 million tonnes of biowaste per year. This shows that there is a possibility that the facilities could, in theory, have 93.8% less capacity than required for the worst-case scenario, indicating a requirement for significant investment in infrastructure if the EU wishes to achieve its targets through further implementation of AD.

In 2010, the UK government made the decision to work towards a “Zero Waste” economy and to initiate measures to enhance energy from waste by encouraging people to recycle more of the waste arising from the household [61]. This change was to be led by businesses, which were given the opportunity to initiate the change through renewable energy technologies such as AD. As a result, the Government’s Structural Reform Plans (2010) included actions to endorse increased energy from waste through AD, and the installation of the AD facilities has resultantly seen a marked increase in recent years, alongside the overall AD capacity. From 2013 to 2015 there was a 57% increase in the number of AD systems and a 62% increase in total capacity [58].

In 2014, there were 161 AD plants in the UK that are outside the water industry and small agricultural plants, with the capacity to produce 143 MW of electrical power, demonstrating an increase of 34% MW capacity from the previous year. It was also identified an estimated 30 MWe of additional capacity would be constructed in 2015 [58]. However, in January 2015, there were only 161 functional AD plants, capable of processing around 628 tonnes of biowaste in an hour, or 5.5 million tonnes per year. Using population and waste arisings statistics from 2014, and accountable for improvements in recycling performance and waste arisings, the waste produced at the household level [12] was approximately 415 kg per person in 2015. Since the population of the UK was approximately 65 million in 2015 [62] , it is estimated that approximately 10.7 million tonnes of biowaste were produced in 2015. This estimation indicates that the AD capacity in the UK could only cover 51% of the required capacity for the biowaste arising in 2015, inferring that further capacity is required to achieve an effective waste management strategy for the entirety of the UK’s biowaste. Furthermore, there is a problem in that centralized systems are already in place in many areas, which require the householder to behave in a specific way in order to achieve biowaste recycling. It may be necessary to emphasize rural regions that are not served by centralizing AD systems to improve biowaste recycling in the future.

In Greater Manchester, there are four Mechanical Biological Treatment (MBT) facilities using AD with a total capacity of 440,000 tpa. As mentioned previously, the average mass of biowaste per household is 380 kg in a year and there are approximately 1,128,000 (2013) households in GM. Based on this data, there is a possibility of the production of 430,000 tpa of biowaste. Therefore, it can be concluded that the current facilities have the potential to recycle all the biowaste that would be produced by GM. However, these facilities also treat all the biodegradable waste produced within GM [34]. When the content of biowaste at the household level reaches 60%, the total mass would be 574 kg (2013) of biowaste at the household level and this indicates a potential total of 650,000 tonnes of biowaste in GM. This shows that the local facilities could, in theory, have 47% less capacity than required for the worst-case scenario. Even in the best-case scenario, it will be extremely difficult to achieve the 2030 recycling target outlined by the government (70%). Although the waste generated by persons in GM is decreasing and the amount of recycling of household waste is increasing (Figure 5), the population size has increased by an average of 0.56% annually since 2001 [63].

Therefore, taking the average rates, and other variables for the coming years constant, the recycling rates will be approximately 54.7% and 62.7% in the years 2020 and 2030, respectively. Thus, without further changes, GM will miss its 2030 waste and recycling target by 7.3%. This could be a damning scenario for GMWDA and VL, which is a waste company in Manchester which is jointly owned by John Laing and Viridor, waste. Therefore, new avenues for recycling must be explored, including the possibility of decentralized AD facilities that can be voluntarily managed by community groups and/or leaders. This can enhance society involvement in the whole waste process.