Introduction

Decentralised or community-scale composting is a promising solution to waste mismanagement and the loss of organic material to landfilling, which has been gaining popularity since the 1990s [1]. Globally, landfills are filling up and regions are running out of land to construct new ones [2]. Queensland is no different, and the state government has developed a plan to reduce waste by 25% and increase recycling to 75% by 2050. Part of this plan involved introducing a landfill levy in 2019, which has incentivised local councils to divert their waste via recycling of both inorganic and organic wastes [3]. Redland City Council is in South-East Queensland and comprises the mainland Redlands Coast as well as the bay islands. Under Queensland Government’s interim target, Redland City Council [4] aims to recover 55% of all waste by 2024.

Four of the six inhabited bay islands fall under the label of Southern Moreton Bay Islands (SMBIs), which produce a total of 5400 tonnes per year of garden waste. This, along with recyclables and general waste (including food scraps), is sent to the mainland to process at Council’s expense. Hence Redland City Council is looking for opportunities to process food and garden organics onsite in a way that adds value to the local community and does not emit the greenhouse gases that their breakdown in landfill does. As Sealey and Smith [5] say, “the ability for small islands to meet sustainability goals is exacerbated by the costs of transporting goods on, and then, wastes off the islands”. Centralised waste management systems common in populous regions of Australia would not be productive in these regions. Hence, Redland City Council has partnered with the Centre for Recycling of Organic Waste and Nutrients at the University of Queensland (UQ) to implement a pilot project on organic waste recycling at a community scale. This was funded by research grant from the Goodman Foundation Moreton Bay [6].

Community-scale composting is usually conducted within a neighbourhood, keeping nutrients cycling locally and creating local enterprise [7]. Karragarra Island, a SMBI comprising 150 residences of mostly 1–2 people each, boasts a functional and organised community garden which proved the ideal location for this pilot project. The final product from the composting project will be used as local organic fertiliser, reducing the need to import it from the mainland. Running Wild, a youth conservation programme, has partnered with the community garden to provide extra labour to implement and sustain this pilot project, and SMBI permaculture has also partnered with this project. The pilot project is called Small is beautiful—how and to what degree it is possible to recover, process and utilise organic garden and kitchen waste on Karragarra Island for environmental and community gains.

This review was conducted as part of UQ’s contribution to support the project. A systematic review was conducted on other decentralised organic waste recycling projects worldwide, with a focus on those in South-East Queensland; in the whole of Australia; and on other small islands. The aims of this study are as follows:

  • To provide background information on community-based composting and organics recycling projects in Australia, their successes and challenges;

  • To analyse the factors for success and failure of community-based composting and what it can and cannot achieve;

  • To give context for the Karragarra Island pilot project in order that it may be compared with existing projects and improved where possible; and

  • To investigate the effectiveness of community-based composting on Karragarra Island in involving all residents in organic waste recycling and thus reducing the solid waste shipped to the mainland.

This review will assist the pilot project by providing information on different situations where something similar has been tried; what worked and what failed in each situation. This will increase the ability of the pilot project to be successful and reproducible when applied to different and unique small communities, especially other islands.

Methodology

Systematic Literature Review

A systematic literature review was employed to collate secondary data from several studies to apply to the pilot project. Data were found via searching on three databases: UQ Library, Google Scholar and Ecosia search engine. The keywords used were “decentralised”, “community”, “organics”, “composting”, and to a lesser extent, “island”. Despite covering multiple key words, sources referring to bacterial or microbial community in composting or organic waste recycling were disregarded. Further sources were found from the references of these documents, and some non-scholarly references were found via word-of-mouth and personal communication. All sources were published in the last 20 years.

Data was then collected to answer the following questions:

  • What are the most relevant similar projects being done in Queensland? Australia? Worldwide?

  • Of the most relevant similar projects, what are the areas they were most successful in? What were their challenges?

  • What are the collection, participation and contamination rates of these projects, where available? Are there unique factors of these communities contributing to these statistics?

  • What aspects of community, circular economy and nutrient cycling can be applied to this project?

  • How much organic waste is diverted from landfill by the pilot project on Karragarra Island? How does Karragarra Island measure up to the 55% recovery target, both before and during the pilot project?

Most of the information gathered was from peer-reviewed, scholarly articles and grey literature. Some websites were used, and one source was found via word of mouth.

Karragarra Island Pilot Project Methodology

Survey

To best apply the literature findings to the pilot project on Karragarra Island, data were collected from an initial survey of island residents on interest in participation in the pilot project and residents’ current food waste recycling or repurposing. The survey also covered residents’ preferred methods of compost collection; however, as these results were used simply to design the project, they will not be included in this report.

Compost Collection, Participation and Contamination

Data on participation and collection rates were obtained as well as approximate contamination rates for the first 7 weeks that the pilot project operated. The compost, collected in compostable caddy liners, was dropped off by residents in two 240 L wheelie bins at different locations on the island. Every week, these bins were collected and weighed to find the amount of food organic waste collected. This organic waste was then tipped into the composting bay: a solar-powered forced aeration system including three bays, each 1.2 m in each direction, was used. After each wheelie bin of food waste was poured in, a garden fork was used to tear open compostable bags to allow for visual checking for contaminants. An approximately equivalent mass of wood chip (usually one wheelbarrow full) was then poured into the bay after each bin of food waste to improve the C:N ratio. The wood chips were visually gauged for moisture content and if they looked dry, a 9-L jug of water was poured over the surface. The far-left bay was filled first, and a thermometer probe was inserted into the bay where it stayed for the duration of the study and was routinely checked.

Results

Systematic Review Sources

Scholarly Sources and Grey Literature

From the database searches, a total of 17 scholarly sources and 11 grey literature sources were found. Eight of the scholarly sources and two pieces of grey literature were found to be irrelevant to this project and discarded, leaving 11 peer-reviewed journal articles and nine grey literature sources to be analysed. Two further scholarly articles were found but were unable to be accessed through the university library and thus could not be analysed.

Of the grey literature, four were conference or workshop proceedings; two were electronic reports; one was a thesis; one was a newsletter, and the other was a scholarly article not published in a journal. Seven sources were published after 2017, five sources between 2012 and 2016 and four sources between 2007 and 2011. The electronic reports were undated. The remaining five sources were published between 2002 and 2006. Figure 1 shows a more detailed analysis of the date and genre of published literature reviewed. No relevant literature found was from Australia. The regional breakdown of studies can be seen in Fig. 2.

Fig. 1
figure 1

Graph of frequency of types of literature reviewed by year published

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Fig. 2
figure 2

Graph of frequency of types of literature reviewed by region studied

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Personal Communication and Other Sources

Notable local projects were Leah Harradine’s compost collection from the community in parts of Victoria Point, and the composting project being designed in Yarrabilba. Both communities are in South-East Queensland, inland from the SMB islands. These projects were both found via word of mouth and did not appear on database searches using the search terms defined above.

Leah is a private residential gardener located in Victoria Point, a suburb on the southern coast of the mainland Redland City Council area. To provide a greater quantity of compost to fertilise her garden, she collected food organics from four households (excluding her own) and a childcare centre. This project was done for a period of 5 months. The suppliers of compost are in the local community of Victoria Point and neighbouring suburb Redland Bay. Leah supplied and collected the buckets of compost from the residents and the childcare centre until despite its success in providing her with more organic fertiliser for her garden, and this workload became too much for her alone [8].

Yarrabilba is an upcoming circular food economy south of Brisbane City and south-west of Redland City which intends to implement a Food Agility project designed by Queensland University of Technology and Lendlease [9]. This programme involves an incentivised food collection scheme using an app at the household level and a wicking bed composting system which digitally measures aspects of the compost including methane emissions, as confirmed by Richards [10], pers. comm., 24 May.

Cassowary Shire Council in Northern Queensland conducts a community composting programme, but no further details on its successes and challenges could be found online [11]. Several city and state council websites were found in Australia which employ centralised composting via a mixed food and garden organics bin (NSW [12]. None of these were further analysed due to the lack of information and the irrelevance of centralised composting to this project.

Hundreds of home and small community composters were found on the ShareWaste [13] website in Australia and Community Composters [14] overseas. These individuals were not followed up further.

Karragarra Island Pilot Project Results

Survey

Of the 150 households surveyed, 42 surveys were returned. Of these respondents, 55% (23 households) had no food waste going out as they already composted at home or fed the scraps to chickens; while the remaining 45% (19 households) had food waste, they were able to donate to the community garden.

Compost Collection, Participation and Contamination

As of the 30th of April 2021, 3 weeks into the pilot project and 7 weeks after the first caddies were handed out, 50 households had taken compost caddies and were donating their food organics to the community composting programme. The amount of waste collected was weighed every week from the 14th of April to the 27th of May 2021—noting the bins were out for collection at the start of the week ending on the 14th of April. It averaged 42 kg per week, diverting a total of 293 kg food organic waste from landfill over this period. Extrapolation to a year of the same weekly collection gives 2.18 tonnes. The total amount of food and organic waste collected on the island every year is 200 tonnes, 16 tonnes of which is food waste that goes to landfill [15]. In its first 7 weeks, the composting facility processed approximately 16.6% of the food waste generated on the island during that time.

Weekly waste collected is graphed in Fig. 3.

Fig. 3
figure 3

Graph of total food waste collected per week in Karragarra Island composting pilot project over 7 weeks in April and May 2021

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Minimal contamination was found over the first 7 weeks of collection. Two plastic bags were found in the first few weeks, and subsequently, only the occasional small wrapper or fruit sticker was found. No prohibited animal products were found in waste collection. If contaminants were estimated to weigh 15 g out of 293 kg wet food waste this is a contamination rate of 0.005%.

A problem noticed when sorting the food waste was the amount of unnecessary food waste that was being discarded for composting. Each week, up to a bag of whole fruits (not just the peel and seeds) was found in the food waste collection. Some of the fruit was still in good condition and appeared to have been fit for human consumption at the time it was discarded. The low temperature of the compost, which fluctuated between 30 and 39°, was also concerning.

Discussion

Relevant Similar Projects, Their Successes and Challenges.

Lessons from Similar Projects in Asia: Malaysia

Keng et al. [16] document a community-scale composting pilot project involving the cooperation of the University of Nottingham, Malaysia, and surrounding residents. This project was a step in the transition towards the circular economy ideal where all organic waste is recycled and eliminated from landfill. Five aerated static pile composting heaps were made. Each pile was 2 m in diameter and 1.5 m tall and deposited on top of a layer of shredded leaves. In total, 130 kg food waste per day was collected from residents and mixed with leaf litter at a 4:1 mass ratio of food waste to leaves (giving a C:N ratio of 30:1). A month’s worth of compost was collected in each pile, and after that they were manually turned twice a week for 5 months. The first of these 5 months are the mesophilic stage, the next 4 being the thermophilic stage. During the mesophilic stage, the compost heated to 50 °C within a month. In the thermophilic stage, the temperature rose to 62 °C due to thermophilic bacteria and stayed there for 3 months. It then cooled down in the final month. After 5 months of aeration, it entered the curing and finishing stage, for which the compost was moved to a covered tank where it was no longer turned or watered. Here, it matured and cooled to an ambient temperature. The overall composting process took 7 months, giving a yield of 11% wet basis and 30% dry basis. It successfully met Malaysian organic fertiliser standards.

Although this experiment was not repeated at a larger scale, a socio-cultural and economic analysis for increasing composting capacity from approximately 4 to 6 tonnes per month was conducted. Local government-organised community-scale composting was found to mitigate difficulties of household composting (such as insufficient room for compost or time to rotate it). Community awareness and participation was a huge factor towards successful composting as the process depends on organic waste being largely separated by the public rather than mixed with inorganic waste and landfilled. While composting was not yet financially more economical in Malaysia, it would be if landfilling cost increased 2.3 times. The increase of landfilling prices is a call for local governments to begin establishing community-scale composting. Decentralised composting has other social and environmental benefits such as reduced smell from an unsanitary landfill, reduced GHG emissions and lower rates of eutrophication from chemical fertiliser use [16].

Karragarra Island has the advantage of a community with high awareness of composting (50 of the 150 households already participated in the pilot project, and at least 23 more do their own composting instead). Composting is very economically viable for the council as it reduces the high cost of sending landfill to the mainland.

Though both Karragarra Island and Keng et al. [16] use aerated static pile composting, there are notable differences in methodology (solar forced aeration rather than manual turning), collection rate (42 kg weekly rather than 130 kg daily) and composition (1:1 compared to 4:1 food waste to leaf litter) on Karragarra Island compared to that of Keng et al. (2020). Furthermore, the duration of the Karragarra Island pilot project was only 7 weeks. On a corresponding timeline, Keng et al. [16] would have filled the first compost pile for the first 4 weeks, then it would have entered the mesophilic stage for 3 weeks. Hence it is likely the Karragarra Island pilot project’s compost did not enter the thermophilic stage during the experiment. However, the difference in food waste to leaves and resulting C:N ratio may also provide insight into reasons for the low temperature of the compost. Nevertheless, we see the success of a similar pilot project in creating a quality compost.

Ridzuan et al. [17] surveyed students in Universiti Sains Malaysia and residents in the surrounding community. While only 42% of participants had previously been exposed to composting initiatives, most agreed to participate in them in the future. A total of 8% of respondents disagreed: mainly because of time commitments or lack of space to do so. Hence, partnerships between community members, organisations, education institutions and governing bodies were recommended to ensure individuals are not left to take on more than they can handle. Such a partnership is seen in the cooperation of the University of Queensland, Redland City Council, Running Wild, SMBI Permaculture and the community garden in Karragarra Island. These, along with the Goodman Foundation Moreton Bay’s research grant, all contributed to the project taking off. The results of the Karragarra Island pilot project show that participation rates of a community composting initiative (50 households in the first 7 weeks) are likely to be much higher than the sum of households composting individually (23 households). This trend is supported by Ridzuan et al. [17].

Bong et al. [18] conducted a life-cycle assessment supported feasibility analysis in Iskandar, Malaysia, of community-based composting in a village of 600 households within a palm oil plantation. It was shown to reduce up to 72% of the GHG emissions of the village due to waste diverted from landfill (preventing emissions associated with anaerobic decomposition),reduce the need for production of nitrogen (N), phosphorous (P) and potassium (K) fertilisers and return carbon (C) to the soil when the compost is used. Karragarra Island is a quarter of the size of this community and in a different geographical and socio-economic setting; however, the basic principles of greenhouse gas emissions, and of C, N, P and K cycling, can be applied. Community composting reduces organic waste in landfill (and its resulting greenhouse gas emissions) while keeping biomass in the island ecosystem. Because it enables us to close the loop in nutrient cycling, it reduces the need for boats (which also emit greenhouse gases) to take the waste off the island and transport fertilisers onto it.

Lessons from Similar Projects in Asia: India and Bangladesh

Zurbrugg et al. [1, 19, 20] report the results of several community-based and decentralised community composting initiatives in Indian cities. Nine sites servicing from 120-1200 households each were analysed. Most use bin-composting and sell the final compost product in the neighbourhood. One of these, done by the Scientific Handling of Waste Society of Bangalore was aerated similarly to Karragarra Island and composted 50kg of waste per day from 180 households (thus also being the closest in size to it). This project was notably able to sell compost to middle- and high-income companies at a high price for use in parks. All other composting initiatives were not actively aerated, either in a passively aerated box or windrows on vacant lots in the neighbourhood. Most composted 100kg or more organic waste per day. Compost is actively composted for 3-5 weeks, then left to mature for 4-6 weeks, depending on the size of the collection. Despite their success in diverting waste from landfill and producing quality compost, most composting initiatives have minimal economic profits if any Zurbrugg et al. [1].

Zurbrugg et al. [1] categorises organic waste recycling initiatives into four categories: citizens’ and community initiatives; business and institution initiatives (operating on their premises); decentralised small- and medium-scale private enterprises; and large-scale centralised public–private partnerships. Despite having minimal economic profit, the network of decentralised organic waste recycling studied was much more economically viable and easily implemented than centralised composting schemes. The many community composting initiatives found by Zurbrugg et al. [1] show that community composting is not new; it simply has not been of interest to scholarly literature until recently.

Because citizens’ and community initiatives did not have government support, they required individuals’ dedication, commitment of time and money and acceptance of the smell of compost bays in their neighbourhoods. The process of organising a composting initiative without government direction requires cooperation, compromise, collaboration and goodwill between all members of a neighbourhood. As a result, it brings people together, facilitates the education of entire communities about the importance of composting and looking after the environment and provides further opportunities for business ventures and other value-adding processes in the future. The compost created is a valuable product that can be sold to residents both within and outside the community it is obtained from, and the government for maintenance of local parks [1]. The benefits of waste being separated, composted, and returned to the soil as nutrients onsite is not limited to nutrient cycling and reduced transport costs and emissions: jobs are created, local enterprise is supported, healthy gardens and parks are supported and environmental awareness is cultivated.

These benefits overlap with the aims of Running Wild Youth Conservation Culture Inc.: a not-for-profit that creates local jobs, provides traineeships, and mentors and supports young people with a focus on conservation and care of the local environment [21]. As a result of their contribution to this project, Karragarra Island’s community composting pilot project provides opportunity for these social, economic and environmental benefits in the SMBIs.

A decentralised organic waste recycling project was done in Dhaka, Bangladesh, involving the partnership of non-governmental research organisation Waste Concern, fertiliser company MAP Agro Industries, and the local community, with a donor grant. 1.7 tonnes of waste from 790 households were treated daily, and the fertiliser company marketed the final quality compost to provide the economic success to sustain this project from 1995 onwards [22]. As typical for most projects, an upfront investment was needed to enable its implementation. Either financial incentives from municipal authorities or the assistance of established businesses may provide this and give an opportunity for successful decentralised composting that later sustains itself economically [23].

Lessons from Similar Projects in Africa

Pilot projects similar in scale and design to that on Karragarra Island are being implemented in peri-urban areas of Zanzibar, Tanzania, and of Swaziland. These projects function as part of the municipal waste management and cater for all the wet waste from households. Each project involves manual labourers to collect sorted waste from households and maintain the aerated box composting system. The waste collectors collect both food waste and recyclables from the 100 households involved in each of these pilot projects; the latter of which is sold for their own profit. These pilot projects both aim to be a model that can be replicated in other nearby communities; however, no clear plan for the ownership of these projects is made [24].

In total, 70–90 kg of wet waste is collected from the households in the Zanzibar pilot daily [25]. Despite only catering for twice as many households as the Karragarra pilot, it takes in more than ten times the amount of food waste. Possible reasons for this include differences in lifestyle across geographic and socioeconomic divides, larger household sizes and the absence of a general waste collection which people may place unsorted waste in. Perhaps a way to increase collection rates on Karragarra Island and beyond is to monitor what residents are putting in their general waste bins—and to make composting the mainstream way of waste management rather than a niche extra.

Another similar pilot project was implemented in Tiassalé, Côte d’Ivoire, a commune in sub-Saharan Africa. Two hundred thirty households’ organic waste fraction is collected door-to-door and processed in a composting plant outside of town. Open-pile composting was used to process 1.5 tonnes of waste per week. Despite some inorganic contamination, the compost successfully matured and generated 24% yield of wet mass and a 36% yield of dry mass. The C:N ratio of organic waste input was 34, and of the final product was 11.3. Compost was tested for quality and found to be rich in nutrients and fell far below the recommended maximum content of all heavy metals [26]. Because the average household contained eight people, the waste collection was over 30 times that of Karragarra Island despite servicing only 4.6 times as many households.

Mhindu et al. [27] documented another successful decentralised composting operation in Harare, Zimbabwe. Food waste was collected at a farm (rather than household consumer level) and composted in bins with straw. In total, 140-day trials were conducted of many different straw contents. A total of 50% or more straw was found necessary to ensure compost heated up to 55 °C for 3 days or more (a requirement to kill weeds and pathogens). The straw used had a C:N ratio from 70 to over 100 whereas the vegetable waste used had a C:N ratio of only 12. Chemical analyses of the compost and similar trials of different amounts of wood chips may provide insight into the low temperature of the compost on Karragarra Island and may make it possible to manipulate the timeframes in which the compost enters mesophilic and thermophilic stages as well as the quality of the final product.

Lessons from Similar Projects in Europe and America

Bruni et al. [7] outline the composting process: the mesophilic stage occurs at 25–40 °C, the thermophilic stage at 35–65 °C (though 40–65 °C is optimal), the cooling (second mesophilic) stage occurs as it cools back down to room temperature and the maturation stage follows. Based on its temperature, the Karragarra Island community compost appears to have remained in the mesophilic stage during its first 7 weeks.

A notable similar project was done at Sandals Emerald Bay, a resort on Great Exuma, in the Bahamas. Bulk composting of food waste generated by the resort was done using a multi-stakeholder partnership to cover the costs. 78.6 tonnes of waste were composted over a 7-month period which diverted it from landfill and reduced the strain on waste management services on the island. However, this project faced considerable challenges in that staff conducting it were not given proper training or adequate staffing to complete the job. Furthermore, no incentive by the local authorities was given to conduct such a project as it simply became another cost on top of waste management that staff felt little inclination to do. Despite 62% of the cost being covered by private businesses, implementing, and sustaining a decentralised composting programme, can be challenging [5]. This should be considered when the Karragarra Island pilot project is left to sustain itself once the research grant runs out; and it should encourage grant application when trying to replicate this project elsewhere.

Another pilot project using a similar collection scheme to that of Karragarra Island was applied in the Tinos Island communities of Pyrgos and Panormos, Greece. A total of 21 m3 of organic waste collected before composting in a bioreactor was chemically analysed and found to be suitable for composting. A successful compost was generated after 20–60 days [28]. Three other community composting initiatives in Spain were found to successfully generate quality compost [29]. However, another community-level composting project in Spain was unsuccessful and had to be stopped due to insufficient organic material being processed to make it economically viable. This project collected only green waste from city parks to create fertiliser and did not collect food waste out of fear of contamination. Yet while it was running, the fertiliser created from this green waste was sold to farmers thus adding value to the local economy and reducing the need for chemical fertilisers [30]. It is not known whether the addition of food waste to this organic waste recycling initiative would have allowed for enough sales to make it economically viable, but, despite the benefits of organic waste recycling projects, cost is a barrier to their implementation and continued success.

A survey done by Marcello et al. [31] showed that residents of Tuscany, Italy are willing to switch to participate in community composting and walk to drop off their wastes as part of a circular economy and ecological transition movement. 54% of respondents would do this for free,however, the other 46% would require a small financial compensation. The biggest factor to the effort residents would be willing to put into this project (i.e. distance walked to collection site) was their perception of composting. Those who already composted were willing to walk further than those who did not, and those who thought composting was a good way to manage food wastes were willing to walk further than those who were indifferent.

Also in Italy, a review by Bruni et al. [7] showed that while community-scale composting was an effective way to get consumers to be aware of and reduce their waste, it needed technical support from local authorities and a high level of public acceptance and enthusiasm to be successful.

Slater et al. [32] surveyed community composters in Britain and found 121 successful composting projects processing 20 500 tonnes of organic material in 2006. A total of 82% of groups had a relationship with their local council, and considerable benefits to the wellbeing of individuals (such as purpose, belonging and health) and communities (environmentally, economically, and socially) were found. Hence the benefits of community-based composting extend much further than its environmental and even economic benefits. It creates a lifestyle and brings people together in communities as well as improving well-being.

Collection, Participation and Contamination Rates

At an average of 42 kg per week, Karragarra Island’s community-scale composting pilot project processes a smaller amount of organic waste than all projects found in literature. Fifty of the 150 households on the island participated, and at least 23 additional houses had no food waste as what was produced was either composted at home or fed to chickens. Seventy-seven houses remain unaccounted for. It is unknown how many of these may compost at home, are unoccupied for parts of the year, or dispose of all their food waste in landfill.

Over its first 7 weeks of operation, the pilot project diverted approximately 16.6% of food waste generated by the island from landfill on the mainland. The other 83.4% may contain non-compostable organics such as animal products or originate from the 77 households who were unaccounted for in terms of their composting. These households should be targeted to increase involvement in community composting as the project goes forward.

If the survey results were extrapolated to the entire island, 55% of food organic waste (excluding animal products) is already composted by individuals and Karragarra Island already meets Redland City Council’s target in that area. This is unlikely to have been the case as the surveys on composting that were returned are likely to have come from people who are already enthusiastic about composting, and the pool of data be biased towards those who compost. True quantities of home composted waste are not known, and it is unknown whether the island meets this target either before or after the implementation of this project. However, 83.4% of the food waste that was sent to landfill before the project is still not recovered. Working towards a circular economy involves incrementally increasing the amount of waste that is recovered. Hence participation in either home or community composting can and should be financially incentivised by the government, such as through offering a discount on rates and other fees for those who participate or introducing a landfill tax for residents that is proportionate to how much general waste they generate. At a community level, higher participation in the organic waste recycling programme can include assisting neighbours to transport their organic waste to the wheelie bins and mentioning the project in conversations to raise awareness about it.

Although Swagemakers et al. [30] avoided collecting food waste in case of contamination, most other literature found the problem to be minimal. Yeo et al. [26] found only a few plastic bags and nappies in 1.5 tonnes of organic waste. Harradine [8] found no contamination. Similarly, the contamination of the Karragarra Island pilot project was minimal and as a result should not prevent accessing the benefits of recycling food waste. An unusual study by Korner et al. [33] found household waste poorly sorted into its organic and inorganic fractions had contamination ranging from 1.5–28%. This may be seen as the upper limit for contamination of sorted waste in communities that are not well educated about waste, but not as the norm. To combat the problem of contamination, Keng et al. [16] suggests incentivising households to properly sort compost by giving out vouchers for well-sorted food waste. The pilot project on Karragarra Island has extremely low rates of contamination. However, this strategy may be used, potentially with the aid of technology (as done in Yarrabilba) when implementing a similar design in locations where residents are not as cooperative as they are on Karragarra Island.

Community-Scale Composting into the Future

Bobeck [34] found that public perception of composting in developing countries could be improved via education linking it to improved garden yields, a cleaner and healthier environment, and a good economic outcome. An increase in public understanding and conversation about compost both increased participation rates and generated the market for the compost product which was necessary to sustain composting projects. This overview also found that compost was required to reach 60 °C to break down effectively, and a C:N ratio of organic waste added of 30:1 was optimal [34].

Feasibility studies conducted on parts of North America and Europe show potential for decentralised composting in local areas. Adhikari et al. [2] found that home and community composting could reduce landfill costs by 50%, 37% and 34% in richer European countries, poorer European countries and Canada, respectively. A total of 3770 hectares of agricultural land in these areas could be preserved rather than converted to landfill. This provides a positive outlook and a great incentive for governments and communities alike to invest in composting. However, success is dependent both on partnerships between multiple stakeholders and economic viability of these projects. A challenge found by Pai et al. [35] when conducting a feasibility analysis for community composting in Chicago, USA, was that decentralised composting technology and knowledge was far behind that of centralised composting. This can be seen in the push of Australian governments to combine kerbside food and organic waste collection, and the relative lack of information on local community-scale composting. Another challenge noted by Pai et al. [35] was the difficulty in finding appropriate land to conduct the composting while engaging the community. One way to do this is to use places that already have frequent public gatherings.

On Karragarra Island, a functioning community garden is both a community hub and a place where workers and volunteers are equipped and capable of learning a skill that on a global scale, is relatively poorly documented. When considering replicating this pilot project in other communities (such as neighbouring SMBIs Lamb, Macleay, and Russel Island) the capacities for technical knowledge, manual labour, community engagement and economic viability should be considered and partnerships with education institutions, community groups (such as Running Wild and SMBI Permaculture) and local authorities should be sought.

Conclusion

The viability of decentralised, community-scale composting has been presented using existing literature and preliminary results from the Karragarra Island community composting pilot project. From literature, factors for success were high levels of public interest (which is increased by education), council support or private funds to finance the initial stages, and sufficient economic benefit to allow the project to continue once funding is exhausted. In Karragarra Island, the decrease in landfilling, waste transporting and fertiliser costs associated with the project can directly support its continued running.

Challenges faced by the pilot project were participation (as only 33% of households took part) and the low temperature of the compost. The quality of the final compost product relies on an effective composting process, and its quantity reflects the amount of food waste diverted from landfill. Thus, further study on ways to increase community engagement (such as through technology or financial incentives for citizens) should be conducted. Finally, extensive data collection chemical composition and temperature of the compost over time should be obtained from more analysis, monitoring and control measures.