Urban composting

Tiger worms used in verminculture CC BY-NC-SA 2.0 Photo by looseends via Flickr

Composting generally is a process in which organic materials are reduced from large volumes of rapidly decomposing material, into smaller volumes of slowly decomposing material. During this process the ratio of carbon to other nutrients (such as nitrogen) is balanced, avoiding the imbalance and immobilisation of nutrients caused by active decomposition (Raabe, n.d.). Instead, finished compost applied to soil gradually releases plant available nutrients over time.

Urban composting works to capture and transform organic waste products (especially food, alongside other cosmopolitan ‘waste’ materials such as arborist wood chips, sawdust, coffee wastes, and leaves) into compost, a valuable resource for agriculture. Organic waste in landfills is an ongoing issue globally and can produce greenhouse gas emissions when it biodegrades in anaerobic landfill environments. In Aotearoa New Zealand for example, food makes up 9% of all waste sent to landfill, but generates 22% of landfill greenhouse gas emissions (Cropp, 2023).

Urban composting is generally associated with urban agriculture, or spaces like community gardens, but can take place across scales, from single household to whole city. Importantly, urban composting infrastructure should be located in a place that can allow the collection of resource input for compost (like food and mulch), as well as ideally provide an end use for the compost. Urban composting is a valuable nature-based solution that can benefit sustainable agriculture, manage urban waste streams as valuable materials, challenge perception and values around food and waste, and provide community cohesion and education (Ayilara et al., 2020; Diprose et al., 2023)

There are three main types of composting used in urban composting:

Hot composting (in vessel).

Hot compost is a form of aerobic composting, decomposing organic materials using microorganisms that require oxygen (Raabe, n.d.). Air is maintained in the pile by regular turning (moving the material around). The process of turning also regulates heat in the compost, which is generated by the respiration of microorganisms as they break down organic materials. To prevent heat loss and maintain a stable temperature (ideally 71c) that allows efficient decomposition and encourages beneficial microorganisms, a minimum volume of material is recommended, usually 1m3. Water is given off as vapor because of the high temperatures of this method but is also required for the desired microorganisms. It is usually recommended to start a compost with approximately 50% water content, and then maintain this as needed during the process of turning. Material to be composted should overall have a ratio of carbon-to-nitrogen of 30:1. This is hard to measure but is usually achieved by mixing equal parts naturally ‘green’ high nitrogen material (such as food, green prunings, weeds) and equal parts naturally ‘brown’ high carbon materials (such as wood mulch, sawdust, woody prunings, and fallen leaves). When a carbon to nitrogen ratio of 30:1 is achieved, with adequate air and water, the process of composting can happen in as little as 21 days, producing a high-quality product for agricultural use (Raabe, n.d.). Most urban compost projects use ‘in vessel’ systems to maintain the composts in a visually appealing and rodent proof way, and can process reasonable quantities of organic material on a community scale.

Vermiculture

Vermiculture or worm farming is a method of composting that uses composting worms such as tiger worms, which consume a mixture of food, garden debris, and carbon rich materials like paper, card, and leaves. The end product produced are the castings (composted material) and a liquid fertiliser made up of the excess liquid drained from the bin often called ‘worm tea’. Both are excellent fertilisers. Vermiculture vessels or ‘bins’ need to be located in appropriate location to function best, shady, cool and sheltered from the sun and weather (Worm Farming, n.d.). Composting worm populations in vermiculture bins will naturally fluctuate depending on the amount of material available to be consumed, it is recommended worms are fed 70% ‘green’ material like food, and 30% ‘brown’ material like cardboard and leaves. Composting worms also need air and water, so the environment in the bin should be damp and well drained (Worm Farming, n.d.). Vermiculture composts are completely self-contained and require little maintenance aside from the adding of organic matter and removal of finished castings. For this reason, they are popular solutions where space is constrained, including in basements and parking buildings.

Bokashi

Bokashi is a method of anaerobic composting that ferments food waste using microorganisms that prefer reduced oxygen environments (including yeasts, and lactic acid bacteria) (Bokashi, n.d.). Originating in Japan, the process is applicable to processing food waste on a small scale from households and workplaces. Once deposited in a bokashi bin, which is a small vessel or bucket that allows liquid to be drained from the base, the microbial process of fermentation produces lactic acid that effectively preserves the food waste and excludes unwanted microorganisms. A smell is produced similar to that of fermented foods or yogurt. Once a bin is full, the preserved food can be buried or incorporated into composts as an effective inoculant. Bokashi accepts only food, and does not require carbon inputs like other composts methods (Bokashi, n.d.). Because of the small form factor of bokashi, the method is useful for apartments or other high density urban settings.

Name of NbS

Urban composting

Type of NbS

Hybrid living/engineered interventions

Location

Urban

Volunteers add material to a compost at Kelmarna Community Farm’s “Soil Factory”, Auckland, New Zealand. Photo provided.

Relationship to Indigenous knowledge

Indigenous perspectives on organic material are often focused on the concept of care, highlighting the role organic material plays in life-sustaining processes, in particular that it is a tangible way of understanding reciprocal human-nature connections (Diprose et al., 2023). Diprose et al. (2023) position that circular organic waste practices can challenge socio-economic paradigms and shape wider positive human-nature relationship changes. Historically, many people would have understood organic material as compostable and as part of wider natural cycles, as opposed to a consumerist linear view on consumption and waste. This way of thinking agrees with the relational view of many Indigenous people worldwide, which centre the sentiments of responsibility, reciprocity and reverence for nature (Celidwen & Keltner, 2023).

Read More

This can be highlighted by the Māori concepts of mauri (life-force) and whakapapa (kinship) which emphasise the interconnectedness and ‘livingness’ and importance of care for the environment. Diprose et al. (2023) also highlight that the concept of organic material as ‘waste’ has occurred as a recent shift and runs counter to both Māori and Pākehā (European New Zealander people) understandings of waste in Aotearoa New Zealand, for example.

Read More
Climate change benefits
  • Indirect health, social, cultural climate change impacts
  • Loss of other ecosystem services
  • Reduced soil quality

Urban composting impacts climate change through the capture and decomposition of organic materials (Waste management through composting challenges and potentials). These include the potential to reduce loss of carbon to the atmosphere through harmful methane emissions from landfills, instead decomposing organic materials in a controlled way, adding carbon rich organic matter to the soil, in the process sequestering carbon dioxide (Ayilara et al., 2020). Often taking place at community gardens, at a local level, urban composting initiatives often rely on volunteers to process and maintain composts (Bakshi et al., 2021). Providing opportunities for community members to come together and actively participate in waste reduction and resource capture can improve health, wellbeing, and social cohesion, as well as providing tangible ways for people to address the seemingly abstract and large scale ‘wicked’ problem of climate change. As an active process, composts need to be monitored to ensure decomposition is occurring in the desired way (usually aerobically), as poorly managed compost can have undesirable effects including odours, leachate, and low quality or contaminated end products.


Compost improves soil structure, increasing its ability to hold water and nutrients, making these available to plants. These are key outcomes in sustainable agriculture, and under a conventional agriculture model (one that employs chemical fertilisers and mechanisation like irrigation) can also reduce the need for high-energy inputs (Ayilara et al., 2020). In a regenerative setting, and when used correctly, applications of compost perpetually increase soil organic matter, soil biology, health and productivity.

Read More
Societal / socio-cultural benefits
  • Climate change adaptation
  • Food security and quality
  • Pressures of urbanization (waste management, hygiene, etc.)

The immediate climate change adaptation benefit provided by urban composting is the reduction of greenhouse gas emissions produced by organic material in landfills (Ayilara et al., 2020; Cropp, 2023). Alongside this, urban composting initiatives can reduce the amount of waste that needs to be collected, transported, and processed in landfills or other industrial waste management facilities. This reduces emissions associated with transport and can release the need for new landfills which are historically associated with environmental degradation and susceptible to weather related natural disasters as the effects of climate change on weather increase.

By generating fertility locally, food security and quality can be improved (Diprose et al., 2023). Urban composting represents an opportunity to produce material that can improve soil macro and micronutrients, which are lacking in many parts of Te Moananui Oceania (Feary, 2011). Ensuring balanced, high organic matter soils for food growing means that the quality of production and therefore diets are improved.

The problem of food waste is an enormous problem one urban areas. This kind of issue requires multi-faceted approaches that tackle the issue at many levels. Proponents of urban composting and organic material diversion in general often promote a hierarchy of solutions in order of desirability and positive outcomes (Platt, 2017):

  1. Prevent surplus food generation in production and consumption.
  2. Divert still edible food to feed people in need or use food not suitable for human consumption as feed for livestock such as chickens.
  3. Home composting deals with food waste as a valuable material in the most immediate way.
  4. Small scale, decentralised solutions like urban composting can accept organic material including food waste from their neighbourhood or simply process their own material. 
  5. Composting at a town or farm scale can process up moderate quantities of organic material and are designed to serve small geographic areas.
  6. Centralised composting or anaerobic digestion facilities can serve large geographic areas, but nutrient rich material generally leaves the community in which it is generated.
  7. Mechanical biological mixed waste treatment recovers recyclables and reduces waste volume using biological process and potential for methane production before landfill disposal.
  8. Landfill or incineration is the unacceptable last resort, involving high capital costs, producing pollution and contributing to emissions.
Read More
Ecological and biodiversity benefits
  • Biological control (regulation of pests and disease)
  • Decomposition
  • Education and knowledge
  • Nutrient cycling
  • Soil building

Ecological and biodiversity benefits provided by urban compost are related to its role in soil health. When applied to soils, compost promotes soil microbiology that generates soil structure, supports plant growth and health, and effectively balances and cycles nutrients (Ayilara et al., 2020). Compost is essentially human aided decomposition, capturing waste material streams as a resource, and sequestering the carbon contained in those materials into the soil. The three compost methods described (hot compost, vermiculture and bokashi) all are processes which create selective environments, essentially excluding harmful soil organisms. Hot composts especially, can sterilise weed seeds, destroy many plant pathogens, and can even break down chemical residues like some pesticides from conventional agriculture practices due to the high temperatures present. Often involving community members through voluntary work, urban composting also contributes to education and knowledge about waste streams, soil, food, and human health (Diprose et al., 2023).

Read More
Bokashi bucket, full and showing typical beneficial fungi CC BY-NC-SA 2.0 by Neal Foley via Flickr.
Finished compost at Kelmarna Community Farm’s “Soil Factory”, Auckland, New Zealand. Photo provided.

Technical requirements

An appropriate compost method should be selected for the context of an urban compost project, including space available, waste material streams available, and capacity to manage composts effectively, leveraging community. The factors that determine a successful compost like material ratios, moisture content and temperature must be adhered to mitigate potential issues such as odor and pests, while optimising microbiological activity. Local regulations should be followed, especially health, safety, and environmental standards. Some urban compost initiatives undertake extensive monitoring and testing, especially for compost quality (including nutrient balance), microbiological activity, contaminants, and value metrics. Additional monitoring might be required by some quality and certification schemes, like organic certification. Community engagement both through direct participation and outreach to neighbouring communities can help with participation and support. There must be a utilisation of the produced finished compost in a valued and safe way.

Issues and Barriers

Pathogens (both human and otherwise) can pose human health risks, as well as issues with plants, with some capable of withstanding high temperatures, like the tobacco mosaic virus (Raabe, n.d.) potentially damaging crops and being perpetuated in composts. Similarly, many chemicals survive the composting process, and can be persistent in soils, including pesticides such as clopyralid and PFAS found in the lining of some compostable packaging products. Bioplastics and compostable packaging are a significant issue and often do not break down completely during the composting process, or still contain plastic that degrades the finished compost. Microplastics are a significant issue in the food system generally, with composts being one of the entry points for plastic contamination to enter the soil, water, and food stream.

Regulations on composting imposed range from place-to-place, but can restrict the types and scales of composting due to concerns about health impacts, environmental and nuisance concerns (such as odor). There can be a public aversion to the sights and smells of composting, especially food, which can deter participation in composting programmes. Compost is only as nutritious as the materials it is made from, so in many parts of Te Moananui Oceania, where soils are depleted and food may be imported and of low nutrient density, the quality of compost may have a lowering nutrient status, and reducing it’s effectiveness as an agricultural input. There is ongoing debate about centralised composting on a city or regional scale and decentralised, community-based composting such as urban composting. Both likely have merits and challenges relating to scale, efficiency, and community outcomes.

In Te Moananui Oceania, Indigenous concepts often emphasis the cyclical nature of materials, and the innate living force that is within things, known is mauri (life-force) to Māori in Aotearoa New Zealand. This way of looking at material is often overlooked in waste policy making (Diprose et al., 2023). Modern lifestyles have led to a disconnect from natural processes like decomposition, which can further effect peoples’ willingness to engage.

Read More

Opportunities

Urban composting can highlight wider opportunities to adopt circular waste practices, including composting but also recycling and reuse (Diprose et al., 2023; Platt, 2017) in Te Moananui Oceania. Engaging communities in collective action to perform real, tangible tasks that mitigate waste but also have measurable climate change impacts are a key strategy for community adaptation, education, and hope. Local, small-scale solutions are readily implementable and actionable, reduce the need for transport of material and can develop circular material and nutrient flows on a small scale, in neighbourhoods, cities, or regions.

Incorporating Indigenous principles into waste management provides the opportunity to align modern waste practices with ecological knowledge, and to contribute to the perpetuation of Indigenous knowledge practices. Education and behaviour change are provided by urban composting as people involved in volunteer or training capacities build up knowledge, inspiration and skills. The area of composting is also one of technological innovation, with groups developing contained, mechanised solutions to composting that reduce labour requirements as well as technologies such as biodigesters, that can produce other useful resources like bio-gas to reduce reliance on imported resources. Within policy and infrastructure development there is also a key opportunity to build a synergy between centralised and decentralised compost solutions that allow capture of organic waste material in the most beneficial way possible.

Read More
Food waste hierarchy from ILSR
Vermin proof compost boxes for ‘in vessel’ hot composting. Photo provided by Kelmarna Community Farm.

Financial case

Linear solutions to waste, such as landfills are associated with high costs, both monetary and environmental. Mismanagement of waste has social and environmental implications (Diprose et al., 2023). Comparatively, urban composting projects as a nature-based solution to waste have relatively low set-up costs for infrastructure, can be more agile and scalar, responding to change in waste production in local communities, and provide numerous valuable benefits, both economically and socially.

Compost projects can generate immediate income through the sale of compost, a valuable source of fertility for agriculture. Indeed, in many parts of Te Moananui Oceania, agriculture is one of the main sources of income and a major source of (Pierre-Louis et al., 2021). Urban composting projects can create green jobs, and train people in key skills related to community waste management. A specific financial case for an urban composting projects is unavailable in Te Moananui, but an evaluation by De Boni et al. (2022) provides a financial case study in southern Italy, showing that an initial investment and infrastructure cost of NZD$424,800 (€236,900) could expect to be paid off after 6 or 7 years. This project sold compost at NZD$1.80 (€1 per kilo) to generate income.

Read More
References
  • Ayilara, M., Olanrewaju, O., Babalola, O., & Odeyemi, O. (2020). Waste Management through Composting: Challenges and Potentials. Sustainability, 12(11), 4456. https://doi.org/10.3390/su12114456
  • Bakshi, N., Chicca, F., Brown, A., & Fletcher, M. (2021, July 13). Facilitating community gardening and urban agriculture. Open Access Victoria University of Wellington | Te Herenga Waka. https://doi.org/10.26686/wgtn.14973894.v1
  • Bokashi. (n.d.). Compost Collective. https://compostcollective.org.nz/compost-factsheets/
  • Celidwen, Y., & Keltner, D. (2023). Kin relationality and ecological belonging: A cultural psychology of Indigenous transcendence. Frontiers in Psychology, 14, 994508. https://doi.org/10.3389/fpsyg.2023.994508
  • Cropp, A. (2023, April 9). Councils are transporting food scraps hundreds of kilometres as NZ tries to avoid dumping 350,000 tonnes of food waste into landfills each year. Stuff NZ. https://www.stuff.co.nz/business/131678424/councils-are-transporting-food-scraps-hundreds-of-kilometres-as-nz-tries-to-avoid-dumping-350000-tonnes-of-food-waste-into-landfills-each-year
  • De Boni, A., Melucci, F. M., Acciani, C., & Roma, R. (2022). Community composting: A multidisciplinary evaluation of an inclusive, participative, and eco-friendly approach to biowaste management. Cleaner Environmental Systems, 6, 100092. https://doi.org/10.1016/j.cesys.2022.100092
  • Diprose, G., Dombroski, K., Sharp, E., Yates, A., Peryman, B., & Barnes, M. (2023). Emerging transitions in organic waste infrastructure in Aotearoa New Zealand. New Zealand Geographer, 79(1), 15–26. https://doi.org/10.1111/nzg.12348
  • Feary, A. (2011). Restoring the Soils of Nauru: Plants as Tools for Ecological Recovery [Open Access Te Herenga Waka-Victoria University of Wellington]. https://doi.org/10.26686/wgtn.16992997
  • Pierre-Louis, R. C., Kader, Md. A., Desai, N. M., & John, E. H. (2021). Potentiality of Vermicomposting in the South Pacific Island Countries: A Review. Agriculture, 11(9), 876. https://doi.org/10.3390/agriculture11090876
  • Platt, B. (2017, April 4). Hierachy to reduce food waste and grow community. Institute for Local Self-Reliance. https://ilsr.org/food-waste-hierarchy/
  • Raabe, R. D. (n.d.). The rapid composting method. University of California Vegetable Research and Information Center. https://vric.ucdavis.edu/pdf/compost_rapidcompost.pdfWorm farming. (n.d.). Compost Collective. https://compostcollective.org.nz/compost-factsheets/

Further resources: