Could nutrition-packed, freshly-picked local produce, grown using energy and fertiliser made from food waste become the norm in cities of the future? Helen Theaker, Mark Walker and Rokiah Yaman have been exploring a circular organic model that could yield social, environmental and economic benefits.
Background
Every year, an estimated 10 million tonnes of food waste in the UK is thrown away – including 5.7 million tonnes from households. In the move towards a low-carbon society, managing this problem needs to be a priority. The best solution is to avoid creating waste, but some cannot be avoided and other solutions need to be explored. Food waste collection by local councils is continually rising and is now provided to 100% of households in Wales and Northern Ireland, 91% in Scotland and 52% in England. This means that diverting food waste away from landfill is becoming more feasible, opening the door to other treatment options, which can help to realise the inherent value contained in organic waste.
An alternative to landfill and the more energy-efficient option of ‘energy from waste’ (normally incineration) is anaerobic digestion (AD). This is a renewable energy system that can be used to break down ‘wet’ organic matter in a controlled way, which allows the products to be captured. Energy is released in the form of biogas, made up of methane and carbon dioxide, which can be collected and used for cooking, electricity generation, heating, transport, or can be upgraded and injected into the national gas grid. A nutrient and fibre-rich fertiliser, known as digestate, is also produced, which can be returned to farmland to boost the health of the soil and replace artificially produced fertilisers.
It has been widely reported that the concentration of nutrients in our fruit and vegetables has declined since the 1950s[1], most probably due to a combination of early harvesting, longer transportation journeys, extended storage times, crop selection for disease resistance and reduced nutrients in the soil.
An innovative system of circular organic resource management could help to address these issues. AD can form an important part of this system, where instead of losing nutrients and energy when food is thrown away, it is put back into the soil, allowing it to re-invigorate. At the same time, emissions from waste in landfill are avoided and renewable energy and heat are created.
There is currently 430 MW of AD installed in the UK, providing about 1.4% of the UK’s electricity demand, with 40% of this coming from foodwaste. However, if all of the food waste produced in the UK was processed by AD, it could supply up to 4% of our electricity demand and satisfy 5% of our fertiliser requirements. In general, AD facilities are, by economic necessity, large-scale plants located in the countryside away from built-upareas. However, there is a growing interest in building smaller plants in urban areas, in an effort to capture more of the available food waste and reduce transport miles.
The micro-AD systems used by LEAP are around 1/1000th the size of their industrial counterparts but are still able to convert food waste into valuable products by the same process.'
The LEAP project
Established in 2012, LEAP (Local Energy ADventure Partnership) has developed a decentralised, urban closed-loop model, where food waste is processed on-site using microscale AD. Biogas is used to supply local energy needs, while the digestate replaces artificial fertiliser supporting local, intensive urban agriculture. The micro-AD systems used by LEAP are around 1/1000th the size of their industrial counterparts but are still able to convert food waste into valuable products by the same process. LEAP’s vision is clear: prioritise local organic resource management to create a recipe for low carbon, resilient, economically viable food production.
In moving towards the organic circular economy, LEAP seeks to embrace not only environmental and economic aims, but also social aspirations, including public engagement, educational, training and employment benefits (Figure 1). Its initial focus on the viability of micro-AD in the urban context addressed issues of odour, cost, user friendliness and lack of food growing space.
During 2012-13, LEAP compiled two feasibility reports. The first focused on the potential for a micro-AD network in the London Borough of Camden and how resources, such as technical expertise and smart data analysis, could be shared. The second narrowed down twelve sites identified in the first study and described four detailed scenarios for implementation.
Currently, three pilot plants have been established, with two in Central London up and running and the third nearing commissioning. Each plant is designed to test different equipment configurations as well as various end-uses of biogas and digestate.
The first plant in Camley Street Natural Park, near Kings Cross (Figure 2), sits overlooking a canal in a beautiful nature reserve. Zero carbon collections within a 1-mile radius use a cargo bike to transport catering and office food waste to the digester (Figure 3).
Biogas is used for cooking on-site by Wildwood Community Café – set up as an intrinsic part of the closed-loop model, while café food waste is fed to the digester. Digestate is used as a fertiliser for food growing on a nearby railway embankment site and the produce supplies the café.
The second plant is situated at the Calthorpe Project, a community centre and garden with an established food-growing area. Here, another closed-loop café has been set up, feeding the digester with food preparation waste, while biogas is used on-site for cooking and generating heat. In colder months, the growing season will be extended in the polytunnel, where hydroponic experiments are underway to see whether soilless or soil-based growing is the way forward for using digestate in limited urban spaces (Figure 4).
The third plant in East London is part of R-Urban, an EU funded project developed in conjunction with public works, an architect and artist collective that specialises in participatory design. The AD system was assembled in a shipping container by participants over a series of knowledge-sharing workshops. This system, sited on a social housing estate as part of a community reuse resource that will include a tool library, workshop space, café and community energy hub, will soon be commissioned (Figure 5).
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Figure 1. A closed-loop visualisation for Camden’s Future Cities Feasibility Study
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Figure 2. LEAP's first pilot system at Camley Street Natural Park
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Figure 3. Zero carbon food waste collections using a cargo bike
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Figure 4. Hydroponic experiments in the Calthorpe polytunnel
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Figure 5. Learning together - practical, participatory workshops attract like-minded individuals across London
Case study
The 2m3 AD plant at Camley Street was commissioned in October 2013 and was monitored by researchers from January to November 2014. The plant was profiled in terms of its stability, ability to digest waste and its energy usage in comparison to larger AD facilities.
The study was the first detailed assessment of a community-based micro-AD in the UK and was recently published by the University of Sheffield in Waste Management Journal[2]. At its target feeding rate, the AD system was fed on an average of 24kg a day of food waste, mostly from local businesses.
As the project progressed, maintenance and improvements were made to the plant to keep it running smoothly. This included the automation of the feed system, which gave the plant a predictable and regular feeding pattern, and the development of a bespoke boiler and hob to burn the gas.
Many measurements were taken from the AD plant during the study, which allowed researchers to confirm its stable and successful operation. In fact the micro-scale system showed very similar operational characteristics to a standard large-scale facility. However, the energy balance of the plant was very different because, unlike a standard large digester, the plant was housed indoors. Anaerobic digesters typically need to be run at 37-40°C, but the heating required by the plant was far less than normal. The carbon emissions savings were calculated and are broken down in the illustration in Figure 6 (p31). The largest carbon saving is from the diversion of waste from landfill, followed closely by the production of green energy, which displaces energy from fossil fuels.
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Figure 6. Carbon emissions savings from the Camley Street AD plant
Lessons learned
Five years into the project, a number of very valuable lessons have been learned from the research carried out:
• Digestate is the single biggest challenge for managing organic waste with AD in the urban context. While this is the case across all scales of AD, cities pose specific challenges with limited growing space and the high value of land. Despite having a number of potential options for digestate deployment, managing the volume produced in real time requires good planning and knowledge of the most effective ways of utilising this valuable byproduct.
• User friendliness of micro-AD systems is key to their success. AD is already a more complex process than composting, so simplifying user operation through good design is essential. Having straightforward procedures, monitoring and maintenance routines makes it easier to maintain a healthy and efficient biological process.
• Odour is another key issue, which good design can help eliminate. Housing digesters within a structure not only helps minimise heat losses but also keeps odour during operation inside the building. Hygiene protocols also reduce smells and maintaining steady operation of the system makes a difference, as irregular feeding causes instability, which can increase odour generation.
• Good engineering design and training are both critical to the success of a plant. While the technology is relatively simple, it benefits hugely from AD design experience and the operator understanding the process, since mistakes can easily be made without knowledge of the subtle interactions between feedstock, process, design and operation. LEAP has found that its operators can become effective trouble-shooters, especially when given ongoing training from experienced AD engineers.
Tomorrow’s circular organic scenarios
Organic resources are too valuable to be wasted in landfill. The potential exists to implement smart urban solutions that integrate the waste, energy and food sectors. Although this would require changes to current systems and infrastructure, LEAP has demonstrated it to be feasible, practicable and capable of providing great benefits, both social and economic. It boils down to a matter of will - a change in thinking at the political, institutional, business and individual level.
LEAP is engaging with researchers from a broad range of fields and with many potential stakeholders. Some examples of potentially feasible scenarios emerging from this work include:
1. Social housing flats and new developments where recycling rates are typically low could incorporate floor-by-floor collections for food waste. This waste digested on-site could supply gas to heat communal areas and/or a polytunnel with food grown using digestate. Energy and waste management savings could finance running costs and residents could be incentivised to separate food waste with the offer of low cost organic food grown on-site.
2. Supermarkets could utilise car parking space to manage unusable food waste on-site. Coupled with intensive food growing in on-site polytunnels, this could provide training and employment opportunities for local people and supply fresh produce to the store. Biogas could be used to power an in-store refrigerator or even a community fridge.
3. Municipal parks have a range of suitable organic waste streams from cafes, offices and horticulture. These could be digested and the liquid fertiliser applied to land for turf strengthening, amenity horticulture and food production, while the biogas could be used to power a park vehicle or provide heat/electricity for park buildings.
4. Office blocks with flat roof space could install rooftop greenhouses or vertical growing walls for
hydroponics, which could supply fresh food to local cafés and restaurants. The structures would also help insulate the building reducing energy costs. Biogas produced from local food waste could be used to generate electricity, heating and cooling to regulate the greenhouse environment as well as providing carbon dioxide to boost plant growth.
5. The final example translates the model into the refugee camp context. Designed for temporary deployment, the average lifespan of a camp is now eight years with some pushing over six decades. Infrastructure in these camps is typically poor and lack of resources means that achieving a good quality of life for residents is a constant struggle. A closed-loop food-energy-waste solution here would allow human waste to be sanitised and converted into energy, fertiliser and food, while providing valuable opportunities for learning, improved health, income generation and nutrient recovery.
Helen Theaker, PhD student, University of Sheffield
Dr Mark Walker, Energy2050 Researcher, University of Sheffield
Rokiah Yaman MA, Director, LEAP micro AD, London
LEAP is looking for partners to help develop this circular organic resource model to achieve both commercial and humanitarian aims. We can provide technical expertise and excellent academic contacts. If you are moving in the same direction and could contribute to the process of realising this vision, do get in touch:
1. Davis, D. R., Epp, M. D. & Riordan, H. D. 2004. Changes in USDA food composition data for 43 garden crops, 1950 to 1999. Journal of the american College of nutrition, 23, 669-682.
2. Walker, M., Theaker, H., Yaman, R., Poggio, D., Nimmo, W., Bywater, A., Blanch, G. & Pourkashanian, M. 2017. Assessment of micro-scale anaerobic digestion for management of urban organic waste: A case study in London, UK. Waste Management, 61, 258-268.
