Self-sustaining system recycles food waste to produce electricity, heat and fertilisers
Researchers at the National University of Singapore (NUS) are waging war against food waste. Led by Associate Professor Tong Yen Wah, who is from the Department of Chemical and Biomolecular Engineering at NUS Faculty of Engineering and Co-programme Director of E2S2-CREATE, researchers from NUS and Shanghai Jiao Tong University have jointly developed an anaerobic digester system that recycles food scraps to produce electrical energy and heat. This digester system is self-sustaining as the generated electricity and heat fully power the system and its processes.
In Singapore, food waste reportedly accounts for about 10 per cent of the total waste generated in the nation, while the recycling rate of such wastes is currently at about 14 per cent.
“We want to play our part in reducing waste in Singapore. Our digester system is easy to operate, and we can now generate electricity, heat and fertilisers from food waste that would otherwise be disposed of. All the processes in the system can be easily controlled and monitored to ensure optimal performance and safety. For instance, we have sensors that are programmed to send out end-of-process updates and flag any safety concerns in real-time directly to the team via mobile phone alerts,” said Assoc Prof Tong, who is also a Principal Investigator at NUS Environmental Research Institute (NERI).
From food waste to useful resources
“An anaerobic digester works like a biochemical stomach that breaks down organic matter in an oxygen-free (anaerobic) environment. Using a specially formulated mix of anaerobic micro-organisms, the digester system efficiently breaks down food waste into biogas that is subsequently converted into heat and electrical energy,” explained Dr Zhang Jingxin, Research Fellow at NERI, who designed the system.
The heat is reused to produce hot water that gets channelled back into the jacket layer surrounding the anaerobic digester tank to ensure that the digestion is maintained at an optimum working temperature of about 50 degree Celsius. From the control computer, sensors and lights, to the engines, pumps and ventilating fans, every component of the system runs entirely on electricity generated from the self-contained system. Excess electrical energy is stored in batteries which can be used to power or charge electronic devices such as mobile phones and tablets. Based on the team’s calculations, one tonne of food waste can generate between 200 kilowatt-hours (kWh) to 400 kWh of electricity, depending on the composition of the food waste. For instance, food waste that is high in carbohydrate, protein and fat content produce more biogas, thus yield more electrical energy.
The anaerobic digester also converts about 80 per cent of the food waste fed into the system into nutrient-rich digestate, which can be processed to produce liquid fertilisers for farming and horticultural needs.
Mobile digester to meet demand for food waste recycling
The researchers have developed a mobile anaerobic digester system which is currently being piloted at Raffles Hall, one of the six halls of residence of NUS, for about six months beginning from late January 2018.
The system is housed in a 20-foot container and can be purposefully carted off to venues to fulfil ad-hoc demands for a food waste recycling facility. This mobile unit can be parked within two outdoor parking lots, and has solar panels on its roof to harness solar energy as a backup power source and a tank for collecting rain water. The team is targeting to process up to 200 kilograms of food waste daily and will be using the system to test out different configurations. Excess electrical energy generated by the system is channelled to a mobile charging station where students can charge their mobile phones and tablets for free.
The researchers from NUS are also working on a larger, stationary anaerobic digester system to cater to the needs of a canteen or food centre. This system would be able to process up to 400 kilograms of waste food daily.
“Unlike composting which is used in a lot of commercial waste food digesters, anaerobic digestion is relatively odourless which makes this approach suitable for an urban city environment. Our system removes moisture and trace gases such as hydrogen sulphide that gives food waste its unpleasant smell,” explained Assoc Prof Tong.
Currently, the team is closely monitoring and studying the feasibility and outcomes of the two anaerobic digester systems. They are also working with other NUS research groups to expand the project, such as processing the digestate into suitable fertilisers for horticulture and crop farming.
“We are keen to see whether this model could work in Singapore’s public residences. The anaerobic waste digester has the potential to help towns reduce waste and greenhouse gas emissions, as well as be more energy and resource efficient,” said Assoc Prof Tong.
The team is also looking to expand their project in China as well.