Singapore is dumping more than 740,000 tons of food waste every year and the recycling rate has never exceeded 20%. Some people are redistributing ugly food while others are looking to innovative ways to use leftover produce. Regardless of whether people are using artificial intelligence (AI), smart sensors, mobile apps, or other means of reducing food waste, it remains one of the biggest waste streams in Singapore. As part of the overall solution to manage food waste, NUS Associate Professor Tong Yen Wah’s research group has been looking into recovering energy and resources from it. Apart from technology process parameter optimization, life cycle assessment and cost-benefit analysis to evaluate the sustainability of the technology, as well as the social and psychological studies to encourage residents to participate in food waste recycling, have also been the focus of the Team. One of which is the development of an on-site self-sustaining anaerobic digester (AD) system that recycles food waste to produce electricity and fertilizers.

Figure 1: Mobile anaerobic digester in a 20-ft container

Food waste to Electricity and Fertilizers

AD is a process of breaking down organic matters, such as food waste by microorganisms, in the absence of oxygen. Through this process, biogas and digestate are generated. Biogas comprises mainly methane (CH₄) and can be used directly as cooking fuel or for the generation of electricity. Besides, nutrient-rich digestate (a by-product of the AD process) can be used as a fertilizer to grow commonly consumed leafy vegetables, such as Xiao Bai Cai (Brassica rapa). In addition to sustainable agriculture, the Team is also investigating the suitability of digestate to cultivate protein-rich microalgae.

The Team has also been looking for ways to further enhance biogas production. For example, a compact three-stage AD bioreactor was designed and shown to enhance biogas production by 24% to 54%. Additives, such as activated carbon and biochar have also been added to enhance biogas production. Currently, the Team is focusing on thermophilic AD, i.e. operating at 55 °C, of food waste to further increase biogas production, reduce the footprint of food waste treatment facilities, increase sludge reduction rate, and also to produce sludge low in pathogens.

Figure 2: A 1,000 L anaerobic digester for pilot-scale testing

Currently, the Team is funded by the Ministry of National Development (MND) and works together with National Environmental Agency (NEA) to scale up the AD system at East Coast Lagoon Food Village (ECLFV), which targets to treat all the food waste generated at ECLFV. The electricity generated by the system will be sent back to support the ECLFV electricity needs, while the digestate will be collected by NParks to use as biofertilizer. The system at ECLFV is planned to be up and running in early 2021.

Figure 3: Microalgae cultivation using digestate

Exploring other waste to resource methods

Like all other technologies, AD is not a silver bullet for all food waste problems. Beside AD, the Team is also exploring other methods of converting food waste to resource. Food waste is being tested to produce organic acids, which can potentially be utilized in various applications such as the production of biofuels and bioplastics. Part of the work involves optimization of operational conditions, improving the process conversion efficiency, and looking for ways to effectively recycle the clean organic acids.

Article contributed by Associate Professor Tong Yen Wah

For more information, please contact:
Associate Professor TONG Yen Wah
E-mail: chetyw@nus.edu.sg

References:

1. Zhang, J., Loh, K.C., Li, W., Lim, J.W., Dai, Y., Tong, Y.W., 2017. Three-stage anaerobic digester for food waste. Appl. Energy 194, 287–295.
2. Zhang, L., Lim, E.Y., Loh, K.C., Ok, Y.S., Lee, J.T.E., Shen, Y., Wang, C.H., Dai, Y., Tong, Y.W., 2020. Biochar enhanced thermophilic anaerobic digestion of food waste: Focusing on biochar particle size, microbial community analysis and pilot-scale application. Energy Convers. Manag. 209.
3. Cheong, J.C., Lee, J.T.E., Lim, J.W., Song, S., Tan, J.K.N., Chiam, Z.Y., Yap, K.Y., Lim, E.Y., Zhang, J., Tan, H.T.W., Tong, Y.W., 2020. Closing the food waste loop: Food waste anaerobic digestate as fertilizer for the cultivation of the leafy vegetable, xiao bai cai (Brassica rapa). Sci. Total Environ. 715, 136789.
4. Lim, E.Y., Tian, H., Chen, Y., Ni, K., Zhang, J., Tong, Y.W., 2020. Methanogenic pathway and microbial succession during start-up and stabilization of thermophilic food waste anaerobic digestion with biochar. Bioresour. Technol. 314.