Dr Chen Jianlin and the microfluidics fabricated by his team.
The global issue of food waste production has been exacerbated by economic and population growth. According to the Food and Agriculture Organization of the United Nations, approximately 1.3 billion tons of food intended for human consumption is lost or wasted annually worldwide. In the case of Hong Kong, the daily production of municipal solid waste in 2020 amounted to 10,809 tons, with 30% of that being attributed to food waste. Most of the food waste ends up in landfills. However, due to the limited landfill capacity, 13 out of 16 landfills have already been closed, leaving only 3 operational. Consequently, it is crucial for the government to reduce food waste disposal in landfills and prioritise proper waste management. To address this issue, the government has implemented various strategies, one of which involves the anaerobic co-digestion of food waste and sewage sludge, a natural process where microorganisms break down organic materials in the absence of oxygen and produce biogas that can be used as a renewable energy source. This method can enhance Hong Kong’s food waste treatment capability and reduce its reliance on landfills.
However, food waste can have a negative effect in an anaerobic digestion system if it is not managed properly. Due to its low pH, the introduction of a large amount of food waste into the system may alter the acidity level, making it more challenging for the system to effectively break down the waste and generate biogas. To mitigate potential instabilities in the anaerobic co-digestion process and enhance system efficiency, real-time monitoring of anaerobic co-digestion is essential. It enables the early detection of potential system failures and facilitates operators to make timely adjustments.
Sludge droplets generated in microfluidics
To address this issue, Dr Chen Jianlin, Assistant Professor in School of Science and Technology at Hong Kong Metropolitan University, conducted a research project supported by the Environmental and Conservation Fund. The project aimed to construct a microfluidics analytical device — a tiny tool with specially designed channels and chambers that helps scientists study small amounts of liquids — for monitoring the metabolic activity of food waste and sewage sludge anaerobic co-digestion.
Fluorescence generated in droplets
Dr Chen’s team took advantage of Polydimethylsiloxane (PDMS) — a commonly used material in microfluidic devices for droplet formation — to construct the microfluidics analytical device. The device employs a strategy that involves the inhibition of methanogenesis through the addition of Fe3+ and the assistance by a special dye known as resazurin. By analysing the intensity and changes of fluorescent signal produced due to the reduction of dye, the research team evaluated the effect of food waste on the anaerobic digestion activity. The team discovered that co-digestion of food waste with anaerobic sludge resulted in a higher level of biogas production compared to anaerobic digestion alone. Specifically, when the ratio of anaerobic sludge to food waste was 1:3, it promoted anaerobic digestion effectively.
The findings highlight the potential of food waste as a substrate for anaerobic digestion that could lead to generation of renewable energy in terms of increased biogas production. The monitoring capabilities of the analytical device could enhance the stability of the anaerobic digestion system, particularly when food waste is co-digested. The device enables the mitigation of potential issues arising from scenarios like sudden or significant changes in the feedstock or operating conditions of the anaerobic digestion system, thereby improving the overall efficiency of the system.
For more details, please refer to the following publications generated from the research project:
‘The multipurpose application of resazurin in micro-analytical techniques: Trends from the microbial, catalysis and single molecule detection assays’, TrAC Trends in Analytical Chemistry.
‘A dye-assisted paper-based assay to rapidly differentiate the stress of chlorophenols and heavy metals on Enterococcus faecalis and Escherichia coli’, Biosensors.
 Source: https://www.fao.org/3/ca6030en/ca6030en.pdf
 Source: https://www.epd.gov.hk/epd/english/environmentinhk/waste/prob_solutions/food_waste_challenge.html