Hong Kong, a densely populated city where over a thousand tonnes of sewage sludge is generated per day, should develop carbon-neutral processes to turn the waste into valuable products as part of its decarbonisation strategy, according to an academic.

Ren has secured three grants from the government to conduct research on converting used medical face masks, poultry litter and sewage sludge into energy and valuable products.

To tackle the problems and lower costs, Ren said his team aims to develop processes that can incorporate multiple feedstocks and produce multiple products, such as using face masks as one of the feedstocks and upgrading the primary product into bio-oil, a clean-burning fuel.

Sludge, the mud-like by-product of sewage treatment, can be used as feedstock to make methanol, a motor fuel and industrial chemical, said Hong Kong Polytechnic University’s industrial and systems engineering professor Ren Jingzheng, the recipient of the 2022 Apec Science Prize for Innovation, Research and Education (Aspire).

The carbon emission per kilogram of methanol produced from sludge is 2.1kg, a quarter less than the 2.9kg emitted if coal is the feedstock, Ren and his research team estimated. “In mainland China, methanol is mostly produced from coal,” Ren said in an interview with the South China Morning Post. “Our process of producing it from sludge could lead to less carbon dioxide emission, but the cost will be high. We are still working on ways to reduce it.”

If all of the around 1,200 tonnes of sludge generated in the city every day is converted to 400 tonnes of methanol, this could generate 400 tonnes of methanol worth US$160,000 a day at current market prices. The process could also reduce 120,000 tonnes of carbon dioxide emission from energy consumption, compared to methanol produced from coal, Ren’s team said. It amounts to 6.7 per cent of Hong Kong’s total emissions from industrial processes and product use, and 0.4 per cent of the city’s total carbon emissions.