According to Hong Kong government statistics, an average of 15,637 tonnes – more than 1,000 doubledecker buses’ weight – of solid waste was disposed of in landfills each day in 2019.
A mean of almost 240 tonnes of this daily total was made up of unavoidable by-products from water and sewage treatment processes. Although a sludge treatment facility was built to alleviate the growing pressure faced by landfills, it can only process some of the sludge.
Dr Chris Tsang Yiu-fai from the Department of Science and Environmental Studies at The Education University of Hong Kong and his research team have come up with an ingenious way of solving this problem. They have been exploring the possibility of reusing and recycling the waste residues into ecoconstruction materials. This eases pressure on waste treatment facilities and reduces energy consumption.
By using waste sludge and combustion by-products from water and wastewater treatment plants, as well as coal-fired power stations, they have produced eco-concrete paving blocks. Instead of sand, which is typically used in making concrete, they have added bottom ash, fly ash, waterworks sludge and sewage sludge. “This method has a two-way benefit: it reduces the use of natural resources and upcycles metropolitan waste,” Dr Tsang explained.
The engineering performance, including compressive strength, of the eco-concrete blocks complies with the General Specification for Civil Engineering Works in Hong Kong and the toxicity characteristics meet both Hong Kong and United States standards. Not only that, the eco-concrete blocks outdo other ecoconstruction blocks for a number of reasons: pre-treatment, transportation and storage costs are lower; and supply and quality of these waste residues are stable.
At the 2020 International Innovation and Invention Competition in Taiwan, the project won Silver Medal; and the Silver Medal at the 2021 International Exhibition of Inventions of Geneva. As a pilot project supported by the Water Supplies Department (WSD), the eco-concrete blocks derived from waterworks sludge will be used to construct a stretch of road next to the buildings at two local schools, one NGO, as well as the WSD visitor centre.
The next step will be to develop a standard treatment for unavoidable by-products from drinking water purification and waste recycling processes, and collaborate with commercial partners to produce this inexpensive source of eco-construction materials at an industrial level.