Construction waste may be the greener, smarter way to cover up Hong Kong’s landfills
Professor’s plan also tackles increased refuse generated from work sites because of recent rise in development projects
A veteran soil mechanics expert says he has found a greener, smarter way to cover the mountains of waste piling up in Hong Kong’s landfills – by using more waste.
Taking construction waste from work sites and laying them on top of regular municipal rubbish as a “final cover” could also free up valuable space at landfills, said Charles Ng Wang-wai, CLP Holdings Professor of Sustainability at the University of Science and Technology.
Ng’s suggestion comes as the Environmental Protection Department’s latest statistics show another increase in construction waste sent to landfills in 2016.
The quantity of overall construction and demolition waste disposed of at landfills climbed to 1.62 million tonnes in 2016 or 4,422 tonnes daily, 5.3 per cent more than in 2015.
The rise was caused by more construction projects taking place. The gross value of such works also increased by 5.6 per cent from 2015.
“More than 25 per cent of landfill space is taken up by construction waste right now,” Ng said. “We can take a bit of that, around 5 to 8 per cent, to use as a final cover.”
Ng and his team of researchers developed and patented a three-layer landfill cover system that can be applied to the city’s landfills when operators decide to cover them up. Such work is likely to be carried out in phases.
Thirteen of 16 landfills in the city have already been covered. The three remaining ones in operation will also have to be sealed in a number of years, Ng said.
His design involves burying mounds of solid waste under multiple layers of coarse and fine ground construction waste.
The design would allow waste mounds to be piled at a steep 30-degree angle in landfills, maximising space in the surrounding areas.
“It’s a bit like building a sandcastle,” he said. “The system is theoretically sound to prevent water infiltration and minimise [greenhouse] gas emissions from a landfill.”
Traditional landfill covers are made with a layer of clay which is lined with a synthetic “geomembrane” and vegetated topsoil.
Ng said synthetic membranes were susceptible to defects and deterioration that could cause soil cover to slide, affecting the stability of the slopes. Maintenance and replacement costs were also very high and fills cannot be stacked too high or steep.
“Using construction waste ... as a cover, is more environmentally-friendly, safer, cheaper and more waterproof,” he said. His team has been conducting more than a year of field trials at the Xiaping landfill in Shenzhen using covers made of recycled concrete.
About 93 per cent of construction waste generated in Hong Kong is inert – sand, bricks and concrete – and is used as public fill for land reclamation, most of which is stored at fill reception facilities or transferred to works for reuse.
The reuse rate of inert materials has remained at above 90 per cent in recent years, despite falling demand for local reclamation. Surplus fill material is shipped to Taishan in Guangdong province. About 100 million tonnes of fill had been delivered there by the end of 2016.
The rest of construction waste is non-inert – bamboo, plastics, glass, wood, paper, vegetation and other organic materials – and sent to landfills.
Yet, this small amount now takes up nearly 30 per cent of space at landfills. Operators are already using thin layers of inert waste to fill small voids in mounds to reduce odour and prevent rain infiltration.
Dr Wilson Lu Weisheng, who studies construction waste issues at the University of Hong Kong’s department of real estate and construction, said the increase in absolute volume of construction waste generated and the ratio sent to landfills in recent years, was alarming.
“We need to be looking at greener construction methods that generate less waste,” he said.
The department expects the increase in construction waste disposal charges last April to lead to reduced disposal rates. But Lu is sceptical about its efficacy.
According to his analysis of the public records of construction waste sent to landfills fortnightly in 2017, there was a slight drop in volume in the run-up to the new disposal charges in April. But since then, Lu said it had been “business as usual”, with the monthly rate increasing steadily from 110,000 tonnes in April to 120,000 in December.
“This is quite embarrassing,” he said. “Hong Kong has to be vigilant in dealing with this waste crisis by either finding new landfill space, reducing its waste sent to such sites, or both.
The Environmental Protection Department said measures were in place to encourage reduction and reuse of construction waste. These include requiring companies to implement material management plans for projects and for contractors to carry out on-site sorting to reduce the amount of waste sent to landfills.
In the private development sector, methods such as selective demolition, modular building design and pre-casting of building components were already being applied, the spokesman said.