Cavern wastewater treatment is emerging as a practical response to land scarcity, ageing infrastructure and rising environmental expectations in dense urban environments. In Hong Kong, this approach is being delivered at scale by relocating the Sha Tin Sewage Treatment Works into a purpose-built cavern, supported by advanced sludge dewatering systems.
SUEZ has been awarded the sludge dewatering contract for the new Sha Tin Cavern Sewage Treatment Works, a major infrastructure project being delivered by the Drainage Services Department of the Hong Kong SAR Government and scheduled for completion in 2030.
Once operational, the facility will be one of Asia’s largest underground sewage treatment plants, setting a new benchmark for compact, high-performance wastewater infrastructure.
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Why cavern wastewater treatment is gaining momentum
The existing Sha Tin Sewage Treatment Works has served around 700,000 residents for more than 40 years. While the facility remains critical to public health and sanitation, modernisation has become increasingly constrained by surrounding urban development and limited surface area.
Relocating the treatment works into a cavern addresses both challenges. Underground construction frees up approximately 28 hectares of surface land for other uses while reducing odour impacts through natural containment, enclosed processes and dedicated deodourisation systems.
For cities facing similar pressures, cavern wastewater treatment offers a way to expand or modernise treatment capacity without competing directly with housing, transport or commercial development.
The role of sludge dewatering in underground facilities
Sludge management is a central consideration in underground sewage treatment plants, where space, access, and safety constraints are particularly challenging.
At Sha Tin, SUEZ will install 16 units of its Dehydris Twist sludge dewatering system. The fully enclosed and highly automated design supports safer operation in confined environments and reduces operator exposure, while the compact footprint lowers space requirements by around 25 per cent.
This allows greater flexibility in plant layout and enables other essential equipment to be accommodated within the cavern.
Performance under local wastewater conditions
Hong Kong’s wastewater system presents additional challenges due to widespread seawater flushing, which raises sewage salinity to around 5,000 to 6,000 milligrams per litre.
To confirm performance under these conditions, the sludge dewatering system was piloted locally over several years. The trials demonstrated consistent outcomes, increasing sludge solids from around 3 per cent to no less than 30 per cent.
This level of dewatering significantly reduces sludge volumes, lowering transport and disposal requirements and associated costs, while supporting more efficient downstream treatment and recovery processes.
The system is expected to produce approximately 120,000 tonnes of dewatered sludge each year, thereby contributing to energy recovery and supporting Hong Kong’s broader circular economy and carbon-reduction goals. These outcomes place the project firmly within the wider discussion on biosolids dewatering in Asia.
Long-term collaboration and technology maturity
The sludge dewatering solution draws on more than 15 years of collaboration between SUEZ and Bucher Unipektin AG, with the technology already deployed across wastewater projects in Europe, Australia and mainland China.
Daniel Schneider, CEO of Bucher Unipektin AG, said the Sha Tin project reflects growing demand for advanced biosolids solutions capable of operating reliably under complex conditions.
“Our technology sets new standards in efficiency and sustainability, demonstrating what is now achievable in modern wastewater treatment,” Schneider said.
Applying global expertise to local challenges
For SUEZ, the project builds on decades of involvement in the Greater Bay Area, including wastewater treatment operations in Hong Kong, drinking water services in Macao and industrial water solutions across Guangdong.
Pierre Pauliac, Executive Vice President International at SUEZ, said the cavern project demonstrates how global expertise can be adapted to local needs.
“This project is an important opportunity for SUEZ to deliver its core sludge dewatering solution for a new cavern-based treatment plant, integrating our global expertise to create local value for Hong Kong,” Pauliac said.
As urban populations continue to grow and land availability tightens, projects like Sha Tin show how cavern wastewater treatment and high-performance sludge dewatering are becoming central to the future of urban sanitation infrastructure across Asia.