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Hong Kong Scientists Pioneer Ultrafast Mesh Bioreactor for Wastewater Treatment

A team of researchers from the Hong Kong University of Science and Technology (HKUST) has developed a groundbreaking wastewater treatment technology that promises to transform how cities and industries manage water. By integrating a mesh bioreactor with an ultrasound-induced transient cavitation cleaning mechanism, the system delivers ultrafast filtration, high efficiency, and reduced operational costs, offering a sustainable solution for wastewater challenges worldwide.

The Challenge of Conventional Wastewater Treatment

Wastewater treatment is essential for public health, environmental protection, and sustainable urban development. Most secondary wastewater treatment systems today rely on membrane bioreactors (MBRs). These systems use aerobic or anaerobic microorganisms to break down organic matter in wastewater and separate the treated water from suspended solids. While effective, conventional MBRs face persistent challenges:

  • Membrane fouling: The buildup of solids and microbial biomass on membranes reduces efficiency.

  • Frequent cleaning and replacement: Fouled membranes require chemical cleaning or replacement, increasing costs.

  • Energy consumption: Maintaining the filtration process and cleaning the membranes consumes significant energy.

For context, Hong Kong’s Drainage Services Department requires that secondary-treated effluent must have total suspended solids (TSS) of 30 mg/L or below. While MBRs can meet this standard, the fouling problem often makes operations expensive and energy-intensive.

HKUST’s Innovative Solution: The Mesh Bioreactor (MeBR)

The HKUST team, led by Prof. Chen Guanghao, Chair Professor of the Department of Civil and Environmental Engineering, along with Dr. Guo Hongxiao and Ph.D. student Luo Yu, designed a mesh bioreactor (MeBR) that addresses these limitations. Their research, published in Nature Water, is titled “Transient cavitation enables ultrafast fouling removal in mesh bioreactors for efficient sludge–liquid separation during wastewater treatment.”

The MeBR uses mesh material with a pore size of 10–200 μm to separate water from solids. Unlike traditional MBRs that rely on pressure to push water through dense membranes, MeBRs form a self-generated biocake on the mesh surface from retained solids and microbial biomass. This biocake acts as a natural filtration layer, improving water separation while reducing clogging.

Ultrasound-Induced Transient Cavitation: The Key Breakthrough

What makes this system revolutionary is its ultrasound cleaning mechanism. The MeBR incorporates piezoelectric ultrasound transducers, which create millions of microbubbles that rapidly form and collapse—a process known as transient cavitation. This action:

  • Removes fouling on the mesh surface almost instantly.

  • Enables complete mesh cleaning within 10 seconds under aerobic conditions and as quickly as 3.8 seconds under anaerobic conditions.

  • Maintains mesh integrity over long-term operations, reducing replacement costs.

This combination allows the system to operate 10–20 times faster than conventional MBRs, while using significantly less energy.

Outstanding Performance Metrics

The HKUST MeBR system has demonstrated remarkable performance in both laboratory and real-world tests:

  1. High Flux: Each square meter of mesh can process 148–307 liters per hour, achieving 10–20 times the flux of traditional MBR systems.

  2. Superior Effluent Quality: The treated water consistently maintains TSS below 20 mg/L, surpassing Hong Kong’s standard and meeting global discharge requirements in around 75% of countries.

  3. Energy Efficiency: The system consumes only 2.5–47 Wh per cubic meter, a fraction of what conventional systems require, drastically reducing operational costs.

  4. Durability: In 120 days of continuous filtration and a 21-day municipal wastewater trial, the mesh maintained structural integrity. Minor physical changes such as slight variations in pore size and surface roughness did not compromise performance.

According to Luo Yu, the first author and Ph.D. student at HKUST, “The meshes retained their mechanical stability even after long-term operation, demonstrating the durability required for practical wastewater treatment applications.”

Advantages Over Conventional Systems

The MeBR technology offers multiple advantages over existing wastewater treatment methods:

  • Ultrahigh fluxes: Water passes through the system much faster, reducing the time and resources required for treatment.

  • Rapid biocake removal: Cavitation ensures the biocake does not hinder performance, maintaining stable water quality.

  • Lower energy consumption: Less energy is needed for filtration and cleaning, which reduces carbon footprint and operating costs.

  • Cost-effective: The system can reduce treatment costs to approximately 50% of conventional MBRs, translating to $0.05 per cubic meter of treated wastewater.

Dr. Guo added, “The ultrahigh fluxes shorten the biocake reformation period to under 10 minutes, overcoming a longstanding challenge in high-efficiency wastewater treatment and ensuring stable effluent quality.”

A Solution for Global Urban Challenges

As cities face rising energy costs, increasing wastewater loads, and climate change pressures, efficient wastewater management is more critical than ever. Prof. Chen emphasized, “Our technology demonstrates that high-flux treatment can be achieved while still meeting stringent global discharge standards. This could reduce the burden on existing treatment facilities and provide a more flexible solution for densely populated urban areas.”

By offering a scalable, cost-effective, and energy-efficient solution, the MeBR system can help municipalities and industries treat more wastewater with fewer resources. It is particularly valuable for cities experiencing rapid population growth or limited space for new treatment plants.

Environmental and Societal Impact

The potential impact of the HKUST MeBR technology extends beyond operational efficiency:

  • Environmental Protection: Cleaner effluent reduces the risk of water pollution in rivers, lakes, and coastal areas.

  • Resource Savings: Reduced energy and chemical use contribute to lower greenhouse gas emissions.

  • Economic Benefit: Cutting treatment costs can save municipalities millions annually, freeing up resources for other infrastructure projects.

  • Public Health: Reliable wastewater treatment prevents the spread of waterborne diseases and protects communities.

Prof. Chen concluded, “Our goal is to deliver research that creates real and meaningful value for society. By improving wastewater treatment efficiency and reducing costs, we hope to contribute to more sustainable cities and a healthier environment.”

Future Prospects

The HKUST team plans to further optimize the MeBR system for different wastewater types, including industrial effluents and high-strength organic wastewater. They are also exploring integration with renewable energy sources to make the system even more sustainable.

If widely adopted, this technology could redefine the standards for wastewater treatment, offering a blueprint for high-efficiency, low-cost, and environmentally friendly water management worldwide.

Reference:
Yu Luo et al., Transient cavitation enables ultrafast fouling removal in mesh bioreactors for efficient sludge‒liquid separation during wastewater treatment, Nature Water (2025). DOI: 10.1038/s44221-025-00531-7

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