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Hong Kong is one of the pioneers at saving water!

Water is the source of life but rising pollution due to population growth, industrialization and urbanization threaten to cause a shortage in access to fresh water. As the situation becomes more precarious, learning how to cherish limited water resources is a problem that everyone around the world needs to ponder.

Led by Professor Guanghao Chen at the Department of Civil and Environmental Engineering, the research team from the Hong Kong University of Science and Technology (HKUST) has developed a cutting edge water recycling system. It has solved the problem of sewage treatment, helping reduce cost as well as meet the increasing demand for fresh water in Hong Kong. Not only that, it has also received international recognition for the technologies used.

problem of sewage treatment

How critical is the water pollution crisis?

Water pollution is causing a global water shortage. It is one of the problems that we must face and solve in the 21st century. How serious is the problem? Let’s understand the hidden crisis by looking at the numbers:

4 billion

4 billion people, nearly two-thirds of the world’s population, face severe water shortage for at least one month every year

2.3 billion

2.3 billion people live in countries where the water supply is inadequate and 733 million live in countries with severe water shortages, such as Pakistan, Eritrea, Turkmenistan, Botswana


By 2030, about 700 million people could be displaced by severe water shortage. In addition, conflicts have occurred in many areas with extreme and high water shortages, and water may be one of the causes of conflicts. These areas include Israel, Libya, Yemen, Afghanistan, Syria and Iraq.


By 2040, about a quarter of the world’s children will be living in areas of extreme water scarcity, for example, eastern and southern Africa


By 2050, half of the world’s population could be living in areas facing water scarcity


What about the water resources in Hong Kong?

Washing face, brushing teeth, washing hands, flushing… every detail of our life involves water. Hong Kong, with a population of more than seven million, produces two hundred and eighty million tons of domestic sewage every day. It is enough to fill 1,120 standard swimming pools every day. This is how much water we consume on a daily basis.

In order to effectively utilize water resources, Hong Kong has a seawater supply system that provides 80% of the population with seawater for flushing. At present, Hong Kong is one of the very few places in the world that exclusively uses seawater saving about 270 million cubic meters of fresh water every year.

Hong Kong also imports fresh water (Dongjiang water) from the Pearl River Delta and uses a dual pipe system to supply seawater for flushing (22% of total fresh water demand) but its maintenance is expensive.
How can we meet the demand for water? How can the resulting water pollution be controlled?

Professor Guanghao Chen’s research team studied Hong Kong’s water problems in two major directions. They provided two major solutions to the water resources problem, namely the “SANI” Process and “The Future Water Factory”. Read on to learn more about them!

The “SANI” Process

Before learning about the “SANI” process, let’s understand how wastewater is treated.

Wastewater treatment plants remove two main types of pollutants from wastewater, which arise mainly from human excreta: organic matter and nitrogen & phosphorus nutrients. Wastewater treatment plants generally use biological treatment methods like using activated sludge to purify wastewater by having microorganisms “eat” the pollutants. However, it produces sludge, which contains overgrown microorganisms, microbial carcasses and pollutants that microorganisms cannot eat. Learning how to effectively treat sludge has been a question for years.

For example, a sewage treatment plant that treats 100,000 tons of sewage per day generates 50 to 70 tons of sludge every day. Direct treatment of sludge in the plant accounts for about 30-50% of the total cost of the plant taking up space. Moreover, after digesting and dewatering the sludge inside the plant, it’s sent out for disposal. However, there are only two options.

(1) Dispose sludge to the landfill - However, it takes up land, produces unpleasant odor, greenhouse gases and leachate.

(2) Incineration - It generates fly ash and waste gases, creating another pollution problem that would then need to be addressed.

How can we treat wastewater in a less polluting and more efficient way? Professor Chen’s research team invented the “SANI” process, which is a promising solution.

The “SANI” Process

The research team is working with Delft University of Technology in the Netherlands, the University of Cape Town in South Africa and the Airport Authority, Water Supplies Department and Drainage Services Department in Hong Kong to reduce sludge production at the source during the sewage treatment process.

To achieve it, the team transforms the environment so that slow-maturing microorganisms replace the fast-maturing ones, introducing a new microbial assemblage that significantly reduces sludge production. Known as “Sulphate reduction, Autotrophic denitrification and Nitrification Integrated (SANI)”, the abbreviation (SANI) , when enunciated, means “sludge killing” in Chinese thereby describing the technology in a different language.

Professor Chen points out that the transformation of the environment is essential to the technology: “These microorganisms themselves exist in nature, we did not add them; we introduce microorganisms that do not grow even if they eat the sludge replacing the original microorganisms that fatten up dramatically changing the equation in sewage treatment.”

The team started the research as early as 2003 and then started to go outside the laboratory in 2007 to test the new technology in the Tung Chung sewage pumping station and the Shatin sewage treatment plant respectively. The results have reduced sludge volume by 70-90%. The new technology was also found to reduce odour, energy consumption and greenhouse gas emissions as well as reduce the cost of sewage treatment and the space occupied by more than 30%. It took 18 years for the research team to achieve a new breakthrough in wastewater treatment technology.

The new “SANI” process has been well-received internationally for its effectiveness in improving wastewater treatment and helping solve wastewater and sludge treatment problems around the world.

The following are some of the accolades for “SANI”.

• “This system, which combines seawater flushing in Hong Kong with “SANI” technology , is one of the most successful water management systems in the world.”-- International Water Association (IWA) :

• “The “SANI” is most suitable to be used together with seawater flushing and wastewater reuse to save a lot of fresh water and reduce sludge production providing a cheap and sustainable solution for future development of seawater flushing in China’s coastal cities due to water shortage. This will provide a cheap and sustainable solution for the future development of coastal cities in China to switch to seawater flushing due to water shortage.” ---Mogens Henze, Editor-in-Chief of the international journal Water Research and Professor at the Department of Environment and Resources of the Technical University of Denmark

• “ The “SANI” process is based on sulfur cycling which is unique for wastewater treatment normally is seen as a problem, and it makes an advantage out of the sulfur cycling. I believe that having now already a pilot plant at a larger scale pilot plant, almost demonstration scale. It’s a very rapid uptake of the technology and a rapid development. And I am very excited to see this development and see also hopefully the next step in the near future into a full scale demonstration of a full scale plant in a more commercial sense.” --- Mark van Loosdrecht, winner of Stockholm Water Prize, and chair professor in Environmental Biotechnology at the Delft University of Technology.

• The technology has been patented in China, the United States and Japan

• The team was invited to apply the technology in China, Japan, Cuba and the Netherlands, with interest from Peru, Spain, Thailand and the Philippines

• It has been recognized by UNESCO as a way to promote sustainable use of water resources

• Professor Chen has also been elected as a Fellow of the International Water Association, becoming the first Hong Kong scholar receiving this prestigious honor

"SANI" was awarded:

2019 Hong Kong Green Innovations Awards – Gold Award 2018 (the Oscar prize in the green and environment field);
2018 International Water Association (IWA) Global Project Innovation Award - Bronze Award for Breakthrough Research (the only recipient from Asia);
2017 China National Innovation Pioneer Award (the first awardee from Hong Kong);
2015 IWA Distinguished Fellow (only 3 by 2015 and 5 by today in whole China including HK, Macao and Taiwan);
2012 IWA Project Innovation Award - East Asia Honor Award in Applied Research;
2012 IWA Project Innovation Award - Global Honor Award in Applied Research;
2012 IWA Sustainability Specialist Group Prize Run-up Award;
2012 Finalist of Smart Cities Award in Project at the Smart City World Congress in Spain;
2012 The International Huber Technology Prize of Germany - Second Prize

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The Future Waterworks

SANI’s success has not stopped Professor Chen from studying water resources. After solving the sludge problem, he came up with another way to enhance the use of water resources with new ideas and new technologies.

“Recycled water” has become an essential part of water supply systems in many major cities around the world, ensuring fresh water supply and improving water safety. There are examples of cities around the world where wastewater is safely recycled directly or indirectly into drinking water, and there are also examples of “recycled water” in Australia and Singapore. These examples show that wastewater can be treated and returned to drinking water status and then be recycled.

Water cycle 4.0+

With the “Future Water Factory”, Professor Chen’s research team developed a new water system that uses key technologies to convert saline effluent into a safe, reliable and affordable fresh water source. The system is named “Water cycle 4.0+”.

The project has been carried out in collaboration with the University of Hong Kong, the City University of Hong Kong, the Hong Kong Polytechnic University and the Chinese University of Hong Kong with an expected completion towards the end of 2024.

Learn more about the Four Steps to “Water cycle 4.0+” on the next page.

Four Steps to “Water cycle 4.0+”

Step One
• An advanced Hybrid Forward osmosis and Reverse Osmosis (hereafter referred to as HFRO) system will be developed to treat the primary wastewater effluent directly.
o A large portion of water containing trace levels of pollutants will pass through this system for potable water production.
o The system will pre-concentrate the feed sewage to 10-50% of its original volume.

Step Two
• The HFRO permeate will be further treated by a new Chloramine/UltraViolet AOP process.
o Degrading the emerging micropollutants and inactivate pathogens as well as antibiotic-resistant bacteria (ARB) in the permeate via chloramine, the process helps post-disinfection in water reuse.

Step Three
• Pre-concentrated sewage will be treated by a new saline microbiology-based biological process called Sulfur-conversion and anamMOX (SMOX) for the removal of residual chemical oxygen demand (COD) and nitrogen. It will produce minimal sludge and require very limited aeration.

Step Four
• The SMOX process produces a tiny amount of excess biological sludge which contains valuable chemicals. The research team invented a technology called Valuable chemIcal Production (VIP) to recover these valuable chemicals from SMOX sludge, which can offset the cost of sludge treatment and disposal.

The entire “four-step” new technology process is being tested to assess its technical and economic feasibility as well as its ecological, environmental and health impact.

Significance of Research

The research is expected to be completed by the end of 2024. In the first 18 months, the research team has published more than 60 academic journal articles and received 6 patents for the research in an inter-university collaboration among five universities.

“Water cycle 4.0+” demonstrates the concept of “water is water and resources are resources” and the research will bring different levels of impact and contribution to the water scarcity issue.

‧ Resource Saving
Compared to conventional technologies, the Water Recycling Plant is expected to save*:
Space 30 - 50%
Energy consumption 20 - 40%
Greenhouse gas emissions 20 - 40%
Operating cost 15 - 35%
(*More accurate values will be provided once the project completed)

‧ Local contribution
The results of the study will help Hong Kong transform traditional conventional waterworks or introduce groundbreaking water supply systems in the development of new towns to provide an alternative to drinking water supply.

‧ International contribution
The research will help alleviate freshwater shortages and related problems faced by islands, coastal areas and agricultural regions around the world that rely on groundwater. It will enable Hong Kong to become a new international hub for water research and help other water-scarce regions achieve UN goals for water, agriculture, public health and sanitation at the same time.

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Being free to explore

Professor Guanghao Chen comes from Jiaxing, a famous town with plentiful access to water in Zhejiang China, where playing and swimming in the river is an important part of his memories growing up. He has an inexplicable affection for water and therefore secretly changed his name with the Chinese character for the element of water so that it remains a part of him throughout life.

He has long focused on water pollution issues in Hong Kong and specialises in the field of environmental engineering. Despite spending a lot of time in sewage treatment plants that are always full of sewage stench, his passion for sewage research never diminishes. His commitment brings far-reaching contributions to humanity’s water resource issues. After a long research career, he encourages future researchers:

It is better to conduct research without a purpose because you will be anxious with it; if you do research purely for exploratory reasons, it does not matter if you fail. Society needs people who are free to explore.