By rethinking what happens to leftovers after they leave our plates, HKUST-led researchers show that sending wet food waste through city sewers can slash emissions and save money. Their new framework could reshape how major cities worldwide handle food waste.
Leftover soup, soggy noodles and vegetable scraps may not look like climate solutions. But a new study from the Hong Kong University of Science and Technology (HKUST), conducted in collaboration with Huazhong University of Science and Technology, suggests that, in many big cities, those wet leftovers could help cut greenhouse gases and lower waste-management bills if they are sent down the drain instead of to the dump.
Analyzing data from 29 large cities worldwide, including Hong Kong, Beijing and New York, the research team found that grinding food waste and diverting it into the sewage system can be more effective than relying mainly on landfills in places where food waste is especially wet.
Their findings, published in the journal Nature Cities, point to a surprisingly powerful lever for city leaders: how much moisture is in the food waste stream.
Most cities still depend on landfilling or incineration to handle food waste, even as urban populations grow and trash volumes climb. That is a problem because food waste is heavy with water, which makes it expensive to collect and transport and less efficient to burn. In landfills, decomposing food is also a major source of methane, a potent greenhouse gas. In the United States, food waste alone accounts for more than half of methane emissions from landfills.
The research team, led by HKUST civil and environmental engineering chair professor Chen Guanghao, postdoctoral fellow Guo Hongxiao and doctoral student Zou Xu, set out to rethink this system.
They compiled detailed data on food waste composition, wastewater generation, energy use and treatment costs from cities around the world. They then built a new analytical framework, called the Urban Biowaste Flux (UBF) model, to compare different ways of handling food waste.
One of their key findings is that the most important factor for treatment efficiency is not how much food waste a city produces or what kind of food it throws away, but how wet that waste is.
Higher moisture loads were linked to higher treatment costs and higher emissions when food waste was handled separately through landfills or incinerators. In contrast, when wet food waste was ground up, sent into the sewage network and treated together with wastewater, overall systems in many cities became cheaper and cleaner.
Using the UBF framework, the team identified a threshold of about 46.8 kilograms per person per year of food waste moisture. Above that level, an integrated system that diverts food scraps into the sewer and combines them with landfill or incineration tends to be less costly than keeping food waste and wastewater completely separate.
That threshold matters for cities where diets are heavy on fresh ingredients, soups and broths, such as Hong Kong, Beijing and Seoul. These eating habits create wetter food waste streams, which are particularly inefficient to truck to landfills.
In Hong Kong, the researchers found that installing food waste grinders and shifting to an integrated system would raise annual operating costs for wastewater and sludge treatment, but those increases would be more than offset by a sharp drop in landfill spending. Overall, they estimate that Hong Kong’s total waste-management costs, including capital, operating and grinder costs, would fall by about 11%.
The climate benefits are even larger. For Hong Kong, the study suggests that integrated treatment could cut direct and indirect greenhouse gas emissions by nearly 47%.
The team’s detailed sampling work in Hong Kong helped explain why the shift makes sense there, according to Zou.
“Using Hong Kong as an example, our analysis of food waste and wastewater samples shows that food waste accounts for 57.78% of the total chemical oxygen demand entering the biowaste treatment system. This demonstrates why we must rethink how food waste is managed,” Zou said in a news release. “The UBF model offers an effective analytical tool for cities with high food waste moisture loads like Hong Kong.”
Chemical oxygen demand is a standard measure of how much organic material is in wastewater. If more than half of that load is coming from food waste, as in Hong Kong, then routing that material through systems designed to recover energy from organic matter can be a major opportunity.
In many wastewater treatment plants, that opportunity comes through anaerobic digestion, a process in which microbes break down organic waste in the absence of oxygen to produce biogas, which can be used as fuel.
Guo noted that some cities are already moving in this direction, especially in North America.
“Compared with traditional separate collection and landfilling, integrated system can reduce greenhouse gas emissions by 24% to 88% across different cities. Around half of all food waste in the U.S. is already managed through this method, but it remains uncommon in Asia,” Guo said in the news release. “The UBF model helps cities like Hong Kong identify more efficient and sustainable approaches to food waste management.”
The study’s global analysis suggests that the potential is widespread.
“Using wastewater systems to process wet food waste, together with anaerobic digestion, allows sludge to be used as fuel, and the heat generated during incineration can be recovered for electricity generation. Our study shows that among 29 cities worldwide, 27 would reduce per‑capita annual energy consumption by about 20.6%, and 26 would reduce per‑capita greenhouse‑gas emissions by about 22.6% if integrated treatment were adopted,” added Chen. “Of course, cities differ, and not all will be suited to the same model. But for those with high food waste moisture load and high solid‑waste processing costs, integrated treatment is a practical path forward. We hope this research provides a scientific foundation for cities to formulate more appropriate strategies for food waste management.”
The work does not suggest that every city should immediately start sending all food waste down the drain. Local infrastructure, regulations, housing types and public behavior all matter. Some cities may be better off expanding composting or improving landfill gas capture, especially where food waste is relatively dry.
But the UBF framework gives planners a way to quantify trade-offs and design systems that match their city’s specific conditions, rather than relying on one-size-fits-all solutions.
For students and young professionals interested in sustainability, the study is a reminder that climate solutions can be found in unexpected places, including kitchen sinks and sewer pipes. It also shows how data and systems thinking can turn a messy problem like food waste into an opportunity to save money, cut emissions and recover energy.
As cities worldwide search for practical ways to meet climate goals while serving growing populations, the HKUST-led team’s message is straightforward: in the right conditions, yesterday’s leftovers can help power tomorrow’s low-carbon cities.