Millions of Chinese citizens are flooding into urban areas throughout the country, with 70 percent of the nation’s population expected to reside in cities by 2030. As cities face an influx of residents, they also face an influx of wastewater and sludge. Currently, 80 percent of sludge in China is improperly dumped-—an increasingly controversial environmental issue that has urban centers scrambling to decrease pollution by improving their wastewater treatment plants (WWTPs). As it grapples with these challenges, China could use an example to follow. New York City’s large size and innovative wastewater system make it one of the only municipalities in the United States that can serve as a useful case study for China’s growing cities.
New York City’s five boroughs must manage and treat the wastewater and waste generated by its 8.6 million residents. The city’s households and businesses typically produce tons of waste, about one quarter of which is organic. Fortunately, this organic waste isn’t actually waste at all; it can be processed in ways that produce energy. Some of New York City’s advanced treatment plants use innovative methods to capture valuable by-products of organic waste, such as methane and digestate, which can then be reused or sold.
ANAEROBIC DIGESTION, CO-DIGESTION AND DIGESTATE, OH MY!
In addition to the standardized multi-stage wastewater treatment process, all NYC wastewater treatment includes an advanced process called a naerobic digestion (AD). This process allows anaerobic bacteria to “eat away” the organics in sludge, generating methane biogas as they do so. This methane—a notorious greenhouse gas that causes 25 percent of man-made global warming-—can be captured in vessels called digesters, preventing it from entering the atmosphere. The captured methane can then be used as a source of energy in the form of heat, renewable natural gas or even vehicle biofuel.
At wastewater treatment plants that don’t use AD technology to capture methane, the organic-rich sludge is sent to landfills, where methane biogas is released directly into the atmosphere.
All 14 wastewater treatment plants in New York City capture methane. At Newtown Creek Wastewater Treatment Plant, the city’s largest treatment facility, methane is captured in giant digester “eggs.” Thanks to a partnership with National Grid, the captured methane will soon be productively used as renewable natural gas (RNG), enough RNG to heat 5,200 homes and reduce greenhouse gas (GHG) emissions by 90,000 metric tons CO2 equivalent.
To make even more clean energy from waste methane gas, WWTPs can co-digest food waste and sludge during the AD process. In 2013, then-Mayor Michael Bloomberg passed co-digestion and composting bills that led to an increase in organic food waste available for co-digestion at New York City’s WWTPs; Newtown Creek is leading the charge.
The private company Waste Management is partnering with Newtown Creek in a pilot co-digestion project. Both partners reap benefits: Newtown Creek receives higher quality methane gas and Waste Management gets a low cost and eco-friendly alternative to disposing food waste into landfills.
Besides biogas, anaerobic bacteria generate another marketable byproduct: the digested sludge, or “digestate.” If disinfected, digestate becomes a nutrient rich biosolid. Thanks to an EPA designation that certifies biosolids are free of any pathogens, Class A biosolids are allowed to be commercially sold and used as fertilizer or soil conditioner.
Unfortunately, most of New York City’s thousand-plus tons of sludge produced daily do not become Class A biosolids, because the city does not yet have large-scale sludge disinfection capacity. Most sludge is currently disposed of in out-of-state landfills. In an effort to curtail this inefficient and environmentally damaging trend, New York City has announced the laudable goal of zero-landfilling of biosolids by 2030.To make this a reality, the city will need to convert more of its own digestate to market-ready Class A biosolids.
ENDING COMBINED SEWER FLOODS
Before wastewater in New York City can undergo these advanced treatment processes, it must be transported to WWTPs via sewers. Many older cities in the United States, including New York, have combined sewers, which receive and transport both stormwater and sewage in a single stream. When these centuries-old sewer systems overflow during storms, a toxic mixture of raw sewage and rainwater floods into rivers and streams. By diluting organic matter in the waste stream, combined sewers also reduce the effectiveness of anaerobic digestion by decreasing the amount of methane that can be captured and productively used.
New York City is plagued by an average of one combined sewer overflow, or CSO, per week. These overflows discharge into the New York Harbor, which directly violates the 1972 Clean Water Act (CWA). Because the city’s waterways have not met the CWA’s “fishable and swimmable” standards for years, nine environmental organizations collectively filed a lawsuit against the U.S. Environmental Agency in 2017. The suit alleges that the agency had not held New York state to CWA standards.
In order to reduce its CSOs and be CWA-compliant, New York City is turning to solutions beyond municipal separate storm sewer systems (colloquially called MS4s, which transport sewage and storm water separately). The city has designed and constructed CSO retention facilities, which collect and hold stormwater during heavy rain events. The city is also utilizing more green infrastructure solutions, such as natural catchment areas, drainage corridors, increasing park space and street trees and using permeable pavements, all of which seek to reduce the amount of storm water that enters drains.
NEW YORK’S CLIMATE RESILIENCY
New York City is a strong model for cities looking to increase their climate resiliency. The city Department of Environmental Protection (DEP) has set up a climate resiliency initiative in response to damage from 2012’s Hurricane Sandy, during which eight of the city’s wastewater treatment plants stopped operating for a time. A DEP study after the event uncovered that the majority of the city’s pumping stations, and virtually all of its wastewater treatment plants, are at risk of flood damage. Since Hurricane Sandy, the city has established a $20 billion resiliency fund, one of the world’s only city-level wastewater resiliency plans, and a city-level Panel on Climate Change. The following year, the DEP published a wastewater resiliency plan.
New York City’s resiliency efforts are supported by multiple political offices. The Mayor’s Office of Sustainability develops annual reports, such as OneNYC, which tracks the city’s progress to become the world’s most resilient, sustainable and equitable city. However, the design and implementation of innovations -—like the ones highlighted in OneNYC -—fall under the jurisdiction of the Mayor’s Office of Sustainability and the Office of Recovery and Resiliency. Both offices, plus the Office of Environmental Coordination, are housed in the Mayor’s Office of Climate Policy and Programs. The three separate offices, plus private-sector contracts, make for a delicate and fragmented management structure that may compromise overall effectiveness.
BIG APPLE LESSONS FOR BEIJING
What can China’s cities learn from all of this? Chinese urban areas currently facing significant wastewater treatment challenges can glean four major takeaways from New York City.
Takeaway 1: Widely implementing anaerobic co-digestion, methane capture and productive use of digestate will allow Chinese cities to transform waste into energy and profit.
Without anaerobic digestion and methane capture, wastewater treatment plants must either incinerate sludge or put it in landfills. This leads to methane entering the atmosphere, as well as costly sludge transportation and disposal. In contrast, by using AD and ideally co-digesting food waste and sludge together, WWTPs can decrease the amount of waste going to landfills and increase the quality of methane and digestate produced. The captured methane can be reused by plants to on-site, significantly reducing operation costs, or can be sold for profit as energy or biofuel. Meanwhile, leftover digestate can be dewatered and disinfected, then used for land application or as fertilizer.
Takeaway 2: CSOs are one of the most severe threats to water quality; they also reduce the effectiveness of AD. Investing in infrastructure solutions that replace CSOs should be a priority for Chinese cities.
China’s combined sewers have been a low priority for policy makers and city officials. Data on the total amount of CSOs in the country is scarce, but combined sewers serve as the main drainage system in many of China’s most populous cities, including the megacities of Guangzhou (which discharges 470,000 tons of sewage into the Liuxi River daily), Shenzhen and Shanghai. As one of the largest contributors to urban pollution, the Chinese government should make addressing CSOs a top priority as a part of its sponge city initiative, which seeks to green China’s urban spaces. This should be done via construction of MS4s, in addition to urban greening, which can help reduce CSOs in the time period before MS4 installation.
Takeaway 3: Wastewater resiliency plans are necessary in the face of a changing climate.
As 12 percent of China’s total population lives in low-elevation coastal zones, excessive groundwater pumping and construction are exacerbating threats from sea level, causing megacities like Shanghai to sink. These coastal cities face drainage challenges, saltwater intrusion into aquifers, and increased flooding. In order to ensure the resiliency of its wastewater infrastructure, China should follow the example of New York City and conduct a wastewater utility hardening campaign, including installation of backup power sources, elevation of critical equipment, and raising or sealing off areas of treatment plants. It should also continue to grow its sponge city program, which will both reduce CSOs and increase cities’ resilience.
Takeaway 4: Too many cooks make an ineffective kitchen.
As evidenced by a crowded resiliency administration, and by the EPA Clean Water Act lawsuit, New York City’s ability to keep its waters clean is reduced by local and federal infighting between various regulatory actors. Despite progressive proposals, the city continues to struggle with timely and effective implementation. As China begins to regulate its new wastewater industry, officials should seek to create streamlined regulatory organizations, whose capacity is not diluted by a plethora of similar organizations vying for influence.