Technical Entity Trends: Trash and the City


New York City, the city AIChE and some 8.5 million people call home, has a dirty little secret. Actually, it’s pretty big: New Yorkers generate over 3 million tons of residential waste per year, most of which is transported by long-haul trucks to out-of-state landfills. This practice is unsustainable due to high costs, emissions, and resource use.

I was first made privy to the magnitude of NYC’s waste management challenges through a collaboration between the AIChE Institute for Sustainability and the Earth Engineering Center at City College of New York (EEC-CCNY). Together we hosted an interactive workshop focused on creating dialogue around this very subject.

In NYC, waste is managed through a combination of reuse — through private and public donation opportunities; recycling — through the New York City Dept. of Sanitation’s (DSNY) mandatory dual-stream recycling program, organic waste collection bins, and recycling stations; waste to energy; and landfilling.

The overarching objective of a good waste management plan is to create less waste in the first place. For the waste that is inevitably generated, the goal is to maximize reuse first and recycling second, reducing as much as possible the waste residue, called discard, that enters landfills. Demetra Tsiamis, Associate Director of the Earth Engineering Center at CCNY, explains that discard typically includes organic waste that has not been diverted through composting or anaerobic digestion; non-recyclable plastics, such as films, bags, and foams; and items that have not been properly recycled. When landfilled, the organics decompose, producing the powerful greenhouse gas methane and possibly leaching toxins into the soil and groundwater.

Landfills are unnatural — in nature, there is no such thing as “waste,” as everything gets reused. Waste-to-energy (WTE) technologies already exist that thermally process discard streams to recover energy and materials, in addition to addressing aforementioned challenges associated with landfilling. “Think of it as thermal recycling,” Tsiamis says of the three main WTE technologies: combustion, gasification, and pyrolysis. These are in different stages of commercial viability and vary in their optimal feedstock composition and outputs.

Combustion facilities take in “as received” municipal solid waste (MSW) and incinerate it in the presence of oxygen to produce heat and electricity. Combustion has a high processing capacity and is the most prevalent of the WTE technologies, with commercial-scale plants operating widely in Europe and parts of Asia, although there are still few in the U.S.

Gasification thermally processes organics mixed with plastics in the presence of lower levels of oxygen to produce syngas. This technology entails more feed pre-processing, such as shredding and drying waste to achieve a more homogeneous feed. There are several large-scale electricity-producing gasification plants in Europe and Asia. Applications for syngas as a feedstock to produce liquid fuels and chemicals are being developed.

During pyrolysis, waste is thermo-chemically broken down at very high temperatures in the absence of oxygen to produce fuel oils This technology can process difficult-to-recycle plastics such as plastic bags. Pyrolysis plants exist mostly at pilot scales and are not yet operating commercially.

The ash or char byproduct of WTE processes contains metals and non-combustibles. Research is ongoing into beneficial uses of the ash and the extraction of metal from it. Ash is already being used for cement production.

Along with producing energy, one of the biggest appeals of WTE technologies is that they can reduce the volume of waste to be disposed by up to 90%. WTE also enhances the circular economy mindset of reusing the old instead of extracting new materials from nature. “Ultimately,” Tsiamis concludes, “it’s looking at waste as a resource before we dispose of it” that is at the heart of a responsible waste management approach.

Despite widespread success in countries like Sweden, Denmark, and Japan, where up to 90% of the MSW is processed in WTE facilities, this practice is not nearly as popular in the U.S., where roughly half of our MSW is landfilled and WTE plants handle less than 15%. So why the lack of enthusiasm?

First, a not-in-my-backyard (NIMBY) attitude stems from the public’s perception of these technologies as unsafe or environmentally harmful and having the potential to undermine recycling programs. Another challenge is the absence of an infrastructure capable of providing a continuous feed stream necessary to make a commercial-scale WTE facility financially viable. In addition, a combination of abundant land and lack of policy incentives make landfilling more economical than WTE in the U.S. The scientific-engineering community can play an important role in educating the public and policymakers on the true opportunities and tradeoffs that modern waste-management technologies offer.

I asked Tsiamis to describe what an Urban Waste Management Utopia would look like to her. Just as with alternative energy solutions and climate change mitigation plans, there is no one-size-fits-all answer to municipal waste management challenges; everything has to be adopted based on local geography, resources, and market constraints. The population, as waste generators, would follow the waste hierarchy described earlier to minimize waste generation and recover maximum value from waste. An infrastructure would be established to allow for more composting and anaerobic digestion, with ample education and time to allow the population to ramp up to the new practices. Whatever refuse could not be digested or recycled would then go to a WTE facility. During the ramp-up time and with sufficient policy support and funding, WTE technologies would be developed further to produce more energy, recover more materials, and perform more efficiently. At the end of the cycle, if there is residual refuse to discard in a landfill, the volume would be drastically less than current volumes.

The DSNY has set an ambitious goal of reducing all of the city’s waste by 90% by 2030 — sending none of it out of state to be buried in landfills. I eagerly look forward to this city’s residents, politicians, and the scientific community coming together to make that vision a reality.

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