Waste

Historically, the principal function of a city is to serve as an efficient manufacturing center and trade hub. They exist to produce goods and, when they can’t, obtain goods from around the world. The natural consequence of this purpose is that cities also consume. A lot. And as globalization and mass consumption reach new heights, consumption is only going to increase. In 2018, the United States produced 292.4 million tonnes of municipal waste, equivalent to 4.9 lbs per person per day [1]. Thinking volumetrically, this is a lot of material that has to be disposed of. Outside of the EU, most of this waste just gets landfilled - taking up valuable land and producing 880 million tons of CO2e per year [2]. Think about how much energy this represents! All that energy used to make things going straight into the landfill. What if there were a way to get that energy back?

    The solution is the appropriately-named waste-to-energy facility (WTE). They basically are what they seem; they take in municipal waste and use it to produce energy. There are a number of different types of WTE facilities, the most widely-used one being the incineration plant. These plants burn waste at high temperatures, heating steam that drives a turbine generator much in the same way a conventional fossil fuel power plant works. The residual heat can also be used to power district heating systems for cities. One important incineration facility is the Amager Bakke plant in Copenhagen. This plant converts 400,000 tonnes of waste from the Copenhagen metro area into 1.3 million MWh of power per year [3]. This is enough electricity for 550k people and enough heating for 140k. Clearly, these systems have the potential to generate highly significant amounts of energy.

    Other types of ‘thermal conversion’ WTE plants utilize either pyrolysis or gasification. Both create controlled high-temperature environments for the waste, which induces the formation of gas - pyrolysis gas in pyrolysis and syngas in gasification. This gas then gets combusted to drive a turbine [4]. These WTE plants require specific ‘input wastes’, so specific pathways for appropriate waste have to be created for them to be functional. However, they are more efficient than incineration plants, and so they have seen success in places around the world, especially in Japan [4]. A last form of WTE plant uses biological conversion techniques of bacteria to produce biogas from waste. Studies find that 1 m^3 of biogas produces around 2.04 kWh of energy, or about 10% of natural gas [4]. 

    Burning waste might intuitively seem terrible for the environment, but fly ash filters and improvements in heavy metal processing have dropped pollution by 97% since the 1990s [5]. WTE plants do produce large amounts of CO2, but large enough facilities generally reduce net CO2 emissions due to reductions in fossil fuel energy generation [6]. Obviously, the goal of the renewable energy transition is to reach 100% renewable energy - and create a carbon-free society - but we must remember that intermediate reductive steps have to be taken to get there. We can not let our fascination with net-zero make our ideology too rigid - we must consider how each initiative contributes to the goal, even if it isn’t the goal itself. 

    Additionally, byproducts of WTE can be reused in society. For example, bottom ash from incineration can be used for road construction [3]. This means that we don’t need to mine as much of the specific types of sands usually needed for roads. Although such byproducts could be seen as promoting car use (and fossil fuel use!), there is no reason that we couldn’t use this technology for constructing pedestrian walkways or bike lanes instead. Finally, biological conversion WTE plants produce a nitrogen- and mineral-rich sludge byproduct. This sludge is a great fertilizer, which can go back to the farms that originally produced the food waste used in the facility.

In this way, we can see why waste-to-energy facilities can become such an important part of a renewable energy transition. While such systems do require external addition of resources, cities are going to consume these resources anyway! As urbanization continues and billions of people globally enter the middle-class, degrowth is an unrealistic expectation. Rather, we must focus on how consumption - even high consumption, like in cities - can be sustainable. 

Citations [Waste]

[1] “National Overview: Facts and Figures on Materials, Wastes, and Recycling,” US EPA, 2020. https://www.epa.gov/facts-and-figures-about-materials-waste-and-recycling/national-overview-facts-and-figures-materials. Accessed 30 Nov, 2020.
[2] “Landfill Methane,” Global Methane Initiative, 2011. https://www.globalmethane.org/documents/landfill-methane. Accessed 30 Nov, 2020.
[3] “Amager Bakke: High-efficiency energy recovery from waste,” Ramboll Energy. https://ramboll.com/projects/rme/copenhill?utm_source=alias?utm_campaign=amager-bakke. Accessed 30 Nov, 2020.
[4] Kumar, A. and Samadder, SR. “A review on technological options of waste to energy for effective management of municipal solid waste,” Waste Management, Vol. 69, 2017.
[5] Brunner, P. and Rehberger, H. “Waste to energy - key element for sustainable waste management,” Waste Management, Vol. 37, 2015. 
[6] Christensen, TH., Damgaard, A., and Astrup, TF. “Waste to energy the carbon perspective,” Waste Management World, 2015.