The antiquated status quo is far from ideal: only 30% of the biogases generated by the NYC’s Department of Environmental Protection’s (DEP) wastewater plants are beneficially used – to provide heat and/or power to the facilities – while the other 70% is flared. This has been a major missed opportunity.
Methane-rich biogas is emitted at the wastewater treatment plant, as sewage and other organic waste streams decompose. The DEP recently signaled its intent to put 100% of this renewable energy resource to use, either on-site or by upgrading it to ultra-low-carbon renewable natural gas (RNG), and then using it to displace fossil fuels. In doing so, NYC would make major strides in addressing two of its most pressing environmental challenges: what to do about its huge stream of food and green waste, and how to meet its ambitious air quality and climate change goals.
Newtown Creek Leads the Way
Newtown Creek is NYC’s largest and most modern wastewater plant, processing 310 million gallons of wastewater per day. It has just undergone an upgrade to co-digest some food waste with sewage; the resulting biogas is captured and refined to RNG at a new facility and then injected into National Grid’s pipeline system. Newtown Creek is thus the first wastewater plant in NYC to adopt the co-digestion and “gas-to-grid” model.
This is a big net gain for the climate. RNG made from wastewater sludge is classified as “net carbon-negative,” because producing the fuel captures more greenhouse gasses (specifically methane) than using it emits (in the form of CO2, which has a far lower warming effect). The plant also has the capacity to process up to 500 tons of food waste (12.5% of NYC’s total) per day, which will be crucial as the City expands organics collection.
Adopting the Model Citywide
By replicating the Newtown Creek model at other large wastewater plants, according to Energy Vision’s research, NYC could reduce municipal greenhouse gas emissions by more than 600,000 tons per year, process over 30% of its food waste, and generate or save $48-72 million dollars annually.
Depending on infrastructure upgrades and potential co-digestion, DEP’s 14 wastewater plants could produce 2.4-3.6 million MMBTUs of RNG annually. This could replace up to 27% of the fossil natural gas that the City currently purchases; or it could instead displace over 25 million gallons of diesel fuel – more than enough to power all 6,000 heavy-duty trucks in NYC’s municipal fleet, which still rely predominantly on diesel fuel today.
Diesel exhaust is especially harmful to children and the elderly, and it affects environmental justice communities disproportionately. Twelve of the 14 DEP wastewater plants are in or next to what New York has designated as “potential environmental justice areas.” Running these trucks on RNG instead of diesel would eliminate more than 90% of health-threatening nitrous oxide emissions, and 60% of particulate emissions.
Adopting the Newtown Creek model systemwide would align with NYC’s ambitious environmental goals, including cutting citywide emissions 40% by 2025 and 50% by 2030 before achieving carbon neutrality by 2050, and sending zero waste to landfills by 2030.
Potential for Replicating the Model Elsewhere
The U.S. has about 15,000 municipal wastewater plants, including 1,132 large ones (with a capacity of at least 5 million gallons per day) that could be feasible candidates for RNG output. While these large plants make up only 8% of the total, they handle about 77% of the wastewater by volume. Thus, they tend to be located near cities, and two-thirds are within 5 miles of natural gas pipelines, making them prime candidates for the “gas-to-grid” model.
Even though the RNG industry has grown rapidly in the last five years, wastewater plants have been slow to get onboard. Only 26 have operational RNG facilities (see map), and few are as big as Newtown Creek. According to Argonne National Laboratory, if all 1,132 large plants produced RNG, their annual capacity would be 27.2 million MMBTUs, with lifecycle net greenhouse gas emissions actually cutting 2 million metric tons of CO2 equivalent. If all were to co-digest food waste with sewage, their RNG capacity and avoided greenhouse gas emissions would both rise further. Even a small portion of that potential pool following Newton Creek’s example would represent major progress toward cleaner air and reduced emissions.
About 90% of all wastewater plants – over 13,000 – do not use anaerobic digestion, meaning the greenhouse gasses from decomposing sewage simply escape into the atmosphere. Of the other roughly 10% that do use anaerobic digestion, around two-thirds utilize the resultant biogas on-site to provide heat and/or power to their facilities, while the other third flares all the biogas. About 475 (42%) of the large plants utilize anaerobic digestion, making them the most straightforward subset to convert to RNG production. Over 130 wastewater plants also co-digest food waste; most of these are large plants where RNG production would be the next logical step.
The up-front capital investments vary by site. However, upgrading wastewater plants to produce RNG leads to big cost savings and new income over the life of a project. Notably, co-digestion of food scraps increases biogas production and allows for greater revenue in the form of “tipping fees” (paid per ton to the plant). Plus, financial incentives in the Inflation Reduction Act make investing in these projects more attractive than ever. The IRA allows direct reimbursement of significant capital costs from the federal gov’t, as well as financial incentives for RNG production. There’s only a limited time window of eligibility through 2024, though, so it’s imperative for cities to act now.