Newtown Creek Wastewater Treatment Plant by Steven Olswang, CC BY-SA 4.0, via Wikimedia Commons
This guide poses a series of straightforward questions that will quickly allow officials and communities to see if their local wastewater plant is suitable for RNG production and what steps to take.
Hundreds of wastewater plants across the U.S. are viable candidates for producing RNG. Almost 30 wastewater plants nationwide have already done so, by upgrading the methane-rich biogas produced by their decomposing sewage sludge in anaerobic digesters (ADs) to RNG and then injecting it into existing natural gas pipelines. This RNG is then sold to end users in the transportation market or to the growing voluntary market of utilities and corporations that seek to decarbonize their operations. Either way, upgrading the biogas to RNG is a far more valuable use than flaring it, which unfortunately is still all too common.
About 475 large wastewater plants across the U.S. already have ADs in place, in part to prevent odor from the decomposing sewage. Of those, some 135 plants just flare their biogas, which causes air pollution and wastes a valuable energy resource. Over 200 other large plants with ADs use their biogas as boiler fuel, which is a productive use but often fails to match the climate and economic benefits of producing RNG. Using the biogas to generate electricity and onsite heat at the plant is relatively uncommon but is a significant step up from boiler fuel. RNG production, however, can deliver the largest climate benefits, especially if used to displace fossil fuels in hard to decarbonize (e.g. difficult to electrify) sectors like heavy-duty transportation and heavy industry (e.g. steel and concrete).
As the primer makes clear, starting with ADs is an enormous advantage on the road to RNG production, saving tens of millions of dollars and years of construction time per project. The average payback time for a wastewater plant retrofitting to produce RNG is just 5-7 years, which is relatively fast compared to most municipal infrastructure projects.
Wastewater plant ADs are also ideal for mixing in or “co-digesting” food waste with sewage sludge. This helps divert food waste from landfills, avoids major methane leakage from landfilled food waste and boosts the amount of biogas generated in ADs, increasing RNG production capacity. Wastewater plants also get paid to take in food waste, offsetting some retrofit costs and shortening the payback time. (Energy Vision’s 2023 report, Gotham Gas Goes Green, details the model “gas to grid” initiative undertaken in Brooklyn, NY.)
Energy Vision found that the 50 largest wastewater plants in the U.S. which already have ADs could potentially co-digest 4.7 million tons per year of food waste, or 7.5% of the nation’s food waste. This would be an impactful and cost-effective way to help meet the U.S. goal to cut methane emissions at least 30% by 2030 as well as municipal targets for diverting food waste from landfills.
