HomeNewsEnergy Vision highlights potential of wastewater plants to produce RNG, follow gas-to-grid model

Energy Vision highlights potential of wastewater plants to produce RNG, follow gas-to-grid model

10/3/2024

The sustainability NGO Energy Vision has released a primer for municipalities on how their local wastewater treatment plant(s) may be able to produce renewable energy and help fight climate change, titled “Can your wastewater plant do more?”

The primer lays out straightforward criteria for determining whether wastewater plants can be retrofitted or equipped to produce renewable natural gas (RNG), what the main costs and challenges are, and whether they could also accept and “co-digest” food scraps and other organic wastes in addition to sewage sludge.

Retrofitting wastewater treatment plants to process food waste and produce RNG could significantly lower municipalities’ emissions. RNG produced from wastewater plus food waste co-digestion is often net carbon-negative, meaning that more greenhouse gases (GHG) are captured in producing the fuel than are ever emitted when it is used, so it slashes GHG emissions on a lifecycle basis.

Of the 15,000 wastewater plants across the US, the primer deems 1,132 large enough to produce RNG. 475 of those large plants already have the basic infrastructure in place: anaerobic digesters (ADs). ADs are sealed tanks where the sewage sludge decomposes, generating methane-rich biogas. The biogas can be utilized onsite for heat and/or power or it can be upgraded to RNG and injected into the pipeline grid, displacing fossil fuels. This strategy for wastewater treatment plants is known as the “gas-to-grid” model.

The 475 large plants are the most viable near-term candidates for adopting food waste co-digestion and the gas-to-grid model, and would be the most impactful in helping the U.S. meet its commitment to cut annual methane emissions at least 30% by 2030.

However, of those 475 only about 30 produce RNG today. Roughly 135 just flare their biogas, which is polluting and wastes a valuable energy resource. They are prime targets for exploring a switch to RNG production. Over 200 utilize their biogas solely as boiler fuel, which is a productive use, but often cannot match the potential climate and economic benefits of RNG. Some of these plants could also consider a switch to producing RNG. 

For plants that already have ADs, the typical payback period for building the additional infrastructure needed to upgrade biogas to RNG is short compared to most municipal infrastructure projects: five to seven years.

Co-digesting food waste with sewage sludge boosts the amount of biogas ADs generate, increasing a wastewater plant’s RNG production capacity. It also helps divert food waste from landfills, avoiding major methane leakage from landfilled food waste. Wastewater plants also get paid to take in food waste, offsetting retrofit costs and shortening payback time.

Energy Vision estimates the 50 largest wastewater plants in the U.S. that already have ADs could potentially co-digest 4.7 MMtpy of food waste, or 7.5% of the nation’s food waste. If most of the 475 large plants with ADs adopted co-digestion, they could handle an estimated 10% of nationwide food waste (~6.3 MMt). Diverting 10% of food waste from landfills would avoid significant methane emissions – about 5 metric MMtpy of carbon dioxide equivalent (CO2e) – while increasing these plants’ RNG output, which could be used to displace fossil fuels and result in even deeper net greenhouse gas emission cuts.

“Gas-to-grid with food waste co-digestion is a highly impactful and cost-effective way to help meet the national goal of cutting methane emissions at least 30% by 2030, and municipal goals for diverting food waste from landfills,” said Michael Lerner, Energy Vision’s Director of Research and Publications, and author of the primer.

 

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