The relationships between wastewater pollution, wastewater treatment, and greenhouse gas emissions are complex, and overwhelmingly negative. While the energy consumption of wastewater treatment plants is considerable, the bigger issue is their contribution to greenhouse gases from the compounds emitted by microbes as they process the waste in wastewater; roughly 3% of global annual emissions (Tseng et al., 2016). Aerobic wastewater treatment processes emit carbon dioxide (CO₂), nitrification-denitrification processes emit nitrous oxide (N₂O, a greenhouse gas 300 times more potent than CO₂), and anaerobic wastewater treatment processes emit methane (34 times more potent than CO₂). Carbon dioxide, methane, and nitrous oxide are the big three greenhouse gases.

In fact, methane produced by wastewater treatment plants represents anywhere from 3% to 19% of global anthropogenic methane emissions (Yang et al., 2017). These numbers will only grow if we work to treat all wastewater, as only 20% of the world’s wastewater is currently treated (UN WWDR, 2017).

Another significant contributor to greenhouse gas emissions is the hypoxic, or low-oxygen zones, created by harmful algal blooms (HABs) that can occur when wastewater is discharged in freshwater and marine systems (see Nutrients). In freshwater systems like lakes, the anaerobic decomposition of the algae produces carbon dioxide, nitrous oxide, and methane. In marine systems, such decomposition emits mainly carbon dioxide and nitrous oxide, the latter at levels that are 10,000 times higher than in nonhypoxic zones (Codispoti, 2010). Given that climate change is creating favorable conditions (warmer waters, more carbon dioxide, higher salinities) for HABs and that HABs release significant amount of greenhouses gases, a climate change feedback loop is likely possible.

In addition to contributing to greenhouse gas emissions, discharging wastewater pollution into the ocean is compromising our ability to draw down carbon in the atmosphere. Coastal habitats that are known to be carbon sinks, such as salt marshes and seagrass beds, store carbon in soil and plant material (Bulseco et al., 2019). But the high concentrations of nitrogen found in wastewater and other types of coastal pollution increase microbial decomposition rates of organic material in the soil, thereby releasing more carbon than the habitats can store. In other words, wastewater pollution has the potential to convert these coastal habitats from carbon sinks to carbon sources.

Wastewater treatment plants hold a key to reducing emissions. It is possible to capture methane and potentially nitrous oxide, and transform emissions from wastewater treatment into usable energy, fertilizer and water (see Resource Recovery). Strategies include developing on-site carbon sequestration technology that runs on renewable energy. There is great potential to transform a whole industry from being a massive consumer of energy and contributor to climate change to one that provides energy and is carbon neutral — or even carbon negative.