Microplastics in sewers may be driving methane emissions

A June 2026 study found that PET and PBAT microplastics age inside low-oxygen sewer networks, reshaping microbes in ways that reduce sulfide but boost methanogenic activity. The findings suggest urban sewer systems may be an overlooked source of greenhouse gas emissions and a new target for microplastics control.

Why it matters: - Sewer networks are not just pipes moving wastewater. They are active bioreactors that can change how pollutants behave. - The study suggests microplastics may increase methane-related risks in urban drainage systems, adding a greenhouse gas concern to existing corrosion and odor problems. - The findings also raise the possibility that microplastics become more reactive before they ever reach wastewater treatment plants.

What happened: - Researchers from Beijing University of Technology and Beijing Waterworks Group Co., Ltd. studied how microplastics interact with sewer conditions. - The team published the findings in June 2026 in Environmental Science and Ecotechnology. - The paper has DOI 10.1016/j.ese.2026.100726. - The experiment ran for 120 days in concrete sewer reactors fed with real domestic sewage. - The reactors were exposed to polyethylene terephthalate (PET) and polybutylene adipate terephthalate (PBAT) microplastics at 30, 100 and 500 particles per liter.

The details: - Microplastics are common in municipal wastewater, with reported concentrations of 10 to 470 particles per liter in sewage. - Low-oxygen, sulfide-rich sewer conditions promoted microplastic aging. - Hydroxyl radicals generated through sulfide autoxidation and redox reactions were identified as the main driver of that aging. - Electron paramagnetic resonance spectroscopy showed hydroxyl radical signals in sewage but not in deoxygenated water. - Microplastics amplified radical generation, creating a feedback loop that increased oxidative stress in the system. - The radicals attacked ester bonds in PET and PBAT, causing chain scission, surface roughness and oxygen-containing functional groups. - PBAT degraded faster than PET. - PBAT particle size shrank by 11.3%, and its carbonyl index rose by 16.3%. - Density functional theory calculations identified ester bonds in the aliphatic segments as the most vulnerable sites. - Two-dimensional correlation spectroscopy showed PET degraded in a slower, stepwise pattern. - PBAT showed faster, broader oxidative cleavage across multiple C–O bands.

Between the lines: - The study shows microplastics are not inert in sewer environments. - Aging plastics changed the microbial community, not just the plastics themselves. - Microplastics weakened microbial co-occurrence networks and reduced the number of interspecies interactions. - Community assembly shifted from stochastic processes toward deterministic, stress-adapted behavior. - Hydrolytic and fermentative bacteria fell by up to 63.4%. - Hydrogen-producing acetogens and methanogenic archaea rose by 48.4% to 67.0%. - Sulfate-reducing bacteria dropped by up to 49.7%. - At 500 particles per liter, genes linked to sulfate reduction, including aprA/B and dsrA/B, fell by 40.4% to 55.5%. - Genes linked to hydrogenotrophic methanogenesis, including fdwG, ftr, mch, mtd and mer, increased. - The microbial shift cut sulfide concentration by 89.5%, but likely increased methane production. - The authors warned that reducing sulfide alone may not improve sewer safety if methane production rises.

What's next: - The researchers said microplastic management needs to move upstream, not just focus on wastewater treatment plants. - They pointed to source reduction, including limiting microfiber release from laundry and pretreating industrial effluents. - They also suggested capture systems at key sewer nodes could help stop downstream transport of pre-aged microplastics. - The study was supported by the Beijing Nova Program (20240484694) and the Beijing Municipal Science and Technology Commission, Administrative Commission of Zhongguancun Science Park (054000543125001).

The bottom line: - Urban sewers may be turning microplastics into a bigger climate and safety problem than expected.