Degradation of PHB and PE microbeads in aerobic and anaerobic biological wastewater treatment microcosms

Abstract

Synthetic plastic microbeads, often found in personal care products such as toothpaste, face scrubbers, hand sanitizers, hand soap, and shower body wash, pose a great threat to aquatic life and the environment. Their size and nature prevent wastewater treatment plants from removing all microbeads, and some are discharged with effluent into the environment. US federal legislation recently banned the use of synthetic and biodegradable plastic microbeads, though the fate and transport of biodegradable plastic microbeads is not proven to be the same as synthetic microbeads. Microcosm experiments were performed to investigate the degradation of PE and PHB microbeads under conditions that mimic municipal biological wastewater treatment processes. A known weight of PE or PHB microbeads sieved to a uniform size range of 500-800 microns was placed in 80 mL of activated sludge (MLSS), return activated sludge (RAS), aerobic digester sludge (AerD), or anaerobic digester sludge (AnD) for incubation periods of 2-23 days. Samples pulled from incubation were oxidized using household bleach (6% sodium hypochlorite) and sieved through 400-micron nylon mesh in Nalgene reusable filter units. Samples were dried, weighed, and characterized into contributing residual sludge and microbead weights. Temperature, pH, total solids and total volatile solids were also collected and analyzed. A linear regression for weight percent recovery over time for each sludge was used to estimate the degradation rate. This research showed partial degradation of PHB microbeads in microcosms of MLSS, RAS, AerD, and AnD, with weight percent recoveries of 72%, 48%, 27%, and 13%, respectively, after 16-day incubation. PE microbeads were confirmed to not degrade under biological wastewater treatment conditions in microcosms. The results demonstrate that microbeads made from PHB degrade in typical municipal biological wastewater treatment microcosms while PE microbeads do not. These results demonstrate that not all plastics should be treated equal, and the recent microbead ban prevents the use of a viable alternative to nondegradable microbeads.