Synergistic control of microplastic and microbial contamination in stormwater harvesting: new insights into the dual functionality of ferrate in coagulation and disinfection.

Stormwater harvesting is a key strategy to alleviate global water scarcity, but the prevalence of microplastics and associated microorganisms in stormwater poses risks for its safe reuse. Thus, there is an urgent need for developing a treatment strategy that can simultaneously address both microplastic and microbial hazards in stormwater. Ferrate, owing to its dual functions of coagulation and disinfection, was investigated in this study for this need. The results showed that ferrate alone had a limited effect on microplastic flocculation, whereas its combination with cationic polyacrylamide (CPAM) under neutral pH conditions achieved high coagulation efficiency, increasing the median floc size by 1.87 times compared with ferrate alone. The DLVO analysis and floc characterization indicated that neutral pH, rather than charge neutralization, is the key factor governing microplastics coagulation in the CPAM-ferrate system. Under such a condition, the combined effects of electrostatic modulation and polymer bridging induced by CPAM and ferrate could be maximized. Nevertheless, both pH adjustment and CPAM introduction diminished the disinfection efficacy of ferrate, either by reducing the contact time or encapsulating microorganisms. Therefore, an appropriate treatment sequence was established to ensure both effective microbial inactivation and microplastics coagulation. Moreover, a subsequent filtration step after CPAM-ferrate coagulation was proven critical to remove light and tiny microplastics which are highly abundant in stormwater. Overall, this study proposed a ferrate-based coagulation-filtration treatment strategy that achieved 98.3% removal of microplastics and 2.7-log removal of microorganisms in real stormwater, with E. coli being undetectable in the effluent. The results of the study demonstrate a promising approach for promoting safe and sustainable stormwater reuse and provide new insights into the dual functionality of ferrate in coagulation and disinfection.