Suspended-growth biological treatment methods keep the microorganisms floating in the wastewater rather than growing on a surface. In this setup, air or mixing keeps the biomass in the liquid so the microbes continuously contact and oxidize the pollutants. Activated sludge and sequence batch reactors are classic examples: air is bubbled through a mix of wastewater and microbes, holding the suspended solids in constant contact with the pollutants for rapid biological treatment. An aerated lagoon operates similarly, with the biomass kept dispersed in the water through mixing and aeration, albeit more slowly. Fixed-film processes, like trickling filters and rotating biological contactors, rely on biofilms attached to media rather than suspended solids, so they’re not suspended-growth systems. The other options are not biological treatment in the suspended-growth sense either—physical/chemical separations or processes that treat sludge rather than the freely suspended wastewater biomass.