Abstract: The present invention relates to a method for chemically modifying a reactant using microbes. The method includes providing a particulate material which includes a plastic carrier and microbes attached to the carrier. The particulate material is dispersed in a dispersing fluid and has a specific gravity less than that of the dispersing fluid. When the microbe is anaerobic the particulate material has an operating interfacial surface area of from about 2,000 to about 240,000 square meters per cubic meter of reactor volume. When the microbe is aerobic the particulate material has an operating interfacial surface area of from about 1,000 to about 30,000 square meters per cubic meter of reactor volume. The method further includes establishing a flow of the reactant through the particulate material effective to contact the reactant with the microbes for a time sufficient to chemically modify the reactant. Use of the methods in wastewater treatment, aquaculture fish production, and ethanol production are disclosed.