· Microorganisms attach to the exposed support media in an aqueous environment
· These immobilized cells grow, divide, and produce a gel-like material (a polysacharide) called a biofilm
· The activity of the biofilm depends on local surface conditions to transport nutrients, substrate, electron acceptors, and donors to the immobilized cells
· This biofilm adsorbs the necessary components for metabolism. Microorganisms for the system are selected on the basis of the material (hazardous
substances they metabolize
· The biofilm collects other microorganisms during the process which changes the nature of the biofilm
· The solution of hazardous wastes must be held in the reactor for long periods of time for metabolism of all toxic substances to take place
· The depth and composition are influenced by many factors:
· Microbial diversity changes due to concentration gradients of electron donors and acceptors and other biological transformation factors including
biological transformation products
· Both anaerobic and aerobic environments can exist within the same biofilm
· The biomass adsorbs organic compounds from the bulk solution yielding an enriched energy source at the biomass film surface
· Oxygen transport through the biofilm is diffusion limited and the concentration gradient for diffusion is significantly reduced by the biofilm's
respiration activity. This results in anaerobic conditions within the biofilm.
· The nature and concentration of the organic substrate and the nature of the microrganisms
· The superficial velocity (the velocity through the reactor if there were nothing inside it)
· The pH and eletrolyte concentrations of the water
· Characteristics of the support media which include surface area, surface roughness, pore volume, pore size, surface charge, adsorption properties,
density, and specific gravity
· It is important to maintain large microbial diversity in stable form to create high mean cell residence time, reduce sludge handling to maintain biomass, and create
better response to toxicity