In fixed-film bioreactors, microbial cells continue to grow by metabolizing the toxic components in the system. Three forms of metabolism that can exist are aerobic, anoxic, and anaerobic. Aerobic systems are limited in their degredation rate (rate of removal of toxic substances) of high organic concentrated wastes due to the rate of oxygen transfer. Heavy biomass buildup creates high oxygen demand. Anoxic metabolism consists of placing the microorganisms into a particular growth substrate (ex. acetate) and placing the combination into an aquifer containing nitrate and sulfate. The system will first use up the nitrate first, which must then be removed in order for the sulfate to perform its required task, dehalogenation. Anaerobic metabolism occurs when the microorganism used toxic compounds as its main energy source and form carbon dioxide and water. Rate of removal of toxic compounds is also limited by mass transfer and diffusion within the biomass.