Biological Wastewater Treatment, Third Edition by C. P. Leslie Grady Jr., Glen T. Daigger, Nancy G. Love,

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2 Attached Growth Bioreactors There are three major types of attached growth bioreactors: packed towers, rotating discs, and fluidized beds. The microorganisms in a packed tower grow as a film on an immobile support, such as plastic media. In aerobic bioreactors the wastewater flows down the media in a thin film. If no recirculation of effluent is practiced, there is considerable change in the reaction environment from top to bottom of the tower as the bacteria remove the pollutants. The recirculation of effluent tends to reduce the severity of that change, and the larger the recirculation flow, the more homogeneous the environment becomes.

Engineering design is greatly facilitated by the application of mathematical models to quantitatively describe system performance. Construction of such models for biochemical operations must be based on a fundamental understanding of the microbiological events occurring in them. Chapter 2 provides that understanding, as well as an appreciation of the complex interactions occurring among the microorganisms that form the ecosystems in the operations. That appreciation is crucial to recognition of the simplified nature of the models, thereby encouraging their appropriate usage.

This is because biochemical operations function more efficiently than chemical and physical ones when the concentrations of reacting constituents are low. In biochemical operations the soluble pollutants are converted either into an innocuous form, such as carbon dioxide or nitrogen gas, or into new microbial biomass, which can be removed by a physical operation because it is a particulate. In addition, as the microorganisms grow, they entrap insoluble organic matter that escaped removal upstream, thereby allowing it to be removed from the wastewater by the physical operation as well.