Biological solids sedimentation: A model incorporating fundamental settling and compression properties

This dissertation developed, calibrated, and verified the accuracy of a one-dimensional, multiphase computational fluid dynamic mathematical model of the sedimentation of biological solids based on the separation of mixed liquor from effluent in an activated sludge secondary clarifier.{09}Model calibration required measuring the dry solids density, the floc density, and the permeability and compressive resistance of the biological solids matrix as a function of porosity. The model applied a finite volume technique with the Carmen-Kozeny equation to represent the permeability of the biological solids and a discontinuous power equation to represent the compressive resistance. The discontinuity occurred at the concentration at which a compressive force could be carried in the biological solids matrix. Below this concentration, the gel concentration, the compressive resistance force does not act.; Model verification included operation of a pilot secondary clarifier fed with mixed liquor from an aeration basin. The pilot clarifier design permitted sample collection from the center of the column at 15 cm intervals.; Steady-state concentration profiles collected at three wastewater treatment facilities compared well to the mathematical model and, overall, the mathematical model behaved in a similar manner to the pilot clarifier. The constitutive equation representing the compressive resistance did not completely capture the variability that occurs in the gel concentration of biological solids, however. Additional study of the factors affecting the compressive resistance of biological solids must be conducted before the parameters applied in this model could replace sludge settling characteristics presently being employed at wastewater treatment facilities.; Model application to wastewater design and operation should improve secondary clarifier capacity estimates as well as permit the calculation of the distribution of solids between the aeration basin and secondary clarifier and the clarifier solids residence time. Incorporation of this model into biological models presently used to design activated sludge systems and into computational fluid dynamic fluid flow techniques could increase activated sludge process design optimization.