Shimin Luo

Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Civil and Environmental Engineering


Based on column settling tests, a dynamic model of final clarifiers is developed. Unlike most current final clarifier models which are based on solids flux theory, the concept of dynamic layers is introduced. Compression settling is assumed to dominate final clarifier behavior. A new equation, termed the compression settling equation, is developed to simulate the compression settling curve. Five basic parameters describing sludge settling characteristics are defined and calculated from a single column settling test. Use of dynamic layers, which emphasizes sludge blanket behavior only, reduces computation time. Different operational scenarios of an activated sludge treatment plant are simulated with the model, and control strategies are discussed. Simulations verify that both recycle rate and wastage rate can affect sludge blanket behavior in final clarifiers. A change of recycle rate can cause an almost immediate response in sludge blanket behavior while the response to a change of wastage rate is much slower. Diurnal flow variations affect the entire activated sludge system and the sludge blanket depth increases and decreases in a diurnal pattern which mimics flow rate changes. Model simulations show that transient peak flows may cause a loss of biomass in the effluent without proper control. Simulations reveal that increasing recycle rate before a peak flow can be a remedy for this situation. The step feeding process proved to be an applicable method for stabilizing blanket height during long term flow increases. Although solutions for bulking sludge are complex, operations which ensure effluent quality are investigated. Increases in both recycle rate and wastage rate proved to be possible control methods.

Library of Congress Subject Headings

Sewage -- Purification -- Activated sludge process -- Computer simulation Sewage clarifiers -- Computer simulation



Number of Pages



South Dakota State University