Secondary Treatment Alternatives: Suspended Growth

by Gordon L. Culp, (M.ASCE), Chairman and Pres.; Culp/Wesner/Culp, El Dorado Hills, Calif.,

Serial Information: Journal of the Environmental Engineering Division, 1979, Vol. 105, Issue 2, Pg. 283-296

Document Type: Journal Paper


The modifications of conventional activated sludge capable of producing secondary effluent when followed by properly designed sedimentation units include extended aeration, oxidation ditches, pure oxygen activated sludge, contact stabilization, and step aeration. The activated biofilter process, which combines a trickling filter with an aeration basin, can be classified as a suspended growth system due to the nature of the solids produced. Classification of activated sludge processes is most easily done according to operational characteristics such as aeration time, food-to-microorganism ratio, mixed liquor solids concentration, and mean cell-residence time. The modified Monod model is the most general of several reaction rate models currently being used for design. The selection of alternatives necessitates assessment of the constraints and tradeoffs of each process from a technical viewpoint. As well, other factors such as performance, cost effectiveness, energy utilization, and local environmental and social considerations must be evaluated during the decision-making process.

Subject Headings: Aeration | Activated sludge | Social factors | Effluents | Oxidation | Oxygen | Biological processes | Filters

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