Domestic Wastewater Treatment Mobilization Construction - AskTOP

EM 1110-3-17211 May 84CHAPTER 11ACTIVATED SLUDGE PLANTS11-1 . General considerations . The activated sludge process is abiological treatment process in which a mixture of wastewater andmicroorganisms is agitated and aerated . The activated sludge mixedliquor is subsequently separated by sedimentation, and the sludge isreturned to the process, with some withdrawn as waste sludge . Thischapter presents the different modifications of the conventionalactivated sludge process, including general bases for design, methodsof aeration, and design factors for aeration tanks, final sedimentationunits, and sludge handling systems . All designed processes willinclude preliminary treatment consisting of screening as a minimum .a . Basic process .' Basically the activated sludge process usesmicroorganisms in suspension to oxidize soluble and colloidal organicsto carbon dioxide and water in the presence of molecular oxygen .During oxidation, portions of the organic material are synthesized intonew cells . Oxygen is required to support the oxidation and synthesisreactions . The solids generated must be separated in a clarifier forrecycle to the aeration tank ; the excess sludge is withdrawn fortreatment and disposal .b . Modifications of the process . Modifications to the conventionalactivated sludge process include step aeration, contact stabilization,plug flow, completely mixed, and extended aeration systems .11-2 . Activated sludge processes . The . two basic reactor types areplug-flow and complete-mix models . In plug flow, fluid particles passthrough the tank and are discharged in the same sequence in which theyenter . Complete mixing occurs when the particles entering the tank areimmediately dispersed throughout the tank . Because of the continuousmixing, completely mixed units are less susceptible to upsets caused bytoxic chemicals or shock loadings than are plug-flow units .a . Conventional activated sludge . In a conventional (plug-flow)activated sludge plant, the primary-treated wastewater and acclimatedmicroorganisms (i .e ., activated sludge or biomass) are aerated in abasin or tank . After a sufficient aeration period the flocculentactivated sludge solids are separated from the wastewater in asecondary clarifier . The clarified wastewater flows forward forfurther treatment or discharge . A portion of the clarifier underflowsludge is returned to the aeration basin for mixing with theprimary-treated influent to the basin, and the remaining sludge isreturned to head of plant . The portion recirculated is determined onthe basis of the ratio of MLVSS to influent wastewater BOD which willproduce the maximum removal of organic material from the wastewater .Recirculation varies from 25 to 50 percent of the raw wastewater flow,