Wastewater engineering treatment and reuse by metcalf

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WASTEWATER ENGINEERING, Treatment and Reuse by Metcalf and Eddy, Inc., McGraw-Hill, Fourth Edition, 2003. Randall, C.W., Barnard, J.L., Stensel, H.D. (1992) Design and retrofit of wastewater treatment plants for biological nutrient removal, by Technomic Publishing Co., Lancaster, Pa.. Primary Treatment Primary Clarifiers: Primary clarifiers may be rectangular or circular. In a circular clarifier the influent flow may be center fed or a peripheral inlet. Circular clarifiers typically are preferred except when space is limited or when the cost savings of shared walls for multiple units is important. Lamella (down flow inclined plate clarifiers) and upflow inclined tube clarifiers have also been developed. Sometimes these are used to upgrade an existing primary clarifier. In domestic wastewater there is a large quantity of influent TSS that is suspended organic and inorganic material. Review typical WW characteristics. Note high influent TSS. The suspended solids in domestic wastewater is a suspension of both organic and inorganic material that may be colloidal in nature or have a significant settling velocity. Often these solids are flocculent in nature (i.e. the particles interact with each other) resulting in higher efficiencies than would be predicted by discrete particle settling alone. Settling column tests can give accurate estimations for design. Sedimentation can be enhanced by preaeration (causing primary solids to flocculate more readily) or chemical addition (i.e. coagulation).
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Primary treatment is used frequently in A.S. processes but not all the time. Can be counterproductive in COD limited BNR systems unless used as a primary prefermenter to produce VFAs. Removes: influent TSS and associated BOD, oil and grease, scum Typical Removals for Primary Sedimentation TSS – 50 to 70 % BOD5 – 25 to 40 % Options : Primary sludge only Cosettling of secondary WAS with primary sludge Use as a static prefermenter for primary sludge hydrolysis and acidogenic fermentation to produce VFAs (to be discussed later) Types of Settling I. For ideal (i.e. discrete) particles settling would be a function of overflow rate only (tank SA). II. For non-ideal (i.e. flocculent) particles both overflow rate and HRT make a different (i.e. depth makes a difference). III. Hindered (zone) settling IV. Compression There are 3 major design parameters for a primary settling tank: 1. HRT: 1.5 to 2.5 hrs typical 2. Surface-Loading Rate: 800-1200 gpd/sq ft day 3. Weir Loading Rate: 10000 to 40000 gal/ft day Primary clarifiers are an economic issue not a process issue. More economical with extended air than with conventional or completely mixed A.S. Design must be adequate to accommodate peak flows, not just average flows (2 to 3 times avg flow rate typically). Determine required area of tank, HRT then governed by depth.
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  • Spring '13
  • Sewage treatment, current time period, primary clarifier, Dr. Randall, eq basin

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