ENV6016_Lect#6&#7phys&chemproc_2013

ENV6016_Lect#6&#7phys&chemproc_2013 - ENV...

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ENV 6016: Biological Treatment Systems in Environmental Engineering Spring, 2013 Dr. Randall Lecture #6: Physical Unit Processes A typical domestic wastewater treatment plant will have the following unit processes: Influent bar rack Comminutor or Grinders (not typically used for the most part) Odor Control (as needed) Grit chamber (sometimes in front of comminutors to prolong their longevity) Equalization (optional) Primary Clarifier (optional) Activated sludge process Secondary Clarifier Tertiary filter (traveling bridge, disk, other) Chlorine contact basin Dechlorination Aeration cascade (optional) Preliminary Treatment Bar Racks (coarse screen): removes coarse debris. May be manually or mechanically cleaned. Typically stops debris of 0.25 to 3 inches across to pass. Fine Screens – e.g. drums for bypass, other uses Comminutors/Grinders: Still used in a lot of small plants (<5 MGD) these created problems of ropes or balls of material clogging equipment, and the presence of shredded material throughout the process and in waste sludge, limiting its' utility. Comminutors have generally fallen into disfavor. Grinders have less of these liabilities but still cause high head loss and other liabilities. They may be used, however, where large quantities of solids and rags are common. Grit Chamber: removes sand and gravel. These large dense particles settle as discrete particles. They are removed since they can result in abrasion and damage to centrifugal pump impellers and other equipment in the process train, and in addition they will deposit in corners and on the bottom of reactor basins and reduce the usable volume. Grit chambers can be rectangular basins or may be of a vortex configuration. They may or may not be aerated. The major types of grit chambers: Aerated grit chamber: diffused air introduced into bottom of basin causes a rolling pattern that keeps organics in suspension while grit and sand drop out. Allows efficient removal over a wide flow range but has the disadvantage of high power consumption. Also may strip odorous compounds into the air causing complaints. Vortex: Mechanically induced vortex causes grit to drop out in center of basin while organics go through. Effective over a wide flow range but has a proprietary design. Horizontal Flow Grit Chamber: Typical flow velocity is 1.0 fps so that grit settles out while organics pass through. Typically there may be a proportional weir that controls the amount of effluent flow so that the 1.0 fps flow velocity can be maintained over a range of influent flows. However it is more difficult to maintain the proper flow velocity for highly variable influent flows so this is a disadvantage relative to other types of grit chambers. Also proportional weirs may cause bottom scour due to the high velocity at the channel bottom. Flow control weirs result in high head loss also.
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Hydrocyclone: Removes grit while keeping organics in suspension due to centrifugal forces generated by tangential flow into the unit.
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  • Spring '13
  • RANDALL
  • Sewage treatment, current time period, primary clarifier, Dr. Randall, eq basin

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