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    WWTP (Primary Treatment) - T

    Uploaded by

    sarfa
    Here are the steps to solve this design problem: 1) Given: Flow (Q) = 7570 m3/d Minimum depth = 3 m Overflow rate (HL) = 36.7 m3/m2.d 2) To find diameter (D): Q = HL * π * (D/2)2 7570 = 36.7 * π * (D/2)2 D = 12 m 3) Depth = Minimum given depth = 3 m 4) Circumference (C) = π * D = π * 12 = 37.7 m 5) Volume (V) = π * D2 * Depth/

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    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd

    WWTP (Primary Treatment) - T

    Uploaded by

    sarfa
    137 views22 pages
    Here are the steps to solve this design problem: 1) Given: Flow (Q) = 7570 m3/d Minimum depth = 3 m Overflow rate (HL) = 36.7 m3/m2.d 2) To find diameter (D): Q = HL * π * (D/2)2 7570 = 36.7 * π * (D/2)2 D = 12 m 3) Depth = Minimum given depth = 3 m 4) Circumference (C) = π * D = π * 12 = 37.7 m 5) Volume (V) = π * D2 * Depth/

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    13. WWTP (Primary Treatment)-T

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    Here are the steps to solve this design problem: 1) Given: Flow (Q) = 7570 m3/d Minimum depth = 3 m Overflow rate (HL) = 36.7 m3/m2.d 2) To find diameter (D): Q = HL * π * (D/2)2 7570 = 36.7 * π * (D/2)2 D = 12 m 3) Depth = Minimum given depth = 3 m 4) Circumference (C) = π * D = π * 12 = 37.7 m 5) Volume (V) = π * D2 * Depth/

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    137 views22 pages

    WWTP (Primary Treatment) - T

    Uploaded by

    sarfa
    Here are the steps to solve this design problem: 1) Given: Flow (Q) = 7570 m3/d Minimum depth = 3 m Overflow rate (HL) = 36.7 m3/m2.d 2) To find diameter (D): Q = HL * π * (D/2)2 7570 = 36.7 * π * (D/2)2 D = 12 m 3) Depth = Minimum given depth = 3 m 4) Circumference (C) = π * D = π * 12 = 37.7 m 5) Volume (V) = π * D2 * Depth/

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 22

    WASTE WATER TREATMENT

    Primary Treatment (Sedimentation)


    • Primary treatment reduces the organic loading, by removing
    settleable, suspended and floatable materials.
    • TSS and BOD are removed
    • Waste water velocity is reduced to facilitate settling
    • Removes floating grease and scum from surface through
    skimmers, and settled sludge through scrappers and pumps for
    disposal/further treatment.
    • Sedimentation tanks are used for primary treatments
    Sedimentation tank
    Sedimentation tank, also called settling tank , is an essential component of a
    modern system of water supply or wastewater treatment.

    A sedimentation tank allows suspended particles to settle out of water or wastewater as


    it flows slowly through the tank, thereby providing some degree of purification.

    A layer of accumulated solids, called sludge, forms at the bottom of the tank and is
    periodically removed.

    In drinking-water treatment, coagulants are added to the water prior to sedimentation in


    order to facilitate the settling process.

    In modern sewage treatment, sedimentation is usually preceded by treatment using bar


    screens and grit chambers to remove large objects and coarse solids.
    Types of sedimentation tanks

    A. Septic Tanks:
    • These serve mixed purpose of settling and skimming as well as
    an anaerobic digester.
    • Long settling times of 6-8 hours.
    • Decomposed solids are not separated from water and once the
    tank gets full, solids are discharged along with water.
    • Suitable for smaller facilities, not for large scale.
    B. Imhoff Tank

    • Similar to septic tank: removal of settleable solids and


    anaerobic digestion
    • Difference: Imhoff is a two story tank: settling compartment
    and digestion tank
    • Gas vents also present
    • A slot in settling tank passed the settled solids into digestion
    compartment
    • Anaerobic decomposition in lower tank, sludge is removed.
    C. Clarifiers

    Clarifiers remove heavy organic solids in order to reduce the


    organic loading on the secondary processes.

    Clarifiers are used to reduce the velocity of the water to a point


    where organic solids will settle to the bottom of the tank.

    The sludge is collected and sent to the solids handling


    processes.

    Clarifiers also contain equipment that is used to remove floating


    solids and greases from the surface.
    Rectangular clarifiers:
    • Water introduced at one end and discharged at other end.
    • Settled sludge removed via hopper at one end, through single scrapper or flights on
    parallel chains.
    Circular clarifiers:
    Effluent enters at the middle of the tank, and flows outward Settled sludge
    pushed towards the hopper in the center, surface skimming also involved
    Detention time in all clarifiers is 2-3 hours
    Zones of CLARIFIERS

    The Influent Zone is inlet to the


    The Settling Zone represents the largest
    clarifier. Water enters the end of a
    portion of the tank. The water velocity is
    rectangular tank, or the center of a
    reduced to 0.03-0.05 fps and the detention time
    circular tank. The influent zone is
    should be about 2 hours. The problem is that
    equipped with a baffle. Circular tanks will
    the flow rises and falls during a 24-hour period.
    have a collar-type circular baffle that
    As the flow increases the detention time
    directs the water down as it enters the
    decreases. Slowing the water down for this long
    center of the tank. Rectangular tanks will
    allows the sludge to settle to the bottom while
    have a perforated wall that spreads the
    the water is removed from the top of the tank.
    water laterally across the influent end of
    Clarifiers are usually only about 8-12 feet deep
    the tank. The purpose of the baffle is to
    and have a surface loading rate of about 800-
    prevent short-circuiting. Short-circuiting
    1400 gpd/sq ft. This keeps the upward velocity
    reduces the detention time in parts of the
    of the water low enough to minimize solids
    tank causing solids to carry over into the
    carryover.
    secondary process and resulting in uneven
    sludge distribution.
    The Skimming Zone is at the surface of The Effluent Zone is the part of the tank where the
    the tank. Solids and greases that have a settled water leaves to go to the secondary treatment
    specific gravity of less than 1.0 will float to processes.
    the surface of the clarifier. A skimmer arm
    is attached to the rake assembly. It skims
    the surface as the rake rotates. In a
    rectangular tank the sludge rakes act as The Sludge Zone is the bottom of the tank where the
    skimmers when the chain brings them to settled sludge collects and compacts. Sludge blanket
    the surface. Floating scum is deposited in a depth should be measured and sludge should be removed
    grease or scum hopper. This grease and at least every shift. Sludge rakes push the sludge to one
    scum must be properly landfilled to prevent end or the center of the tank so that it can be pumped
    odor problems. out.
    Key Factors in Sedimentation Operation
    Process control calculations are used in the sedimentation process to
    determine:

    1. Percent removal
    2. Hydraulic detention time
    3. Surface loading rate
    5. Percent total solids (% TS)
    Percent Removal
     
    • The expected range of percent removal for a primary clarifier
    is:
    Settleable solids 90–95%
    Suspended solids 40–60%
    BOD 25–35%
    • Percentage removal Efficiency: (In-Out) x 100
    In
    Hydraulic Detention Time

    • The
      length of time the water stays in the tank is called the
    hydraulic detention time (HDT) measured in hours or
    minutes 1440min/day in case of
      minutes

    HDT=
     
    Hydraulic Loading Rate (Surface Settling Rate/ Surface
    Overflow Rate)

    • Surface loading rate is the number of gallons of waste water


    passing over 1 square feet of a tank in one day.

    HL= Q/A
    Design Considerations
    • The tanks are designed on the bases of overflow rate or the hydraulic surface
    loading rate (HL) expressed as : Q/A (m3/m2.d), where A is the surface area of
    the tank.

    • The overflow rate affects the efficiency of the tank as shown in the figure
    below. The range of (HL) is 30-50 m3/m2.d at the average design flow.

    • The hydraulic detention time should not be less than 1.5 hrs based on the
    average flow. The hydraulic detention time is defined as Θ = V/Q.
    • The diameter of circular tanks is in the range of 3 to 60 m, and their
    depth is in the range of 3 to 6 m.

    • The length of rectangular tanks is in the range of 10 to 100m, and the


    depth is in the range of 2.5 to 5 m.

    • The ratio of length to width is in the range of 1.0 to 7.50.

    • The ratio of length to depth is in the range of 4.2 to 25.


    Design Problem:

    A circular primary sedimentation tank/Clarifier for a municipal wastewater treatment


    plant is to be designed for a flow of 7570 m3/d. The minimum side wall depth is 3
    m. Assume the overflow rate (HL) as 36.7 m3/m2.d.

    Design Parameters

    1. The diameter of the tank.


    2. The depth of the tank.
    3. Check for detention time
    4. The Circumference of the tank.

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