L10 -PRELIMIARY AND PRIMARY TREATMENT OF SEWAGE.pptx

Wastewater Engineering
TREATMENT OF SEWAGE
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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Classification and Application of
Wastewater Treatment Methods
The individual treatment methods are usually
classified as:
Physical unit operations
Chemical unit processes
Biological unit processes.
The different treatment methods used in
wastewater treatment plant are classified in
three different categories as:
Preliminary Treatment
Primary Treatment
Secondary Treatment
Tertiary Treatment
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Physical Unit Operations:
◦ Most of these methods are based on physical forces
◦ Screening, mixing, sedimentation, flotation, and filtration.
Chemical Unit Processes:
◦ Addition of chemicals or by other chemical reaction
◦ Precipitation, gas transfer, adsorption, and disinfection.
Biological Unit Processes:
◦ treatment by biological activity.
◦ Removes biodegradable organic substances from the wastewater, either in colloidal or dissolved form.
◦ Trickling Filters, ASP, Oxidation Ponds, Oxidation Ditches, Aerated Lagoons
screens
Sedimentation tanks
Trickling filters
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Preliminary Treatment:
◦ Separate large floating materials (e.g. Dead Animals, Tree Branches, Papers, Inorganic Materials –
sand, Oil)
◦ BOD removal – 15 – 30 %
◦ Screening, Grit Chambers or Detritus Tanks, Skimming Tanks.
Primary Treatment:
◦ Large Suspended organic solids.
◦ Sedimentation Tanks
Secondary Treatment:
◦ treatment by biological activity (aerobic/anaerobic).
◦ Trickling Filters, ASP, Oxidation Ponds, Oxidation Ditches, Aerated Lagoons
Tertiary Treatment: Chlorination, UV, Ozone
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TYPICALFLOW SHEET OFA SEWAGE TREATMENT
PLANT
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PRELIMINARY TREATMENT UNITS
Solely separating the floating materials and also the heavy settable organic and inorganic solids.
Removing the oils and grease from the sewage.
Reduces the BOD of the wastewater by about 15 to 30%.
Grit chambers or detritus tanks - remove grit and sand
Skimming tanks - remove oils and grease
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DESIGN OF SCREENS
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SCREENING
Trapping and removal of floating matter
Protection of pumps – by preventing choking
Placed before Grit Chamber
Types of Screens
Coarse Screens
Racks (opening 50 mm or more)
Rags, woods, paper ( burial and
incineration)
Medium Screens
Bar screens - steel bars parallel to each other
20 mm to 50 mm
Some inclined to about 30ᵒ to 60ᵒ to increase
opening area.
Cleaning Mechanical / Manual
Fine screens
06 mm to 20 mm
20% suspended solids from water
Brass or Bronze plates and wire mesh
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DESIGN CONSIDERATIONS FOR
SCREENS
VELOCITY OF FLOW
Low velocity – greater amount of screening
Design velocity should permit 100% removal of solids without settlement
IS 6280 – 1971 ( SPECIFICATIONS FOR DESIGN OF SCREENS)
0.6 to 1.2 m/sec found to be satisfactory
Minimum Approach Velocity – 0.3 m/sec
Materials
Steel Bars of flat rods
Clear Spacing between bars = 15 mm – 75 mm
Minimum Cross section = 10 mm ˟ 50 mm (* Larger dimensions always parallel to the flow)
Maximum screen width = 1.5 m
Angle of inclination = 45ᵒ - 60ᵒ (Manual ); 60ᵒ -90ᵒ (Automatic)
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DESIGN OF SCREENS
1. PEAK SEWAGE FLOW = VELOCITY OF FLOW THROUGH SCREEN Χ NET PROJECTED AREA
2. EFFICIENCY COEFFICENT OF BARS =
𝐂𝐋𝐄𝐀𝐑 𝐒𝐏𝐀𝐂𝐈𝐍𝐆 𝐁𝐄𝐓𝐖𝐄𝐄𝐍 𝐁𝐀𝐑𝐒
𝐂𝐋𝐄𝐀𝐑 𝐒𝐏𝐀𝐂𝐈𝐍𝐆 𝐁𝐄𝐓𝐖𝐄𝐄𝐍 𝐁𝐀𝐑𝐒+𝐓𝐇𝐈𝐂𝐊𝐍𝐄𝐒𝐒 𝐎𝐅 𝐓𝐇𝐄 𝐁𝐀𝐑𝐒
3. EFFICIENCY COEFFICENT OF BARS =
𝐍𝐄𝐓 𝐏𝐑𝐎𝐉𝐄𝐂𝐓𝐄𝐃 𝐀𝐑𝐄𝐀
𝐆𝐑𝐎𝐒𝐒 𝐏𝐑𝐎𝐉𝐄𝐂𝐓𝐄𝐃 𝐀𝐑𝐄𝐀
4. NO. OF OPENINGS = NO. OF BARS + 1
5. GROSS WIDTH OF THE BAR = (NO.OF BARS X WIDTH OF BAR) + (NO. OF OPENINGS X WIDTH OF OPENINGS)
6. GROSS AREA = GROSS WIDTH X GROSS DEPTH
7. HEAD LOSS = 0.0729 (V2 – v2 )
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Numerical
Estimate Screen requirement for a plant treating a peak flow of 60 MLD of sewage.
1. Peak Flow = 60 Ml/day =
60,000
24 𝑋 60 𝑋 60
= 0.694 cu-m/sec
2. Assume velocity through the screen as 0.8 m/sec
3. Net Area of the screen = Q/A =
0.694
0.8
= 0.87 m2
4. Efficiency coefficent of bars =
𝐂𝐋𝐄𝐀𝐑 𝐒𝐏𝐀𝐂𝐈𝐍𝐆 𝐁𝐄𝐓𝐖𝐄𝐄𝐍 𝐁𝐀𝐑𝐒
𝐂𝐋𝐄𝐀𝐑 𝐒𝐏𝐀𝐂𝐈𝐍𝐆 𝐁𝐄𝐓𝐖𝐄𝐄𝐍 𝐁𝐀𝐑𝐒+𝐓𝐇𝐈𝐂𝐊𝐍𝐄𝐒𝐒 𝐎𝐅 𝐓𝐇𝐄 𝐁𝐀𝐑𝐒
Assume that the rectangular steel abrs have a screen width of 1 cm and are placed with a clear spacing of 5
cm
=
5
5+1
=
5
6
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Numerical
5. Efficiency coefficent of bars =
𝐍𝐄𝐓 𝐏𝐑𝐎𝐉𝐄𝐂𝐓𝐄𝐃 𝐀𝐑𝐄𝐀
5
6
=
0.87
Gross Projected Area = 1.04 m2
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DESIGN OF GRIT CHAMBERS
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Detritus tanks
 Screens → GRIT → Sedimentation
Removes Inorganic materials – Sand, Silt , Clay e.t.c
Prevents damage of Pumps – due to abrasion
Removes particle size - 0.2 mm
Modifications – Aeration
Types
Horizontal Grit Chambers
Aerated Grit Chambers
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PEAK SEWAGE FLOW = HORIZONTAL VELOCITY Χ CROSS SECTION AREA
DETENTION PERIOD =
𝐃𝐄𝐏𝐓𝐇 𝐎𝐅 𝐖𝐀𝐓𝐄𝐑 𝐈𝐍 𝐁𝐀𝐒𝐈𝐍
𝐒𝐄𝐓𝐓𝐋𝐈𝐍𝐆 𝐕𝐄𝐋𝐎𝐂𝐈𝐓𝐘
◦ ASSUME SETTLING VELOCITY FOR GRIT CHAMBERS = 0.016 to 0.022 m/sec
◦ DETENTION PERIOD = 1-2 mins.
◦ ASSUME WATER DEPTH = 1 m
*For design purpose 2 units of Grit chambers have to be provided in a Sewage treatment plant.
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AERATED GRIT CHAMBERS
Spiral flow aeration tanks
Detention period = 3 mins
Hopper bottom (0.9 m) with steep slopes
Helical or Spiral flow
Air feed rate = 0.15 – 0.45 m3 /min –m ( 0.3
m3 /min –m)
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DETRITUS TANKS
Rectangular chambers ( lesser flow velocity – 0.09 m/sec)
Longer Detention periods – 3 to 4 minutes
Separate fine as well as large Grit materials
Baffle walls and Aeration Systems
Grit is removed by scraper mechanism.
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DESIGN OF SEDIMENTATION TANKS
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Sedimentation Tanks
After removal of larger floating and suspended
materials (through screening) and heavier grit
materials (in grit chamber)
Directed to Primary Clarifier (Sedimentation)
for the removal of suspended organics as well as
finer inorganic solids.
Sedimentation is essentially a phase separation
operation for separating liquid and solids.
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Types of Settling
TYPE I ‐ Discrete (or Free) settling: The particles settle without interaction
and occurs under low solids concentration. A typical occurrence of this
type of settling is the removal of sand particles.
o TYPE II ‐ Flocculent settling: This is defined as a condition where
particles initially settle independently, but flocculate in the depth of the
clarification unit. The velocity of settling particles are usually increasing as
the particles aggregates. The mechanisms of flocculent settling are not well
understood.
o TYPE III ‐ Hindered (or Zone) settling: Inter‐particle forces are sufficient
to hinder the settling of neighboring particles. The particles tend to remain
in a fixed positions with respect to each others. This type of settling is
typical in the settler for the activated sludge process (secondary clarifier).
o TYPE IV ‐ Compression settling: This occurs when the particle
concentration is so high that so that particles at one level are mechanically
influenced by particles on lower levels. The settling velocity then
drastically reduces.
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Sedimentation Basin Zones
Inlet zone:
The inlet or influent zone should provide a smooth transition of water and should distribute the
flow uniformly across the inlet to the tank.
The normal design includes baffles that gently spread the flow across the total inlet of the tank and
prevent short circuiting in the tank.
The baffle could include a wall across the inlet, perforated with holes across the width of the tank.
Settling Zone
The settling zone is the largest portion of the sedimentation basin.
This zone provides the calm area necessary for the suspended particles to settle.
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Sedimentation Basin Zones
Sludge zone:
Located at the bottom of the tank, provides a storage area for the sludge before it is removed for additional
treatment or disposal.
High flow velocities near the sludge zone should be minimized.
Sludge is removed for further treatment from the sludge zone by scraper or vacuum devices which move
along the bottom.
Outlet Zone
The basin outlet zone or launder should provide a smooth transition from the sedimentation zone to the
outlet from the tank.
This area of the tank also controls the depth of water in the basin.
Weirs are set at the end of the tank to control the overflow rate and prevent the solids from leaving the
tank before they settle out.
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Sedimentation Basin Types
Intermittent and Continuous Types:
o The intermittent tanks, also called quiescent type tanks, are those which store water for a certain
period and keep it in complete rest.
o In a continuous flow type tank, the flow velocity is only reduced and the water is not brought to
complete rest as is done in an intermittent type.
Rectangular or Circular Types:
o Settling basins may be either long rectangular or circular in plan.
o Long narrow rectangular tanks with horizontal flow are generally preferred to the circular tanks
with radial or spiral flow.
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L10 -PRELIMIARY AND PRIMARY TREATMENT OF SEWAGE.pptx

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