Requirements for settling tank (clarifier equipment) for water treatment plant
1982 Edition

Overview and Application

• Defines the scope for materials and construction methods of settling tanks used as clarifiers in water treatment.
• Emphasizes durability, corrosion resistance, and adherence to Indian Standards.

Materials Overview (Refer Table 1)

Painting Details

• Steel surfaces require red oxide primer plus three finish coats.
• Refer IS 1477 (Parts I & II) for painting procedures.

graph TD
  InfluentPipe[Influent Pipe] --> PipeFittings[Pipe Fittings]
  PipeFittings --> SludgePipe[Sludge Draw Off Pipe]
  SludgePipe --> SluiceValve[Sluice Valve]
  SluiceValve --> ScraperBlades[Scraper Blades & Frame]
  ScraperBlades --> Bridge[Rotating Bridge & Walkway]
  Bridge --> DriveEquip[Driving Equipment & Gears]
  DriveEquip --> Bearings[Bearing & Couplings]

General Specifications for Settling Tanks

• Material selections focus on long-term strength and resistance to corrosion.
• Protective finishes such as epoxy coatings or galvanization are recommended for mild steel parts.

Materials for Settling Tank Fabrication

Painting Instructions

• All steel components must be cleaned, dried, and free from rust and grease.
• Apply one coat of red oxide primer followed by three coats of finish paint as per IS 1477.

Structural and Design Considerations

• Anchor bolts should be galvanized steel to resist corrosion.
• Rectangular tanks longer than 20 meters require multiple collecting launders interconnected at the outlet end to ensure uniform effluent weir loading.
• Launders may incorporate mild steel weir plates or submerged orifice holes for flow distribution.
• Walkways and handrails must be fabricated from anti-corrosive painted or galvanized mild steel.

flowchart TD
  InfluentPipe --> SettlingTankBody
  SettlingTankBody --> SludgeDrawOff
  SettlingTankBody --> ScraperBlades
  ScraperBlades --> RotatingBridge
  RotatingBridge --> DrivingEquipment
  SettlingTankBody --> EffluentLaunders
  EffluentLaunders --> WeirPlatesOrOrifices

Mechanical Equipment and Drive Systems

1. Centre Drive System:

• Includes a drive unit with overload alarm, tipping mechanism, steel scraper arms, bridge, handrails, and walkway extending to the tank center.
• Drive train consists of motor, reduction gearboxes, spur gears, and scraper arms; chain drives permitted if required.
• Gearboxes preferably oil-immersed.
• Motors comply with IS 325-1970 (3-phase induction) or IS 996-1964 (single-phase/universal).

2. End Drive System:

• Mounted on the end carriage with rotating bridge.
• Motor drives reduction gearbox which transmits power via spur or bevel gears or chains to traction wheels.
• Recommended tip speed for circular tanks is approximately 0.3 m/min or less.

3. Materials for Mechanical Parts: | Component | Material | IS Standard Reference | |-----------------------|---------------------------------|----------------------------| | Scraper blades | Mild steel | IS 226-1975 | | Bridge and rails | Mild steel or RCC | IS 226-1975 | | Driving equipment | Cast iron, cast steel, alloy steel | IS 210-1978, IS 1030-1974, IS 1570-1961 | | Bearings | High carbon steel | IS 2898-1976 | | Couplings | Cast iron | IS 2693-1964 | | Chain sprocket | Steel | IS 2403-1975 | | Traction wheel | Rubber/chrome-nickel or carbon steel | N/A |

4. Painting Requirements:

• All fabricated steel parts must be cleaned, dried, free from rust and grease.
• Apply one coat of red oxide primer plus three coats of finish paint according to IS 1477.

Sludge Removal Systems as per IS 10313

• Scraper blades constructed from mild steel are arranged at an angle to the rake arm axis to direct sludge towards the tank center.
• Blades may be straight or inwardly curved, overlapping horizontally, covering from three-quarters to full tank diameter depending on size and peripheral speed.
• Scraper arms connect to a rotating bridge driven at the tank periphery or a center drive cage; large tanks (≥55 m diameter) may have bridges driven at both ends supported vertically.
• Square tanks utilize fixed bridges with pivoting pentograph extensions and corner blades guided on corrosion-resistant wheels.
• Rectangular tanks employ scrapers mounted on traveling bridges or endless chains.
• For long rectangular tanks (>20 m), a series of collecting launders with interconnections at the outlet end facilitate sludge collection.

Design Notes:

• Scraper blades maintain continuous sludge contact.
• Peripheral speed and blade overlap are vital for effective sludge removal.
• Dual-end driven bridges are recommended for tanks larger than 55 meters in diameter.
• Long rectangular tanks may require manual cleaning or duplicate tanks if scrapers are absent.

Operational Testing and Verification

• For bridges with a height less than 90 cm, a handrail must be installed on the top beam, ensuring a minimum total height of 1.0 m.
• Mechanical scrapers should be functionally tested for smooth operation, proper clearance, and sludge removal efficiency.
• Materials for components must align with Table 1 specifications, including steel grades and coatings.
• Painting system requires a red oxide primer coat plus three finishing coats as per IS 1477.

Detention Time and Weir Loading Parameters

Floor Slopes

• Circular tanks with mechanical scraper: slope ≥ 1:12
• Circular tanks with manual cleaning: slope approximately 1:10
• Rectangular tanks: floor slopes about 10% crosswise and ≥ 5% longitudinally
• Sludge hopper slope: ≥ 60° for vertical flow tanks, or ≥ 55° for gravity sludge withdrawal

flowchart TD
  Inlet --> RectangularTankFloor
  RectangularTankFloor --> FloorSlopes
  FloorSlopes --> Outlet
  DetentionTime --> PlainSettling[3-4 hrs]
  DetentionTime --> CoagulatedWater[2-2.5 hrs]
  DetentionTime --> SolidContact[1-1.5 hrs]

Painting and Anti-Corrosion Measures

• All fabricated steel surfaces must be cleaned thoroughly to remove rust, grease, and moisture before painting.
• The painting system includes one coat of red oxide primer, followed by three coats of finish paint.
• Refer to IS 1477 (Part I & II) for detailed painting procedures tailored for ferrous metals.
• Proper surface preparation is critical to ensure paint adhesion and long-term corrosion resistance.
• Use epoxy or other anti-corrosive paints for enhanced protection, especially in aggressive environments.

flowchart TD
  FabricatedSurface --> CleanDry
  CleanDry --> RedOxidePrimer
  RedOxidePrimer --> FinishPaintCoats
  FinishPaintCoats --> ProtectedSurface

Electrical and Motor Protection Requirements

• Overload protection devices are mandatory to prevent motor damage due to excessive current.
• Common protective elements include thermal overload relays, magnetic circuit breakers, and electronic motor protection relays.
• Motors should adhere to IS 325 (three-phase induction motors) and IS 996 (single-phase/universal motors).
• Motor frames and structural parts use mild steel or cast iron; bearings use high carbon steel per IS 2898.
• Couplings and gears follow relevant IS standards for material and durability.

Overload Relay Setting Formula

[ I_{set} = k \times I_{FL} ] where

• (I_{set}) = overload relay trip current
• (I_{FL}) = motor full load current
• (k) = multiplier factor, typically between 1.15 and 1.25

graph LR
  Motor --> OverloadRelay
  OverloadRelay --> CircuitBreaker
  CircuitBreaker --> PowerSupply
  OverloadRelay -.-> TripSignal

Procedures for Hydraulic Testing of Pipes

• Pipes must be tested at twice the maximum working hydrostatic pressure after installation and jointing, before embedding in concrete or floor laying.
• Testing includes both internal and external pressure applications to verify integrity.
• Materials for pipes and fittings comply with IS standards such as IS 1536/1537 for cast iron pipes, IS 1538 for fittings, and IS 780 for sluice valves.

Testing Steps:

1. Lay and joint pipes as per specifications.
2. Fill with water and expel air.
3. Apply test pressure equal to twice the maximum operating pressure.
4. Maintain pressure for the prescribed duration (usually 2 hours).
5. Inspect for leaks or pressure drop.

Formula for Test Pressure

[ P_{test} = 2 \times P_{working} ]

flowchart TD
  PipeLaying --> HydraulicTesting
  HydraulicTesting --> PressureApplication
  PressureApplication --> LeakCheck
  LeakCheck --> Pass{Passed?}
  Pass -- Yes --> EmbedPipe
  Pass -- No --> RepairRetest

Specifications for Weir and Launder Systems

• Circular and square tanks feature a peripheral collecting launder.
• Rectangular tanks longer than 20 meters have a sequence of interconnected launders at the outlet to maintain even flow.
• Launders may include mild steel weir plates with 90° 'V' notches or aperture orifices for flow regulation.

flowchart TB
  SettlingTank --> PeripheralLaunder
  PeripheralLaunder --> WeirPlates
  PeripheralLaunder --> OrificeHoles
  WeirPlates --> UniformOverflow
  OrificeHoles --> UniformOverflow

Key Parameters for Detention Time and Hydraulic Loading

1. Detention Time:

• Plain settling tanks: 3 to 4 hours
• Coagulated water tanks: 2 to 2.5 hours
• Solid contact (vertical flow) units: 1 to 1.5 hours

2. Overflow Rate:

• Circular horizontal flow tanks: 30 to 40 m³/day/m²
• Vertical flow tanks: 40 to 50 m³/day/m²

3. Minimum water depth at sides:

• At least 2.5 meters for rectangular and circular tanks

4. Weir Loading:

• Normal maximum weir loading is 300 m³/day/m

5. Floor Slopes:

• Circular tanks with mechanical scraper: slope ≥ 1:12
• Circular tanks with manual cleaning: slope approximately 1:10
• Rectangular tanks: floor slope about 10% crosswise, longitudinal slope ≥ 5%
• Hopper slope: ≥ 60° for sludge blanket tanks, ≥ 55° for gravity sludge withdrawal

flowchart TD
  RawWater --> PlainSettling
  PlainSettling --> CoagulatedWater
  CoagulatedWater --> SolidContact

Design Guidelines for Floor Slopes and Hopper Bottoms

• Bottom slopes enable efficient sludge movement towards collection points without disturbing sedimentation.

• Typical slopes:

  • Circular tanks with mechanical scrapers: minimum slope 1:12 (~8.3%)
  • Circular tanks cleaned manually: about 1:10 (10%)
  • Rectangular tanks: about 10% cross slope from sides to center, longitudinal slope ≥ 5%

• Hopper design:

  • Inlet pipe runs across tank center, descending vertically into hopper.
  • Sludge draw-off pipe follows hopper slope and exits vertically through tank wall.
  • Hopper bottom inclined to facilitate sludge collection and removal.

flowchart TD
  InletPipe --> HopperBottom
  HopperBottom --> SludgeDrawOffPipe
  SludgeDrawOffPipe --> SludgeOutlet

Installation and Maintenance Requirements

• Materials for components must conform to IS standards (e.g., cast iron pipes IS 1536/1537, fittings IS 1538, sluice valves IS 780, mild steel parts IS 226).
• All steel surfaces should be cleaned, dried, and free from corrosion before application of protective coatings.
• Painting involves one coat of red oxide primer followed by three finish coats as per IS 1477.
• Regular inspection is necessary for sludge draw-off pipes and sluice valves to prevent blockages.
• Bearings and gears require periodic lubrication; bearings made per IS 2898 standards.
• Protective paint layers should be maintained to prevent corrosion.

graph LR
  InfluentPipe --> PipeFittings
  PipeFittings --> SludgeDrawOffPipe
  SludgeDrawOffPipe --> SluiceValve
  SluiceValve --> ScraperBlades
  ScraperBlades --> RotatingBridge
  RotatingBridge --> DrivingEquipment

Related Indian Standards Referenced by IS 10313

• Painting Codes:

  • IS 1477 (Part I) - 1971: Code of Practice for Painting Ferrous Metals - Surface preparation
  • IS 1477 (Part II) - 1971: Code of Practice for Painting Ferrous Metals - Painting procedure

• Motor Specifications:

  • IS 325 (Third Revision): Specification for three-phase induction motors
  • IS 996 (Revised): Specification for single-phase small AC and universal motors

Painting Requirements Summary

• Steel surfaces must be properly cleaned and dried.
• Apply one coat of red oxide primer followed by three coats of finish paint.

flowchart LR
  SteelSurfacePreparation --> DryClean
  DryClean --> RedOxidePrimer
  RedOxidePrimer --> FinishPaint
  FinishPaint --> DurableCoating