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Wastewater Collection

Wastewater Collection. Types of Sewer Systems. Combined In wet weather, both wastewater and stormwater flow in same system If flow is too high, untreated water is discharged into river (or channels in tehran) Separate Wastewater and stormwater flow in separate sewer systems.

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Wastewater Collection

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  1. Wastewater Collection

  2. Types of Sewer Systems • Combined • In wet weather, both wastewater and stormwater flow in same system • If flow is too high, untreated water is discharged into river (or channels in tehran) • Separate • Wastewater and stormwater flow in separate sewer systems

  3. Sewer systems

  4. Sewer Basics • Collection and transport of wastewater from each home/building to the point where treatment occurs. • Wastewater Characterization • Solids • Liquids • Pipe System • Plastic • Ductile iron • Concrete/lined • Bricks

  5. Satellite Wastewater Management • Also called “decentralized” or “distributed” • Undertaken by utilities for a variety of reasons: • Economics • To reuse water locally • To avoid expanding “centralized facilities • As communities expand, the distances from the new developments to existing wastewater treatment facilities becomes so great as to not be economically feasible • Sustainable • Cost Effective, good for the public and the environment

  6. Collection System Alternatives • Conventional Gravity sewers • Septic Tank Effluent Gravity (STEG) • Septic Tank Effluent Pump (STEP) • Pressure Sewers with Grinder Pumps • Vacuum Sewers

  7. Gravity Flow • Least expensive method of operation • Gravity sewers are never full • More vulnerable to H2S corrosion

  8. Conventional Gravity Sewer • Large pipe (200 minimum), manholes spaced 90-150 m • Designed to transport solids • Minimum velocity > 0.6 m/s (to avoid the deposition of solids) • Max velocity = 4.5 m/s • Infiltration and Inflow (I & I) • Uniform Slope between manholes • Sewer sizeMinimum slope (m/100 m) • 8-inch (200) 0.40 • 10-inch (250) 0.28 • 12-inch 0.22 • 14-inch 0.17 • 16-inch 0.14 • 18-inch 0.12 • 24-inch 0.08

  9. Conventional Gravity Sewer

  10. Conventional Gravity Sewer Alignment:24-inch sewers -600 (or smaller) should be laid with straight alignment between manholes. Changes in pipe size: When a smaller sewer joins a larger one (at a manhole), the invert (bottom) of the larger sewer should be lowered sufficiently to overcome head losses. An approximate method is to place the 0.8 depth point of both sewers at the same elevation. Sewer Materials: many types, materials and bedding shall prevent damage from external loads, and joints shall prevent leakage 0.8 depths are aligned

  11. Manholes * Located at changes in sewer size, direction, or slope * Or every 90 – 150 m * Provides access for maintenance (cleanout, etc.) * Problematic because of Infiltration and Inflow (I&I)

  12. Small Diameter Gravity Sewer Systems • Wastewater flows from home to interceptor (septic) tank where settleable solids and grease are removed • Wastewater flows by gravity to central collector pipe • Central collector pipe can flow full in certain areas • Pipe sizes are typically 4 and 6-inch diameter (100 – 150 mm)

  13. Small Diameter Sewer Layout Pipe will flow full, under pressure in these areas

  14. Interceptor Tank (same as Septic Tank)

  15. Most Sewers Rely on Gravity Flow • Most sewers are designed to flow by gravity (water flows down-hill) • Includes sewer pipe from home to septic tank or to a municipal collector pipe • Gravity sewers must follow the topography of the land • Where gravity flow is not possible, pumps are used • Individual unit pump • Large municipal lift (pump) station

  16. Hydraulics of Sewers • Most sewers designed to flow at a velocity of 0.6 m/sec when flowing half-full • Do not want pipe to flow full at peak flows • Do not want pipe flow velocity too low—can’t transport solids • Most designs are complex, but use basic hydraulic equations for computing necessary size and slope

  17. Manning Equation for Pipe Flow • Manning Equation V = velocity (m/sec) n = coefficient of roughness (dependent upon pipe material/condition) R = hydraulic radius = area/wetted perimeter (m) S = hydraulic slope (assumed to be slope of pipe) (m/m)

  18. Basic Sewer Design • Collector pipes (pipe in street) is minimum 8 inches (200) diameter (to allow cleaning) • Service pipes (home or building to collector is 4 to 6 inches (100-150 mm)diameter • Gravity sewer pipes have no bends, manholes used to make transitions in direction and pipe size • Pipe sections between manholes are at a constant grade or slope (S)

  19. Proportional velocity and discharge

  20. d/D radian  Ad Pd v/V q/Q 0.015 0.4911 28 5.48648E-05 0.0368 0.1164 0.00036 0.016 0.5073 29 6.04235E-05 0.0380 0.1215 0.00042 0.017 0.5230 30 6.61557E-05 0.0392 0.1265 0.00047 Proportional geometry elements

  21. Conventional Gravity Sewer Bedding: specified by an engineer for pipe type and anticipated loads

  22. Conventional Gravity Sewer Backfill:suitable material, free of debris, stones, etc. DO NOT DISTURB SEWER ALIGNMENT Separation from Water Lines: • 3 m horizontal • Water line 0.5 m above sewer

  23. Crown Corrosion H2S + H2O    H2SO4 H2SO4 H2S

  24. Pressure Flow • Wastewater is actively pumped • Used when gravity flow will not work, due to terrain or other factors • Construction and operation usually more expensive than gravity flow

  25. Pressure Sewers • Each home/building has individual pump • Wastewater pumped to a central treatment location • Pumps “grind” sewage solids

  26. Pressure Systems Can Pump Wastewater Treated by Septic Tank – Used for Homes Previously on Septic Systems

  27. Wet Well Design

  28. Conventional Gravity vs. Pressure Sewers

  29. Sewer Alternatives and Characteristics

  30. Septic Tank Effluent GravitySTEG • Small diameter plastic pipe • Conveys effluent from a septic tank • with an effluent filter • No solids to transport (no minimum velocity required) • Can be installed at variable (flat) grades • No manholes • Air-release valves needed at high points

  31. Septic Tank Effluent GravitySTEG

  32. STEG Sewer Components

  33. Pipe Size and Velocity

  34. Septic Tank Effluent PumpSTEP • High-head turbine pump used to pump screened septic tank effluent into a pressurized collection system. • Small diameter, plastic pipe (50 mm) • No Solids transport (no minimum velocities required) • Installed shallow • Can follow terrain • Air-release valves incorporated

  35. STEP Components • Building sewer (from house to septic tank) • Septic Tank (or interceptor tank) • Vaults/pump basins (effluent filter and pump) • Pumps (submersible, high-head, turbine) • Service lateral (1.25-inch typical) • Check Valves (at pump outlet and at edge of property)

  36. Septic Tank Effluent PumpSTEP

  37. STEP System Design Data

  38. STEP System Interceptor Tank

  39. Pressure Sewer with Grinder Pumps • Discharge pump with chopper blades in a small pump basin • Small diameter, pressure line, installed shallow • Solids and greases are transported • Relatively simple installation • Somewhat higher O&M • No I&I

  40. Pressure SewerGrinder Pump basin

  41. Grinder PumpBasin

  42. Vacuum Flow • Not very common, but use is gradually increasing • Vacuum pumps used to move waste through sewer • Same advantages and disadvantages as pressure sewers • Less water needed to transport waste

  43. Vacuum Sewers • Wastewater flows by gravity to a central collector well (up to four homes per well) • When well fills a vacuum lines pulls wastewater to a central vacuum tank • Wastewater pumped from central vacuum tank to treatment or a gravity sewer

  44. Vacuum System Components Could also comefrom existingseptic tank

  45. Vacuum Sewer • Central vacuum source maintains a vacuum on a small diameter sewer • Pulls wastewater to a central location • Ideal application: • Flat terrain • High water table • 70-100 connections to be economical

  46. Vacuum Sewer

  47. Vacuum Sewer Components

  48. Vacuum Station

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