1. Wastewater Collection

2. Types of Sewer Systems Combined Separate
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 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 size Minimum slope (m/100 m)
8-inch (200) 0.40
10-inch (250) 0.28
12-inch
14-inch
16-inch
18-inch
24-inch

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

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 Gravity STEG
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 Gravity STEG

32. STEG Sewer Components

33. Pipe Size and Velocity

35. Septic Tank Effluent Pump STEP
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

36. 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)

37. Septic Tank Effluent Pump STEP

38. STEP System Design Data

39. STEP System Interceptor Tank

40. 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

41. Pressure Sewer Grinder Pump basin

42. Grinder Pump Basin

43. 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

44. 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

45. Vacuum System Components
Could also come from existing septic tank

46. 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
connections to be economical

47. Vacuum Sewer

48. Vacuum Sewer Components

49. Vacuum Station

51. Estimating Flow rate Equivalent Dwelling Unit (EDU)
A residence with a given number of people (say 4)
Represents the average household flowrate
Design Peak Flowrate (DPF)
Flowrate expected in the collection system, assuming a given number of EDUs are discharging at the same time
Typical values for systems with >50 EDUs is 1.3 to 1.9 L/min-EDU
Total DPF QDP = 0.5 N

52. How to Obtain Quantity (lpd) numbers
Estimated design numbers in codes
These are generally higher than actual use
They provide a factor of safety
Metered flow
Add a factor of safety
Average flows from similar facilities
Peak flows
Design

53. Estimated
Wastewater
Flows

54. How much wastewater do we produce each day?
These values are rough estimates only and vary greatly by locale.
Wastewater Characteristics

55. Typical data on unit loading factor and expected
wastewater contaminant loads for individual residences. (Taken from Wisc. Comm. 83 code)

56. Daily flow pattern for a home
Peak flow
Flow – gallons/hr/person
Average flow
6 pm MN
6 AM
Noon MN
Time of day

57. What are the wastewater flows?
Domestic (sanitary)
Industrial
Infiltration/inflow
Stormwater

58. Wastewater flow rates Rule of thumb:
Iran domestic is 250 lpd/capita (ref. Tbl. 3-1 M&E)
Developing countries: 20 to 200 lpd/capita ( Tbl. 3-9 M&E)
Other: depends on facility (industry, commercial, etc.)
I/I can be significant (Fig. 3.2 M&E)

59. Factors affecting flow rates
Geographical location & socioeconomic conditions
Type of development
Season
Time of Day
Climate (rain or dry)

60. “Diurnal variations” in domestic wastewater flows

61. Wastewater flows
Flows are either normally distributed or log-normal (log of flows are normally distributed)
Statistical procedures based on flow history used to determine average (dry weather, wet weather, annual daily), peak (instantaneous, hour), maximum (day, month), minimum (hour, day, month) (Table 3-11 M&E)

62. Wastewater loads
Treatment is to decrease chemical constituents, but quantity to be treated depends on:
Concentration (typ. mg/L)
Flow rate (typ. lpd or MLD)
Mass loading:
Concentration x Flow = mass /time

63. Weekly flow Peak flow Average flow Flow – gallons/day M T Day S W T F

64. Commercial Sources Many sources Variable from site to site
High organic matter
High fat, oil and grease
Nitrogen
Phosphorus
Cleaning agents
Heat (high water temperature)

65. Influences on Wastewater Characteristics
Organic matter
BOD and TSS
Nitrogen
Organic nitrogen, ammonia, nitrate
Pathogens
Bacterial –use fecal coliform as indicator organism
Viral – coliphage as indicator organisms
Fats, oils and greases
Phosphorus
Cleaning agents/antibacterial/VOC
Excessive laundry wastes
Medications
What else????

66. Many styles of low
Flow toilets today.
All toilets sold today
has 1.6 gal./flush.

69. Manholes Needed for: Spaced every 100 to 120 m along the sewer
Change in direction
Change in pipe size
To route sewer under roads, rivers, etc.
Clean-out and inspection
Spaced every 100 to 120 m along the sewer

70. Manhole Construction Brick was once common
Most manholes are now concrete
Smaller sizes are pre-cast concrete
Large manholes are constructed in-place

71. Typical Manhole
Fig 5.1, p 134

73. Problems in Sewers Blockage Corrosion in sewers made of Inflow
Concrete
Iron
Inflow
Infiltration
Leakage

74. Blockage in Sewers Caused by sand, rocks, tree roots, various wastes
Can cause flooding
In streets
In basements

75. Corrosion in Sewers Concrete is made of cement, which is basic
H2S gas formed by anaerobic bacteria
H2S converted to sulfuric acid (H2SO4), chemically or by bacteria
Sulfuric acid reacts with cement (neutralization)
Result: cement is dissolved by sulfuric acid

76. Effects of long detention times

77. Sources of Inflow Water from roof and cellar drains
Water leaking around manhole covers
Cause excess hydraulic load on treatment plant

78. Sources of Infiltration
Seepage of groundwater into pipe joints and manholes
Controlled by
Good construction materials and practices
Avoiding construction in saturated soil, if possible

79. Leakage from Sewers Caused by
Cracked, broken, or corroded pipe
Bad connections at joints
Can cause groundwater contamination, and damage street and building foundations

81. Pump Stations
Pump (lift) sewage from low to higher elevation, generally from end of one gravity sewer section to another, higher section
Consist of a wet well and pumps
Wet well forms a place for wastewater to collect and be pumped from

82. Large Pump Station
Source: Metcalf & Eddy, Inc. Wastewater Engineering: Collection, Treatment and Disposal. McGraw-Hill:New York, 1972.

83. Small Pump Station

84. Pump Station

85. Pump Station Details

86. Main Pump Station