Sewerage and Drainage Systems
1. I ntroduction 1.1 History of Sewers Combined sewer ± to convey both sanitary sewage and storm water runoff. Disposal of the combined sewage: a). Without treatment if conditions permitted; b). With treatment if conditions do not permit raw discharge. Combined sewer interceptor systems and sewage treatment plants.
1.2 Combined Sewer System Performance Overflow - usually about 97% of the sanitary sewage is intercepted. First flush - Although only 3% of sanitary sewage is lost, more than 35% of suspended solids (SS) are lost in overflows. Conclusion - combined sewer system performance is poor, therefore change is necessary.
1.3 Solution to CSO (1) I ncrease interceptor capacity (2) Off-system Storage (3) I n-system Storage and Real Time Control (4) Outfall Treatment (5) Sewer Separation
2. Sanitary Sewage Quantities 2.1 Composition of Wastewater (Sanitary Sewage) (1) Spent Water (domestic waste) - expressed as gpcd or lpcd - 60 ~ 80 % of domestic water demand (2) Commercial and Industrial Wastes - wide variations in strength and volumes - expressed as gallons per acre per day (gpad) or population equivalent
(3) Extraneous Flows - may be the major flow component in extreme cases (4) Infiltration ± Exfiltration - infiltration is the groundwater seepage into joints and cracks - exfiltration is the sewage leakage out of the joints and cracks - expressed as gpd/acre or gpd/mile of sewer pipes
2.3 Residential Flow Rates (1) Small Residential Areas - estimate from population density and gpcd. (2) Large Residential Areas - estimate from land area and per acre contributions. (3) Measurements - whenever possible, base flow rates on actual measurements from selected typical residential areas. (4) Empirical Formulae - develop formulae from regression analysis with actual data.
2.4 Variations in Dry-Weather Flow Rates (Residential Areas) Similar to water demand. Typical ratios (Peak Factors): Max. Day/Avg. Max. Hour/Avg. Min. Day/Avg. Min. Hour/Avg.
2.5/1 3.0/1 0.67/1 0.33/1
2.5 Extraneous Flows Expressed as: (1) peak factor of avg. DWF: 3)¶VRIaUHSRUWHG (2) flow per unit area: reports as high as 0.02 cfs/acre Caused by erroneous/illegal connections, etc. Should be measured at or near the design location.
2.6 I nfiltration/I nflow Rates vary from 1000 ± 150,000 gpd/mile. Specifications vary: e.g., 1600 ± 12,000 gpd/mi for 8-inch sewer, 4000 ± 36,000 gpd/mi for 24-inch sewer. Consult local standards.
3. Hydraulics of Sewers 3.1 I ntroduction Wastewater collection differs from water distribution in two ways: (1) Conduits do not flow under pressure except in special situations. (2) Wastewater contains significant floating, suspended and dissolved solids.
The design of sewers is based on: (1) open channel flow while partially or barely full. (2) limiting velocities so as to: - reduce deposition at low velocities, - avoid erosion at high velocities.
Sanitary Sewer Design Problems: (1) Long design lifetime, (2) Hydraulic gradients are inflexible, (3) Velocity limits must be met. 0DQQLQJ¶VIRUPXOD: most widely used.
V
1.49 2 / 3 1 / 2 R S n
Values for n: (E.g., Metro Toronto) Vitrified tile, asbestos cement, n = 0.013 Concrete > 24-inch diameter (600 mm), n = 0.013 Concrete < 24-inch diameter (600 mm), n = 0.015. CMP ± paved invert, n = 0.019. CMP ± plain galvanized, n = 0.021.
2. Hydraulic Calculations To determine the diameter and slope of a pipe to transport a given flow. Pipes Flowing Full: - With slope selected first, a direct solution from the Manning equation is possible because the relationship between A and R is fixed, the relationship between D and R is also fixed. Therefore Q is a function of R only. Solve for R first for a given Q, and then determine D.
Pipes Flowing Partially Full:
d
- A direct solution from the Manning equation is not possible, because
d = f(Q), Q = f(R), R = f(d). A one-step solution is made possible with information on the Ratios of Hydraulic Elements.
Recall Manning¶V Equation:
V
Q A
1.49 2 / 3 1/ 2 R S n
Containing 4 variables (hydraulic elements):
Q or V;
R or A/Wp;
S or hL;
n
(1) For a fixed shape, n, and S; the other elements vary with depth of flow, i.e.,
v /V
(r / R)
2/3
q / Q (a / A)(r / R)
2/3
where lower case variables are for partly filled pipes, and upper case variables are for full pipes.
From the above, it can be seen that the ratios of partly-filled and full cross-section hydraulic elements are related for fixed n and S. A figure was developed from these relationships.
Use of the Ratios of Hydraulic Elements: (i) If depth of flow is known, the corresponding ratios of other hydraulic elements (area, wetted perimeter, hydraulic radius, discharge, velocity) can be read off directly from the individual curves. (ii) If the ratios of v/v-Full or Q/Q-Full are known, determine the % of depth of flow first, then determine the corresponding ratios of other hydraulic elements as in (i).
Note: If n = f (depth), corrections may be made to improve accuracy. Refer to, e.g., ASCE Manuals of Practice. Example: Determine the pipe size necessary to transport a flow of 15 cfs at a slope of 1 ft/1000 ft and n = 0.015. Also find the depth of flow in the pipe.
Solution: (1) From the Manning equation with Q = 15 cfs, S = 0.001, and n = 0.015, we find ) = 33-inch, running full; go to the next largest pipe size = 36-inch. (2) From the Manning equation with ) = ¶¶S = 0.001, and n = 0.015, we find Q = 19 cfs, running full.
(3) Thus, we know that 15 cfs will run partly IXOOLQD¶¶SLSH&DOFXODWHIORZUDWLR q/Q = 15 cfs/19 cfs = 0.79. )URP³5DWLRRI+\GUDXOLF(OHPHQWs &KDUW´ knowing q/Q = 0.79, we find d/D = 0.67. 7KXVWKHGHSWKRIIORZLQD¶¶SLSH carrying 15 cfs is: d = 0.67D = 0.67u¶¶ 23-inch. Determine velocity v in a similar manner.
3.3 Limiting Velocities of Flow Both deposition and erosion are functions of the tractive force of flowing wastewater. Tractive force: force exerted by moving liquid on channel bed. To prevent deposition, minimum velocities are 2 and 3 ft/s for sanitary sewers and storm drains, respectively.
Moving water carrying solids (sand, grit) at high velocities is very abrasive. Maximum velocities are also specified. Commonly, the maximum velocity is specified as 10 ft/s (or 3 m/s) from experience.
To prevent erosion, especially in large concrete or brick pipes, invert liners may be used. The design flows for sanitary sewers are usually the maximum hourly flow in residential areas, or 3 u Ave. Daily Flow.
3.4 Specific Energy and Critical Depth An open channel may transport a given rate of flow in one of three uniform flow conditions depending on its slope. Which condition the flow is at can be assessed by examining its specific energy, defined as: 2
E
v y 2g
where y = depth of flow, v = mean velocity.
Specific Energy Diagram: Constructed by continuously changing the slope of the channel and determining the corresponding uniform flow depth and velocity to transport a given flow rate.
For a constant rate of flow, the depth at which the specific energy is minimum is known as the critical depth (yc), the corresponding velocity is known as critical velocity (vc). Depending on the flow depth, the flow can be Critical, Subcritical or Supercritical. Critical flows are highly unstable. Sewers are designed for subcritical flows.
Ways to determine critical depth and critical velocity
At critical flow:
Q
3
g( A / B) , V
g( A / B)
A = cross-sectional area of channel; B = channel width at water surface. Easy to determine yc if B is constant; not the case for circular pipes.
4. Design of Sanitary Sewer Systems 4.1 I nformation Requirements (1) Topographic Surveys Containing: - contours, - zoning (commercial, residential, industrial; etc.), - road surfaces and right-of-ways, - land use information (for siting pump stations, STPs, etc.)
(2) Detailed Plans and Profiles containing: - basement (cellar) or sill elevations, - location of building drains, - other utilities (water, gas, electric, telephone, etc.), - soil information (type, rock, GWT, etc.)
4.2 Elements of Sewer Profile Much less flexibility in sewer plan design. Objective in Profile Design: minimize the size and depth of sewer pipe consistent with: (1) slope for min. and max. velocities, (2) min. sewer depth (pipe loading and frost penetration), (3) max. distance between manholes, (4) street grade.
The elements are related:
h1 h2
l ( g s)
h1, h2 ± depth in excess of minimum; l ± distance between manholes; g ± street grade; s ± sewer grade; x ± minimum depth of pipe.
Possible Cases: (1) g < smin, sewer become progressively deeper until pumping necessary. (2) g = smin, ideal, depth remains constant at the minimum. (3) g > smin and h1 = 0, design s = g, steeper grade permits small pipe.
(4) g > smin and h1 > 0, (4.1) If possible to get h2 = 0, Let smin < s < g, use flatter slope to get back to minimum depth. (4.2) If impossible to get h2 = 0, Let s = smin, use flattest slope to minimize h2. (4.3) If minimum grade is used already to avoid high velocities and that results in h2 < 0, use drop manhole such that h2 = 0. May need to move manholes closer.
4.3 Layout of Sanitary Sewer Systems General guidelines: (1) Obtain a topographic map of the area; (2) Locate the drainage outlet; (3) Sketch a preliminary pipe system to serve all the contributors; (4) Pipes and manholes are generally placed under streets or rights-of-ways so that all users (present and future) can readily tap on; number manholes moving upstream from the outlet;
(5) Establish preliminary pipe sizes; (6) Revise layout to optimize flow-carrying capacity at minimum cost; (7) Try to avoid pumping across drainage boundaries. (8) To obtain minimum-sized sewers, get as much flow as possible through pipes before joining the main sewer.
Small hills
Design slope is the larger of Smin and street slope, unless the street is to steep.
Detailed Calculations 1. For circular pipe flowing full (British Units: ft, s) (1)
With Q (design flow), n, S known, the pipe diameter D for full flow can be calculated using Eq. (1). This D is the pipe size recorded in Column 8. 2. Selected pipe size in Column 9 is slightly greater than the D in Column 8, its full flow rate Q-Full (Column 10) is also calculated using Eq. (1)
ĞƚĂŝůĞĚĂůĐƵůĂƚŝŽŶƐ͙ 3. The ratio of Q (design flow) and Q-Full is calculated in Column 11, the corresponding ratio of d/d-max is read from the Ratios of Hydraulic Elements chart. 4. The design flow depth d is calculated in Column 13 from the selected D and d/d-Max. 5. V-Full is calculated using the following equation with the selected D:
ĞƚĂŝůĞĚĂůĐƵůĂƚŝŽŶƐ͙ 6. V/V-Full in Column 15 is read from the Ratio of Hydraulic Elements chart corresponding to the d/dMax ratio in Column 12. 7. The design flow velocity V in Column 16 is then calculated using values in Columns 14 and 15. 8. If the V in Column 16 is not within the required range, the slope or the size of the pipe may be changed, all the calculations have to be repeated.
Experience: Ɣ6HZHUslopes be made to conform to street slopes in order to minimize excavation. x Velocities and flow depths are generally not problematic except in the upstream pipes.
4.4 Typical Design Standards Pipe M aterials: (1) Gravity lines: concrete, R/C, Asbestos Cement, VC; (2) Pressure lines (Force mains): ductile iron, steel, R/C All pipe classes are specified by ASTM by wall thickness and strength.
Pipe Sizes: (1) diameter no less than 8-inch for residential areas and no less than 10-inch for industrial and commercial areas; (2) larger pipes never feed to smaller pipes. I nvert Progression: (1) crown-to-crown (resulting in invert drop at the manhole); (2) matching 0.8udepth lines.
M anholes: (1) at every change of grade, direction, elevation, or pipe size; (2) placed every 200 ± 500 ft (60 ± 150 m) for small sewers; (3) use drop manhole for drops > 2 to 3 ft; (4) manhole losses (see Text, on storm sewers).
Trench Depth and Pipe Cover: (1) enough depth to prevent frost penetration (Toronto, 2 ft); (2) enough cover to permit load distribution (also depends on bedding). Trench Bedding:
Different Classes
4.5 Design Computations Design information kept in tabular form. Essential elements: - Peak design flows be carried at velocities great enough to prevent sedimentation, yet small enough to prevent erosion. - Not to run full but half full to full under design flow conditions. (Some airspace is desirable for ventilation and suppression of sulfide generation.) 4.6 Computer-Aided Design
