WASTEWATER & STORM WATER COLLECTION AND REMOVAL

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1 CVE 471 WATER RESOURCES ENGINEERING WASTEWATER & STORM WATER COLLECTION AND REMOVAL Assist. Prof. Dr. Bertuğ Akıntuğ Civil Engineering Program Middle East Technical University Northern Cyprus Campus CVE 471 Water Resources Engineering 1/44

2 Overview Introduction Flow in Sewers Design of Separate Systems Design of Storm Water Sewer Systems Design of Sanitary Sewer Systems Constructional Details of Sewer Systems CVE 471 Water Resources Engineering 2/44

3 Introduction Sewerage: The process of collection, transmission, treatment and disposal of wastewater is called sewerage. Sewerage is required in order to maintain hygienic conditions in the environmental and infra-structural systems of cities. Wastewater can be disposed into a water recipient body. The quality of disposed water must satisfy the universal standards. Wastewater treatment may be needed if the quality is lower than the tolerable limits. CVE 471 Water Resources Engineering 3/44

4 Introduction Sewer System: The discharge (pipe size) increase in the direction of flow. Gravity system (open channel flow). Water Supply System: The discharge (pipe size) decrease in the direction of flow. Pressurized system (pipe flow). Sewage is transmitted in a closed conduit called sewer, Sewer normally flows partially filled. CVE 471 Water Resources Engineering 4/44

5 Introduction Combined Systems In conventional practice, common system is used to collect and transmit: the domestic sewage, the public sewage, the industrial waste, and the storm runoff. However, the capacity of the combined system is usually not enough to carry all waste and storm water. CVE 471 Water Resources Engineering 5/44

6 Introduction Separate Systems Two different separate systems to carry the sanitary sewage, and storm runoff. This makes sanitary sewage treatment more effective. As urbanization increases, the storm runoff increases due to higher impervious surfaces. Therefore, separate systems are better for effective management of both sanitary sewage and storm runoff. CVE 471 Water Resources Engineering 6/44

7 Overview Introduction Flow in Sewers Design of Separate Systems Design of Storm Water Sewer Systems Design of Sanitary Sewer Systems Constructional Details of Sewer Systems CVE 471 Water Resources Engineering 7/44

8 Flow in Sewers Intended to be a gravitational flow. Pumping may required depending on topographic conditions. Designed as open channels Flowing partly full or Flowing full (at most) Because of hydraulic requirements, designed with circular cross-sections. The percent fullness of a sewer is defined by the depth ratio (d/d) The geometric elements: The average cross-sectional velocity (m/s): S f : the friction slope in case of partially filled sewers. S f =S slope of the pipe (uniform flow) CVE 471 Water Resources Engineering 8/44

9 Flow in Sewers Using Manning s equation, the dimensionless velocity: the dimensionless discharge: where for full flow case A: Flow area, N: Manning coefficient, R: Hydraulic radius In practice, to inhibit the flow instabilities, sanitary sewers are designed for: d/d < 0.75 CVE 471 Water Resources Engineering 9/44

10 Flow in Sewers The low flow in sewers the suspended material may settle down Suspended material decrease the hydraulic efficiency Therefore, the min. allowable velocity or min, tractive force should be maintained. Equal self-cleaning properties at all depths can be achieved if the tractive forces are the same for all depths. CVE 471 Water Resources Engineering 10/44

11 Flow in Sewers To ensure the same self-cleaning: For flowing full S (slope of the sewer) For flow depths < 50% fullness s = 2 x S The low flow depths in sewers large slope (increases the cost) To increase the system safety against self cleaning: Small slope must be selected and effective flushing system can be considered CVE 471 Water Resources Engineering 11/44

12 Flow in Sewers The low flow depths in sewers large slope (increases the cost) Suspended material decrease the hydraulic efficiency Therefore, the min. allowable velocity or min, tractive force should be maintained. Equal self-cleaning properties at all depths can be achieved if the tractive forces are the same for all depths. To ensure the same self-cleaning: For flowing full S (slope of the sewer) For flow depths < 50% fullness s = 2 x S CVE 471 Water Resources Engineering 12/44

13 Overview Introduction Flow in Sewers Design of Separate Systems Design of Storm Water Sewer Systems Design of Sanitary Sewer Systems Constructional Details of Sewer Systems CVE 471 Water Resources Engineering 13/44

14 Design of Separate Systems Design of Storm Sewer Systems The size of an inlet is dictated by the amount and depth of storm runoff as well as the thickness of bars. Design discharge of a storm sewer system is determined from the surface runoff having a high return period. The amount of storm sewer depends on the duration and intensity of the rain as well as the size and surface characteristics of the drainage area. CVE 471 Water Resources Engineering 14/44

Design of Separate Systems Design of Storm Sewer Systems In the determination of the storm runoff, rainfall-runoff relations of the are is required.

15 Design of Separate Systems Design of Storm Sewer Systems In the determination of the storm runoff, rainfall-runoff relations of the are is required. In case of lack of relevant hydrologic information, synthetic unit hydrograph or simply the rational method can be used. The rational method is used satisfactorily for small drainage areas having sizes up to 8 km 2. where Q p : Peak surface runoff (m 3 /s), i: rainfall intensity (mm/hr) C: Runoff coefficient (Table 8.1), A: the area of the section of city contributing to the sewer (km 2 ) CVE 471 Water Resources Engineering 15/44

16 Design of Separate Systems Design of Storm Sewer Systems The design rainfall intensity for the selected return period is determined from the rainfall-duration-frequency curve of the city as a function of time of concentration, t c. t c : The time traveled by a single runoff particle from the most remote point in the drainage area to the point of interest. The maximum rate of storm runoff, Q p, is obtained at the time of concentration. CVE 471 Water Resources Engineering 16/44

17 Design of Separate Systems Design of Storm Sewer Systems Time of Concentration t c = t 0 + t f t 0 : The inlet time. The time it takes for flow from the remotest point to reach the sewer inlet. t f : The flow time in the upstream sewers connected to the outer point. t f = n i= 1 L u i i L i : the length of the i th sewer along the flow path u i : the flow velocity of the i th sewer along the flow path Suggested Minimum Velocities to prevent accumulation: m/s (Fair et al., 1971) m/s (The Turkish Bank of Provinces, STBP 1991) CVE 471 Water Resources Engineering 17/44

18 Design of Separate Systems Design of Storm Sewer Systems Slope of the storm sewer can be selected as the street slope and minimum and maximum velocities under this slope are checked. In case of steep-sloped streets, the velocities may exceed the maximum allowable values. In this case, the slope giving maximum allowable velocity may be used with sets of drops. CVE 471 Water Resources Engineering 18/44

19 Design of Separate Systems Design of Storm Sewer Systems CVE 471 Water Resources Engineering 19/44

20 Design of Separate Systems Design of Storm Sewer Systems CVE 471 Water Resources Engineering 20/44

21 Design of Separate Systems CVE 471 Water Resources Engineering 21/44

22 Overview Introduction Flow in Sewers Design of Separate Systems Design of Storm Water Sewer Systems Design of Sanitary Sewer Systems Constructional Details of Sewer Systems CVE 471 Water Resources Engineering 22/44

23 Design of Separate Systems Design of Sanitary Sewer Systems Sanitary sewer and industrial wastes are collected and removed by sanitary sewer systems. Average sanitary sewage, Q av Q av = % of the average daily water consumption. CVE 471 Water Resources Engineering 23/44

24 Design of Separate Systems Design of Sanitary Sewer Systems By examining the wastewater flows of Ankara province, curves are proposed in Figure 8.6. PF max : the max. possible variation of the wastewater flow over the design period as a function of the average wastewater discharge. PF max = (Q des /Q max ) PF dry : the max. daily variation of dry weather flows in sewage flow as a function of the average wastewater discharge. PF dry = (Q des /Q dry(max) ) CVE 471 Water Resources Engineering 24/44

25 Design of Separate Systems Design of Sanitary Sewer Systems The min. full flow velocity required for self cleansing may be taken as 0.6 m/s. The slope, S 0.6 necessary for the min. full flow velocity of 0.6 m/s under N=0.016 are given in the figure. The max. allowable velocity proposed by the Turkish Bank of Provinces is m/s. CVE 471 Water Resources Engineering 25/44

26 Design of Separate Systems Design of Sanitary Sewer Systems High groundwater table elevations above sanitary sewers may cause leakage of water to the system which increases the capacity. Infiltration into the sewer system depends on the quality of sewer installation, elevation of the groundwater table relative to that of the sewer, and the properties of the soil. The storm water may also leak from manholes and improper connections of the storm water collection system. Since storm sewer systems are places at higher elevations than sanitary sewers, possible rainfall contribution to the sanitary sewer systems should be considered. CVE 471 Water Resources Engineering 26/44

27 Design of Separate Systems Design of Sanitary Sewer Systems For a conservative approach, Q des = (Q av x PF max ) + (Groundwater infiltration) + (Rainfall) If (percent fullness under design flow) > 0.75 (Full Area) or If (design velocity) > (maximum allowable value) increase diameter to the next commercial size The system performance is also check for detecting whether solid wastes deposit at a low discharge (dry weather flow, Q low ) or not. Q low = (Q av x PF dry ) + (Groundwater infiltration) CVE 471 Water Resources Engineering 27/44

28 Design of Separate Systems Design of Sanitary Sewer Systems The slope which brings the downstream end of the sewer to the required minimum soil cover ( 2.0 m), is the most desire sewer slope since it minimizes the excavation cost of the sewer. In practice, (sewer slope) = max (street slope, self-cleansing slope dry ) If the dry weather flow fills the sewer more than 50%, the slope that provides required self-cleansing at full flow under a velocity of 0.6 m/s can be used (Figure 8.7). CVE 471 Water Resources Engineering 28/44

29 Design of Separate Systems Design of Sanitary Sewer Systems Steep-slope streets: If (velocity) > (max allowable velocity) Series of drops are used Nearly zero-slope streets: Upstream: Minimum allowable depth (2 m) Downstream: Minimum allowable slope that gives a self-cleansing velocity of 0.6 m/s CVE 471 Water Resources Engineering 29/44

Design of Separate Systems Design of Sanitary Sewer Systems Sewage Pump A sewage pump is placed at the end of the sewer where the invert elevation corresponds to the maximum permissible depth of

30 Design of Separate Systems Design of Sanitary Sewer Systems Sewage Pump A sewage pump is placed at the end of the sewer where the invert elevation corresponds to the maximum permissible depth of cover, and the sewage level is increased until the minimum allowable depth of cover from which sewage flows again under gravity. Another possibility is to place the sewage pump at the beginning of the pipe and transmitting the sewage under pressurized flow conditions. Submersible Pump CVE 471 Water Resources Engineering 30/44

Design of Separate Systems Design of Sanitary Sewer Systems Sewage Pump http://www.

31 Design of Separate Systems Design of Sanitary Sewer Systems Sewage Pump CVE 471 Water Resources Engineering 31/44

32 Overview Introduction Flow in Sewers Design of Separate Systems Design of Storm Water Sewer Systems Design of Sanitary Sewer Systems Constructional Details of Sewer Systems CVE 471 Water Resources Engineering 32/44

Constructional Details of Sewer Systems First Step: The decision for the layout of the system. Underground survey is required to detect the location of existing systems.

33 Constructional Details of Sewer Systems First Step: The decision for the layout of the system. Underground survey is required to detect the location of existing systems. Construction is initiated with the excavation of trenches. It is recommended to bury the sewer in the bed of the trench. In case of weak foundations, the sewer is to be protected by a concrete cover. CVE 471 Water Resources Engineering 33/44

34 Constructional Details of Sewer Systems Sewers are laid deep enough to protect them against: breakage, traffic load, and freezing. Sewers are laid deep enough to permit them to drain the lowest basements of the buildings in the region. Common sewer depth: 1.0 m below basement flow or about 3.0 m below the top of the foundation. Water pipes and sanitary sewers be separated by placing them on both sides of the street. Because of some local restrictions, if they are placed in a common trench, there must be enough spacing between them. CVE 471 Water Resources Engineering 34/44

35 Constructional Details of Sewer Systems The following sewers are used in the wastewater collection systems in the consecutive order. Building and house sewers, Lateral (branch) sewers, Trunk sewers, Intercepting sewers The collected wastewater is disposed into a water recipient body. CVE 471 Water Resources Engineering 35/44

36 Constructional Details of Sewer Systems Manholes are the enlarged compartments used for inspection and cleaning purposes. They should be located in places where there is a change in diameter, a change in slope, and the street junctions CVE 471 Water Resources Engineering 36/44

37 Constructional Details of Sewer Systems The code of the Turkish Bank of Provinces propose following spacing between two successive manholes 50 m for Φ sewers 70 m for Φ sewers Spacing between manholes < 150 m. A typical manhole: CVE 471 Water Resources Engineering 37/44

38 Constructional Details of Sewer Systems Wastewater systems are intended to be operated by gravity. However, pumping may also be required in case of very low basement elevation with respect to the level of street sewers. highly rolling terrains, and the transmission of wastewater from one treatment plant to anloter. Centrifugal pumps are mainly used in pressurized systems. Grinders must be used in order to break the solid wastes into smaller pieces before pumping station. CVE 471 Water Resources Engineering 38/44

39 Constructional Details of Sewer Systems Sewages are usually made of plain concrete, reinforced concrete, asbestos cement, cast iron, or corrugated steel. The selection of a specific type of sewer material is governed by the quantity of sewage and the stress applied. The common practice is to use less expensive materials since sewers are rarely subject to pressurized flow. CVE 471 Water Resources Engineering 39/44

40 Constructional Details of Sewer Systems CVE 471 Water Resources Engineering 40/44

41 Constructional Details of Sewer Systems Cleaning of sewer systems having low slopes can be accomplished by flushing them with water. Mechanical instruments, which are driven by electric motors can also be used for brushing the accumulated material. Toxic gas may be produced in sanitary sewers by biological activity. The gas content of manholes should be tested before any repair work and cleaning. CVE 471 Water Resources Engineering 41/44

42 Example 8.4 CVE 471 Water Resources Engineering 42/44

43 Example 8.4 CVE 471 Water Resources Engineering 43/44

44 Example 8.5 CVE 471 Water Resources Engineering 44/44

45 Example 8.5 Solution: CVE 471 Water Resources Engineering 45/44

Learning objectives. Upon successful completion of this lecture, the participants will be able to:

Learning objectives. Upon successful completion of this lecture, the participants will be able to: Solomon Seyoum Learning objectives Upon successful completion of this lecture, the participants will be able to: Describe and perform the required step for designing sewer system networks Outline Design