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CVE 471 Water Resources Engineering 1/44

Assist. Prof. Dr. Bertuğ Akıntuğ

Civil Engineering ProgramMiddle East Technical University

Northern Cyprus Campus

CVE 471CVE 471WATER RESOURCES ENGINEERINGWATER RESOURCES ENGINEERING

WASTEWATER & STORM WATER WASTEWATER & STORM WATER COLLECTION AND REMOVALCOLLECTION AND REMOVAL


66. WASTEWATER COLLECTION AND REMOVAL. WASTEWATER COLLECTION AND REMOVAL

Overview

Introduction

Flow in Sewers

Design of Separate SystemsDesign of Storm Water Sewer SystemsDesign of Sanitary Sewer Systems

Constructional Details of Sewer Systems



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.



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.



Introduction

Combined SystemsIn 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.



Introduction

Separate SystemsTwo different separate systems to carry

the sanitary sewage, andstorm 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.



Overview

Introduction

Flow in Sewers





Flow in Sewers

Intended to be a gravitational flow.Pumping may required depending on topographic conditions.Designed as open channels

Flowing partly full orFlowing 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):

Sf: the friction slope in case of partially filled sewers. Sf=S slope of the pipe (uniform flow)



Flow in Sewers

Using Manning’s equation,the dimensionless velocity:

the dimensionless discharge:

In practice, to inhibit the flow instabilities, sanitary sewers are designed for: d/D < 0.75

where for full flow case A: Flow area,N: Manning coefficient,R: Hydraulic radius



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.



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 andeffective flushing system can be considered



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



Overview

Introduction

Flow in Sewers





Design of Separate Systems

Design of Storm Sewer SystemsThe 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.




Design of Storm Sewer SystemsIn 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 km2.

where Qp: Peak surface runoff (m3/s), i: rainfall intensity (mm/hr)C: Runoff coefficient (Table 8.1),A: the area of the section of city contributing to the sewer (km2)




Design of Storm Sewer SystemsThe design rainfall intensity for the selected return period is determined from the rainfall-duration-frequencycurve of the city as a function of time of concentration, tc.tc: 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, Qp, is obtained at the time of concentration.




Design of Storm Sewer SystemsTime of Concentration

tc = t0 + tf

t0 : The inlet time. The time it takes for flow from the remotest point to reach the sewer inlet.tf : The flow time in the upstream sewers connected to the outer point.

Suggested Minimum Velocities to prevent accumulation:0.75-0.90 m/s (Fair et al., 1971)0.5 – 5 m/s (The Turkish Bank of Provinces, STBP 1991)

∑=

=n

i i

if u

Lt1

Li: the length of the ith sewer along the flow pathui: the flow velocity of the ith sewer along the flow path




Design of Storm Sewer SystemsSlope 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 beused with sets of drops.




Design of Storm Sewer Systems



Overview

Introduction

Flow in Sewers






Design of Sanitary Sewer SystemsSanitary sewer and industrial wastes are collected and removed by sanitary sewer systems.Average sanitary sewage, Qav

Qav= 70-130% of the average daily water consumption.




Design of Sanitary Sewer SystemsBy examining the wastewater flows of Ankara province, curves are proposed in Figure 8.6.

PFmax : the max. possible variation of the wastewater flow over the design period as a function of the average wastewater discharge. PFmax = (Qdes/Qmax)

PFdry: the max. daily variation of dry weather flows in sewage flow as a function of the average wastewater discharge.PFdry = (Qdes/Qdry(max))




Design of Sanitary Sewer SystemsThe min. full flow velocity required for self cleansing may be taken as 0.6 m/s. The slope, S0.6 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 2.5-3.0 m/s.




Design of Sanitary Sewer SystemsHigh 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, andthe 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.




Design of Sanitary Sewer SystemsFor a conservative approach,

Qdes = (Qav x PFmax) + (Groundwater infiltration) + (Rainfall)

If (percent fullness under design flow) > 0.75 (Full Area)or

If (design velocity) > (maximum allowable value)

The system performance is also check for detecting whether solidwastes deposit at a low discharge (dry weather flow, Qlow) or not.

Qlow = (Qav x PFdry) + (Groundwater infiltration)

increase diameter to the

next commercial size




Design of Sanitary Sewer SystemsThe 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 slopedry)

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/scan be used (Figure 8.7).




Design of Sanitary Sewer SystemsSteep-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




Design of Sanitary Sewer SystemsSewage Pump

A sewage pump is placed at the endof 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 Pumphttp://www.discount-pumps.biz/grinder-pump.htm




Design of Sanitary Sewer SystemsSewage Pump

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Overview

Introduction

Flow in Sewers






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.




Sewers are laid deep enough to protect them against:breakage,traffic load, andfreezing.

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.




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.




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




The code of the Turkish Bank of Provinces propose following spacing between two successive manholes

50 m for Φ200-500 sewers70 m for Φ600-800 sewers

Spacing between manholes < 150 m.A typical manhole:




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, andthe 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.

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Sewages are usually made ofplain concrete,reinforced concrete,asbestos cement,cast iron, orcorrugated steel.

The selection of a specific type of sewer material is governed by the quantity of sewage andthe stress applied.

The common practice is to use less expensive materials since sewers are rarely subject to pressurized flow.




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.



Example 8.4



Example 8.5



Example 8.5

Solution:

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