Module IIICollection and Estimation of Sewage Bibhabasu Mohanty Asst. Prof. Dept. of civil Engineering SALITER, Ahmedabad
Course Content Different types of sewers, design period, variations in sewage flow, estimation of waste water discharge.
Introduction… Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains.
The system of pipes used to collect and carry rain, waste water and trade waste away for treatment and disposal is called the sewerage or the waste water system. There are three types of public sewers, each classified according to the kind of wastes it is required to handle.
Combined sewers Combination public sewers are the oldest variety of the three types of sewers and they are required to carry storm and sanitary wastes to some safe terminal. Rain water should be carried to some terminal not associated with the disposal plant
Sanitary sewers Sanitary sewers are those which are required to carry domestic sanitary wastes only. All rain water must be excluded from them. Combination and sanitary sewers generally are placed about ten feet under the street grade and usually are found below the centre line of the street.
Strom sewersStorm sewers are a comparatively new installation, made necessary because of sewage treatment.These sewers are made to carry only storm water and may terminate at any natural drainage area.
Problems in sewer systemThe per capita water supply (including the local and individual sources) to the population is less than the minimum requirement of water supply for the efficient performance of the sewerage system, i.e., 100 Lpcd.This will cause the operation and maintenance problems with frequent clogging, due to silting in the absence of self cleansing velocity;
The topography of the town is flat necessitating deeper excavation of trenches and more number of sewage pumping stations. Either ground water table is high or the substrata are made up of hard rock, leading to difficult construction of sewerage system.
Sanitary sewer systemPublic sanitary sewers perform two primary functions: Safely carry the design peak discharge, Transport suspended materials to prevent deposition in the sewer.3 types of sanitary wastewater collection systems based on hydraulic characteristics and purpose:
Gravity Sanitary Sewer Most common. Wastewater transported by gravity. Used to collect wastewater from residential, commercial, industrial, and institutional sources. Conveyance capacity allowances must be made for groundwater infiltration and unavoidable inflow.
Pressure (Pumped) Sanitary Sewer Economical and environmentally friendly way of collecting, transporting and disposing of wastewater from households. They are often used in areas when the landscape is either very hilly or very flat, in areas that regularly flood or have high water tables, or where it is impractical to install other types of sewerage systems.
A pressure sewer system is made up of a network of fully sealed pipes which are fed by pumping units located at each connected property. The pumping unit processes the household wastewater and transfers it to the pressure sewer located in the street via a small pipeline within the property.
The pressure sewer system is made of four key elements. These elements are: The pumping unit The boundary valve kit The house service line The control panel
Advantages Being smaller in diameter are installed at shallower depths than conventional sewers. They need not be laid on a precise grade as in gravity sewers, but can often go over hill below the hydraulic gradient line. Since there are no elements such as access manholes and sewer line is under pressure, the inflow from storm runoff is virtually eliminated. The system is cheaper than conventional sewerage where unfavourable conditions prevail.
Disadvantages They are to be differentiated from the water supply distribution system with proper colour code. Awareness among the public is required about the hazard risk of the pressure sewer system if they are tampered. Each and every house should have a collection sump and pumping facilities.
Vacuum Sewerage System The wastewater is being delivered by a gravity system to the pre tank of the domestic shaft. While the pre-tank being filled, an electronic sensor opens the interface valve. During the opening air flows into the mixing chamber and is being mixed with the wastewater and leaves the valve flowing into the vacuum pipe network as a water- air mixture.
There are also pneumatically controlled valves that open and close depending on the vacuum in the pipe network. The vacuum pump produces a vacuum in the wastewater collection tank as well as the pipe network by which the wastewater is sucked from the pipe network to the collection tank at the pumping station.
Two main categories: Sanitary Wastewater Wastewater from residential, commercial, institutional and industrial sources. Storm water Runoff Wastewater resulting from rainfall running off streets, roofs, and other impervious surfaces.
Components of a Community’s Wastewater Domestic (sanitary) wastewater – wastewater discharged from residences and from commercial, institutional and similar facilities. Industrial wastewater – wastewater in which industrial wastes predominate.
Infiltration/Inflow (I/I) – extraneous water that enters the sewer system from the ground through various means, and storm water that is discharged from sources such as roof leaders, foundation drains, and storm sewers. Storm water – runoff resulting from rainfall and snow melt
Infiltration to Sanitary Sewer Systems Groundwater/percolating water in the subsurface entering a sewer system through: Defective pipes Leaking pipe joints Poor connections Cracked manhole walls etc.
Inflow to Sanitary Sewer SystemsWater entering a sewer system from surface sources such as: Leaking manhole covers Directly connected roof gutters Cellar or foundation drains Cross connections from storm drains and combined sewers Yard and area drains Cooling-water discharges Drains from springs and swampy areas Street wash water
Sanitary Sewer Overflows Heavy rains overload the system though inflow and infiltration into cracks, ill-fitting joints, and leaky manholes. To prevent hydraulic overload of treatment plants, the excessive sewage bypasses the plant and is discharged without treatment.
Effects of SSOsEnvironmentalNutrients and toxicants may cause algal blooms and harm wildlife. Algal blooms remove O2 from water, smothering aquatic life.Decrease in water quality reduces number and range of plants and fish.
Public Health Direct contact with water containing sewage can cause skin and ear infections and gastroenteritis, and cuts become infected. Illnesses result from eating fish/shellfish that swim in sewage contaminated waters.Inhalation and skin absorption can also cause disease.
How much wastewater flow is there going to be?
Sources and Rates of Domestic WastewaterFlows Small residential districts – wastewater flows determined based on population density and average per capita contribution of wastewater. Large residential districts – wastewater flows developed based on land use areas and anticipated population density (typically rates are based on wastewater flows from nearby areas).If data is unavailable, estimate 70% of the domestic water-withdrawal rate is returned to the sanitary sewer system.
The quantity of sewage produced depends upon the quantity of water use.Generally;Average daily flow = (70 – 80) % average water consumption i.e.Average Daily Flow (ADF) of Sewage = 75%Average Daily Demand (ADD) of water consumption = 0.75 ADD
The flows in sanitary sewers vary seasonably monthly, daily, hourly. For areas of moderate sizes be expressed as;Maximum Daily Flow = 1.5 x ADF Where1.5 varies from place to placeMaximum hourly flow = (2 – 4) ADFThis is actually the peak flow
Sewers are designed on peak flow basis, however the minimum flow passing through the sewer is also important in the design of a particular sewer because at low flow the velocity will be reduced considerably which may cause silting. So the sewers must be checked for minimum velocities at their minimum hourly flows. Generally for a moderate area the following minimum flows may be assumed. Minimum Daily Flow = 2/3 ADF Minimum Hourly Flow = 1/3 ADF
Daily/Weakly variations in residential waste water flows fordry/ wet periods
Typical hourly variations in residential area waste water flows
Average Wastewater Flows from ResidentialSources
Average Wastewater Flows fromcommercial Sources
Industrial Wastewater Estimation Industries without internal reuse programs: approximately 85 to 95% of water used will be returned to the sanitary sewer system. Large industries with internal-water-reuse programs: need data on how much water is reused internally.
Average Wastewater Flows fromInstitutional Sources
How is the wastewater going to beremoved and treated?
Types of Sewer Pipes in a Typical SeparateSanitary Collection System Sanitary sewers must be laid near all occupied buildings in order to collect wastewater. Building Connecting Pipes Connects the building plumbing to the public sanitary wastewater collection system. Convey wastewater from the buildings to lateral or branch sewer, or any other sewer except another building sewer. Normally begins outside the building foundation
Lateral or Branch Sewers Forms the first element of a wastewater collection system. Usually in streets or special utility easements. Used to collect wastewater from one or more building sewers and convey it to a main sewer.Main Sewers Main sewers are used to convey wastewater from one or more lateral sewers to trunk sewers or to intercepting sewers.
Trunk Sewers Trunk sewers are large sewers that are used to convey wastewater from main sewers to treatment or other disposal facilities, or to large intercepting sewers.Interceptor Sewers Intercepting sewers are large sewers that are used to intercept a number of main or trunk sewers and convey the wastewater to treatment or other disposal facilities