Wastewater Collection System

Transcription

1 WASTEWATER COLLECTION SYSTEM CE Wastewater Collection System The function of the collection system is to collect the wastewater from residential, commercial, and industrial areas within the service area and transport it to the treatment plant or disposal area. The system poses a different challenge than the water distribution system. Wastewater must be transported from the point of collection to the treatment plant as quickly as possible to prevent septic conditions. Waste flows are highly variable Waste flows contain cores solids System may carry storm runoff 2 1

2 Hydraulic Considerations Wastewater systems are usually designed as open channel except where lift stations are required to overcome topographic barriers. The driving force for open-channel flow is gravity. The velocity in an open-channel is determined by Manning s equation: / 3 1/ 2 V = R S n Where; V = velocity of flow, fps n = coefficient of roughness R = hydraulic radius, ft S = slope of energy grade line 3 Hydraulic Considerations Sanitary sewers are usually circular in cross section, and therefore it is cumbersome to compute values for the hydraulic radius and the cross sectional area for conditions when the pipe is partially full. The following figure is used to compute the partial-flow values from full flow conditions. 4 2

3 5 Hydraulic Considerations It is important from design point of view to maintain uniform flow. Uniform flow for a given slope and discharge may occur at less than or grater than critical depth. At or near critical depth, the flow is highly unstable. Therefore, it is important to avoid flow at critical depth. Critical depth occurs when the specific energy is a minimum. Specific energy is defined as: E s = d + (V 2 /2g) Where; d = depth of flow; V = mean velocity The critical depth is calculated by taking the derivative of the above equation with respect to depth, setting it equal to zero, and solving for d c. 6 3

4 Design of Sanitary Sewer House and building connections: Connection from the main sewer to houses or other buildings are commonly constructed of vitrified clay, concrete, or asbestos cement pipe. Building connections are usually made on about a 2% grade with a minimum of 6-in or larger pipe. Collecting sewer: Collecting sewer gather flows from individual buildings and transport them to an interceptor or main sewer. Location of these sewers is dictated by local standards. The collecting sewer should be capable to carry the flow of the present and future population of the area it is serving. The design flows are the sum of the peak domestic, commercial, and industrial flows in addition to infiltration flow. The collecting sewer must transport this design flow when flowing full. 7 Design of Sanitary Sewer Collecting sewer: (continue) Manholes are normally located at change in direction, grade, pipe size, or at intersections of collecting sewers. Manholes are usually spaced no more than 400 ft apart to permit for inspection and cleaning when necessary. The minimum pipe size is usually 8-in. Intercepting sewer: Intercepting sewer are expected to carry flows from the collector sewers in the service area to the point of treatment or disposal. They usually follow the valleys or natural stream beds of the drainage area. For 15- to 27-in sewers, manholes are constructed at least every 600 ft. Grades should be designed so that the criteria regarding maximum and minimum velocities are satisfied. 8 4

5 Flow to treatment plant 9 Design of Sanitary Sewer Materials: Collecting and intercepting sewers are constructed of the following materials: Asbestos cement pipe. Concrete pipe. Vitrified clay pipe. Brick. Plastic or PVC. Care should be taken not to exceed permissible structural loading on the material selected. Information on pipe loadings are readily available from the manufactures. 10 5

6 Design of Sanitary Sewer System Layout: The first step in designing a sewerage system is to establish an overall system layout that includes a plan of the area to be sewered, showing roads, streets, buildings, other utilities, topography, soil type, etc Care should be taken to include undeveloped areas. A tentative layout of collecting sewers and intercepting sewer should be made. The sewer location should minimize the length required while providing service to the entire area. Normally, the sewer slope should follow the ground surface so that flows can follow the approximate path of the area s surface drainage. In some instances, it may be necessary to lay the sewer slope in opposition to the surface slope, or to pump the wastes across a drainage divide. 11 Ground surface Sewer Ground surface Sewer 12 6

7 13 Design of Sanitary Sewer Hydraulic design: The main consideration for the hydraulic design is that the peak design flows be carried by the flowing full at velocities greater enough to prevent sedimentation, yet small enough to prevent erosion. The hydraulic gradient must not change abruptly at changes in horizontal direction, pipe size, or quantity of flow. This is in order to minimize head loss. In case pumping is required, force mains (pressure flow) must be designed to carry the flows. It should be noted that the cost of pumping is an important consideration. In pressure flow, velocity should be designed to prevent sedimentation and erosion. Values ranging between 3-5 fps are common. 14 7

8 Design of Sanitary Sewer Protecting against floodwaters Because the volume of sanitary wastewater is small compared to flood flows, it is important to prevent admittance of large surface-runoff volumes to the sewers. If large quantities of floodwater enter the sewers, the treatment plant will be overloaded resulting in loss of degree of treatment. To prevent or reduce floodwater from entering the sewers: The manhole stacks are raised above the flood level (for interceptors sewers) if possible. Watertight manhole covers are employed. 15 Design of Sanitary Sewer Inverted Siphons An inverted siphon is a section of sewer constructed below the hydraulic gradient due to some obstruction and operates under pressure. Usually two or more pipes are needed to handle flow variability. The minimum flow in a siphon must be great enough to prevent deposition of suspended solids. When more than one pipe is required; The small pipe handle the low flows Intermediate flows are handled by the small pipe and the a larger one Maximum flow may require the use of three or more pipes. By subdividing the flow in this manner, adequate cleaning velocities are insured for all flow magnitudes. 16 8

9 Inlet Structure of inverted siphon Excess flow 2 Inflow Minimum flow 1 Flow exceeding 1 & 2 Water Level 3 Water Level 2 Water Level