Wastewater Collection and Pumping

• The presentation discusses the collection and pumping of wastewater.

Wastewater Disposal Modes

• There are several modes of wastewater disposal.
• One mode involves having no system at all.
• On-site systems are another mode of disposal.
• Pit-latrines fall under the umbrella of on-site systems
• Septic tanks and soak away systems are also on-site systems.
• Satellite Wastewater Treatment Plants are a disposal method.
• Centralized Wastewater Treatment Plants handle wastewater for populations greater than 100,000 people.

Types of Sewers

• Building sewers, sometimes called building connections, connect building plumbing and convey wastewater to lateral sewers.
• Lateral sewers form the first element of a wastewater collection system, typically in streets or easements.
• Main sewers convey wastewater from one or more lateral sewers to trunk or intercepting sewers.
• Trunk sewers are large sewers that convey wastewater from main sewers to treatment facilities.
• Intercepting sewers are large sewers intercepting main or trunk sewers, conveying wastewater to treatment.
• Rising mains transport wastewater from a pumping station to a WWTP (Wastewater Treatment Plant) or Trunk sewer.

Design Example: Covell Park

• A preliminary design for a trunk sewer in Covell Park includes flowrates, pipe sizes, and slopes, assuming a flat ground slope and an established location.
• Planning data includes expected saturation population densities and wastewater flows for various housing types within the Covell Park Development.
• Commercial and industrial wastewater flowrate allowances are estimated at 20 to 30 m³/ha.
• Average peaking factors are 1.8 for commercial and 2.1 for individual flows.
• Peaking factors for residential flows are obtained from Fig. 3-4.
• A planned school in Covell Park will serve 2000 students at capacity.
• The school's average flow is 75 L/student/day, with a peaking factor of 4.0.

Summary of Basic Design Criteria

• The design period should use the saturation period (time to reach saturation population).
• Population densities should use the data in the table provided.
• Residential wastewater flows should use the data in the table.
• Commercial flows are designed for 20 m³/ha-day.
• Industrial flows are designed for 30 m³/ha-day.
• Institutional flows (schools) are designed for 150 m3/day, calculated as (2000 students x 75 L/student/day) / (1000 L/m3).
• For residential areas, the peak infiltration values can be derived from curve B for new sewers in Fig. 3-5.
• For commercial, industrial, and institutional areas, use peak values from curve B in Fig. 3-5, but apply only 50% of the actual area when computing the infiltration allowance, as the total length of sewers is generally less than that in residential areas.
• Assume that steady-flow inflow is accounted for in the infiltration allowance.
• For residential areas, use the curve in Fig. 3-4 for peaking factors.
• The peaking factor for commercial areas is 1.8.
• The peaking factor for industrial areas is 2.1.
• The peaking factor for institutional (school) areas is 4.0.
• The peaking factor for infiltration is 1.6.
• Use the Manning equation with an n value of 0.013 for hydraulic design.
• Use Figs. 2-12 and 2-13 to simplify computations.
• To prevent solids deposition at low flows, maintain a minimum velocity of 0.75 m/s during peak flow conditions.
• Minimum cover is established by the local community building code

Tabulation Form for Preliminary Design

• The line under consideration is identified in column 1.
• Columns 2 and 3 are used for manhole numbers. The example uses numbers from Fig. 4-4. Additional manholes may be needed in a detailed design.
• The subarea is identified in column 4.
• Use separate entries for each subarea when multiple subareas contribute to a line.
• Column 5 is used for the area, in hectares, for residential subareas.
• Population density data from the table are entered in column 6.
• The population density value for subarea A-4 was derived by assuming equal areas for each residential development type.
• Incremental and cumulative population data are in cols. 7 and 8.
• Average unit flow data from the table are placed in column 9. For flow area A-4, the data was calculated by averaging the data in Table 4-6. A simple average was considered conservative.
• Cumulative average residential flow is given in column 10
  • The cumulative average flow is equal to 3756 m³/d (2417 m³/d + 5952 per x 225 L/cap.d/103 l/m³).
• The peaking factor derived from Fig. based on the cumulative av. flow data is in column 11.
• In cols. 13, 14, and 15, enter commercial area, the corresponding unit flow, and the cumulative average flows
• The given peaking factor for the commercial area is in col. 16. The computed cumulative peak commercial flows are entered in col. 17.
• The entries in columns 18 through 22 concern Industrial Flows.
• Columns 23 through 27 detail the institutional flows.
• The cumulative average and peak domestic and industrial wastewater flows are summarized in columns 28 and 29.
• To determine the infiltration allowance, list the area for each subarea in col. 30, with the cumulative area in col. 31.
• The unit infiltration allowance, obtained from curve 23 in Fig. 3-5, using cumulative area data (col. 31), is placed in col. 32, with cumulative infiltration allowance in col. 33.
• These values include the domestic, industrial, and infiltration inflow contributions.
• The total cumulative average and cumulative peak design flows for various lines are in cols. 34 and 35.
• Col. 36 contains the peak design flows, expressed in cubic meters per second.
• Sewer design data are determined through col. 36 in cols. 37 through 40.
• Assess sewer pipe diameter, slope, capacity, and velocity.
• Select a pipe diameter line nearest to the part found.
• Move along the diameter line to a point that satisfies the minimum velocity and peak discharge requirement.
• The required information is entered in cols. 37 through 40 of Table.
• The final step in designing a sanitary sewer involves determining the invert elevations of the sewers.
• A computation table saves time and displays data and computed results for subsequent use.
• Specific columns in a given computation table depend on factors in arriving at peak design flows.
• Most sanitary and civil engineering consulting firms develop their own tabulation forms for computations.
• Forms may differ, but information is usually the same.
• Some engineering firms have also developed computer programs for sewer design.