2. Design Basis : Generally in larger communities having wastewater flows of one (1) MGD or more, a person will generate 100 gallons of wastewater per day containing 0.17 pounds of BOD5. From a hydraulic loading and organic loading standpoint, this represents "One population equivalent (P.E.)." This equivalent population flow, 100 gallons per captia per day (gpcd), is generally accepted as the upper limit for domestic flow projection on a daily average basis for new wastewater systems serving large communities. Suspended solids (SS) concentrations from domestic flows can be reasonably assumed to contain 0.2 pounds of suspended solids per capita per day. If the use of garbage grinders in a system is significant, then the design basis should be increased to 0.22 pounds of BOD5 and 0.25 pounds of suspended solids per capita per day. If wastewater flow data are not available, the design average daily flow shall be based on 75 gpcd for communities with flows less than 1 MGD. An alternate method to determine design capacity could be justified by local water consumption records but shall not be less than 60 gpcd. These records should generally be based on the average monthly usage for the months of December, January, and February. Special consideration may be needed for communities with highly varying seasonal consumptive water use such as tourism and certain seasonal industrial uses. Projected wastewater flows for a community could be calculated by using 80 percent of the actual water consumption. Allowances shall be made for infiltration, inflow and significant commercial / industrial waste flows which will be added to the per capita flow rate. If no actual data on infiltration is available, an assumed infiltration design allowance for existing sewers should be added to the design flow. For existing systems, the minimum infiltration design allowance for the existing sewers and service lines shall be no less than 200 gallons per inch of pipe diameter per mile of pipe per day. The department recommends that actual flow measurements to determine infiltration rates be conducted for those communities that have collector sewers that lie below the groundwater table.
3. Design Flows :
a. Average Daily Flow : This rate is determined by totaling all of the average flows for existing contributing sources based on a 24-hour period in which these flows are all generated during dry weather conditions. This rate is generally used to determine items such as costs, solids loading and organics loading.
b. Minimum Flow : This rate will be based on 50 percent of the average daily flow if flow measurements are not available. c. Design Average Flow: This rate is determined to be the total of the projected population design domestic flow, additional maximum design wastewater flow from commercial/industrial plants, existing inflow and groundwater infiltration during wet weather flows.
d. Peak Design : This rate is generally used to determine hydraulic sizing and mass loading of treatment units by multiplying a factor of 2.0 to 2.5 times the design average flow rate.
e. Peak Hourly Flow : This rate shall be the ratio of peak diurnal flow to daily average flow shall be multiplied times the daily average flow as determined by Figure 1. Use of other values for peak hourly flow will be considered if justified on the basis of extensive documentation.
The design for various components of the wastewater collection system (collector sewers, interceptor sewers, pumping stations) will be based on peak hourly flow. Force mains from pumping stations will be sized to handle the pumping capacity of the station.
The design for various components of the treatment plant will be based upon either peak design flow or peak hourly flow. Generally, the organic loading of a wastewater treatment unit is based on the design average flow and the hydraulic loading of a unit is based on the peak design flow. Where recirculation is provided, the recirculation rate shall be added as required.
The design criteria for each flow rate will be designated in the Chapter pertaining to that particular unit or component.
4. New Process or Equipment : Consideration should be given whenever a new process or equipment is involved in the design, documentation of reliability and applicability in full-scale operations is recommended. Consideration should also be given to the availability of maintenance services or a convenient alternative in the event of mechanical failures. When an existing treatment works is to be upgraded or expanded, the component parts of the facility should be arranged for convenience of operation and maintenance, flexibility, economy and continuity of equipment.
5. Safety : All mechanical and treatment units should have adequate safety provisions for the personnel concerning occupational health and physical safety.
