It should be known that for areas served with sewers, waste-water flow rates are commonly determined from existing records or by direct field measurement.
On the other hand, for new developments, waste-water flow rates are derived from an analysis of population data and corresponding projected unit rates of water consumption or from estimates of per capita waste-water flow rates from similar communities.
If field measurements of waste-water flow rates are not possible and actual waste-water flow rate data are not available, water supply records can often be used as an aid to estimate waste-water flow rates.
Where water supply records are not available, useful data for various types of establishments and water – using devices are provided for making estimates of waste-water flow rates.
Analysis of Waste-water Flow Rate Data
Because the hydraulic design of both collection and treatment facilities is affected by variations in waste-water flow rates, the flow rate characteristics have to be analyzed carefully from existing records.
Where flow records are kept for treatment plants and pumping stations, at least two years of the most recent data should be analyzed. Long term record may be analyzed to determine changes or trends in waste-water generation rates. Important information that needs to be obtained through the analysis of waste-water flow rate data includes the following:
Average Daily flow – Occurring over 24 hour period based on annual flow rate. The average daily flow rate is used in evaluation treatment plant capacity and in developing flow rate ratios used in design.
Maximum daily flow – Calculated on over a 24 hour period based on annual operating data. The maximum daily flow rate is important particularly in the design of facilities involving retention time such as equalization basins.
Peak hourly flow – The peak sustained hourly flow rate occurring during a 24 hour period based on annual operating data. Data on peak hourly flows are needed for the design of collection and interceptor sewers, waste-water – pumping stations, waste-water flow meters, sedimentation tanks and channels in the treatment plants.
Minimum daily flow– The flow rate occurs over a 24 hour period based on annual operating data. Minimum flow rates are important in the sizing of the conduits where solids deposition might occur at low flow rates.
Minimum hourly flow – The minimum sustained hourly flow rate occurring over 24-hour period based on annual operating data. Data on the minimum hourly flow rate are needed to determine possible process effects and for sizing of waste-water flow meters, particularly those that pace chemical-feed systems.
At some treatment facilities, such as those using tricking filters, recirculation of effluent is required to sustain the process during low-flow period. For waste-water pumping, minimum flow rates are important to ensure that the pumping systems have adequate turn down to match the low flow rates.
Forecasting Average Flow Rates
The development and forecasting of average daily flow rates is necessary to determine the design capacity as well as the hydraulic requirements of the treatment system.
Average flow rates need to be developed both for the initial period of operation and for the future (design) period. In determining the design flow rate, elements to be considered are:
- The current base flows
- Estimated future flows for residential, commercial, institutional and industrial sources and
- No excessive infiltration/inflow.