Relevant wastewater characteristics are discussed and appropriate methodologies are proposed to suit different circumstances. Global flow and load parameters recommended for use in design are also included.
The contributing population is an important factor in flow and load estimation and a methodology for population and employment forecasts is also presented.
Sewerage systems are designed to accommodate peak flows which are determined from the product of the average dry weather flow (ADWF) and a peaking factor (PF). The ADWF is made up of the following components :
Groundwater Infiltration (IG)
The commercial component is made up of flows from offices, shops and restaurants. The institutional component includes flows from schools and hospitals. Industrial flows are predominantly flows derived from “wet” industrial processes.
Peaking factors comprise the following components :
Diurnal Peaks (PD)
Industrial Peaks (P,)
Seasonal Peaks (Ps)
Stormwater Infiltration (Is)
The diurnal peaks reflect the general pattern of water usage throughout the day and the magnitude will generally depend on the total contributing population. Industrial peaks may be variable and affected by batch discharges. Seasonal peaks reflect variations in water usage at different times of the year. Stormwater infiltration is primarily due to expedient or illegal stormwater connections and will depend on the impermeable area connected and the storm intensity.
In practice it is often difficult to determine IG, Is, Ps and РР The equation for the peak flow, QPEAK is simplified as follows, where P is the global peaking factor:
QPEAK — (Q ] + Q2 + Q3 + QA) (P)
In this case IG is normally assumed to be included in the ADWF. Is may either be included in the ADWF or the peaking factor (P).
The most important loads to be taken into account in the design of sewage treatment works are as follows
Suspended Solids (SS);
Biochemical Oxygen Demand (BOD);
Chemical Oxygen Demand (COD);
Total Kjeldahl Nitrogen (TKN);
Ammoniacal Nitrogen (NH3N);
Escherichia coli (E. coli).
Loads are normally determined on a per capita basis except for industrial loads which must be determined according to the industrial process involved
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