It is estimated that the volume of water used daily in England and Wales (exclusive of water abstracted for cooling purposes) amounts to 5000 x 106 gal (23 x 106 m3) or approximately
95 gal (430 l) per capita per day. Domestic use accounts for nearly 1800 x 106 gal (8 x 106 m3) of this average daily total. Nearly all of the water used domestically and approximately
1500 x 106 gal (6.8 x 106 m3) of the water used by industry each day is discharged to the
sewers, yielding a total sewage flow of 3100 x 106 gal (14.1 x 106 m3) or about 60 gal (275
1) per capita per day. Nearly 96% of the UK population is connected to sewers leading to wastewater treatment works, while the rest is served by small private treatment works, cesspits or septic tanks. To achieve this degree of wastewater treatment requires some
5000 sewage treatment works serving populations in excess of 10000; these are distributed
throughout the ten Water Plc’s in England and Wales. The sewerage systems which carry the sewage to the site of treatment, or point of discharge, are of two types. Foul sewers carry only domestic and industrial effluent. In areas serviced in this way there are entirely separate systems for the collection of stormwater which is discharged directly to natural water courses. However, in older towns and cities considerable use has been made of combined foul and stormwater systems. The use of combined sewerage systems leads to very significant changes in the flow of sewage during storms. However, even in foul sewers significant changes in the flow occur due to variations in the pattern of domestic and industrial water usage which is essentially diurnal, and at its greatest during the day. Infiltration will also influence the flow in the sewage system. Although a properly laid sewer is watertight when constructed, ground movement and aging may allow water to enter the sewer if it is below the water table. The combined total of average daily flows to a sewage treatment works is called the dry weather flow (DWF). The DWF is an important value in the design and operation of the sewage treatment works and other flows are expressed in terms of it. DWF is defined as the daily rate of flow of sewage (including both domestic and trade waste), together with infiltration, if any, in a sewer in dry weather. This may be measured after a period of 7 consecutive days during which the rainfall has not exceeded
0.25 mm. The DWF may be calculated from the following formula: DWF = PQ+ I + E where, P
= population served Q = average domestic water consumption (I d-1 ) I = rate of infiltration (ld-1) E = volume (in litres) of industrial effluent discharged to sewers in 24 hours.