The amount of waste water produced over time varies depending on water consumption from industry and households, precipitation (in particular rain), ... . Variations can be daily, weekly and seasonal and depend a great deal on the size of the catchment area. Whereas for example in small settlements without any industry the waste water peaks are very pronounced in the morning, at midday and in the evening, cities have only slight peaks.
Large sewer systems delay and lessen the peaks arising in the sewage plants – the amount of waste water arriving at sewage plants in cities is levelled. In a small (short) sewer system, peaks which arise largely arrive directly at the sewage plant. The sewer system must be designed for the directly arising quantities taking into account peak drainage values and variations – dimensioning to the maximum arising quantities would however be uneconomical. Dimensioning values are defined for the individual structures.
water demand per unit and day:
| municipal size [PE]
water demand incl. business (no industry)
|< 5.000||60 – 150|
|5.000 – 10.000||100 – 200|
|10.000 – 250.000||150 – 250|
|> 250.000||150 - 300|
| P: person E: employee CH: cattle and horses unity PA: patient
G: guest S: scholar T: teacher
The water demand of a region is determined by a number of factors. For example, climate, water supply, other sources of water, water composition, water price, local sewers, economic structure, standard of living, piping network losses and the like. Consumption Q is indicated by year, month, day, hour and second (indices a, m, d, h and s). The mean daily consumption is calculated by dividing Qa by 365 and is approximately 200 l/(P x d) in Austria. In tourist regions consumption can however be much higher (frequent washing of laundry, many restaurants, etc.).
water delivery-duration curve
|water consumption per month -- seasonal fluctuation [% of Qa]|
The consumption in april is about the annual mean.
Qd: average daily water delivery
Qd,max: water delivery on the day with the highest consumption Qh,max: max. hourly water delivery
fS(d): daily peak factor, dependent on the size of the supply area, the precipitation distribution and - height and the fraction of industry and business
fS(h): hourly peak factor, dependent on the size oft he supply area, the structure of the supply area and the tourism
Peak consumption at weekends is 0.80 – 0.95 of that during the week.
|sewage pipe||on Qh,max||sewage plants with equalizing reservoir||on Qd,max|
|rainwater buffer basin (bei Mischwasserkanalsystemen z.B. vor Kläranlagen)||on Qd||sewage plants without equal. reservoir||on Qh,max|
|supply area||fS(d)||fS(h)||fS(d)*fS(h)||supply area||fS(d)||fS(h)||fS(d)*fS(h)|
|country community||2,0-3,0||2,9||5,8-8,7||bigger town||1,5-2,5||2,0||3,0-5,0|
|small town||1,7-2,7||2,4||4,1-6,5||big town||1,5-2,0||1,5||2,3-3,0|