However, you should consider existing levels of infiltration and the type of development. The infiltration allowance for the increase in population is normally at 50% of the per capita rate of the existing sewerage system. Make an allowance for infiltration from the new population. The Environment Agency may accept lower than consented trade values where you can show that the flows have permanently reduced to a lower value. For E, use the sum of the current permitted maximum daily flow and any future projections. Then using the DWF formula increase P and G according to your projections. I DWF = measured Q80 – current PG – current E This approach assumes all points on the distribution of daily flows will increase from the current values by the same percentage.Īlternatively, you can use the DWF formula. For example, a projected population increase of 15% will lead to a 15% increase in the mean flow, the standard deviation and the Q80 you need to apply and design for. The simpler approach is to use measured data to determine the Q80 value and then extrapolate it to the future Q80 value. You must take into account future growth. Calculate future DWF for existing dischargesįor existing discharges, you can use either the DWF formula or the Q80 method to determine DWF permit limits at the design horizon. If the estimated dry weather infiltration is above that determined by BTKNEEC, you should investigate and take remedial action to reduce infiltration. Although you should use realistic estimates of infiltration, you should also limit infiltration according to best technical knowledge not entailing excessive cost ( BTKNEEC). Where possible, use the measured dry weather infiltration from nearby discharges to estimate likely infiltration. The design horizon is a future date used to determine the size of the WWTW and takes account of growth in a catchment. N = number of good measurements of TDV in the yearįor example, if N = 332, then n = integer (0.2 x 332) = integer (66.4) = 66 Calculate DWF for new dischargesīase the values, such as population, on predictions that will take account of any expected increases up to the WWTW future design horizon rather than using current values. Integer = round down to the nearest integer N = nth value of TDV data ranked from lowest to highest If your data set contains less than 365 values, calculate Q80 by: If you have 365 measured values of TDV in a year ranked from the lowest to the highest, the Q80 is the 73rd value. The 20-percentile figure is that value exceeded by 80% of the recorded daily values. The nonparametric 20-percentile value of a time series of measured total daily volume ( TDV) data provides a good estimate of DWF.
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