Guidance

Calculating dry weather flow (DWF) at waste water treatment works

Published 8 May 2018

Dry weather flow (DWF) is the average daily flow to a waste water treatment works (WWTW) during a period without rain.

The flow in a combined sewerage system will increase when it rains. This flow may vary seasonally due to changing levels of sewer infiltration and population numbers. You need to design your WWTW with enough capacity to treat the flows from the sewerage collection systems it serves.

The Environment Agency sets limits on the quality and quantity of treated effluent from WWTW so that WWTW do not cause an unacceptable impact on the environment. The flow that may be discharged in dry weather is one of these limits.

When you apply for an environmental permit for a WWTW you need to predict the DWF.

When an operator at a permitted WWTW applies to increase the DWF, the Environment Agency will usually tighten the numeric discharge quality limits. At WWTW with storm overflows limiting the maximum flow that is fully treated, an increase in permitted DWF will usually lead to an increase in the overflow pass forward flow settings and storm tank capacity requirements.

Calculate the current DWF

There are 2 ways to calculate the current DWF:

1. DWF formula.
2. Nonparametric 80% exceeded measured daily flow (Q80) in a year.

1. DWF formula

DWF = PG + I_DWF + E

Where:

DWF = total dry weather flow (l/d)
P = catchment population (number)
G = per capita domestic flow (l/hd/d)
I_DWF = dry weather infiltration (l/d)
E = trade effluent flow (l/d)

2. Nonparametric 80% exceeded flow (Q80)

The nonparametric 20-percentile value of a time series of measured total daily volume (TDV) data provides a good estimate of DWF. The 20-percentile figure is that value exceeded by 80% of the recorded daily values. It’s also known as the Q80.

If you have 365 measured values of TDV in a year ranked from the lowest to the highest, the Q80 is the 73rd value.

If your data set contains less than 365 values, calculate Q80 by:
n = integer (0.2N)

Where:

n = nth value of TDV data ranked from lowest to highest
Integer = round down to the nearest integer
N = number of good measurements of TDV in the year

For example, if N = 332, then n = integer (0.2 x 332) = integer (66.4) = 66

Calculate DWF for new discharges

Use the DWF formula for new discharges.

Base 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. The design horizon is a future date used to determine the size of the WWTW and takes account of growth in a catchment.

Where possible, use the measured dry weather infiltration from nearby discharges to estimate likely infiltration. Although you should use realistic estimates of infiltration, you should also limit infiltration according to best technical knowledge not entailing excessive cost (BTKNEEC). If the estimated dry weather infiltration is above that determined by BTKNEEC, you should investigate and take remedial action to reduce infiltration.

Calculate future DWF for existing discharges

For existing discharges, you can use either the DWF formula or the Q80 method to determine DWF permit limits at the design horizon. You must take into account future growth.

The simpler approach is to use measured data to determine the Q80 value and then extrapolate it to the future Q80 value. 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. This approach assumes all points on the distribution of daily flows will increase from the current values by the same percentage.

Alternatively, you can use the DWF formula. Use your current values for P, G, and E. Calculate dry weather infiltration as:

I_DWF = measured Q80 – current PG – current E

Then using the DWF formula increase P and G according to your projections. For E, use the sum of the current permitted maximum daily flow and any future projections. The Environment Agency may accept lower than consented trade values where you can show that the flows have permanently reduced to a lower value.

Make an allowance for infiltration from the new population. The infiltration allowance for the increase in population is normally at 50% of the per capita rate of the existing sewerage system. However, you should consider existing levels of infiltration and the type of development. For example, population increases via infill or high-rise developments may not increase sewer lengths and infiltration significantly.