Report No FR0203



June 1991



To undertake quality assurance of the sewer flow quality data archived at WRc and to identify a suite of storm events suitable for statistical analysis.

To produce an improved interim technique for the estimation of polluting loads associated with storm sewage spills.


There is scope for improvement to the current interim procedure for determining the polluting load associated with storm sewage spills, as defined in the Sewerage Rehabilitation Manual, Second Edition (SRM 2). The sewer flow quality model, MOSQITO, which is to be used in conjunction with the WALLRUS sewer flow model, is still being developed. An improved interim measure, which makes use of additional sewer flow and quality data which are now available is, therefore, desirable.


Archived sewer flow and quality data have been screened and a series of storm events have been identified for appropriate statistical analysis.

The statistical analysis included the calculation of factors to relate determinand concentrations in base flows to storm flows, and mean concentration values and flow weighted mean concentration values for chemical oxygen demand, total suspended solids, ammoniacal nitrogen, total dissolved solids and volatile suspended solids. These parameters can be used to replace (or introduce in the case of volatile solids), some of the existing figures currently given in the SRM 2, and thus be utilised to provide enhanced estimates of the polluting loads associated with volumes of spill.


The revised factors and average flow weighted mean concentration values given in Tables 5.1 and 5.2 should be used to estimate the pollutant load discharged from combined sewer systems, in preference to Tables F.1 and F.2 in SRM 2.

Further analysis would be appropriate when/if data from more sites and for additional parameters become available.

The first foul flush is a difficult parameter to characterise. Additional work should be undertaken to ensure that the chosen method of determining the level of flush is the most suitable. Procurement of sufficient data pertaining to the first 30 minutes of the storm should also be a priority when selecting suitable events for further analysis.


Section 1 describes the background and objectives of this report. In Section 2, the method for screening the sewer flow quality data is described. Rainfall and flow characteristics are calculated for each of the selected storm events and data transformations are applied prior to statistical analysis. Results of the statistical analyses are presented in Section 3. The conclusions from the study are given in Section 4. Section 5 gives recommendations and the potential for further work.

The full report can be obtained on application to the Foundation

Copies of the report are available from FWR, price 15.00, less 20% to FWR Members.