Report No FR/CL0008



March 1998




Continuous river quality standards are based on percentiles of sanitary determinand concentrations applying over full 1 2-monthly periods (i.e. annual percentiles). These percentiles are calculated from spot-sample data using the standard Agency method for parametric percentile estimation.

Intermittent standards, on the other hand, are based on the environment's ability to tolerate brief episodes of poor water quality provided that the episodes are not too frequent. The standards are expressed as a three-dimensional measurement: threshold concentrations; the durations of these episodes in excess of these thresholds; and frequency of the episodes (usually given in terms of the average return period of the episodes - equivalently, the average time between successive episodes). The definitions of the intermittent standards are defined in the Urban Pollution Management (UPM) Manual (FWR Report FR/CL 0002, 1994).


The aims of the project were to

  1. examine and assess the relationships between continuous and river quality standards;
  2. demonstrate the circumstances in which fixed annual percentile standards might be an acceptable practical substitute for the full set of UPM intermittent standards.


These were

  1. confirmation that trade-offs between decisions on continuous and intermittent discharges do not waste money and do not put the environment at risk;
  2. increased confidence that SWQOs will be met; and
  3. possibility of establishing a sound policy for the setting of upper tier limits and for determining the extent of flows for full treatment.


These can be summarised as follows.

  1. Foundation for Water Research (FWR): The FWR was responsible for the management of the overall programme through the appointment of a Programme Manager whose roles were to liaise with all participants and undertake report production and dissemination.
  2. Water Research Centre (WRc): WRc was appointed as the Specialist Sub-Contractor to undertake the statistical analysis and interpretation of the data and to prepare the report on the Project.
  3. Regulators: The Regulators, comprising the Environment Agency, the Scottish Environment Protection Agency, and the Department of the Environment for Northern Ireland represented by SNIFFER (the Scotland and Northern Ireland Forum for Environment Research) funded the programme management by FWR and the work of the Specialist Sub-Contractor, WRc.
  4. Operators: The Operators, comprising the Water Services Companies of England and Wales, together with the Water and Sewage Treatment Operators in Scotland and Northern Ireland, permitted the data generated during Phase I to be used for this Project, and provided additional information where this was available.
  5. Steering Group: A Steering Group representing all participants in the Project was formed to oversee the study and approve outputs.


In order to be able to examine the relationship between continuous and intermittent standards it is necessary to observe river quality just downstream of a treatment works (effluent outfalls and sewer overflows). The observations of river quality need to be high frequency ('continuous monitoring'), at most hourly intervals. As the relationship must be examined under a variety of circumstances (to study its behaviour and sensitivity), observations are also needed at many different sites each with differing combinations of upstream river conditions and treatment works operation configurations. Real data, such as described above, do not exist at present and to gather these data would be a large and costly study to set up. Consequently, the proposed method for getting the required information was to simulate downstream river quality.

The simulation of downstream river quality has been achieved using a combination of stochastic simulation, deterministic modelling, and a Warn-Brew mass balance approach (Figure 1). Enough data were generated by this simulation process to enable the relationship between continuous and intermittent river quality standards to be examined in detail under a range of conditions, since all the aspects of the simulation are controlled.

The upstream river quality and flow were generated by randomly picking values from appropriate Log-Normal distributions with cross-correlations and autocorrelation. The effluent flow and CSO flow and quality were produced by SIMPOL, driven by STORMPAC-g enerated rainfall time series. Effluent quality was simulated using a statistical model with seasonal and diurnal components, various cross-correlations, and autocorrelation. Hourly values were drawn form each of the modelling stages and combined using mass balance to produce time series of hourly downstream quality values.

Each simulation 'run' produced a single downstream time series of quality for both BOD and total ammonia, which was then reduced to a UPM statistic and an annual percentile. Therefore each run gave a pair of statistics, one an intermittent and one a continuous measure. Simulations were done for different river and treatment works catchment area conditions producing around 700 pairs of intermittent and continuous statistics for each determinand.

ii These pairs of statistics were analysed to identify any relationship there might be between them. The results of the analyses were validated using continuous monitoring data from sites downstream of treatment works.

Figure 1 Data flow diagram showing the dependence of the modelling components.


At present river quality standards are expressed in the following way.

Note that the fundamental intermittent standards used for UPM are given for DO and unionised ammonia and that the continuous standards are given for BOD, AmmN and DO. Because of this, and because of the nature of the data which are available from the first phase of the NCTP, this study concentrates on BOD and AmmN. it was therefore necessary to use the derived intermittent standards for BOD and AmmN from the UPM manual. These derived standards are quoted as concentration thresholds for episodes with six-hour durations and one-year return periods.


The main points emerging from the study are:

On this basis it appears that there is a slight discrepancy between the two methods of protecting rivers; the river ecosystems classification seems to demand a more stringent river quality than the derived UPM standards. An important point to note is that while having a 99th annual percentile which is better than RE class 2/3 boundary should give a UPM-type measurement within the derived standards (for any river type), having a river with quality which complies with the UPM standards will not ensure that the river is in RE class 2 or above. This is the case even when there is no effluent discharge and only intermittent discharges operating.

A possible explanation for this is that the RE classes are based on the long-term requirements for maintaining river aquatic life, whilst the derived UPM standards offer protection for aquatic life from short-term exposures. The river ecosystems classification is tighter because it has to encompass a greater range considerations (the long-term needs) and covers all river types. The UPM standards, on the other hand, are tailored according to the physical nature of the receiving water, and so are more lenient because they allow for the possible improvements in quality due to the river 'self cleaning' further downstream. In addition to this, the UPM fundamental standards are based on a matrix of thresholds for different return periods and durations of episodes; all of these standards must be met in order to meet the overall standard. It may be, therefore, that the derived standards by themselves do not offer sufficient protection, but that the fundamental standards do.


The fact that the two approaches to protecting receiving waters do not marry-up as well as one might expect does not imply one is 'more correct' than the other, although in practical terms, the RE classes have been extensively applied in the field and are generally considered to reflect the status of rivers well. The UPM standards have very few instances of empirical validation as they are used mainly for planning purposes. It would be interesting to investigate this matter further (starting with overviews of existing work and historical data, and possibly moving on to specifically planned monitoring exercises) to determine whether one or both systems need amending.

A possible spin-off from the linkage between the two types of standards is in the setting of upper-tier limits for discharges. These are normally considered in terms of very high percentiles such as the 99th or 99.5th. It should be feasible to calculate an upper-tier limit based on an equivalent UPM-type threshold in the receiving water.

Copies of the report are available from FWR, price 25.00, less 20% to FWR Members.