Water Quality Training Day, 8 Dec 2016(30-40 min)

APPENDIX A: NOTES ON WATER QUALITY MONITORING IN RELATION TO SEWAGE TREATMENT WORKS

The data for the exercise on "Regular, spot and continuous sampling at river and sewage treatment works" is taken from a paper written by Peter Lloyd (Some Observations on Chemical Water Quality Monitoring, 2016). The following text describes some key factors when considering the water quality of discharges from STW's. Much of this is based on Peter Lloyd's paper, a copy of which can be found in Peter Lloyd's paper on the CaBA website.

  1. A pattern of diurnal variability. The water quality of the discharges from a STW tend to vary during the day, and in some cases markedly. This is because the flow entering the works at different times of day affects the time that the sewage spends in each stage of treatment (the retention) time, and this in turn affects the quality of theeffluent discharged. It is often observed that there is a pattern to this daily variability for a particular sewage works. This means that a some works (such as Beddington STW in the exercise) may consistently discharge lower concentrations during the daytime with peak concentrations occurring during the evening and overnight. So any monitoring scheme would need to take this into account.
  2. The age of the STW. The diurnal variation at a sewage treatment works depends on the loading factor relative to the design load at the works. New works, because they initially have spare capacity, tend to exhibit very much lower diurnal variability, because sufficient retention time is provided, even for periods of peak flow. As a works becomes increasingly overloaded, and plant starts to deteriorate, retention times become critical, and diurnal patterns will become more obvious. So continuous monitoring to observe the peak concentrations can be a useful indicator of an overloaded works.
  3. Rainfall. Increased rainfall causes an increase in flow to STWs, which is likely to exert extra pressure on treatment processes and result in variable effluent quality throughout the rainfall event. Normally, quality will deteriorate initially and then recover later in the event. The quality deterioration at some works may contribute to significant downstream problems.
  4. River flow. Rainfall obviously also affects river flow and three important effects of river flow on water quality are:
  5. An increased volume of water will dilute some substances that are normally present in river water.
  6. Wet weather discharges and run-off to the river will increase the level of certain substances that are associated with run-off or sewage overflows.
  7. Increase in river velocity will re-suspend river sediment and increase concentrations of substances contained in the sediment.

This means that it is important to take account of rainfall and river flows when analysing water quality data.

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