In the UK, the Environment Agency Regulator requires Water Utilities to self-monitor effluent discharges from Wastewater Treatment Works (WwTW's), to comply with the Urban Waste Water Treatment Directive. At WwTW's with dry weather flows >50m3/day, continuous flow monitoring is required by the Regulator, to an accuracy of +- 8 % of the total daily flow. It was understood that more rigorous enforcement of checking this monitoring was being planned by the Environment Agency for 2003.
At most WwTW's, hydraulic structures are included in the infrastructure to allow flow depth to be monitored for conversion to flows. Many of these structures have been constructed broadly in compliance with the British Standard BS3680. The standard discusses errors in flow measurement relating to construction and measuring issues. These need to be evaluated for each individual site and no overall accuracy for flow estimation is quoted in BS3680. However, it is likely that if BS3680 is complied with, then the accuracy of the total daily flow measurement will be substantially within +- 8% and probably 2-3%.
The construction of structures for depth measurement at WwTW's frequently did not comply in all details with BS3680, due to site constraints, quality of construction, methods of measurement etc. What was not understood is how far the variation in structure characteristics influence the instantaneous flow (either individually or in combination) and how this effected accuracy of integrated daily flow volume estimates.
Where the +- 8% requirement wasnot met, Water Utilities undertook remedial work on structures or considered alternative methods of flow measurement. The associated costs of this work was minimised by the structure characteristics being better understood in relation to the required accuracy.
This research explored the sensitivity of instantaneous depth/flow measurement, for a range of flows over the diurnal pattern of dry weather flow typically experienced at WwTW's, to gauging structure characteristics including:
- Approach conditions
- Exit conditions
- Roughness conditions.
This approach involved:
- Numerical modelling of representative structure(s) - initially a rectangular flat bottomed flume
- Field measurement at typical installations.
- Objective information on non-compliant gauging structures which can be used by Utilities in negotiations with the Regulator.
- Large potential savings in Wastewater Treatment Works upgrades.