Project: Mixing Efficiency Testing
Site: Blackminster STW, Worcestershire
Client: Morganest
When: July 2006

WEBS undertook a mixing efficiency test on the selector zone serving the new double oxidation ditch arrangement at Blackminster STW. 

Testing was conducted to meet Severn Trent Water’s ME18F requirements.  Final effluent was used to fill the selector zone, and sufficient final effluent was stored to provide a forward flow rate of average settled sewage flow plus RAS at one dry weather flow for a test period of 1.5 times the hydraulic retention time of the selector zone at this flow rate without the need to re-circulate the test water. 

Rhodamine WT was used as the tracer, and three Fluorimeters connected to a laptop computer provided real time Rhodamine concentrations at the outlet of the selector zone. 

Data was analysed to provide information on short circuiting and dead space, and number of tanks in series according to the
O. Levenspiel model from ‘Chemical Reaction Engineering’.

Project: Mixing Efficiency Testing
Site: Belper STW, Derbyshire
Client: Norwest Holst
When: May 2006

WEBS undertook a mixing efficiency test on one lane of the recently refurbished ASP at Belper STW. 

The lane tested had four new surface aerators, the mixing efficiency of which was to be tested.  Testing was conducted to meet Severn Trent Water’s ME18F specification for mixing efficiency of surface aeration plant.  Due to the size of the tank being tested and the length of the test (28 hours), it was not possible to conduct this test using final effluent and the test had to be conducted with the ASP in process. 

Lithium chloride was selected as the tracer due to relatively low background Lithium concentrations, the inert nature of Lithium chloride and hence negligible absorbance to the sludge floc.  A temporary pumping arrangement with flow monitoring was set up to the lane tested to provide a constant flow of settled sewage and RAS at average settled sewage flow plus RAS at 1 dry weather flow for the 28 hour test period. 

Samples were taken for Lithium analysis from three points across the overflow weir at the end of the lane tested, and because the test was conducted on a live works with the recirculation of the tracer in the RAS line, samples were taken at the inlet to the lane tested so that the recirculation of the tracer could be taken into account in the calculations.  Data was analysed to provide information on short circuiting and dead space, and number of tanks in series according to the O. Levenspiel model from ‘Chemical Reaction Engineering’.

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