Application: Reduction in filter backwash frequency.

Customer: Anglian Water Limited

Service: CFD (Computational Fluid Dynamics) Modelling and Site testing

Problem: As part of an efficiency drive at Grafham Water Treatment Works, Anglian Water Services set itself an objective of reducing the quantity of water lost due to filter backwashing. During periods of maximum demand, when plant throughput was necessarily high, the performance of the 24 upflow clarifiers fell severely. The drop in performance was evidenced by the increase in solids carryover and higher settled water turbidity. This resulted in faster blinding of the rapid gravity filters, an increase in filter backwash frequency, causing water wastage and an increase in pumping costs.

BHRSolutions was asked to investigate the cause of blanket instability in the clarifiers and if possible to improve hydraulic performance.

Objectives: To model a single clarifier using CFD (Computational Fluid Dynamics) and compare predicted results with tracer test results undertaken on site.

Velocity contours developed by the standard clarifier

Approach: The computational model identified jetting in corners, recirculation through the blanket and poor upward velocity distribution in the existing clarifier. The site test results obtained by BHRSolutions were used to validate the CFD model by comparing residence time distributions and overall flow patterns. Having demonstrated close agreement with the site data, the CFD model was then used to develop an innovative yet relatively inexpensive modification giving more uniform upward velocities with no recirculation or jetting in the corners of the clarifier.

Velocity contours developed by the modified clarifier

Outcome/Benefits: The novel design modification was initially installed in a single clarifier for controlled trials. This demonstrated a significantly improved flow pattern within the clarifier that encouraged a more stable sludge blanket and lower settled water turbidity levels. The subsequent result was a reduction in backwash frequency and savings on water and pumping costs.

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