Practical Hydraulics on a Large Wastewater Treatment Works

Transcription

1 Practical Hydraulics on a Large Wastewater Treatment Works Rob Wilson Examples Flow distribution chambers with weirs Flow distribution without weirs Manifold distribution Flow distribution to inlet screens

2 Flow distribution chamber with weirs 6 m deep chamber 6 degree jet angle 2 m diameter pipe velocity requirement of 1.1 m/s Large chamber width to ensure maximum average velocity of 0.35 m/s Flow distribution chamber with weirs Construction Photograph

3 Flow distribution chamber with weirs Conditions at the weirs Flow distribution without weirs Flow splitter

4 Flow distribution without weirs Flow splitter Flow distribution without weirs Flow splitter Equal flow split in each feed channel verified by downstream flumes Flow splitter is over to one side

5 Flow Distribution Without Weirs Investigation Flow splitter Partial blockage in the feed channel Difference in geometry e.g. channel widths Partial blockage in the feed pipes Unequal outlet weir levels on the tanks Flow Distribution Without Weirs Investigation Detailed level survey showed 30 mm discrepancy in weir levels between PST 7 and PST 5, 6 & 8 Flow Split to PST 5-8 Comparison Plot 35 Flow to PST as a percentage of the total flow Primary Settlment Tank Dry Weather Flow Peak Flow Intermediate Average Day Flow Unequal flow distribution is more pronounced at low flows than at high flows

6 Flow Distribution Without Weirs Investigation Velocity meter used to check flow rate from each primary tank Flow Distribution Without Weirs Investigation - Further work Automate the flow splitter on flow measurement Monitor the position of the flow meter Advise Process Controller that PST 7 will be treating approximately 9% less flow than the other tanks during dry weather 6% less in average day conditions Only real solution is to adjust the weirs

7 Manifold Distribution Electromagnetic flow meter Plug valve Manifold Distribution Commissioning Issues Adjustment of one valve to achieve the target flow (equal split) produced unequal distribution to the other lanes 5 Operators with radios to coordinate Approximately equal flow distribution was achieved at a given total flow rate and with all 12 lanes operating Achieving a equal flow distribution at a lower total flow rate or with lanes out for maintenance is not practical

8 Manifold Distribution Alternatives Ideally have a distribution chamber with a weir (space, cost) Physical modelling or CFD to size manifold pipework (valves only used for fine tuning) Automate the valves to control using the flow measurement. Screw the valves right down to minimise the velocity in the manifold issues with the pump Distribution to inlet screens Low Flows Storm Conditions

9 Possible Solutions Raise Inlet Overflow and Coping Levels Increase Flooding in the Network Storage Required Network Pumping Modifications to Pipe Bridges Inlet PS Site is a complete gravity works Pump up to 4500 l/s - OPEX Carbon footprint Available space Chosen Solution Existing screen bay Preferential feed to the existing bank of screens Inflow from the catchment New screen bay

10 Physical Modelling Physical Modelling Grit Settlement Sediment Build Up

11 Engineering Success? Positives No more flooding Can pass consented flow rate before spill Greater flexibility/redundancy with screens No increase in network flooding or expensive modifications to the network Gravity solution Negatives Long term issues with grit i.e. grit in the primary sludges, wear on mechanical equipment, reduction in hydraulic capacity of inlet works Further work with Operations to resolve the grit issue Summary Variety of methods for distributing flow on a large WwTW When it comes to flow distribution simpler is usually better passive control over active Be aware of the impact of hydraulic design on other engineering disciplines When selecting a solution you may not be able to satisfy every design requirement but the affect of not meeting that requirement can be far reaching

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