practical hydraulics on a large wastewater treatment works
TRANSCRIPT
Practical Hydraulics on a Large Wastewater Treatment WorksRob Wilson
Flow distribution chambers with weirs
Flow distribution without weirs
Manifold distribution
Flow distribution to inlet screens
Examples
Flow distribution chamber with weirs
2 m diameter pipe – velocity requirement of 1.1 m/s
6 m deep chamber –6 degree jet angle
Large chamber width to ensure maximum average velocity of 0.35 m/s
Flow distribution chamber with weirsConstruction Photograph
Flow distribution chamber with weirsConditions at the weirs
Flow distribution without weirs
Flow splitter
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
Flow Distribution Without WeirsInvestigation
Flow splitter
Unequal outlet weir levels on the tanks
Partial blockage in the feed channel
Partial blockage in the feed pipes
Difference in geometry e.g. channel widths
Flow Distribution Without WeirsInvestigation
Detailed level survey showed 30 mm discrepancy in weir levels between PST 7 and PST 5, 6 & 8
Unequal flow distribution is more pronounced at low flows than at high flows
Flow Split to PST 5-8 Comparison Plot
0
5
10
15
20
25
30
35
5 6 7 8
Primary Settlment Tank
Flow
to P
ST
as a
per
cent
age
of th
e to
tal f
low
Dry Weather FlowPeak FlowIntermediateAverage Day Flow
Flow Distribution Without WeirsInvestigation
Velocity meter used to check flow rate from each primary tank
Flow Distribution Without WeirsInvestigation - 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
Manifold Distribution
Electromagnetic flow meter
Plug valve
Manifold DistributionCommissioning 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
Manifold DistributionAlternatives
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
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
Possible Solutions
Chosen Solution
New screen bay
Existing screen bay
Inflow from the catchment
Preferential feed to the existing bank of screens
Physical Modelling
Physical Modelling – Grit Settlement
Sediment Build Up
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
Questions