Real Time Control to increase Hydraulic
Capacity of Wastewater Treatment Plants
during rain
Anders Lynggaard-Jensen,
Hans Peter Hansen, DHI
Flemming Husum, Jakob Kaltoft,
Morten Nygaard, Aarhus Water
Prepared enabling change
Aarhus, 21 – 23 , January 2014
1
Focus: Downstream boundary
Cros cutting
overview/reporting
Early warning
systems
Distributed
rainfall
Monitoring,
modeling, control
SCADA
Monitoring
Station
Monitoring
and control
SCADA
Sensors
PLC
Communication
Control handles
Aarhus, 21 – 23 , January 2014
Overview
• Use balanced overall return sludge rate (Qr,min):
Less energy for pumping, high and stable SSr
saving on polymers for dewatering
• Compensate skew distribution on clarifiers:
Secure the use of the built capacity
• Increase max. hydraulic load Qbiomax during rain:
Flushing SS to clarifiers using these as sludge
storage and maintain efficiency in aeration tanks
• Enhanced sludge storage control using groups
of clarifiers and virtual sludge blankets
Aarhus, 21 – 23 , January 2014
Min. return sludge rate Qr,min and
max. hydraulic load Qbiomax
Qrec
Qbio
Process tanks
Qtot
Clarifiers
SB
SS g/l
Qr
Qtot = Qbio + Qr; (Qs << Qbio and Qr)
Aarhus, 21 – 23 , January 2014
Qbio
Qs
Overall return sludge rate
• Ref: WST, 54(11-12), pp.249–256, 2006
• Estimate initial settling velocity, ISV
• Calculate the balanced (minimum) return sludge rate,
Qr,min from:
Fup
– Vsed = ISV * Exp ( -nv * X )
– nv = K1 * Exp ( K2 * SVI ) + K3
– Flux equations
• Qbiomax = Vsed * A
Fin
Qbio m3/h
X kg/m3
Vsed m/h
A m2
SS kg/m3
Qbio + Qr m3/h
Fdown
Aarhus, 21 – 23 , January 2014
Qr m3/h
Settling Velocity
Settling Velocity
Vsed = ISV * Exp ( -nv * X )
ISV: Initial Settling Velocity
nv = K1 * Exp ( K2 * SVI ) + K3
Vsed [m/h]
K1= -0,9834;
K2= -0,00581;
K3= 1,043;
Methods for ISV:
1. User input
2. Lab. procedure
3. On-line estimation
4. Derived from SV-sensor
Suspended solids, X [g/l]
Vsed
Aarhus, 21 – 23 , January 2014
Clarifier state diagram –
overview
Clarifier State Diagram
Fsettling (X) = X * ISV * Exp ( -nv * X )
Freturned (X) = (( Qbio + Qr ) * SS – X * Qr ) / A
Fupwards (X) = X * Qbio / A
Flux [kg/(h*m2)]
Slope = Qbio/A
State Point
Slope = -Qr/A
SS Process tanks
SS Return Sludge
Suspended solids, X [g/l]
Settling Flux
Returned Flux
Upward Flux
Aarhus, 21 – 23 , January 2014
Clarifier state diagram – min.
return sludge
Flux [kg/(h*m2)]
Clarifier State Diagram; Qr=Qr,min
State Point
Slope = -Qr,min/A
Balance: Settling
Flux = Returned Flux
Balance point:
Fsettling (X) = Freturned (X)
and
F’settling (X) = F’returned (X)
ISV * nv * (X2 - SS * X ) +
SS * (ISV – Qbio * Exp ( nv * X ) / A)
=0
Qr =
ISV * A * ( nv * X – 1 ) * Exp (-nv * X )
Suspended solids, X [g/l]
Settling Flux
Returned Flux
Upward Flux
Aarhus, 21 – 23 , January 2014
Clarifier state diagram – max.
inlet to WWTP
Clarifier State Diagram; Qbio=Qbiomax
Flux [kg/(h*m2)]
Slope = Qbiomax/A
Fsettling = Fupward (= Freturned )
X * Vsed =
(( Qbio + Qr ) * SS – X * Qr ) / A
and X = SS:
Qbiomax = Vsed * A
or
Qbiomax = ISV * Exp ( -nv * SS ) * A
Suspended solids, X [g/l]
Settling Flux
Returned Flux
Upward Flux
Aarhus, 21 – 23 , January 2014
Clarifier state diagrams –
decrease SS -> increase Qbiomax
Flux [kg/(h*m2)]
Clarifier State Diagram; Qr=Qr,min
Suspended solids, X [g/l]
Settling Flux
Returned Flux
Upward Flux
Aarhus, 21 – 23 , January 2014
Clarifier state diagrams –
decrease SS -> increase Qbiomax
Flux [kg/(h*m2)]
Clarifier State Diagram; Qbio=Qbiomax
Suspended solids, X [g/l]
Settling Flux
Returned Flux
Upward Flux
Aarhus, 21 – 23 , January 2014
WWTP distribution to clarifiers
Clr.1
Process tanks
Qbio
Qr,1
Clr.2
Qtot
SS g/l
Qr,2
Clr.3
Qr
Aarhus, 21 – 23 , January 2014
Qr,3
Qbio
Distribution between clarifiers
• Ref: WST, 60(9), pp.2439–2445, 2009
• Measure the sludge blanket, SBi, in each of the
clarifiers (i = 1 to nClr) and calculate the average
sludge blanket, SBAvg
• Calculate the compensated percentage, Qr,i pct, for
each clarifier as:
– Qr,i pct = 100 / nClr + K * (SBAvg - SBi ) / SBAvg
• Normalise the percentages and calculate the
required set-point, Qr,iSP, for each clarifier as:
– Qr,iSP = Qr,i normpct * QrSP ,
• Write the set-points, Qr,iSP, to the local control loops
of the return sludge pumps
Aarhus, 21 – 23 , January 2014
Distribution between 10 clarifiers
Colours on plots for sludge blanket measurements and distribution of return sludge pumping
follows the colour spectrum – violet for clarifier1 and dark red for clarifier10, whereas the average
sludge blanket level is black.
Aarhus, 21 – 23 , January 2014
Enhanced sludge storage
control
• Secure that sludge is flushed to the most efficient
clarifiers
• The use of virtual sludge blankets between clarifier
lines at Marselisborg WWTP
Aarhus, 21 – 23 , January 2014
Real time monitoring/control
principle
DIMS
Software sensors, early
warning, control
algorithms, models
Validated data
Clarifiers 1-10
Area: 4400 m2
Depth: 3.0 m
Results incl.
warnings /setpoints
Data Validation, Filters,
Aggregation,
Visualisation, Reporting
Measurements
and set-points
Set-points
Clarifiers 11-12
Area: 2092 m2
Depth: 4.0 m
SCADA/PLC/Logger
Sensors/actuators
Aarhus, 21 – 23 , January 2014
DIMS secondary clarifier control
Aarhus, 21 – 23 , January 2014
Distribution of load between sets
of clarifiers
% Qtot
Gate
position
SB
averages
Aarhus, 21 – 23 , January 2014
Dynamic Hydraulic Capacity
Vsed
SS
Qbiomax
Qhydmax
Qbiodim
Qbio
Aarhus, 21 – 23 , January 2014
Conclusion
• Efficient control of secondary clarifiers makes it
possible to increase the hydraulic load during rain
considerably above the dimensioned hydraulic load
• The presented controller does not have any lead
time – which often is the case for this type of
controller
• The controller does not affect the operation and
control of the upstream biological process
Aarhus, 21 – 23 , January 2014
Thank you for your attention!!
Prepared enabling change
Aarhus, 21 – 23 , January 2014
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