WASTEWATER TREATMENT PROCESS DESIGN CALCULATOR | NO 4

Activated sludge process – Secondary clarifier sizing

Objectives:

  • Calculate size of secondary clarifier

  • Calculate blower discharge head required

  • Convert blower size to field condition suction side conditions

 

Theoretical basis:

 

Design Philosophy (Courtesy: reprint from CPHEEO manual on wastewater engineering 2013)

 

 

Calculator

 

(Note – all Yellow are user defined value, all white are fixed values,
all green are calculated value, all orange is calculated not to be
shown)

Instructions

Enter values highlighted in the
Yellow

Green highlights
are the results

In case of
any invalid result of non-functioning of the calculator, please write
to us at urvpatel@swaenviro.com. We will try and help you with your
sizing and calculation

INPUT

Parameter Value Unit Remarks
Flowrate MLD Ultimate build out capacity

41.67 cum/hr
Recirculated flow 5.00 % Assumed 5 %
Recirculated flowrate 2.08 cum/hr
Average flowrate 0.01 cum/s
Peak factor
3 for <2 MLD, 2.5 for <3.5 MLD , 2.25 for 5MLD
Peak flowrate 0.035 cum/s
Assumed RAS flowrate fraction of plant flow rate Maximum assumed for peak loading ; In reality it would range from 0.3-1 of the average plat capacity
RAS flow 1000 cum/day

41.67

Design Calculations
Tank sizing -As per conventional ASP
Solids loading rate kg/sqm-day 25-120 kg/sqm-day for average flow
Surface area required as per SLR 26.25 sqm Based on 100 % RAS flow rate of average and above SLR
Solids loading rate (Peak) kg/sqm-day 170 kg/sqm-day for peak flow;
Surface area required as per SLR (Peak) 71.47 sqm Based on 100% RAS + Peak & Recirculated
Surface overflow rate cum/sqm-day 8-12 cum/sqm-day for average flow; As per CPHEEO Nov-2013 Section 5.7.4.2.5, Table 5.8, Page 5.53 8-15 cum/sqm-day for average & 25-35 cum/sqm-day for peak;
Surface area required as per SOR 70.00 sqm Average + Recirculated
Surface overflow rate (Peak) cum/sqm-day As per CPHEEO Nov-2013 Section 5.7.4.2.5, Table 5.8, Page 5.53 8-15 cum/sqm-day for average & 25-35 cum/sqm-day for peak;
Surface area required as per SOR (Peak) 87.14 sqm Peak + Recirculated
Surface area selected 87.14 sqm Maximum of all of the above
Number of clarifier

Surface area of each clarifier 87.14 sqm
Diameter 10.54 m
Side water depth (Sewage) m >3-3.5 As per CPHEEO Nov-2013 Section 5.7.4.2.5, Table 5.8, Page 5.53
Sludge storage depth m
Total SWD 3.50
Length 10.89 m In case if rectangular is selected
Breadth m
Diameter 10.54 m In case if circular is selected
Bottom Slope 1:1 for rectangular or square and ‘1:10 for circular
Volume of each clarifier 305.22 cum
Hydraulic Retention Time (Q+ RQ) 3.49 hrs Considering RAS flow + Avg Q + Recirculated
Free board m 0.3-0.5
Check Weir loading rate at average flow + Recirculated 92.16 cum/m-day Should be <185 cum/day-m
Check OK

Secondary clarifier detailing
Launder (outside) Overflow in launder is from 1 side
Flowrate 0.04 cum/s Including 100% RAS of Peak flow + Recirculated
Velocity of flow in the launder to avoid deposition of solids m/s 0.6-1.2 m/s as per CPHEEO manual
C/S area of flow 0.06 sqm
Depth of flow m
Width of launder 0.29 m
Width of launder provided m
Free fall provided m
Free Board m
Total depth of launder 0.53 m
Diameter of each orifice mm 40, 50, 75 mm
Flow Thorugh all orifice cum/hr
Head over orifice cm
Cc

Cv

Area of orifice required for design flow & head 0.066 sqm
Area of each orifice 0.001256 sqm
Number of orifice 52.2 nos
Number of orifice (Rounded) nos
Degree spacing between each orifice 6.79 deg C/C
Electro-mechanical equipment sizing for RAS pump and WAS pump
Installed Max capacity


Max RAS flowrate required per basin or per process train 41.7 cum/hr
Pumping head required m As per P&ID and mechanical
Pumping power required 1.43 BkW
Efficiency of the pump % Efficiency assumed 70-85 %
Shaft power required 2.74 hp
Shaft power provided hp
Provide common pump for RAS & WAS application with tapping in RAS header for WAS withdrawal 42 cum/hr @ 12 meter head
Clarifier – Circular Schematic
Bottom Slope
As per CPHEEO manual 1:8 to 1:12
A 3.50 m Including 0.5 m F.B.
B m 0.5 clearance around central shaft
C m
D 1.48 m
E 0.48 m
F 4.77 m
Secondary clarifier dimensions (May change in actual GAD – this is only tentative)



Feed Well Tentative – verify with vendor
Diameter of feed well 2.11 m 10-20 % of tank diameter as per CPHEEO Page 5-58 Section 5.7.4.2.7 Nov,2013
Height of feed well 2 m 1-2 m as per CPHEEO Page 5-58 Section 5.7.4.2.7 Nov,2013
Downward velocity 0.02 m/s
CHECK velocity OK
No specifications in CPHEEO; <0.75 m/s as per Metcalf and Eddy Edition-5, Page-387, Section-5.6
Volume of feed well drum 6.98 cum
HRT of feed well 1.65 min
Sludge piping (Secondary Clarifier to RAS pump)
Sludge RAS flow per clarifier 0.0116 cum/sec
Velocity of flow in pipe m/s
C/S area of sludge removal pipe 0.01 sqm
Diameter 0.111 m
Diameter provided 0.200 m
C/S area 0.0314 sqm
Velocity during peak flowrate 0.37 m/s
Velocity during 50% RAS of Average flowrate 0.18 m/s Assuming 50% RAS flowrate; Should be 0.3-0.6 m/s as per CPHEEO manual
Velocity during 30 % RAS 0.11 m/s Assuming 30% RAS flowrate; Should be 0.3-0.6 m/s as per CPHEEO manual
At 50 % RAS flow of Average flow Intermittent RAS flow Regulation required

At 30 % RAS flow of Average flow Intermittent RAS flow Regulation required

RAS flow duration per hours @ 30% RAS flowrate 18.00 min/hr
RAS flow duration per hours @ 50% RAS flowrate 30.00 min/hr
Summary
Diameter 10.54 m
Side water depth 3.50 m
Free board 0.3 m
Feed well diameter 2.11 m
Feed well depth 2 m
Width of launder 0.29 m
Depth of launder incl free fall 0.53 m
RAS pump capacity 41.7 cum/hr
Bottom Slope 0.10
As per CPHEEO manual 1:8 to 1:12
A 3.50 m Including 0.5 m F.B.
B 1.00 m 0.5 clearance around central shaft
C 1.00 m
D 1.48 m
E 0.48 m
F 4.77 m
No of orifice 53.00 nos Equally spaced along the launder inner side
Orifice diameter 40.00 mm


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