Explanatory Design

Prepared by: Philip Nathan B.Sc Eng, UMIST, UK

Design of Wet Well

CALCULATION SHEET Design Category : Sewerage & Waste Water Engineering Title : Design of Wet Well for Sewage Pump Station. Designed by: Philip Nathan (BScEng, Salford, UK) Senior Engineer, Associate WEO

Design No:

SWWE-1

Project Brief A small community of 9,500 population require a central sewerage system to collect and treat domestic sewerage. The system will consist of underground pipes which will collect domestic sewage via gravity to several pump wells in low lying areas, and taken to a Central Treatment Plant where Treatment is provided via a series of Oxidation Ponds. Effluent is discharged into a nearby river Objective To determine the size of Pumping Well and Pump Capacity Design Criteria (1) Maximum and Minimum velocity in mains to be 2.4m/s and 0.9m/s respectively, to avoid excessive scouring and siltation. (2) Average Dry Weather Flow (ADWF) = 50gal/head/day Peak Dry Weather Flow (PDWF) = 3 x ADWF Peak Wet Weather Flow (PWWF) = 1.5 x PDWF) (3) Pump Capacity = 2 x PWWF (A) Population / Built-Up Total Population = Pno

=

9500 persons (given above)

= = = =

50 gal/head/day(design assumption) 475000 gal/day = 330 gal/min = 1425000 gal/day = 990 gal/min = 2137500 gal/day = 1484 gal/min =

0 m3/s 0.1 m3/s 0.1 m3/s

0.2 m3/s

(B) Sewage Flows Sewage Flow Rate ADWF = Pno x 50 PDWF = 3 x ADWF PWWF= 1.5 x PDWF

(D) Determine Wet Well Size & Pumps Pump Capacity = 2 x PWWF

=

4275000 gal/day =

2969 gal/min =

Try Wet Well Dimension * Length * Width * Water Depth Volume of Water

= = = =

7 3.5 1.5 36.8

8085 gal

metres metres metres cu metres

1/2

=

Prepared by: Philip Nathan B.Sc Eng, UMIST, UK

Design of Wet Well

Check Retention Times in Wet Well :At Peak Flows : Tp = 8085 + 8085 = 2969 -1484 1484 At Average Flows : Tav = 8085 + 8085 -329.9

8085 = 330

formula used 11 >=

10 mins

OK

26 <=

30 mins

OK

Tp =

V V − P − Fp Fp

Tp =

V V − P − Fp Fp

Determine Delivery Main Diameter :* Dia of Delivery Mains = 350 mm 0 M R 0 m2 * Pumped Velocity = 2.34 m/sec OK * Velocity of Flow must be within the ranges: >0.9m/s<2.4m/s Determine Head Losses :Darcy-Weisback = 4X0.01 Length of Pipe Frictional Loses hf

Static Head Velocity Heads Minor Head Loss Pumping Heads Rqd Total Heads

= = = = = = = =

0.04 60 K 2 1.9 4.65 0.5 0.09 7 14.2

m m (length of delivery pipe to Inlet of Treatment Plant) * L * V2 Where: *g *D K = Darcy-Weisback Constant m L = Pipe Length in Metres m V = Pipe Velocity in M/s m g = 10 m D = Pipe Diameter in M m m * Static Head = GL - Wet Well Level * Velocity heads normally negligible * Minor losses, assume 2% of Static Head

PROVIDE : * Wet Well- 2 Cells - 3.5M X 3.5M X1.5M * Pumps 2 Nos @ 3103gal/min @ 15M Heads, with 1 Duty & 1 Standby or 5 Nos @ 756gal/min @ 15M Heads with 4 Duty & 1 Standby Use Flygt, complete with Auto Controls, Floats, Lift Chain, Guide rails, fittings, etc * Delivery Main 350mm Diameter Ductile Iron Pipes from Pump Station to outlet * Rising Main for Each Pump Max 200mm DI Pipes, or to suit Pump Sizes (100 to 200mm) = END=

GENERAL NOTES Questions & Comments, please post in "Calculation Forum" and quoting the Title and Design No above Whats available for this Calculations:(1) MS Excel File for auto calculations (2) Complete Calculations for whole project consisting of sewer pipes sizing, sumps, structural, oxidation ponds designs, are available for Paid Downloads or Members Subscription (3) AutoCad Drawings are available for Paid Download or Members Subscription (3) The Author is available for Commercial Design, please contact him in WEO Member Site. Philip Nathan (BScEng, Salford, UK)

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