Water Supply Scheme Design

i Base Year 2008ii Current Total Population 106469

iii Adopted Growth rate 4 %iv Domestic Water Demand 50 Lpdv Other Uses of Water:a Commerce and Industry 20 %b Institutions 20 %c Fire Services 5 %d Horticulture 5 %

Total Other Uses 50 %

vi Allowance for:

a Backwashing of Treatment Facilities

b

c

d Staff Quarters Use etc.Total Allowance = 10 %

Adopted Operation Hour 22 hrs

vii Population Estimation

YearDomestic Demand Other Uses Demand Total Water Demand

mgd mgd mgd

a 2008 106469 50 5323 1.17 2662 0.59 7985 1.76b 2010 115157 50 5758 1.27 2879 0.63 8637 1.90c 2015 140106 50 7005 1.54 3503 0.77 10508 2.31d 2020 170460 50 8523 1.87 4262 0.94 12785 2.81e 2025 207391 50 10370 2.28 5185 1.14 15554 3.42f 2030 252323 50 12616 2.78 6308 1.39 18924 4.16

viii Total Water Demand:a 4.16 mgdb 0.42 mgd

Total = 4.58 mgd

ix Scheme Design:a Intake Works

Raw Water Draft 946.21

0.263262.84 L/s

Detention Time in well = 0.15 hStorage in Well 141.93

Adopted Height of Water in Well 4.5 mAre of Well 31.54

No of Chambers 4Area of each Chamber 7.89

Width of Chamber 2 mLength of Chamber 3.94 m

Chamber DimensionsLength 4.5 mBreath 2 m

Assumed Freeboard 0.5 mAdopted Height 5 m

Raw Water Mains:Capacity 0.263

Diameter of Mains 500 mm0.5 m

Area of Mains 0.20Length of Mains 1000 mVelocity of Flow 1.34 m/s

Hazen Williams Constant 1303.1933 m/Km

Total Head Loss in mains line 3.19 m

Raw Water PumpsNo of Pumps 3

96.55 mNote: This is Determined From Spot heigths from Survey Report

Potential Head at Intake Works 50.96 mStatic Head Difference (SHD) 45.59 m

Fittings Losses 10 % of SHD4.559 m

Flushing of Sludge in Sedimentation Tank

Laboratory Works (Chemical Mixing etc)

Population Figure at 4% annual growth rate

Per Capita Water Demand (Lpd) m3/day m3/day m3/day

m3/h

m3/s

m3

m2

m2

m3/s

m2

Note: Decrease in the Constant will increase the Head Loss in mains

Head Loss (Hi)

Potential Head at Treatment Works

167.1

852.1

.819.6

D

C

V

wh

Total D Head (TDH) 53.34 mPump Min. Specification

No of Duty 3No of Standby 1

Capacity of Each 315.40

0.08887.61 L/s

Min. Allowable Head 53.34 m

Assumed Pump Efficiency 0.70

Acc. Due to Gravity (g) 9.81

1.00Pump Power Consumption 65.49 KW

Conversion Factor to Horse Power (Divide by) 745.7Pump Power Consumption 87.83 HP

Generator Power Rating 0.80 use 0.75 or 0.8 or 0.85Genset Power Rating 327.47412825213 KW

ii AeratorsType of Aerator Spray AeratorsNo. of Aerator 2 NosCapacity of each 0.13Assumed Diameter of Riser Pipe 350 mm

0.35 mArea of Mains 0.10

Length of Mains 5 mVelocity of Flow 1.37 m/s



Header PipeNo of Header Pipe 2.00

Flow Through each 0.07Assumed Diameter of Riser Pipe 250 mm





LateralsNo of Laterals 5.00

Flow Through each 0.01Assumed Diameter of Riser Pipe 100 mm





NozzlesLateral Length 4.00 m

Spacing of Laterals 0.25 mNo of Nozzles 32.00Flow / Nozzle 0.00

1.48Flash Mixer Design

No of Flash Mixer 2Capacity of each Chamber 473.105407148888

Assumed Detention Time 1 mins 30 - 60 secsVolume 7.89

Assumed Depth 1.5 mArea 5.25672674609876

Length 4.7 mWidth 4.3 mArea 20.21 Ok

Flocculators

m3/h

m3/s

(Adopt between 0.7 and 0.9)

m/s2

Density of liquid (ℓ) ton/m3

m3/s

m2


Head Loss (Hi)

m3/s

m2


Head Loss (Hi)

m3/s

m2


Head Loss (Hi)

m3/s

m3/h

m3/h

m3

m2

m2

167.1

852.1

.819.6

D

C

V

wh

167.1

852.1

.819.6

D

C

V

wh

167.1

852.1

.819.6

D

C

V

wh

Number of Flocculators 2Assumed Detention Time 15 mins

Capacity of each Chamber 473.105407148888

Volume 118.28Assumed Tank Height 4.5 m

Area 26.2836337304938Length 4.7 mWidth 4.7 mArea 22.09 Error!!!: Redesign

Sedimentation Tank DesignNo of Sedimat 4

Capacity of Each Chamber 236.552703574444Assumed Detention Time 3 Hrs 3 - 4 hrs

Volume 709.66Assumed Depth of Tank 3.3

Surface Area 215.047912340404Length 18.3 mBreath 6.5 m

Area 118.95 Error!!!: RedsignSurface Loading (overflow Rate) 1.98867342223156 m/hr Ok

Rapid Sand FilterNo of Beds 2

Assumed filtration rate 9 OkArea of Filter 105.134534921975

Area of Each Bed 52.5672674609876Adopted Length 9.1 m OkAdopted Width 7 m

Area of Bed 63.7 OkAssumed Depth of Sand 80 cm

Underdrainage System:Total Area of Holes = 0.2 to 0.5% of Bed Area

Assumption 0.25 %Area of Lateral = 2 (Area of Holes of Lateral)Area of Manifold 2 (Area of Laterals)

So, area of Manifold = 4 (Area of Holes)Area of Holes = 0.15925

Area of Manifold = 0.637Diameter of Manifold = 0.90 m

90.0 cm

30 cm Ok

30.3333333333333 on either side

4.09979802512448 m

Take dia of holes 12 mm Ok0.012 m

1.2 cmTotal Area of Holes = 0.15925

Number of Holes 1407.51262626263 NumbersAdopt 1410 Numbers

Number of Holes/ Lateral 17.625

Area of Perforations/lateral 1994.14285714286

0.232612653907772 m

23.2612653907772 cmCS area of Lateral 39.8828571428571

Diamter of Lateral = 7.12460525222275 cm

Check: Length of Lateral < 60 d = 427.476315133365 cm

4.27476315133365 m Ok

50 cm/min

0.5 m/minWashwater discharge/bed= 0.530833333333333

Velocity of Flow through Lateral 1.66372650858466 m/s OkManifold Velocity = 1.33098120686773 m/s Ok

WashWater GutterDischarge of Washwater/ bed 0.530833333333333

Size of Bed:Length 9.1 mWidth 7 m

2.33333333333333 m c/c

m3/h

m3

m2

m2

m3/h

m3

Note: Depth of Sedimat is 0.6 to 1.2m much than the depth of flocculatorm2

m2

m3/m2/h

m2

m2

m2

Assume c/c of Lateral (Btw 15 and 30) =

Total Number (Length/c/c of Lateral) =

Length of lateral ((Width of Bed/no of beds) - (Diameter of Manifold/no

of Beds) =

CS area of Lateral (2 x area of perforation per Lateral)

m2

(Number of Holes /Depth of Sand Bed

mm2

Spacing of Holes = (Length of Lateral/Number of Laterals)

cm2

Rising Washwater Velocity in Bed =

m3/s

m3/s

Assume 3 troughs running lengthwise at =

Discharge of each trough = (Q/3) 0.176944444444444Q = 1.71.b.h^(3/2)

Assume b = 0.3 mh = 0.640833062301179 m

Free Board = 0.05 mAdopted h = 0.69 m

For 4 Hour Filter Capacity, 4Capacity of Tank = 4586.4

Assume Depth = 4 mSurface Area = 1146.6

B = 23.9436839270819 mL = 47.8873678541638 m

150 mm

Velocity = < 0.6m/s

67.5 cm

Air Compressor unit 1For 5 mins

Air required = 10.6166666666667

Rapid Pressure Filter Design:

Flow Rate = 946.21

Filter Loading Rate = 35 Ok

27.0345946942222

No of Filters = 42 Sets in Parallel/ Sed Tank

Area of each Filter = 6.76Diameter of each filter = 2.9329024009701 m

Adopted Diameter = 2.9 m

Transfer PumpsNo of Duty pumps = 2

No of Standby = 1Pump Rate = 236.552703574444

Assumed Head = 15 m

Backwashing:Backwashing Water rate = 35 Ok

Filter Surface Area = 6.76

Backwash Pumping Rate = 236.55 Allowable Head 15.00 m

Assumed Pump Efficiency 0.70

Acc. Due to Gravity (g) 9.81

1.00Pump Power Consumption 13.81 KW

Conversion Factor to Horse Power (Divide by) 745.7Pump Power Consumption 18.52 HP

Genset Power Rating

Clear Water Tank:Detention Time = 3 Hrs

Volume = 2838.63244289333No of Tanks = 2

Capacity of each = 1419.31622144667

Circular Ground Tank:Heigth of Tank = 9 m

Surface Area of Tank = 157.701802382963Diameter of Tank = 14.1672645507537 m

Adopted Diameter = 14 mFree Board = 0.6 m

Total Height = 9.6 m

Chemical Dosing:Chlorine Dosing =

Normal Water = 4.5 - 5 mg/L

10 mg/L

Alum Dosing = 40 - 60 mg/LLime Dosing = 45 mg/L

m3/s

Clear Water Reservoir for Backwashing

m3

m2

Diameter of Inlet Pipe coming from two filter

Diameter of washwater pipe to overhead tank

air/min/m2

m3/ of air

m3/h

m3/m2/h

With Combined sand & anthracite media m2

m2

m3/m2/h

m2

m3/h

(Adopt between 0.7 and 0.9)

m/s2

Density of liquid (ℓ) Kg/m3

m3

m3

for efficient treatment works, the clear water tank should have a minimum storage of 3 hrs. this will allow for a timely warning to the communities being served during any emergency situation at the plant.

m2

Heavily Polluted Water =

25555

Design of Air Scour Blower

Design Parameters

1 Filtration Rate 6 m3/hr/m2

Air Scour Flow Rate 1 m3/min/m2

Recommended Flow mean Velocity 0.01666 m/s

Filter Type

Filter Dimensions

B 5 m

L 7 m

Number of Filters 4

Number to backwash simultaneously 1

Air Density 1.225 Kg/m3

1.1839 Kg/m3

1.1644 Kg/m3

1.1455 Kg/m3

Effective diamter of Sand 1 mm

Dynamic Viscosity of Air 1.85E-05 Ns/m2

1.96E-05 Ns/m2

2.08E-05 Ns/m2

0.55

Height of Sand Bed, l 1.3 m

Head Losses Within Filter Tank using Carman - Kozeny developed equation which predicts the headloss due to fluid flow through media such that

Delta P 93.0950986747

Head Losses along the Pipe Length

Diameter of air Pipe 80 mm

Density of Air 0.96725 Kg/m3

Porosity of Sand, ᵋ

For the fluuidized (when passing a fluid upwards through a packed bad of solids the point at which fluid drag force is equal to the bed weigth the and particles no longer rest on each other; this is the point of fluidization] state of the bed the heith increases hnce the porosity increases to 0.55

Velocity 7 m/s

K 0.15 mm

Length of Pipe 48 m

Reynold's Number 27607.54

Roughness Ratio 0.001875

Lander 0.024 from Moody Diagram

Head Loss based on Darcy Weishbach Equation becomes

g 9.81 m2/s

h 35.9633027523 m

Pressure Loss, Hp in N/mm2 341.2458 N/m2

Atmospheric Pressure

For Barometic Reading

Ha 24.45 inches of mercury

Density of Mecury 0.0254 Kg/m3

Barometic Head 13600

Ha 82855.33848 Pa

Total Head to be overcome by the air between the compressor nd the free surface of the water in the filter tank can be expressed as

H 83289.6793787

Air flow required

Total Air flow to be supplied Flow demand per M2 X cross sectional Area

Volume flow rate through the layer is Q determined from the continuity equation

Q Au

Where U mean velocity of air through the bed

A Cross sectional Area of porous portion of bed

A Total Area cross section area * bed porosity

Therefore

Q 0.250733 m3/s

902.6388

Air Blower Sizing 20883.4711797

20.8834711797 KW

Range

6 - 7 m3/hr/m2 Design Manual for Water Supply in Kenya

@ 15oC

@ 25oC

@ 30oC

@ 30oC

@ 300K

@ 325K

@ 350K

0.43 0.602

Assumed

Head Losses Within Filter Tank using Carman - Kozeny developed equation which predicts the headloss due to fluid flow through media such that

Total Head to be overcome by the air between the compressor nd the free surface of the water in the filter tank can be expressed as

6 - 10 m/s sufficiently low to prevent excessive pressure drops on the most systems and also will allow moisture to precipate

2590

Documents

Water Supply Scheme Design