Wastewater Engineering

5th week Lectures

BY
Dr. Mohamed Fekry
‫محطات الرفع‬
Pumping Stations
Purpose:

1- To rise the sewage from the level of last manhole to the level of the
first tank in the wastewater treatment plant (deceleration tank =
Slowdown chamber).

2- If the excavation depth exceeds 5m – 7m (depends on the type of soil)

sewage pump station (Lift pump) is needed to rise the sewage to a
manhole of level 1.2m)
‫ صفحة‬401 ‫ – فى‬2005 ‫ طبعة‬- ‫الكود المصرى لمحطات الرفع‬
https://drive.google.com/file/d/0BwTqC1X60rp0Q2FueEFNYU9IaWM/view?usp=sharing
The sewage pump station consists of:
Pumps, Wet well, Dry well, and rising main.

- Dry well: For housing the pumps

- Wet well: For incoming sewage
- Rising main: To led the pumped sewage to
high leveled gravity sewer
Shapes of sewage pump station:
Rectangular and circular depends on type of soil and the
available area.

Types of pumps:

1- Dry pump.
- Horizontal pump.
- Vertical pump.
2- Submerged pump.
3- Screw pump.
Vertical & Submerged pump
Horizontal pump
Screw Sewage Pump
1- pumps:

- The discharge of the pump doesn’t exceed 300 l/s and the
head of the pump doesn’t exceed 90 m.

- The total number of pumps must not be less than 3 pumps

(one pump is working and one pump is standing by and the
other one is in maintenance ).
Types of pumps:
- Centrifugal pumps
- Submersible wastewater pumps

Force Main

Gravity Pipe

Pump
Total head of pump:
HT=H static+h friction+h minor Losses +h Losses in P.S
Hstatic = H.W.L – L.W.L
= water level in the deceleration tank – L.W.L in the wet sump
Friction losses hf
flv 2 8 flQ 2
hf  
2 gd g 2 d 5
Minor Losses = 10 % of hf
Losses in P.S = 2 – 5 m.
Qdes = Qmax ( the bigger of Q max summer and Q max winter)

Electrical power of the pump:

QH
HP  1HP= 75 kg.m/s = 0.746 kw
75 1 2
2- Wet sump:

Purpose:
Collects and distributes the wastewater uniformly on the total number of pumps
Design criteria:

- T at Q max = 5 – 10 minutes
T at Q min ≤ 30 minutes

- vs ≤ 1.5 m/s ( velocity in suction pipe)

vd ≤ 2 m/s ( velocity in delivery pipe)

- Vmin wet = Q * T/ 4
- V wet = 2* V min wet
- H = (0.8 – 3) m (distance between stop and start level inside the wet sump)
- A wet = V wet/H
- Circular well 1Wet: 1 Dry
2𝐴𝑤𝑒𝑡
- r1-1 =
𝜋
- Circular well 1 Wet: 2 Dry
𝐴𝑤𝑒𝑡
- r1-2 =
0.906

- The distance between pumps = 1.5 – 2.5 m

3- Rising main:

Purpose:
Transmit the wastewater from P.S to the
deceleration tank.
Design criteria:
- Qdes = A x v
- v = 1 - 2.5 m/s v ≥ 1 m/s
- minimum Φ = 100 mm (ductile iron)
- n≥2
Design velocity and the optimal pipe diameter for rising main

X-axis represents pipe diameter

Y-axis represents the costs

Draw relation between operating cost and diameter ???

Draw relation between construction cost and diameter ???
 ‫‪ -‬أنواع المواسير المستخدمة فى خطوط الطرد‬
‫‪ -‬صمامات خطوط الطرد‬
‫‪ -‬الضغوط واإلجهادات على خطوط الطرد‬

‫كما فى شبكات مياه الشرب التى سبق دراستها‬

 Pump Operation

 When asked how a pump operates, most reply that it

“sucks.” While not a false statement, it’s easy to see
why so many pump operators still struggle with pump
problems. Fluid flows from areas of high pressure to
areas of low pressure. Pumps operate by creating low
pressure at the inlet which allows the liquid to be
pushed into the pump by atmospheric or head
pressure (pressure due to the liquid’s surface being
above the centerline of the pump).
 Net Positive Suction Head (NPSH)
 NPSH can be defined as two parts:

NPSH Available (NPSHA): The absolute pressure at the suction port of

the pump.

NPSH Required (NPSHR): The minimum pressure required at the suction

port of the pump to keep the pump from cavitation.

 NPSHA is a function of your system and must be calculated, whereas

NPSHR is a function of the pump and must be provided by the pump
manufacturer. NPSHA MUST be greater than NPSHR for the pump
system to operate without cavitation. Put another way, you must have
more suction side pressure available than the pump requires.
Pump cavitation occurs when the pressure in the pump inlet drops
below the vapour pressure of the liquid. Vapour bubbles form at
the inlet of the pump and are moved to the discharge of the pump
where they collapse, often taking small pieces of the pump with
them.

Cavitation is often characterized by:

- Loud noise often described as a grinding or “marbles” in the pump

- Loss of capacity (bubbles are now taking up space where liquid should be)
- Pitting damage to parts as material is removed by the collapsing bubbles

Noise is a nuisance and lower flows will slow your process, but
pitting damage will ultimately decrease the life of the pump.
Vapour Pressure and Cavitation
To understand Cavitation,
you must first understand
vapour pressure. Vapour
pressure is the pressure
required to boil a liquid at a
given temperature. Soda
water is a good example of
a high vapour pressure
liquid. Even at room
temperature the carbon
dioxide entrained in the
soda is released. In a
closed container, the soda
is pressurized, keeping the
vapour entrained.
Calculating NPSHA

No engineer wants to be responsible for installing a

noisy, slow, damaged pump. It’s critical to get the
NPSHR value from the pump manufacturer AND to
insure that your NPSHA pressure will be adequate to
cover that requirement.

 The formula for calculating NPSHA:

 NPSHA = HA ± HZ - HF + HV - HVP
The formula for calculating NPSHA:
NPSHAav = Habs ± HZ - HF + HV - HVP

‫ال يمكن لمضخة ان‬

‫تنزح الماء من بئر‬
‫عميقة بعمق اكبر من‬
‫ متر؟‬10
 ‫طلمبات الرفع‬
‫‪ ‬تصمم على التصرفات القصوى ساعة الذروة عند سنة التصميم شاملة مدة التنفيذ‬
‫ويمكن حسابها بالمعادلة التالية‪:‬‬

‫‪14‬‬
‫‪𝑄𝑝 = (1 + 4+ 𝑝)Qav.waste + Qinf ‬‬

‫‪ ‬تعتبر المضخات الطاردة المركزية هى الكثير شيوعاً فى مجال الصرف الصحى والتى‬
‫يعتمد نوعها على السرعة النوعية (‪ )Ns …… rpm‬وهى السرعة التى يكون عندها‬
‫تصرف المضخة ‪ 1‬م‪/3‬ث مع رفع ‪ 1‬متر ماء عندى اقصى كفاءة للمضخة وتحسب من‬
‫المعادلة‪:‬‬
‫𝑁‬
‫= 𝑠𝑁‬ ‫‪𝑄‬‬
‫‪𝐻 0.75‬‬

‫‪ :N ‬سرعة دوران المضخة (لفة‪/‬دقيقة)‬

‫‪ : Q ‬تصرف المضخة (م‪/3‬ث)‬
‫‪ :H ‬الرفع الكلى (م)‬
 ‫وباالستعانة بالجدول التالى يمكن تحديد واختيار نوع المضخة الطاردة المركزية‬
:‫والتى تنقسم الى‬

-Radial flow pump

for high H …… greater than 40 m

- Axial flow
for small H …… less than 10 m & greater Q ……. More than 600 l/sec

- Mixed flow
in-between the previous kinds, H from 10 to 40 m
 ‫نوع المضخة الطاردة المركزية‬ ‫السرعة النوعية )‪(Ns …. rpm‬‬
‫بطيئة السرعة ذات تصرف قطرى‬ ‫‪30 - 10‬‬
‫متوسطة السرعة ذات تصرف قطرى‬ ‫‪50 – 30‬‬
‫عالية السرعة ذات تصرف قطرى‬ ‫‪80 – 50‬‬
‫عالية السرعة ذات تصرف مختلط‬ ‫‪160 – 80‬‬
‫عالية السرعة ذات تصرف محورى‬ ‫‪500 – 160‬‬
‫مضخات ذات سرعات عالية جداً‬ ‫اكبر من ‪500‬‬

‫‪For single stage centrifugal pump, Ht should not exceed 65 m‬‬