**Tutorials 3 and 4 **

Based
on the discussions of tutorial number 2, following process train for the
treatment of water was decided upon:

·
Assume 30 % reduction in turbidity due to storage in raw water storage
tanks, i.e., due to the settling and consequent removal of a part of the
settleable solids.

·
Also assume that each NTU of remaining turbidity is equivalent to 3
mg/L of suspended solids.

Chlorine
Addition Raw
Water from Storage Tanks

## Prechlorination

**Prechlorination:
**Chlorine
dose required for prechlorination is 1 mg/L per mg/L BOD to be destroyed. Liquified Chlorine stored in tanks is
directly applied using jet mixers.
Caculate the chlorine required per day.
A contact time of 45 – 90 seconds in an open channel is required for
chlorine reaction to be complete. Design
a suitable open channel for this purpose.

**Rapid Mix: **Assume the required
coagulant (Alum) dose to vary between 20 – 40 mg/L depending on the raw water
turbidity. Design conventional
vertical-shaft rapid mix unit.
Appreciable decrease in pH due to addition of alum, if any, must be
prevented by adding soda (sodium carbonate) along with
alum.

*Design
Parameters:*

Detention time (t): 20
– 60 s

Ratio of tank height to diameter: (1:1 to 1:3)

Ratio of impeller diameter to tank diameter: (0.2:1 to 0.4:1)

Velocity gradient (G): >300 /s

Gt: 1000
– 2000

Tank diameter: <
3 m

Paddle tip speed: 1.75
– 2.0 m/s

Velocity of paddle relative to water: 0.75 x paddle tip
speed

Paddle area/Tank section area: 15:100

Coefficient of drag on impeller blade: 1.8

Maximum length of each impeller blade: 0.25 x impeller diameter

Maximum width of impeller blade: 0.20 x impeller
diameter

Impeller height from bottom: 1.0 x impeller diameter

Use the following equation to calculate the soda dose to be added (if reqd):

_{}

**Coagulation –
Flocculation: ** Design horizontal paddle
flocculation unit using conventional design parameters.

*Design
Parameters:*

Detention time: 10
– 30 minutes

Velocity gradient (G): 20 – 75 /s

Gt: 2x10^{4}
– 6x10^{4}

Basin depth: <
5 m

Paddle tip speed: 0.25
– 0.75 m/s

Velocity of paddle relative to water: 0.75 x paddle tip
speed

Paddle area/Tank section area: 15:100

Coefficient of drag of paddle: 1.8

Maximum length of each paddle: 5 m

Maximum width: 50
cm

Thickness: 5
cm

**Secondary
Sedimentation: **Assume suitable surface overflow rate to design circular settling
tank. Calculate the amount of sludge
generated, both on volume any dry weight basis.
Assume specific gravity of sludge to be 1.1 and solids content to be 5
percent. Assume 90 percent removal of
turbidity in secondary sedimentation.
Also assume that 100 percent of alum added has precipitated as aluminium
hydroxide.

*Design
Parameters:*

Surface over flow rate: 30 – 40 m^{3}/m^{2}/d

Depth: 5
– 6 m

Diameter: 20
– 80 m

Detention time: 30
– 45 minutes

**Rapid Sand
Filtration: **Assume
filter depth to be 60 cm. Filter media
is 0.5 mm sand particles. Calculate
clean bed headloss using Karmen – Kozeny equation. Terminal headloss is 2.5 m. Average filter run length is 8 hours, and
filtration rate is 8 m^{3}/m^{2}/hr. Calculate the water required for backwash,
and hence calculate the size of the backwash tank. Also calculate the effective filtration rate,
i.e., after accounting for filter down-time due to backwashing and water
requirement for backwashing. Assume the
turbidity of water post-filtration to be 1 NTU.
Based on this information calculate the solids concentration in backwash
water.

*Design
Parameters:*

Length to width ratio: 1.3 – 1.5 :
1

Length: <
7 m

Free Board: 0.5
m

Underdrainage system depth: 1.5 m

Backwash rate: 1.0
m^{3}/m^{2}/min

Backwash time: 5
minutes

Filter down time due to backwashing: 30 minutes

**Post-Chlorination**: Chlorine is added using
jet mixers in closed conduits in a manner similar to during
prechlorination. Assume that the initial
demand for chlorine has been satisfied due to prechlorination. Calculate the breakpoint chlorination dose to
destroy ammonia. A free chlorine
residual of 2 mg/L must be maintained in the treated water.** **

Use the following equation to calculate the breakpoint chlorination dose:

_{}

Assume the CT value for 6 Log kill of coliform
organism is 120, and that for 5 log kill is 96.
Based on this information, calculate the minimum time of contact
required to achieve complete disinfection.

**Other Remarks:**

When designing water treatment plants, it must be remembered that it is advisable to put several small unit processes in parallel, rather than designing a single large unit. However construction of very large number of small unit processes will result in an increase in operation and maintenance costs. Proper provisions must be made for units being under repairs, and hence out of commission.

** **