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:                                                                                2x104 – 6x104             

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 m3/m2/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 m3/m2/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

Water Depth on top of Filter:                            3 m

Free Board:                                                      0.5 m

Underdrainage system depth:                            1.5 m

Backwash rate:                                     1.0 m3/m2/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.