Classification of water carriage systems/sewerage systems/advantages and disadvantages of each systems/sanitary engineering.

Classification of water carriage system/sewerage systems

Separate system

In  this  system,  storm  water  and  sewage  are  conveyed  separately  in  two  different  sets  of conduits.

Advantages

1.The  size  of  pipes  is  much  smaller  than  the  combined  system  sewers.  This  gives  the advantage  of  good  hydraulics  in  the  pipe  (the  pipe is  Designed  to  have  a  minimum velocity to prevent sedimentation of sand)

2.Separation of wastewater from storm water minimize the total quantity of sewage which has the following advantages :

     a. Smaller pumping stations are needed. 

    b. Smaller and more efficient treatment plants are needed. 

    c. Overflow  of  combined  sewers  in  the  storm  events  produces  pollution  to  environment which         is not the case in separate sewer. Only unavoidable Storm water inters the system which protects             the system from the accumulation of sand in the sewers in the non-paved areas.

Disadvantages:

1.Separate set of plumbing work required for collection of storm water and sewage, starting from the household unit.

2.If  the  sewer  length  is  very  high  in  compare  to  the quantity,  laying  two  sets  of  pipe  can increase the total project cost.

Combined system

In this system, single pipe is used to carry both storm water and sewage.

Advantage:

1.Unlike separate system, there is no need of two set of plumbing work.

2.Suitable for regions where wet seasons is short.

3.Only option in the areas where it is difficult to construct two pipelines. crowded streets.

4.Cost of the overall system is less than separate system.

5.Sewage is diluted.

6.Self  cleaning  velocity  is  maintained  most  of  the  time.  Especially  in  the  cases  where sewage quantity is low.

Disadvantages:

1.Since  the  system  carries  both  storm  and  sanitary  waste,  the  pipe  size  is  generally  large and hence it is difficult to meet the requirement of hydraulic characteristics (such as self cleaning velocity) when only sewage is considered.

2.In dry season, sedimentation of organic matter may lead to anaerobic digestion expelling hydrogen sulphide and other gases. These gases corrode the pipe material and as thus the life of pipe is decreased.

3.Unnecessary  pollution  of  rain  water  which  could  have  been  discharge  directly  to  the river.

 

Partially combined system

In this system a portion of storm water is allowed to enter in the sanitary sewage carrying sewer and remaining storm sewage into another sewer. Storm water from roof, pavement and yards are allowed to enter sewer and remaining sewage is drained off from other drain. 

Advantage: 

• •This is the most preferred system as it offers benefits of both systems. 
• •There is no need of separate plumbing work in each household or receiving units. 
• •The storm water collected from the road and public spaces can be directly discharged to the river through open drain. 
• •The  storm  water  entering  to  the  sewer  helps  in  flushing  deposited  sediments  that  can possibly occur at the beginning part of the sewer line due to low sewage quantity