La “vuelta a Kant”

8. 1 Introduction

If untreated wastes are accumulated or spread everywhere, the decomposition of the organic materials within wastes can lead to the production of malodorous gases, pathogenic organisms or may contain toxic compounds and will pollute our environmental (land, air, water).

Therefore, all the solid and liquid wastes have to be collected and conveyed to suitable places for treatment, and are disposed safely.

Definition of Terms:

Sewage: a combination of the water carried wastes removed from residences, institutions, and commercial and industrial establishments, together with such ground water, surface water, and storm water as may be present.

Sewer: Underground conduit (drain) through which sewage is conveyed Sanitary sewer: A sewer carrying domestic sewage only

Storm sewer : A sewer carrying storm water only

Combined sewer: A sewer carrying domestic sewage and storm water Sullage : Wastewater from bathroom, kitchens, etc.

Night soil: Human excreta

Sewerage system: The entire science of collecting and conveying sewage by water carriage system through sewers.

BOD (Biochemical Oxygen Demand): the quantity of oxygen utilized by a mixed population of microorganisms in the anaerobic oxidation ( of the organic matter in a sample of waste water ) at a temperature of 20⁰C

8.2 Methods of collection and Conveyance There are two methods/systems:

1. Water-carriage system

2. Conservancy system ( Non-water Carriage) Water Carriage System

 In this system, water is used as a media of conveying the sewage to the points of treatment and disposal. Therefore it is called water carriage

 In this system the excremental matter are mixed up with large quantity of water, are takeout from the city thought properly designed sewerage systems, and disposed after necessary treatment.

Merits and Demerits of the Water-carriage system

Adama Science and Technology University- Department of Civil Engineering 2011/12

25 Merits:

1. It is hygienic method because all the excremental matters are collected and conveyed by water only and no human agency is employed for it

2. There is no nuisance in the streets of the towns because all the sewage goes in closed sewers under the ground, the of epidemics (spreading of disease )is reduced

3. Due to more quantity of sewage self cleaning velocity can be obtained even at less gradient

4. Buildings can be designed as one compact unit

5. The usual water supply is sufficient and no additional water is required in water carriage system.

6. This system does not depend on manual labor at every time except when sewers get chocked

Demerits

 Costly in initial cost

 Maintenances of this system is also costly

 During monsoon, (wet season) large volume of sewage is to be treated (where as very small volume is to be treated in the remaining period of the year).

(CEng-4607): Water Supply & Sewerage Lecture Notes (Water supply project planning)

Adama Science and Technology University- Department of Civil Engineering 2011/12

26 Water Carriage system may be divided in two:

a. Sewerage Systems b. Septic Tank

8.3 Types of Sewerage Systems

These are broadly classified as follows:

1. Combined system 2. Separate system

3. Partially separate system

Combined system : when only one set of sewers are laid, carrying both the sanitary sewage and storm water it is known as combined system. This system is most useful in areas having less sewage to obtain self- cleanings velocity and crowded area (where it is difficult to provided two sewers)

Industrial wastes

Segregation &

Pretreatment

Residential &

Commercial Wastewater

Municipal Wastewater treatment plant

Treated Effluent

Combined storm &

Sanitary Wastewater

Overflow by- pass

Natural watercourse

Surface drainage

Infiltration & Inflow

Sanitary Sewers Separate

disposal

Unpolluted Cooling water

Interceptor sewer

Fig 8.1 Source of municipal wastewater in relation to collector sewers and treatment

Adama Science and Technology University- Department of Civil Engineering 2011/12

27 Separate system: When the domestic and industrial sewage are taken in another set of

sewers, it is known as separate system.

If the rainfall is heavy and it is for short time, it is better to provide separate system, because in combined system self –cleaning velocity will not be available for most of the period of years.

Partially separate system: - for this type of system, if a portion of storm water is allowed to enter in the sewers carrying sewage and remaining storm flows in separate system.

Merits and demerits of separate system Merits:-

1. The sewage flows in separate sewer, therefore the quantity of sewage to be treated is less, which results in economical design of treatment plants

2. It is cheaper than combined system, because only sanitary sewage flows in closed sewers and the storm water, which is a foul (unclean), in nature can be taken through open shutters or drains.

3. During disposal, if the sewage is to be pumped, the separate system is cheaper.

Demerits

1. Self-cleaning velocity is not available, due to small quantity of sewage; therefore flushing is required at various points.

2. There is always risk that storm water may enter the sanitary sewer and cause overflowing of sewer and heavy load on treatment plant.

3. As two sets of sewers are laid, therefore the maintenance cost is more

4. In busily lanes, lying of two sewers are difficult, which also causes great inconvenience to the traffic during repair.

Merits and Demerits of the Combined System Merits:

1. There is no need of flushing, because self cleaning velocity easily available at every place due to more quantity of sewerage.

2. Rainwater dilutes the sewage therefore; it can be easily and economically treated Demerits:

1. Initial cost is high as compared with separate system.

2. It is no suitable system for areas having rainfall for small periods of the year. Because dry weather flow (rain water) will be small due to which self-cleaning velocity will not be available , resulting in the silting up of the sewers,

3. If the whole sewage is tube disposed of by pumping , it is un economical 4. During heavy rains the over following of sewers will endanger the pubic heath 8.3.1 Separate System

(CEng-4607): Water Supply & Sewerage Lecture Notes (Water supply project planning)

Adama Science and Technology University- Department of Civil Engineering 2011/12

28 This system comprises of :

 Sanitary system

 Storm water system

 Domestic and industrial sewage are taken to one set of sewers  sanitary sewers.

 Storm water is taken to another set of sewers  Storm sewers

 Sanitary sewage is carried to treatment plants and storm water discharged directly to natural water bodies (rivers, lakes, etc.)