SEWAGE TREATMENT SYSTEMS AND TECHNOLOGY

 

History of Toilets

Toilet Technologies

Pathogens in Human Excreta

Sulabh Technology

Water Reclamation and Reuse

Status in Delhi

  

Sewerage refers to the collection, treatment and disposal of liquid waste. Sewage systems include all the physical structures required for collection, treatment and disposal of the wastes. In other words, discharged waste water's that are collected in large sewerage networks, transporting the waste from the site of production to the site of treatment comprise Sewage treatment networks (Sewerage system).

SEWAGE: Sewage consists of liquid wastes produced in residences, commercial establishments and institutions; Liquid Wastes discharged from industries; and any subsurface, surface or storm-water which enters the sewer. Hence basically sewage contains three components:

  1. Sanitary or domestic sewage

  2. Industrial wastes

  3. Infiltration, Inflow and storm-water.

SEWER: A Sewer is a pipe or conduit, normally closed, flowing full or partially full, which carries sewage. Classification based on use of Sewers or Sewage systems can be done as follows:

  1. Sanitary Sewers: These sewers  transport domestic sewage, industrial wastes to the treatment plant. Storm-water does not normally enter these sewers except through joints, manhole covers and defaults in the system.

  2. Storm sewers: These carry the surface and storm-water passing through or generated in the area that they usually serve.

  3. Combined Sewers: These sewers carry all types of wastes - domestic sewage, industrial wastes and storm water in the same conduit.

 For Sewage Treatment Systems, large civil and infra-structural investments (like electricity supply) are required, generally paid by the authorities from the income tax collected from the citizens. In order to prevent clogging of sewers, a substantial amount of clean, and often potable water is required, which subsequently results in a dilution of the concentrated waste. The treatment systems, which need to be developed to treat the huge flows of diluted waste water, are extremely expensive and consume large amounts of energy. Moreover, the installed sewerage systems often suffer from lack of maintenance leading to huge losses of waste water (particularly in the developing countries).  

A sewage network is a transportation system for human excreta and/or industrial wastes to a central discharge point and/or treatment system, with valuable drinking water as the transport medium. The water demand for such sewer system is extremely high and absurd in conditions where safe drinking water is hardly available, like those prevalent in most cities and towns in India. Also from the environmental engineering point of view, the generally applied centralised sanitation concept can be questioned, because concentrated wastes are relatively easy to manage while management of diluted wastes requires large investments because of the increased quantity of wastewater to be treated.

             Many cities are served with sewer systems though the coverage and the treatment given to domestic and industrial wastewaters before discharge into surface water bodies is often inadequate.

 

BASIC DESIGN CONSIDERATIONS FOR SEWAGE TREATMENT SYSTEMS

1. DEGREE OF TREATMENT TO BE GIVEN TO THE WASTEWATER:

The degree of treatment in most cases is decided by the regulatory authorities [which in the case of India is the Central Pollution Control Board (CPCB) along with the various State Pollution Control Boards (SPCB's)] and the application for which the final effluent from the treatment plant is to be utilised/ disposal method used for the final effluent.

2. DESIGN PERIOD:

The sewage treatment system is normally designed to meet the requirements for a period of 30 years or more after the completion of construction activities. The system should not be under-loaded in the initial years nor be over-loaded in the dying stages. A provision has also to be made for installation of civil structures at any later date depending upon the requirement.

3. POPULATION TO BE SERVED:

A thorough estimate of the population to be served by the treatment system throughout its life period is necessary for the estimation of the quantity of sewage treatment that would require treatment.

4. SEWAGE FLOWS:

The quantity and quality (characteristics) of sewage show a marked  hourly variation and hence peak, average and minimum flows are important considerations from design point of view. The process loadings in the sewage treatment plant are based on daily average flows and the average characteristics are determined from a 24 hour weighted composite sample.

5. POPULATION EQUIVALENT:

The population equivalent is useful for conversion of the contribution of wastes from industrial establishments as compared to the domestic wastes, for acceptance in the sewage system. The average daily per capita contribution of suspended solids and BOD5 are 90 grams and 45 grams respectively, for estimation of population equivalent in Indian conditions.

 


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UNIT OPERATIONS/ PROCESSES IN SEWAGE TREATMENT:

*Screening

*Grit removal

*Primary Sedimentation

*Aerobic Biological Processes:

*Attached Growth Process

*Suspended Growth Process

*Anaerobic Biological Processes

*Sludge Digestion and Stabilisation

 

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SEWAGE CHARACTERISTICS

*Temperature

*pH (Hydrogen ion concentration)

*Colour and Odour

*Solids (Volatile, Suspended, Dissolved)

*Nitrogen and Phosphorus

*Chlorides

*Biochemical Oxygen Demand (BOD)

*Chemical Oxygen Demand (COD)

*Toxic chemicals and compounds

 

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