Disposal of Sewage Effluents

General methods of disposing of the sewage effluents:

• Dilution i.e. disposal in water, and
• Effluents Irrigation or Broad Irrigation or Sewage Farming. i.e. disposal on land

Disposal by Dilution

• Discharged into a river stream. or a large body water, such as a lake or sea.
• It is purified by "self purification process" of natural waters.
• The degree of treatment to be given to raw sewage before disposing depends on quality of raw sewage, the self purification capacity of the river stream and the intended use of its water.
• The ratio of the quantity of the diluting water to that of the sewage is known as the dilution factor

Standard of Dilution for discharge of Waste waters into Rivers:

 

Dilution Factor	Standards of Purification required
Above 500	No treatment is required
Between 300-500	Primary treatment such as plain sedimentation is to be given
Between 150-300	Treatments such as sedimentation screening and essentially chemical precipitation are required.
Less than 150	Complete through treatment should be given to sewage

• The BOD5 of sewage effluents discharged into surface waters,shall not exceed 20 mg/l as per BIS
• The BOD5 industrial effluents discharged into public sewer,shall not exceed 500 mg/l as per BIS
• The BOD5 of industrial effluents discharged into Marine Coasts shall not exceed 100 mg/l as per BIS.
• The BOD5 of effluents used for sewage farming shall not exceed 500 mg/l.

Various natural forces of Self purification:

Physical forces are

• Dilution and dispersion
• Sedimentation
• Sunlight

Chemical forces aided by biological forces

• Oxidation
• Reduction

Dilution and Dispersion : When sewage of concentration Cs flows at a rate Qs into a river stream with concentration CR flowing at a rate QR, the concentration C of the resulting mixture is given by

The above equation is applicable to the concentrations of D.O.,B.O.D etc.

Sedimentation : Settleable Solids will settle down into bed of the river.

Sun-light: Algae plants by absorbing carbon dioxide  and releasing oxygen by a process as photo Synthesis.

Oxidation: Oxidation will continue till the organic matter has been completely oxidized. This is the most important action responsible for effecting self purification of rivers.

Reduction: Anaerobic bacteria at the bottom of river bed will help in splitting the complex organic constituents of sewage into liquids and gases, and thus paving the way for their ultimate stabilization by oxidation.

The various factor on which these natural forces of purification depend are: Temperature, turbulence, hydrography, dissolved oxygen, rate of re aeration etc.

At higher temperatures the capacity to maintain the D.O. Concentration is low, while the rate of biological and chemical activities are high, causing thereby rapid deletion of D.O at higher temp.

Zones of pollution in a River-Stream:

• Zone of degradation
• Zone of active decomposition
• Zone of recovery
• Zone of cleaner water

Zone of degradation or zone of pollution: This zone is found for a certain length just below the point where sewage is discharged. Water becomes dark and turbid with formation of sludge deposits at the bottom DO. is reduced to about 40% of the saturation value.

Zone of active decomposition: This zone is marked by heavy pollution. D.O concentration falls down to zero, and anaerobic conditions may set in. Fish life will be absent

Zone of recovery: Stream tries to recover B.O.D falls down and DO. content rises above

40% of the saturation value. The organic material will be mineralized to form nitrates, sulphates, phosphates, carbonates, etc

Zone of cleaner water: The river attains its original conditions with D.O. rising up to the saturation value. Fish ( requires at least 4 mg/l of D.O) and usual aquatic life prevails.

Disposal of waste Water in Sea Water: Sea water normally contains 20% less oxygen than that contained in fresh water of a river stream.

Sludge tanks: Sewage solids thrown into sea water, chemically react with the dissolved matter of sea water, resulting in precipitating some of the sewage solids, giving a milky appearance to sea water and forming sludge banks.

The specific gravity of sea water is greater than that of sewage, and temperature of sea water is lower than that of sewage.

Sleek: The lighter and the warmer sewage will rise up to the surface when thrown into the sea water and will result in spreading of the sewage at the top surface of sea in a thin film of ‘sleek’.

Disposal of Sewage Effluents on Land for irrigation

Disposing of the sewage, may help in increasing crop yields (by 33% or so)

The BOD5 of effluents used for sewage farming shall not exceed 500 mg/l

The oxygen Deficit of a Polluted River-stream

Oxygen deficit (D) = saturation D.O. - Actual D.O

Re oxygenation: Oxygen is absorbed from the atmosphere to counteract the depiction of D.O. content.

Oxygen Deficit Curve: (Oxygen Sag Curve):

• Obtained by algebraically adding the deoxygenation and reoxygenation curves.
• Deoxygenation Rate > Reoxygenation Rate - Deficit increases.
• When both the rates are equal 4 Critical deficit occurs.
• Reoxygenation > Deoxygenation Rate - Deficit decreases.

Self Purification constant:

                     f= KR / KD

KD = Deoxygenation Rate

KR = Reoxygenation Rate

Streeter - Phelps equation,Dt

Dt = The D.O. deficit of the mixture in mg/l after ‘t’ days

L = Ultimate first stage D.O.D. of mixture at the point of waste discharge in mg/l

Do = Initial oxygen deficit of mixture in mg/I at the mixing point

Critical or maximum oxygen deficit,Dc 

Sewage Sickness : When untreated sewage is continuously applied on a piece of land. during course of time the soil voids get clogged thereby free circulation of air is prevented and anaerobic conditions develop. Then the land can not take any further sewage load and foul gases will evolve. This phenomenon is known as’ Sewage Sickness.