Environmental Engg. 3(2+1)

Lesson-20 Introduction to domestic waste water treatment

The raw sewage must be treated before it is discharged into the river stream. The extent of treatment required to be given depends not only upon the characteristics and quality of the sewage but also upon the source of disposal, its quality and capacity to tolerate the impurities present in the sewage effluents without itself getting potentially polluted.

The unit operations and processes commonly employed in domestic wastewater treatment, their functions and units used to achieve these functions are as follows:

Screening

The main purpose of the installation of screens is to remove floating matter of comparatively large size.  If such materials are not removed, they will choke up the small pipes or affect seriously the working of sewage pumps.  Thus, the main idea of providing screens is to prevent the pumps and other equipment from the possible damage due to floating matter of the sewage. Screens should preferably be located just before grit chambers.  If the quality of grit is of not much importance, as in case of land filling, screens may even be placed after the grit chambers.  Screens are usually placed in an inclined position with an angle of about 30) to 60) with the direction of flow.  Screens are sometimes accommodated in the body of grit chambers.

Grit removal

Sewage contains both types of material, namely, organic and inorganic.  The purpose of providing grit chamber in the sewage treatment process is to remove grit, sand and such other inorganic matter from sewage.  To achieve this purpose, velocity of flow in grit chamber is decreased to such an extent that the heavier inorganic materials settle down at bottom of grit chamber and lighter organic materials are carried forward for further treatment. 

In general, grit chambers are placed after pumping stations and before the screens.  But, there is no fixed rule regarding the location of grit chambers.  

Primary Sedimentation

Sedimentation tanks are also known as settling tanks or clarifier and the overall features of these tanks are more or less the same as for those tanks which are provided in water supply schemes.

Following are the objects of installing sedimentation tanks in sewage treatment works

• The process of sedimentation reduces the strength of sewage to the extent of about 30 to 35 per cent.

• Quantity of settleable solids in sewage is reduced to the extent of about 80 to 90 per cent.

• There is reduction in B.O.D. to the extent of about 30 to 35 per cent.

• Sewage after being treated in sedimentation tanks becomes fit for further treatment processes.

Sedimentation process

When velocity of flow is decreased or when sewage is allowed to stand at rest, the suspended particles carried by the sewage tend to settle at the bottom of tanks.  Material collected at the bottom of sedimentation tanks is knows as sludge and the partially treated sewage is known as effluent.  Sludge and effluent both require further additional treatment to make them unobjectionable.

Aerobic biological suspended growth process

Its function is to convert the colloidal, dissolved and residual suspended organic matter into settleable biofloc and stable inorganics.  This can be achieved by activated sludge process, waste stabilization ponds and aerated lagoons

Aerobic biological attached growth process

Its function is similar to that of the previous process, viz., aerobic biological suspended growth process.  This can be achieved by trickling filter and rotating biological contactor

Anaerobic biological growth processes

The purpose of this process is to convert organic matter into methane and carbondioxide and relatively stable organic residues.  Anaerobic filter, Fluid bed submerged media anaerobic reactor, Upflow anaerobic sludge blanket reactor, Anaerobic rotating biological contactor are some of the treatment methods adopted for this process.