The primary treatment of sewage is a ___i____ process while the secondary stage of treatment is ___ii____.

The correct option is B i - physical, ii - biological
The large quantities of waste water that is generated every day in cities and towns is known as sewage. It contains human excreta, large amounts of organic matter (food waste) and pathogenic microbes. It is treated in sewage treatment plants (STPs) before it is discharged into natural water bodies to reduce pollution. The first stage of treatment is called primary treatment. It involves the physical removal of particles from the sewage through filtration and sedimentation.

Filtration: The floating debris is removed by sequential filtration during which sewage is passed through more than one filter of varying pore sizes in series. The debris forms the retentate.

Sedimentation: The filtrate obtained from sequential filtration is allowed to stand undisturbed for a certain time. The heavier particles such as soil and small pebbles settle down under the effect of gravity and form the sediment which is referred to as primary sludge. The supernatant formed is called the effluent and undergoes further secondary treatment.

In the secondary treatment the enzymatic digestion of the effluent takes place by living organisms like heterotrophic aerobic microbes (bacteria and fungi). Therefore, it is also known as biological treatment. The microbes use the organic pollutants in sewage as a source of nutrition and thereby reduce its pollution potential.

Some STPs consist of a series of tanks, along with pumps, blowers, screens, grinders, and several other mechanical components to remove coarse insoluble impurities in sewage before biological treatment. It is referred to as mechanical treatment.