In the primary treatment of sewage, floating debris is removed by ___I__ and the grit is removed by ___II___.

The correct option is C I - sequential filtration, II - sedimentation
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 sewage 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.

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 (grit) 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 biological treatment/secondary treatment.

The enzymatic digestion of the effluent by heterotrophic aerobic microbes such as bacteria and fungi takes place during the secondary/ biological treatment.