First of all filtration is taken out so that the garbages of bigger size can get separated easily after that sedimentation takes place in which the dissolved particles are settled down then biological activity takes place and then chlorination takes place so that water can get free from harmful microbes...

The correct sequence of steps involved in sewage treatment is step i, iii, ii, and iv, which is option B. Let's understand each step in detail:

i. Sedimentation:
- The first step in sewage treatment is sedimentation, also known as primary treatment.
- In this step, sewage is allowed to settle in large tanks called sedimentation tanks or clarifiers.
- During sedimentation, heavy solids such as grit, sand, and gravel settle down at the bottom, forming a layer called sludge.
- Lighter solids like oil and grease float on the surface and are skimmed off.

ii. Filtration:
- The second step in sewage treatment is filtration, also known as secondary treatment.
- After sedimentation, the partially treated wastewater is passed through different types of filters such as sand filters or activated carbon filters.
- These filters help in removing finer suspended solids, bacteria, and other microorganisms from the wastewater.
- Filtration plays a crucial role in improving the quality of water before it undergoes further treatment.

iii. Biological treatment:
- The third step in sewage treatment is biological treatment, also known as secondary treatment.
- In this step, the wastewater is treated using biological processes to remove organic matter and nutrients.
- The most commonly used biological treatment method is the activated sludge process.
- In this process, the wastewater is mixed with a culture of microorganisms called activated sludge in aeration tanks.
- The microorganisms break down the organic matter present in the wastewater into simpler compounds through aerobic digestion.
- The treated water is then separated from the sludge and moves on to the next step.

iv. Chlorination:
- The final step in sewage treatment is chlorination, also known as disinfection.
- In this step, the treated water is disinfected to kill any remaining harmful bacteria, viruses, and pathogens.
- Chlorine or other disinfectants are added to the water in a controlled manner to ensure that all harmful microorganisms are destroyed.
- After chlorination, the water is usually dechlorinated using chemicals like sulfur dioxide to remove any excess chlorine before it is discharged into water bodies or reused for non-potable purposes.

Overall, the correct sequence of steps involved in sewage treatment is sedimentation, filtration, biological treatment, and chlorination. This sequence ensures the removal of physical impurities, organic matter, and harmful microorganisms, resulting in the safe and environmentally friendly disposal or reuse of wastewater.