The correct option is B Treatment of sewage by natural processes
Friends of the Arcata Marsh (FOAM) is credited with the treatment of sewage by natural processes. The town people of Arcata created an integrated waste water treatment process in collaboration with biologists from the Humboldt State University. The cleaning occurs in two stages:
i) The conventional sedimentation, filtering and chlorine treatments are given. After this stage, lots of dangerous pollutants like dissolved heavy metals still remain. To combat this, an innovative approach was taken, which happens to be the second stage of cleaning.
ii) The biologists developed a series of six connected marshes over 60 hectares of marshland. Appropriate plants, algae, fungi and bacteria were seeded into this area, which neutralise, absorb and assimilate the pollutants. Hence, as the water flows through the marshes, it gets purified naturally.