Select the correct statement from the following.

Explanation

Sewage water can be purified by passing it through sewage treatment plants with the action of heterotrophic microorganisms. There are three stages of this treatment - primary, secondary, and tertiary. Primary treatment removes floating and suspended solids from sewage through two processes filtration and sedimentation. First, the floating matter is removed through sequential filtration. The filtrate is kept in large open settling tanks where grit settles down. The sediment is called primary sludge while the supernatant is called effluent. The primary sludge traps a lot of microbes and debris. In secondary treatment, the primary effluent is taken to aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs. Flocs are masses of bacteria held together by slime and fungal filaments to form mesh-like structures. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As the BOD of the waste matter is reduced to 10-­15% of raw sewage, it is passed into the settling tank. Thus secondary treatment is more or less biological. The sediment of the settling tank is called activated sludge. The remaining is passed into a large tank called an anaerobic sludge digester. It is designed for continuous operation. The aerobic microbes present in the sludge get killed. Anaerobic microbes digest the organic mass as well as aerobic microbes of the sludge. They are of two types, nonmethanogenic and methanogenic. Methanogenic bacteria produce a mixture of gases containing methane, H2S, and CO2.