Question

Describe how biogas is generated from activated sludge. List the components of biogas.

Answer 1

Hint:
Treatment of a lot of waste water is done by the microbes naturally present in the sewage. This treatment includes two stages - primary treatment ( physical removal of particles from waste ) and secondary treatment ( biological treatment).
The secondary treatment leads to production of biogas. Biogas is a mixture of gases produced by microbial activity which may be used as a fuel.

Complete step by step answer:
During physical treatment of sewage, large and small particulate matter is filtered. All the solids that settle form the primary sludge and the remaining effluent is further taken for secondary treatment.
-During secondary treatment growth of useful aerobic microbes occurs due to constant supply of oxygen.
-Due to this, biological oxygen demand or BOD (amount of oxygen consumed by microbes to oxidise whole waste products) decreases.
-The sewage water is treated until the BOD is reduced. Once the BOD is reduced , this effluent is further transferred to a Settling tank.
-In the Settling tank microbial masses are allowed to sediment . This sediment is then called activated sludge.
-Then the major part of sludge is transferred to large tanks (anaerobic sludge digesters). Here anaerobic bacterias start growing and then digests the aerobic bacterias and fungi already present in activated sludge.
-During this digestion, bacteria produce a mixture of gases. These gases form the Biogas.
Thus biogas is produced from activated sludge during secondary sewage treatment.

Note:
Biogas is also called "Gobar gas" as it can be produced by the cow dung (potent fertilizer).
The major composition of biogas is:
-Methane = 50-70%
-Carbon dioxide = 30-40%
-Traces of hydrogen sulphide and hydrogen.
-It can also be used as fuel as it is a mixture of highly inflammable gases predominantly methane.