Question

Why and how is the sewage is treated before disposal?

Answer 1

• Sewage has to be treated before it is discarded into the environment so that it causes less harm to the health and environment.
  
• The goal of this treatment is to reduce harm as much as possible when discarded into the environment. It involves following stages:
  
• Primary treatment is usually done to remove suspended and floating solids. It is done by removing suspended solids on basis of gravity which is done in suspension tanks. When water is allowed to settle for a long time solids move down and oil and grease that moved to the surface are skimmed and also can be removed by saponification. Suspension tanks are provided with scrappers that remove sludge and send it to sludge removal tank.
  
• Secondary treatment of water involves removal of dissolved and colloidal compounds by different processes:
  
• Biofiltration: Most commonly used is trickling filters. These are made of packed bed of plastic, broken rock, gravel etc. the effluent is made to pass through the bed to remove solids.
  
• Aeration: This is also called as activated sludge. In this water is treated with flocculent suspension of microbes and aeration. Water is adsorbed onto the microbial aggregates and these microbes metabolize contaminants into simpler compounds.
  
• Oxidation ponds: This makes use of natural water bodies like lagoons where water is allowed to stand for about three weeks. Organic contaminants undergo decomposition and carbon dioxide, ammonia and nitrate are released for use by algal community. Organic sludge settles at the bottom and methane is finally released. But this water may prone to harbour pathogens.
  
• Tertiary treatment: the secondary effluent obtained is made to undergo tertiary treatment for removal of remaining inorganic compounds. The process essentially removes phosphates and nitrates from water.
  
• Bacteria, viruses and parasites are also removed at this stage. It uses sand filters which has alum due to which particles aggregate to form floc. This floc is arrested in the sand and water is passed into chlorinated tank by action of gravity which is located below. Chlorine removes microorganisms in treated wastewater including bacteria, viruses and parasites such as Giardia and Cryptosporidium.
  
• Floc is removed from time to time and any chlorine present in water is removed by sodium bisulphite.