Question

What are the stages involved in the secondary and tertiary treatment of water?

Answer 1

• 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 metabolizes 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.