STP Disinfection and Reuse


India being a water scarce nation with rapidly increasing population requires innovative solutions to either cut down the water usage or reuse/recycle the wastewater generated by human activities. There is an urgent need to reduce the forever increasing gap of the supply and demand of potable water. Due to long distance transportation to water scarce localities, the billing charges of freshwater usage takes a heavy toll. Reduction of groundwater extraction will lead to the natural conservation of water resources.


Challenges Faced

  • Reusing Sewage water often leads to the risk of F.Coli contamination.
  • Although, due to the perception of the society wherein it is unacceptable to ‘consume’ sewage water as potable; some level of aesthetics needs to be maintained.
  • Problems of colour, odour, COD (Endocrine Disrupters in Medicines, Shampoos, etc) are often the kind of challenges faced when reusing sewage.
    Remove pathogens thus reducing challenges at final disinfection especially Cryptosporidium which is highly resistant to chlorination

  • Common STP Problems in Today’s Times
  • A lot of STPs in India were designed just to comply with the pollution control norms.
  • Majority of factors were neglected while designing these STPs; hence we often come across STPs which are running over capacity and are unable to produce the desired results.
  • With the industrial revolution and advancement in every sector including household items , the household discharge these days are getting even more complex.
  • Typical problems of biological systems failing to give any reduction in COD/BOD etc have become widespread.
  • Majority of the STPs that were built decades ago are not capable enough to comply with revised norms. Hence the delta COD which remains after tertiary treatment has to be treated in some way to meet the required standards.


  • Disadvantages of Excessive Chlorination

  • The chlorine residual, even at low concentrations, is toxic to aquatic life.
  • All forms of chlorine are highly corrosive and toxic. Thus, storage, shipping, and handling pose safety risks.
  • Chlorine oxidizes organic matter in wastewater, sometimes creating compounds that could be harmful to humans and the environment. (Example: Trihalomethanes (THMs))
  • The level of total dissolved solids is increased in the treated effluent.
  • The chloride content of the wastewater is increased.
  • Certain types of microorganisms have shown resistance to low doses of chlorine.

  • Disinfection by Ozone

  • Ozone is more effective than chlorine in destroying viruses and bacteria.
  • The ozonation process utilizes a short contact time (approximately 10 to 30 minutes).
  • There are no harmful residuals that need to be removed after ozonation because ozone decomposes rapidly.
  • After ozonation, there is no regrowth of microorganisms, except for those protected by the particulates in the wastewater stream.
  • Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling.
  • Ozonation elevates the dissolved oxygen (DO) concentration of the effluent. The increase in DO can eliminate the need for reaeration and also raise the level of DO in the receiving stream.
  • Ozone can further reduce the COD due to its oxidizing property and help meet the new norms without much changes.