A Practical Method to Conserve Fresh Water

Many of our clients have indicated a willingness to reuse treated water from a Sewage Treatment Plant (STP).  The main indicated use of this water being car washing, flushing, landscape irrigation, cleaning of basement and bathrooms.  The intent is noble and the solution to reuse is logical.  The idea should be strongly encouraged.  The main advantage using treated water is a significant reduction in consumption of fresh water and the cost associated with fresh water, especially if it comes from an expensive source like tankers.    A number of precautions are needed before this water can be safely used, or else it will turn into a nightmare for health and cost.

Simply put, the function of Sewage Treatment Plant is to treat sewage water to get rid of organic matter, nitrogen, phosphorous and biological impurities such as bacteria and viruses which are predominant.  The thus treated water has to be clear, colorless, odor free, free of suspended matter and neutral i.e. its pH should be in the range of 6.5-8.5.  It also needs to meet the standards of local pollution control board.

In a well-run STP, pH, organic matter, nitrogen and phosphorous are very well treated.  Salinity or dissolved salts are not treated by an STP.  To achieve clarity, make the treated water odor free, remove the traces of organic matter and disinfect treated water, a tertiary treatment for treated sewage water with a disinfection unit is required.  The tertiary treatment typically consist of a well-designed sand and activated carbon filters.  The clarity and odor removal will be achieved when treated effluent is passed through these filters.  The filters need to be periodically regenerated.  The carbon needs to be replaced on a regular interval, approximately every year.  The bacteria need to be destroyed properly through a disinfection unit.  Chlorine or Ozone based disinfection units are very effective for killing bacteria.

High salinity levels is detrimental for irrigation and other use.  High salinity promotes water retention in plants and will affect their growth.  A high salinity is also associated with corrosion.  High salinity water can damage the metallic mechanism of flush tanks and reduce the life of metallic pipes.  It is best to not use it.  In event it needs to be used a Reverse Osmosis system will be required to treat the STP water.

Reverse Osmosis is an expensive proposition with respect to capital, operating and maintenance costs.  A through cost benefit analysis should be done before installing a RO system.

What happens if the STP does not treat the sewage water well?

In a poorly designed or operated STP, the organic matter, nitrates and phosphates will not be completely removed.  The water may have a fain color or odor and bacteria or viruses may be present in the treated water.  When such water is used for any applications, the organic matter can deposit on surfaces along with nitrates and phosphates and act as food for bacteria to grow on.  The bacteria promote a bloom or an uncontrolled growth on surfaces where this water is used.  Which in turn can lead to infections or dieses.  Hence it is of great important to treat the sewage water appropriately and used.

Precautions to handle treated sewage water

  • Have a separate pump and tank to store treated sewage water for further use.
  • Ensure STP works as per the manufacturer’s recommendations.  Do not operate above the rated capacity.  Do not operate way below the rated capacity.
  • Treated sewage water should be clear, colorless and odor free. If not, you may need to regenerate the filters or replace the activated carbon in carbon filter.  If the treated water is gray or blackish in color, do not use it.  It needs to be treated and discarded.
  • Dissolved Oxygen (DO) levels in treated sewage water should be routinely measured. This is an excellent surrogate measure which tells us if the STP is working properly.  Typically, it should be above 5 mg/L but may be less in hot weather.  You should talk to the STP supplier for details.  Adjust aeration to ensure full degradation of organic matter.  A good STP should have a DO indicator.
  • For treated and disinfected water, Oxidative Reductive Potential (ORP) should be 650-700 mV.  This value ensures that all bacteria are destroyed and water is disinfected1.  Typically, a good STP should have an indicator for ORP.
  • Periodically, test your treated water for the parameters indicated by the local pollution control board.  Typically, these are Color, Odor, Turbidity, pH, Phosphorous, Nitrates, Biological Oxygen Demand (BOD) and bacterial count such as coliform through an external laboratory.  This is a good measure which tells us if STP is functioning well.
  • The dissolved salt (salinity) levels need to be checked for use of treated water for landscape irrigation.  The key parameters for irrigation are salinity measured as electrical conductivity of water, sodium adsorption ratio, boron content.  These should be within limits or the growth of trees will be affected.