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