1.2    Sewage
        Sewage is  correctly the subset of wastewater, that is contaminated with faeces or urine , but is often used to mean only wastewater. Sewage is released by homes, industries, hospitals, offices etc. It also includes rainwater that has run down the street            during a storm or heavy rain. Sewage has dissolved and suspended impurities. These impurities are called “contaminants”.

      Wastewater constituents : Wastewater is a complex mixture containing suspended  solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes. A partial list of what it may contain is given below -
     (i)    Organic pollutants : Human faeces, animal waste, oil, urea (urine), pesticides,     herbicides,
        fruit and vegetable waste, etc.

        The organic matter of sewage is measured by determining its biological oxygen demand (BOD) or the chemical oxygen demand (COD).
        BOD (Biological Oxygen Demand) : It is a measure of the oxygen utilised by microorganisms during oxidation of organic materials.

        COD (Chemical Oxygen Demand) : It is an index of the waste (organic and inorganic) of water which can be oxidised  by strong oxidising agents. 

   (ii)    Inorganic pollutants : Nitrates, Phosphates, metals.  

   (iii)    Nutrients : Phosphorus and Nitrogen.     
           Eutrophication : The excessive growth of algae and aquatic plants due to added nutrients is called eutrophication.

    (iv)    Pathogen and micro-pollutants : Bacteria  Which causes cholera and typhoid.
                                                                    Other microbes  Which causes dysentery

1.3    Potable and Non-potable water    
        Potable water means water which is fit for drinking by humans and other animals. It can be consumed with low risk of immediate or long term harm. Non-potable water is that which is not safe for drinking. It may carry disease causing microbes, and high          levels of dissolved salts and minerals, heavy metals and suspended  solids. Drinking or using such water for cooking leads to illnesses and may even cause death.
       Contaminated or non-potable water can be treated to turn it into potable or drinking water. Let us learn about simple methods of purifying water.