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    Preliminary treatment

    Sewage contains a lot of materials that have to be removed before treatment can begin.
    These include paper, rubber, plastics, cotton and grit washed from roads. The larger
    items are removed by passing the sewage through a sieving device called a screen that
    traps anything larger than 6mm in diameter


                                                      6 mm diameter
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    Primary treatment

    The first treatment process removes the fine solid material suspended in the sewage. The
    sewage is put into large settlement tanks and left for several hours to allow the fine
    particles to sink to the bottom of the tank. Alternately, the sewage is passed through a
    rectangular pool containing vertical baffles. As the sewage passes over the baffles solid
    material settles to the bottom.

    The cleaner water is removed from the top of the tank and goes to secondary treatment.
    The sludge that settles on the bottom of the tank is pumped out into holding tanks before
    being sent for processing.
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    Secondary treatment

    The second treatment process removes organic substances dissolved in the water. The
    water is either trickled through a biological filter bed or is put into tanks and air is
    bubbled through it. Both methods provide microorganisms occurring naturally in the
    sewage with the oxygen and food they need to grow. As the microorganisms grow, they
    breakdown the organic material present in the sewage.

    The mixture is then put into settlement tanks so that the microorganisms, and their waste
    products, can settle out as sludge. In the tank method - called activated sludge - some of
    this sludge is returned to the treatment tanks, providing more microorganisms to feed on
    the sewage. The surplus is pumped to sludge holding tanks. The water from the
    secondary treatment process is clean enough to be returned to the environment in most
    cases.
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    Tertiary treatment

    A third stage of treatment is sometimes added at plants that discharge into sensitive water
    supplies. The process reduces the amount of phosphates and nitrates, further reduces the
    amount of insoluble particulate matter and kills most of the remaining microorganisms.

    First the water from secondary treatment is pumped into a tank and a flocculant is added.
    The most common flocculants are Alum (aluminum sulfate) and Lime (calcium oxide).
    These chemicals have three effects on the treated water.

       1. They raise the pH to an alkaline level, which kills most of the microorganisms.

       2. They react with phosphates and nitrates and form insoluble compounds that
           precipitate.

       3. They form a gel-like substance that coagulates suspended material causing them
           to precipitate.

       The clarified water at the surface is pumped over sand and activated charcoal filters to
       remove the remaining particles and organic compounds. The effluent is sometimes
       treated with ultraviolet light or chlorine to kill any remaining microorganism.
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    Sludge processing and recycling

    The sludge from the primary and secondary treatment processes is pumped to storage
    tanks for thickening. At some plants the thickened sludge (about the consistency of
    tomato soup) is put into special tanks called digesters.

    Here the sludge is warmed and the bacteria in the sludge grow anaerobically (in the
    absence of oxygen). This creates gases that can be burned in a combined heat and power
    plant - the heat is used to warm more sludge and the power is used to drive the plant.

    Once digested the sludge is safe for agricultural use and much of it is injected into
    farmers' fields.

    At other plants, the sludge is dried into granules or pellets that can be used as a fuel, as a
    carbon source in manufacturing or as a soil conditioner in agriculture or land reclamation.

				
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