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