VPE 511: Environment and Environmental Hygiene (2+1)

PRELIMINARY TREATMENT

• Storm water
• The quantum of storm of upto three times the dry weather flow should be dealt with in the works as ordinary sewage. Anything over this amount should be diverted to special storm tanks.
•  Screening
• This is necessary to remove gross solids such as lumps of faeces or paper etc. The process may consist of coarse or fine screening. In the former method the sewage is passed through fixed screens made up of vertical iron bars which are 1 cm to 5 cm apart. These screens may remove as much as 10 percent of the total suspended matter in the sewage. Fine screens are of various types, the opening being from 0.5 to 1 cm in wide.
•  Detritus or Grit tanks
•  These are intended to remove the heavier inorganic matter such as road grit which is easily settled.
•  Sedimentation or Precipitation tanks
• These are tanks usually built of concrete, which may be worked with or without the addition of chemical precipitants, and in which finer solids in the sewage are allowed to settle by sedimentation the resultant precipitate or sludge is pumped out daily or weekly as may be found necessary in practice.
• Tanks may be operated on the fill and draw method or on the continuous method. In the former the top liquid is drawn off leaving the sludge. Continuous flow tanks may be designed for horizontal flow or vertical flow.
• Horizontal flow tanks are usually long in relation to width. Where the bottom is flat the sludge may be removed after the removal of the supernatant liquid. The capacity of a sedimentation tank is usually fixed to give a period of detention of 6 to 8 hours in the tank.
• Chemical precipitation of the suspended matter is adopted where sewage is very concentrated or where trade wastes have to be removed. Lime or lime and ferrous sulphate or sulphates of aluminum are used as precipitants. The amount of sludge produced by the aid of precipitants is greater than with simple sedimentation.
• Disposal of sludge
• One of the most troublesome parts of sewage treatment is the disposal of sludge including screenings and detritus. Screenings and detritus are always very offensive and are usually dealt with by shallow burial The screenings may also be burnt. The sludge may also be discharged into the open sea .Where sea disposal is not possible, disposal on land is resorted to either direct or after partial dehydration. Where direct disposal is adopted the sludge is pumped on the land and after drying is ploughed in.
• Sludge has considerable manurial value but before it can handle the water content must be reduced. This may be done on sludge filter beds , where the sludge is spread on comparatively thin layers of porous material, such as ashes well under drained. In some cities the sludge may be pressed into cake and afterwards converted into manure.
• Sludge may be digested in deep tanks during which process a considerable amount of gas is given off, consisting chiefly of methane with a small proportion of Co₂ and H₂. These gases are collected and utilised for power generation. Digested sludge contains of course considerably less organic material than the original and is non-putrefactive. It may be used for raising the level of low lying areas of land without causing any offence. In some cases the digested sludge is used as a base in the manufacture of artificial manure.
• Septic tanks
•  If sewage were held sufficiently long in a tank the greater proportion of the organic matter would pass into solution and little or sludge remain such a tank would then become a septic tank. This is due to putrefaction of organic matter under anaerobic condition. Septic tanks constitute a suitable means of sewage treatment for single houses or for all communities provided that not too much inorganic solids are carried in the sewage.