Domestic waste water (Sewage) treatment
The waste water from kitchen, bathroom lavatory, laundry,laboratory etc is called domestic sewage. It contains undissolved, suspended and dissolved solids in water. The sewage treatment involves four stages:
1. Preliminary or Mechanical treatment
2. Primnary or Physical treatment
3. Secondary or Biological treatment
4. Tertiary or Chemical treatment
1. Preliminary treatment (Mechanical treatment)
The main adjectives of preliminary treatment are the removel of
i) floating solids such as paper, rags, wood chips. etc by screening.The screen is made of metal bars or heavy wires.
i) grit (sand) in grit chambers.
iii) oils and greases as scum. The waste water is aerated and stirred well. Oils and greases which float on the water surface as, scum are easily skimmed off. This is called shimming.
2. Primary treatment (Physical treatment)
In the primary treatment, finely divided suspended organic solids and colloidal particles are removed. The waste water is Treated with alum or lime when the suspended and colloidal particles are precipitated and settle down in the sedimentation tank. This process is called coagulation and the chemical used is known as coagulant. The liquid sewage coming out of the sedimentation tank is known as effluent while the thick viscous liquid settled at the bottom of the tank is called sludge.
3. Secondary treatment (Biological treatment)
This treatment is employed for the removel of dissolved organic matter by bacteria and other micro-organism,. It is carried out in large tanks of different designs, of which the Activated sludge process (ASP) is the most versatile method.
The waste water is aerated in a tank when aerobic oxidation of organic matter occurs slowly. However, the oxidation is quickened by adding some microbial floc (sludge) obtained from the previous oxidation process. The bacterial flora present in the waste water grows and remains suspended in the form of a floc, which is called activated sludge. The effluent is separated from the sludge by settling and is discarded after keeping a part for recycling.
The bacteria and other micro-organisms present in the sludge bring about the following sequential changes:
i) Oxidation of the dissolved organic matter to CO2 and H20
ii) Degradation of nitrogenous organic matter to ammonia) which is then converted into nitrite and nitrate.
4. Tertiary treatment (Chemical treatment)
Tertiary treatment is necessary when the waste water is to be reused. The aim of tertiary treatment is to improve the quality of water by removing nitrogen and also by killing the disease causing micro-organisms. The various stages of tertiary treatment are:
i) Precipitation : The effluent received after the secondary treatment is mixed with lime when phosphorus compounds are precipitated as calcium phosphate in the settling tank
from where it is filtered out.
ii) Nirogen stripping : Nitrogen is present in the form of ammonia gas, nitrites and nitrates. When air is forced through the effluent, ammonia is removed.
iii) Chlorination : After the removal of phosphorus and nitrogen, the waste water is chlorinated to kill disease causing microbes. The treated water is then discharged into rivers or lakes and reused for irrigation and industrial purposes.
DESALINATION OR DESALTATION
Sea water as a source of Drinking water. The method by which salt is removed from sea water is known as desalination or desaltation.
Methods of desalination
1. Electro dialysis method: The separation of salt from water be achieved by employing an ion selective membrane.
Sea water is taken in a rectangular iron tank fitted with two ion selective membranes. One membrane consists ofa cation exchange resin while the other is made up of anion exchange resin. The former is permeable to cations whereas the latter permits only anions to pass through it.
On passing electric current, Na* ions move towards the cathode and CI ions move towards the anode.
Thus, the salinity (concentration. of NaCI) of sea water is reduced in the middle compartment and increased in the outer compartments. Water taken from the middle compartment is now suitable for drining purpose. Also, big molecules of the industrial effluents are retained by the membrane.
2. Reverse osnosis method: When a salt solution (sea water) is separated from water by a semipermeable membrane, water diffuses through the membrane into the saline side. This process is called osmosis. When an external pressure considerably greater than the osmotic pressure of the solution is applied on the saline side, diffusion of water takes place from the salt solution. This is called reverse Osmosis.
Sea water is placed between two semipermeable membranes made of polyamides (e.g. nylon) arnd pure water is fed into the outer Compartments.
An extermal pressure considerably greater than the osmotic pressure of the solution is applied on the solution in the middle compartment. Water moves into the outer compartments through the membranes leaving behind the concentrated salt water in the middle compartment.
