Sewage Treatment process are often classified as
1) Preliminary treatment
2) Primary treatment
3) Secondary(Biological) treatment
The preliminary treatment works on separating the floating materials and also the heavy settleable inorganic solids. This treatment reduces the BOD of wastewater by about 15-30%
Primary treatment consists of removing large, suspended organic solids. The organic solids, which are separated out in the
sedimentation tank are often stabilized by anaerobic decomposition and indigestion tank. The treatment reactors, in which the organic matter is destroyed & stabilized by anaerobic bacteria, are known as an anaerobic biological unit and may consist of anaerobic lagoons, septic tanks, Imhoff tank,s, etc.
The various treatment units are as follows:
There are 3 types of screens used for screening purposes.
a) Coarse Screen- Also known as racks. Spacing between the bars is about 50mm or more. Generally uniform in size placed across the flow, to retain floating coarse solids.
b) Medium Screen- The spacing between the bars is about 6 to 40 mm
c) Fine Screen- Size between 1.5mm to 3mm. These screens prove very effective.
COMMINUTORS – These devices are also used as shredders which break the large sewage solids to about 6 mm in size.
- The approaching velocity of sewage in the screen chamber shall not fall below the self – cleaning velocity, (i.e) 0.6 m/s and shall not be more than 1 m/s, so that the trapped screening is not dislodged.
- The head loss through clean flat bar screen is calculated from
Where =Head loss, m;
Methods of Disposal of Screenings
- Land burial:
- Incineration (burning):
B) GRIT CHAMBERS
Also called grid basins are intended to remove the inorganic particle, such as sand gravel, grit egg shells, etc. of size 2mm or larger to prevent the damage of pumps.
Principle of Working of Grit Chamber
Critical Scour velocity given by Shield’s as,
Where u is Horizontal velocity of flow in cm/s, g is Gravity constant cm/s; Ss is specific gravity off grit particle, and d is the Diameter of grit particle in cm.
Design Criteria for Grit Chambers
- Surface loading: The grit particle sizes lie between 0.1 mm to 1.0 mm. stoke’s law setting of 0.1 mm diameter and less that of size particles.
Ss = Specific gravity of the particle, and g= Gravity constant
If diameter is 1.0 mm and more then
Hazen’s modified equation (for transition zone that is 0.1 mm to 1.0 mm size particles) gives a relationship between settling velocity, the density of particles and density, the temperature of liquid medium as
For particle size 0.1 mm to 1.0 mm
Where, T = Temperature of liquid medium in ° C, Us = setting velocity in cm,/s, and d= Diameter of particles in cm.
SEDIMENTATION AIDED WITH COAGULATION
Very fine suspended particles present in wastewater cannot be removed in plain sedimentation may sometimes be settled by increasing their size be changing them into flocculated particle. For this purpose, certain chemical compounds (like ferric chloride, ferric sulphate with lime, alum chlorinated copperas etc) called coagulant are added to wastewater which on thorough mixing
form a ppt called floc. The Process of Sedimentation is same as explained in the Water Treatment Process.
Biological treatment is the most important stage in the processing of sewage because, in primary treatment as in the process of screening, sedimentation only 30 -40% of BOD and 60-70% of suspended solids are removed. The remaining pollution load should be removed by biological methods.There are two different types of Biological treatment that exists
AEROBIC TREATMENT Process requires supply of oxygen. The detention period is 4-6 hr. Sludge generated is more as compare to anaerobic process. End products are Carbon dioxide and water. No bad odour produced. Process is economical on large scale. Units are Trickling Filter; Aeration Tank; Activated sludge process. Oxidation Ponds
ANAEROBIC PROCESS Process does not require air. Longer retention periods are required (about 60 days). Less sludge is produced as compared to the Aerobic process. The main end products are Hydrogen sulphide, ammonia, Carbon dioxide and volatile organic compounds. It gives foul odours due to end products. They are economical and desirable on a small scale.