Sewage Treatment Processes : Primary, Secondary and Tertiary!
The treatment of sewage is one of the important measures, which aims in the removal of BOD, phosphorous, nitrogen, solids and bacteria.
The composition of sewage is complex, and it differs depending upon the sources, the type of treatment or lack of it. The process of treating sewage is broadly classified as primary; secondary and tertiary (see Figures 8.3, 8.4 and 8.5).
1. Primary Treatment:
Primary treatment consists of removing floating and suspended solids by mechanical means. More than half of the suspended solids can be removed by primary treatment as shown in Figure 8.4.
In this process, first, the large solids are screened out and grease and scum are removed. Screening is done through metal bars first spaced 25 to 50 mm apart and may range down to 0.8 mm openings. Sand and other coarse material is removed by grit chambers. After screening and removal of grit, the waste water is run directly into settling or sedimentation tanks, the process by which the suspended solids are removed by gravitational setting.
Primary sludge is a problem because it is bulky and must be removed. It also contains 94 to 99 per cent water. In some cases, sludge is dried in beds with some water removed by filtration. A report of the American Chemical Society indicates that primary treatment reduces 60 per cent suspended solids, 35 per cent BOD, 30 per cent COD (Chemical Oxygen Demand), 20 per cent nitrogen and 10 per cent phosphorus.
2. Secondary Treatment:
Secondary treatment of waste involves the biological degradation of organic material by micro-organisms under controlled conditions.
The usual method is to bring about the biological oxidation of the organic material under aerobic conditions, in which the waste is aerated to supply oxygen for the micro-organisms. The degraded material settles out in secondary settling tanks and afterwards removed by sedimentation. The clarified waste water is discharged in the outfalls to rivers, lakes or oceans (see Figure 8.5).
3. Tertiary Treatment:
Tertiary treatment aims at further purification of waste water and also for its recycling.
The number of methods used for tertiary treatment is:
(i) Chemical coagulation and filtration,
(iii) Chemical oxidation.
(iv) Desalination, and
(v) Oxidation ponds.
However, there is a growing need for advanced procedures that will provide a product capable of being reused for various purposes. The recycled waste water can be used for irrigation, mainly for non-food crops such as grasslands, lawns, play grounds, etc. The renovation of waste water to a quality that would permit its reuse for a variety of purposes is a major objective of current research in this field.