Society and Environment - Study Notes



•  How should we protect the ozone layer? Write in brief.

Ans: So far the only method we know to protect the stratospheric ozone layer is to limit the emission of those substances (mainly CFCs) that can destroy it. No one knows of another material we could send into the stratosphere to protect the ozone layer from CFCs. The threat to the ozone layer is so severe that international conferences have been held and declarations and treaties have been adopted that commit the nations to restrict and eventually eliminate the use of CFCs. For some applications of the CFCs, satisfactory replacements are alread available and for others, they are being sought on an emergency basis. Many of the proposed substitutes for CFCs are hydrochlorofluorocarbons (HCFCs), which contain atleast one hydrogen atom; so they are susceptible to attack by OH radicals in the stratosphere.

Huge amounts of money and effort are being spent now to develop suitable substitutes for the CFCs. The original CFCs were designed to be non- toxic, non-flammable and chemically inert. There replacements should have all these properties and, in addition, a low ozone depletion potential (ODP). The ODP is a relative measure of the ability of a gas or vapour to attack and destroy the stratospheric ozone layer. It is expressed as a function of the percentage weight of chlorine in the halogenated hydrocarbon (to be used a CFC substitute) and its lifetime in the stratosphere.

The chemical industry' is developing two major types of CFC substitutes. One of these, known as the hydrochlorofluorocarbon (HCFC) family, has a fairly low ODP. The other group, known as the hydrofluorocarbon (HFC) family, has zero ozone depletion potential since the members of this family have no chlorine atoms in their molecules. It may be noted, however, that the members of both families (HCFC and HFC) contribute to the greenhouse effect and global warming. Many chemical companies, including the industrial giants like Du Font (USA) and the Imperial Chemical Industries (UK), argue that the world should switch over to the HCFCs as an interim step because they are cheaper to produce and simpler to use.

Compared to the CFCs, the hydrofluorocarbon (HFCs) are less stable and readily hydrolysed in the troposphere by the OH attack; so they are believed to be more environment tally acceptable than the CFCs. The data on the new HFCs are, however, scarce and of Limited accuracy and this is a major deterrent to their adoption as CFC substitutes.

Another problem with the CFC substitutes is that they are not as good as the CFCs. For example, one HCFC (known as HCFC-2), which, is already being used in large-scale refrigerator installations in supermarkets, etc., cannot be used in domestic refrigerators. Another substitute, a hydrofluorocarbon known as HFC-134a, is under development for domestic refrigerators; but it is likely to cost at least five times more than the CFCs used currently for this purpose.

In addition to the problems mentioned above, the CFC substitutes will require a substantial amount of re-engineering in many applications. In case of domestic refrigerators, for example, larger compressors and pumps may be needed when CFC replacements are used. These substitutes may also require more energy, thus forcing us to burn larger amount of fossil fuel, leading to more global warming.


•  Describe some methods to treat and dispose the municipal sewage.

Ans: In developing countries, the organic pollutional load (contributed mainly by municipal sewage) is a major problem in connection with water pollution. In India, it has been estimated that municipal sewage contributes 90% organic pollutants to our river systems, 7% is contributed by large and medium industries, while the remaining 3% comes from small-scale industries. Thus treatment, utilization and proper disposal of municipal sewage are very important steps towards the control of water pollution. Some of the important methods used for the disposal of sewage are as follows :

(1) Land Disposal and Sewage Irrigation. In this method, the sewage is disposed off on land (often for the purpose of irrigation) instead of discharging it into a water body. In this case, the land area acts as a crude filter and stabilizes the sewage by aerobic filtration. The sewage is usually given primary treatment before its disposal on land. This method is very similar to intermittent sand filtration. In case of sewage irrigation, the water and the fertilizing elements in the sewage, viz., nitrogen, phosphorous and potassium, are utilized to raise crops. Hence, this method has the special advantage of fertilizing the land.

(2) Dilution. In this method, the sewage is discharged into a large body of water like river lake or sea. The sewage is usually treated before discharging it into a water body to ensure that the condition of the receiving water body does not deteriorate to the extent of impacting on its normal use. I he cities arid towns which are situated near a large river (Allahabad, Kanpur, Varanasi and Patna, all of which are situated near the river Ganga) mainly use the method of dilution to dispose off their sewage. This method is considered quite satisfactory during the rainy season when the dilutionfactor is extremely high, but not suitable for the period from November to June, when the volume of river flow or the volume of water in the river is low,

Treatment of Sewage

Primary Treatment: The primary treatment of sewage mainly consists of physical separation of floating and suspended solids (that are settleable) from the sewage. The main equipment used for the primary treatment are screens, grit chambers, detritus tanks, skimming tanks and settling tanks.

Secondary Treatment : The secondary treatment of sewage includes flocculation and precipitation of the remaining materials in the sewage with the help of biological agencies, and their physical separation in secondary settling tanks. The by-products of the secondary treatment process are screenings, grit and sludges of different kinds. Out of these, the first two can be easily disposed off by burial or burning. Sludge, on the other hand, contains unstable volatile organic substances; so it may be treated by the method of digestion. In the process of digestion of sludges, various gases are produced with high calorific value which can be successfully utilized for heating or power generation. The solid materials left behind (i.e., the digested sludge) contain many fertilizing materials and are useful as manure or soil builder.

Sludge Digestion : The process of sludge digestion consists of liquefaction of organic materials in the sludge by anaerobic bacteria, which produce an alkaline reaction. When the sludge is first placed in the digesting tank., acid digestion with the production of noxious gases results and eventually, alkaline digestion prevails. Once established, the alkaline condition remains in the digestor for an indefinite period. The gases given off by the sludge are mainly methane (CH~) with some carbon dioxide (CO;) and small quantities of other gases. Among the sludge gases, methane has a calorific value of 8,000 kcal/m3 Since methane forms about 67% of the sludge gases, the calorific value of the sludge gas can be taken to be approximately 5,334 kcal/m3 The sludge gas occupies about 0.95 m3/kg of volume at normal temperature and atmospheric pressure. The main use of the sludge gas, apart from heating the digestion tanks, is for generating power. The power thus generated is enough to pump the entire sewage. Dried digested sludge can be used as a fertilizer since it contains 0.8% to 3.5.% nitrogen, about 1.6% phosphorous and 0.35% potassium on the basis of the dry weight of solids. The digested sludge may also be used for land filling, incinerated to produce heat and power, or dumped into large bodies of water.






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