CHAPTER –9 WASTE MANAGEMENT
Long Type Questions:
• Describe in brief the sanitary land-filling method for solid wastes disposal.
Ans: The basic principle of a landfill operation is to deposit the refuse, compact it with the help of bulldozers and then cover the material with at least 15cm of dirt at the conclusion of each day's operation. When the area is full, a final cover of about 60cm of dirt is applied, which is necessary to prevent rodents from burrowing into the refuse. The selection of a proper land-fill site is a difficult problem.
The engineering aspects of land-fill site selection include:
(1) Drainage: Rapid runoff reduces the mosquito problems, but close proximity to streams or dug-wells may result in water pollution.
(2) Wind: It is preferable that the sanitary land-fill be situated in the down wind direction from the city or town.
(3) Size: A small land-fill site with limited capacity is generally not acceptable because finding a new site involves considerable amount of trouble.
(4) Ultimate Use: At the planning stage, one has to consider whether the area can be utilised for public or private use after the land-filling operation is completed.
The above engineering problems are quite important, but even more important are the social and psychological problems of sanitary land-fills. No one in his right mind will be happy about having a sanitary land-fill near his house. A community may be rewarded with playgrounds, tennis courts, golf courses, etc., for tolerating a land-fill operation for a few years. If this operation is conducted according to accepted practice, there will be very little adverse environmental impact; but it is very difficult to convince and explain this to the people living near a land-fill. The reason is that most of the sanitary land-fills in the past were nothing but glorified garbage dumps.
The sanitary land-fill operation is, in fact, a biological method of waste treatment. In the absence of oxygen, anaerobic decomposition steadily degrades the organic material to more stable form. This process, however, is very slow. The decomposition may still be going on after 25 years. The end products of anaerobic decomposition are mostly gases like carbon dioxide (CO 2 ), methane (CH 4 ), aminoma (NH 3 ), and a small amount of hydrogen sulphide (H 2 S). Since these gases have to find an escape, it is a good practice to install vents in land-fills to prevent the build-up of the above gases. The decomposition reactions are self-sustaining and temperature inside the land-fill often reaches 55-60° C.
The biological aspects of the land-fill as well as the structural properties of the compacted solid waste dictate the ultimate use of sanitary land-fill sites. It is suggested that nothing should be constructed on a land-fill site for at least two years because uneven setting may often create problems. If initial compaction is poor, it may take as long as five years for just 50% settling to occur. If pilings are used as foundations on a land-fill, they should extend through the fill and onto rock or some other sufficiently strong material.
9.2 Describe a modem composting method.
Ans: composting of solid wastes is an aerobic method of decomposing the wastes. Many types of micro-organisms, which are already present in the wastes, stabilize the organic matters in it to produce a soil conditioner. The micro-organisms that decompose the wastes include bacteria (which predominate at all stages of the process), fungi (which often appear after the first week) and actinomycetes (which exist during the final stages).
Initially, the process starts with the mesophilic bacteria, which oxidize the organic matters in the wastes to carbon dioxide (CO 2 ) and liberate heat. As a result, the temperature rises to about 45°C. At this point, the thermophilic bacteria take over and continue the decomposition. During this phase of decomposition, the temperature rises further to about 60°C. The refuse is periodically turned over to a How enough oxygen to penetrate all parts of the material to support the process. The compost is stabilized after about three week's. The end point of a composting operation can be determined by noting a drop in the temperature, earthy smell and a dark brown colour.
The moisture content of the wastes is a critical factor in the aerobic composting process. Excessive moisture makes it difficult to maintain aerobic conditions. Dearth of moisture, on the other hand, inhibits the growth of microorganisms. For an optimum rate of composting, a moisture content of about 55% is required, and it may be necessary to add water in order to maintain a satisfactory moisture content.
Modern methods of composting may be classified into two broad categories:
(1) Manual method.
(2) Mechanical method.
The manual method is more attractive for towns with a rural bias. Manual methods are widely practiced in India and many other developing countries. In India, two manual methods have been developed for composting the refuse/night-soil mixture. These are:
(1) The Indore process.
(2) The Bangalore process.
In the Indore process, layers of vegetable waste and night-soil are alternated, each about 8cm thick, to depth of about 1.5m in a trench, or form a mound above the ground. The mixture is kept aerobic by turning it at regular intervals for two or three months. The resulting compost is then left for another month or so without turning. The whole process takes about 3-4 months. The Bangalore process of composting is a modified form of the hid ore process. The Bangalore process has been widely adopted now by various municipalities all over India. In this process, the refuse/ night-soil mixture is placed in layers (as in the Indore process) in a trench of about one metre deep. In contrast to the Indore process, however, the material is not himed in the Bangalore process, but is digested under essentially anaerobic conditions, whereby the decomposition is complete in about 4-5 months. Though the Bangalore process avoids turning the material altogether, it lays great emphasis on the carbon to nitrogen (C/N) ratio of the compost heap and its initial moisture content.
The resulting compost is free from pathogenic (i.e., disease-causing) organisms. On dry basis, the compost contains approximately 1.5% nitrogen, 1.1% phosphorous (as P 2 0 5 ;) and 1.5% potassium (as K 2 0) and proves to be a valuable nutrient for the soil. Fully mechanized composting plants involve shred ding, grinding and mechanical separation of high density solids.
There are basically four processes of mechanical composting available in India;
(1) The Buhler process.
(2) The Dano process.
( 3 ) The Tollemache process.
(4) The Nusoil process.
In the Buhler process, the waste material is ground in two stages in hammer mills. The non-compos table inorganic materials (metals and glass) are separated from the waste by the strong sifting action on circular swinging sieves. The remaining corn postable materialis then decomposed aerobicatly in open trenches or mounds. Stabilisation by this process may take about 2-3 months.
The Dane process uses a long rotating drum, called a bio-stabiliser unit, for decomposing the waste. The rotating drum is inclined so that the waste flows from one end to the other. The refuse is partially decomposed in the drum and the outcoming refuse is generally free from odour and pathogenic micro-organisms. This partially decomposed refuse is then completely decomposed in open trenches or mounds. The entire operation may take about a month.
In the Tollemache process, the waste is pulverized in a vertical pulveriser and then passed through a screening plant to screen out plastics, etc. The pulverised and screened refuse is allowed to decompose in trenches mounds for about three weeks, with three or four turnings. The resulting compost is then cured for 4-5 weeks. The complete stabilization, therefore, takes about two months.
In the Nusoil process the non-compostable inorganic material is separated from the waste, the resultant waste is pulverised and the pulverized waste is led to a vertical digester where decomposition takes place. The vertical digester is a cylindrical unit having seven sections. The refuse moves downward through each section of the digester. The rates of air flow and water addition are regulated so that the decomposition of the waste takes place under optimum conditions. The refuse is kept for about a day in each section of the digester and thus, the digestion process is completed in about seven days. The resulting compost is quite satisfactory' for direct field application without addition of supplementary nutrients. Next
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