Here we are going to study that how solid waste or refuse collection from each individuals house or apartments takes place. Then the process of removal of solid waste or refuse, we will discuss. Further the transportation of solid waste or refuse carried through which vehicles. Then a detailed phenomenon of disposal / decomposition of solid waste or refuse. At the end composting is explained and its various methods are mentioned.
COLLECTION AND REMOVAL OF REFUSE OR SOLID WASTE
The frequency of solid waste depends upon the quantity of the refuse and the season. Generally, refuse collected in individual houses in small containers or cans. This should kept outside the premises of the house, from where it thus removed daily by sweepers. Sometimes, public dust-bins are provided by the municipalities/local boards at convenient places by the sides of roads. Dry refuse fallen on the public streets and roads, along with road sweepings. These usually collected once or twice a day by the sweepers.
For this purpose, a portable galvanised iron receptable with a closely fitting lid. And having a capacity of 0.02 to 0.1 m3 is generally used. The containers or cans used for refuse collection should be clean, without any dirt left in. Otherwise fresh refuse gets seeded with the putrefactive organisms and start giving foul smell. The frequency of refuse collection kept such that the refuse may not start giving bad odour and fly breeding. The collection of refuse from the business areas should done during non-working hours.
TRANSPORT OF REFUSE OR SOLID WASTE
The refuse collected in the public dust bins located by the sides of roads, transported to the disposal site for solid waste. This should transported by means of following vehicles –
(i) Auto-rickshaws (ii) Trailers (iii) Trucks
- Auto rickshaws: These, having three or four wheels, have covered bodies. Since their capacity limited to 1/2 to 3/4 tonnes. These are use only for those narrow localities where other heavy vehicles cannot go.
- Trailors: Trailors have slightly larger capacity (2 to 3 tonnes). They also used for localities where trucks cannot go. Loading of trailors done manually. However, they are of tilting-tipping type and hence their unloading done automatically with the help of hydraulically operated jacks.
- Trucks: Trucks have larger capacity (5-10 tonnes). They are generally of tilting-tipping type so that unloading is automatic. Special types of trucks, capable of bodily lifting covered skip boxes (in place of ordinary dust bins). These now available, and should used so as to avoid nuisance of flies.
The vehicles employed for the transport of refuse should be of such pattern and design. So that collected garbage does not fall once again on the road during the transport. The transport vehicle should be strong durable and water tight. They should made of steel. It Should have smooth interior surface and round edge and corners. So that they can be kept clean.
DISPOSAL OF REFUSE OR SOLID WASTE
Refuse or solid waste can be finally disposed of by the following methods –
- Controlled tipping
- Filling of low lying areas (Land filling)
- Dumping into sea
1. Controlled tipping method in disposal
This method is useful where adequate site for redevelopment is available. The method consists of tipping the refuse in hollows to a depth of 1 to 2 m. While tipping, coarse material is tipped at the bottom while fine material is the tipped on the top. These tips are covered with soil, so as to provide a seal under which bacterial decomposition takes place. At the end of about 12 months period, the decomposition is complete. Due to which the tip settles down to a height of 30 cm only. Normally, an area of 0.3 to 0.5 m2 per capita per year is required.
2. Filling of low lying area (land-filling)
This method is quite common. The garbage dumped into low lying areas or depressions available nearby. Dumping is done in layers of 1 to 2 m. And each layer thus covered by 0.2 m thickness of good earth. A rest is of 2 to 3 weeks given before dumping the second layer. If dry refuse is loosely pack, it may give rise to health hazards. Hence each layer should compacted by movement of dumping vehicles for its settlement. Is should done before starting filling the second layer of refuse or solid waste.
The advantages of this method are –
- It is simple and economical
- No plant/equipment is reqd.
- Separation of various materials of the refuse is not reqd.
- There are no by-products and hence there is no problem of the disposal of the by-products
- The low lying areas can therefore reclaimed and put to better use by this method.
The disadvantages of this method are –
- proper site may not be available nearby
- wind direction may not be favorable
- large land areas are reqd.
- It may be difficult to get large quantities of covering material
- dumped garbage containing carcinogenic non-biodegradable matter. Such as plastics, unused medicines, paints, insecticides, sanitary napkins etc. These may cause trouble later because of leachate coming out of the dump during rainy season
- Leachate, from the dumped garbage may pollute surface water as well as ground water.
3. Trenching method in disposal
This method is generally adopt when low lying areas are not available. Trenches of size 4 to 10 m long, 2 to 3 m wide, 1 to 2 m deep excavated. It is having a clear spacing of 2 m. These trenches then filled with refuse/garbage in layers of 15 cm. On the top of each layer, 5 cm thick sandwiching layer of night soil/animal dung then spread off. This layer is in semi liquid form. On the top layer protuding 0.3 m above ground surface, 10 cm layer spread. It is of good earth or other non-combustible material. This is spread to act as a seal. So that flies do not get access and wind does not blow the refuse off.
The dumped garbage thus converted into a type of compost by the fermentation. It is carried out by anaerobic bacteria, within a period of 6 months. The compost available from trenching is of low agricultural value. On the contrary, it may sometimes have elements which may be harmful to the soil.
4. Dumping into the sea
Solid waste/refuse can also be disposed of by bargeing out into the sea. After carrying it at reasonable distance (say 15 to 20 km) into the sea. This is necessary to prevent the shores from refuse nuisance. This is because sea waves can carry back the refuse to the shores. The depth at such disposal point should not be less than 30 m.
The following are the defects of this method
- bulky and lighter matter in refuse may float, spread out and tend to return to the shores during high tides
- during stormy weather and monsoons, it is not possible to send barges out into the sea
- Inspite of best care, some portion of refuse may return the shores and spoil them. Also, the method is possible only in cases of coastal cities.
5. Pulverization method of disposal of solid waste
In this method, the dry refuse is pulverised into powder form, without changing its chemical form. The powder can either be used as poor quality manure, or else be disposed of by land filling. In some countries the pulverised refuse is discharged in the sewer. The method is quite costly and hence not commonly used in India.
6. Incineration method in disposal
This consists of burning the refuse in the incinerator plant. This commonly used in disposing of garbage from hospitals and industrial plants. Before incineration, non-combustible and inert material like earth, broken glass, chinaware, metal etc. separated. This is done so as to reduce the load on the hearth. The by-product of this method is ash and clinker which can be easily disposed of by land filling. The heat generated by burning the dry refuse may be utilized for raising steam power.
The quantity and quality of refuse is, however, changing and hence the power generated will fluctuate. Emission of air pollutants from incinerators includes particulates such as fly ash, unburnt fuel and others. Permissible level of particulate emissions from large incinerators is 0.23 g per standard cubic metre of exhaust gas. Smoke includes all liquid and solid matter in the exhaust hinders visibility. Smoke can be eliminated by mixing the exhaust with hot air to complete combustion.
The following points should be carefully observed during incineration
- The refuse charging should be thorough, rapid and as nearly continuous as possible
- Each batch of refuse entering furnace should well mixed.
- Auxiliary burners are usually install above the refuse to ignite it. And to establish the draft at the beginning of the cycle. This is all the more necessary when the moisture content of air is high.
- Minimum temperature in the combustion chamber should be sufficient (> 670°C). So that all the organic matter incinerated and foul smelling gases thus oxidized.
- After burners are sometimes reqd., together with particulate removal devices such as settling chambers or scrubbers.
Furnaces can be
(i) vertical (ii) circular (iii) rotary multicell (iv) rectangular in their design.
- In the vertical and circular furnace, the refuse is charge through a door in the ceiling. And drops into a central cone grate surrounded by a circular grate. Primary combustion is support by underfire air.
- In the rotary kiln furnace, the wastes are partially burn in a rectangular furnace. And then fed, via grates, to a rotating kiln. The rotary action exposes the unburnt material for combustion. Final combustion takes place in the chamber after the kiln discharge point.
- The multicell furnace has cells side by side. Each cell has grates for moving the refuse across them. Several cells have a common combustion chamber and residue hopper.
- In rectangular furnace unit, two or more grates are arrange in tiers. So that the refuse is agitate as it falls from one level to the next. Secondary combustion usually employed. Pathological waste incinerator handles organic wastes of human or animal origin and crematory furnaces. Such incinerators have multiple chamber units, which release fluids as the material is being destroy. These fluids don’t evaporate quickly and therefore solid hearth rather than grating is reqd.
Advantages of incineration
- This is most hygienic method, since it ensures complete destruction of pathogens
- There is no odour trouble or dust nuisance
- The heat generated can be use for raising steam power
- Clinker produced can be use for road purposes
- The disposal site (i.e. incineration site) can be located at a convenient distance
- Lesser space is reqd. for disposal of residues
- Modern incinerators can burn a great variety of refuse materials which are otherwise not biodegradable
- Adverse weather conditions have no effect on the incinerator’s operation.
Disadvantages of incineration
- Large initial expenditure
- Improper operation results in air pollution problems and incomplete reduction of the waste materials
- Disposal of the remaining residue of solid waste is reqd.
- High stacks needed for natural draft chimneys present safety problems.
It should be clearly noted that municipal incineration of solid waste or refuse is a volume reduction process. It is not one of complete or ultimate disposal of solid waste. Safe disposal of remaining residue is an essential requirement. Also, the plant need to operate properly so that the gases are completely burn and a stable residue produced.
Composting is a method in which putrescible organic matter in the solid waste/refuse digested anaerobically. Further converted into humus and stable mineral compounds. It is a hygienic method which coverts the refuse into manure through the bacterial agencies. Compost is widely use as manure which is rich in nitrogen content. Due to composting, the volume of refuse is very much reduced. And the resulting matter can be safely handle since it becomes free from pathogenic organisms. In India normally, night soil of the conservancy system is also dispose of simultaneously along with refuse, producing valuable manure.
There are three methods of composting
- Composting by trenching
- Open window composting
- Mechanical Composting