Environmental Science

LESSON 32. SOLID WASTE DISPOSAL

' Solid wastes' is the term now used internationally to describe non-liquid waste materials arising from domestic, trade, commercial, industrial, agriculture and mining activities and from the public services. The principal sources of solid wastes are domestic, commercial, industrial and agricultural activities.  Solid wastes comprises countless different materials. dust, food wastes, packaging in the form of paper, metals, plastics or glass, discarded clothing and furnishing, garden wastes and hazardous and radioactive wastes. Domestic and commercial wastes are considered together as the so-called urban wastes.  Main constituents of urban wastes differ acc. to the level of economic development geographic location, weather and social conditions.  As the personal income rises, kitchen wastes decline but the paper, metals and glass wastes increase.  The total weight generated rises but the density of the waste declines.

Domestic wastes have three main characteristics :

            Weight generated, density and constituents.

- the range of weight generated / person/day usually lies between 250 and 1000g worldwide.

- density varies from 100kg/comet. to 600 kg/comet.

- this volume may range between 1/2lit and 10 lit/person/day

Main constituents of domestic wastes vary as

- Vegetable / putrescible matter 20% to 75%

- Inter 2% to 66%

- glass 0% to 10%

- metals 0% to 15%

Solid waste management may be defined as the discipline associated with the control of generation, storage. collection, transfer and transport, processing and disposal of solid wastes in a manner that is in accord with the best principles of public health, economics,  engineering, conservation, aesthetics and other environmental considerations, and that also is responsible to public attitudes.

Knowledge of the sources and types of solid wastes, along with data on the composition and rates of generation, is basic to the design and operation of the functional elements associated with the management of solid wastes.

Sources of Solid wastes :        1) Residential      2) commercial    3) municipal      4) industrial       5) Open areas   6) treatment    and   7) agricultural

The method and capacity of storage, the correct type of collection vehicles, the optimum size of crew and the frequency of collection mainly depend on volume and density, climate also has some influence. The four main aspects of solid wate management are :

- Storage at or near the point of generation

- Collection

- Street cleansing

- disposal

Classification of solid wastes

The solid waste can be broadly classified as:

• Residential or household waste consisting of leaves, food wastes, paper, glass, etc.

• Commercial wastes due to activity of offices, markets, hotels, etc.

• Industrial waste including food-processing residue, ash, plastic, packaging waste, etc.

• Building construction wastes such as bricks, sand, stones, etc.

• Hospital wastes composed of blood, limbs, parts of human body, etc.

• Bulky waste including trees, furniture, telephone poles, etc.

• Hazardous wastes comprising of explosives, radioactive materials, toxic materials, etc.

            These wastes have to be collected, transported and disposed off.

Characteristics of waste and their importance

For a systematic planning of MSW management, it is necessary to know the quantity and quality of waste generated. The physical and chemical properties such as density, moisture, heating value and chemical composition are some of the important waste characteristics. These depend on the components and constituents in the waste such as garbage, paper, glass, metal, plastic, construction waste and others. Table 3.1 summarizes the components which are generally present in MSW. Additionally chemical analyses of MSW is undertaken to know the amounts of various chemical constituents in it. The important general parameters in chemical analysis are shown in Table 3.2.

Table 3.1: Components Of Municipal Solid Waste (All the values are given in percentage and calculated on wet weight basis as received at source.)

Paper	Rags/ Textile	Plastics	Rubber & Leather	Metals	Glass & Crockery	Stones & Bricks	Ash & earth	Compo -stable matter	Woody matter	Moisture

Table 3.2: Chemical Parameters of Municipal Solid Waste (All the values are calculated on dry weight basis)

pH	Organic matter %	Carbon %	Nitrogen % as total N	Phosphorous % as P2O5	Potash % as K2O	C/N ratio	HCV in Kcal/kg

The information on composition, physical and chemical characteristics and the quantities of waste generated forms the basis on which the management system needs to be planned, designed and operated.

Item	Indian City	N.Eropean City
Paper	4%	27%
Vegetable/Pursuable matter	75%	30%
Dust etc. under 10 mm	12%	16%
Metals	0.4%	7%
Glass	0.4%	11%
Textiles	3%	3%
Plastics	0.7%	3%
Other stones, ceramics, etc.	7%	3%
Weight/Person/Day	414 g	845 g
Weight/dwelling/Day	2.5 kg (6 person)	2.5 kg (3 person)
Density kg/m3	570	132

Collection methods

All cities in India have a combination of collection systems. The most common collection systems are the following.

• Door to door collection

• Curbside collection

• Block collection

• Communal bins

Door to door collection: The households place the filled containers or disposable plastic bags outside their doors when the waste collectors arrive. The collection vehicle stops on the street near the houses and the waste collectors go door-to-door to collect the waste from the containers or the plastic bags. For this they carry a large bag into which they empty all the waste. In colonies with multi story buildings or apartment complexes the waste collectors may have to travel a considerable distance from the vehicle with this bag. In many multi story buildings dust-chutes are used for the collection of wastes. With this system the collector/s enters the chute at the bottom and fill up many bags which they then carry to the vehicle.

Curbside collection: This method is used rarely in India and can be seen in smaller more enclosed communities. The house owners leave the containers outside the house on the edge of the road. The waste collectors empty the containers into the vehicle as it passes through the street at a set time every day, and returns the container to the curb.

Block collection: In another solution in smaller settlements the vehicles arrives at a set time to collect the wastes from the households. Households bring their waste containers and empty directly into the vehicle.

Communal bins: By far the most common method. Communal bins are provided at convenient locations, where the community members or a private collector carry their waste and throw it into bins. One type of communal bin is a permanently walled enclosure from where the waste collectors load wastes into the vehicle. Other types are movable containers of various sizes and designs which are emptied manually, or the container is lifted manually into the truck and hauled away to the disposal site. A new container is placed in place of the removed container.

Disposal

The solid waste is collected, transported and disposed off in one of the following ways:

• Open dumping

• Sanitary landfill

• Incineration    

• Composting

i) Open Dumping: In this method, the solid waste collected from the town is deposited in low-lying land, usually on the ourskirts of the town.  Since the 'open dumps' are uncovered, these attract files, insects and rodents (rats) and odours are produced. This method is unscientific and causes nuisance to the public and is subject to fire hazard. At the same time it causes health and pollution hazards and is not suitable aesthetically. Yet, this being an easy method, is adopted in many Indian towns.

ii) Sanitary Land filling : This is a modified form of open dumping. Waste is deposited in 0.9-4.5 m thick layers in depressions and then compacted and covered at least once a day by earth with bulldozers. The covering prevents breeding of files, rats, etc. While selecting site for land filling, it is necessary to examine if any underground potable water source in the vicinity will be polluted and if the neighborhood habitation would be affected by odour or fire.

Sanitary land filling is the cheapest method of refuse disposal

Advantages of landfill method:

• The process is completely sanitary

• Highly skilled personnel are not required

• Land can be safely overloaded without causing any problems

• Converts low-lying, marshy wasteland into useful area

• Natural resources are returned to soil and recycled.

Disadvantages:

• A large area is required

• Since land is available away from town, transportation cost is heavy

• Causes fire hazard due to formation of methane in wet weather.

iii) Incineration: In this method the refuse is burnt in a controlled manner. Incinerators are built with lined furnaces, grate area for burning, air blowers for aiding combustion and oil burners to provide additional heat to burn wet garbage.The primary products of combustion are carbon dioxide, water vapour and nitrogen and a solid residue of glass, ceramics, mineral ash, etc. 90% of the volume of refuse is done away with and 10% , ash is left.  Air has to be supplied to carry away the gaseous products and smoke treatment is necessary to keep pollution hazards under control. For Indian refuse, additional fuel is required for complete combustion and, therefore, the cost is high. Hence incineration method is commonly adopted for disposing hazardous toxic wastes.

Advantages:

• The residue is only 20-25% of original weight; the clinker can be used after treatment.

• It requires very little space

• Cost of transportation is not high as incinerators located within city limits.

• Safest from hygienic point of view.

Disadvantages:

• Its capital and operating cost is high

• Needs skilled personnel

• Air pollution may be caused

Composting: Composting is the biological decomposition of organic substances available in the waste, under controlled conditions. Rotting, putrefaction, etc., are natural processes that take place in a controlled manner. The compost thus formed under controlled conditions is a brown peaty material. For centuries, refuse has been used for producing manures. However, systematic ways of preparing compost was developed by Lord Howard and is known as the 'Indore process'. This involves the laying of alternate layers of sort and the putrescible materials such as night soil, animal dung, garbage, etc. The mass is usually turned double during the bacteria in the absence of oxygen.

There are two systems by which compost can be produced:

• Anaerobic

• Aerobic

In the anaerobic system, anaerobic bacteria perform the work in the absence of oxygen; hence oxygen has to supplied. The disadvantages of anaerobic system are:

• The process is slow, extending over a period of 4-12 months

• It is low-temperature process.

• It produces offensive odour.

In the aerobic process, the compost is produced by aerobic bacteria. High temperature is produced, but bad odours are absent and the compost is formed rapidly

There are two methods by which compost is prepared in the aerobic process:

• Non-mechanical methods

• Mechanical methods

In both the methods the following steps are involved:

• Sorting

• Grinding or shredding

• Windrow making       

• Tuming

• Aerating.

i) Sorting: The materials like empty tins, metals, glass, rags, plastics, etc., have to be removed before grinding as they are dangerous when pulverised. These are removed manually except tin and ferrous materials which are removed by a ballastic seperator.

ii) Shredding of Refuse: The refuse is chopped into small pieces of size ranging from 25 to 50 mm. The shredding (cutting into thin long pieces) increases rate of decomposition and aeration.

iii) Windrow System: The refuse is stacked in piles or mounds in rows of 1.5-1.8m height (if more than this, the reaction may become anaerobic) and width of 2.4 x 3.0m. The windows are kept in open or in sheds. The bacterial activity begins resulting in increase in temperature of the mass. The colour of the  refuse changes from green to brown or bluish-grey. It takes 11-31 days to prepare this compost.

iv) Turning: The windrow composting pile is tuned frequently so that the aerobic process is continued. The mixing is effected from both sides of the pile and is normally done at intervals of 2-3 days.

v) Bottom aeration:  Several methods are adopted for aeration but the one most commonly adopted is stacking the material on a perforated floor and forcing the air into the pile by way of bottom aeration. Non-mechanical methods are employed when the quantity of refuse to be handled is small.

Compost Equipment

A highly mechanized compost plant consists of the following machinery and processes:

• The compost is unloaded by tipping trucks.

• Reception and storage usually in deep bunkers

• A picking belt for removal of contraries such as rags, plastics, card-board etc.,

• Pulveriser to shred the refuse.

• An enclosed vessel for fermentation and mixing.

• Vibrating screens to remove oversized elements from compost

• Ballastic separator for removing dense particles.

Sometimes, equipments for drying, supplementing the compost with nutrients, and packing bags, is also provided.

Advantages of composting:

• When added to soil, it increases water retention and ion-exchange capacity of soil

• A number of industrial solid wastes can also be treated by this method

• It can be sold thereby reducing the cost of disposing of wastes

• Re-cycling occurs.

Disadvantages:

• When mechanized, it involves high investment and hence cost of compost is exorbitant

• The non-consumables have to be disposed of separately

• Use of compost has not yet caught up with farmers and hence no assured market.