Monday, November 4, 2013

Solid waste management - sources, effects and methods of disposal

Solid waste management

Rapid population growth and urbanization in developing countries has led to people generating enormous quantities of solid waste and consequent environmental degradation. The waste is normally disposed in open dumpscreating nuisance and environmental degradation. Solid wastes cause a major risk to public health and the environment. Management of solid wastes is important in order to minimize the adverse effects posed by their indiscriminate disposal.

Types of solid wastes: Depending on the nature of origin, solid wastes are classified into
Urban wastes include the following wastes:
Domestic wastes containing a variety of materials thrown out from homes
Ex: Food waste, Cloth, Waste paper, Glass bottles, Polythene bags, Waste metals, etc.
Commercial wastes: It includes wastes coming out from shops, markets, hotels, offices, institutions, etc.
Ex: Waste paper, packaging material, cans, bottle, polythene bags, etc.
Construction wastes: It includes wastes of construction materials.
Ex: Wood, Concrete, Debris, etc.
Biomedical wastes: It includes mostly waste organic materials
Ex: Anatomical wastes, Infectious wastes, etc.

Classification of urban wastes
Urban wastes are classified into:
Bio-degradable wastes - Those wastes that can be degraded by micro organisms are called bio-degradable wastes
Ex: Food, vegetables, tea leaves, dry leaves, etc.
Non-biodegradable wastes: Urban solid waste materials that cannot be degraded by micro organisms are called non-biodegradable wastes.
Ex: Polythene bags, scrap materials, glass bottles, etc.

The main source of industrial wastes are chemical industries, metal and mineral processing industries.
Nuclear plants: It generated radioactive wastes
Thermal power plants: It produces fly ash in large quantities
Chemical Industries: It produces large quantities of hazardous and toxic materials.
Other industries: Other industries produce packing materials, rubbish, organic wastes, acid, alkali, scrap metals, rubber, plastic, paper, glass, wood, oils, paints, dyes, etc.

  1. Due to improper disposal of municipal solid waste on the roads and immediate surroundings, biodegradable materials undergo decomposition producing foul smell and become a breeding ground for disease vectors.
  2. Industrial solid wastes are the source for toxic metals and hazardous wastes that affect soil characteristics and productivity of soils when they are dumped on the soil
  3. Toxic substances may percolate into the ground and contaminate the groundwater.
  4. Burning of industrial or domestic wastes (cans, pesticides, plastics, radioactive materials and batteries) produce furans, dioxins and polychlorinated biphenyls that are harmful to human beings.
Solid waste management involves waste generation, mode of collection, transportation, segregation of wastes and disposal techniques.

Two important steps involved in solid waste management are:
Reduce, Reuse and Recycle of Raw Materials
Discarding wastes

Reduce - If usage of raw materials is reduced, the generation of waste also gets reduced
Reuse  - Refillable containers that are discarded after use can be reused
               Rubber rings can be made from discarded cucle tubes and this reduces waste generation during      manufacture of rubber bands.
Recycle- Recycling is the reprocessing of discarded materials into new useful products
Ex:     Old aluminium cans and glass bottles are melted and recast into new cans and bottles
          Preparation of cellulose insulation from paper
          Preparation of automobile body and construction material from steel cans
This method (Reduce, Reuse & Recycle), i.e, 3R's help save money, energy, raw materials and reduces pollution.

The following methods are adopted for discarding wastes:
  1. Landfill
  2. Incineration and
  3. Composting
LANDFILL: Solid wastes are placed in a sanitary landfill in which alternate layers of 80 cm thick refuse is covered with selected earth-fill of 20 cm thickness. After 2-3 years solid waste volume shrinks by 25-30% and land is used for parks, roads and small buildings. This is the most common and cheapest cheapest method of waste disposal and is mostly employed in Indian cities.
It is simple and economical
Segregation of wastes is not required
Landfilled areas can be reclaimed and used for other purposes
Converts low-lying, marshy waste-land into useful areas.
Natural resources are returned to soil and recycled.
Large area is required
Land availability is away from the town, tansportation costs are high
Leads to bad odour if landfill is not properly managed.
Land filled areas will be sources of mosquitoes and flies requiring application of insecticides and pesticides at regular intervals.
Causes fire hazard due to formation of methane in wet weather.

It is a hygenic way of disposing solid waste. It is suitable if waste contains more hazardous material and organic content. It is a thermal process and very effective for detoxification of all combustible pathogens. It is expensive when compared to composting or land-filling.
In this method municipal solid wastes are burnt in a furnace called incinerator. Combustibe substances such as rubbish, garbage, dead organisms and non-combustibe matter such as glass, porcelain and metals are separated before feeding to incinerators. The non-combustible materials can be left out for recycling and reuse. The leftover ashes and clinkers may account for about 10 to 20% which need further disposal by sanitary landfill or some other means.

The heat produced in the incinerator during burning of refuse is used in the form of steam power for generation of electricity through turbines. Municipal solid waste is generally wet and has a high calorific value. Therefore, it has to be dried first before burning. Waste is dried in a preheater from where it is taken to a large incinerating furnace called "destructor" which can incinerate about 100 to 150 tonnes per hour. Temperature normally maintained in a combustion chamber is about 700 C which may be increased to 1000 C when electricity is to be generated.

Residue is only 20-25% of the original and can be used as clinker after treatment
Requires very little space
Cost of transportation is not high if the incinerator is located within city limits
Safest from hygenic point of view
An incinerator plant of 3000 tonnes per day capacity can generate 3MW of power.

Its capital and operating cost is high
Operation needs skilled personnel
Formation of smoke, dust and ashes needs further disposal and that may cause air pollution.

It is another popular method practiced in many cities in our country. In this method, bulk organic waste is converted into fertilizer by biological action.
Separated compostible waste is dumped in underground trenches in layers of 1.5m and finally covered with earth of 20cm and left for decomposition. Sometimes, actinomycetes are introduced for active decomposition. Within 2 to 3 days biological action starts. Organic matter is destroyed by actinomycetes and lot of heat is liberated increasing the temperature of compost by 75C and the refuse is finally converted into powdery brown coloured odourless mass called humus that has a fertilizing value and can be used in agriculture. Humus contains lot of Nitrogen essential for plant growth apart from phosphates and other minerals.
Manure added to soil increases water retention and ion-exchange capacity of soil.
This method can be used to treat several industrial solid wastes.
Manure can be sold thereby reducing cost of disposing wastes
Recycling can be done
Non-consumables have to be disposed separately
The technology has not caught-up with the farmers and hence does not have an assured market.


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