Environmental Engg.

Lesson-27 Solid waste disposal in rural and urban areas

Disposal is the final element in the solid waste mangement system. It is the ultimate fate of all solid wastes, be the residential wastes collected and transported directly to a landfill site, semisolid waste (sludge) from municipal and industrial treatment plants, incinerator residue, compost or other substances from various solid waste processing plants that are of no further use to society. It is, therefore, imperative to have a proper plan in place for safe disposal of solid wastes, which involves appropriate handling of residual matter after solid wastes have been processed and the recovery of conversion products/energy has been achieved.

Problems due to improper disposal of wastes

• health hazards (e.g., residents in the vicinity of wastes inhale dust and smoke when the wastes are burnt; workers and rag pickers come into direct contact with wastes, etc.);

• pollution due to smoke;

• pollution from waste leachate and gas;

• blockage of open drains and sewers.

Hence it is very much imminent that safe disposal of solid wastes is important for safeguarding both public health and the environment.

Some of the disposal methods of solid wastes are

1. Open dumping

2. Composting

3. Land filling

4. Incineration

5. Gasification

6. Refuse-derived fuel

7. Pyrolysis

Open dumping

Open dumping is an illegal process, in which any type of the waste such as household trash, garbage, tires, demolition/construction waste, metal or any other material dump at any location like along the roadside, vacant lots on public or private property even in parks other than a permitted landfill or facility.  Open dumping poses a threat to human health and the environment because it causes land pollution.  In the developing countries, municipal solid waste is commonly disposed off by discharge the waste in open dumps around 60-90%, which are environmentally unsafe. Open dumping of nondegradable component like burning of plastic waste is added to create air pollution and uncollected waste pose serious health hazards.  As a result of illegal dumping, land area such as property value may decrease and also put negative impact on the scarcity of land in future

 Composting

Composting is a natural biological process that carried out under controlled aerobic (requires oxygen) or anaerobic conditions (without oxygen). Organic waste is biodegradable and can be processed in the presence of oxygen or in the absence of oxygen using anaerobic digestion. Anaerobic composting is not common because of the slow degradation rate and produce odorous intermediate product. Anaerobic digestion however also produces methane gas which is an important source of bio-energy.

Composting is an efficient method to break down organic materials into an end product which is beneficial for soil and plants. Compost is used as an organic amendment to improve the physical, chemical and biological properties of soil. Adding compost helps to increase the ability of the soil to hold and release essential nutrients.

Composting has a long tradition particularly in rural India. Composting is difficult process because the waste arrives in a mixed form and contains a lot of non-organic material. When mixed waste is composted, the end product is of poor quality. The presence of plastic objects in the waste stream is especially problematic, since these materials do not get recycled or have a secondary market. In the absence of segregation, even the best waste management system or plant will be rendered useless. In India, composting is used around 10-12% because composting needs segregation of waste and sorting is not widely practiced

Landfills

A landfill is an area of land onto or into which waste is deposited. The aim is to avoid any contact between the waste and the surrounding environment, particularly the groundwater.

Landfilling will be done for the following types of waste:

• Mixed waste not found suitable for waste processing;

• Pre-processing and post-processing wastes from waste processing sites;

• Non-hazardous waste not being processed or recycled.

Landfilling will usually not be done for the following waste streams in the municipal solid waste:

• Biowaste/garden waste

• Dry recyclables

Landfills minimise the harmful impact of solid waste on the environment by the following mechanisms:

•  Isolation of waste through containment;

• Elimination of polluting pathways;

• Controlled collection and treatment of products of physical, chemical and biological changes within a waste dump – both liquids and gases; and

• Environmental monitoring till the waste becomes stable.

Essential components of municipal solid waste landfill

1. A liner system at the base and sides of the landfill which prevents migration of leachate or gas to the surrounding soil.

2. A leachate collection and control facility which collects and extracts leachate from within and from the base of the landfill and then treats the leachate.

3. A gas collection and control facility (optional for small landfills) which collects and extracts gas from within and from the top of the landfill and then treats it or uses it for energy recovery.

4. A final cover system at the top of the landfill which enhances surface drainage, prevents infiltrating water and supports surface vegetation.

5. A surface water drainage system which collects and removes all surface runoff from the landfill site.

6. An environmental monitoring system which periodically collects and analyses air, surface water, soil-gas and ground water samples around the landfill site.

7. A closure and post-closure plan which lists the steps that must be taken to close and secure a landfill site once the filling operation has been completed and the activities for long-term monitoring, operation and maintenance of the completed landfill.

Incineration

Incineration is one of the waste treatment technologies that involve the combustion of organic materials and other substances. This refers to the controlled burning of wastes, at a high temperature (roughly 1200 – 1500°C), which sterilises and stabilises the waste in addition to reducing its volume. Hence, Incineration waste treatment system is normally described as ‘thermal treatment’. Incinerator process converts the waste into bottom ash, particulates and heat, which can be used to generate the electric power. The volume of ash is usually 10% of the original volume of the waste.  Finally, the ash is typically disposed off in the landfill site. In developing countries, the use of incineration is in few amounts to around 1-5% respectively.

In India the incineration is a poor option as the waste consists mainly high organic material (40–60%) and high inert content (30–50%) also low calorific value content (800–1100 kcal/kg), high moisture content (40–60%) in MSW and the high costs of setting up and running the plants.

Gasification

This is the partial combustion of carbonaceous material (through combustion) at high temperature (roughly 1000°C) forming a gas, comprising mainly carbon dioxide, carbon monoxide, nitrogen, hydrogen, water vapour and methane, which can be used as fuel.  Gasification is the solid waste incineration under oxygen deficient conditions, to produce fuel gas. In India, there are very few gasifiers in operation, but they are mostly for burning of biomass such as agro-residues, sawmill dust, and forest wastes. Gasification can also be used for MSW treatment after drying, removing the inert and shredding for size reduction.

 Refuse-derived fuel (RDF)

This is the combustible part of raw waste, separated for burning as fuel. Various physical processes such as screening, size reduction, magnetic separation, etc., are used to separate the combustibles

Pyrolysis

This is the thermal degradation of carbonaceous material to gaseous, liquid and solid fraction in the absence of oxygen. This occurs at a temperature between 200 and 900°C. The product of pyrolysis is a gas of relatively high calorific value of 20,000 joules per gram with oils, tars and solid burned residue