Solid waste management

Introduction to solid waste management

Solid waste is the unwanted or useless solid materials generated from combined residential, industrial and commercial activities in a given area. It may be categorized according to its origin (domestic, industrial, commercial, construction or institutional); according to its contents (organic material, glass, metal, plastic paper etc.); or according to hazard potential (toxic, non-toxin, flammable, radioactive, infectious).

Management of solid waste reduces or eliminates adverse impacts on the environment and human health and supports economic development and improved quality of life. A number of processes are involved in effectively managing waste for a municipality. These include monitoring, collection, transport, processing, recycling and disposal.

CLASSIFICATION

Typical classification of solid waste as follows:

1. Garbage: Putrescible wastes from food, slaughterhouses, canning and freezing industries.

2. Rubbish: non-putrescible wastes either combustible or non-combustible. These include wood, paper, rubber, leather and garden wastes as combustible wastes whereas the non-combustible wastes include glass, metal, ceramics, stones and soil.

3. Ashes: Residues of combustion, solid products after heating and cooking or incineration by the municipal, industrial, hospital and apartments areas.

4. Large wastes: Demolition and construction wastes, automobiles, furniture’s, refrigerators and other home appliances, trees, fires etc.

5. Dead animals: House holds pets, birds, rodents, zoo animals, and anatomical and pathological tissues from hospitals.

6. Sewage sludges: These include screening wastes, settled solids and sludges.

7. Industrial wastes: Chemicals, paints, sand and explosives. 8. Mining wastes: Tailings, slug ropes, culm piles at mine areas 9. Agricultural wastes: Farm animal manure, crop residues and others.

Traditionally these wastes are categorized into the following five types.

1. Residential: It refers to wastes generated mainly from dwelling, apartments, and consisted of left over food scrapes, vegetables, peeled material, plastics, wood pieces, clothes and ashes.

2. Commercial: This mainly consists of grocery materials, leftover food, glasses, and metals, ashes generated from stores, hotels, markets, shops and medical facilities.

3. Institutional: The wastes generated from schools, colleges and offices include, paper, plastics, and glasses.

4. Municipal: This includes dust, leaf litter, building debris, and treatment plant sediments. These arise from various activities like demolition, construction, street cleaning, land scraping etc.

5. Agricultural: This mainly includes spoiled food grains, vegetables, grass, litter etc., generated from fields and farms.

Classification of Solid Wastes Based on Types

These wastes may have reuse values in some other places, but these are of no value to the possessor who wishes to dispose them. The knowledge about sources of solid wastes along with the information of the composition and rate of generation, will help in the process of design and operation of the functional elements associated with the disposal and management of solid wastes. Therefore it is important to define various types of solid waste that are generated from various sources (Pheleps et al., 1995).

1. Refuse: This is all putrescible and non-putrescible waste except body wastes. It includes all types of rubbish and garbage.

2. Rubbish: This refers to that portion of the refuse, which is non-putrescible solid waste such as packaging materials.

3. Garbage: This refers to that portion of the refuse, which is putrescible component of solid waste. These are produced during cooking and storage of meet, fruits and vegetables.

4. Bulky wastes: These include household wastes, which cannot be accommodated in the normal storage containers and need a special collection mechanism. These include, household appliances such as refrigerators, washing machine, furniture, vehicle parts, tyres, trees, wood branches etc.

5. Street wastes: This includes wastes collected from streets, walkways, parks, playgrounds, which include paper, cardboard, plastics, leaves and other vegetable matter in large quantities.

6. Dead animals: These include dead animals those die naturally or accidentally killed on the road. This category does not include carcasses and animal parts from slaughterhouses, which may be regarded as commercial or industrial components. Many times as in India the large animals if died and are not lifted on right time then they may pose a threat to public health through attracting flies and produce bad odour and create an unhygienic scene.

7. Hazardous Wastes: Hazardous wastes are those produced in the industries, institutes, hospitals and laboratories. These are dangerous to the living organisms immediately or in the long run to the environment in which they are disposed. The hazard may be due to their physical, chemical, biological and radioactive characteristics like, ignitibility, corrosively, reactivity and toxicity. In some cases various chemicals and their mixtures act as hazardous wastes. Those may be pesticides, solvents, acids and bases. Certain hazardous wastes may cause explosions in the incinerators and fires at the landfill sites. Other hazardous waste includes pathological wastes from hospitals and radioactive wastes, which require special handling. A good

management practice should ensure that hazardous wastes are stored, collected, transported and disposed separately after suitable treatment.

8. Sewage Sludge: The sewage treatment plants produce huge amounts of sludge during primary and secondary phase of treatment, these are sticky and rich in pathogens require proper treatment. These are both inorganic and organic. The bulk of dewatered and digested sludge can be used as organic fertilizer or it may be burnt to produce energy.

Objectives of Waste Management

The main goal of solid waste management is reducing and eliminating adverse impacts of waste materials on human health and environment to support economic development and superior quality of life. 

Risks and Problems Associated with Solid wastes

If solid wastes are not managed properly, there are many negative impacts that may result. Some of the most important are mentioned in the following list. The relative importance of each depends very much on local conditions.

Uncollected wastes often end up in drains, causing blockages, which result in flooding and unsanitary conditions.

Flies breed in some constituents of solid wastes, and flies are very effective vectors that spread disease.

Mosquitoes breed in blocked drains and in rainwater that is retained in discarded cans, tires and other objects. Mosquitoes spread disease, including malaria and dengue.

Rats find shelter and food in waste dumps. Rats consume and spoil food, spread disease, damage electrical cables and other materials and inflict unpleasant bites.

The open burning of waste causes air pollution; the products of combustion include dioxins, which are particularly hazardous.

Aerosols and dusts can spread fungi and pathogens from uncollected and decomposing wastes.

Uncollected waste degrades the urban environment, discouraging efforts to keep streets and open spaces in a clean and attractive condition. Solid waste management is a clear indicator of the effectiveness of a municipal administration - if the provision of this service is inadequate large numbers of citizens (voters) are aware of it. Plastic bags are a particular aesthetic nuisance and they cause the death of grazing animals, which eat them.

Waste collection workers face particular occupational hazards, including strains from lifting, injuries from sharp objects and traffic accidents.

Dumps of waste and abandoned vehicles block streets and other access ways.

Dangerous items (such as broken glass, razor blades, hypodermic needles and other healthcare wastes, aerosol cans and potentially explosive containers and chemicals from industries) may pose risks of injury or poisoning, particularly to children and people who sort through the waste.

Heavy refuse collection trucks can cause significant damage to the surfaces of roads that were not designed for such weights.

Waste items that are recycled without being cleaned effectively or sterilized can transmit infection to later users. (Examples are bottles and medical supplies.)

Polluted water (leachate) flowing from waste dumps and disposal sites can cause serious pollution of water supplies. Chemical wastes (especially persistent organics) may be fatal or have serious effects if ingested, inhaled or touched and can cause widespread pollution of water supplies.

Large quantities of waste that have not been placed according to good engineering practice can slip and collapse, burying and killing people.

Waste that is treated or disposed of in unsatisfactory ways can cause a severe aesthetic nuisance in terms of smell and appearance.

Liquids and fumes, escaping from deposits of chemical wastes (perhaps formed as a result of chemical reactions between components in the wastes), can have fatal or other serious effects.

Landfill gas (which is produced by the decomposition of wastes) can be explosive if it is allowed to accumulate in confined spaces (such as the cellars of buildings).

Methane (one of the main components of landfill gas) is much more effective than carbon dioxide as a greenhouse gas, leading to climate change.

Fires on disposal sites can cause major air pollution, causing illness and reducing visibility, making disposal sites dangerously unstable, causing explosions of cans, and possibly spreading to adjacent property.

Former disposal sites provide very poor foundation support for large buildings, so buildings constructed on former sites are prone to collapse.

Functional Elements of Solid Waste Management

The activities associated with the management of municipal solid wastes from the point of generation to final disposal can be grouped into the six functional elements: (a) waste generation; (b) waste handling and sorting, storage, and processing at the source; (c) collection; (d) sorting, processing and transformation; (e) transfer and transport; and (f) disposal.

1. Waste Generation: Waste generation encompasses activities in which materials are identified as no longer being of value (in their present form) and are either thrown away or gathered together for disposal. Waste generation is, at present, an activity that is not very controllable. In the future, however, more control is likely to be exercised over the generation of wastes. Reduction of waste at source, although not controlled by solid waste managers, is now included in system evaluations as a method of limiting the quantity of waste generated.

2. Waste Handling, Sorting, Storage, and Processing at the Source: The second of the six functional elements in the solid waste management system is waste handling, sorting, storage, and processing at the source. Waste handling and sorting involves the activities associated with management of wastes until they are placed in storage containers for collection. Handling also encompasses the movement of loaded containers to the point of collection. Sorting of waste components is an important step in the handling and storage of solid waste at the source. For example, the best place to separate waste materials for reuse and recycling is at the source of generation. Households are becoming more aware of the importance of separating newspaper and cardboard, bottles/glass, kitchen wastes and ferrous and non-ferrous materials.

On-site storage is of primary importance because of public health concerns and aesthetic consideration. Unsightly makeshift containers and even open ground storage, both of which are undesirable, are often seen at many residential and commercial sites. The cost of providing storage for solid wastes at the source is normally borne by the household in the case of individuals, or by the management of commercial and industrial properties. Processing at the source involves activities such as backyard waste composting.

3. Collection: The functional element of collection includes not only the gathering of solid wastes and recyclable materials, but also the transport of these materials, after collection, to the location where the collection vehicle is emptied. This location may be materials processing facility, a transfer station, or a landfill disposal site.

4. Sorting, Processing and Transformation of Solid Waste: The sorting, processing and transformation of solid waste materials is the fourth of the functional elements. The recovery of sorted materials, processing of solid waste and transformation of solid waste that occurs primarily in locations away from the source of waste generation are encompassed by this functional element. Sorting of commingled (mixed) wastes usually occurs at a materials recovery facility, transfer stations, combustion facilities, and disposal sites. Sorting often includes the separation of bulky items, separation of waste components by size using screens, manual separation of waste components, and separation of ferrous and non-ferrous metals.

Waste processing is undertaken to recover conversion products and energy. The organic fraction of Municipal Solid Waste (MSW) can be transformed by a variety of biological and thermal processes. The most

commonly used biological transformation process is aerobic composting. The most commonly used thermal transformation process is incineration.

Waste transformation is undertaken to reduce the volume, weight, size or toxicity of waste without resource recovery. Transformation may be done by a variety of mechanical (e.g. shredding), thermal (e.g. incineration without energy recovery) or chemical (e.g. encapsulation) techniques.

5. Transfer and Transport: The functional element of transfer and transport involves two steps: (i) the transfer of wastes from the smaller collection vehicle to the larger transport equipment and (ii) the subsequent transport of the wastes, usually over long distances, to a processing or disposal site. The transfer usually takes place at a transfer station.

6. Disposal: The final functional element in the solid waste management system is disposal. Today the disposal of wastes by landfilling or uncontrolled dumping is the ultimate fate of all solid wastes, whether they are residential wastes collected and transported directly to a landfill site, residual materials from Materials Recovery Facilities (MRFs), residue from the combustion of solid waste, rejects of composting, or other substances from various solid waste-processing facilities. A municipal solid waste landfill plant is an engineered facility used for disposing of solid wastes on land or within the earth’s mantle without creating nuisance or hazard to public health or safety, such as breeding of rodents and insects and contamination of groundwater.

Reduce, Reuse, Recycle

Methods of waste reduction, waste reuse and recycling are the preferred options when managing waste. There are many environmental benefits that can be derived from the use of these methods. They reduce or prevent green house gas emissions, reduce the release of pollutants, conserve resources, save energy and reduce the demand for waste treatment technology and landfill space. Therefore it is advisable that these methods be adopted and incorporated as part of the waste management plan.

Waste reduction and reuse

Waste reduction and reuse of products are both methods of waste prevention. They eliminate the production of waste at the source of usual generation and reduce the demands for large scale treatment and disposal facilities. Methods of waste reduction include manufacturing products with less packaging, encouraging customers to bring their own reusable bags for packaging, encouraging the public to choose reusable products such as cloth napkins and reusable plastic and glass containers, backyard composting and sharing and donating any unwanted items rather than discarding them. All of the methods of waste prevention mentioned require public participation. In order to get the public onboard, training and educational programmes need to be undertaken to educate the public about their role in the process. Also the government may need to regulate the types and amount of packaging used by manufacturers and make the reuse of shopping bags mandatory.

Recycling

Recycling refers to the removal of items from the waste stream to be used as raw materials in the manufacture of new products. Thus from this definition recycling occurs in three phases: first the waste is sorted and recyclables collected, the recyclables are used to create raw materials. These raw materials are then used in the production of new products.

The sorting of recyclables may be done at the source (i.e. within the household or office) for selective collection by the municipality or to be dropped off by the waste producer at recycling centres. The pre-sorting at the source requires public participation, which may not be forthcoming if there are no benefits to be derived. Also a system of selective collection by the government can be costly. It would require more frequent circulation of trucks within a neighbourhood or the importation of more vehicles to facilitate the collection.Another option is to mix the recyclables with the general waste stream for collection and then sorting and the municipality at a suitable site can perform recovery of the recyclable materials. The sorting by the municipality has the advantage of eliminating the dependence on the public and ensuring that the recycling does occur. The disadvantage however, is that the value of the recyclable materials is reduced since being mixed in and compacted with other garbage can have adverse effects on the quality of the recyclable material.

Waste Treatment and Disposal Methods

Following are the detailed discussions on various waste treatment and disposal methods:

Thermal Treatment

Thermal waste treatment refers to the processes that use heat to treat waste materials. Following are some of the most commonly used thermal waste treatment techniques:  

Incineration is one of the most common waste treatments. This involves combustion of waste material in the presence of oxygen. The incineration process converts wastes into ash, flue, gas, water vapor and carbon dioxide. This thermal treatment method is commonly used as a means of recovering energy to be used in the supply of electricity or heating. This method has a number of advantages such as it quickly reduces waste volume, lessens transportation costs and decreases harmful greenhouse gas emission.    

Gasification and Pyrolysis are two similar methods both of which decompose organic waste materials by exposing wastes to low amounts of oxygen and very high temperatures. Pyrolysis uses absolutely no oxygen while gasification allows very low amount of oxygen in the process. Gasification is particularly more advantageous as it allows the burning

process recover energy without causing air pollution.

Open Burning is another common thermal waste treatment, which involves burning waste materials in a way that causes smoke to be released into the open air directly without passing through a stack or chimney. This particular method is regarded very harmful, environmentally, as the incinerators used in such process have no pollution control devices. The harmful gas released by this method of waste treatment include hexachlorobenzene, dioxins, carbon monoxide,  articulate matter, volatile organic compounds, polycyclic aromatic compounds, and ash. Still many local authorities to reduce volume of wastes very quickly, without spending much collection practice this method and transportation costs. 

Dumps and Landfills

Sanitary landfills are used as the most common waste disposal fields. These landfills are desired to eliminate or reduce the risk of environmental or public health hazards due to waste disposal. These landfills are placed usually in the areas where land features work as natural buffers between the environment and the landfill. For instance, the landfill area can be comprised of clay soil, which is quite resistant to hazardous wastes or is characterized by an absence of surface water bodies or a low water table, preventing the risk of water pollution. Use of sanitary landfills presents the least health and environmental risk but the cost of establishing such landfills are comparatively higher than other waste disposal methods.

Controlled dumps are more or less the same as sanitary landfills. These dumps comply with almost all the requirements for being a sanitary landfill but generally lack one or two. These dumps may have a well-planned capacity but no cell planning, there may be no or partial gas management, basic record keeping, or regular cover. These dumps too have reduced chances of harming the natural environment. Initial costs for establishing such dumps are low and operating costs are moderate.

Bioreactor Landfills are the result of recent technological research. These landfills use superior microbiological processes to speed up waste decomposition. The actual controlling feature is the continuous addition of liquid to sustain optimal moisture for microbial digestion. The liquid is generally added by re-circulating the landfill leachate and when the amounts of leachate in not adequate liquid wastes for example sewage sludge are used.

Biological Waste Treatment

Composting is another most frequently used waste disposal or treatment method, which is the controlled aerobic decomposition of organic waste materials by the action of small invertebrates and microorganisms. The most common composting techniques include static pile composting, Vermin-

composting, windrow composting and in vessel composting.

Anaerobic Digestion is almost similar to composting as it also uses biological processes to decompose organic materials. The main difference is Anaerobic Digestion uses Oxygen and bacteria free environment to decompose the waste materials where composting must have air and uses a number of microbes.