Processes or Stages of Municipal Solid Waste Management
Municipal solid waste management comprises various stages from the generation of waste to its final disposal. Planning for proper management must consider all these stages. For a complete system of solid waste, all the stages need to be in place:
1. Storage SystemKeeping the waste in containers is hygienic, and minimises handling and loading times, but under some economic conditions it appears that less satisfactory collection systems, that deposit waste on the ground and then load it, cost less per ton. Containers can be a very significant part of the costs of a collection system, particularly if the containers quickly become corroded or damaged or need to be replaced frequently for other reasons. Solid waste storage facilities for domestic wastes may be classified as household (that is, household bins or bags, sometimes known as primary storage) and community (that is, containers or bunkers, each used by many households, known as secondary storage).
2. Collection of Municipal Solid WasteThe term “solid waste collection” is taken to include the initial storage of waste at the household, shop or business premises, the loading, unloading and transfer of waste, and all stages of transporting the waste until it reaches its final destination disposal site (the sweeping of streets and public places, the cleaning of open storm drains and the removal of these wastes are also included). This destination may be a material processing facility, a transfer station or a landfill, a treatment plant or disposal site. Collection of generated solid waste is the crucial part in MSW management
Stages of municipal solid waste collection include: collection from various non-point sources and point sources and transportation to disposal sites.
3. Transfer SystemThe requirements of a vehicle for collecting wastes are very different from the requirements for transporting the wastes that it has collected to a transfer station, disposal site or processing plant. For collecting, the vehicle should be small, easy to maneuver along narrow streets. For transporting the wastes to the disposal site a large vehicle is required with a higher-powered engine for fast travel.
Conventional split level transfer stations normally have ramps which the primary collection vehicles drive up to discharge their loads, either directly into containers or secondary transport vehicles below or into stationary compactors which compact the waste into hook lift containers or truck bodies.
4. Recycling and Resource Recovery and Treatment of Solid WasteThe stages involved in recycling may include picking, transporting, trading, sorting, cleaning and processing. In some cases manufacturing may also be involved. Reuse of items for the same purpose as that for which they were originally used (such as drink bottles) is included. To these activities is added the recovery of energy from wastes for economic purposes. The term “resource recovery” is strictly more accurate, but less well-known. The treatment of solid wastes includes any processes that are implemented to reduce the costs of transporting or disposing of wastes, and any processes that reduce the risks posed by the wastes. There are clear overlaps with recycling.
Composting can be regarded as recycling when there is a market for the product and as treatment when the purpose is to reduce the pollution arising from the disposal stage. Incineration is generally regarded as treatment, but can be considered as a resource recovery process when energy from the burning of the waste is put to economic use.
• Composting: is the converting, by aerobic bacteria, of biodegradable wastes (such as food waste) into a good soil conditioner. Another benefit of composting is that it reduces the amount of biodegradable wastes in the waste stream, resulting in less pollution. In general, mechanised composting has not proved sustainable in developing countries due to the high costs and short life of the equipment, as well as the problems of finding a market for the compost large enough to justify a mechanised operation. However, small-scale manual composting of selected wastes (typically market and abattoir wastes) can be viable.
Composting and anaerobic digestion uses natural microbial organisms to decompose the organic fraction of MSW. These methods reduce the volume of waste that must be landfilled or incinerated, and end products can potentially be used as agricultural fertilizers, or processed into fuels for motor vehicles. However, like incineration, project implementation can be too expensive for poor communities.
• Incineration: is the high-temperature combustion of wastes. Noncombustible must be sorted out before incineration. Benefits of incineration include reduction of volume of waste and production of energy in the form of electricity and heat. Inadequately designed and poorly operated incinerators can cause dangerous air pollution. Incinerators are used for municipal wastes and for selected hazardous wastes. However, construction and start-up costs of incineration facilities can be prohibitively expensive for developing nations.
5. Final DisposalFinal disposal is the last stage in the waste management stream. This is the stage when all the collected waste requires a safe disposal. Despite all the efforts to reduce, recycle and reuse the waste, there are always certain quantities of waste requiring final disposal. At the final disposal stage we need to deal with the larger and accumulated quantities of waste.
The two most common methods of municipal solid waste disposal are open dumping and sanitary landfilling. Incineration, composting and anaerobic digestion are volume reducing technologies; ultimately, residues from these methods must be landfilled.
- Open or Crude Dumping: waste is dumped at a designated site without any environmental control measures. This is not a disposal option but a common practice in developing countries. It has high environmental health risks. Waste is unloaded wherever the driver of the collection truck finds a convenient space. Access to parts of the site may be blocked by piles of waste, accumulations of water or rough terrain. Usually there are many smoldering small fires. These fires may be started by waste pickers for various reasons or by municipal employees in an attempt to discourage fly breeding and reduce the volume of the waste. Neighbouring householders may light fires to control the insects and rats. Some fires may also start as the result of natural processes, scraps of glass focusing the sun’s rays, or the depositing of burning loads.
- Sanitary Landfilling: landfilling is the only true “disposal” method of managing MSW. It is also the most economical, especially in developing countries where it typically involves pitching refuse into a depression or closed mining site. Landfills produce landfill gases and leachate which can harm human and natural systems. Landfill gases (LFGs), produced when methanogens decompose complex molecules, are primarily methane and carbon dioxide (up to 90%), but also include CO, N2, alcohols, hydrocarbons, organosulfur compounds, and heavy metals.
Leachate forms as water percolates intermittently through the refuse pile, and can contain high levels of nutrients (nitrogen, phosphorous, potassium), heavy metals, toxins such as cyanide, and dissolved organics.
The objective of sanitary landfilling is to dispose of solid waste in a way that causes minimum impact on the environment, and at minimum cost. In order for the operation to be economical, the site must be well managed so that the maximum amount of waste can be placed on the site. Sanitary landfills can be used to fill holes below ground level or form hills higher than the surrounding ground. Intermediate stages between crude dumping and full sanitary landfilling are called controlled dumping or managed (or engineered) landfilling.