Different scientific studies claim that solid waste has had diverse effects on both the natural environment and human society. Besides, lack of plausible solid waste handling systems is as well postulated to be a major challenge, mainly in developing countries. For instance, the waste handling systems found in Kigali, Rwanda, are substandard compared to those highlighted by different studies in developed countries such as Sweden and USA. In general, municipal authorities and other private companies, mainly in Rwanda, are keen to keep streets and some suburbs clean with the help of open landfill disposal system. They often collect and dump solid waste, mixed of organic and inorganic materials to the only municipal dump site located at Nyanza Hill in the outskirts of Kigali City.
The exploratory study conducted in Kigali, Rwanda tested the plausibility of briquetting, composting, incineration, cullet pulverisation, and plasma pyrolysis systems to handle solid waste. The study started with the review of various literatures on solid waste management, and thereafter surveyed 400 residents of Kigali City for their opinions. Computer software Web-Hipre was used to analyze public opinions on the five systems to handle solid waste. The results indicate briquetting as the optimal system to handle solid waste in Kigali City. Composting considered suboptimal; incineration, plasma pyrolysis, and cullet pulverisation systems were not considered.
Keywords: Composting, Briquetting, Incineration, Cullet pulverisation, Plasma pyrolysis.
Homesteads and workplaces are, worldwide, crucial for people in their daily activities to meet their needs. They are thereby producing waste, which includes non-liquid materials thrown away by the people or companies (Ngoc., Schnitzer, 2009). These materials can be categorised into two fractions, namely inorganic and organic solid waste (Dimitris., Ham, 2006). Both fractions may end up at garbage disposal sites. However, there is a need to respect environmental hygiene at these sites which receive tons of solid waste day after day from various places. There is also need to undertake formal procedures in order to have such hygiene at all garbage dump sites in every country (Serrona., Yu, 2009).
In the developed countries, such as USA and Sweden, there are legal provisions that regulate the flow of solid waste to various handling sites. These provisions, which local administration authorities undertake, consider apportioning waste handling fee to every producer of solid waste (National Renewable Energy Laboratory, 1995). In the developing countries, such as Rwanda, flow of solid waste to landfills occurs in a less restricted manner. However, the environmental law, for instance, enacted by the Government of Rwanda in 2005 prohibit free flow of solid waste to dump sites which pollute the environment.
The open landfills, such as one at Nyanza hill located in the outskirts of Kigali City in Rwanda, are the source of rotting garbage threatening residents around with toxic fumes and polluted rainfall runoff (Beeman, 2009). The threats, mainly from Nyanza dump site, which Beeman considers being multiplying each year is a complex issue in the expansion of Kigali City. The Rwanda Environment Management Authority (2009) report links garbage threats to lack of integrated waste management system in the Kigali City development plan. Report explain how some parts of Kigali, residential and commercial areas, evolve in a way that causes irregular municipal waste collection and disposal system. This is due to lack of a centralised system for waste collection. Lack of such system prompts some institutions to practise inefficient handling of some solid waste from their premises. If not, municipal authorities and few private companies collect and dump large volumes of solid waste at Nyanza dump site; now at climax of its carrying capacity. The rapid population growth also increases solid waste predicament. As population increases in Kigali City with activities which produce more solid waste, residents still think too little about possibilities for developing a useful waste management system (Drakenberg, 2008).
1.1 Waste management theory
It helps to channel environmental management into engineering design. This theory is a cohesive body of knowledge about waste management system. It considers the framework of industrial ecology. This framework, built with interdisciplinary method, is comprised of plausible subsystems that handle solid waste. Some of these subsystems include composting, briquetting, incineration, cullet pulverisation, and plasma pyrolysis (Pongracz et al., 2005).
Composting involves the natural break down of organic solid materials into a component of soil comparable substance called compost (Bertoldi., European Commission, 1996). Humans thereby replicated the process by placing pile of organic solid waste on the topsoil or in a silo for microbes to degrade while synthesising their own food (McNelly, 2009., Pommier et al., 2008). Composting process for organic solid waste occurs under the influence of micro and macro organisms such as actinomycetes, fungi, bacteria, mites and snails (College of Agriculture Consumer and Environmental Science, 2010). Some of the organic solid materials to compost include cardboard rolls, news papers, fruits, vegetables, yard trims, sawdust, and coffee husks (United States Environmental Protection Agency, 2000).
In traditional composting systems, organic material is just piled together and then left for a year; while more elaborated composting systems take 2 to 3 weeks for complete decomposition. This system necessitates that the materials to compost be chopped up, into small pieces, and carbon (C) to nitrogen (N) ratio not exceeding 30:1. It also requires moisture content in the compost between 40 to 60%, achieved by adding in water during construction and turning of the compost pile. Modern composting needs to take place at the temperature range of 71o C, and regular turning of the compost pile to prevent overheat that would kill microbes (Raabe, 2001). Another important factor to achieve rapid composting, which Raabe warns for, is to avoid adding more organic materials into the bin once the composting process is in progress. The reason is that the effective breakdown of materials takes place within a fixed period. He also discourages the adding of materials, for instance soil and ashes from the fireplace or cooking stoves, into the bin. These materials add weight to the compost pile, thus making the turning of the solid waste pile more strenuous. Ashes may also influence the power of hydrogen (pH).
The modern composting system requires effective environmental conditions to help microbes break down organic materials. The three significant factors, which determine these conditions, include; (a) quality and quantity of carbon (food) as an energy source and a source of minerals (b) shape and physical dimensions of the organic ingredients; (c) the appropriate population of organisms engaged in the decomposition of organic solid materials into substantial manure (Cooperband, 2000).
The modern composting system, in which microbes play big role, occurs within the recommendable carbon to nitrogen ratio in waste materials. The recommendable high mass of carbon is 30; and the recommendable low mass of nitrogen is 1. The 30:1 ratio enables microbes to keep the amount of energy and protein to speed up the decomposition process. The carbon to nitrogen ratio helps to avoid the lengthy decay in case of excess carbon. The ratio also prevents stinky compost pile when nitrogen content is too high (Ebeling, 2003).
In Kigali City, Rwanda, companies such as Amizero, Cooped, Agruni and Cosen are composting. They are aware that composting enables production of manure needed for croplands. However, lack of proper solid waste handling systems and designed garbage trucks to transport solid waste cause production of poor composts. The current waste collection and transportation systems mix inorganic and organic solid materials. This makes the whole composting process difficult (Paulin Buregeya, unpublished observations).
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