We became environmental engineers and have devoted our careers to finding ways to minimize adverse environmental impacts created by our fellow citizens in their routine daily lives. Solid waste generation and the management (or, in too many cases, mismanagement) of such wastes have become two of the most visible negative impacts of humans on earth over the past 100 years, particularly since populations have been growing in urban centers (where solid waste is usually referred to as municipal solid waste (MSW)). For example, by 1920, more Americans lived in cities than in rural areas for the first time in US history. Worldwide, by 2008, an equal number of people lived in both urban and rural areas (about 3.4 billion in each). The trend towards population increase in general, and urbanization in particular, is projected to continue, such that about 6.3 billion people (69% of the world's population) will be living in urban areas by 2050.
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