A city or town can be envisioned as a thermodynamic system with a continuous input of materials and energy, generating work and wastes as output. Most material chains in today's globalized cities are open loops, starting as raw materials and ending up as wastes in landfills. They are not sustainable as the materials move in a single direction resulting in significant depletion of natural resources and causing environmental degradation over time. The creation of natural resources involves energy inputs which are embodied in the wastes that are generated at the end of material loops. Thus, closed material cycles where materials and energy are recycled or recovered hold a key to sustainable world.
According to the waste management hierarchy, reducing wastes and reusing products are the most sustainable techniques. The effectiveness of these techniques can be increased by fostering innovation in the design of materials. Materials recovery or refilling the feed stocks falls next on the hierarchy and involves recycling of some materials and composting of others. Quality of the recycled products depends on upstream phenomena like separating wastes and can be improved in a short span of time but only with changes in life style or by introducing incentive models. Recycling of source-separated metals, paper, and some plastics has a higher chemical footprint when compared to other processes and can be improved by green chemistry and sustainable design of products both of which are still being researched. They will need decades of research to hold any promise of reinventing and transforming majority of our daily-use materials. Energy recovery is the next best alternative to handle wastes and holds better promise. Technologies like mass-burn incineration are a reality today and will be relevant for the next few decades. They need to be supplemented by smaller scale technologies like gasification and RDF that can be blended into existing infrastructure and recover energy and also metals. Most of these supplementary techniques will be practical in India within the next decade. Practical sustainable solutions to waste management for the next few generations will be composting, recycling and energy recovery technologies. Landfill gas recovery is a method of abating the effects of a waste management method of the past, landfilling.