Urban populations have grown unabated since the advent of cities. Some urban centers have such high populations (> 10 million) that the term, megacity, is used. The impact of megacities on the natural environment has reached unprecedented scales in human history. Growing resource and energy consumption, with corresponding waste generation, has exerted a toll on the regional and planetary environment.

Overexploitation of natural resources, air and water pollution and solid waste mismanagement are undermining sustainability in many cities. For example, the United Nations estimated that cities consume about 75 percent of globally generated power, which produce 70 percent of anthropogenic greenhouse gas emissions.

The problems are expected to exacerbate by 2050, when two-thirds of the world’s population is projected to live in cities. Therefore, sustainable urban solutions are required.

Researchers of various disciplines in the Energy and Environmental Sustainability Solutions for Megacities (E2S2) program—a collaboration between Shanghai Jiao Tong University, in China, and the National University of Singapore—have conducted research and data collection in two cities of different size and complexity, the megacity of Shanghai and land-scarce Singapore.

A major problem in both cities involves the incineration of food waste, which comprises more than 22 percent of incinerable waste but is only 16 percent recycled in Singapore. E2S2 found solutions that coupled the problem of waste management with energy and material production.

These include a high-efficiency three-stage anaerobic digester (AD) that converts food waste to biogas, and a high-solids anaerobic digester that is water-efficient. We have developed gasification technology that can convert solid wastes and the digestates from the AD process into syngas and biochar. Pilot tests are now being run at eateries in Singapore to prove the feasibility of AD in an urban environment.

Other solutions include technologies for using low-quality waste heat in adsorptive cooling and dehumidification, wireless air-quality sensor network and monitoring of emerging contaminants in reservoirs in real-time, by a network of water drones. Studies were also done on toxicology and risks assessments for contaminants in air, water and land environments in the city.

These sustainable solutions are applicable for Singapore and other megacities in Asia and around the world, to provide a clean and healthy urban environment, while maintaining environmental sustainability.