The world generates over 2 billion tonnes of MSW annually, with at least 1/3 not managed in an environmentally safe manner. Due to population growth, urbanization, and economic growth, worldwide MSW generation is expected to increase to 3.4 billion tonnes by 2050. Waste generated per person per day averages 0.74 kg but ranges widely, from 0.11 to 4.54 kg. Typically, richer countries produce more MSW than poorer countries. With proper waste recycling schemes, the organic waste, plastic, paper, cardboard, metal, and glass can be sorted and recycled, reducing the volume of MSW for disposal and closing the material loop. However, less than 20% of waste is recycled each year in the world, with huge quantities still sent to incineration plants or landfills. Waste is also often disposed of at open dump sites, especially in developing nations.

Landfill is the oldest and most common approach for waste disposal. If organic waste co-exists in the landfill, leachate and methane gas will be generated over time, posing risks to human and the eco-system. Sanitary landfill was introduced in 1940s to solve the issues with open dumps and other unsanitary landfill. It is an engineered facility with liner and capping that isolates and confines waste. The leachate is collected and treated, and the methane gas is burned out or utilized as an energy source. In addition, more and more countries are banning organic waste from landfills. The ultimate solutions to mitigate the damages caused by landfills are: reducing our waste generation, improving waste sorting and recycling rate, and separation of organic waste for composting. However, landfill will continue to be the unavoidable final destination of some toxic wastes for a period of time.

The population growth and booming economic activities have resulted in a concomitant growth in municipal solid waste (MSW) generation. Since landfill space for waste disposal is decreasing fast, as an alternative for MSW disposal, incineration is becoming prevalent. Incineration recovers the energy in MSW and reduces the waste volume by 90%. Nevertheless, incineration is not the final step for MSW disposal. The residual incineration bottom ash (IBA), around 17% by mass of the MSW, still needs to be landfilled. To prolong the lifespan of landfill, converting IBA into value-added construction materials is being practised in more and more countries. National legislations and standards have been established to promote and regulate the utilization of IBA in road, concrete, bricks, and land reclamation. Incineration fly ash (IFA), as a hazardous by-product generated from incineration, amounts to 3% of the MSW mass. The common practice for IFA disposal is to treat and landfill. In recent years, due to landfill space shortage, utilization of IFA is getting more attention and has been commercialized in some cities.

Soil contamination is typically caused by industrial activities, agricultural chemicals or improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons, solvents, pesticides, and heavy metals. The concern over soil contamination stems primarily from health risks, from direct contact with the contaminated soil, vapour from the contaminants, or from secondary contamination of water and food supplies within and underlying the soil. Additionally, the CO2 and N2O emissions from contaminated soils accelerate climate change. Soil pollution contributes to land degradation and loss of terrestrial and aquatic biodiversity.

Industrial waste is the residues or by-products produced by industrial activities. Types of industrial waste include demolition waste, scrap metals, chemicals, sludge, ash, slag, and used catalyst. Industrial waste may be solid, semi-solid or liquid in form. It may be hazardous or non-hazardous. The world has been generating increasing amounts of industrial waste in recent decades due to fast industrialization, but little information is available on the generation and treatment of industrial waste. Some industrial solid wastes are being treated and reused, but most are handled improperly. The various types and levels of pollutants in untreated industrial waste may pollute the nearby soil or water bodies, causing human health risks and ecotoxicity. E-waste has become one of the fastest growing waste streams in the world and a major environmental challenge of the 21st century. It contains both hazardous substances and valuable metals such as copper, gold, palladium, and rare earth that can be recovered.

Every day, more than 1 billion m3 of wastewater is being generated in the world from human’s domestic, industrial, commercial and agricultural activities. Most of the wastewater is discharged into the earth’s hydrosphere, creating health, environmental, and climate-related hazards. However, wastewater is one of the most under-exploited resources. It contains energy, valuable metals, organics, phosphates, nitrogen, cellulose, and other resources. Recovering the water, energy, metals, nutrients and other precious materials entrained in wastewater is crucial for sustaining the human civilization. Today, technologies are making resource recovery from wastewater increasingly viable, including biogas, fertilizer, metals, paper and, it is also a source of ‘new’ water.