Environmental engineering has a leading role in the elimination of ecological threats, and deals, in brief, with securing technically the conditions which create a safe environment for mankind to live in. Due to its interdisciplinary character it can deal with a wide range of technical and technological problems. Since environmental engineering uses the knowledge of the basic sciences – biology, chemistry, biochemistry and physics – it is able to neutralise pollution in all the elements of the environment, i.e. the hydrosphere, atmosphere and lithosphere. Moreover, environmental engineering deals with the design and maintenance of systems of water supply, sewage disposal, heating, ventilation and air-conditioning in buildings. Environmental Engineering IV contains 77 peer reviewed papers selected from 527 presented at the 4th Congress of Environmental Engineering (Lublin, Poland, 2-5 September 2012). The contributions are divided into 7 chapters: • Water supply • Water and wastewater treatment • Neutralization of solid wastes and sludge • Air protection and quality • Indoor microclimate • Energy • Biology and technology Environmental Engineering IV assesses the state of scientific research in various areas of environmental engineering, evaluates the organizational, technical and technological progress made in contributing to ecological security, and determines the place of environmental engineering in sustainable development, taking into account current political and economic conditions, and is a valuable source of information for the environmental engineering professional and academic community.