Phosphorus is an essential nutrient for every organism on the Earth, yet it is also a potential environmental pollutant, which may cause eutrophication of water bodies. Wastewater treatment plants worldwide are struggling to eliminate phosphorus from effluents, at great cost, yet current research suggests that the world may deplete the more available phosphorus reserves by around 2300. This, in addition to environmental concerns, evokes the need for new phosphorus recovery techniques to be developed, to meet future generations needs for renewable phosphorus supply. Many studies have been, and are, carried out on phosphorus recovery from wastewater and its sludge, due to their high phosphorus content. Chemical precipitation is the main process for achieving a phosphorus-containing mineral suitable for reuse as a fertilizer, such as struvite. This paper reviews the current status and future trends of phosphorus production and consumption, and summarizes current recovery technologies, discussing their possible integration into wastewater treatment processes, according to a more sustainable water-energy-nutrient nexus.

Fossil fuel continued overuse and carbon emissions issues have prompted increased research efforts on sustainable and renewable energy sources as alternative to fossil fuels. Biofuels include products derived from biomasses or their residues, such as: biogas, biodiesel, bioethanol, biomethanol, synthetic biofuels, biohydrogen and vegetable oil. Due to their properties, all of these may be used in conventional engines as produced, or blended with fossil fuels. First and second generation biofuels may be produced from a large variety of feedstocks, including food crops such as sugar beet, sugar cane, sorghum, energy-crops, such as lignocellulosic masses, and waste, like the organic fraction of municipal solid waste, or landfill leachate. In the last decade, a third generation of biofuels has emerged, i.e. those derived from microalgae. Even though their yield is higher, and their greenhouse gas footprint lower than previous generations’, they still need large amounts of water and nutrients for production. In this paper, the most common biofuels (biogas, syngas, biodiesel, bioethanol and biobutanol) are reviewed and analyzed with particular attention on their carbon and water footprints and economic sustainability perspective.

Urban water systems and, in particular, wastewater treatment facilities are among the major energy consumers at municipal level worldwide. Estimates indicate that on average these facilities alone may require about 1% to 3% of the total electric energy output of a country, representing a significant fraction of municipal energy bills. Specific power consumption of state-of-the-art facilities should range between 20 and 45 kWh per population-equivalent served, per year, even though older plants may have even higher demands. This figure does not include wastewater conveyance (pumping) and residues post-processing. On the other hand, wastewater and its byproducts contain energy in different forms: chemical, thermal and potential. Until very recently, the only form of energy recovery from most facilities consisted of anaerobic post-digestion of process residuals (waste sludge), by which chemical energy methane is obtained as biogas, in amounts generally sufficient to cover about half of plant requirements. Implementation of new technologies may allow more efficient strategies of energy savings and recovery from sewage treatment. Besides wastewater valorization by exploitation of its chemical and thermal energy contents, closure of the wastewater cycle by recovery of the energy content of process residuals could allow significant additional energy recovery and increased greenhouse emissions abatement.

Collection and treatment of wastewater have a huge impact on the environment and economy, both at the local and global levels. Eco-innovation may play a paramount role in the reduction of the environmental impact of such systems, and in their greater sustainability in economic, environmental, and social terms. Decentralization appears as a logical solution to tackle sustainability problems of wastewater management systems, as it focuses on the on-site treatment of wastewater and on local recycling and reuse of resources contained in domestic wastewater (in primus, water itself). This paper analyses the needs, technological options and contribution to water management of decentralized systems. Decentralized solutions in general will tend to be compatible with local water use and reuse requirements, where locally treated water could support agricultural productivity or (in more urban areas) be used as a substitute for drinking-quality supply water for compatible uses. In analyzing sustainability of technology, different dimensions should be taken into account (in particular, local issues). There is no fixed or universal solution to the technological issue; to the contrary, all relevant studies demonstrated there are varying degrees of sustainability in the way a technology is selected and operated, to avoid exporting problems over time or space.

In many Countries, small communities are required to treat wastewater discharges to increasing standards of lesser environmental impacts, but must achieve that goal at locally sustainable costs. While biological membrane treatment (membrane bio-reactors (MBRs)) is quickly becoming the industry standard for centralized wastewater treatment plants, and would also be ideally suited also for small plants potentially subject to relatively large hydraulic load variations, its investment and operating costs are usually high for that class of applications. Consequently, small treatment plants are generally configured as anoxic or aerated biological tanks with little sedimentation, making them quite susceptible to hydraulic loads transient and sludge quality changes. As an alternative, Constructed Wetlands Systems (CWSs) are gradually and successfully being introduced in many Countries. CWSs are designed to utilise the natural functions of wetland vegetation, soils and their microbiological populations to treat wastewater. Pretreatment occurs by filtration and settling, followed by bacterial decomposition in a natural-looking lined marsh. A new technology, a new type of membrane-like aerobic reactor initially designed for the degradation of hydrocarbon-derived groundwater contaminants, was recently tested for treating domestic, with performance similar to that of MBRs. Examples from the above applications are illustrated and compared in this paper. The paper also discusses merits and drawbacks of the various illustrated technologies, in view of their sustainability potential, and according to the new development paradigms for urban water systems, that encourage the development of local water-cycle clusters with local reuse and recycle of the resource, and possible local recovery of energy and/or materials.