Résumé
Modern wastewater treatment plants feature both aerobic and anaerobic biological processes. It is also possible to combine anaerobic and aerobic biological breakdown in one system by exploiting the ability of plant cells to photosynthesize and release oxygen for the microbial aerobic metabolism of organic matter and to reduce the amount of CO2 released from microbial anaerobic metabolism. Wastewater treatment systems based on constructed wetlands offer enormous opportunities to reduce energy inputs associated with gaseous exchanges. They also offer opportunities for tailoring organic matter degradation according to the nature of organic matter present in wastewater, by exploiting the cooperative and mutualistic metabolism evidenced between plants and bacterial communities in both the plant rhizosphere and endosphere. Advances in algal pond treatment systems that incorporate the utilization of microalgae have demonstrated increases in municipal wastewater treatment capacity without associated economic, energy, and environmental costs. They will now deliver water for irrigation or discharge into a water resource, methane-rich biogas, and biomass for bioenergy and feed (where permitted). Constructed wetlands and algal pond treatment systems sit at the forefront of innovations in contemporary wastewater treatment aimed at the food-water-energy nexus and are the subject of this chapter.