Introduction

Eutrophication and hypoxia are processes that occur by the increase in the concentration of nitrogenous and / or phosphorus nutrients and organic matter in surface waters. These processes cause an exacerbated growth of marketer species that jeopardize the biodiversity of the ecosystem they invade. The increase in nutrient concentration in water bodies may have a natural origin, but in many cases, it is directly related to human activity. For example, it might be due to agricultural land runoff with high fertilizer content; or due to the emission of urban and industrial waste water to the environment. In the last decades, sewage plants manage to eliminate much of these nutrients, however, obsolete or ineffective treatments may still cause that the outflow effluent of these treatment plants can still lead to eutrophication and hypoxia of surface water bodies.

As well as jeopardizing the biodiversity of the place affected by eutrophication and hypoxia, these phenomena may have direct effects on human health. In many cases, the proliferation of marketer organisms implies the growth of cyanobacteria and other species of microscopic algae that generate toxins with their metabolism. If the waters affected by this bloom of algae are used for the production of drinking water, these toxins can pass into the distribution networks and cause a public health problem. On top of this, the excess of organic matter in these waters, technically complicates the water treatment process and may provoke generation of carcinogenic byproducts, caused by the reaction between organic matter and disinfectant products used during the process.

On the other hand, focusing the view in Spain again, the purification of 3,000 hm³/year of urban wastewater treatment represents 1 % of national energy consumption, being the biological reactors’ areation, the higher enegy consumption process, approximately 50 % of the total. Although the larger plants are designed with a minimizing energy philosophy, including aeration systems that allow them to consume between 20-30 kWh/(hours/year), its estimated that sewage treatment plants in Spain, have an average consumption of around 50 kWh/hours/year). These data show that there is a large number of treatment plants in wich energy consumption is not optimized. For this reason, while technological trends of urban water purification point to an increase in energy demand, due to the greather demands on the quality of treated water, it is necessary to obtain this increase through processes that consume less energy, which are more efficient and/or generate energy-efficient products.