Limnology of natural systems for wastewater treatment. Ten years of experiences at the Experimental Field for Low-Cost Sanitation in Mansilla de las Mulas (León, Spain)

Abstract

The first experimental field in Spain for low-cost wastewater treatment was constructed in 1998 in Mansilla de las Mulas (Len). The project was funded by the Diputacin de Len and was run scientifically by the Department of Ecology at the University of Len until 1999. The objective of the field was to compare performances and to study the biological fundamentals of systems such as constructed wetlands and algal ponds which had been adapted for wastewater treatment in rural areas. The experiences carried out on constructed wetland systems proved that macrophytes had a significant role in the treatment of diluted wastewaters. The role of vegetation varied depending on the technology used, being important for organic matter removal in free water surface systems, whereas plants were only significant for nutrients and faecal bacteria in subsurface flow systems. There were no differences among plant species in the performance of free water surface systems. Algae-based systems such as high rate algal ponds proved to be a highly efficient technology for wastewater disinfection. Studies on decay and inactivation of faecal bacteria and parasites (helmiths and protozoan oocysts) demonstrated for the first time that the physico-chemical conditions created by the algae are powerful mechanisms for pathogen destruction. Wastewater treatment plants are technologically-confined ecosystems in which limnological studies should be further encouraged both examining basic knowledge on natural species and processes and leading to a better understanding of the biological foundations for their design and operation.