by natural processes similar to those that take place in the surface waters, carry out stabilization of the organic matter inside them, as part of the treatment. This is why they received the name of waste stabilization ponds. Microorganisms The reactions take place by the joint participation of several microorganisms living within the pond. The organic matter is measured as biochemical oxygen demand (BOD). BOD values in the pond effluent are lower than in the influent, reflecting the removal of organic matter. This pond biome uses organic matter from the wastewater as food. Nutrients are converted to cell material and energy for life processes including reproduction and growth of living cells. Some of these living cells will be consumed by organisms at higher trophic levels within the pond. In ponds, the most important group of microorganisms are bacteria, which utilize most of the organic matter from the wastewater, but also consume oxygen. Algae are another essential group of microorganisms. They do not depend on the organic material from the influent. Instead, they undertake photosynthesis, in which they produce the organic matter for their own consumption and, very importantly here, they release oxygen. The excess oxygen released supports the respiration done by the aerobic organisms in the pond. Atmospheric oxygen is also dissolved into the liquid, which assists in maintaining an aerobic layer on the top of the pond surface. Oxygen levels The oxygen concentration varies in the liquid column: Close to the surface, concentrations are high and support the growth of aerobic organisms. Close to the pond bottom, sunlight penetration is low, and thus photosynthetic activity is reduced. This causes oxygen concentrations to be low there. Finally, inside the sediments in the bottom layer, there is no oxygen at all. Here, organic matter is removed by digestion undertaken by anaerobic organisms. Removal of pathogens Pathogens can be efficiently removed in waste stabilization ponds. The process relies mostly on maturation ponds for removal of pathogens, although some removal also takes place in the other ponds of the system. The higher the number of ponds in the series, the more efficient the pathogen removal. Removal of pathogenic bacteria and viruses occurs mainly by inactivation. Pathogens are inactivated as a result of a complex interaction of mechanisms that involve pH (the pH value in ponds is high because of algal photosynthesis), temperature, ultraviolet radiation present in the sunlight that reaches the pond surface