Nutrient content in sludge profiles from a full-scale stabilization pond in a temperate location.

The inorganic nutrient concentrations in sludge profiles from a full-scale municipal facultative pond in Puerto Madryn City (Argentina) were measured. Sludge samples were collected with cores during autumn, winter, and summer at three sites: inlet, intermediate, and outlet. In general, the sludge accumulates NH4+ and PO4(3-), increasing their concentrations with depth. However, NH4+ presented a different behavior at the outlet station during the summer, when the lower concentrations were recorded. This finding reflects a nutrient release, originating in their greater demand from the water column. In the sludge, the NO3- followed the spatial and seasonal pattern recorded in the surface water: detectable concentrations in the warmer months at the outlet. The vertical reduction of NO3- could be an indication of denitrification. The study supported the hypothesis that the sludge can act as a nutrient trap or source, depending on factors such as the temperature, nutrients/oxygen concentration, mixing processes, and location.