Relationships between abiotic and biotic variables in a maturation pond and their influence on E. coli removal.

The effects of depth and climate seasonality on zooplankton, algal biomass, coliforms and Escherichia coli in a small full-scale shallow maturation pond receiving pre-treated domestic wastewater were evaluated during a tropical climatic seasonal cycle. The experiment revealed that the zooplankton community was dominated by rotifers and protozoans, and concentrations were influenced by seasonality. A negative correlation between zooplankton, and pH, dissolved oxygen, temperature and ultraviolet radiation, and chlorophyll-a and Escherichia coli were observed at all depths. The major driving forces influencing Escherichia coli were pH, dissolved oxygen, ultraviolet radiation and the zooplankton. A significant difference between Escherichia coli removal throughout the three different depths were observed. Both bacterial and zooplankton concentrations were greater closer to the bottom of the pond, therefore reinforcing the integral role of solar radiation on bacterial removal. These results give an insight on the dynamics of these groups in pond systems treating domestic wastewater, by correlating the variation of zooplankton with biotic and abiotic variables and seasonal changes in a tropical climate, where few studies have been performed on this topic.

Vertical profiling and modelling of Escherichia coli decay in a shallow maturation pond operating in a tropical climate.

Maturation ponds are excellent natural treatment systems for disinfecting domestic wastewater from pathogenic bacteria because of their great sunlight exposure through depth and high pH and dissolved oxygen values - all considered important factors. Escherichia coli (E. coli) removal is higher at closer to the surface of the pond and proceeds to decrease due to the limited amount of sunlight at deeper depths. To quantify the decay coefficients (K b) of E. coli in a shallow maturation pond, quartz vessels were used to create isolated batch experiments and submerged at different depths inside the pond during different periods of the day (morning and afternoon). K b values ranged from 0.48 to 0.28 h-1, considering different depths and both periods. A dark control vessel was also used to quantify dark decay rates (K d) at the same depth and periods as the quartz vessels, and ranged from 0.14 to 0.04 h-1. Environmental variables did not appear to influence overall disinfection. Two models for estimating final E. coli concentrations were proposed, considering the kinetic coefficients obtained in the batch experiments and the dispersed flow regime for continuous flow ponds, which resulted in very good fittings with monitoring data and demonstrated the vertical profiling of E. coli concentration.

Solar radiation (PAR, UV-A, UV-B) penetration in a shallow maturation pond operating in a tropical climate.

Solar radiation is considered the primary route for disinfection of pathogenic bacteria in maturation ponds. There is scarce information on depth profiling and attenuation of photosynthetically active radiation (PAR), UV-A and UV-B in shallow maturation ponds operating in tropical climates. Measurements of solar irradiance of the three wavelength ranges, together with turbidity, have been acquired from different depths for over 1 year in a shallow maturation pond (44 cm of depth) operating in Brazil. UV-A and UV-B were still detected at 10 cm from the surface, but from 15 cm both were undetectable. PAR was still detected at 30 cm of depth. Irradiation attenuation showed to be related to turbidity. Attenuation coefficients were calculated and simple models without turbidity (traditional structure) or including log10 of turbidity are proposed for predicting PAR irradiance attenuation as a function of depth.