Abstract
In northeastern Brazil, stabilization ponds are very suitable for wastewater treatment because of the relative great land availability and environmental conditions (e.g., high temperature) favorable for microorganism optimal development. However, blooms of potentially toxic cyanobacteria may affect the use of these treatment ponds due to resulting effluent poor quality. The objective of this study was to evaluate the dynamics of phytoplankton communities and the occurrence of cyanobacteria in a maturation pond located immediately after a series of two ponds. Temperature, dissolved oxygen, pH, BOD, N, and P were measured during a period of four months when samples were collected from the surface and the bottom of 7 sampling points distributed inside the pond. The phytoplankton of collected samples was also identified and classified using a conventional optical microscopy. Analysis of variance and Tukey test were used to evaluate the results. The three phytoplankton divisions found (Cyanophyta, Chlorophyta, and Euglenophyta) did not change considerably through surface and bottom. However, they changed greatly over the sampled months; great dominance of Cyanophyta was found at April and October, while Chlorophyta dominated the lagoon in September. Low superficial organic loads (between 78 and 109 kg BOD.ha-1.d-1) and N:P ≤ 10 were the determinant factors that favored the predominance of Cyanophyta. The presence of two potentially toxic species of Cyanophyta, Oscillatoria sp. and Microcystis aeruginosa, indicates that caution is required when considering the final destination of treated effluent and suggests a need to assess the risks and benefits associated with the use of the treatment technology.
Highlights
Biological processes are usually applied for the treatment of domestic sewage
The WWTP includes a series of three stabilization ponds, the first of which is a facultative pond followed by two maturation ponds with a total volume of 31,850 m3
This work was conducted in the second maturation pond because the final treated effluent is discharged from this unit into the receiving body (River São Francisco, Itaparica reservoir) or reused in two agricultural pilot areas (2 hectares each)
Summary
Biological processes are usually applied for the treatment of domestic sewage. Anaerobic reactor followed by aerobic reactor or stabilization pond is the main technology used in tropical countries, due to climate favorable conditions. In Brazil, for example, several wastewater treatment plants (WWTP) use this technology due to the climate and the availability of land at a relatively low cost (Kellner and Pires, 2002; Pearson et al, 2005; Von‐Sperling and Mascarenhas, 2005). Cyanobacteria blooms have occurred in water reservoirs in northeastern Brazil (Molica et al, 2005; Bittencourt‐Oliveira et al, 2010, 2012) These blooms are associated with certain abiotic factors, as reported by Paerl and Paul (2012), such as vertical stratification, nutrient availability and high temperatures throughout the year. These abiotic factors are commonly observed in stabilization ponds (Kellner and Pires, 2002)
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