No abstract

The water reservoir of Vargem das Flores, Brazil, has been subject to eutrophication problems. The occurrence of cyanobacterial blooms in this water body has raised concerns regarding the presence of toxic metabolites. In order to understand the factors that promote the proliferation and distribution of cyanobacteria in the reservoir, physicochemical parameters, species composition, and toxicity of the reservoir water column were analyzed at monthly intervals over a 3-year period (2004-2007) using a range of analytical techniques. The results showed that cyanobacteria were absent in the dry period, returning after the onset of stratification. A succession pattern was observed within this group. The Aphanocapsa genus was dominant in the last rainy period of the study (October 2006 to March 2007). However toxicity was only detected when the genera Microcystis and Sphaerocavum were present (October 2005). The principal components analysis showed negative correlation between cyanobacteria and orthophosphate. The change of dominant cyanobacteria along the 3 years of study and the final prevalence of nontoxic genera in the reservoir is an evidence of water quality improvement in response to the management techniques implemented in the reservoir basin.

Potential problems arising from the presence of cyanobacteria in water intended for human consumption have been reported by several researchers. Regarding water treatment plants, intact cells of cyanobacteria should be removed to avoid the release of cyanotoxins due to cell lysis. Water treatment techniques with different degrees of complexity can be employed but, whenever possible, the method of easiest installation, operation and maintenance should be selected, especially for non-industrialized countries and rural communities. In this context, research was carried out to evaluate the efficiency of slow sand filtration to treat water from Gavião reservoir in the city of Pacatuba, Ceara, Brazil, which has exhibited phytoplankton density of approximately 105 cells/mL with a prevalence of cyanobacteria representing over 90% of total cells. The results have demonstrated that slow sand filtration can be used to achieve water purification that meets federal standards. However, it was established that filtration through beds of gravel (prefilter) before the slow sand filtration is essential. The removal of phytoplankton reached values of approximately 97% and the filter run duration was more than 70 days. Furthermore, the slow sand filter was very efficient in removing total coliforms, with removal of up to 99.98%.

The current study monitored the water quality of a tropical reservoir (Vargem das Flores) used to supply potable water to seven cities in the metropolitan area of Belo Horizonte, Minas Gerais, southeast Brazil.