RESUMO
Forty-eight hours acute toxicity tests were employed with Daphnia magna and Lecane quadridentata to assess the influents and effluents of the seven most important wastewater treatment plants (WTP) in the state of Aguascalientes, Mexico, during the 2006 dry and rainy seasons. The WTP of the City of Aguascalientes treated 1948 Ls(-1). The remaining six plants treated wastewater in the range from 28 to 93 Ls(-1). Plants efficiently removed toxicity when Daphnia magna was used as a model organism, but performed poorly when the freshwater rotifer Lecane quadridentata was employed. It was observed that biochemical oxygen demand (BOD(5)), chemical oxygen demand (COD), conductivity, dissolved oxygen (DO), pH, phenol, methylene blue active substances (MBAS), temperature, and total suspended solids (TSS) were within the maximum allowed levels (MAL) in the effluents during the rainy season. Whereas the BOD(5), total nitrogen, total dissolved solids (TDS) and TSS showed levels greater than the MAL in effluents during the dry season. The levels of BOD, MBAS, and total nitrogen were greater than the MAL in influents. In contrast, the values of TDS and TSS in influents were above the MAL during the rainy season. In the dry season the levels of aluminum (Al), iron (Fe), zinc (Zn), manganese (Mn), chromium (Cr), mercury (Hg) and lead (Pb) in influents and Fe, Mn and Pb in effluents were above the MAL. During the rainy season the levels of Mn, Cr and Hg in influents and those of Mn in effluents were above the MAL. When D. magna was used as model organism, acute toxicity units (aTU) for influents ranged from 0.2 to 4.4 and from 0.1 to 0.2 for effluents. These values for effluents are acceptable according to international guidelines. However, when L. quadridentata was the model organism, ranges were from 2.0 to 8.3 aTU in influents and from 1.6 to 2.6 aTU in effluents. The treated water that discharges into the San Pedro River would be considered toxic. The results of the toxicity tests demonstrated that the freshwater rotifer Lecane quadridentata was more sensitive (in 89.3% of the samples) than the cladoceran Daphnia magna when both organisms were exposed to the influents and effluents of the wastewater treatment plants.
