Evaluation of the efficiency of the sewage treatment plant from the University of Santa Cruz do Sul (UNISC), RS, Brazil

Abstract

AIM: The main objective was to evaluate the efficiency of the sewage treatment plant (STP) of UNISC, through the ecotoxicological characterization and the analyses of the physical, chemical and microbiological variables from the raw and treated effluent. METHODS: Samples were collected during 2008 and 2009 for performing acute toxicity (Daphnia magna), chronic toxicity (Ceriodaphnia dubia), as well as the determination of environmental variables. RESULTS: The results indicated acute toxicity with an EC(I)50 48 hours average of 64.1 ± 9.9%, characterized as moderately toxic, and an average chronic toxicity CI(I)25 of 8,1 ± 2.6%, characterized as highly toxic. The total phosphorus (3.6 ± 1.4 mg L-1) and the ammonia nitrogen (77.8 ± 22.5 mg L-1) showed high concentration in the treated effluent, highlighting the STP inefficiency and, specially, the large amount of nutrients that the system is introducing into the receiving waterbody, the Lajeado Stream, condition that characterizes a large potential environmental impact known as eutrophication. The treated effluent showed high levels of thermotolerant coliforms, reaching an average of 6.4 × 10(5) ± 8.6 × 10(5) NMP 100 mL-1, corresponding to a potential pollution load of high impact in the receiving waterbody, characterizing therefore a public health problem. CONCLUSIONS: The Lajeado Stream has no supporting capacity for preventing acute and chronic effects on biota.