Abstract
As sewerage has spread in Japan, evaluation of the effects of sewage and treated sewage on aquatic organisms has been becoming important. In this paper, applicability of two bioassays to sewage samples was discussed. One is an algal growth inhibition test using the green alga, Selenastrum capricornutum, proposed by Environment Canada that is conducted in a microplate for simplicity of handling. Because sewage samples include high concentration of nutrients, the test medium was modified here. The other is teratogenicity test with embryos of the frog, Xenopus laevis proposed by ASTM. Application of both the assays to sewage samples has been little reported. Therefore, we applied these bioassays to sewage, treated sewage and its receiving water at three sewage treatment plants (STPs). MICROTOX acute toxicity test was also conducted for comparison with the above bioassays. As a result from all the bioassays, the following findings were observed : i) significant reduction of the toxicity of sewage during biological treatment in all the STPs, ii) no significant toxicity of disinfection with ozone and low-chlorine-dose, iii) high toxicity due to disinfection with high-chlorine dose from MICROTOX test, iv) toxicity due to high-chlorine-dose from algal test even after dechlorination, v) no significant toxicity of high-chlorine-dose from frog embryo test. Considering the above results, these bioassays are useful for evaluating the toxicity of sewage samples to each organism.
