Abstract
Chlorine-disinfected sewage effluent poses a risk to aquatic organisms, prompting the need for an innocuous alternative disinfection method. In this study, the toxicity of secondary effluent disinfected with chlorine, chlorine dioxide, ozone and UV irradiation was evaluated by the algal growth test using the green alga Selenastrum capricornutum. Algal growth was significantly inhibited in test media containing 10% chlorinated water. In the chlorination process, about 50% of chlorine remained in the disinfected water as oxidant, while about 80% of residual oxidant was monochloramine (NH2Cl) . NH2Cl is the most toxic substance that inhibits algal growth in chlorinated sewage effluent. On the other hand, chlorine dioxide and ozone disinfection apparently did not inhibit algal growth, but the possibility of chlorophyll biosynthesis inhibition was indicated. Effluent dechlorinated with sodium sulfite, which removes residual oxidants such as NH2Cl, did not inhibit algal growth. Our results have shown that the algal toxicity of the disinfection methods follows this order: chlorine >> chlorine dioxide > ozone. In contrast, UV-irradiated effluent did not harm the algae. This shows that UV irradiation, which is harmless to aquatic organisms, is adequate enough for disinfecting treated sewage effluent.
