Assessment of acute toxicity of Modjo tannery effluent using Daphnia magna bioassay

Abstract

Tannery wastewater contains large quantities of organic and inorganic compounds, including toxic substances such as sulfides and chromium salts. There are many established facts about the effects of tannery effluent on water quality, on living biota and to the environment. Employing bioassay for evaluation of industrial effluent helps better understanding and picture of possible effects of industrial effluents. In Ethiopia, chemical analysis based factory effluent evaluation and monitoring has been commonly employed. The aim of this research was to  determine  the  toxicity  of  Modjo  tannery  effluent  to  the  receiving system (Modjo river) (also known as Mojo River) using Daphnia bioassay at  24-h  and  48-h  acute  toxicity  test, and  to suggest  the  possibility  of employing Daphnia bioassay for evaluation and monitoring of factory effluents in the country. Two effluent samples from two different tannery stages (beam-house stage and tanning stage) were considered. The studyresult showed that both effluents caused moderately acutely toxic effects on D. magna at both 24-h and 48-h of exposure with LC50  values ranging from  4.51%  to  8.36%  dilution.  Therefore,  proper  treatment  of  the currently discharged effluent concentration to <4.51 % is recommended before  direct  release  into  the  river.    The  result  also  showed that  the effluent from tanning stage caused more sensitivity to the test organisms at both 24-h (EC50= 4.65%) and 48-h (EC50= 4.51%) tests than the effluent from beam-house stage, which has EC50= 8.36% at 24-h and EC50= 7.91% at  48-h  tests.  This toxicity  level  difference might be  attributed to the variation in chemical composition of tanning stage and beam-house stage effluents.