Chemical Composition of Industrial Effluents and Their Effect on the Survival of Fish and Eutrophication of Lake Hawassa, Southern Ethiopia

摘要

Growing trends in industrialization in Ethiopia have raised concerns about pollution of water bodies particularly of lakes. This study was therefore conducted to 1) characterize the chemical contents of major industrial effluents (namely textile ceramic and soft drink factories) that reached Lake Hawassa and 2) investigate the effects of the above mentioned factory effluents on survival of larvae fish and growth of algae. Effluent samples were collected from the outlet lagoons of each factory in December, 2009. Then, effluent samples were analyzed for total N, NO3-N, NH4-N, S2-, , , COD, total dissolved solids (TDS) and heavy metals at the federal Environmental Protection Agency (EPA) Laboratory, Addis Ababa. The experiments on the impact of effluents on survival of fish larvae and growth of algae were conducted using six concrete paved ponds (with different concentration of effluent). The results of chemical analysis showed that textile effluent had high COD (nearly 3 times higher), TDS (19 times higher) as well as (39 times higher) than the maximum permissible limits (MPL) set by Environmental Protection Authority (EPA). Among heavy metals, Zn and Fe of textile effluent were much higher (41 and 1.5 times higher, respectively) than the MPL set by EPA. Ceramic effluent also contained high concentration of (24.5 times), S2-, (2 times) and Zn (14 times) exceeding the limit of EPA. Regarding soft drink factory, high values of COD, and Zn were found in the effluent than the limit specified by EPA. The biological treatment lagoons of the respective source were not effective since the effluents were taken from the last treatment lagoon (outlet lagoon) and some of the measured parameters were higher than MPL. Results of the pond experiment showed that 5%, 10% and 20% concentration levels of the HTF effluent killed significantly high proportion of the fry (65%, 86.8% and 88.7%, respectively). In contrast, fry mortality in ceramic and soft drink effluent treatments, even at 10 and 20% concentration levels, stayed nearly below 10%, which were not far off from the mortality of the control groups. Algal biomass in the treated ponds varied with the type of effluent treatment and concentration level. Compared with the first day, chlorophyll “a” concentration measured on the 7th day had increased by 51%, 48%, 74%, 27% and 31% at 0%, 1%, 5%, 10% and 20% concentrations, respectively of textile effluent. The 5% treatment level of textile effluent caused the highest rate of algal growth above the growth rate observed in the control pond, but further increased in concentration to 10% and 20% resulted in a lower growth of algae than in the control pond. Therefore, textile effluent may boost up algae growth at lower concentrations (~5%) but at higher concentration levels its toxic effect may become inhibitory. Regarding the effluent of soft drink factory, algal growth progressively diminished with increasing concentration of the effluent. The overall change (between initial and day 7) in chlorophyll “a” concentration was highest in the control pond (36.2%) and decreased with increasing effluent concentration (1% to 20%) from 21.7% to -9.4%. On the other hand, algal growth under ceramic effluent treatment was comparable with that observed in the control pond.