Technology, Performance and Design Evaluation of Tannery Wastewater Treatment Plants in the vicinity of Addis Ababa, Ethiopia (The Case of China Africa Tannery)
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Date
2014-12
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Addis Ababa University
Abstract
Industrial water pollution is among the major environmental problems in and around Addis Ababa, Ethiopia while leather-tanneries are among the heavy polluters. Effluent treatment plants already exist in some tanneries with huge concern on their performance. The aim of the study was to evaluate the technology, design and treatment efficiency of tannery wastewater treatment plants in Addis Ababa area based on a case study. Morning hours (10-12 A.M) wastewater sampling for a maximum of five days and then laboratory analysis were conducted at five selected sites in the treatment line for BOD5, TSS, TDS, NH3-N and pH in both the dry (end of January-February, 2014) and wet (end of June-July, 2014) seasons while COD, chromium, sulfide and R-PO4 for the dry season and total phosphorous for the wet season were also analyzed. Dissolved oxygen (in the AS aeration basin) and temperature were also measured on-site in both seasons while other additional data were also collected using questionnaire/checklist. Pollutant removal efficiencies were compared with the actual plant design efficiencies, international benchmark efficiencies as well as other similar studies whereas the final effluent quality was compared with national and international effluent release standards to surface waters. The day-to-day and seasonal treatment efficiency variations were also assessed. Technology evaluation for secondary treatment was conducted based on multi-criteria analysis technique (MCA) through stakeholders’ participation. The collected data were analyzed by Design Expert Software Version 7.0.0 and MS-Excel 2007. For the overall treatment plant, the dry season mean daily removal efficiencies were 56.3%, 74.9%, 89.5% and 99.7% for BOD5, COD, TSS and sulfide respectively while the figures for the wet season were 37.4% for BOD5, 80.5% for TSS and 28.3% for total phosphorous. NH3-N and TDS were significantly enhanced rather than reduced in both seasons. No statistically significant day-to-day (ANOVA; p > 0.05) efficiency variations were observed. Nor was there statistically significant seasonal efficiency variation (t-test; p > 0.05) except for the primary-secondary combined treatment in the removal of NH3-N and TSS. The mean daily final effluent quality was well within the national standard for BOD5, COD, sulfide, total phosphorous, pH and temperature while the standard was significantly exceeded for TSS, NH3-N and total nitrogen. The treatment plant was well designed and flexible for the liquid treatment while it was by far deficient for the sludge treatment. WSPs followed by CWs are better than AS and UASBs for secondary treatment of tannery effluent in Ethiopia though the choice depends also on specific local conditions. Efficiency determining factors such as cleaner production are inadequately addressed. To conclude, the treatment plant is well functional, reasonably efficient and is achieving national compliance requirements of major parameters while it has huge limitations in TSS, nitrogen and TDS removal coupled with very poor sludge management.
Key word: Ethiopia, tannery wastewater, treatment technology, design, treatment efficiency, efficiency determining factors, seasonal variation,
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Keywords
Ethiopia, Tannery wastewater, Treatment technology, Design, Treatment efficiency, Efficiency determining factors, Seasonal variation, Day-to-day variation