Abstract
A culture-independent approach was used to elucidate the microbial diversity and structure in the anaerobic-aerobic reactors integrated with a constructed wetland for the treatment of tannery wastewater in Modjo town, Ethiopia. The system has been running with removal efficiencies ranging from 94%–96% for COD, 91%–100% for SO42- and S2-, 92%–94% for BOD, 56%–82% for total Nitrogen and 2%–90% for NH3-N. 16S rRNA gene clone libraries were constructed and microbial community assemblies were determined by analysis of a total of 801 unique clone sequences from all the sites. Operational Taxonomic Unit (OTU) - based analysis of the sequences revealed highly diverse communities in each of the reactors and the constructed wetland. A total of 32 phylotypes were identified with the dominant members affiliated to Clostridia (33%), Betaproteobacteria (10%), Bacteroidia (10%), Deltaproteobacteria (9%) and Gammaproteobacteria (6%). Sequences affiliated to the class Clostridia were the most abundant across all sites. The 801 sequences were assigned to 255 OTUs, of which 3 OTUs were shared among the clone libraries from all sites. The shared OTUs comprised 80 sequences belonging to Clostridiales Family XIII Incertae Sedis, Bacteroidetes and unclassified bacterial group. Significantly different communities were harbored by the anaerobic, aerobic and rhizosphere sites of the constructed wetland. Numerous representative genera of the dominant bacterial classes obtained from the different sample sites of the integrated system have been implicated in the removal of various carbon- containing pollutants of natural and synthetic origins. To our knowledge, this is the first report of microbial community structure in tannery wastewater treatment plant from Ethiopia.
Highlights
The leather industry is the second largest economic sector that contributes to foreign exchange earnings in Ethiopia [1]
Performance of the integrated biological tannery wastewater treatment plant The untreated tannery wastewater channelled into the integrated system was characterized by its high concentration of Biological Oxygen Demands (BOD), Chemical Oxygen Demands (COD) and Sulphate ranging from 4551–5201 mg/l, 11180–13770 mg/l and 200–1600 mg/l, respectively
The effluent parameters obtained for the BOD, COD, sulphate (SO42-), sulphide (S2-), nitrate-nitrogen (NO3-N) and ammonia –nitrogen (NH3-N) were in line with the provisional emission limit values set for tannery effluents in Ethiopia (EPA 2003)
Summary
The leather industry is the second largest economic sector that contributes to foreign exchange earnings in Ethiopia [1]. 26 tanning industries are present in the country producing semi-finished and finished hides and skins [2]. Due to the complex nature and excessive levels of the pollutants, treatment of tannery wastewater has become an important issue for pollution control in leather producing countries [5, 6]. Untreated tannery effluents can cause severe environmental pollution affecting surface and underground water resources [7, 4], human habitats and living systems [6]. Very few of the existing tanning industries in Ethiopia have treatment plants that enables them meet the requirements for effluent quality standards [8, 9]. Surface and underground water pollution has become a major problem that needs to be addressed by establishing cost- effective wastewater treatment options
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