Abstract
BackgroundHigh-strength wastewater defined by elevated levels of hazardous pollutants measured in BOD, heavy metals, nutrients and other toxic substances. This kind of wastewater discharged to water body without treatment from different industrial sectors that adversely affects aquatic environment and downstream water consumers. The general objective of this study is to investigate efficient substrate with selected plant type for constructed wetland to remove hazardous pollutants from tannery wastewater. This study was conducted at Modjo town, Modjo tannery PLC. Plug flow experimental study design was carried out. The substrate (Pumice) was collected around the study area and chemical characteristics were determined. Chrysopogon zizanioides was planted and grown for 5 months before running tannery wastewater for the treatment. The composite wastewater was introduced to the constructed wetland from Modjo leather industry, Ethiopia. The physicochemical analysis of the sample wastewater was done before and after treatment at four different hydraulic retention time.ResultsCharacterization of the untreated tannery wastewater revealed that the mean concentration of BOD5, COD, TSS, PO4-P, TP, NO3-N, TN and total chromium were 1641 ± 373.6, 6953.33 ± 339.4, 1868 ± 863.1, 88.06 ± 40.8, 144.53 ± 20.8, 116.66 ± 26.6, 650.33 ± 93.6 and 18.33 ± 6.7 mg/l respectively beyond the permissible limits. The maximum removal efficiency of the constructed wetland in pumice bed revealed that BOD5 at HRT 7and 9 days (96.42%, 96.30%), COD at HRT 5 and 7 days (96.76%, 96.91%), NO3-N at HRT 5 and 7 days (99.99%, 99.68%), TN (98.67%, 99.00%), PO4-P HRT 7and 9 days (96.97%,100%), TP at HRT 5 and 7 days (94.79%, 96.17%) and total Chromium at HRT 5 and 7 days (98.36%, 98.91%) respectively. Whereas, the removal efficiency of constructed wetland bed with gravel substrate used as a control subject with similar condition to pumice showed lower performance. The result between pumice and gravel bed was tested for their significance difference using two sample t-test statistics. Based on the test statistics, the pumice substrate perform better than the gravel significantly at 95% confidence interval, p-value = 0.01.ConclusionPumice substrate and Chrysopogon zizanioides have a potential ability to remove hazardous pollutants from tannery wastewater in horizontal subsurface constructed wetlands.
Highlights
High-strength wastewater defined by elevated levels of hazardous pollutants measured in Biochemical Oxygen demand (BOD), heavy metals, nutrients and other toxic substances
This study revealed that the mean concentration of BOD5, Chemical Oxygen Demand (COD) and Total Suspended Solid (TSS) were 1641 ± 375.6, 6953 ± 339.4 and 1868 ± 863.1 mg/l respectively (Table 1)
This result is basically similar to different studies in Ethiopia with slight difference for different parameters for example a study done at the same tannery industry indicated that the mean concentration of COD was laid between 7950 and 15,240 mg/l with the mean of 11,123 ± 563.9 mg/l (Seyoum Leta et al 2003)
Summary
High-strength wastewater defined by elevated levels of hazardous pollutants measured in BOD, heavy metals, nutrients and other toxic substances. This kind of wastewater discharged to water body without treatment from different industrial sectors that adversely affects aquatic environment and downstream water consumers. The general objective of this study is to investigate efficient substrate with selected plant type for constructed wetland to remove hazardous pollutants from tannery wastewater. The discharge of high strength wastewaters with high concentration of nutrients and heavy metals from industrial sectors. Tanneries are typically characterized as pollution intensive industrial complexes which generate widely varying, high-strength wastewaters. 30 m3 of wastewater is generated during processing of one tone of raw skin/hide (Suthanthararajan et al 2004)
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