Developing Horizontal Subsurface Flow Constructed Wetland using Pumice and Chrysopogon zizanioides for Tannery Wastewater Treatment

Abstract

Background: 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. This study was conducted at Modjo town from 2016 to 2018. 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 five 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.Results: Characterization 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.Conclusion: Pumice substrate and Chrysopogon zizanioides have a potential ability to remove hazardous pollutants from tannery wastewater in horizontal subsurface constructed wetlands.