Abstract

Background: The discharge of untreated tannery wastewater causes serious public and environmental health problems. Wastewater discharged to water body without treatment from different industries like tannery adversely affect the aquatic biota and downstream water users. To come up with the best efficiency of wastewater treatment using constructed wetland, selecting best efficient substrates is essential. Hence, in the present study, the combined adsorption potential of the two substrates (Pumice and Scoria) was evaluated in both planted and unplanted constructed wetland cells.

Objective: The general objective of this study is to design and investigate the efficiency of mixed substrate to remove pollutants from tannery wastewater in subsurface flow constructed wetland.

Methods: This study was conducted at Modjo tannery, Modjo from 2016 to 2019. The substrates were collected around Modjo town and their chemical characteristics were determined. Composite samples before and after treatment of four different hydraulic retention time (HRT) were collected. All the physiochemical analysis has been carried out according to the standard methods for the examination of water and wastewater. R statistical software and OriginPro 2017 were used for statistical analysis. Vetiveria zizanioides was planted and grown for five months before running industrial wastewater for the treatment. The composite wastewater was introduced to the treatment plant from Modjo leather industry, Ethiopia.

Result: Based on this investigation, characterization of the untreated tannery wastewater revealed that the mean concentration of BOD_5, COD, TSS, TDS, orthophosphate, TP, ammonium, nitrate, TN, sulfide, sulfate, chloride, and total chromium were 1641±373.6, 6953.33±339.4, 1868±863.1, 5877.3±2294.8, 88.06 ±40.8, 144.53 ±20.8, 253.33±11.6, 116.66 ±26.6, 650.33 ±93.6, 241.33 ±101.2, 1072.82 ±352.7, 1919±1271.6 and 18.33±6.7 mg/l respectively beyond the permissible limits. The maximum removal efficiency of the treatment plant revealed that BOD₅ at HRT 7 days (96.38%), COD at HRT 7 days (98.14%), NO₃-N at HRT 7 days (99.76%), TN (95.80%), PO₄-P HRT 9 days (99.9%), TP at HRT 7 days (95.00%), Sulfide at HRT 7 days (99.9%), sulfate at HRT 9 days (91.81%), chloride at HRT 9 days (80.16%) and total Chromium at HRT 7days (99.45%) respectively. The removal efficiency of treatment plant used as a control subject with the commonly used gravel substrate showed lower performance. By considering removal of selected wastewater pollutant parameters within the four hydraulic retention time, the efficiency between the study and control bed was tested, the result showed that significantly difference at 95% confidence interval, p-value = 0.001. Hence, the mixed substrate performs better than the gravel one in which it can be useful for the treatment of tannery wastewater in horizontal subsurface constructed wetland.

Conclusion and Recommendation: The identified substrates with Vetiveria zizanioides have a good potential ability to remove the major pollutants including heavy metals from tannery wastewater. Further investigation on the efficiency of this substrate with other plant types is recommended.

Key Words: wastewater, Pumice, scoria,substrate, Constructed wetland, Vetiveria zizanioides