Abstract

Background: wastewater with elevated levels of hazardous pollutants measured in heavy metals, nutrients and other toxic substances is the most important cause for environmental pollution. This kind of wastewater discharged to water body without treatment from different industries like tannery, textile, slaughterhouse, distilleries, breweries and others that adversely affect the aquatic biota and downstream water users. Therefore, identifying locally available adsorbents and potential plant for the removal of those pollutants from wastewater is vital.

Objective: The general objective of this study is to investigate locally available, low-cost and efficient adsorbents as well potential plant to remove pollutants from tannery wastewater.

Methods: This study was conducted at the Modjo town from September 2016 to March 2018. Plug flow experimental study design was carried out. The adsorbent (red-ash) was collected from the study area and chemical characteristics were determined using X-ray fluorescence 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. The physicochemical analysis of the sample wastewater was done before and after treatment at four different hydraulic retention time (HRT) using American public health association (APHA) standard methods. Phytoremediation capacity of the plant and adsorption potential of the adsorbent were determined by Ash-Modified AOAC 923.03 and Minerals-Modified AOAAC 985.35. Mean and standard deviations were calculated for each parameter. Originlab 2017 and R statistical software were run for graphing and data analysis.

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 7and 9 days (97.48%, 97.38%), COD at HRT 5 and 7 days (98.25%, 98.31%), NO₃-N at HRT 5 and 7 days (99.55%, 99.37%), TN (98.15%, 98.21%), PO₄-P HRT 7and 9 days (99.74%,100%), TP at HRT 5 and 7 days (99.86%, 99.81%), Sulfide at both at HRT 5 and 7 days (99.99%), sulfate at HRT 5 and 7 days (89.99%, 91.05%), chloride at HRT 5 and 7 days (90.08%, 89.94) and total Chromium at HRT 7 and 9 days (99.29%, 98.75%) respectively. The removal efficiency of treatment plant used as a control subject with the commonly used substrate showed lower performance. Considering all the 15 wastewater parameters reduction result from all four hydraulic retention time between study and control bed was tested for their significance difference using two sample t-test statistics. Based on the test statistics, the treatment bed with the study adsorbent perform better than the control bed significantly at 95% confidence interval,  p-value = 0.02.

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

Key Words: wastewater, Substrate, Constructed wetland, Vetiveria zizanioides