Last modified: 2019-02-19
Abstract
Abstract
Background: Industrial wastewater is High-strength wastewater which is 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 like tannery that adversely affects the health of downstream water consumers. Therefore, developing low-cost and ecofriendly wastewater treatment technology using locally available material for the removal of hazardous pollutants is critical.
Objective: The general objective of this study is to investigate locally available, low-cost and efficient substrate for constructed wetland to remove hazardous pollutants from high-strength wastewater in ecofriendly manner.
Methods: This study was conducted at Modjo town from September 2016 to March 2018. Plug flow experimental study design was carried out. The substrate (Pumice) was collected from the study area and chemical characteristics were determined using X-ray fluorescence analysis. Chrysopogon zizanioides was planted and grown for five months before running high-strength 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 (HRT) using American public health association standard methods. Heavy metal uptake capacity of the plant and adsorption potential of the pumice 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: Characterization of the untreated tannery wastewater revealed that the mean concentration of BOD5, COD, TSS, PO4-P, TP, NO3-N, TN, sulfide, sulfate 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, 241.33 ±101.2, 1072.82 ±352.7and 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%), Sulfide at HRT 7 and 9 days (99.59%, 99.76), sulfate at HRT 5 and 7 days (94.07%, 94.72%) 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 ˂ 0.05.
Conclusion and Recommendation: Pumice substrate and Chrysopogon zizanioides have a potential ability to remove hazardous pollutants from industrial wastewater in horizontal subsurface constructed wetlands.
Key Words: High-strength wastewater, Pumice, Constructed wetland, Chrysopogon zizanioides
Acronyms: BOD5: 5 days Biochemical Oxygen Demand; COD: Chemical Oxygen Demand; TN: Total Nitrogen; TP: Total Phosphorus.