Treatment Performance Assessment of Natural and Constructed Wetlands on Wastewater From Kege Wet Coffee Processing Plant in Dale Woreda, Sidama Regional State, Ethiopia
Authors: Berego Yohannes Seifu , Sota Solomon Sorsa , Ulsido Mihret Dananto , Beyene Embialle Mengistie 
Published: December 2022

Constructed wetlands are engineered systems built to use natural processes and remove pollutants from contaminated water in a more controlled environment. The research was an experimental research carried out to assess the effectiveness of natural and constructed wetland systems in the treatment of coffee wastewater. The 2 vertical flow constructed wetland was built. The first wetland covered an area of 132 m2. It has 12 m width and 11 m length. Open space is constructed between 2 constructed wetlands with a dimension of 11 m × 3 m × 1 m. The second wetland was constructed and its function is similar to the first one, from this wetland water is discharged to the river. The construction of the wetland is accomplished by constructing 20 cm wide furrows with a spacing of 30 cm. Vetiver grasses have planted with a spacing of 20 cm intervals. The physicochemical data were recorded, organized, and analyzed using R software (version 4.1) and Microsoft Excel. Data were processed using parametric (one-way ANOVA) and nonparametric (Mann-Whitney’s U test) statistical tests of homogeneity. One-way analysis of Variance (ANOVA) was used to determine the significance of differences in variations in physicochemical variables within the constructed wetland sites. Tukey’s multiple comparisons for differences between means were also assessed. Findings indicated that a natural wetland had a mean influent and effluent of total suspended solids (TSS) of 2190.78 ± 448.46 mg/l and 972.67 ± 234.312 mg/l, respectively. A Mann-Whitney U test revealed that TSS were significantly higher in natural wetland (median = 1551.50) compared to constructed wetland (median = 922.5), U = 676.5, z = −2.435, P = .015, r = .257. Natural wetlands had a mean influent of biological oxygen demand (BOD) was 4277.94 ± 157.02 mg/l, while in the effluent of BOD it was 326.83 ± 112.24 mg/l. While in constructed wetland it was 4192.4 ± 191.3 mg/l, 782.72 ± 507.6 mg/l, and 88.28 ± 20.08 mg/l in influent, middle, and effluent respectively. Average chemical oxygen demand (COD) value at influent in natural wetlands was 8085.61 ± 536.99 mg/l and in the effluent it was 675.33 ± 201.4 mg/l. In constructed wetland, it was found to be 8409.8 ± 592.9, 1372.6 ± 387.94, and 249.0 ± 7.68 for influent, middle, and effluent respectively. Comparatively, the purification efficiency of organic pollutants (TSS, BOD, and COD) of constructed wetlands was better than natural wetlands, whereas natural wetlands had better purification efficiency of nitrogen compounds such as ammonium, nitrite, and nitrate. On average, removal rates for nitrogen compounds were 39.53% and −24.41% for ammonium, 79.44% and 55.4% for nitrite, and 68.90% and 60.6% for nitrate in natural and constructed wetlands respectively, while the phosphate removal rate was 43.17% and 58.7% in natural and constructed wetlands, respectively. A Mann-Whitney U test revealed that there is no significance difference in nitrite, nitrate, ammonium, and phosphate concentration between natural and constructed wetlands( P > .05). Based on these results, both systems of treatment were effective in treating the coffee effluent since most of the values obtained were below the permissible EEPA limits. Even though the constructed wetland treatment plant performed better overall, in comparison, the natural wetlands had better purification efficiency for nitrogen compounds like ammonium, nitrite, and nitrate and the constructed wetlands had better purification efficiency for organic pollutants (TSS, BOD, and COD).