Efficiency of a horizontal subsurface flow constructed wetland treatment system for the removal of dissolved pollutants of the wastewater of the benefit of coffee

Abstract

The efficiency of a horizontal subsurface flow constructed wetland planted with the macrophyte Phragmites australis was evaluated for the purification of wastewater from wet coffee processing, WWWCP, measured as total suspended solids (TSS), chemical oxygen demand (COD), nitrites (NO₂^-) and phosphates (PO₄^3-). A descriptive, exploratory and graphic statistical analysis of the response variables was carried out by implementing the IBM SPSS statistical package and Microsoft Excel 2016 and, subsequently, the non-parametric Wilcoxon hypothesis test was applied to the medians of the distributions of the output concentrations. The results showed that the distributions of removal percentages were asymmetric to the left with a predominance of high removal efficiencies: TSS, COD, nitrites and phosphates had median removal percentages of 80, 77, 84, 75, and 85%, respectively. Additionally, the Wilcoxon test when evaluating the null hypotheses raised for the distributions of output concentrations of COD and nutrients showed p levels higher than the reliability level (α) of 5%, which indicated that the system must increase its stability and efficiency to comply with the corresponding Colombian guide values. Regarding the TSS and pH, the test showed p levels lower than α, indicating that the wetland efficiently removes the first compounds through physical mechanisms, behaving as a buffered system, by increasing the pH by 2 units through the contact of the residual water with the filtering material and the interaction between gravel and biofilm that produces carbon dioxide.