Evaluation of selected wetland plants for removal of chromium from tannery wastewater in constructed wetlands, Ethiopia

Abstract

Wastewater from leather processing industries is very complex and leads to water pollution if discharged untreated, especially due to its high organic loading and chromium content. In this study, the phytoremediation efficiency of selected wetland plant species in subsurface flow (SSF) constructed wetlands receiving tannery wastewater was investigated. Four pilot units were vegetated with Cyprus alternifolius, Typha domingensis, Parawaldeckia karaka and Borassus aethiopum and a fifth unit was left as unvegetated (control). The treatment performance of the systems for total Cr, chemical oxygen demand (COD), biochemical oxygen demand (BOD) and nitrogen under a 5 day hydraulic retention time were analyzed based on HACH manual. The Cr in the plant tissue was analyzed through oven dried milled, weighed, digested and analyzed using atomic absorption spectrophotometer (AAS.) The wastewater analysis showed that Cr in the effluent was reduced up to 99.3% for an inlet average Cr loading rate of 40 mg/L, COD was reduced up to 80% for an inlet organic loading varying between 2202 and 8100 mg/L and BOD5 was reduced up to 77% for an inlet organic loading varying between 650 and 1950 mg/L. NO3 and NH3-N removal achieved 57 and 82%, respectively. Roots accumulate significantly higher Cr in all plant species when compared with shoots. B. aethiopium and P. karaka shows higher Cr translocation factor than the others. Constructed wetlands are cost effective and environmentally friendly treatment methods in tropical climate hence can be used as an alternative treatment method in developing countries.

 Key words: Species in subsurface flow (SSF) constructed wetland, tannery wastewater, chromium uptake, chromium translocation, bioaccumulation factor.