Chromium removal efficiency of plant, microbe and media in experimental VSSF constructed wetlands under monocropped and co-cropped conditions

Abstract

Chromium (Cr), one of the most abundant and hazardous heavy metals, is generally observed to be widely distributed in environment, primarily due to the inter-mixing of the untreated domestic and industrial wastewaters. There has been an increased interest to replace conventional centralized treatment technologies with the low energy, low cost, and zero sludge producing decentralized constructed wetland technology. Therefore, a long-term investigation on the comparative metal removal efficiency of the experimental vertical sub-surface flow (VSSF) constructed wetland systems, irrigated with Cr-spiked ground waters, under both mono and mixed-culture conditions planted with five different macrophytes viz. Typha (T), Phragmites (P), Acorus (V), Arundo (A), and Vetiver (K), in as mono- and [viz. (TP), (PA), (KV), (AT), and (VT)] as co-cropped combinations along with unplanted (U) systems as controls was conducted at the ICAR-Indian Agricultural Research Institute, New Delhi, India. Long-term investigations revealed significant differences between metal removal efficiencies of the planted (61.6% to 78.5%) and the unplanted systems (32.8% to 47.9%). However, these long-term average metal removal efficiencies were found to be insignificantly different for the mono (78.5%) and the co-cropped systems (77.6%). On further compartmentalization of the experimental wetland system’s Cr-removal efficiencies amongst the major components viz. plant, microbe, and substrate, it was observed that vegetation contributed the maximum (i.e., 33– 48%) while the microbes and the substrate contributed only 4–20% and 8–28%, respectively. It was further observed that due to reduced microbial diversity under unplanted conditions, the planted systems were associated with 2–7% higher microbial and equivalently lower substrate removal efficiencies. Thus, microbial activity-mediated metal mobilization and plant uptake were observed to be the principal processes governing Cr removal in the test VSSF constructed wetland systems exposed to varying Cr concentrations. Amongst all test macrophytes and their combinations, Arundo (81.9%) and Acorus (84.5%) based monocropped systems and Arundo+Typha (89.3%) based co-cropped systems emerged to be the most superior Cr-removing systems.