Quantification of oxygen release by plant roots in constructed wetlands

The present research is an effort to estimate the amount of oxygen contribution by wetland plant (bulrush, Scirpus validus) roots towards wastewater treatment. The research was carried out in the backdrop of conflicting reports about the role wetland plants play in wastewater treatment. The plants\u27 ability to release oxygen through their roots to the wastewater environment was investigated in the field and in the laboratory subsurface flow and vertical flow wetlands. Dissolved oxygen (DO), pH, and oxidation reduction potential (ORP) microenvironments in the Scirpus validus rhizosphere were studied using microelectrodes (3–10 μm tip size). The DO value on the surface of the bulrush lateral roots increased from 0.64 to 2.04 mg L−1 as the bulk BOD5 increased from 24 to 1267 mg L−1. A layer of oxygen was detected around the lateral roots, and its thickness (760–1160 μm) also increased with bulk BOD5. Bulrush was found to have two different types of main roots having different appearance (white and brown) and different radial oxygen loss (ROL) characteristics associated with their lateral roots. The active root lengths (i.e., the length of the root that exhibits ROL) in brown and white laterals were ∼40 μm and ∼1675 μm, respectively. The ROL value was in the range of ∼62 to ∼176 ng O2 cm−2 (of root surface) min −1 for the white lateral roots for various bulk BOD5 concentrations. The ROL value was ∼97 and ∼313 ng O2 cm−2 min−1, respectively, for white and brown lateral roots in primary settled municipal wastewater. The amount of oxygen released by bulrush (Scirpus validus) was estimated to be ∼5.0 to 7.65 mg O2 m−2 (of wetland surface) d−1. Of this amount ∼63% was contributed by the brown roots which made up ∼95% of total roots. An oxygen release of ∼5.0 to 7.65 mg O2 m−2 d −1 by wetland plants may not have any practical implication in wastewater degradation