Browsing by Author "Zillur Rahman, Khaja"

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Open Access
Treatment of arsenic containing artificial wastewater in different laboratory-scale constructed wetlands
(2009) Zillur Rahman, Khaja; Rott, Ulrich (Prof. Dr.-Ing.)
Knowledge regarding dynamics of As-species and their interactions under gradient redox conditions in the rhizosphere of treatment wetlands is still insufficient. Therefore, it is necessary to understand the fundamental processes and mechanisms operative in treatment wetlands to realise long-term stability and high As-removal efficiencies, both in the presence and absence of wetland plants. In the past, little attention has been paid to the biotransformation processes and metabolism of As in constructed wetlands. Hence, the aim of this investigation was to gain more information on the biotransformation of As and the dynamics of As- species in predominantly multi-gradient (both micro- and macro-) horizontal subsurface-flow constructed wetlands. Experiments were carried out in laboratory-scale wetland systems, two planted with Juncus effusus and one unplanted, using an artificial wastewater containing As (0.2 mg l−1) under defined conditions of organic C- and SO42−-loading. Immobilization of As was found in all systems (both planted and unplanted) under conditions of limited C, obviously due to adsorption and/or co-precipitation. Removal efficiencies were substantially higher in the planted systems (60-70%) as compared to the unplanted system (37% on average). Immobilization under such conditions appeared to decrease over time in all systems. At the beginning, the dosage of organic carbon (COD~340 mg l−1; SO42− ~75 mg l−1) immediately caused intensive microbial dissimilatory sulphate reduction in all systems (in the range of 85-95%) and highly efficient removal of total arsenic (81-96% on average), most likely as As2S3 precipitation. A significant amount of reduced As [As(III)] was found in the effluent of the planted systems (>75% of total As) during this period of efficient microbial sulphate reduction, compared to the unplanted system (>25% of total As). Later on in this operation period, the intensity of sulphate reduction and simultaneous removal of As decreased, particularly in the planted wetlands (ranging from 30-46%). One reason could be the re-oxidation of reduced compounds due to oxygenation of the rhizosphere by the emergent water plants (helophytes). Only traces (2-3 µg As l−1) of DMA were found in the temporal dynamics of the planted wetlands under these conditions. The immobilization of arsenic was found to be more stable in the planted beds than in the unplanted bed in a more oxidised environment with and restricted microbial sulphate reduction (stopping of inflow C-dosage; SO42− ~ 0.2 mg l−1). In principle, both systems (planted and unplanted) were suitable to treat wastewater containing As, particularly under sulphate reducing conditions. The unplanted system seemed to be more efficient regarding the immobilization of As, but planted systems showed a better stability of immobilized As. Total As-mass retention in planted wetlands was substantially higher (nearly 60% of the inflow) than in the unplanted bed (only 43%). In general, a higher level of As was found in the roots than in the shoots in the planted beds. Only 1% of the inflow As-mass was retained in the shoots while more than 55% was sequestered in and/or on the roots and sediments. In conclusion, the obtained results demonstrated that horizontal subsurface-flow constructed wetlands seem to be viable alternatives for effective elimination of elevated As concentration from secondary domestic wastewater effluents prior to disposal to the receiving water bodies (rivers, lakes etc.) or application for agricultural field irrigation purposes. Long-term accumulation of As in the wetland vegetation (mostly in the below-ground biomass) and soil sediments may reduce widespread distribution of As in the environment.