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Mechanisms and factors regulating organic carbon removal in surface flow constructed wetlands receiving woodwaste leachate Tao, Wendong

Abstract

An amber leachate was generated by rainfall on an uncovered woodwaste pile beside Fraser River in Mission, BC, Canada. The "young" leachate in the pile's placement period was acidic (pH 3.4-3.7) and nutrient-poor, with high concentrations of chemical oxygen demand (COD 12559-14254 mg L⁻¹), tannin and lignin (T&L 3066-5150 mg L⁻¹) and volatile fatty acids (VFAs 1564-2132 mg L⁻¹). The leachate at 1.5 years into the closure period became less acidic, ammonia-rich, more refractory, and lower in organic strength. More than 98% of leachate constituents were soluble. Laboratory tests demonstrated insignificant volatilization, limited effect of sediment adsorption, and the major role of biological degradation for organic carbon removal from woodwaste leachate in surface flow constructed wetlands (CWs). The activity of the microbial community was investigated by measuring ³H-leucine incorporation into bacterial protein, ¹⁴C-glucose turnover, and bacterial assimilation and mineralization of ¹⁴C-acetate in pilot-scale CWs receiving the young leachate. The effects of hydraulic retention time (HRT), influent strength and mass loading rate on treatment performance, microbial biomass and heterotrophic activities were examined in mesocosm wetlands receiving the "aging" leachate. There were insignificant longitudinal variations in heterotrophic activities and insignificant vertical variation in biomass. It could take up to 6 weeks for microbial maturation. A HRT value of 5 d was enough to avoid adverse effects on bacterial communities. When a VFAs-rich influent was fed, COD and T&L reduction efficiencies increased significantly with HRT. When a refractory influent of aged leachate was fed, reduction efficiencies increased slightly with HRT. Heterotrophic activities were regulated by the availability of organic substrates, electron acceptors and inorganic nutrients, which varied with influent strength and HRT. Nanoflagellate grazing controlled a proportion of active bacteria. The influents with up to 40% of the organic strength of young leachate did not inhibit the acclimatized microorganisms. Vegetation made little or no difference in performance. Reduction rates and rate constants were correlated to the heterotrophic activities of planktonic, epiphytic and sedimentary bacteria. Bacterioplankton and sedimentary bacteria usually contributed 77-99% to the total heterotrophic activities. The relative importance of sedimentary bacteria increased under substrate-limiting conditions, while bacterioplankton was favored by VF As-rich influents.

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