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The improvement effect of Tubifex tubifex on the pollutant removal efficiencies (REs) of vertical flow constructed wetlands (VF-CWs) treating wastewater with various C/N ratios was explored. The experiment was conducted in pilot-scale saturated VF-CWs, being added different densities of T. tubifex and fed synthetic wastewater with successive C/N ratios of 0.5, 1.5, 3.0 and 6.0. The results suggest that T. tubifex addition and the influent C/N ratio had an interactive effect, i.e., T. tubifex addition improved NO x −-N, NH 4 +-N, TN and COD REs by 36.7%, 56.5%, 22.6%, and 10.0%, respectively, under low C/N ratios, while high C/N ratios inhibited this improvement. Low-density T. tubifex addition significantly increased substrate dissolved oxygen (DO) by retarding excessive soil organic matter (OM) accumulation. With T. tubifex addition, an improvement in bacterial diversity, the relative abundance of N-cycle and fermentative bacteria, and N-cycle functional genes was only observed in substrates under low C/N ratios. T. tubifex can improve the purification function of saturated VF-CWs, but this strategy strongly depends on both the influent C/N ratio and density of T. tubifex addition. [Display omitted] • The function of T. tubifex in saturated VF-CW was influenced by C/N ratio. • T. tubifex enhanced substrate DO by avoiding excessive OM accumulation in soil. • Extensive DO consumption at high C/N ratios inhibited T. tubifex function. • Low-density T. tubifex was less sensitive to influent C/N ratio. • T. tubifex improved saturated VF-CW performance for effluents with low C/N ratios. [ABSTRACT FROM AUTHOR]

• T. tubifex enhanced saturated VF-CWs nitrogen and COD removal efficiency. • There existed significant interactions between macrophytes and T. tubifex. • The connectivity and DO were improved by T. tubifex in the saturated VF-CWs. • Ammonia oxidation and anammox were the main pathways for increased N removal. • T. tubifex is a useful tool for increasing pollutant removal in saturated VF-CWs. Pilot-scale saturated vertical flow constructed wetlands (VF-CWs) were established to identify whether T. tubifex has the similar performance in saturated VF-CWs to that in surface flow CWs in improving pollutant removal efficiency (RE). The saturated VF-CWs with T. tubifex achieved REs of 67.3% total nitrogen (TN) and 39.8% chemical oxygen demand (COD), which were significantly higher than treatments without T. tubifex (42.2% TN and 31.4% COD). There existed significant interactions between macrophytes and T. tubifex. T. tubifex greatly improved the dissolved oxygen by increasing the connectivity between layers, and enhanced dehydrogenase activity and fluorescein diacetate. Adding T. tubifex improved the bacterial diversity and relative abundance of both N-cycle bacteria and fermentation bacteria in the biofilms. The improvements of ammonia oxidation and anammox were the main pathways for the increased nitrogen removal by T. tubifex. Therefore, T. tubifex is a useful tool for improving pollutant REs in saturated VF-CWs. [ABSTRACT FROM AUTHOR]

The role of Tubifex tubifex in organic matter (OM) decomposition in aquatic ecosystems has been widely studied, but considerable uncertainties exist in terms of the effect mechanism. The effect of T. tubifex on sediment OM decomposition in laboratory-scale microcosms was quantified, and possible pathways were identified. In the first 7 days of the decomposition of OM mixed in sediment, no significant effect of T. tubifex on organic matter loss (OML) was observed for both low- and high-OM treatments; meanwhile, from day 7–60, T. tubifex addition significantly improved OML from 55.0%-57.5% to 71.8%–77.7% in the low-OM treatments and from 55.5%-56.6% to 64.1%–68.7% in the high-OM treatments. The enhanced OML observed with T. tubifex was mainly due to the promoted decomposition of refractory organic components, e.g., cellulose, hemicellulose and lignin. The proportion of refractory components in the gut of T. tubifex was significantly lower than that in the sediments (p < 0.01), indicating a pathway corresponding to the ingestion and digestion of refractory components by T. tubifex. Although T. tubifex reduced the water dissolved oxygen (DO) by increasing the water chemical oxygen demand (COD), the oxygen supply was improved by T. tubifex, and this could be affected by the increase in the relative abundance of aerobic to anaerobic bacteria in the sediments. T. tubifex significantly increased the diversity of the bacterial and fungal communities in the sediments. Moreover, the community structure of bacteria and fungi was substantially different between gut and sediment. Therefore, multiple pathways of the effect of T. tubifex on OM decomposition were established, and the results have great significance for the artificial manipulation of OM circulation using T. tubifex and the restoration of damaged aquatic ecosystems. Image 1 • T. tubifex promoted OM decomposition mainly by increasing refractory component loss. • T. tubifex, with special microbial community in gut, digested refractory component. • T. tubifex promoted sediment bacteria and fungi diversity. • Although water DO decreased, T. tubifex improved sediment oxic-microenvironments. • Multiple pathways of the effect of T. tubifex on OM decomposition were established. [ABSTRACT FROM AUTHOR]