Effect of Tubifex tubifex on the purification function of saturated vertical flow constructed wetlands for effluents with varying C/N ratios.

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 NOx--N, NH4+-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.

Mutual promotion of submerged macrophytes and biofilms on artificial macrophytes for nitrogen and COD removal improvement in eutrophic water.

Both submerged macrophytes (SMs) and artificial macrophytes (AMs) have been widely used to improve water quality in eutrophic water. However, in heavily eutrophic aquatic ecosystems, the purification function of SMs is often restricted by the poor growth state due to competition from algae, while the purification function of AMs is often restricted by the limited carbon source supply for biofilm microbes attached to the AM surface. The objective of this study was to develop a new strategy to increase pollutant removal efficiency (RE) by combining the use of SMs and AMs. Pilot-scale microcosms, including treatments with both SMs and AMs (S&A), only SMs (SO) and only AMs (AO), were established to identify the performance of the new strategy. The results suggest that treatment S&A obtained REs of 88.9% for total nitrogen (TN) and 48.1% for chemical oxygen demand (COD); as comparison, treatments SO and AO obtained REs of 77.4% and 81.2% for TN and REs of -13.7% and 39.0% for COD, respectively. Compared with SO, the S&A treatment benefited SM growth in biomass, leaf chlorophyll concentration and root activity by inhibiting algae growth. In addition, compared with treatment AO, S&A increased the biofilm microbial biomass and the relative abundance of nitrifiers of families Nitrosomonadaceae and Nitrospira attached to AM surfaces. Therefore, by the mutual promotion of SMs and biofilms on AMs, the synergic application of SMs and AMs is a useful strategy for improving TN and COD REs in eutrophic water bodies such as rivers and constructed wetlands. A strategy was developed to increase nitrogen and COD removal in eutrophic water by the mutual promotion of submerged macrophytes and biofilms on artificial macrophytes.

Effect of influent salinity on the selection of macrophyte species in floating constructed wetlands.

Pilot-scale floating constructed wetlands (FCWs) under varying influent salinities were implemented, and the effects of influent salinity on pollutant removal efficiency (RE) and macrophyte species selection were identified. The results suggest that a salinity increase generally decreased pollutant REs, while some macrophytes, such as Iris pseudacorus, could effectively resist this decrease. The average coefficients of variation between macrophyte species in REs of chemical oxygen demand, ammonium nitrogen, nitrate nitrogen and total phosphorus increased from 28.6% at low salinity to 91.3% at high salinity, which suggests the greater importance of macrophyte selection under high salinity. With an increase in salinity, the rhizosphere bacterial community showed convergent evolution or convergence followed by slight divergent evolution between macrophyte species, while the importance of macrophyte parameter selection in characterizing pollutant REs decreased. Therefore, influent salinity is a key factor to consider in macrophyte selection and application, especially in FCWs without soil.

Accelerated nitrogen consumption in sediment by Tubifex tubifex and its significance in eutrophic sediment remediation.

Sediment remediation in eutrophic aquatic ecosystems is imperative, but effective ecological measures are scarce. A pilot-scale trial investigated sediment remediation by the addition of Tubifex tubifex. The results showed that the addition of T. tubifex accelerated sediment organic matter (OM) and nitrogen (N) loss, with averages of 7.7% and 75.1% increased loss (IL) compared to treatments without T. tubifex in the 60-day experiment, respectively. The percentages of the increased in water to the IL in sediment were only 0.6%, 0.21%, 2.1% and 6.3% for NH4+-N, NOx--N, TN and COD, respectively, at the end of the experiment. The absolute abundances of the nitrifying genes AOA and AOB; the denitrifying genes napA, nirS, nirK, cnorB and nosZ; and the anaerobic ammonia oxidation gene anammox increased 2.3- to 11.0-fold with the addition of T. tubifex. Therefore, the addition of T. tubifex is an effective strategy for sediment remediation by accelerating OM and N loss in sediment without substantially increasing the water N concentration.

The improvement of pollutant removal efficiency in saturated vertical flow constructed wetlands by tubifex tubifex.

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.

Operation strategy for constructed wetlands in dry seasons with insufficient influent wastewater.

Using vertical flow constructed wetlands (VFCWs) with different influent wastewater volumes and feeding modes, this study aimed to identify the optimal operation strategy for dry seasons under wastewater deficiency. Using half the influent wastewater volume (HIWV) did not necessarily improve the removal efficiency (RE) of the chemical oxygen demand (COD), NH4+-N, NO3--N and total nitrogen. In the HIWV treatments, intermittent resting did not result in significantly different pollutant REs, whereas strategies involving partial saturation and prolongation of the hydraulic retention time (HRT) slightly decreased the pollutant REs compared with those obtained in the constant feeding mode. Of the three HIWV strategies, the intermittent resting mode achieved the highest anaerobic ammoxidation, the dominant pathway for nitrogen removal in the systems, and thus stimulated nitrogen transformation. The intermittent resting mode forms part of the recommended operation strategy for VFCWs in dry seasons with wastewater deficiency.

Enhanced organic matter decomposition in sediment by Tubifex tubifex and its pathway.

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.

The effect of re-startup strategies on the recovery of constructed wetlands after long-term resting operation.

The objective of this study was to identify the proper re-startup strategies (RSSs) for constructed wetlands (CWs) after long-term resting operation in terms of the recovery of pollutant removal efficiency (RE) and N-cycle gene abundance. The results suggested that backwashing increased the gene abundance without shortening the recovery time of gene abundance. The RSS involving excavation and washing performed better in terms of chemical oxygen demand (COD) RE, especially at the beginning, and performed slightly better or similarly in terms of N-cycle gene abundance and the REs of ammonia nitrogen (NH4+-N) and total nitrogen (TN). The abundance of the Amox gene was 66.1-92.8, 76.3-161.8 and 1550-2492 times larger than that of the napA, narG and amoA genes, respectively, and the anammox process was the dominant N removal pathway. Therefore, excavation and washing is recommended as the RSS for CWs with a long-term rest period.