Effects of cattail biomass on sulfate removal and carbon sources competition in subsurface-flow constructed wetlands treating secondary effluent

Weakening of sulfate removal by aquatic plants in iron-based constructed wetlands: The rhizosphere is a sink or source of sulfur?

The fate of sulfur (S) was controlled by a complex interaction of abiotic and microbial reactions in constructed wetlands (CWs). Although zero-valent iron (ZVI) was generally considered to promote nitrogen (N) and S cycle by providing electrons, but its binding effect on sulfate (SO42--S) removal with the rhizosphere oscillating redox conditions had not been determined. This study found that the presence of plants increased SO42-_S removal in Con-CW, while decreased it by 3.93 % in ZVI-CW accompanied by the decrease of S content in the rhizosphere substrates. The enrichment of S oxidation genes (soxA/Y and yedZ), organic S decomposition genes (aslA) and plants radial oxygen loss (ROL) accelerated the transformation of solid-phase S to SO42--S, resulting in ZVI-CW turn from S sink to S source. Overall, the source-sink transformation provided a theoretical guidance for comprehending S cycling in CWs.

Revealing sulfur-iron coupling mechanism for enhanced autotrophic denitrification in ecological floating beds

A sulfur-iron coupled ecological floating bed (EFB-SFe) was developed to enhance the denitrification capability of sulfur-based ecological floating beds (EFB-S). The denitrification performance, kinetic process and microbial community composition were explored. Results showed that sulfur-iron coupling effectively enhanced the denitrification performance of EFB, surpassing the sum of their individual effects. The average total nitrogen removal rate ranged from 1.56 to 4.56 g·m-2·d-1, with a removal efficiency of 22-84 %. The k value for the S + Fe group increased from 0.04 to 0.18 d-1 to 0.40-0.46 d-1 relative to the S group. The sulfur-iron coupling promoted the enrichment of denitrifying bacteria (Thiobacillus and Ferritrophicum). The denitrification genes in EFB-SFe were upregulated, being 12-22 times more abundant than in EFB-S. Sulfur and iron autotrophic denitrification were identified as the main nitrogen removal processes in EFB-SFe. Overall, sulfur-iron coupling showed the potential to enhance the denitrification capacity of EFB-S for treating low-pollution water.

Henna plant biomass enhanced azo dye removal: Operating performance, microbial community and machine learning modeling

The bio-reduction of azo dyes is significantly dependent on the availability of electron donors and external redox mediators. In this study, the natural henna plant biomass was supplemented to promote the biological reduction of an azo dye of Acid Orange 7 (AO7). Besides, the machine learning (ML) approach was applied to decipher the intricate process of henna-assisted azo dye removal. The experimental results indicated that the hydrolysis and fermentation of henna plant biomass provided both electron donors such as volatile fatty acid (VFA) and redox mediator of lawsone to drive the bio-reduction of AO7 to sulfanilic acid (SA). The high henna dosage selectively enriched certain bacteria, such as Firmicutes phylum, Levilinea and Paludibacter genera, functioning in both the henna fermentation and AO7 reduction processes simultaneously. Among the three tested ML algorithms, eXtreme Gradient Boosting (XGBoost) presented exceptional accuracy and generalization ability in predicting the effluent AO7 concentrations with pH, oxidation-reduction potential (ORP), soluble chemical oxygen demand (SCOD), VFA, lawsone, henna dosage, and cumulative henna as input variables. The validating experiments with tailored optimal operating conditions and henna dosage (pH 7.5, henna dosage of 2 g/L, and cumulative henna of 14 g/L) confirmed that XGBoost was an effective ML model to predict the efficient AO7 removal (91.6%), with a negligible calculating error of 3.95%. Overall, henna plant biomass addition was a cost-effective and robust method to improve the bio-reduction of AO7, which had been demonstrated by long-term operation, ML modeling, and experimental validation.

Constructed wetlands for the treatment of household organic micropollutants with contrasting degradation behaviour: Partially-saturated systems as a performance all-rounder

The degradability of specific organic micropollutants in constructed wetlands (CWs) may differ depending on the prevalence of oxic or anoxic conditions. These conditions are governed, among other factors, by the water saturation level in the system. This study investigated the removal of three environmentally-relevant organic micropollutants: bisphenol-group plasticizer bisphenol S (BPS), household-use insecticide fipronil (FPN) and non-steroidal anti-inflammatory drug ketoprofen (KTP) in the model CWs set up in an outdoor column system. BPS and KTP, in contrast to FPN, exhibit higher biodegradability potential under oxic conditions. The experimental CWs were operated under various saturation conditions: unsaturated, partially saturated and saturated, and mimicked the conditions occurring in unsaturated, partially-saturated intermittent vertical-flow CWs and in horizontal-flow CWs, respectively. The CWs were fed with synthetic household wastewater with the concentration of the micropollutants at the level of 30-45 μg/L. BPS and KTP exhibited contrasting behaviour against FPN in the CWs in the present experiment. Namely, BPS and KTP were almost completely removed in the unsaturated CWs without a considerable effect of plants, but their removal in saturated CWs was only moderate (approx. 50%). The plants had only a pronounced effect on the removal of BPS in saturated systems, in which they enhanced the removal by 46%. The removal of FPN (approx. 90%) was the highest in the saturated and partially-saturated CWs, with moderate removal (66.7%) in unsaturated systems. Noteworthy, partially-saturated CWs provided high or very high removal of all three studied substances despite their contrasting degradability under saturated and unsaturated conditions. Namely, their removal efficiencies in planted CWs were 95.9%, 94.5% and 81.6%, for BPS, KTP and FPN, respectively. The removal of the micropollutants in partially-saturated CWs was comparable or only slightly lower than in the best treatment option making it the performance all-rounder for the compounds with contrasting biodegradability properties.

Partial nitrification in free nitrous acid-treated sediment planting Myriophyllum aquaticum constructed wetland strengthens the treatment of black-odor water

Black-odor water pollution in rural areas, especially swine wastewater, can lead to the deterioration of water quality and thus seriously affect the daily life of people in the area. However, there is a lack of effective treatment measures with simultaneous attention to carbon, nitrogen and sulfur pollution in rural black-odor water bodies. This study evaluated the feasibility of an in-situ pilot-scale constructed wetland (CW) for the synchronous removal of COD, ammonium, and sulfur compounds in the swine wastewater. In this study, the operation strategy of CW sediment pretreated with free nitrous acid (FNA) and Myriophyllum aquaticum plantation was established. Throughout the 114-day operation, the total removal efficiencies of COD and ammonium nitrogen in experimental groups were 81.2 ± 4.2 % and 72.8 ± 1.8 %, respectively, which were significantly higher than CW without any treatment. Removal efficiencies of Sulfur compounds, i.e. sulfide, sulfate, thiosulfate, and sulfite, were 92.3 ± 1.9 % (61.2 % higher than the no-treatment group), 42.1 ± 3.8 %, 97.9 ± 1.7 %, and 42.7 ± 4.5 % respectively. High-throughput sequencing and qPCR revealed that experimental group significantly increased denitrification genes (nirK, nosZ) and sulfur oxidation genes (soxB, fccAB) and enriched the corresponding microbial taxa (Bacillus, Conexibacter and Clostridium sensu stricto). Moreover, metabolic pathways related to nitrogen and sulfur and the degradation of organic matter were up-regulated. These results indicated that partial nitrification in CW planted with M. aquaticum promoted sulfur oxidation denitrification and heterotrophic denitrification. Overall, the in-situ pilot-scale study revealed that the cultivation of M. aquaticum in FNA-treated CW can be a sustainable approach to treat black-odor water bodies.

Iris pseudacorus as precursor affecting ecological transformation of graphene oxide and performance of constructed wetland

The role of plants is largely unknown in constructed wetlands (CWs) exposed to phytotoxic nanomaterials. Present study investigated transformation of graphene oxide (GO) and performance of CWs with Iris pseudacorus as precursor. GO was trapped by CWs without dependence on plants. GO could move to lower substrate layer and present increases on defects/disorders with stronger effects in planted CW. Before adding GO, planted CW achieved better removal both of phosphorus and nitrogen. After adding GO, phosphorus removal in planted CW was 93.23-95.71% higher than 82.55-90.07% in unplanted CW. However, total nitrogen removal was not improved, showing 48.20-56.66% and 53.44-56.04% in planted and unplanted CWs. Plant improved urease, phosphatase, and arylsulfatase, but it decreased β-glucosidase and had less effects on dehydrogenase and catalase. Pearson correlation matrix revealed that plant enhanced microbial interaction with high degree of positive correlation. Moreover, there were obvious shifts in microbial community at phylum and genus level, which presented closely positive action on substrate enzyme activities. The functional profile was less affected due to functional redundancy in microbial system, but time effects were obvious in CWs, especially in planted CW. These findings could provide the basis on understanding role of plants in CWs for treating nanoparticles wastewater.

Impact of microplastics on the treatment performance of constructed wetlands: Based on substrate characteristics and microbial activities

Presence of microplastics (MPs) in wastewater has posed a huge ecosystem risk. Constructed wetlands (CWs) can effectively intercept MPs, while with MPs accumulation the response of CWs' performance is still unclear. In order to evaluate those effects, we conducted a 370-day experiment using CW microcosms fed with different levels (0, 10, 100, and 1000 μg/L) of polystyrene (PS) MPs (diameter: 50-100 μm). Results showed that nitrogen removal efficiency was increased (by 3.9%-24.7%) during the first 60 days and then decreased (by 7.1%-41.3%) with MPs accumulating, but no obvious change in COD and TP removal was observed. From further analysis, MPs accumulation changed the biofilm composition (TOC content increased from 41.4% to 52.7%), substrate porosity (electrical resistivity increased by 1.2-2.4 folds), and oxygen mass transfer (|K_La,O2| increased from 3.5% to 18.6%). Moreover, the microbial dynamics presented a higher abundance of nitrifiers (Nitrospira and Nitrosomonas) during the 60-day experiment and a lower abundance in the last days, while denitrifiers (Thauera, Thiobacillus, and Anaerolinea) had a high relative abundance throughout the experiment, being consistent with the variation of nitrification and denitrification rates. Finally, structural equation model analysis proved that due to the changes of substrate characteristics and microbial compositions and activities, the obvious decrease in nitrification efficiency was a direct reason for the decline of nitrogen removal during 370-day MPs accumulation. Overall, our study first prove that MPs accumulation can cause a series of changes in physicochemical and microbial characteristics of substrate, and ultimately affect the nitrogen-transforming process in CWs. Although our conclusions were based on the lab-scale CWs being different from the real wetlands, we hope that the conclusions can provide the effective regulatory strategies to guide the control of MPs in the actual wetlands.

Response of constructed wetland for wastewater treatment to graphene oxide: Perspectives on plant and microbe

The wide application of graphene oxide (GO) increases its release into environment with less known on environmental effects. This work investigated 120-day interaction between GO (500 and 5000 μg/L) and constructed wetlands (CWs) planted with Iris pseudacorus. CWs showed the effective retention for GO via mature biofilm but less biodegradation. GO significantly induced enzyme activities (urease, neutral phosphatase, and catalase), which was attributed to increases in ecological association and enzyme abundance. GO decreased microbial biomass on day 30, but it had no impacts on day 120. The microbial community showed gradual self-adaption with time due to protection of antioxidant defense system (L-ascorbate oxidase, superoxide reductase, and glutathione related enzyme). The antioxidant enzymes (superoxide dismutase and peroxidase) and lipid peroxidation of Iris pseudacorus were increased by GO, accompanied by reduction on chlorophyll biosynthesis. Overall, the separate effects of GO on micro-regions and individual bodies in CWs were obvious, but it was acceptable that variations in pollutant removal were not evident due to synergetic role of plant-substrate-microbe. Organic matter and phosphorus removals reached to above 93%, and ammonia and total nitrogen removals in GO groups were reduced by 7-8% and 9-13%, respectively.

Metagenomic insights into the effects of submerged plants on functional potential of microbial communities in wetland sediments

Submerged plants in wetlands play important roles as ecosystem engineers to improve self-purification and promote elemental cycling. However, their effects on the functional capacity of microbial communities in wetland sediments remain poorly understood. Here, we provide detailed metagenomic insights into the biogeochemical potential of microbial communities in wetland sediments with and without submerged plants (i.e., Vallisneria natans). A large number of functional genes involved in carbon (C), nitrogen (N) and sulfur (S) cycling were detected in the wetland sediments. However, most functional genes showed higher abundance in sediments with submerged plants than in those without plants. Based on the comparison of annotated functional genes in the N and S cycling databases (i.e., NCycDB and SCycDB), we found that genes involved in nitrogen fixation (e.g., nifD/H/K/W), assimilatory nitrate reduction (e.g., nasA and nirA), denitrification (e.g., nirK/S and nosZ), assimilatory sulfate reduction (e.g., cysD/H/J/N/Q and sir), and sulfur oxidation (e.g., glpE, soeA, sqr and sseA) were significantly higher (corrected p < 0.05) in vegetated vs. unvegetated sediments. This could be mainly driven by environmental factors including total phosphorus, total nitrogen, and C:N ratio. The binning of metagenomes further revealed that some archaeal taxa could have the potential of methane metabolism including hydrogenotrophic, acetoclastic, and methylotrophic methanogenesis, which are crucial to the wetland methane budget and carbon cycling. This study opens a new avenue for linking submerged plants with microbial functions, and has further implications for understanding global carbon, nitrogen and sulfur cycling in wetland ecosystems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00100-3.

Effects of Cattails and Hydraulic Loading on Heavy Metal Removal from Closed Mine Drainage by Pilot-Scale Constructed Wetlands

This study demonstrated heavy metal removal from neutral mine drainage of a closed mine in Kyoto prefecture in pilot-scale constructed wetlands (CWs). The CWs filled with loamy soil and limestone were unplanted or planted with cattails. The hydraulic retention time (HRT) in the CWs was shortened gradually from 3.8 days to 1.2 days during 3.5 months of operation. A short HRT of 1.2 days in the CWs was sufficient to achieve the effluent standard for Cd (0.03 mg/L). The unplanted and the cattail-planted CWs reduced the average concentrations of Cd from 0.031 to 0.01 and 0.005 mg/L, Zn from 0.52 to 0.14 and 0.08 mg/L, Cu from 0.07 to 0.04 and 0.03 mg/L, and As from 0.011 to 0.006 and 0.006 mg/L, respectively. Heavy metals were removed mainly by adsorption to the soil in both CWs. The biological concentration factors in cattails were over 2 for Cd, Zn, and Cu. The translocation factors of cattails for all metals were 0.5–0.81. Sulfate-reducing bacteria (SRB) belonging to Deltaproteobacteria were detected only from soil in the planted CW. Although cattails were a minor sink, the plants contributed to metal removal by rhizofiltration and incubation of SRB, possibly producing sulfide precipitates in the rhizosphere.

ALLODUST augmented activated sludge single batch anaerobic reactor (AS-SBAnR) for high concentration nitrate removal from agricultural wastewater

Nitrate is among the most widespread contaminants that threaten water bodies and waterways. Under favourable environmental conditions, high nitrate concentrations in water can contribute to eutrophication, thus presenting a high potential for risk to ecosystems and human health. Low-cost allophanic soil material and carbon-based bio-wastes have great potential to reduce nutrient concentrations from contaminated waters. This study investigated the mechanisms that underpin the reduction of nitrate concentrations and nitrous oxide (N₂O) emission in the presence of novel developed media in an activated sludge process. A new operating approach, employing a newly developed media (ALLODUST), was evaluated for enhanced NO^-₃-N removal from agricultural wastewater. Two anaerobic-aerobic batch reactors were developed, where the coupled bottom aeration method was used for efficient agitation and aeration in the aerobic reactor. The reactor was run at high NO^-₃-N concentrations (110 mg L^-1), under anoxic conditions at low- to long-term contact times (2, 12, and 22 h), while the aerobic period (clarification) was constant for all the experimental designs (2 h). ALLODUST retained its integrity and stability over the long-term operation. Low ALLODUST concentrations (5.95 g L^-1) removed 87% of the NO^-₃-N from the wastewater within 12 h. Further exploration revealed that the same amount of the media was optimal for decreasing N₂O emissions from the anaerobic activated sludge reactor by 80%.

Impacts of carbon-based nanomaterials on nutrient removal in constructed wetlands: Microbial community structure, enzyme activities, and metabolism process

The increasing use of raw carbon-based nanomaterials (CBNs) will inevitably affect wastewater treatment systems. Constructed wetlands (CWs) are ecological wastewater treatment facilities and can intercept the vast particles pollutant, including CBNs. However, the impacts of CBNs on the treatment performance of CWs have no available knowledge. Therefore, we systematically inspected the effects of single-walled and multi-walled carbon nanotubes (SWCNTs and MWCNTs) and fullerene nanoparticles (C₆₀) on CW performance under 180-day exposure to 0, 10 and 1000 μg/L concentrations. The results showed that CBNs had marginally adverse impacts on chemical oxygen demand (COD) and total phosphorus (TP) removal, whereas nitrogen removal declined by 24.1 %-42.7 % following long-term exposure to CBNs. MWCNTs had the greatest inhibition effect on nitrogen removal, followed by SWCNTs and C₆₀. The CBNs also induced reactive oxygen species (ROS) overproduction as the increasing concentration, which confirmed that CBNs have biotoxic effects in CWs. The variation of functional microbial community and the inhibition of enzyme activities were the dominant reasons for the decline in nitrogen removal efficiency. Furthermore, predictive functional profiling showed that CBNs affected functional gene abundance, and caused a decline in the enzymes abundance connected to nitrogen removal by the end of the 180-day exposure period.

Biological iron nitrogen cycle in ecological floating bed: Nitrogen removal improvement and nitrous oxide emission reduction

Ecological floating beds (EFBs) have become a superior method for treating secondary effluent from wastewater treatment plant. However, insufficient electron donor limited its denitrification efficiency. Iron scraps from lathe cutting waste consist of more than 95% iron could be used as electron donors to enhance denitrification. In this study, EFBs with and without iron scraps supplementation (EFB-Fe and EFB, respectively) were conducted to explore the impacts of iron scraps addition on nitrogen removal, nitrous oxide (N₂O) emissions and microbial communities. Results showed the total nitrogen (TN) removal in EFB-Fe improved to 79% while that in EFB was 56%. N₂O emission was 0-6.20 mg m^-2 d^-1 (EFB-Fe) and 1.74-15.2 mg m^-2 d^-1 (EFB). Iron scraps could not only improve nitrogen removal efficiency, but also reduce N₂O emissions. In addition, high-throughput sequencing analysis revealed that adding iron scraps could improve the sum of denitrification related genera, among which Novosphingobium accounted for the highest proportion (6.75% of PFe1, 4.24% of PFe2, 3.18% of PFe3). Iron-oxidizing bacteria and iron-respiring bacteria associated with and nitrate reducing bacteria mainly concentrated on the surface of iron scraps. Principal co-ordinates analysis (PCoA) indicated that iron scraps were the key factor affecting microbial community composition. The mechanism of iron scraps enhanced nitrogen removal was realized by enhanced biological denitrification process. Iron release dynamic from iron scraps was detected in bench-scale experiment and the electron transfer mechanism was that Fe⁰ transferred electrons directly to NO₃^--N, and biological iron nitrogen cycle occurred in EFB-Fe without secondary pollution.

Differences in bacterial N, P, and COD removal in pilot-scale constructed wetlands with varying flow types

The mechanisms of bacterial nitrogen (N), phosphorus (P), and chemical oxygen demand (COD) removal in pilot-scale constructed wetlands (CWs) were investigated in the present work. Three types of CWs were assessed: vertical flow (VF), horizontal flow (HF), and surface flow (SF), each with three planting conditions, with either Thalia, Canna or without plants. The results show that construction types affected microbes more than planting conditions. VF CWs promoted the aerobic processing of total N, total P, COD, and NH₃-N, increasing the respective removal efficiencies by 4-19%, 13-32%, 19-29%, and 75-80%, respectively, compared with SF CWs. The relative abundance of nitrifying, denitrifying, methanotrophic and dephosphorized bacteria, and functional genes such as nxrA, nirK, nosZ, mmoX, and phoD were higher in VF CWs. Positive and simple gene networks in VF CWs can effectively reduce the redundancy in functional genes, enhance bacterial function and gene interactions, thus promoting nutrient removal.

Sulfur Cycle in a Wetland Microcosm: Extended 34S-Stable Isotope Analysis and Mass Balance

The sulfur cycle is an important part of constructed wetland biogeochemistry because it is intimately intertwined with the carbon, nitrogen, and iron cycles. However, to date, no quantitative investigation has been conducted on the sulfur cycle in constructed wetlands because of the complexity of wetland systems and the deficiencies in experimental methodology. In this study, ³⁴S-stable isotope analysis was extended in terms of the calculation for the enrichment factor and the kinetic analysis for bacterial sulfate reduction. With this extended method, we attempted for the first time to assess the true rate of bacterial sulfate reduction when sulfide oxidation co-occurs. The joint application of the extended ³⁴S-stable isotope and mass balance analyses made it possible to quantitatively investigate the primary sulfur transformation in a wetland microcosm. Accordingly, a sulfur cycle model for constructed wetlands was quantified and validated. Approximately 75% of the input sulfur was discharged. The remainder was mainly removed through deposition as acid volatile sulfide, pyrite, and elemental sulfur. Plant uptake was negligible. These findings improve our understanding of the physical, chemical, and biological transformations of sulfur among plants, sediments, and microorganisms, and their interactions with carbon, nitrogen, and iron cycles, in constructed wetlands and similar systems.

The Influence of Waste Composition on Landfill Gas Generation in a Pilot-Scale Lysimeter

The Sudokwon landfill site in Korea, is one of the largest landfill sites in the world, and consists of a first landfill site and second landfill site. The second landfill site generates 3–30 times more H2S than that of the first landfill site. However, the cause of the increase in H2S has not been identified. In this study, the main causes of H2S concentration increase were investigated in the second landfill site in the Sudokwon landfill site. We classified wastes at the Sudokwon landfill site into seven types including Construction and demolition (C&D) debris waste. A lysimeter reactor was designed as a similar environment to the Sudokwon landfill site for simulation. In addition, the experiment was conducted under the same conditions. Three components and elements were analyzed to identify the composition of waste in the landfill site. Leachate was analyzed through a chemical oxygen demand and SO42− standard method. For landfill gas, a gas analyzer was used. The trend in the generation of leachate and landfill gas depending on waste composition at the landfill site was observed and the cause of the increase in H2S was examined. As a result, landfilling of C&D debris waste is recommended as a single landfill.

Simultaneous improving nitrogen removal and decreasing greenhouse gas emission with biofilm carriers addition in ecological floating bed

Ecological floating bed (EFB) is a green technology for treatment of micro-polluted wastewater. However, its nitrogen removal efficiency is still unsatisfactory. In this study, two EFBs with additional carbon source were established to explore biofilm carriers addition on nitrogen removal and greenhouse gas (GHG) emissions at different C/N ratios and temperatures. Results showed that biofilm carriers addition increased nitrification and nitrogen removal efficiencies in EFB, and more denitrifying and nitrifying bacteria were attached to the biofilm carriers. Higher N₂O and CH₄ emissions were found in control EFB without biofilm carriers addition which was consistent with higher nitrite accumulation. In addition, high-throughput sequencing analysis revealed that adding biofilm carriers could improve the richness and diversity of biological communities. For EFB with additional carbon source treating secondary effluent, adding biofilm carrier can obtain higher TN removal efficiency and lower greenhouse gas emission.

Nitrogen removal in response to plants harvesting in two kinds of enhanced hydroponic root mats treating secondary effluent

Hydroponic root mats (HRMs) are a green technology for various wastewaters purification. However, plants wilting will inevitably reduce the purification efficiency of HRMs. Harvesting as an important way of plant management for a better understanding of sustainability of HRMs has always been highly controversial. The goal of this study was to investigate the impacts and sustainability of harvesting on nitrogen removal and greenhouse gases (CH₄, N₂O) emissions of the two kinds of enhanced hydroponic root mats: autotrophic hydroponic root mat (AHRM) and heterotrophic hydroponic root mat (HHRM) for treating secondary effluent. The results showed that harvesting temporarily decreased nitrogen removal efficiency of the two systems, and removal efficiency recovered quickly because of the existence of external electron donors. The effects of harvesting are ordered as: HHRM > AHRM, NO₃^--N > NH₄⁺-N. Increasing C/N, S/N would reduce the impact of temperature on harvesting systems. Harvesting also increased the emission of greenhouse gases, and increasing C/N(=6), S/N(=1.1) could significantly reduce greenhouse gases emission of the harvesting systems at low temperature. In addition, composition analysis of the shoots of the harvested plants was also conducted, and the results showed that N contents of growing shoots were significantly higher than that of withered shoots. In order to make the hydroponic root mats sustainable, harvesting before the plants wilt is more effective in removing nitrogen from the system permanently and maintain a sustainable system.

Effect of corn cobs as external carbon sources on nitrogen removal in constructed wetlands treating micro-polluted river water

Micro-polluted river water is characterized as having limited biodegradability, low carbon to nitrogen ratio and little organic carbon supply, all of which makes it hard to further purify. Two bench scale constructed wetlands (CWs) with a horizontal subsurface flow mode were set up in the laboratory to evaluate their feasibility and efficiency on denitrification with and without corn cobs as external carbon sources. Micro-polluted river water was used as feed solution. The CW without corn cobs substrates possessed a good performance in removing chemical oxygen demand (COD, <40 mg/L) and ammonia nitrogen (NH₃-N, <0.65 mg/L), but less efficiency in removing total nitrogen (TN) and nitrate nitrogen (NO₃-N). In marked contrast, the CW with 1% (w/w) corn cobs substrates as external carbon sources achieved a significant improvement in the removal efficiency of TN (increased from 34.2% to 71.9%) and NO₃-N (increased from 19% to 71.9%). The incorporation of corn cobs substrates did not cause any obvious increase in the concentrations of COD and NH₃-N in the effluent. This improvement in the denitrification efficiency was owing to the released organic carbon from corn cobs substrates, which facilitated the growth of abundant microbes on the surface and pores of the substrate. The open area of the used corn chips is larger than that of the pristine ones, and corn cobs can continue to provide a carbon fiber source for denitrification.

The nitrogen removal performance and microbial communities in a two-stage deep sequencing constructed wetland for advanced treatment of secondary effluent

The advanced treatment of secondary effluent was conducted in a two-stage deep sequencing constructed wetland (DSCW) which comprised a denitrification chamber (W1) and a nitrification chamber (W2). The results showed that a superior NO₃^--N removal rate was observed in W1 with a C/N ratio of 6.5, and a high NH₄⁺-N removal rate was obtained when the W2 was operated with 6-h duration of idle. In the long-term operation for 45days, the two-stage DSCW pilot system achieved high and stable removal of TN, NH₄⁺-N and NO₃^--N, which were 92.9%, 83.7% and 95.6% in average, respectively. The microbial communities between W1 and W2 were significant different. Rich diversity of the microbial community and the high proportion of denitrifying bacteria in the W1 were essential for nitrogen removal in this treatment system. AOB in the W2 played a major role in NH₄⁺-N removal in W2.

Natural treatment system models for wastewater management: a study from Hyderabad, India

Wastewater generated on a global scale has become a significant source of water resources which necessitates appropriate management strategies. However, the complexities associated with wastewater are lack of economically viable treatment systems, especially in low- and middle-income countries. While many types of treatment systems are needed to serve the various local issues, we propose natural treatment systems (NTS) such as natural wetlands that are eco-friendly, cost-effective, and can be jointly driven by public bodies and communities. In order for it to be part of wastewater management, this study explores the NTS potential for removal of pollutants, cost-effectiveness, and reuse options for the 1.20 million m³/day of wastewater generated in Hyderabad, India. The pilot study includes hydro-geophysical characterization of natural wetland to determine pollutant removal efficiency and its effective utilization for treated wastewater in the peri-urban habitat. The results show the removal of organic content (76-78%), nutrients (77-97%), and microbes (99.5-99.9%) from the wetland-treated wastewater and its suitability for agriculture applications. Furthermore, the wetland efficiency integrated with engineered interventions led to the development of NTS models with different application scenarios: (i) constructed wetlands, (ii) minimized community wetlands, and (iii) single outlet system, suitable for urban, peri-urban and rural areas, respectively.

Performance evaluation of semi continuous vertical flow constructed wetlands (SC-VF-CWs) for municipal wastewater treatment

The present study demonstrated the understating of municipal wastewater treatment in five types of CWs operated under semi continuous vertical flow mode. All CWs treatment conditions show the significantly lower pollutants concentrations. The average NH₄⁺-N, TN, NO₂^--N, NO₃^--N, SO₄^2-, and PO₄^3- removal efficiency in the ISs-CWs were 83.60%, 82.43%, 15.61%, 48.93%, 80.45%, and 78.94% respectively. The average NO₂^--N removal efficiency shows that highest nitrite accumulation occurred in the Cont-CWs followed by C-CWs. The lowest increase in the biomass (127.5%) was observed in the Eichhornia crassipes planted in the ISs-CWs. The ISs filtration barrier created in the constructed wetlands was sufficient enough to remove all the pollutants. Principal components EFA 2D deformation plots show the distribution of the various nitrogenous species in the constructed wetlands along different components.

Long-term assessment at field scale of Floating Treatment Wetlands for improvement of water quality and provision of ecosystem services in a eutrophic urban pond

Pollution of urban water bodies requires stringent control measures and the development of low-cost and highly efficient alternative technologies. In contrast to Constructed Wetlands, Floating Treatment Wetlands (FTWs) have the advantage of not requiring large surface of land since they operate in situ. However, there is limited information about their long-term evaluation while operating at field scale. The aim of this work was to assess the performance of FTWs using a combination of Pontederia sagittata and Cyperus papyrus for the improvement of the water quality and provision of ecosystem services of a eutrophic urban pond. The FTWs were built with low-cost material easy to acquire and to ensemble. Two FTWs (17.5m² and 33m²) located in Pond 1 within a complex of 4 urban artificial ponds were evaluated for two years. They promoted an increase in the dissolved oxygen (D.O.) within a range of 15 to 67%, a removal of fecal coliforms in the range of 9 to 86% and a nitrate removal in the range of 9 to 76%. The plant productivity reached a maximum of 363g_dmm^-2d^-1 in the FTW1 and 536g_dmm^-2d^-1 in the FTW2 during the period March-June 2016. The TKN and the TP content in the plant were in the range of 18.3 to 28.1 and of 0.05 to 0.196gkg^-1 dry matter, respectively. In conclusion, the tested FTWs have proved to be a very beneficial low-cost technology for the improvement of water quality and provision of ecosystem services.

Enhanced nutrient removal and mechanisms study in benthic fauna added surface-flow constructed wetlands: The role of Tubifex tubifex

This study designed a combined benthic fauna-T. orientalis-substrate-microbes surface-flow constructed wetlands (SFCWs) through the addition of T. tubifex. Results showed that, the removal efficiencies of nitrogen and phosphorus in the tested SFCWs achieved 81.14±4.16% and 70.49±7.60%, which were 22.27% and 27.35% higher than that without T. tubifex. Lower nitrate (2.11±0.79mg/L) and ammonium (0.75±0.64mg/L) were also observed in the tested SFCWs, which were 3.46mg/L and 0.52mg/L lower than that without T. tubifex. Microbial study confirmed the increased denitrifiers with T. tubifex. The lower nitrogen in effluent was also attributed to higher contents of nitrogen storage in sediment and T. orientalis due to the bioturbation of T. tubifex. Furthermore, with T. tubifex, higher proportions of particulate (22.66±3.96%) and colloidal phosphorus (20.57±3.39%) observed promoted phosphorus settlement and further absorption by T. orientalis. The outcomes of this study provides an ecological and economical strategy for improving the performance of SFCWs.

Greenhouse wastewater treatment by baffled subsurface-flow constructed wetlands supplemented with flower straws as carbon source in different modes

Four laboratory-scale baffled subsurface-flow constructed wetlands (BSCWs) were established for the treatment of greenhouse wastewater containing high levels of nitrate and sulfate in the present study. Each BSCW microcosm involved a treatment zone and another post-treatment zone with a surface area ratio of 2:1. Evenly mixed straws of carnation and rose (w/w: 1/1), two common ornamental flowers, were supplemented as an organic carbon source into the treatment zone through a hydrolysis zone (CW 1), decentralized vertically installed perforated pipes (CW 2), and centralized pipes (CW 3 in the figures), except the blank system. Removals and transformations of nitrogen and sulfate as well as carbon release in the BSCWs were investigated and comparatively assessed. Results showed that the supplements of flower straws could greatly enhance both the nitrate and sulfate removals, and good performance was achieved during the beginning operation period of 30 days, followed by decline due to insufficient organic carbon supply. Nitrate removal efficiency was significantly higher and more stable compared to sulfate. The highest removal rates of nitrate and sulfate were achieved in the CW 3, with a mean value of 4.33 g NO₃^--N·m^-2 d^-1 and 2.74 g SO₄^2--S·m^-2 d^-1, respectively, although the differences among the experimental microcosms were not statistically significant. However, almost the same TN removal rate (3.40-3.47 g N·m^-2 d^-1) was obtained due to the productions of NO₂^--N and NH₄⁺-N and leaching of organic N from the straws. High contents of organic carbon and colored substance were leached from the straws during the initial 10 days, but dropped rapidly to low levels, and could hardly determined after 30 days operation. The post-treatment zone could further eliminate various contaminants, but the capability was limited. Inorganic carbon (IC) concentration was detected to be a highly good indicator for the estimation of nitrate and sulfate removal efficiencies of the BSCWs, particularly for nitrate.

Sulfate removal and sulfur transformation in constructed wetlands: The roles of filling material and plant biomass

Sulfate in effluent is a challenging issue for wastewater reuse around the world. In this study, sulfur (S) removal and transformation in five batch constructed wetlands (CWs) treating secondary effluent were investigated. The results showed that the presence of the plant cattail (Typha latifolia) had little effect on sulfate removal, while the carbon-rich litter it generated greatly improved sulfate removal, but with limited sulfide accumulation in the pore-water. After sulfate removal, most of the S was deposited with the valence states S (-II) and S (0) on the iron-rich gravel surface, and acid volatile sulfide was the main S sink in the litter-added CWs. High-throughput pyrosequencing revealed that sulfate-reducing bacteria (i.e. Desulfobacter) and sulfide-oxidizing bacteria (i.e. Thiobacillus) were dominant in the litter-added CWs, which led to a sustainable S cycle between sulfate and sulfide. Overall, this study suggests that recycling plant litter and iron-rich filling material in CWs gives an opportunity to utilize the S in the wastewater as both an electron acceptor for sulfate reduction and as an electron donor for nitrate reduction coupled with sulfide oxidation. This leads to the simultaneous removal of sulfate, nitrate, and organics without discharging toxic sulfide into the receiving water body.

Evaluation of an innovative approach based on prototype engineered wetland to control and manage boron (B) mine effluent pollution

A major environmental problem associated with boron (B) mining in many parts of the world is B pollution, which can become a point source of B mine effluent pollution to aquatic habitats. In this study, a cost-effective, environment-friendly, and sustainable prototype engineered wetland was evaluated and tested to prevent B mine effluent from spilling into adjoining waterways in the largest B reserve in the world. According to the results, average B concentrations in mine effluent significantly decreased from 17.5 to 5.7 mg l(-1) after passing through the prototype with a hydraulic retention time of 14 days. The results of the present experiment, in which different doses of B had been introduced into the prototype, also demonstrated that Typha latifolia (selected as donor species in the prototype) showed a good resistance to alterations against B mine effluent loading rates. Moreover, we found that soil enzymes activities gradually decreased with increasing B dosages during the experiment. Boron mass balance model further showed that 60 % of total B was stored in the filtration media, and only 7 % of B was removed by plant uptake. Consequently, we suggested that application of the prototype in the vicinity of mining site may potentially become an innovative model and integral part of the overall landscape plan of B mine reserve areas worldwide. Graphical Abstract ᅟ.

A Hardy Plant Facilitates Nitrogen Removal via Microbial Communities in Subsurface Flow Constructed Wetlands in Winter

The plants effect in subsurface flow constructed wetlands (SSF-CWs) is controversial, especially at low temperatures. Consequently, several SSF-CWs planted with Iris pseudacorus (CWI) or Typha orientalis Presl. (CWT) and several unplanted ones (CWC) were set up and fed with secondary effluent of sewage treatment plant during the winter in Eastern China. The 16S rDNA Illumina Miseq sequencing analysis indicated the positive effects of I. pseudacorus on the bacterial community richness and diversity in the substrate. Moreover, the community compositions of the bacteria involved with denitrification presented a significant difference in the three systems. Additionally, higher relative abundances of nitrifying bacteria (0.4140%, 0.2402% and 0.4318% for Nitrosomonas, Nitrosospira and Nitrospira, respectively) were recorded in CWI compared with CWT (0.2074%, 0.0648% and 0.0181%, respectively) and CWC (0.3013%, 0.1107% and 0.1185%, respectively). Meanwhile, the average removal rates of NH4(+)-N and TN in CWI showed a prominent advantage compared to CWC, but no distinct advantage was found in CWT. The hardy plant I. pseudacorus, which still had active root oxygen release in cold temperatures, positively affected the abundance of nitrifying bacteria in the substrate, and accordingly was supposed to contribute to a comparatively high nitrogen removal efficiency of the system during the winter.

Remediation of nitrate-contaminated wastewater using denitrification biofilters with straws of ornamental flowers added as carbon source

Straws of four ornamental flowers (carnation, rose, lily, and violet) were added into denitrification biofilters using gravel as matrix through vertically installed perforated polyvinylchloride pipes to provide organic carbon for the treatment of nitrate-contaminated wastewater operating in batch mode. Removal efficiencies of nitrate and phosphate, as well as temporal variations of nitrogen and carbon during batches 10 and 19, were investigated and assessed. Nitrate removal was efficiently enhanced by the addition of flower straws, but decreased gradually as the organic substances were consumed. Phosphate removal was also improved, although this very limited. High nitrate removal rates were achieved during the initial 12 h in the two batches each lasting for 3 days, along with the depletion of influent dissolved oxygen due to aerobic degradation of the organic compounds. NO2(-)-N of 0.01-2.83 mg/L and NH4(+)-N of 0.02-1.69 mg/L were formed and both positively correlated to the nitrate reduced. Inorganic carbon (IC) concentrations increased during the batches and varied conversely with the nitrate contents, and could be indicative of nitrate removal due to the highly significant positive correlation between NO3(-)-N removed and IC concentration (r(2) = 0.881, p < 0.0001). It is feasible and economical to use the denitrification biofilter to treat nitrate-contaminated wastewater, although further optimization of carbon source addition is still required.

Enhancement of the performance of constructed wetlands for wastewater treatment in winter: the effect of Tubifex tubifex

An improved constructed wetland (CW) with the addition ofTubifex tubifexin winter was studied in laboratory batch systems. The outcomes of this study indicate that the potential use ofTubifex tubifexcould improve the ecosystem and water purification by CWs in winter.

Enhanced azo dye removal in a continuously operated up-flow anaerobic filter packed with henna plant biomass

Effects of henna plant biomass (stem) packed in an up-flow anaerobic bio-filter (UAF) on an azo dye (AO7) removal were investigated. AO7 removal, sulfanilic acid (SA) formation, and pseudo first-order kinetic constants for these reactions (kAO7 and kSA) were higher in the henna-added UAF (R2) than in the control UAF without henna (R1). The maximum kAO7 in R1 and R2 were 0.0345 and 0.2024 cm(-1), respectively, on day 18; the corresponding molar ratios of SA formation to AO7 removal were 0.582 and 0.990. Adsorption and endogenous bio-reduction were the main AO7 removal pathways in R1, while in R2 bio-reduction was the dominant. Organics in henna could be released and fermented to volatile fatty acids, acting as effective electron donors for AO7 reduction, which was accelerated by soluble and/or fixed lawsone. Afterwards, the removal process weakened over time, indicating the demand of electron donation and lawsone-releasing during the long-term operation of UAF.

Transformation of chloroform in model treatment wetlands: from mass balance to microbial analysis

Chloroform is one of the common disinfection byproducts, which is not susceptible to degradation and poses great health concern. In this study, the chloroform removal efficiencies and contributions of sorption, microbial degradation, plant uptake, and volatilization were evaluated in six model constructed wetlands (CWs). The highest chloroform removal efficiency was achieved in litter-added CWs (99%), followed by planted (46-54%) and unplanted CWs (39%). Mass balance study revealed that sorption (73.5-81.2%) and microbial degradation (17.6-26.2%) were the main chloroform removal processes in litter-added CWs, and that sorption (53.6-66.1%) and plant uptake (25.3-36.2%) were the primary contributors to chloroform removal in planted CWs. Around 60% of chloroform got accumulated in the roots after plant uptake, and both transpiration and gas-phase transport were expected to be the drivers for the plant uptake. Sulfate-reducing bacteria and methanogens were found to be the key microorganisms for chloroform biodegradation through cometabolic dechlorination, and positive correlations were observed between functional genes (dsrA, mcrA) and biodegradation rates. Overall, this study suggests that wetland is an efficient ecosystem for sustainable chloroform removal, and that plant and litter can enhance the removal performance through root uptake and microbial degradation stimulation, respectively.