Effects of hydraulic loading rate and substrate on ammonium removal in tidal flow constructed wetlands treating black and odorous water bodies

Enhanced ammonia removal in tidal flow constructed wetland by incorporating steel slag: Performance, microbial community, and heavy metal release

Utilizing alkaline solid wastes, such as steel slag, as substrates in tidal flow constructed wetlands (TFCWs) can effectively neutralize the acidity generated by nitrification. However, the impacts of steel slag on microbial communities and the potential risk of heavy metal release remain poorly understood. To address these knowledge gaps, this study compared the performance and microbial community structure of TFCWs filled with a mixture of steel slag and zeolite (TFCW-S) to those filled with zeolite alone (TFCW-Z). TFCW-S exhibited a much higher NH4+-N removal efficiency (98.35 %) than TFCW-Z (55.26 %). Additionally, TFCW-S also achieved better TN and TP removal. The steel slag addition helped maintain the TFCW-S effluent pH at around 7.5, while the TFCW-Z effluent pH varied from 3.74 to 6.25. The nitrification and denitrification intensities in TFCW-S substrates were significantly higher than those in TFCW-Z, consistent with the observed removal performance. Moreover, steel slag did not cause excessive heavy metal release, as the effluent concentrations were below the standard limits. Microbial community analysis revealed that ammonia-oxidizing bacteria, ammonia-oxidizing archaea, and complete ammonia-oxidizing bacteria coexisted in both TFCWs, albeit with different compositions. Furthermore, the enrichment of heterotrophic nitrification-aerobic denitrification bacteria in TFCW-S likely contributed to the high NH4+-N removal. In summary, these findings demonstrate that the combined use of steel slag and zeolite in TFCWs creates favorable pH conditions for ammonia-oxidizing microorganisms, leading to efficient ammonia removal in an environmentally friendly manner.

Impact analysis of hydraulic loading rate and antibiotics on hybrid constructed wetland systems: Insight into the response to decontamination performance and environmental-associated microbiota

Hybrid constructed wetlands (HCWs) are a promising solution for water ecology and environmental treatment, not only for conventional types of water pollution but also for antibiotics. Among the critical parameters for wetlands, the hydraulic loading rate (HLR) is especially important given the challenges of antibiotics treatment and frequent extreme rainfall. To investigate the removal performance of different HLRs on nutrients and antibiotics, as well as the response of antibiotics to nutrient removal, and the impact of HLRs on microbial communities, new HCWs with vertical flow constructed wetlands (VFCWs) and floating constructed wetlands (FCWs) in series were built. The results of the study showed that: (1) HCWs are highly effective in removing chemical oxygen demand (COD), NH4+-N, NO2--N, and total phosphorus (TP) at low HLR (L_HLR), with removal efficiencies as high as 97.8%, 99.6%, 100%, and 80.5%. However, high HLR (H_HLR) reduced their removal efficiencies; (2) The average removal efficiency of fluoroquinolones (FQs) under different HLRs was consistently high, at 99.9%, while the average removal efficiency of macrolides (MLs) was 96.3% (L_HLR) and 88.4% (H_HLR). The removal efficiency of sulfonamides (SAs) was susceptible to HLRs, and the removal of antibiotics occurred mainly in the rhizosphere zone of wetland; (3) High concentrations of antibiotics in HCWs were found to inhibit and poison plant growth and to reduce the removal efficiency of TP by 12%. However, they had a minor effect on the removal efficiency of carbon and nitrogen nutrients; (4) H_HLR altered the diversity and abundance of microbial communities in different compartments of the wetland and also reduced the relative abundance of Bacillus, Hydrogenophaga, Nakamurella, Denitratisoma and Acidovorax genera, which are involved in denitrification and phosphorus removal processes. This alteration in microbial communities was one of the main reasons for the reduced performance of nitrogen and phosphorus removal.

Effects of different aeration strategies and ammonia-nitrogen loads on nitrification performance and microbial community succession of mangrove constructed wetlands for saline wastewater treatment

In highly salinized environments, nitrification is the process that limits the rate of nitrogen transformation and removal. Therefore, this study concentrated on the impacts of different aeration strategies and NH4+-N loads on the nitrification performance of mangrove constructed wetlands (CWs), as well as investigating the succession mechanism of ammonia-oxidizing microorganisms (AOMs). The results showed that both the CW with continuous aeration (CA-CW) and intermittent aeration (IA-CW) achieved a nitrification efficiency of more than 98% under an NH4+-N loading of 1.25-4.7 g/(m2·d). However, the total nitrogen removal rates of IA-CW under low and high ammonia-nitrogen loads (LAL, 20.09 ± 4.4% and HAL, 8.77 ± 1.35%, respectively) were higher than those of CA-CW (16.11 ± 4.7% and 3.32 ± 2.3%, respectively), especially under HAL (p < 0.05). Pearson correlation analysis showed that under different operating conditions, the differential secretion of Kandelia candel rhizosphere organic matter had a certain regulatory effect on nitrification and denitrification groups such as Candidatus Nitrocosmicus, Nitrancea, Truepera, Pontibacter, Halomonas, and Sulfurovum in the wetland root layer. The quantitative polymerase chain reaction revealed that the NH4+-N load rate was the primary factor driving the succession of the AOMs, with different aeration strategies exacerbating this process. Overall, this study revealed that the dominant AOMs in mangrove CWs could be significantly altered by regulating the aeration modes and pollution loads to adjust the rhizosphere organic matter in situ, thereby resulting in more efficient nitrification.

Indigenous microbial community governs the survival of Escherichia coli O157:H7 in constructed wetlands

The survival pattern of Escherichia coli O157:H7 (E. coli O157:H7) and its regulatory factors in natural environments have been widely studied. However, there is little information about the survival of E. coli O157:H7 in artificial environments, especially in wastewater treatment facilities. In this study, a contamination experiment was performed to explore the survival pattern of E. coli O157:H7 and its central control factors in two constructed wetlands (CWs) under different hydraulic loading rates (HLRs). The results showed that the survival time of E. coli O157:H7 was longer in the CW under the higher HLR. Substrate ammonium nitrogen and available phosphorus were the main factors that influenced the survival of E. coli O157:H7 in CWs. Despite the minimal effect of microbial α-diversity, some keystone taxa, such as Aeromonas, Selenomonas, and Paramecium, governed the survival of E. coli O157:H7. In addition, the prokaryotic community had a more significant impact on the survival of E. coli O157:H7 than the eukaryotic community. The biotic properties had a more substantial direct power on the survival of E. coli O157:H7 than the abiotic factors in CWs. Collectively, this study comprehensively disclosed the survival pattern of E. coli O157:H7 in CWs, which is an essential addition to the environmental behavior of E. coli O157:H7, providing a theoretical basis for the prevention and control of biological contamination in wastewater treatment processes.

Long-term synergic removal performance of N, P, and CuO nanoparticles in constructed wetlands along with temporal record of Cu pollution in substrate-biofilm

With continued exposure to CuO nanoparticles (NPs) which were toxic to organisms, the performance of wastewater treatment facility might be affected. In present study, the feasibility of constructed wetlands (CWs) for wastewater treatment containing CuO NPs and common pollutants was comprehensively explored. It was found that CWs removed 98.80-99.84% CuO NPs and 90.91-91.83% COD within 300 days. However, N and P removals were affected to varying degrees by CuO NPs. N removal was inhibited only by 0.5 mg/L CuO NPs with 19.75% decreases on the mean from day 200-300. P removal was reduced by 3.80-50.75% and 1.92-7.19% under exposure of 0.5 and 5 mg/L CuO NPs throughout the experiment. Moreover, CuO NPs changed the adsorption potential of P and ammonium-N on sand-biofilm. Cu concentrations in spatial distribution decreased, while they in temporal distribution increased from 36.94 to 97.78 μg/g and from 70.92 to 282.66 μg/g at middle sand layer exposed to 0.5 and 5 mg/L CuO NPs. Mass balance model showed that substrate-biofilm was main pollutant sink for CuO NPs, N, and P. The minor Cu was absorbed by plants exposed to 0.5 and 5 mg/L CuO NPs, which decreased N by 53.40% and 18.51%，and P by 52.35% and 21.62%. Sequencing analysis indicated that CuO NPs also altered spatial microbial community. N-degrading bacteria (Rhodanobacter, Thauera, Nitrospira) changed differently, while phosphate accumulation organisms (Acinetobacter, Pseudomonas, Microlunatus) reduced. Overall, the negative effects of CuO NPs on N and P removal should be noted when CWs as ecological technologies are used to treat CuO NPs-containing wastewater.

Fate and toxicity of triclosan in tidal flow constructed wetlands amended with cow dung biochar

Triclosan (TC) is one of the threats to the environment due to its bioaccumulative nature, persistency, combined toxicity in aquatic biota, and endocrine-disrupting nature. This study revealed the removal of TC via three distinct setups of vertical flow constructed wetlands (VFCW: B-VFCW (with biochar); PB-VFCW (with plant Colocasia and biochar); C-VFCW (without biochar but with plant)) operated with normal flow and tidal-flow (flooding/drying cycles of 72 h/24 h: B-TFCW; PB-TFCW; C-TFCW) mode for 216 h of the operation cycle. The effluent was analyzed for changes in TC load and wastewater parameters (COD, NO₃-N, NH₄⁺-N, and DO). TC reduction efficiency (%) was found to be higher in PB-TFCW (98.41) followed by, C-TFCW (82.41), B-TFCW (77.51), PB-VFCW (71.83), C-VFCW (64.25), and B-VFCW (52.19) (p < 0.001). Reduction efficiency for COD (29-75 - 53.10%), and NH₄⁺-N (86.5-97.9%) was better in TFCWs than that of setups with a normal mode of operation. TFCWs showed higher DO (3.87-4.89 mg L^-1) during the operation period than that of VFCWs. The toxic impact of TC in plant stand was also assessed and results suggested low phototoxic and oxidative enzyme activities (catalase, CAT; superoxide dismutase, SOD; hydrogen peroxide, H₂O₂; malondialdehyde, MDA) in TFCWs. In summary, biochar addition and tidal flow operation played a significant role in oxidative- and microbial-mediated removals of TC in wastewater. This study provides an alternative strategy for the efficient removals of TC in constructed wetland systems and new insights into the toxic impact of pharmaceuticals on wetland plants.

The importance of rest phase and pollutant removal mechanism of tidal flow constructed wetlands (TFCW) in rural grey water treatment

This paper explored the effects of the rest phase of tidal flow constructed wetlands (TFCW) on pollutant removal and microbial communities, and further analyzed the mechanism of TFCW removal of pollutants from grey water. The results showed that the removal rate of organic matter was 69.91 ± 2.44% in the control group (NR-TFCW) without the rest phase, 94.95 ± 1.17% in the experimental group (TFCW), and 96.95 ± 2.43% in the control group (P-TFCW) with the ventilation pipe enhanced rest phase. Limiting and enhancing the oxygen supply in the emptying stage of TFCW will enhance the overlap rate of microorganisms in the upper, middle and lower layers of the reactor. Enhancing the rest phase of TFCW leaded to better aerobic removal of organic matter in the microbial community, while limiting the rest phase of TFCW results in the opposite. In addition, the species overlap rate of the top, middle and bottom layers of NR-TFCW (69.98%) and P-TFCW (54.29%) was higher than that of TFCW (11.34%). The removal of organic matter by TFCW mainly relied on the adsorption of biochar in the flood phase, and the microorganisms aerobic degraded the organic matter adsorbed on the biochar in the rest phase. And thus form a continuous cycle of adsorption and biological regeneration. The microbial community in TFCW did not have the ability to nitrify, but had the ability to remove phosphorus. Ammonia nitrogen in the influent was adsorbed by biochar or converted into cytoplasm. While the phosphorus in the influent was adsorbed by the biochar, it was also being biologically removed.

Effect of steel slag on ammonia removal and ammonia-oxidizing microorganisms in zeolite-based tidal flow constructed wetlands

The ammonia removal performance of tidal flow constructed wetlands (TFCWs) requires to be improved under high hydraulic loading rates (HLRs). The pH decrease caused by nitrification may adversely affect the NH₄⁺-N removal and ammonia-oxidizing microorganisms (AOMs) of TFCWs. Herein, TFCWs with zeolite (TFCW_Z) and a mixture of zeolite and steel slag (TFCW_S) were built to investigate the influence of steel slag on NH₄⁺-N removal and AOMs. Both TFCWs were operated under short flooding/drying (F/D) cycles and high HLRs (3.13 and 4.69 m³/(m² d)). The results revealed that a neutral effluent pH (6.98-7.82) was achieved in TFCW_S owing to the CaO dissolution of steel slag. The NH₄⁺-N removal efficiencies in TFCW_S (91.2 ± 5.1%) were much higher than those in TFCW_Z (73.2 ± 7.1%). Total nitrogen (TN) removal was poor in both TFCWs mainly due to the low influent COD/TN. Phosphorus removal in TFCW_S was unsatisfactory because of the short hydraulic retention time. The addition of steel slag stimulated the flourishing AOMs, including Nitrosomonas (ammonia-oxidizing bacteria, AOB), Candidatus_Nitrocosmicus (ammonia-oxidizing archaea, AOA), and comammox Nitrospira, which may be responsible for the better ammonia removal performance in TFCW_S. PICRUSt2 showed that steel slag also enriched the relative abundance of functional genes involved in nitrification (amoCAB, hao, and nxrAB) but inhibited genes related to denitrification (nirK, norB, and nosZ). Quantitative polymerase chain reaction (qPCR) revealed that complete AOB (CAOB) and AOB contributed more to the amoA genes in TFCW_S and TFCW_Z, respectively. Therefore, this study revealed that the dominant AOMs could be significantly changed in zeolite-based TFCW by adding steel slag to regulate the pH in situ, resulting in a more efficient NH₄⁺-N removal performance.

Optimization of constructed wetlands on purifying black-odorous water and their potential purification mechanism

Black-odorous water has become a common and widespread problem in recent decades. In this study, nine constructed wetlands (CWs) with different flow types, filters, plants, and hydraulic loadings were designed according to an orthogonal array (L₉ (3⁴), and were used for the purification of black-odorous water in summer and winter. The results showed that CWs are regarded as effective to purify black-odorous water in both seasons. Microbial degradation is the major removal pathway of pollutants in CWs during summer, while the joint effect of biodegradation and adsorption is the main treatment route during winter. Flow type and hydraulic loading appear to be the most important factors impacting the purification performance of CWs, by changing the redox condition of systems and retention time of contaminants, respectively. 'Vertical flow-zeolite filter-high loading' is proposed as the best parameter selection for CWs on the purification of black-odorous water: among them, CWs with vertical flow have better oxygen transport capacity that is conductive to aerobic processes of pollutants, zeolite substrates may adsorb more nitrogen via ion exchange, higher hydraulic loadings can extend the contact time between contaminants and filters, and regulate the water temperature for microbial activity.

Influence of iron species on the simultaneous nitrate and sulfate removal in constructed wetlands under low/high COD concentrations

Nitrate and sulfate are crucial factors of eutrophication and black and odorous water in the surface water and thus have raised increasing environmental concerns. Constructed wetlands (CWs) are the last ecological barrier before effluent enters the natural water body. To explore the simultaneous removal of nitrate and sulfate, the CW microcosms of CW-Con (with quartz sand), CW-ZVI (quartz sand and zero-valent iron), CW-Mag (quartz sand and magnetite), CW-ZVI + Mag (quartz sand, ZVI and magnetite) groups were set up under the low (100 mg/L)/high (300 mg/L) chemical oxygen demand (COD) concentration. Under the high COD condition, CW-ZVI group showed the best performance in nitrate (97.1%) and sulfate (96.9%) removal. Under the low COD concentration, the removal content of nitrate and sulfate in CW-ZVI group was better than CW-Mag group. The reason for this result was that zero-valent iron (ZVI) could be the electron donor for nitrate and sulfate reduction. Meanwhile, ZVI promoted chemical denitrification under high COD concentration according to PCA analysis. In addition, the produced sulfides inhibited the relative abundance of denitrifying bacteria, resulting in the lowest nitrate removal rate in CW-Mag group with sufficient electron donors. This study provided an alternative method to enhance simultaneous sulfate and nitrate removal in CWs.

Enhancement of nitrogen removal and energy recovery from low C/N ratio sewage by multi-electrode electrochemical technology and tidal flow via siphon aeration

In view of the difficulty in denitrification of low C/N ratio wastewater, electrochemical technology with multiple electrodes and tidal flow method via siphon aeration were used to enhance the denitrification process. At the same time, because of the low phosphorus removal efficiency in traditional activated sludge process, the constructed wetland and microbial fuel cell (CW-MFC) reactor with dewatered alum sludge (DAS) as substrate were constructed. In addition, the REDOX conditions of the reactor were changed by siphon, which significantly improved the removal efficiency of N and P and the energy recovery capacity of the reactor. In the 172 d, the Tidal Flow Constructed Wetland-Microbial Fuel Cell (TF CW-MFC) had the highest removal efficiency of COD and total nitrogen (TN), which were 97.4% and 83.4%, respectively. Although the removal rate of total phosphorus (TP) by TF CW-MFC was lower than artificial aeration, it can still reached 89.0%. The removal effect of aromatic protein substances in water was also significant. The amount of electrons generated by the artificial aeration anode and the amount of oxygen generated by the cathode were not enough to match. The voltage of TF CW-MFC was significantly higher than artificial aeration, around 350 mV, and the maximum power density was 98.16 mW m^-3. In addition, MFC had an inhibitory effect on CW methane emissions. The analysis of the microbial community structure showed that most of the dominant bacteria of TF CW-MFC belonged to the Proteobacteria, Actinobacteria and Chloroflexi. These results showed that the TF CW-MFC technology as a zero-energy oxygen supply mode had high efficiency in the treatment of low C/N ratio wastewater and also had the environmental effect of reducing methane emissions. This study suggests that this green wastewater treatment technology has potential application value.

Stereoselective degradation pathway of amide chiral herbicides and its impacts on plant and bacterial communities in integrated vertical flow constructed wetlands

This study demonstrates the stereoselective degradation patterns and biodegradation mechanisms of metolachlor (MET) and napropamide (NAP) in integrated vertical flow constructed wetlands (IVCW). The higher interphase transferability of NAP resulted in higher degradation rates of 90.60 ± 4.09%. The enantiomeric fraction (EF) values of 0.38 ± 0.02 and 0.54 ± 0.03, respectively, recorded for the enantiomers S-MET and R-NAP, with higher herbicidal activities, demonstrated their highly selective biodegradation patterns. The antioxidant enzyme activities and fluorescence parameters of plants showed positive correlations with the degradation efficiency and enantioselectivity of MET and NAP. Adaptive regulations by plants promoted the proliferation of microbial genera like Enterobacter and unclassified_Burkholderiales, which could facilitate plant growth. Moreover, enrichment of the herbicide-degrading functional bacteria Terrimonas (5.10%), Comamonas (4.05%) Pseudoxanthomonas (4.49%) and Mycobacterium (1.42%) demonstrably promoted the preferential degradation of S-MET and R-NAP. Furthermore, the abundance of Ferruginibacter favored the use of R-NAP as carbon source to achieve co-removal of R-NAP and NO₃^--N.

A Review on Microorganisms in Constructed Wetlands for Typical Pollutant Removal: Species, Function, and Diversity

Constructed wetlands (CWs) have been proven as a reliable alternative to traditional wastewater treatment technologies. Microorganisms in CWs, as an important component, play a key role in processes such as pollutant degradation and nutrient transformation. Therefore, an in-depth analysis of the community structure and diversity of microorganisms, especially for functional microorganisms, in CWs is important to understand its performance patterns and explore optimized strategies. With advances in molecular biotechnology, it is now possible to analyze and study microbial communities and species composition in complex environments. This review performed bibliometric analysis of microbial studies in CWs to evaluate research trends and identify the most studied pollutants. On this basis, the main functional microorganisms of CWs involved in the removal of these pollutants are summarized, and the effects of these pollutants on microbial diversity are investigated. The result showed that the main phylum involved in functional microorganisms in CWs include Proteobacteria, Bacteroidetes, Actinobacteria and Firmicutes. These functional microorganisms can remove pollutants from CWs by catalyzing chemical reactions, biodegradation, biosorption, and supporting plant growth, etc. Regarding microbial alpha diversity, heavy metals and high concentrations of nitrogen and phosphorus significantly reduce microbial richness and diversity, whereas antibiotics can cause large fluctuations in alpha diversity. Overall, this review can provide new ideas and directions for the research of microorganisms in CWs.

Comparing the effects of plant diversity on the nitrogen removal and stability in floating and sand-based constructed wetlands under ammonium/nitrate ratio disturbance

Maintaining efficient and stable nitrogen (N) removal in constructed wetlands (CWs) that experience disturbance from their influent pollutant variations is crucial. The ammonium/nitrate (NH₄⁺/NO₃^-) ratio of influent in CWs often varies widely. The N removal and stability in floating CWs have been found to be enhanced by manipulating plant species diversity. However, whether the positive effects occur in sand-based CWs remains unknown. Here, we established sand-based and hydroponic microcosms to investigate the differences in the responses of N removal and stability to plant species diversity under the disturbance of increasing influent NH₄⁺/NO₃^- ratio in late period of plant growth. Results indicated that, (1) increasing plant species richness enhanced N removal but did not affect N removal stability in sand-based CWs under disturbance; (2) sand-based CWs had 46% higher average N removal stability than floating CWs, but the stability in floating CWs reached that in sand-based CWs at higher species richness levels; (3) under disturbed conditions, floating CWs with Phragmites australis or Typha latifolia achieved N removal and stability equivalent to those in sand-based CWs. This study indicates that, when treating wastewater with a variable NH₄⁺/NO₃^- ratio, floating CWs with high plant species richness and specific species can achieve a win-win situation for high and stable N removal and bioenergy production.

An integrated biological-electrocatalytic process for highly-efficient treatment of coking wastewater

Coking wastewater is typically refractory, mainly due to its biological toxicity and complex composition. In this study, a novel integrated biological-electrocatalytic process consisting of two three-dimensional electrochemical reactors (3DERs), two biological aerated filters (BAFs), and a three-dimensional biofilm electrode reactor (3DBER) is developed for the advanced treatment of coking wastewater. 73.21% of chemical oxygen demand (COD), 38.02% of ammonium nitrogen (NH₄⁺-N) and 91.46% of nitrate nitrogen (NO₃^--N) are removed by 3DERs. BAFs mainly convert NH₄⁺-N to NO₃^--N through microbial nitrification. The 3DBER removes the residual NO₃^--N by bio-electrochemical denitrification. The integrated system can eliminate 74.72-83.27% of COD, 99.38-99.74% of NH₄⁺-N, and 69.64-99.83% of total nitrogen from coking wastewater during the continuous operation, as well as significantly reducing the toxicity of the wastewater. The superiorities of the integrated 3DERs/BAFs/3DBER system recommend the application of such biological-electrocatalytic technology in the treatment of highly toxic wastewater.