Constructed wetlands for rural domestic wastewater treatment: A coupling of tidal strategy, in-situ bio-regeneration of zeolite and Fe(Ⅱ)-oxygen denitrification

Integration of steel slag and zeolite enhances simultaneous nitrification and autotrophic denitrification in ultra-low carbon/nitrogen ratio wastewater: Remodeling microbiota and iron metabolism

Constructed wetlands (CWs) are widely used for nitrogen pollution control in rural aquatic environments, yet their nitrogen removal efficiency often remains suboptimal. This study firstly examines how zeolite robustly stimulates Fe-utilization of steelmaking waste (i.e., steel slag) to improve nitrification and autotrophic denitrification of low carbon-to-nitrogen (C/N) ratio wastewater (C/N ≈ 1). Steel slag, by providing alkalinity for nitrification, also serves as an electron donor for denitrification due to its low-valent iron content. As a result, the total nitrogen (TN) removal efficiency was increased by 153.5% compared to the control group. Zeolite reshaped the microbial consortia, enriching iron autotrophic denitrifying bacteria and aerobic denitrifying bacteria. More importantly, zeolite facilitated microbial iron utilization by enhancing transmembrane iron transport and intracellular iron oxidation to boost nitrification and autotrophic denitrification without additional aeration, external carbon sources, or pH regulation. Our work advances understanding the development of low carbon technologies for wastewater nitrogen removal.

The introduction of nano zero-valent iron in constructed wetlands simultaneously enhanced the removal of perfluorooctanoic acid (PFOA) and nutrients

Constructed wetland (CW) serve as the final ecological barrier for hazardous materials entering the natural water environment. Due to the ecological toxicity and difficult bioutilization characteristics of perfluorooctanoic acid (PFOA) itself, CW technology faces great challenges in the field of PFOA remediation. In this study, nano zero-valent iron (nZVI) was introduced into CWs to explore the mechanism of the synergistic removal of PFOA and nutrients in nZVI-CW system. The results indicated that the addition of 10 mg/L nZVI improved the removal efficiency of CW for 1 and 10 mg/L PFOA, with an average removal rate increased by 3.53-8.70%. The transformation products in CW effluents were qualitatively detected using HPLC-Q-TOF-MS, suggesting that the degradation of PFOA may involve decarboxylation, hydrolysis, redox, elimination, substitution and intramolecular rearrangement processes. The presence of nZVI enhanced the average removal rates of NH4+-N, NO3--N and TP by 2.78-18.4% in CWs. The increase in key substrate enzyme activity confirmed the stimulating effect of nZVI on microbial activity. The addition of nZVI facilitated the growth and enrichment of hydroautotrophic denitrifying bacteria, nitrat-dependent iron-oxidizing bacteria, and dissimilatory iron-reducing bacteria. Two types of dissimilatory iron-reducing bacteria (Geobacter and Acinetobacter) may be potential PFOA-degrading bacteria. Additionally, signaling pathways related to carbohydrate metabolism, energy metabolism, and xenobiotic degradation and metabolism exhibited higher abundance in the nZVI treated groups.

Enhanced remediation of real agricultural runoff in surface-flow constructed wetlands by coupling composite substrate-packed bio-balls, submerged plants and functional bacteria: Performance and mechanisms

Import of agricultural runoff containing nutrients considerably contributes to eutrophication of receiving water bodies. Surface-flow constructed wetlands (SFCWs) are commonly applied for agricultural runoff purification, but the performance is usually unsatisfactory. In this study, suspended bio-balls filled with zeolite and iron-carbon (Fe-C) composite substrates, submerged macrophyte (Ceratophyllum demersum) and functional denitrifying bacteria were collectively added into SFCW microcosms to enhance the remediation efficiency for real agricultural runoff with high nutrient concentrations and low content of bioavailable organic matter. The bio-ball added SFCWs achieved notably higher pollutant removal efficiencies (21.1%, 80.2% and 47.5% for chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP), respectively) than the control (COD: 6.9%, TN: 64.4%, TP: 27.9%), because of the versatile functions of filling materials for pollutant removal. C. demersum plantation (COD: 44.2%, TN: 82.8% and TP: 53.7%) and functional bacteria inoculation (COD: 51.8%, TN: 85.8% and TP: 55.1%) further enhanced the efficiency of the SFCWs for agricultural runoff remediation. Bio-ball addition and C. demersum plantation significantly increased the humification degree and reduced the molecular weight of dissolved organic matter (DOM) in the agricultural runoff. Moreover, the two intensification measures also notably reduced organic and nitrogen contents in the wetland sediment. Remarkable distinction in bacterial community distribution patterns was observed in the SFCW sediment and filling substrates in bio-balls. Keystone genera including Clostridium_sensu_stricto_1 and Bacillus in the zeolite, Sphingomonas and Exiguobacterium in the Fe-C substrates and Sediminibacterium in the sediment might be critical for agricultural runoff remediation in the SFCW microcosms. The study highlights a high potential of the intensified SFCWs by these coupling measures for agricultural runoff remediation.

Constructed wetlands using recycled aggregates for the improved treatment of tailwater

Recycled aggregates from the construction industry could be effective materials for constructed wetland (CW) wastewater treatment systems. However, whether the plants or which kinds of plants can survive in CWs with recycled aggregates or not is still scarce and urgent to study. The effects of different plant species, and several operation parameters on tailwater treatment from the sewage plants by CWs with different substrates (mixed construction recycled aggregate [MCRA], red brick [RB], and gravel) are systemically studied. The results showed that the average removal efficiencies of the chemical parameters in MCRA-CWs with seven plants, such as Cyperus alternifolius, were higher than those of RB-CWs and Gravel-CWs. In the MCRA-CWs, the average removal efficiencies of Typha orientalis, Cyperus alternifolius, and Phragmites australis were 77.58%, 75.09%, and 73.16%, respectively, which were higher than those of the other plants. Meanwhile, Proteobacteria had the highest relative abundances in MCRA and RB at 54.98% and 69.22%, respectively, whereas Cyanobacteria (35.21%) were the most abundant in the gravel. The influence of season on water purification was significant (p < 0.05) in the MCRA. Overall, water quality purification was dependent on season, HRT, and C/N, which accounted for 86.1%, 13%, and 7.1%, respectively. The highest average removal efficiencies of the MCRA-CWs were 71.98% (COD), 85.58% (NH4+-N), 95.01% (TN), and 84.11% (TP) when the HRT was 3 d and C/N ratio was 2.5 in the summer. This indicates that treated recycled construction aggregates could be used as substrates in CWs and have both wastewater purification and environmental improvement effects, thus achieving the purpose of "treating the wastes with wastes".

Simultaneous removal of nitrogen, phosphorus, and organic matter from oligotrophic water in a system containing biochar and construction waste iron: Performances and biotic community analysis

The issue of combined pollution in oligotrophic water has garnered increasing attention in recent years. To enhance the pollutant removal efficiency in oligotrophic water, the system containing Zoogloea sp. FY6 was constructed using polyester fiber wrapped sugarcane biochar and construction waste iron (PWSI), and the denitrification test of simulated water and actual oligotrophic water was carried out for 35 days. The experimental findings from the systems indicated that the removal efficiencies of nitrate (NO3--N), total nitrogen (TN), chemical oxygen demand (COD), and total phosphorus (TP) in simulated water were 88.61%, 85.23%, 94.28%, and 98.90%, respectively. The removal efficiencies of actual oligotrophic water were 83.06%, 81.39%, 81.66%, and 97.82%, respectively. Furthermore, the high-throughput sequencing data demonstrated that strain FY6 was successfully loaded onto the biological carrier. According to functional gene predictions derived from the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, the introduction of PWSI enhanced intracellular iron cycling and nitrogen metabolism.

Research on the purification effect of major pollutants in water by modular constructed wetlands with different filler combinations

Constructed wetland systems have been widely used in China due to their advantages of good treatment effect, low cost and environmental friendliness. However, traditional constructed wetlands have challenges in application such as deactivation due to filler clogging, difficulty in filler replacement and low adaptability. To address the above problems, this research proposes a modular filler design constructed wetland based on the concept of assembly construction, which can quickly replace the clogged filler without destroying the overall structure of the wetland. Four commonly used fillers were selected and applied to the pilot system of the assembled constructed wetland in this study, in order to investigate the purification effect of the constructed wetland system with different filler module combinations (CW1, CW2, CW3) on the simulated wastewater. The results showed that the filler combination CW1 was the best for the removal of NH4+-N, and for TP and COD, CW2 has the best removal effect. Therefore, the assembled constructed wetland is adjustable and substantially reduces the maintenance cost, which provides technical guidance for its application in engineering.

Enhanced treatment of low C/N ratio rural sewage by a modified multi-stage tidal flow constructed wetland at low temperature: Quantitative contributions of key functional genera

Rural sewage treatment was traditionally faced contradiction between low-treatment rates and the need for low-cost development. To address this challenge, we explored the coupling of effluent circulation and step-feeding strategies in a multi-stage tidal flow constructed wetland (TFCW) to achieve stable nitrogen (N) removal performance under conditions of low carbon-to-nitrogen (C/N) ratios and low temperatures. The modified multi-stage TFCW demonstrated the ability to significantly reduce the concentrations of effluent NH4+-N and NO3--N by 33.9 % and 54.8 % respectively, resulting in values of 7.47 mg/L and 3.93 mg/L. Additionally, it achieved an average TN removal efficiency of 69.2 %. The improved N removal performance of rural sewage by the modified multi-stage TFCW at low temperatures was primarily attributed to autotrophic nitrification, heterotrophic nitrification, and autotrophic denitrification. Among the identified functional genera, Nitrosomonas and Nitrosospira played key roles as autotrophic nitrification bacteria (ANB), contributing to 28.2 % of NH4+-N removal. The key heterotrophic nitrification bacteria (HNB) Acidovorax and Rudaea were mainly responsible for 71.3 % of NH4+-N removal via the two-step ammonia assimilation through the organic nitrogen pathway. Furthermore, Rhodanobacter and Acinetobacter emerged as key autotrophic denitrification bacteria (ADNB), accounting for 79.9 % of NO3--N conversion and removal. In summary, this study provides valuable theoretical insights and supports ongoing efforts in biological regulation to address the challenges associated with rural sewage treatment.

Performance and mechanism of sacrificed iron anode coupled with constructed wetlands (E-Fe) for simultaneous nitrogen and phosphorus removal

Three anodic biofilm electrode coupled CWs (BECWs) with graphite (E-C), aluminum (E-Al), and iron (E-Fe), respectively, and a control system (CK) were constructed to evaluate the removal performance of N and P in the secondary effluent of wastewater treatment plants (WWTPs) under different hydraulic retention time (HRT), electrified time (ET), and current density (CD). Microbial communities, and different P speciation, were analyzed to reveal the potential removal pathways and mechanism of N and P in BECWs. Results showed that the optimal average TN and TP removal rates of CK (34.10% and 55.66%), E-C (66.77% and 71.33%), E-Al (63.46% and 84.93%), and E-Fe (74.93% and 91.22%) were obtained under the optimum conditions (HRT 10 h, ET 4 h, CD 0.13 mA/cm²), which demonstrated that the biofilm electrode could significantly improve N and P removal. Microbial community analysis showed that E-Fe owned the highest abundance of chemotrophic Fe(II) (Dechloromonas) and hydrogen autotrophic denitrifying bacteria (Hydrogenophaga). N was mainly removed by hydrogen and iron autotrophic denitrification in E-Fe. Moreover, the highest TP removal rate of E-Fe was attributed to the iron ion formed on the anode, causing co-precipitation of Fe(II) or Fe(III) with PO₄^3--P. The Fe released from the anode acted as carriers for electron transport and accelerated the efficiency of biological and chemical reactions to enhance the simultaneous removal of N and P. Thus, BECWs provide a new perspective for the treatment of the secondary effluent from WWTPs.

Effects of iron-based substrate on coupling of nitrification, aerobic denitrification and Fe(II) autotrophic denitrification in tidal flow constructed wetlands

The aerobic properties of nitrification and the anaerobic properties of denitrification in constructed wetlands are difficult to reconcile. In this study, two constructed wetlands were constructed with pyrite and steel slag in combination with zeolite, and their respective nitrification and denitrification capacities were evaluated under different tidal strategies. The steel slag wetland achieved 70.89 % and 46.04 % removal rates of NH₄⁺-N and total nitrogen (TN), and the carbon consumption of denitrification was 1.51 mg BOD/mgN, which was better than pyrite wetland. Microbial analysis showed that Fe(II) autotrophic denitrification and aerobic denitrification occurred in both wetlands, and they were coupled with nitrification to achieve simultaneous removal of NH₄⁺-N and TN. Microbial co-occurrence network and k-core decomposition analysis indicated that the core genus of steel slag wetlands was nitrifying bacteria. This study provides new insights into the application of tidal flow wetlands to treat rural sewage.

Nitrogen and sulfamethoxazole removal in a partially saturated vertical flow constructed wetland treating synthetic mariculture wastewater

This study investigated the removal of nitrogen and sulfamethoxazole (SMX), and the microbial communities in a partially saturated vertical flow constructed wetland (PS-VFCW) fed with synthetic mariculture wastewater operated at different saturated zone depths (SZDs), i.e. 51, 70, and 60 cm. Removal efficiencies were 99.8%-100.0% for COD, 34.1%-100.0% for NH4+-N, 67.8%-97.3% for total inorganic nitrogen (TIN), and 29.8%-57.2% for SMX. Excellent nitrification performance was achieved at the SZDs of 51 and 60 cm. Denitrification performed well at 70 and 60 cm SZDs. The highest TIN removal efficiency (97.3%) was achieved as the SZD was 60 cm. SMX removal was significantly influenced by SZD and was promoted by higher SZD. The removal of organics, nitrogen, and SMX mainly occurred in the unsaturated zone. Ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, denitrifying bacteria, and SMX-degrading bacteria were detected in the unsaturated and saturated zones, and showed an increasing trend in abundance along the depth.