Subsurface constructed wetlands with modified biochar added for advanced treatment of tailwater: Performance and microbial communities

The impact of biochar and activated carbon on the purification efficiency of two wetland systems under varying pollution loads

The water quality purification of constructed wetlands (CWs) with adsorbent materials has been extensively studied, yet the impact on the improvement in water quality of natural sediment wetlands (SWs) remains unclear. In this study, CWs and SWs were established with biochar and activated carbon substrates, and high and low nitrogen (N) and phosphorus (P) pollution loads were applied. Additionally, relevant studies on the purification efficiencies of artificial wetlands under different pollution loads were reviewed. Results showed that the purification capacities of biochar and activated carbon were affected by the pollution load, and that there was significant interaction between the pollution load and wetland type (P = 0.000). The removal efficiencies of NH4+, NO3-, and total P in CWs were significantly higher than those in SWs. In particular, the removal stability of NO3- was higher in the BCW (77.12%) and ACW (75.94%) under high pollution loads (P = 0.000). As the pollution load increased, the TP removal efficiency of SWs decreased from 86.56% to 75.97%, while the addition of biochar significantly enhanced removal efficiency (94.57%). The BSW experiment was more efficient in purifying P and NH4+ than ASW or SW. ACWs presented lowest microbial diversity and richness while higher metabolism of chemoheterotrophy and nitrate reduction. This study confirms that the addition of B can promote the sustainability of SW purification, and that both biochar and activated carbon exhibited varying adsorption characteristics in different wetland types. It demonstrates the improved purification potential in biochar added SWs.

Biochar-amended constructed wetlands enhance sulfadiazine removal and reduce resistance genes accumulation in treatment of mariculture wastewater

With the rapid development of mariculture, an increasing amount of antibiotics are being discharged into the marine environment. Effectively removing antibiotics and antibiotic resistance genes (ARGs) in mariculture wastewater with a relatively high salinity and low C/N presents challenges. Biochar-amended constructed wetlands (CWs) can effectively remove antibiotics, However, few studies have compared the impacts of biochar-amended CWs pyrolyzed at different temperatures on the treatment of mariculture wastewater. Thus, this study utilized biochar prepared at three temperatures as substrate for CWs (CW-300, CW-500, and CW-700), aiming to evaluate their efficiency to treat mariculture wastewater containing antibiotic sulfadiazine (SDZ). The results demonstrated that compared to traditional quartz sand-filled CW (NCW), the addition of biochar with a larger specific surface area significantly enhanced the removal efficiency of SDZ by 21.72%-46.96%. Additionally, the addition of biochar effectively reduced the relative abundance of one integron gene (int1) and antibiotic resistance genes (ARGs) including sul1, sul2, and sul3 in both effluent and substrates. The addition of biochar reduced the accumulation of extracellular polymeric substances within the substrate of CWs, thereby mitigating the proliferation and spread of ARGs. The microbial community structure indicated that the addition of biochar increased the abundance of the potential antibiotic-degrading bacteria such as Proteobacteria and Bacteroidota, facilitating the degradation of SDZ and mitigating the accumulation of ARGs. This study demonstrated that biochar can be a promising substrate in CWs for treating mariculture wastewater containing antibiotics.

Manganese-modified reed biochar decreased nutrients and methane release from algae debris-contaminated sediments

Biochar is one of the ways for carbon storage, pollution control and biosolid reuse. Aquatic plant reeds are widely used in nutrient removal in wetlands and have huge biomass. Nonetheless, little is known regarding the effects of reed-based biochar on sediments. In this study, reed-based biochar (BC) modified with or without Mn-oxidizers (MBC) was prepared to investigate their impacts on nutrient removal, methane (CH4) emission fluxes, and CH4 concentration and microbial community in sediments for 20 days. We found that BC and MBC significantly reduced CH4 emission fluxes by 56.84 ± 10.47% and 69.95 ± 0.76% (p < 0.05) compared to control (CK), respectively. In addition, BC and MBC had a higher efficiency of nutrient removal, and the removal rate increased by 4.4% for NH4+-N and 10.13% for TN in BC and by 3.21%, 8.43% and 18.29% for NH4+-N, TN and TP in MBC, respectively. Proteobacteria, Chloroflexi, Bacteroidota, Firmicutes, Desulfobacterota and Acidobacteriota were the predominant phyla in sediments and might contribute to nutrient removal. Network analysis revealed that biochar addition promoted interspecific competition in sediments, which could be more beneficial for enhancing the stability of microbial community structures. The decreased mcrA (CH4 bioproduction) abundance but increased pomA (CH4 oxidation) abundance was detected in BC and MBC compared to CK, explaining biochar-reduced CH4 emissions. This study highlights that reed straw-based biochar can be used in the in-situ remediation of polluted sediments and provides a choice for carbon storage and pollution control for managers.

Simplification and simulation of evaluation process for low efficiency constructed wetlands based on principal component analysis and machine learning

The existing performance evaluation process of constructed wetlands (CWs) is complex, with shortcomings in both simplification of method and construction of simulation model, especially for low-efficiency CWs (LECWs, with an average close-degree calculated by the entropy weight method being <0.6). This study presents a case study of LECWs in the Ningxia region (comprising 13 subsurface flow constructed wetlands (SSF CWs) and 7 surface flow constructed wetlands (SF CWs)), employs the entropy weight method (EWM) to construct an evaluation of CW operational efficiency, simplifies evaluation indicators through principal component analysis (PCA), develops two random forest (RF) models to validate the rationality of the simplified indicators, and establishes simulation models by logistic regression (LR). The results demonstrate that the evaluation indicators of CWs can be simplified to chemical oxygen demand (COD) and total nitrogen (TN), with no significant difference observed between the evaluation results and the original model (P < 0.05), thereby indicating reliability. Moreover, the simulation model performs well with R2 values for fitting SSF CWs and SF CWs exceeding 0.8. According to the simulated results of the model, the operational efficiency of LECWs is more significantly affected by the COD removal rates compared to the TN removal rates. In comparison to influent with 0 < COD/TN < 3 and 5 < COD/TN < 8, the operational efficiency of SSF CWs and SF CWs is optimal when COD/TN is between 3 and 5. These research findings may provide valuable support for streamlining evaluation processes and daily management for LECWs.

Construction of an ideotype root system architecture of subsurface flow constructed wetland macrophytes by vertical spatial stress: strengthening of rhizosphere effects and determination of appropriate substrate depth

Strengthening rhizosphere effects to enhance pollutant removal is a hotspot of constructed wetlands (CWs) research in recent years, and improving the root traits and metabolic capacity of macrophytes is crucial for strengthening rhizosphere effects. In the field experiment, two types of subsurface flow (SSF) CWs (CW10 and CW20, with substrate depths of 10 and 20 cm, respectively) under the vertical spatial stress of roots (VSSR) and two types of non-VSSR SSF CWs (CW40 and CW60) were adopted with Typha orientalis as cultivated plants to investigate the variability of root development, metabolism, and pollutant removal at different substrate depths. VSSR induced substantial redundant root development, which significantly increased root-shoot ratio, fine and lateral root biomass, root porosity, and root activity, with lateral and fine root biomass of CW20 reaching 409.17 and 237.42 g/m2, respectively, which were 3.18 and 5.28 times those of CW60. The radical oxygen loss (ROL) and dissolved organic carbon (DOC) levels of CW20 single plant were 1.36 and 4.57 times higher than those of CW60, respectively, and more types of root exudates were determined (e.g., aldehydes, ketones and amides). More aerobic heterotrophs (e.g., Massilia, Planomicrobium), nitrification bacteria (e.g., Ellin6067, Nitrospira), aerobic denitrification bacteria (e.g., Bacillu, Chryseobacterium, Pseudomonas) and denitrification phosphorus accumulating organisms (e.g., Flavobacterium) were enriched in the rhizosphere of CW20. This changed the main transformation pathways of pollutants and enhanced the removal of pollutants, with the COD, TN and TP average removal rates of CW20 increasing by 9.99%, 13.28% and 8.92%, respectively, compared with CW60. The ideotype root system architecture CW (RSACW; CW20) constructed in this study, which consists of a large number of fine and lateral roots, can stimulate more efficient rhizosphere effects stably and continuously.

Composition of the microbial community in surface flow-constructed wetlands for wastewater treatment

Traditionally constructed wetlands face significant limitations in treating tailwater from wastewater treatment plants, especially those associated with sugar mills. However, the advent of novel modified surface flow constructed wetlands offer a promising solution. This study aimed to assess the microbial community composition and compare the efficiencies of contaminant removal across different treatment wetlands: CW1 (Brick rubble, lignite, and Lemna minor L.), CW2 (Brick rubble and lignite), and CW3 (Lemna minor L.). The study also examined the impact of substrate and vegetation on the wetland systems. For a hydraulic retention time of 7 days, CW1 successfully removed more pollutants than CW2 and CW3. CW1 demonstrated removal rates of 72.19% for biochemical oxygen demand (BOD), 74.82% for chemical oxygen demand (COD), 79.62% for NH4 +-N, 77.84% for NO3 --N, 87.73% for ortho phosphorous (OP), 78% for total dissolved solids (TDS), 74.1% for total nitrogen (TN), 81.07% for total phosphorous (TP), and 72.90% for total suspended solids (TSS). Furthermore, high-throughput sequencing analysis of the 16S rRNA gene revealed that CW1 exhibited elevated Chao1, Shannon, and Simpson indices, with values of 1324.46, 8.8172, and 0.9941, respectively. The most common bacterial species in the wetland system were Proteobacteria, Spirochaetota, Bacteroidota, Desulfobacterota, and Chloroflexi. The denitrifying bacterial class Rhodobacteriaceae also had the highest content ratio within the wetland system. These results confirm that CW1 significantly improves the performance of water filtration. Therefore, this research provides valuable insights for wastewater treatment facilities aiming to incorporate surface flow-constructed wetland tailwater enhancement initiatives.

Maximizing pollutant removal and greenhouse gas emission reduction in vertical flow constructed wetlands: an orthogonal experimental approach

Owing to the impact of the effluent C/N from the secondary structures of urban domestic wastewater treatment plants, the denitrification efficiency in constructed wetlands (CWs) is not satisfactory, limiting their widespread application in the deep treatment of urban domestic wastewater. To address this issue, we constructed enhanced CWs and conducted orthogonal experiments to investigate the effects of different factors (C/N, fillers, and plants) on the removal of conventional pollutants and the reduction of greenhouse gas (GHG) emission. The experimental results indicated that a C/N of 8, manganese sand, and calamus achieved the best denitrification efficiencies with removal efficiencies of 85.7%, 95.9%, and 88.6% for TN, NH4+-N, and COD, respectively. In terms of GHG emission reduction, this combination resulted in the lowest global warming potential (176.8 mg/m2·day), with N2O and CH4 emissions of 0.53 and 1.25 mg/m2·day, respectively. Characterization of the fillers revealed the formation of small spherical clusters of phosphates on the surfaces of manganese sand and pyrite and iron oxide crystals on the surface of pyrite. Additionally, the surface Mn (II) content of the manganese sand increased by 8.8%, and the Fe (III)/Fe (II) and SO42-/S2- on pyrite increased by 2.05 and 0.26, respectively, compared to pre-experiment levels. High-throughput sequencing indicated the presence of abundant autotrophic denitrifying bacteria (Sulfuriferula, Sulfuritalea, and Thiobacillus) in the CWs, which explains denitrification performance of the enhanced CWs. This study aimed to explore the mechanism of efficient denitrification and GHG emission reduction in the enhanced CWs, providing theoretical guidance for the deep treatment of urban domestic wastewater.

Recent advances in swine wastewater treatment technologies for resource recovery: A comprehensive review

Swine wastewater (SW), characterized by highly complex organic and nutrient substances, poses serious impacts on aquatic environment and public health. Furthermore, SW harbors valuable resources that possess substantial economic potential. As such, SW treatment technologies place increased emphasis on resource recycling, while progressively advancing towards energy saving, sustainability, and circular economy principles. This review comprehensively encapsulates the state-of-the-art knowledge for treating SW, including conventional (i.e., constructed wetlands, air stripping and aerobic system) and resource-utilization-based (i.e., anaerobic digestion, membrane separation, anaerobic ammonium oxidation, microbial fuel cells, and microalgal-based system) technologies. Furthermore, this research also elaborates the key factors influencing the SW treatment performance, such as pH, temperature, dissolved oxygen, hydraulic retention time and organic loading rate. The potentials for reutilizing energy, biomass and digestate produced during the SW treatment processes are also summarized. Moreover, the obstacles associated with full-scale implementation, long-term treatment, energy-efficient design, and nutrient recovery of various resource-utilization-based SW treatment technologies are emphasized. In addition, future research prospective, such as prioritization of process optimization, in-depth exploration of microbial mechanisms, enhancement of energy conversion efficiency, and integration of diverse technologies, are highlighted to expand engineering applications and establish a sustainable SW treatment system.

Biochar imparted constructed wetlands (CWs) for enhanced biodegradation of organic and inorganic pollutants along with its limitation

The remediation of polluted soil and water stands as a paramount task in safeguarding environmental sustainability and ensuring a dependable water source. Biochar, celebrated for its capacity to enhance soil quality, stimulate plant growth, and adsorb a wide spectrum of contaminants, including organic and inorganic pollutants, within constructed wetlands, emerges as a promising solution. This review article is dedicated to examining the effects of biochar amendments on the efficiency of wastewater purification within constructed wetlands. This comprehensive review entails an extensive investigation of biochar's feedstock selection, production processes, characterization methods, and its application within constructed wetlands. It also encompasses an exploration of the design criteria necessary for the integration of biochar into constructed wetland systems. Moreover, a comprehensive analysis of recent research findings pertains to the role of biochar-based wetlands in the removal of both organic and inorganic pollutants. The principal objectives of this review are to provide novel and thorough perspectives on the conceptualization and implementation of biochar-based constructed wetlands for the treatment of organic and inorganic pollutants. Additionally, it seeks to identify potential directions for future research and application while addressing prevailing gaps in knowledge and limitations. Furthermore, the study delves into the potential limitations and risks associated with employing biochar in environmental remediation. Nevertheless, it is crucial to highlight that there is a significant paucity of data regarding the influence of biochar on the efficiency of wastewater treatment in constructed wetlands, with particular regard to its impact on the removal of both organic and inorganic pollutants.