Effects of plant roots on the hydraulic performance during the clogging process in mesocosm vertical flow constructed wetlands

Development and trends of constructed wetland substrates over the past 30 years: a literature visualization analysis based on CiteSpace

Constructed wetland substrates (CWSs) have received considerable attention owing to their importance in adsorbing and degrading pollutants, providing growth attachment points for microorganisms, and supporting wetland plants. There are differences in the configurations and functions of constructed wetlands (CWs) for treating different water bodies and sewage, resulting in a wide variety of substrates. Research on the application and mechanism of CWSs is not sufficiently systematic. Therefore, the current research advancements and hotspots must be identified. Hence, we used CiteSpace to analyze 1955 English publications from the core collection database of the Web of Science to assess the current state of the CWS research field. Based on the cooperative network analysis, the roles of various countries, institutions, and authors in research on CWSs were reviewed. Keyword co-occurrence and cluster analyses were used to discuss the transformation of CWSs from removing traditional pollutants to emerging pollutants and the transition from incorporating natural substrates to artificial substrates. Finally, we underscored the need for more emphasis to be placed on the collocation and application of the CWSs at different latitudes. Furthermore, the substrate micro-interface process and its effects on the interaction patterns of pollutants and microorganisms should be thoroughly investigated to provide theoretical guidance for the development of wetland applications and mechanisms.

Bio-clogging mitigation in constructed wetland using microbial fuel cells with novel hybrid air-photocathode

Excessive accumulation of extracellular polymeric substances (EPS) in constructed wetland (CW) substrate can lead to bio-clogging and affect the long-term stable operation of CW. In this study, a microbial fuel cell (MFC) was coupled with air-photocathode to mitigate CW bio-clogging by enhancing the micro-electric field environment. Because TiO₂/biochar could catalyze and accelerate oxygen reduction reaction, further promoting the gain of electric energy, the electricity generation of the tandem CW-photocatalytic fuel cell (CW-PFC) reached 90.78 mW m^-3. After bio-clogging was mitigated in situ in tandem CW-PFC, the porosity of CW could be restored to about 62.5 % of the initial porosity, and the zeta potential of EPS showed an obvious increase (-14.98 mV). The removal efficiencies of NH₄⁺-N and chemical oxygen demand (COD) in tandem CW-PFC were respectively 31.8 ± 7.2 % and 86.1 ± 6.8 %, higher than those in control system (21.1 ± 11.0 % and 73.3 ± 5.6 %). Tandem CW-PFC could accelerate the degradation of EPS into small molecules (such as aromatic protein) by enhancing the electron transfer. Furthermore, microbiome structure analysis indicated that the enrichment of characteristic microorganisms (Anaerovorax) for degradation of protein-related pollutants, and electroactive bacteria (Geobacter and Trichococcus) promoted EPS degradation and electron transfer. The degradation of EPS might be attributed to the up-regulation of the abundances of carbohydrate and amino acid metabolism. This study provided a promising new strategy for synergic mitigation and prevention of bio-clogging in CW by coupling with MFC and photocatalysis.

Increasing plant diversity offsets the influence of coarse sand on ecosystem services in microcosms of constructed wetlands

As wastewater treatment systems that strengthen natural processes, constructed wetlands provide both ecosystem services and disservices. Manipulating both the physical and ecological structures of constructed wetlands has been the key to improve ecosystem services while reducing disservices. Herein, an experiment using simulated constructed wetlands was conducted to explore the effect of two different substrate sizes (fine sand or coarse sand), plant richness (1, 3, or 4 species), and plant species identity on ecosystem services. Results indicated that (1) only in microcosms with coarse sand, species richness enhanced nitrogen removal efficiency while reduced nitrous oxide emissions and that (2) the presence of Phalaris arundinacea increased nitrogen removal rate, and the presence of Rumex japonicus or Oenanthe javanica decreased nitrous oxide emissions; (3) however, the net ecosystem services (nitrogen removal, greenhouse gas emissions, biofuel production) of microcosms with fine sand were higher than those of microcosms with coarse sand, and (4) interestingly, there was no difference in net ecosystem services between microcosms with coarse sand (1033 yuan ha-1 day-1; 1 yuan ≈ 0.14 USD) and those with fine sand (1071 yuan ha-1 day-1) for the four-species mixtures. Hence, in practice, ensuring plant species richness with appropriate species in microcosms with coarse sand can improve ecosystem services to a level equal to that of microcosms with fine sand and help to prevent constructed wetlands from clogging.

Annual study of hydraulic characteristics in surface flow constructed wetlands using hydrogen and oxygen stable isotope technology

Complex flow patterns and hydraulic characteristics could reduce the utilization efficiency of constructed wetland (CW), and consequently, its pollutant removal performance. Thus, it is of great importance to explore the internal flow patterns of CWs. Isotopic molecules exist naturally in CWs and have special properties under liquid conditions; using hydrogen and oxygen isotope technology cannot only reduce secondary pollution but also reflect the hydraulic characteristics of CWs. In the present study, the annual variation of isotopic composition in field-scale CW was investigated to evaluate the long-term feasibility of stable isotopic technology characterizing hydraulic flow patterns. The relationship between nutrients concentration distribution and flow pattern variation in CW under different seasons was discussed as well. Results demonstrated that isotope ¹⁸O/¹⁶O distribution could be used to determine the internal flow pattern of CW throughout the year, except for preferential flow area of CWs in winter, since more hydraulic retention time is needed to ensure the change of water isotopes due to the small evaporation in winter. Lower ammonia nitrogen concentration was observed in the stagnant area, while the total phosphorus concentration of the stagnant area increased during winter. And more attention should be paid to aquatic plants during the CW design, since it has significant influence on the hydraulic flow patterns of CW.

The influence mechanism of bioclogging on pollution removal efficiency of vertical flow constructed wetland

The effect of change of hydraulic characteristic and microbial community on pollution removal efficiency of the infiltration systems in the bioclogging development process remain poorly understood. In this study, therefore, the pollutant removal as a response to hydraulic conductivity reduction and the change of diversity and structure of microbial communities in vertical flow constructed wetlands (VFCWs) was investigated. The results indicated that the richness and diversity of the bacterial communities in the columns at different depths were decreased, and the microbial communities of the genus level were changed in the process of bioclogging. However, the variation of microbial communities has a low impact on the purification performance of VFCWs because the abundance of function groups, respiratory activity, and degradation potentiality of microorganisms remain steady or even get improved in the columns after bioclogging. On the contrary, the hydraulic efficiency of VFCWs decreased greatly by 16.9%, 9.9%, and 57.1% for VFCWs filled with zeolite (Column I), gravel (Column II), and ceramsite (Column III), respectively. The existence of short-circuiting and dead zones in the filter media cause the poor pollution removal efficiency of VFCWs due to the short contact time and decrease of oxygenation renewal, as well as low activity in the dead zone.

Performance of different macrophytes in the decontamination of and electricity generation from swine wastewater via an integrated constructed wetland-microbial fuel cell process

Plants constitute a major element of constructed wetlands (CWs). In this study, a coupled system comprising an integrated vertical flow CW (IVCW) and a microbial fuel cell (MFC) for swine wastewater treatment was developed to research the effects of macrophytes commonly employed in CWs, Canna indica, Acorus calamus, and Ipomoea aquatica, on decontamination and electricity production in the system. Because of the different root types and amounts of oxygen released by the roots, the rates of chemical oxygen demand (COD) and ammonium nitrogen (NH₄⁺-N) removal from the swine wastewater differed as well. In the unplanted, Canna indica, Acorus calamus, and Ipomoea aquatica systems, the COD removal rates were 80.20%, 88.07%, 84.70%, and 82.20%, respectively, and the NH₄⁺-N removal rates were 49.96%, 75.02%, 70.25%, and 68.47%, respectively. The decontamination capability of the Canna indica system was better than those of the other systems. The average output voltages were 520±42, 715±20, 660±27, and 752±26mV for the unplanted, Canna indica, Acorus calamus, and Ipomoea aquatica systems, respectively, and the maximum power densities were 0.2230, 0.4136, 0.3614, and 0.4964W/m³, respectively. Ipomoea aquatica had the largest effect on bioelectricity generation promotion. In addition, electrochemically active bacteria, Geobacter and Desulfuromonas, were detected in the anodic biofilm by high-throughput sequencing analysis, and Comamonas (Proteobacteria), which is widely found in MFCs, was also detected in the anodic biofilm. These results confirmed the important role of plants in IVCW-MFCs.

The State of the Art of Clogging in Vertical Flow Wetlands

Clogging in vertical flow (VF) wetlands is an important process influencing water purification processes. The main contributing factors are the growth of microorganisms within the filter media, the accumulation of suspended solids on top of the wetland, as well as within the filter media. Both processes lead to a decrease of the available pore space, hence changing the soil’s hydraulic properties. This will alter the water flow and cause malfunctioning of the system. This paper summarizes the state of the art of the prevailing physical, biological and chemical processes influencing clogging in VF wetlands. Different design and operational parameters are discussed to give a better understanding on their influence to prevent malfunctioning. Based on a literature review, a detailed overview on experimental as well as modelling studies carried out is presented. The main conclusions are that on the one hand, important insights on clogging processes in VF wetlands have been gained but, on the other hand, design parameters such as intermittent loading operation and the grain size of the filter media are not well represented in those studies. Clogging models use different conceptual approaches ranging from black box models to process based models.

Parameter-efficient bioclogging model: calibration and comparison with laboratory data

A parameter-efficient bioclogging model coupled with hydrodynamics was developed with a stepwise numerical calculation. Column lab tests were carried out to calibrate and verify the bioclogging model developed in this paper. The results showed that the experimental data fit well with the simulation data, which indicated that the developed model was reasonable. According to the sensitivity analysis of the parameters, the BOD (biochemical oxygen demand) loading rate and deposition coefficient are the key parameters for bioclogging. The results illustrate how the clogging is impacted by changing the BOD loading rate and can predict the biofilm accumulation within the substrate, the microbial saturation along the substrate profile over time, and the biofilter longevity based on the biomass growth. The model could dynamically describe the entire process of biological clogging and could quantitatively predict the amount of biofilm accumulated in the pores with the increasing operation time, which provides a basis for the prediction of biological clogging. Graphical abstract.

The Effects of Plants on Pollutant Removal, Clogging, and Bacterial Community Structure in Palm Mulch-Based Vertical Flow Constructed Wetlands

In this study, the effects of plants on the performance and bacterial community structure of palm mulch-based vertical flow constructed wetlands was studied. The wetlands were built in August 2013; one of them was planted with Canna indica and Xanthosoma sp., and the other one was not planted and used as a control. The experimental period started in September 2014 and finished in June 2015. The influent was domestic wastewater, and the average hydraulic surface loading was 208 L/m2d, and those of COD, BOD, and TSS were 77, 57, and 19 g/m2d, respectively. Although the bed without plants initially performed better, the first symptoms of clogging appeared in December 2014, and then, its performance started to fail. Afterwards, the wetland with plants provided better removals. The terminal restriction fragment length polymorphism (T-RFLP) analysis of Enterococci and Escherichia coli in the effluents suggests that a reduction in their biodiversity was caused by the presence of the plants. Thus, it can be concluded that the plants helped achieve better removals, delay clogging, and reduce Enterococci and E. coli biodiversity in the effluents.

Influence of clogging and resting processes on flow patterns in vertical flow constructed wetlands

Vertical flow constructed wetlands are widely used for removing pollutants from wastewater. Substrate clogging is an operational challenge of constructed wetlands, which can result in impeded water flow and finally a significant decline in the ability of the system to treat the wastewater. The entire clogging process in a vertical flow constructed wetland (VFCW) was quantitatively analyzed by measurements of hydraulic conductivity. Tracer tests and model simulations were carried out to investigate internal flow patterns during the clogging and resting processes. This analysis revealed that hydraulic conductivity gradually decreased with operation time. Further, the distribution time of the flow field was different under different degrees of clogging. Non-uniformity in water flow was primarily observed in the first 400min after adding the tracer (NaCl) in the early clogging stage, as opposed to the last 400min in the late clogging stage. Variation in water flow divergence was closely correlated with piston flow; the reaction efficiency was highest in the early stages of clogging. In the later stages, stronger flow mixing was observed. Resting operations can reduce the dispersion of internal flow and improve reaction efficiency. After resting for approximately 15days, tracer concentration fluctuations decreased and internal flow back-mixing was alleviated. A simulation further described the internal flow pattern and elaborated and validated the tracer experiment. The outcomes of this study will assist in understanding how internal flow behavior varies in response to the clogging process and reveal details of the internal clogging mechanism in VFCWs.

Clogging development and hydraulic performance of the horizontal subsurface flow stormwater constructed wetlands: a laboratory study

The horizontal subsurface constructed wetland (HSSF CW) is a highly effective technique for stormwater treatment. However, progressive clogging in HSSF CW is a widespread operational problem. The aim of this study was to understand the clogging development of HSSF CWs during stormwater treatment and to assess the influence of microorganisms and vegetation on the clogging. Moreover, the hydraulic performance of HSSF CWs in the process of clogging was evaluated in a tracer experiment. The results show that the HSSF CW can be divided into two sections, section I (circa 0-35 cm) and section II (circa 35-110 cm). The clogging is induced primarily by solid entrapment in section I and development of biofilm and vegetation roots in section II, respectively. The influence of vegetation and microorganisms on the clogging appears to differ in sections I and II. The tracer experiment shows that the hydraulic efficiency (λ) and the mean hydraulic retention time (t _mean) increase with the clogging development; although, the short-circuiting region (S) extends slightly. In addition, the presence of vegetation can influence the hydraulic performance of the CWs, and their impact depends on the characteristics of the roots.