Clogging in subsurface-flow treatment wetlands: measurement, modeling and management

A critical review of characteristics of domestic wastewater and key treatment techniques in Chinese villages

As of 2022, China's rural sewage treatment rate is only approximately 31 %. Rapid rural development has led to higher demand. However, China's rural areas are complex and face many problems, such as uneven economic development, population distribution, and water availability. Long-lasting and low-cost wastewater treatment measures are needed for application in rural areas. The quantity and quality of rural domestic wastewater in China were characterized first. Next, the hot topic of domestic wastewater in Chinese villages was confirmed via bibliometric analysis using CiteSpace, and the treatment technologies for rural domestic wastewater were compared. Specifically, the technical status and challenges of the most common technology in rural domestic wastewater treatment, constructed wetlands, were summarized.

A review on the fate of micro and nano plastics (MNPs) and their implication in regulating nutrient cycling in constructed wetland systems

This review discusses the micro-nano plastics (MNPs) and their interaction with physical, chemical and biological processes in a constructed wetland (CW) system that is typically used as a nature-based tertiary wastewater treatment for municipal as well as industrial applications. Individual components of the CW system such as substrate, microorganisms and plants were considered to assess how MNPs influence the CW processes. One of the main functions of a CW system is removal of nutrients like nitrogen (N) and phosphorus (P) and here we highlight the pathways through which the MNPs influence CW's efficacy of nutrient removal. The presence of morphologically (size and shape) and chemically different MNPs influence the growth rate of microorganisms important in N and P cycling, invertebrates, decomposers, and the plants which affect the overall efficiency of a CW treatment system. Certain plant species take up the MNPs, and some toxicity has been observed. This review focuses on two significant aspects: (1) the presence of MNPs in a significant concentration affects the efficiency of N and P removal, and (2) the removal of MNPs. Because MNPs reduce the enzyme activities in abundance and overproduction of ROS oxidizes the enzyme active sites, resulting in the depletion of proteins, ultimately inhibiting nitrogen and phosphorus removal within the substrate layer. The review found that the majority of the studies used sand-activated carbon (SAC), granular-activated carbon (GAC), rice straw, granular limestone, and calcium carbonate, as a substrate for CW treatment systems. Common plant species used in the CW include Phragmites, Arabidopsis thaliana, Lepidium sativum, Thalia dealbata, and Canna indica, which were also found to be dominant in the uptake of the MNPs in the CWs. The MNPs were found to affect earthworms such as Eisenia fetida, Caenorhabditis elegans, and, Enchytraeus crypticus, whereas Metaphire vulgaris were found unaffected. Though various mechanisms take place during the removal process, adsorption and uptake mechanism effectively emphasize the removal of MNPs and nitrogen and phosphorus in CW. The MNPs characteristics (type, size, and concentration) play a crucial role in the removal efficiency of nano-plastics (NPs) and micro-plastics (MPs). The enhanced removal efficiency of NPs compared to MPs can be attributed to their smaller size, resulting in a faster reaction rate. However, NPs dose variation showed fluctuating removal efficiency, whereas MPs dose increment reduces removal efficiency. MP and NPs dose variation also affected toxicity to plants and earthworms as observed from data. Understanding the fate and removal of microplastics in wetland systems will help determine the reuse potential of wastewater and restrict the release of microplastics. This study provides information on various aspects and highlights future gaps and needs for MNP fate study in CW systems.

Vertical flow constructed wetlands as green facades and gardens for on-site greywater treatment in buildings: Two-year mesocosm study on removal performance

This study focuses on the performance and clogging of vertical flow constructed wetlands (VFCWs) planted with climbing ornamentals and ornamental plants for greywater treatment, after two years of operation at mesocosm level. Different substrate (sand, vermiculite) and vegetation (Trachelospermum jasminoides, Lonicera japonica, Callistemon laevis) types were evaluated to determine the optimal removal of pollutants. Results revealed that, during the second year of operation, removal efficiencies of turbidity and COD were significantly higher (1st year: 54-94 %; 71-89 %, 2nd year: 82-98 %; 86-95 %, respectively) for both studied planted substrates, compared to the first year. Moreover, it was found that sand systems from each studied plant as well as from the unplanted systems, were more effective compared to vermiculite for most of the studied parameters (turbidity, TSS, COD, anionic surfactants, pathogens). Sand systems were also quite effective in removing total coliforms (5 log reduction) and Escherichia coli (4 log reduction). At the end of the two-year experiment, all planted systems with sand had significantly higher hydraulic conductivity than the unplanted ones. With reference to evapotranspiration, even though planted systems had significantly higher losses, C. laevis systems demonstrated less water losses than the other vegetated systems. According to the findings, the studied plants managed to continue growing without facing added stress. Therefore, the application of climbing and ornamental plants in VFCWs for greywater treatment in buildings seems a promising option for developing green infrastructures in urban areas and enhancing the removal efficiency of such systems.

Structural and functional perspectives of carbon filter media in constructed wetlands for pollutants abatement from wastewater

Constructed wetlands (CWs) represent the most viable artificial wastewater treatment system that works on the principles of natural wetlands. Filter media are integrally linked to CWs and have substantial impacts on their performance for pollutant removal. Carbon-derived substrates have been in the spotlight for decades due to their abundance, sustainability, reusability, and potential to treat complex contaminants. However, the efficiency and feasibility of carbon substrates have not been fully explored, and there are only a few studies that have rigorously analyzed their performance for wastewater treatment. This critical synthesis of the literature review offers comprehensive insights into the utilization of carbon-derived substrates in the context of pollutant removal, intending to enhance the efficiency and sustainability of CWs. It also compares several carbon-based substrates with non-carbon substrates with respect to physiochemical properties, pollutant removal efficiency, and cost-benefit analysis. Furthermore, it addresses the concerns and possible remedies about carbon filtration materials such as configuration, clogging minimization, modification, and reusability to improve the efficacy of substrates and CWs. Recommendations made to address these challenges include pretreatment of wastewater, use of a substrate with smaller pore size, incorporation of multiple filter media, the introduction of earthworms, and cultivation of plants. A current scientific scenario has been presented for identifying the research gaps to investigate the functional mechanisms of modified carbon substrates and their interaction with other CW components.

Rhamnolipid-induced alleviation of bioclogging in Managed Aquifer Recharge (MAR): Interactions with bacteria and porous media

The prevention and treatment of bioclogging is of great significance to the application of Managed Aquifer Recharge (MAR). This study investigated the alleviating effect of biosurfactant rhamnolipid (RL) on bioclogging by laboratory-scale percolation experiments. The results show that the addition of RL greatly reduced bioclogging. Compared with the group without RL, the relative hydraulic conductivity (K') of the 100 mg/L RL group increased 5 times at the end of the experiment (23 h), while the bacterial cell amount and extracellular polymeric substances (EPS) content on the sand column surface (0-2 cm) decreased by 60.8% and 85.7%, respectively. In addition, the richness and diversity of the microbial communities within the clogging matter decreased after the addition of RL. A variety of bacterial phyla were found, among which Proteobacteria were predominant in all groups. At the genus level, RL reduced the relative abundance of Acinetobacter, Bacillus, Klebsiella, and Pseudomonas. These microbes are known as strong adhesion, large size, and easy to form biofilms, therefore playing a critical role during MAR bioclogging. Moreover, RL changed the surface properties of bacteria and porous media, which results in the increase of electrostatic repulsion and decrease of hydrophobic interaction between them. Therefore, RL mediated the bacteria-porous media interaction to reduce biomass in porous media, thereby alleviating bioclogging. This study implies that RL's addition is an environmentally friendly and effective method to alleviate the bioclogging in MAR.

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.

Effects of wastewater pre-treatment on clogging of an intermittent sand filter

Intermittent sand filters (ISFs) are widely used in rural areas to treat domestic and dilute agricultural wastewater due to their simplicity, efficacy and relative low cost. However, filter clogging reduces their operational lifetime and sustainability. To reduce the potential of filter clogging, this study examined pre-treatment of dairy wastewater (DWW) by coagulation with ferric chloride (FeCl₃) prior to treatment in replicated, pilot-scale ISFs. Over the study duration and at the end of the study, the extent of clogging across hybrid coagulation-ISFs was quantified, and the results were compared to ISFs treating raw DWW without a coagulation pre-treatment, but otherwise operated under the same conditions. During operation, ISFs receiving raw DWW recorded higher volumetric moisture content (θ_v) than ISFs treating pre-treated DWW, which indicated that biomass growth and clogging rate was higher in ISFs treating raw DWW, which were fully clogged after 280 days of operation. The hybrid coagulation-ISFs remained fully operational until the end of the study. Examination of the field-saturated hydraulic conductivity (K_fs) showed that ISFs treating raw DWW lost approximately 85 % of their infiltration capacity in the uppermost layer due to biomass build-up versus 40 % loss for hybrid coagulation-ISFs. Furthermore, loss on ignition (LOI) results indicated that conventional ISFs developed five times the organic matter (OM) in the uppermost layer compared to ISFs treating pre-treated DWW. Similar trends were observed for phosphorus, nitrogen and sulphur, where proportionally higher values were observed for raw DWW ISFs than pre-treated DWW ISFs, with values decreasing with depth. Scanning electron microscopy (SEM) showed a clogging biofilm layer on the surface of raw DWW ISFs, while pre-treated ISFs maintained distinguishable sand grains on the surface. Overall, hybrid coagulation-ISFs are likely to sustain infiltration capacity for a longer period than filters treating raw wastewater; therefore, requiring smaller surface area for treatment and minimal maintenance.

Efficient carbon removal and excellent anti-clogging performance have been achieved in multilayer quartz sand horizontal subsurface flow constructed wetland for domestic sewage treatment

The present study aimed to investigate the application of a multilayer quartz sand substrate horizontal subsurface flow constructed wetland (HSFCW) for campus sewage treatment. It aimed to assess the pollutant removal efficiency and anti-clogging performance under the suggested maximum organic loading rate (250 g/m²/d). The results of the multilayer HSFCW (CW6) were compared to the mololayer HSFCW (CW1) for the removal of the chemical oxygen demand (COD), solid accumulation, and microbial communities. During operation, the combination conditions of high hydraulic loading rate (HLR) with low COD concentration were better for COD removal under a high organic loading rate (OLR) of 200-300 g/m²/d. The maximum removal rate reached 80.4% in CW6 under high HLR, which was 13.8% higher than that in CW1, showing better adsorption and biodegradation ability of organic matter. Impressive clogging resistance capacity was found in CW6 due to the lower contents of the insoluble organic matter (IOM) that are prone to clogging, indicating full degradation of organic matters, particularly IOM, in CW6 under high HLR. Less abundance of unclassified Chitinophagaceae (under low HLR), Pedobacter and Saccharibacteria_genera_incertae_sedis (under high HLR) in CW6, which contributed to aerobic membrane fouling, helped to prevent clogging. Moreover, Brevundimonas, Cloacibacterium, Citrobacter, Luteimonas contributed to IOM degradation, thus further enhancing the anti-clogging performance. In view of the better clogging resistance performance, the application of CW6 operated under high HLR and low COD concentrations was recommended to achieve economical, efficient, and steady COD removal for domestic sewage treatment in long-term operation.

Applying lysozyme, alkaline protease, and sodium hypochlorite to reduce bioclogging during managed aquifer recharge: A laboratory study

Alleviating bacterial-induced clogging is of great importance to improve the efficiency of managed aquifer recharge (MAR). Enzymes (lysozyme and alkaline protease) and sodium hypochlorite (NaClO) are common biological and chemical reagents for inhibiting bacterial growth and activity. To investigate the applicability of these reagents to reduce bioclogging, percolation experiments were performed to simulate a weak alkaline recharge water infiltration through laboratory-scale sand columns, with adding 10 mg/L lysozyme, alkaline protease, and NaClO, respectively. The results showed that, with the addition of lysozyme, alkaline protease, and NaClO, the average clogging rates (the reduced percentages of relative saturated hydraulic conductivity of the sand columns per hour during the percolation experiments) were 0.53%/h, 0.32%/h and 0.06%/h, respectively, which were much lower than that in the control group (0.99%/h). This implied that bioclogging could be alleviated to some extent following the treatments. For further analyzing the mechanisms of the regents on alleviating bioclogging, the bacterial cell amount and extracellular polymeric substances (EPS) concentration were also measured to study the effects of lysozyme, alkaline protease, and NaClO on bacterial growth and EPS secretion. Lysozyme and alkaline protease could disintegrate bacterial EPS by hydrolyzing polysaccharides and proteins, respectively, while they had little effect on the bacterial cell amount. The addition of NaClO significantly decreased the bacterial cell amount (P < 0.05) and thus greatly alleviated bioclogging. Although the lowest average clogging rate was achieved in the NaClO group, it can generate disinfection by-products that are potentially harmful to the environment and human health. Therefore, the biological-based method, i.e., enzyme treatment, could be a promising option for bioclogging control. Our results provide insights for understanding the mechanisms of lysozyme, alkaline protease, and NaClO to alleviate bioclogging, which is of great importance for addressing the clogging problem during MAR activities and achieving groundwater resources sustainable utilization.

Towards Effective, Sustainable Solution for Hospital Wastewater Treatment to Cope with the Post-Pandemic Era

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has spread across the globe since the end of 2019, posing significant challenges for global medical facilities and human health. Treatment of hospital wastewater is vitally important under this special circumstance. However, there is a shortage of studies on the sustainable wastewater treatment processes utilized by hospitals. Based on a review of the research trends regarding hospital wastewater treatment in the past three years of the COVID-19 outbreak, this review overviews the existing hospital wastewater treatment processes. It is clear that activated sludge processes (ASPs) and the use of membrane bioreactors (MBRs) are the major and effective treatment techniques applied to hospital wastewater. Advanced technology (such as Fenton oxidation, electrocoagulation, etc.) has also achieved good results, but the use of such technology remains small scale for the moment and poses some side effects, including increased cost. More interestingly, this review reveals the increased use of constructed wetlands (CWs) as an eco-solution for hospital wastewater treatment and then focuses in slightly more detail on examining the roles and mechanisms of CWs' components with respect to purifying hospital wastewater and compares their removal efficiency with other treatment processes. It is believed that a multi-stage CW system with various intensifications or CWs incorporated with other treatment processes constitute an effective, sustainable solution for hospital wastewater treatment in order to cope with the post-pandemic era.

Bioclogging alleviation for constructed wetland based on the interaction among biofilm growth and hydrodynamics

Bioclogging is the most crucial operation problem of the constructed wetlands, which reduce its removal efficiency and life span. A strategy through properly increasing hydraulic loading is proposed in this study to alleviate the bioclogging for CWs. The two-dimensional porous media flow cell (2D PMFC) test indicated that a quadratic correlation was found between local biofilms growth rate and the near-wall Reynolds number (r > 0.765, p < 0.05). The biofilm growth rate declined with the flowrate when Re exceeded about 6.0. It was also found that the higher flowrate (6 mL/min) lead to the homogeneous biofilm and velocity distribution in the PMFC. The column test indicated that the highest hydraulic loading (9.2 cm/h) produced the smallest decrease in hydraulic conductivity, which was 80 times more than that of low hydraulic load (3.0 cm/h) at the end (40 days) of experiment. Moreover, the relatively homogenized distribution of biofilm was found along the column with the highest hydraulic loading, which confirmed that the proper increase in hydraulic loading can alleviate bioclogging.

Impact of clogging on accumulation and stability of phosphorus in the subsurface flow constructed wetland

Substrate clogging is one of the major operation challenges of subsurface flow constructed wetlands (SSF-CWs). And the phosphorus (P) removal performance and stability of P accumulation of SSF-CWs would be varied with the development of substrate clogging. In this study, three horizontal SSF-CWs microcosms with different clogging degrees were conducted to explore the mechanism of P accumulation behavior influenced by substrate clogging. Increase in clogging degree resulted in hydraulic retention time (HRT) diminution and adsorption sites increase, which jointly led to reduced P removal efficiency at low clogging degree (L-CW), however, higher P removal efficiency was obtained as adsorption sites increase offset HRT diminution at high clogging degree (H-CW). Substrate adsorption was the primary removal pathway in all SSF-CW systems. It accounted for 77.86 ± 2.63% of the P input in the H-CW, significantly higher than the control (60.08 ± 4.79%). This was attributed to a higher proportion of Fe/Al-P accumulated on the substrate of H-CW, since clogging aggravated the anaerobic condition and promoted the generation of Fe ions. The increase in clogging degree also elevated the release risk of the accrued P in SSF-CWs, since Fe/Al-P was considered bioavailable and readily released under environmental disturbance. The obtained results provide new insights into the P transport and transformation in SSF-CWs and would be helpful to optimize substrate clogging management.

Use of constructed wetlands to prevent overloading of wastewater treatment plants

The fluctuation in the number of people in tourist areas affects the wastewater quality and quantity. Constructed wetlands (CWs) aim to simulate physical, chemical, and biological processes occurring in natural environments for wastewater treatment and are considered a sustainable system. The current study aimed at evaluating the effectiveness of in-vessel CWs for supporting the wastewaters treatment plants in periods of overloading. Such approach can be quickly implementable, economic, and the CWs can be fast regenerated in the framework of sustainable good practices. Three pilot scale CWs were prepared in as many containers layering 10 cm of gravel, 60 cm of sand and 10 cm of gravel, and placing pieces of giant reed rhizomes in the upper layers. The bottom of each CW had a tap, and CWs were irrigated with a real municipal sewage three times a week. Before each new irrigation, the tap was opened, and the effluent collected for determining gross parameters, elemental composition, and contaminants of emerging concern (CECs). CWs significantly reduced almost all gross parameters considered and half the CECs, except for a couple of metabolites of corresponding parental compounds. With regards to the potentially toxic elements, all reduced their concentration from the influents to the effluents. The results of this study were promising and highlighted good efficiency of constructed wetlands as pre-treatment of real municipal sewage to reduce the overloading of the wastewater treatment plant.

How to select substrate for alleviating clogging in the subsurface flow constructed wetland?

Constructed wetland (CW) is a cost-effective and environmentally friendly ecological technology for contaminated water remediation, especially in dispersed communities and rural areas. Plants grow, biofilms form, and pollutants attach to the substrate, which is the main supporting structure of a subsurface flow CW (SSFCW) system. After long-term operation, the accumulation of clogs from physical, chemical, and biological processes in SSFCW substrates can easily cause clogging, thus reducing treatment efficiency reduction and service life and causing no discharge of sewage by intermittent until last indicates in the CW surface. Subsequently, stench and mosquito breeding occur, thus influencing environmental sanitation. Substrate clogging is the most serious, challenging, and inevitable problem in the long-term operation of SSFCWs. The present study reviews the effects of substrates on clogging categorized into physical, chemical, and biological clogging and analyzes the substrates that can alleviate/aggravate clogging in CWs. The recommended substrates that can relieve clogging include plastic, rubber, soil mixture, walnut shell, biochar, organic waste, alum sludge, and lightweight aggregate, while shell, steel slag, blast furnace slag, zeolite, and soil may easily generate phosphorus-clogging substances. CW substrate clogging is a mixture of three clogs with synergistic effects, and the corresponding clogging mitigation substrates mentioned above can be used to alleviate the most severe among the three types of clogs to reduce the synergy, and thus to promote stable operation and technology level of CWs. This review aims to promote the scientific selection of substrates for the stable operation and technical level of CW through targeted recommendations for substrates that relieve clogging. Future studies should focus the effects of influent water quality and substrate type on clogging, and waste as substrate to alleviate clogging, while mitigating the negative environmental impact of waste treatment.

Evaluation of substrate clogging in a full-scale horizontal subsurface flow treatment wetland using electrical resistivity tomography with an optimized electrode configuration

This study investigated the spatial distribution of clogging matter in a full-scale horizontal subsurface flow treatment wetland (HSSF TW) based on an electrical resistivity tomography (ERT) method, comparing the performance of two different electrode configurations (i.e., Schlumberger and Wenner arrays). The results indicated that during the draining phase, the substrate apparent resistivities of the full-scale HSSF TWs were negatively correlated with the clogging matter fraction (v/v), and a functional relationship between the two parameters was established using a first-order k-C* model. The detected clogging matter fraction (v/v) based on the Schlumberger array showed higher accuracy (linear slope = 0.900, R-squared = 0.902) than the Wenner array (linear slope = 0.685, R-squared = 0.685). Most of the severe substrate clogging in the full-scale HSSF TW occurred within a 10-m flow distance, and the distribution of the clogging matter showed different characteristics at different substrate depths. From a cross section positioned 1 m from the inlet, the average clogging matter fraction (v/v) at a 0-0.30 m depth (23.1 ± 14.9%) was significantly higher than that at a 0.30-0.80 m depth (5.0 ± 2.1%). The clogging matter at a 5-m flow distance was evenly distributed at different substrate depths. Only a few localized clogging zones were observed in the cross section at a 10-m flow distance. This study provided an accurate and feasible method for investigating the volume fraction of clogging matters containing different organic contents and demonstrates the spatial heterogeneity of clogging matter in HSSF TWs.

Constructed wetland substrates: A review on development, function mechanisms, and application in contaminants removal

Constructed wetlands (CWs) are economical, efficient, and sustainable wastewater treatment method. Substrates in CWs inextricably link with the other key components and significantly influence the performance and sustainability of CWs. Gradually, CWs have been applied to treat more complex contaminants from different fields, thus has brought forward new demand on substrates for enhancing the performance and sustainability of CWs. Various materials have been used as substrates in CWs, and their individual characteristics and application advantages have been extensively studied in recent years. Therefore, this review summarizes the development, function mechanisms (e.g., filtration, adsorption, electron supply, supporting plant growth and microbial reproduction), categories, and applications of substrates in CWs. The interaction mechanisms of substrates with contaminants/plants/microorganisms are comprehensively described, and the characteristics and advantages of different substrate categories (e.g., Natural mineral materials, chemical products, biomass materials, industrial and municipal by-products, modified functional materials, and novel materials) are critically evaluated. Meanwhile, the influences of substrate layer arrangement and synergism on contaminants removal are firstly systematically reviewed. Furthermore, further research about substrates (e.g., clogging, life cycle assessment/management, internal relationship between components) should be systematically carried out for improving efficiency and sustainability of CWs.

Synergistic effects of natural ventilation and animal disturbance on oxygen transfer, pollutants removal and microbial activity in constructed wetlands

This study investigated the feasibility of combining natural ventilation and animal disturbance in constructed wetlands (CWs) and the joint effects on oxygen transfer, microbial activity, organics, and nitrogen removal. The results showed that natural ventilation extended the habitat depth of earthworms by approximately 10 cm by significantly improving oxygen transfer in CWs; in turn, the earthworms slightly promoted the addition of oxygen inside CWs through burrowing activity. Therefore, the interaction between natural ventilation and animal disturbance in CWs mutually reinforced oxygen transfer, enzymatic activity, and the ammonification, nitrification, and aerobic degradation of organics. Additionally, the combination of natural ventilation and animal disturbance in CWs promoted the oxygen transfer rate by 42.1%-68.2%; promoted catalase, urease, and dehydrogenase activity by 19.3%-24.8%, 17.4%-22.3%, and 18.1%-25.6%, respectively; and promoted COD and NH₃-N removal loads by 48.6%-74.2% and 94.9%-135.3%, respectively. To achieve higher total nitrogen removal, moderate wind speeds (≤1 m/s in this study) are recommended to simultaneously create aerobic and anoxic/anaerobic conditions. Although natural ventilation reduced the microbial diversity in CWs by promoting the abundance of aerobes, the combination of natural ventilation and animal disturbance was generally conducive to improving microbial diversity. The relationship between wind speed and oxygen transfer rate and COD and NH₃-N removal loads in naturally ventilated CWs conformed to cubic equations.

Phosphorus cycling in freshwater lake sediments: Influence of seasonal water level fluctuations

Freshwater lakes experience drastic water level fluctuations because of climate change and human activities. However, the influence of such fluctuations on phosphorus cycling in sediments has rarely been investigated. We conducted a geochemical investigation on the phosphorus cycle in a shallow freshwater lake, Dongting Lake; under the influence of human activities and climate change, its water regime undergoes drastic changes. Irrespective of the permanent inundation zone (PIZ) or seasonal inundation zone (SIZ), the phosphorus cycle in sediments was found to be dominated by the reductive dissolution of iron (Fe) (oxyhydr)oxides, degradation of organic matters, and conversion between authigenic phosphorus (Ca-P) and detrital phosphorus in individual seasons. From winter to summer, with increasing water level, the content of Fe-bound phosphorus and organic phosphorus increase due to the deposition of suspended matter, thus increasing total phosphorus in PIZ. Moreover, the rising water level also reduces the dissolved oxygen content and promotes the reductive dissolution of Fe (oxyhydr)oxides. The mineralization of increased organic matter can release CO₂ and reduce pH in the vicinity, which can further result in the acidic dissolution of detrital apatite. In turn, most of the released phosphorus can be adsorbed or co-precipitated with calcium minerals, resulting in the significant increase of Ca-P. The mechanisms of phosphorus transformation in SIZ are similar to those in PIZ, but most of the increased organic matter and total P in a core from SIZ are attributable to the decomposition of plant matter. Therefore, the water level rise not only changes the conservative speciation of phosphorus in sediments to active speciation, but also triggers the release of phosphorus adsorbed to oxides and further increases the risk of phosphorus release from sediments to overlying water. Thus, our findings have major implications for freshwater shallow lakes and their P-driven productivity.

Performance and stratified microbial community of vermi-filter affected by Acorus calamus and Epipremnum aureum during recycling of concentrated excess sludge

Vermi-filter combined with wetland plant (VFP) is an eco-friendly and sustainable approach for recycling of excess sludge by joint action of earthworms, wetland plants and microorganisms. However, the effects of wetland plants on sludge decomposition and involved microorganisms are remain unclear. This study investigated the effects of wetland plants on the treatment performance of VFP for concentrated excess sludge and microbial community distribution inside the VFP. The wetland plants' species of Acorus calamus and Epipremnum aureum were separately planted on the surface layer of vermi-reactors by earthworms Eisenia fetida. Results showed that the growth rate of E. fetida in VFPs significantly (P < 0.001) increased by 75%, compared to VF. In addition, the removal rates of total solids and chemical oxygen demand in VFPs could reach 61%-79% and 36%-68%, respectively, displaying a better performance of sludge reduction than in the VF. The surface layer of VFPs was a hotspot for degradation of organic matter, where bacteria played an important role in bio-decomposition rather than eukaryotes. Moreover, wetland plants could significantly (P < 0.001) enrich the eukaryotic population, rather than bacterial population. Compared to the VF, the wetland plants could promote the diversities of bacterial community in VFPs, showing specific functioned genus in different layers. In contrast, A. calamus could be a better candidate than E. aureum in the VFP. This study suggests that the inoculation of wetland plants can improve the performance for treating concentrated excess sludge by changing the biomass of earthworms and the structure of microbial community within the VFP.

Improving the performance of vertical flow constructed wetlands by modifying the filtering media structure

The aim of this research was to study the influence of the bed media configuration and particle size on the treatment efficiency of subsurface vertical flow (VF) constructed wetlands (CWs) treating municipal wastewater. Two outdoor pilot units (VF1 and VF2, planted with Phragmites australis) with the configuration C1 were operated in parallel for 2 years at similar surface loading rates of 9.7 ± 3.2 (VF1) and 10.1 ± 3.3 (VF2) g biological oxygen demand (BOD₅)/m²·day (19.5 ± 6.4 (VF1) and 20.4 ± 6.2 (VF2) g chemical oxygen demand (COD)/m²·day). A different configuration C2 was used during the third year at 16.9 ± 4.6 (VF1) and 18.2 ± 3.0 (VF2) g BOD₅/m²·day and 26.0 ± 7.2 (VF1) and 28.0 ± 4.7 (VF2) g COD/m²·day. Two different filtering materials (1-3-mm sand for VF1 and 2-6-mm fine gravel for VF2) were used for configuration C1. The same units were modified after 2 years of operation by adding a 10-cm layer of fine sand (0-2 mm) on the top (configuration C2). In C1 conditions, the unit with the coarse material VF2 showed significantly (p < 0.05) lower removal efficiencies of total suspended solids (TSS) and BOD₅ than VF1, and both units failed to meet the BOD₅ discharge limit. In C2 conditions, removal efficiencies reached 82% TSS, 97% BOD₅, 76-81% ammonia, and 60-66% TN, without significant differences between VF1 and VF2 units. Removal efficiencies were significantly higher for configuration C2 than that for C1, due to the positive effect of the upper fine sand layer. The presence of this fine sand layer doubled the water retention time and increased the removal rates, while the infiltration rates were high enough for an operation free of clogging.

Modeling the impacts of plants and internal organic carbon on remediation performance in the integrated vertical flow constructed wetland

The integrated vertical flow (IVF) constructed wetland consists of two or more chambers with heterogeneous flow patterns and strong aeration capability, possesses favorable remediation performance. The Constructed Wetland Model No.1 (CWM1) embedded in the OpenGeoSys # IPHREEQC was applied to investigate the wetland plant effects on treatment efficiency. Two fundamental functions of the plant roots (i) the radial oxygen loss (ROL) and (ii) exudation of internal organic carbon (IOC), are developed and implemented in the model to simulate the treating processes of planted laboratory-scale IVF wetlands fed by the synthetic wastewater. The good agreement between simulated results and measurements of the planted IVF wetland and the unplanted filters mimicking wetland demonstrates the combined effects of ROL and IOC and the model reliability. In summer the ammonia (NH₄-N) and total nitrogen (TN) removals are high as above 90% in both IVF wetlands, and in winter they decline significantly to around 55% and 45% in unplanted wetland, contrastively to about 85% and 78% in the planted wetland. The nitrogen removal - COD/N ratio relation curves of IVF wetlands are proposed and obtained by modeling to evaluate organic carbon loading status. Based on the curves, the COD/N ratios of unplanted and planted wetlands are about 3∼7 and 3∼10 gCOD/gN for high TN removal respectively. Planted wetlands can tolerate a wider range of COD/N ratio influents than unplanted ones. The ROL in the unplanted wetland promotes COD and NH₄-N removal, while may inhibit denitrification under low-temperature conditions. The single addition of IOC enhances the oxygen-consuming and restrains the nitrification under the full loaded COD condition. Summing up all organic carbon releases from substrate and roots as IOC, the quantification of IOC acts on nitrogen treatment was simulated and compared with the external organic carbon (EOC) loading from influent. IOC performs higher efficiency on TN removal than EOC at the same organic loading rates. The results provide the thoughts of the solution for low TN removal in the carbon deficient constructed wetlands.

Changes of microbial community structure during the initial stage of biological clogging in horizontal subsurface flow constructed wetlands

The clogging is a universal problem in constructed wetlands, where microorganisms play an essential role. However, the implication of micro-organism variation due to the clogging is not clear. Four horizontal subsurface flow constructed wetlands (HFCWs) were designed and operated to simulate the process of clogging. The wetland treatment performance and microbial community variation were investigated by regularly monitoring. Results showed the substrate filtration rate and the total phosphorous (TP) removal efficiency consistently decreased and the chemical oxygen demand (COD) and total nitrogen (TN) removal efficiency were at the range of 50%-85% and 10-20%, respectively. The sequencing results indicated that the clogging could affect the richness of bacterial community. The bacterial variation could be attributed to the dissolved oxygen decreasing and organic matter accumulation in the initial clogging period. These findings are expected to provide some theoretical reference for developing the biological methods to indicate the initial clogging in constructed wetlands.

Simultaneous Carbon and Nitrogen Removal from Domestic Wastewater using High Rate Vermifilter

Being a cost-effective and environmentally benign technology, vermifiltration has significantly replaced the available conventional wastewater remediation methods in many cases over the last few decades. The present work emphasizes on the investigation of the nitrogen transformation dynamics, in addition to the organic carbon abatement in the designed high rate hybrid vermifilter. Moreover, the economical sustainability of the vermifiltration technology has also been enlightened by creating a bridge with the concept of circular bio-economy. The designed high rate macrophyte-assisted vermifilter (MAVF) ascertained significant high nitrogen and organic carbon removal efficiencies from the real domestic sewage, considering the chemical oxygen demand (COD) of the influent and hydraulic loading rate (HLR) as the input variables. The designed MAVF facilitated the maximum ammonium nitrogen (NH4 +-N), organic nitrogen, and total kjeldahl nitrogen removal efficiencies up to 98.2 ± 0.70%, 100%, and 99 ± 0.47%, respectively when COD of the influent and HLR were 200 ± 25 mg/L and 3 ± 0.1 m3/m2-d, respectively. On the other hand, substantial enhancement in the nitrate nitrogen (NO3 --N) in the effluent (73 ± 10.55 times its influent concentration) was observed with influent COD of 200 ± 25 mg/L and HLR of 7 ± 0.2 m3/m2-d. When the influent COD and HLR were maintained at 700 ± 45 mg/L and 3 ± 0.1 m3/m2-d, respectively, the highest total nitrogen removal of 87 ± 2.25% was obtained. Alternatively, the influent COD of 200 ± 25 mg/L and HLR of 3 ± 0.1 m3/m2-d yielded the highest COD removal efficiency of 77 ± 1.59%. Hence, the outcome of the present research work strengthens the suitability of the vermifiltration technology as an economically and ecologically sound natural wastewater bio-remediation technology for the treatment of domestic wastewater.

Evaluation of wastewater treatment performance to a field-scale constructed wetland system at clogged condition: A case study of ammonia manufacturing plant

Assessment of the treatment performance in the field-scale hybrid constructed wetland (CW) for ammonia manufacturing plant remains limited. After being in operations running on and off since 2014, the hybrid CW which treats effluent from the ammonia manufacturing plant in Peninsular, Malaysia has recently demonstrated the full clogging to the CW. It takes only 8 months to demonstrate a big deterioration of performance in 2019. Though the mechanism of clogging is not clear, which can be partially from inherent design problems or operational issues, nonetheless, it is important to evaluate how this clogging has impacted the effluent treatment performance and the continuous utilization of the CW. The purpose of this study is to evaluate the impact of the treatment performance on the ammoniacal nitrogen and COD removal when the CW is clogged. The result revealed that there is no impact on COD removal, but it has a substantial impact on the ammoniacal nitrogen removal. The ammoniacal nitrogen removal dropped to negative (outlet concentration is higher than inlet concentration) during the clogged period. Another observation is, the low removal rate also coincides with a high COD/N ratio, when the COD/N ratio increased to >2, the ammoniacal nitrogen removal rate dropped substantially, with the coefficient of determination, R² of 40.5%. The root cause for the clogging to develop in a short period of time is unidentified. However, it is still worth noting that COD and ammoniacal nitrogen efficiency did not behave the same at the clogged CW.

Laboratory investigation on Bacillus subtilis addition to alleviate bio-clogging for constructed wetlands

Bio-clogging is a major problem in the operation of constructed wetlands (CWs) and is caused by accumulation of biofilm and extracellular polymeric substances (EPS) in the substrate. B. subtilis can successfully produce α-amylase and endoglucanase, which can degrade polysaccharides and, consequently, disperse the EPS. Therefore, the addition of B. subtilis was used to decrease the bio-clogging of lab-scale vertical-flow constructed wetlands (VFCW) in this study, and the feasibility and performance of VFCWs were assessed. The results indicate that the addition of B. subtilis can degrade the polysaccharides in the clogging matter and thereby increase the porosity of the substrate. The hydraulic conductivity of Column 1 (with addition) increased by six times, which was 57 times that of control (Column 2). Meanwhile, the chemical oxygen demand (COD) removal rate also increased after the addition of B. subtilis. The microbial communities show that the richness and diversity within the substrate increased after addition. The relative abundance of functional groups of chemoheterotrophy, aerobic chemoheterotrophy, as well as that connected to N cycles also increased, which implied the improvement of the pollution removal efficiency. Meanwhile, the copy number of α-amylase and endoglucanase increased significantly in Column 1 with the addition of B. subtilis, which offers further support for a hydrolase-induced reduction of polysaccharides and the efficiency of B. subtilis on bio-clogging alleviation. The results showed that B. subtilis addition is an effective and safe solution to control the bio-clogging for CWs. However, further research about long-term effect assessment and dosing strategy optimization should be conducted.

Nitrogen removal in vermifiltration: Mechanisms, influencing factors, and future research needs

To meet global health and sanitation goals, there is a continued need for sustainable wastewater treatment alternatives that require minimal energy and investment. Vermifiltration, a technology gaining relevance in Africa and Asia, may be an alternative to traditional wastewater treatment systems due to its cost-effectiveness, ease of application and maintenance, and sustainability. However, nitrogen removal in vermifiltration is not well understood since most of the prior research focuses on organics removal. Thus, a state of the art review is necessary to separately focus on the mechanisms associated with nitrogen removal in vermifiltration, along with the factors affecting nitrogen removal. For the first time, this review attempts to present the types of vermifilter based on their flow pattern. The review further discusses the current status of the application of vermifiltration, along with the benefits and limitations associated with the adoption of this technology. It also explores possible strategies that could be adopted to maximize the nitrogen removal potential of vermifilters as optimizing nitrogen removal is critical for improving the performance of vermifiltration based treatment systems.

Monitoring of horizontal subsurface flow constructed wetlands for tertiary treatment of municipal wastewater

This paper explores the feasibility of the electrical resistivity (ER) method as non-invasive technique to detect dysfunctions of full-scale tertiary wastewater treatment wetlands, such as clogging, that ultimately limit the system lifetime and performance. ER measurements were carried out according to the Werner’s method on two (overloaded) horizontal subsurface wetlands (HSF-CW1 and HSF-CW2) operated in parallel since the end of 2017, while still achieving satisfactory treatment efficiencies. Layered clog-induced preferential flow pathways through the HSF-CW beds and premature deterioration of the wetland’s liner with a possible development of dead zones associated with a low/heterogeneous density of vegetation could be confirmed by analysis of ER profiles. These results suggest that the ER method is a promising and feasible technique, as simplified then adapted/adopted to local context, for better monitoring and assessment of treatment wetlands with early process failure.

Modeling the impact of evolving biofilms on flow in porous media inside a microfluidic channel

This study integrates microfluidic experiments and mathematical modeling to study the impacts of biofilms on flow in porous media and to explore approaches to simplify modeling permeability with complicated biofilm geometries. E. coli biofilms were grown in a microfluidic channel packed with a single layer of glass beads to reach three biofilm levels: low, intermediate, and high, with biofilm ratios (β_r) of 2.7%, 17.6%, and 55.2%, respectively. Two-dimensional biofilm structures and distributions in the porous medium were modeled by digitizing confocal images and considering broad ranges of biofilm permeability (k_b) (from 10^-15 m² to 10^-7 m²) and biofilm porosity (ε_b) (from 0.2 to 0.8). The overall permeability of the porous medium (k), the flow pathways and the overall/local pressure gradients were found to be highly dependent on β_r and k_b but were moderately impacted by ε_b when the biofilm levels were high and intermediate with k_b>10^-11 m². When biofilm structures are well developed, simplified biofilm geometries, such as uniform coating and symmetric contact filling, can provide reasonable approximations of k.

The effect of re-startup strategies on the recovery of constructed wetlands after long-term resting operation

The objective of this study was to identify the proper re-startup strategies (RSSs) for constructed wetlands (CWs) after long-term resting operation in terms of the recovery of pollutant removal efficiency (RE) and N-cycle gene abundance. The results suggested that backwashing increased the gene abundance without shortening the recovery time of gene abundance. The RSS involving excavation and washing performed better in terms of chemical oxygen demand (COD) RE, especially at the beginning, and performed slightly better or similarly in terms of N-cycle gene abundance and the REs of ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN). The abundance of the Amox gene was 66.1-92.8, 76.3-161.8 and 1550-2492 times larger than that of the napA, narG and amoA genes, respectively, and the anammox process was the dominant N removal pathway. Therefore, excavation and washing is recommended as the RSS for CWs with a long-term rest period.

Evaluation of Different Methods to Assess the Hydraulic Behavior in Horizontal Treatment Wetlands

While there have been numerous studies on the rate and development of clogging in horizontal subsurface treatment wetlands (HSTWs) and, consequently, the effects on its hydraulic characteristics, research has not shown a clear understanding of the processes. The existing methods for measuring the impact of clogging provide limited information on the extension and degree of the phenomenon. This study aimed to evaluate the capacity of various measurement techniques to assess the degree and variation in space and time of clogging in HSTWs. Hydraulic conductivity at saturation (Ks) measurements were conducted using a newly implemented scheme, the drainage equation method, and traditional tracer tests, which were carried out in a full-scale HSTW system, located in Sicily, Italy, during 2019. After five years of operation, the results highlighted a severe decrease in Ks (<1000 m day−1) in the inlet zone (despite the fact that the filter gravel was replaced in 2017), a very high reduction of Ks along the central path inside the bed, a nonuniform flow through the HSTW, the presence of stagnant zones, and a reduction of the porosity of the bed gravel. Nonetheless, the mean values of the physical–chemical and bacteriological parameters at the hybrid treatment wetland (hybrid TW) outlet indicated that the partial clogging had no significant effect on the quality of the discharged water. Moreover, the results obtained using the different measurement techniques (in terms of both the Ks values and the flow distribution inside the bed) were consistent with each other and with results obtained previously for the same system. Finally, the most efficient combination of methods to assess clogging in HSTWs was identified.

Time-domain induced polarization as a tool to image clogging in treatment wetlands

During the last decade, treatment (artificial) wetlands have flourished all over Europe for the treatment of sewages from small communities thanks to their low cost of operation. The clogging of the filter of these wetlands is an issue affecting their efficiency and considered as their main operational problem. The present work shows the results of the application of a geophysical method called time-domain induced polarization. It is used to non-intrusively image, in 3D, the clogging of the gravel filters in a quick and efficient way. Induced polarization characterizes the ability of a porous material to reversibly store electrical charges when submitted to an electrical field. The material property characterizing this ability is called normalized chargeability. A set of laboratory experiments allows to determine an empirical relationship between the normalized chargeability and the weight amount of clogging. Induced polarization measurements have been performed in the field over a treatment wetland to get a 3D reconstructed image (tomography) of the normalized chargeability. From this tomography and the previously defined relationship, we are able to image in 3D the distribution of clogging and where it is concentrated in the filter. We can therefore identify the areas requiring preventive measures to minimize this clogging issue.

Multistage Horizontal Subsurface Flow vs. Hybrid Constructed Wetlands for the Treatment of Raw Urban Wastewater

In this study, pilot-scale hybrid constructed wetlands (CWs) and multistage horizontal subsurface flow CWs (HF CWs) have been studied and compared for the treatment of raw urban wastewater. In the hybrid CWs, the first stage was a mulch-based horizontal subsurface flow CW and the second stage was a vertical subsurface flow CW (VF CW). The VF CWs were used to determine if sand could improve the performance of the hybrid CW with respect to the mulch. In the multistage HFs, mulch, gravel and sand were used as substrates. The effect of water height (HF10: 10 cm vs. HF40: 40 cm) and surface loading rate (SLR: 12 vs. 24 g Chemical Oxygen Demand (COD)/m2d) has been studied. The results show that the use of sand in the vertical flow stage of the hybrid CW did not improve the average performance. Additionally, the sand became clogged, while the mulch did not. The effect of water height on average pollutant removal was not determined but HF10 performed better regarding compliance with legal regulations. With a SLR of 12 g COD/m2d, removals of HF10 were: 79% for COD, 75% for NH4+-N, 53% for dissolved molybdate-reactive phosphate-P (DRP), 99% for turbidity and 99.998% for E. coli and total coliforms. When SLR was doubled, removals decreased for NH4+-N: 49%, DRP: −20%, E coli and total coliforms: 99.5–99.9%, but not for COD (85%) and turbidity (99%). Considering the obtained results and the simplicity of the construction and operation of HFs, HF10 would be the most suitable choice for the treatment of raw urban wastewater without clogging problems.

The Importance of Biological and Ecological Properties of Phragmites Australis (Cav.) Trin. Ex Steud., in Phytoremendiation of Aquatic Ecosystems—The Review

Phragmites australis (common reed) is one of the most extensively distributed species of emergent plant worldwide. The adaptive features of this plant show its competitive character. Owing to high intraspecific diversity of common reed, as well as its phenotypic plasticity, the plant shows a broad ecological amplitude. Moreover, the plant exhibits a high capacity for acclimatization to environmental conditions which are considered adverse. This plant has been used for many years in phytoremediation to purify various types of wastewater. Phragmites australis has a high ability to accumulate various nutrients, heavy metals, and micropollutants, and in this respect, it is superior to other aquatic plants. This review examines the existing literature on the biological and ecological properties of common reed, the use of common reed in wastewater treatment for removing pollutants and tolerance for metals, and in hydrophyte treatment systems. It seems vital to conduct further research on the physiology and biochemistry of the common reed, with the aim of increasing the plant’s efficiency for pollutants removal.

Chemical Clogging and Evolution of Head Losses in Steel Slag Filters Used for Phosphorus Removal

The objective of this study was to propose a conceptual model of clogging in alkaline granular filters. Two slag columns were operated for 600 days and monitored using piezometers and tracer tested at regular intervals. The type of influent (organic or inorganic) affected the loss of effective porosity in the filters. Well organized and loose crystal structures were observed by scanning electron microscopy in columns with inorganic and organic influents, respectively. It was postulated that the formation of crystals in unorganized structures results in confined voids that are not accessible for water flow, thus accelerating porosity loss. The effect of the combination of chemical clogging and biofilm on the porosity loss is higher than the effect of these two factors separately. The Kozeny-Carman equation for hydraulic conductivity could not efficiently predict the evolution of head losses in the column fed with an inorganic influent. The crystal structure and connectivity in the presence of homogeneous or heterogeneous precipitation are concepts that could improve predictions of hydraulic conductivity. The results of this study highlighted the importance of the inlet zone on the development of pressure head in alkaline granular filters. Future research on clogging should focus on precipitation mechanisms in the inlet zone and on the design of the feeding system.

Recent Advances in the Application, Design, and Operations & Maintenance of Aerated Treatment Wetlands

This paper outlines recent advances in the design, application, and operations and maintenance (O&M) of aerated treatment wetland systems as well as current research trends. We provide the first-ever comprehensive estimate of the number and geographical distribution of aerated treatment wetlands worldwide and review new developments in aerated wetland design and application. This paper also presents and discusses first-hand experiences and challenges with the O&M of full-scale aerated treatment wetland systems, which is an important aspect that is currently not well reported in the literature. Knowledge gaps and suggestions for future research on aerated treatment wetlands are provided.

Assessment of spatial representativity of X-ray tomography to study Vertical Flow Treatment wetlands

French Vertical Flow (VF) treatment wetlands receive raw wastewater and provide simultaneous sludge and wastewater treatment. For proper sludge handling, the treatment wetland must be designed adequately and specific operational conditions must be maintained. When these conditions are not met, accumulation of biosolids may lead to clogging. Filtration in French VF Treatment wetlands is governed by mechanisms at the pore-scale. They must be better understood to predict reliably biosolid accumulation. X-ray Computed Tomography (Xray-CT) is a promising technique to characterize in detail the morphology of the filtering media in treatment wetlands. In order to set a solid basis for the use of Xray-CT, the spatial representativity of measurements must be assessed. This issue is addressed in this study by successively analyzing spatial properties at the filter scale using Frequency Domain Electromagnetic Measurements (FDEMs), and at the pore scale using Xray-CT. A map of the electric conductivity at the surface of a French VF Treatment wetland is obtained by FDEM that indicates a homogeneous distribution of biosolids to which electrical conductivity is highly correlated. Different morphological properties were computed from Xray-CT after phase segmentation: phase volume fraction profiles, Specific Surface Area profiles and pore size distributions. Samples show several similarities of pore scale properties obtained by Xray-CT independently of the sampling region and especially the same vertical gradients. FDEM measurements and Xray-CT analysis are in agreement to indicate a good influent distribution at the surface of a full-scale mature French VF Treatment wetland. A criterion to define the limits of the deposit layer and gravel layer is introduced. This division allows to compare layers independently. Finally, a 2D-REV analysis suggests that the selected sample diameter of 5 cm is large enough to be representative of the heterogeneous distribution of phases at the pore-scale as long as no Phragmites are present.

Hydraulic performance evaluation of a quasi-two dimensional constructed wetland microcosm using tracer tests and Visual MODFLOW simulation

Hydraulic conditions of constructed wetlands (CWs) are crucial to pollutant removal and are influenced by factors such as influent loading rates, influent or effluent position or porous media size. The performance evaluation of CWs in real application, however, is difficult and a visualization analysis is difficult due to the black-box effect. In this paper, a nonopaque microcosmic horizontal subsurface flow constructed wetland (HSSFCW) reactor was built in the laboratory for an efficient and intuitive assessment of the influences of such parameters on the hydraulic performance of CWs. Chloride tracer tests were carried out to obtain parameters on hydraulic performance by considering hydraulic loading rates and different flow inlet-outlet configurations, while dye tracer tests were designed for the visualization of solute transport and diffusion. In parallel, an identical design of the HSSFCW reactor was modeled using the Visual MODFLOW (VM) software in order to detect the tracer movement and solute concentration field. Results show that the inflow rate and the inlet-outlet configuration have significant impacts on the hydraulic performance of a CW influencing RTD curve shape, flow path, dead zone distribution and hydraulic efficiency. The dead zone and flow path have been visualized and analyzed by comparing the dye tracer experiments of VM solute concentration field and particulate path-line tracking. In addition, the feasibility and reliability of the VM simulation has been verified. The application of VM in this study has been robust indicating a possible application for further investigations on the influencing factors of real CWs.

Phosphorus recovery from sewage with a sustainable and low-cost treatment system

This study proposes a technology conceived based on an integrative approach that aims to promote phosphorus recovery and to recycle ferric water treatment sludge (FWTS), using it as a phosphorus adsorbent which may be applied as a soil ameliorant after reaching saturation. The assessed pilot plant operated with a daily influent flow of 360 litres and presented a removal efficiency of 94.4% ± 3.2% for chemical oxygen demand (COD) and of 91.2% ± 7.8% for suspended solids. It also presented promising results for phosphorus removal. The maximum efficiency of dissolved reactive phosphorus removal was 95% on the first day and it decreased until reaching adsorbent saturation. The estimated breakthrough time was one year in the condition in which the filling medium of a second constructed wetland was only FWTS. In this situation, the effluent phosphorus concentration was 0.2 mg·L^-1. The authors concluded that the application of FWTS in a constructed wetland bed is an interesting alternative. Batch adsorption experiments were run using phosphorus stock solution. Langmuir and Freundlich adsorption isotherm models were obtained for different initial pH values. The maximum adsorption capacity decreased as the initial pH was increased; values ranged from 4.76 mg P·g^-1 (pH = 3.9) to 1.44 mg P·g^-1 (pH = 9.0).

Clogging in constructed wetlands: Indirect estimation of medium porosity by analysis of ground-penetrating radar images

Identification of the degree of porous medium clogging in constructed wetlands (CWs) is a complex procedure because bed obstruction is a phenomenon that occurs in the subsurface and involves several factors. Ideal methods must allow for subsurface analysis and be non-invasive, what is the case of ground penetrating radar (GPR or georadar) used for soil characterization. Thus, this work aimed to evaluate the use of GPR for clogging characterization of two full-scale horizontal subsurface flow constructed wetlands (HSSF-CWs), one unit planted with cattail (Typha latifolia) and the other unit with no vegetation. Both units received municipal sewage (50 p.e. each) previously treated in an anaerobic reactor, had been in operation for seven years (at the time the GPR was used) and showed signs of heavy clogging, leading to surface flow. In order to produce a standard for GPR image (created by the response of the reflected wave in the passage through different media) identification and association of colors with the environmental conditions inside the medium (cleaner or more obstructed condition), a clean granular rock filter (in operation for only one month), similar to the unplanted unit, was used. Equations developed for indirectly estimating the porosity, based on the RGB (red, green and blue) color scale, indicated that the methodology was in agreement with the visual conditions of surface flow occurrence in the HSSF-CW, showing to be a suitable non-invasive method to characterize the advancement of clogging in CWs.

A bilayer coarse-fine infiltration system minimizes bioclogging: The relevance of depth-dynamics

Bioclogging is a main concern in infiltration systems as it may significantly shorten the service life of these low-technology water treatment methods. In porous media, biofilms grow to clog partially or totally the pore network. Dynamics of biofilm accumulation (e.g., by attachment, detachment, advective transport in depth) and their impact on both surface and deep bioclogging are still not yet fully understood. To address this concern, a 104 day-long outdoor infiltration experiment in sand tanks was performed, using secondary treated wastewater and two grain size distributions (GSDs): a monolayer system filled with fine sand, and a bilayer one composed by a layer of coarse sand placed on top of a layer of fine sand. Biofilm dynamics as a function of GSD and depth were studied through cross-correlations and multivariate statistical analyses using different parameters from biofilm biomass and activity indices, plus hydraulic parameters measured at different depths. Bioclogging (both surface and deep) was found more significant in the monolayer fine system than in the bilayer coarse-fine one, possibly due to an early low-cohesive biofilm formation in the former, driven by lower porosity and lower fluxes; under such conditions biomass is favorably detached from the top layer, transported and accumulated in depth, so that new biomass might colonize the surface. On the other hand, in the bilayer system, fluxes are highest, and the biofilm is still in a growing phase, with low biofilm detachment capability from the top sand layer and high microbial activity in depth, resulting in low bioclogging. Overall, the bilayer coarse-fine system allows infiltrating higher volume of water per unit of surface area than the monolayer fine one, minimizing surface and deep bioclogging, and thus increasing the longevity and efficiency of infiltration systems.

Electroactive biofilm-based constructed wetland (EABB-CW): A mesocosm-scale test of an innovative setup for wastewater treatment

Constructed wetlands (CWs) performance enhancement can be done with intensification strategies. A recent strategy still in study is the coupling with Microbial Electrochemical Technologies (MET). An alternative system using electro-conductive biofilters instead of electrodes and circuits used in MET, resulted in the development of a Microbial Electrochemical-based CW (METland). This system relies on electroactive bacteria (EAB) metabolism to transfer electrons to an electro-conductive material, thus boosting substrate consumption, and diminishing electron availability for biomass build-up and methane generation. In previous studies this biofilters have shown an improvement in biodegradation rates in comparison with subsurface flow CW. However, this set-up is still in development, hence there are uncertainties regarding the dynamics involve in the removal of pollutants. Considering that, this work aimed at establishing the capacity and removal kinetics of organic matter and nutrients in an Electroactive Biofilm-Based CW (EABB-CW). Two electro-conductive materials were tested (PK-A and PK-LSN) in planted and non-planted mesocosms and compared with sand. The systems were operated in a continuous upflow mode for 32 weeks and fed with real wastewater. The electro-conductive systems reached removal efficiencies up to 88% for BOD₅, 90% for COD, 46% for NH₄-N, and 86% for PO₄-P. Organic matter removal in electro-conductive systems was possible even at loading rates 10-fold higher than recommended for horizontal flow CWs. First-order area-based removal constants (k), calculated for organic matter and nutrients are higher than values typically reported for saturated CW and in certain cases comparable with vertical flow CW. The organic removal was correlated with electron current densities measures, as indicator of the presence of EAB. The tested EABB-CW profiles as a promising CW type for the removal of organic matter and PO₄-P with margin for modifications to improve nitrogen removal. Future studies with pilot/real scale systems are proposed to validate the findings of this study.

Effect of earthworms and plants on the efficiency of vertical flow systems treating university wastewater

One of the possible ways to improve the operation efficiency of constructed wetlands and to prevent their clogging is the application of earthworms. They have already been successfully applied for vermicomposting and for sludge dewatering and treatment. A few studies have already examined the effect of earthworms on the treatment of wastewater by vertical flow constructed wetlands (VFCWs), but none of them have provided a yearlong research result from an open-air system or compared the effect that different seasons in a temperate climate area can have on these invertebrates. The goal of this research was to estimate the effect that earthworms and plants have on VFCW's operation. Four mesocosms (a filter, a filter with earthworms, a VFCW and a VFCW with earthworms) were built and their influent and effluent water quality was monitored for a period of 1 year. They were fed with wastewater coming from a building of the University of Bologna (Italy). The results have shown that the presence of earthworms in this specific system did not reduce the organic matter content of the substrate, but it has positively influenced plants' growth. However, since neither earthworms nor plants had a statistically significant effect on the effluent quality, it can be concluded that the integration of these invertebrates cannot improve wastewater treatment of vertical flow filters or constructed wetlands.

Assessment of X-ray Computed Tomography to characterize filtering media from Vertical Flow Treatment Wetlands at the pore scale

Computed Tomography is a non-destructive technique often used in earth sciences for the description of porous media at the pore scale. This paper shows the feasibility of this technique to obtain 3D descriptions of filtering media in Vertical Flow Treatment Wetlands (VFTW). Three different samples from two full-scale VFTW were scanned. The samples vary in moisture content and gravel size distribution. The 3D images show three characteristic phases of unsaturated media: voids, fouling material and gravel. The gray contrast level is good enough to perform phase segmentation successfully using region growing algorithms. In this study the results from segmentation are used (i) to compute profiles of phase volume fraction and specific surface at high resolution, (ii) to observe 3D distribution of isolated elements, (iii) and to draw the void's skeleton and to perform a percolation pathway study. This method highlights the presence of a transition zone between the deposit cake and the dense gravel layer. In this zone, mechanical interactions between gravels and filtered solids tend to promote a heterogeneous layer of gravel, fouling material and open porosity. The presence of isolated gravels in the deposit layer is clearly evidenced. The effect of drying to enhance the contrast between phases has been analyzed for one sample by a direct comparison of images obtained before and after drying. The resulting opening of the void phase tends to increase significantly the void-fouling material specific surface and the number and size of percolating pathways computed as the skeleton of the void phase. Finally, a first analysis of filtration processes is proposed. It consists in analyzing the percolation pathways for a class of void size by applying the distance map and skeleton concepts to the void phase.

Optimization of bioclogging in vermifilters: A statistical approach

In the present research, an experiment was conducted with the objective of optimization of the role of earthworms in alleviating the bioclogging of a horizontal subsurface flow vermifilter (HSSFVF), caused due to the application of organics rich brewery wastewater. In this experiment, for the optimization of bioclogging of the vermifilters, the Box-Behnken Design (BBD) and response surface methodology (RSM) were involved. Hydraulic loading rate (HLR), influent COD and earthworm density (EWD) are the variables against which the bioclogging of the HSSFVF has been optimized. EWD of 9475 earthworms/m³, HLR of 1.84 m³/m²-d and influent COD of 3701 mg/L have been observed as the optimized values for the minimum bioclogging in the vermifiltration of brewery wastewater. At this optimum boundary conditions, the reduction in hydraulic conductivity was obtained as 1.49%, against the predicted value of 1.67% based upon the BBD model. The verification of the model against real brewery wastewater yielded insignificant error and thus very strongly portrays the suitability of the derived BBD model. The study indicates that the bioclogging from the vermifilters can be minimized, if the variables are optimized using the response surface methodology.

On the performance of a pilot hybrid constructed wetland for stormwater recovery in Mediterranean climate

The overall efficiency of a pilot-scale hybrid constructed wetland (H-CW), located on a retail store's parking area in Eastern Sicily, for alternative treatment of stormwater runoff and of sequential batch reactor (SBR) effluent was evaluated. Experimental activities were focused on system performances, including wastewater (WW) quality and hydraulic monitoring. System design, macrophyte growth and seasonal factors influenced the pilot plant performance. Very high removal efficiency for microbial indicators were reported within the subsurface horizontal flow unit (HF), playing a strategic role for Clostridium perfringens. The algal growth occurred in the free water surface (FWS) unit and inhibited removal efficiencies of total suspended solids (TSS), biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD), impairing water quality. The whole H-CW showed good efficiency in trace metals removal, especially for Pb, Zn, and Cu. Preliminary results suggested the reliability of the H-CW technology in decentralised water treatment facilities for enhancing water recovery and reuse.

Nitrification and total nitrogen removal in a super-oxygenated wetland

A novel surface flow wetland demonstrates high-rate nitrification by recirculating water supersaturated with oxygen. Design of the wetland was based on two hypotheses: (1) Oxygen supersaturation of a recirculating sidestream would support nitrification in a surface flow wetland at rates comparable to aerated subsurface flow wetlands, and; (2) Nitrification would be maintained in cold water conditions (0.5-5.0 °C) with minimal bioaugmentation using commercially available liquid suspensions of nitrifying bacteria. The wetland has an area of 23,000 m², of which the final 21,000 m² is oxygenated. Mean flow was 863 m³ d^-1. A downflow oxygen contactor (Speece cone) maintains dissolved oxygen at or over saturation in the wetland. Mean inflow NH₃-N was 8.0 mg L^-1, mean outflow 0.6 mg L^-1. In winter, nitrification began to degrade slowly at a water temperature of 0.5 °C to about half the summer rate but was restored by bioaugmentation with nitrifying bacteria in one month at water temperatures less than 3.3 °C. Determination of nitrification rates by a first-order, tanks-in-series model (P-k-C*) was hindered by the method detection limit (0.2 mg L^-1 NH₃-N) of analytical methods and the inherent limitations of first order models for complete nitrification. The median monthly nitrification areal rate coefficient from April 2016 through June 2017 was at least 216 m y^-1, which is 15 times greater than median passive wetland literature rates. High nitrification rates allow for at least a 90% reduction of required treatment area for nitrification at approximately one third the capital cost of aerated subsurface flow wetlands. Denitrification rates were well within observed literature values. The peak monthly mean denitrification rate, observed in June 2017, was 76 m y^-1, but winter denitrification rates were close to zero.

Biotechnological approach of greywater treatment and reuse for landscape irrigation in small communities

A level of water quality intended for human consumption does not seem necessary for domestic uses such as irrigation of green spaces. Alternative water supplies like the use of greywater (GW) can thus be considered. However, GW contains pathogenic microorganisms and organic compounds which can cause environmental and health risks. As the risks related to recycling are unknown, GW treatment is necessary before reusing. To describe the risks related to GW reuses, the scientific approach performed in this study was to characterize domestic GW in order to select an appropriate treatment. The biotechnology chosen is a Horizontal sub-surface flow constructed wetland reactor. In order to minimize health risks, an optimization step based on UV disinfection was performed. The treatment performances were then determined. The treated GW produced in this study reached the threshold values expected by the Moroccan regulation for irrigation of green spaces with treated wastewater. Indeed, the COD and the TSS obtained in treated GW without disinfection are respectively 16.6 mg O₂ L⁻¹ and 0.40 mg L⁻¹. The horizontal sub-surface flow constructed wetland (HSSF CW) reactor has been used to treat 1.2 m³/d of GW for 100 days. Three lawn plots have been irrigated respectively with raw GW, treated GW and tap water as a reference. Contrary to the lawn plot irrigated with raw GW, the risk analysis performed in this study has shown no significant difference between the law plots irrigated with treated GW combined with UV disinfection and the one irrigated with tap water. Overall, UV disinfection treated GW produced from the HSSF CW reactor developed in this experiment is thought to be an effective and feasible alternative for agricultural reuse.

Comparative characterization of surface sludge deposits from fourteen French Vertical Flow Constructed Wetlands sewage treatment plants using biological, chemical and thermal indices

Due to their design and mode of operation, French Vertical Flow Constructed Wetlands (VFCWs) accumulate suspended solids from the inflow wastewater in the form of a sludge layer at the surface of the first filter. In order to maintain the treatment performance over the long term, the characteristics of the sludge deposits and their evolution have to be well described. In this objective, a panel of sludge deposit samples taken from 14 French VFCW sewage treatment plants was investigated. Elemental composition and organic matter content, nature and reactivity were analyzed. Results clearly revealed two categories of sludge deposits, namely the "young-age plants" type (1 year of operation and less) and the "mature plants" type (3 years of operation and more). Sludge deposits from the "mature plants" exhibited same biological, physical and chemical properties. Their organic matter was globally less abundant, more humified and less biodegradable than in the young-age plants type. Their overall contents in trace metals were also higher, although in a limited manner. The effect of additional treatments, particularly FeCl₃ injection for phosphorus precipitation, was observable in the "young-age plants" group. Finally, the sludge deposits sampled from one particular plant with specific operating conditions were found to exhibit very different characteristics from those of either groups identified. This observation underlined the influence of local conditions on the typology of the sludge deposits.