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

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.

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.

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.

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.

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.

Study on clogging mechanisms of constructed wetlands from the perspective of wastewater electrical conductivity change under different substrate conditions

Constructed wetland (CW) has obvious advantages in wastewater treatment of medium and small towns. However, there is a lack of health monitoring research on CW system clogging. The electrical conductivity (EC) of wastewater purified by CW is related to the concentration of pollutants, which can reflect the CW clogging. The objectives of this study are to reveal the mechanisms of CWs substrate clogging from the perspective of wastewater EC changes, and provide an important reference for the health evaluation of CWs. The EC changes of nine CWs substrates (quartz sand, zeolite, gravel, coarse sand, straw biochar, sludge biochar, clay ceramsite, fly ash ceramsite and shale ceramsite) under different conditions (purified water, wastewater and wastewater + NaCl) were tested, and comparative analysis was used to reveal the influence of different substrate materials on the change of wastewater EC. The results show that the adsorption ability of substrate material isn't the main factor affecting the EC of wastewater, and the soluble component in the material is the important factor to cause the difference of EC increment. Under the condition of 0.4-1.0 g L^-1 NaCl concentration, the adsorption of substrate materials had little effect on the EC of wastewater, and the effect of NaCl used in CW tracer experiment was good. Quartz sand, coarse sand, gravel and sludge biochar have little influence on the change of wastewater EC. Other materials that have great influence on the change of wastewater EC can be treated by modifying or controlling the mixing ratio. The results are of great significance to reveal the clogging state of CW system and to carry out health assessment research.

Efficiency and kinetics of conventional pollutants and tetracyclines removal in integrated vertical-flow constructed wetlands enhanced by aeration

The integrated vertical-flow constructed wetland (IVCW) is considered as a potential alternative for domestic wastewater treatment of towns and small cities. Oxygen supply is the main limitation of pollutants removal in IVCWs. In the present study, a field experiment was conducted to evaluate the capacity and kinetics of pollutants removal in IVCWs with/without artificial aeration. Two IVCWs constructed with Canna indica and Phragmites australis were running in continuous flow to remove high concentrations of conventional pollutants and low concentrations of tetracyclines (TETs), which are at similar levels of domestic wastewater. The results showed that IVCWs had a good performance on COD, phosphorus, and TETs with removal efficiencies over 80%, 64%, and 75%, respectively, with a hydraulic retention time (HRT) of 3.0 d. However, the removal of nitrogen was limited, showing as TN removal efficiency of about 30%. The IVCW with Phragmites australis had a higher removal efficiency and rate. A kinetics based on Monod Equation and solved with Matlab 2018a could describe the degradation of conventional pollutants. Artificial aeration improved the oxygen supply and remarkably raised the removal capacity for COD, N, and P in IVCWs. The q_1/2 values, which was defined as the average removal loading before half of the pollutants was removed and represented the removal capacity without limitation of pollutants concentration, were increased by 5-30 times after aeration. In conclusion, IVCWs could remove conventional pollutants and TETs simultaneously showing a great potential in domestic wastewater treatment. Artificial aeration enhanced removal capacity of IVCWs on conventional pollutants while showed little influence on TETs.

Adsorption Media for the Removal of Soluble Phosphorus from Subsurface Drainage Water

Phosphorus (P) is a valuable, nonrenewable resource in agriculture promoting crop growth. P losses through surface runoff and subsurface drainage discharge beneath the root zone is a loss of investment. P entering surface water contributes to eutrophication of freshwater environments, impacting tourism, human health, environmental safety, and property values. Soluble P (SP) from subsurface drainage is nearly all bioavailable and is a significant contributor to freshwater eutrophication. The research objective was to select phosphorus sorbing media (PSM) best suited for removing SP from subsurface drainage discharge. From the preliminary research and literature, PSM with this potential were steel furnace slag (SFS) and a nano-engineered media (NEM). The PSM were evaluated using typical subsurface drainage P concentrations in column experiments, then with an economic analysis for a study site in Michigan. Both the SFS and generalized NEM (GNEM) removed soluble reactive phosphorus from 0.50 to below 0.05 mg/L in laboratory column experiments. The most cost-effective option from the study site was the use of the SFS, then disposing it each year, costing $906/hectare/year for the case study. GNEM that was regenerated onsite had a very similar cost. The most expensive option was the use of GNEM to remove P, including regeneration at the manufacturer, costing $1641/hectare/year. This study suggests that both SFS and NEM are both suited for treating drainage discharge. The use of SFS was more economical for the study site, but each site needs to be individually considered.

Diversity and functional annotation of microorganisms in French vertical flow constructed wetland treating greywater

Constructed wetlands form a unique ecosystem having plants, soil, microbes in which microorganisms play a vital role in the transformation and degradation of pollutants from wastewater. In the present study, French type two-stage vertical flow constructed wetland (VFCW) was used for the treatment of single household greywater (GW). Pilot-scale VFCW having sand and gravel as the filter substrate was constructed with Canna indica plantation for treating GW. To understand the pollutant removal mechanism in VFCW, microbial diversity and functional annotation was carried out by metagenomics analysis of sequences obtained from illumina platform. Efficiency of VFCW was measured with respect to water quality parameters like COD, BOD₅, Total Nitrogen, Nitrate, Nitrite, Ammoniacal-N, ortho-phosphate and TOC from inlet and outlet of system. The removal efficiency was 90%, 93%, 34%, 26%, 89%, 68%, 80%, and 80% for COD, BOD₅, Total Nitrogen, Nitrate, Nitrite, Ammoniacal-N, ortho-phosphate and TOC respectively. Microbial diversity was much more diversified and unique in VFCW compared to GW. Metagenomes exhibited Proteobacteria and Bacteroidetes as major phyla in GW whereas Actinobacteria, Proteobacteria, Nitrospirae abundance in VFCW layers. Total of 809 and 695 genus were found in VFCW and GW respectively with minimum abundance of 10 hits. From functional annotation of sequences, VFCW microbes have the potential to transform various aromatic and xenobiotic compounds along with the removal of pollutants present in the form of Carbon, Nitrogen, and Phosphorus. These data reveal French type VFCW can efficiently treat GW and with its own unique, variable habitat VFCW harbours diverse community of microorganisms that transform and degrade the pollutants in GW.

Numerical Models of Subsurface Flow Constructed Wetlands: Review and Future Development

Numerical model as a simulation tool was used to describe the pollutants transformation and degradation process in constructed wetlands (CWs). It can help provide insight into the “black box” and increase the understanding of the complex processes in CWs. In the last few decades, several process-based numerical models were developed to depict the pollutants removal processes in CWs, which include biochemical model, hydraulic model, reactive-transport model, plants model, clogging model, and coupling model combining two or more sub-models. However, there was a long way to go before fully understanding the decontamination mechanisms of CWs. On the one hand, single or a composite model coupling a small number of sub-models cannot fully reveal the decontamination processes. On the other hand, a comprehensive model including all sub-models of current cognition involves numerous parameters, most of which are interaction and cannot quantitatively determined, thus making the model complex and leading to diffuse interaction. Therefore, in order to describe the reaction processes in CWs more accurately, it is expected that all parameters should be quantified as far as possible in the future model. This study aims to provide a review of the numerical models of CWs and to reveal mechanism of decontamination. Based on the advantages and disadvantages of existing models, the study presented the improvement method and future research direction: (1) new detection/monitoring technique or computing method to quantitatively assess the parameters in CWs models, (2) correcting the simulation errors caused by the assumption of Activated Sludge Models (ASMs) and developing a complete biofilm reaction sub-model, (3) simplification of the comprehensive model, and (4) need of emerging pollutants modeling.

Hydrological and hydraulic behaviour of a surface flow constructed wetland treating agricultural drainage water in northern Italy

A surface flow constructed wetland (SFCW) treating agricultural drainage water was investigated with the aim to detect modifications in hydrological and hydraulic characteristics after more than a decade of operation. Ponded infiltration tests were conducted to estimate the saturated hydraulic conductivity, K_s, of the surface soil layer at the point scale. At the global scale, infiltration rate, i, was computed from the water balance to detect leakages from the pervious wetland surface. Tracer tests were conducted to analyse the existence of preferential flow inside the system and to estimate its hydraulic retention time (HRT). Clogging phenomena occurred given a mean K_s value of 30 mm h^-1 was measured near the SFCW inlet, that was 9.61 times lower than the value at the outlet zone. The estimated infiltration losses were two orders of magnitude lower than infiltration measured at the point scale. The results also confirmed the existence of a moderate amount of preferential flow paths and dead zones in the SFCW as the actual HRT (6.7 days) was shorter than the nominal one (8.1 days). Despite this, it can be concluded that the system performance is still good after 17 years of operation.

Designing process and operational effect of modified septic tank for the pre-treatment of rural domestic sewage

Structure and performance of a newly designed four-chamber septic tank were investigated in current study. A three-layer orifice plate was fitted in the first compartment in an upward direction, and a baffle was arranged in the second compartment of the septic tank. The filter placed in the third and fourth compartment was used to collect water for field irrigation or connect subsequent devices for further processing. Flow field distribution in the first chamber was numerically simulated by Fluent software, and simulation results were verified by residence time distribution tracer experiment. The improved treatment effects of the modified septic tank with optimal parameters of 20 mm (diameter), 60 mm (gap) and 180 mm (distance) were validated by simulation experiments. High removal rates of chemical and biological oxygen demand reflected the effectiveness of modified septic tanks.

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.

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.

Pollutant removal performance of an integrated system that combines a baffled vertical-flow wetland and a scenic water body

Stormwater treatment requires effective control measures and development of low-cost and high-efficiency technologies. An integrated system is developed by combining a baffled vertical-flow constructed wetland (BVFCW) and a scenic water body for stormwater quality control purpose. The objectives of the study are to compare the pollutant removal performance of the full-scale integrated system with four groups of wetland-to-scenic water body area ratios (WSARs) including 1/11, 2/11, 3/11, 4/11 and investigate its treatment efficiency. Results show that the system performs better in the removal of chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), nitrate-nitrogen (NO₃-N), and total phosphorus (TP) at the WSAR of 4/11 than that at 3/11 in sixteen-day operation, while it reaches the highest total nitrogen (TN) removal efficiency of 74.0% at the WSAR of 2/11 due to relatively rich carbon source and high influent TN concentration. The integrated system may prove the most effective COD removal at the WSAR of 4/11 for four-time aerobic/anaerobic alternating conditions, a longer flow path and more time to contact with substrates, although the influent COD is lower than that at 2/11 and 1/11. After sixteen-day operation, BVFCW achieved COD removal rate of 90.3%, NH₄⁺-N removal rate of 85.7%, NO₃-N removal rate of 68.6%, and TP removal rate of 52.5% at the WSAR of 4/11. At the WSAR of 1/11, effluent met the Class IV requirements in Chinese standards after one-week operation, while effluent met the Class III requirements under the rest conditions. Since effluent in all WSARs met the standards, WSARs of 1/11 and 2/11 were recommended.