Long-term investigation of constructed wetland wastewater treatment and reuse: Selection of adapted plant species for metaremediation

Recent progress, challenges, and future prospects in constructed wetlands employing biochar as a substrate: a comprehensive review

Constructed wetlands (CWs) are a cost-effective, efficient, and long-term wastewater treatment solution in various countries. The efficacy and performance of constructed wetlands are greatly influenced by the substrate. Recently, biochar as a substrate, along with sand and gravel in constructed wetlands, has gained importance due to its various physical, chemical, and biological properties. This review presents a detailed study of biochar as a substrate in CWs and the mechanism involved in efficiency enhancement in pollutant removal. Different methods for producing biochar using various types of biomasses are also addressed. The effect of biochar in removing pollutants like biological oxygen demand (BOD), chemical oxygen demand (COD), nitrogen, heavy metals, and non-conventional pollutants (microcystin, phenanthrene, antibiotics, etc.) are also discussed. Furthermore, post-harvest utilization of constructed wetland macrophytic biomass via bioenergy production, biochar formation, and biosorbent formation is explained. Various challenges and future prospects in biochar-amended constructed wetlands are also discussed. Biochar proved to be an effective substrate in the removal of pollutants and proved to be a promising technique for wastewater treatment, especially for developing countries where the cost of treatment is a constraint. Biochar is an effective substrate; further modification in biochar with the right plant combination for different wastewater needs to be explored in the future. Future researchers in the field of constructed wetlands will benefit from this review during the utilization of biochar in constructed wetlands and optimization of biochar characteristics, viz., quantity, size, preparation method, and other biochar modifications.

In situ phytoextraction of Mn and NH4+-N from aqueous electrolytic manganese residue solution by Pistia stratiotes: Effects of Fe/Co presence and rhizospheric microbe synergistic involvement

The electrolytic manganese industry produces a large amount of electrolytic manganese residue (EMR). Soluble Mn, NH4+-N, and other pollutants may be released from the open-air stacked EMR and transported to the environment along with rainfall or surface runoff. Aqueous EMR solution (AES) generally contains various elements required for plant growth, and phytoremediation can be applied to remove these pollutants from AES. Since the contents of Fe and Co vary greatly in AES depending on the ore sources as well as the pre-treatment processes, the presence of bioavailable Fe and Co at different levels may affect plant growth, the rhizosphere microbes, and pollutant removal. The present study investigated the in-situ removal of Mn(II) and NH4+-N from AES solution using free floating aquatic plant Pistia stratiotes, focusing especially on the effects of Fe/Co presence and rhizospheric microbe synergistic involvement on contaminant removal. The results showed that 69.08% of Mn and 94.99% of NH4+-N were removed by P. stratiotes in 24 d. Both the presence of Fe(II) and Co(II) facilitated the Mn(II) immobilization and increased Mn(II) removal by 19-31% due to the enhanced peroxidase activity and the increased Mn accumulating in roots The complete removal of Mn from AES was found in the presence of Fe(II) at 2 mg L-1 or Co(II) at 0.5 mg L-1 and more than 51% accumulated Mn in the roots was stored in the vacuole and cytoplasm. BioMnOx was found on the surface of the roots, revealing that rhizofiltration, rhizospheric plaque/biofilm formation, and Mn biogeochemical cycle exert synergic effects on Mn(II) immobilization. The findings of the present study demonstrate the feasibility of using P. stratiotes in the treatment of aqueous EMR solutions and the presence of an appropriate amount of bio-available Fe and Co can promote the removal of Mn(II) and NH4+-N.

A survey on constructed wetland publications in the past three decades

Decentralised wastewater treatment systems, such as constructed wetlands, are becoming increasingly popular these days because they are more economical and cost-effective than conventional plants. The primary objective of this review paper is to determine the number of studies that have been conducted on constructed wetlands, specifically 'free water surface flow constructed wetlands', 'horizontal subsurface flow constructed wetlands', 'vertical subsurface flow constructed wetlands', and 'hybrid constructed wetlands'. In addition, the paper examines the status of research publications on constructed wetlands by country, author, and journal. Based on the review, it has been found that although constructed wetland technology is economical and cost-effective, it is still not among the top 10 effluent treatment methods. Compared to other constructed wetland systems, 'hybrid constructed wetlands' have received minimal attention. Based on the search results, 4639 documents published between 1989 and 2021 have been extracted from the online edition of SCI-EXPANDED, Web of Science. The documents associated with constructed wetlands are divided into eight main document types. Articles and proceedings papers are the most common document type, accounting for 93% of all publications, followed by reviews (4%), meeting abstracts (1.3%), corrections (0.56%), editorial materials (0.38%), news items (0.2%), letters (0.04%), and book reviews (0.02%).

A three-year study on the treatment of domestic-industrial mixed wastewater using a full-scale hybrid constructed wetland

Three full-scale constructed wetlands (CWs), namely vertical flow (VFCW), surface flow (SFCW), and horizontal flow (HFCW) systems, were combined in a series process to form a hybrid CW, which was used for the treatment performance of domestic-industrial mixed wastewater and investigated over a three-year period. The hybrid CW demonstrated that it is effective and stable during the long-term treatment of high-loading mixed wastewater under different operation years, season changes, and technology processes, with the average removal efficiencies of suspended solids, chemical oxygen demand, biological oxygen demand, total nitrogen, ammonia nitrogen, nitrate nitrogen, and total phosphorous being 84, 40, 54, 54, 70, 40, and 46%, respectively. The effluent quality of the hybrid CW reached the highest discharge standard for wastewater treatment plants. First, a variety of pollutants from the mixed wastewater were effectively removed in the subsurface processes (VFCW and HFCW) via substrate adsorption and degradation of the attached biofilm. The higher dissolved oxygen content and oxygen transfer capacity values in the VFCW were favourable for the occurrence of aerobic pathways (such as nitrification and inorganic phosphorus oxidation). In addition, with the large consumption of oxygen in the previous process, the oxygen-enriching capacity of the SFCW processes, provided aerobic potential for the next stage. In particular, the plant debris in the SFCW temporarily increased the organics and suspended solids, further increasing the C/N ratio, which was beneficial for denitrification as the main nitrogen removal pathway in the HFCW.

Bio-purification of domestic wastewater through constructed wetland planted with Paspalidium flavidum

The current study was aimed to designed laboratory scale constructed wetland (CW) for the treatment of domestic wastewater under temperature range (18-38°C) and hydraulic retention times (24 and 48 h). Besides, the soil of vegetative unit of CW was assessed using conventional culturing techniques, and 13 different bacterial species (Escherichia coli, Micrococcus, Pseudomonas, Proteus, Klebsiella, Streptococcus, Alcaligenes, Salmonella, Bacillus, Enterobacter, Staphylococcus, Shigella and Corynebacterium spp.) were determined. The pathogenic microbial load was high in influent samples, but after treatment, about 73.1-99.7% and 43.5-86.7% reduction in CFU/ml and MPN/100 ml index, respectively, were observed. Moreover, the organic loads in terms of COD, TDS, TSS, and turbidity were high in all influent samples, but after treatment, average percentage removal in different physico-chemical parameters was observed during overall treatment operations, that is, COD (59.7-65.6%), TDS (59.6-76.8%), TSS (64.9-76.7%), and turbidity (72.7-91.6%), while pH of the effluent samples was observed in the prescribed limits. It was concluded that laboratory scale CW using natural flora Paspalidium flavidum and bacterial species was efficient in the reduction of different pollution indicators and hence a best option to be modified on pilot-scale for wastewater treatment in the rural regions of Peshawar. PRACTITIONER POINTS: Role of retention times on performance of CW were studied. The commonly existing vegetation was utilized to treat domestic waste water. Both vegetations and HRT are key ingredients in obtaining high treatment efficiency.

Four Years Continuous Monitoring Reveals Different Effects of Urban Constructed Wetlands on Bats

Proactive artificial wetland constructions have been implemented to mitigate the loss of wetlands and their ecosystem services. As wetlands are habitats for bats, short-term (one or two years) studies find that constructed wetlands can immediately increase local bat activity and diversity. However, it is not clear how constructed wetlands affect bats through time while the wetlands are aging. We collected four years of continuous bat acoustic monitoring data at two constructed wetlands in an urban park in Greensboro, NC, USA. We examined bat activity and community composition patterns at these wetlands and compared them with reference sites in the city. With four years of data, we found that the effects of constructed wetlands were both habitat- and species-specific. The wetland in forests significantly increased bat activity, while the wetland in the open grass altered bat community composition. Specifically, in terms of species, we found that over time, constructed wetlands no longer attracted more big brown, silver-haired, or evening bats than control sites while the wetlands aged, highlighting the need to study broadly how each bat species uses natural and artificial wetlands. We emphasize the importance of long-term monitoring and the periodical evaluation of wildlife conservation actions.

Aquatic Macrophytes in Constructed Wetlands: A Fight against Water Pollution

There is growing concern among health institutions worldwide to supply clean water to their populations, especially to more vulnerable communities. Although sewage treatment systems can remove most contaminants, they are not efficient at removing certain substances that can be detected in significant quantities even after standard treatments. Considering the necessity of perfecting techniques that can remove waterborne contaminants, constructed wetland systems have emerged as an effective bioremediation solution for degrading and removing contaminants. In spite of their environmentally friendly appearance and efficiency in treating residual waters, one of the limiting factors to structure efficient artificial wetlands is the choice of plant species that can both tolerate and remove contaminants. For sometimes, the chosen plants composing a system were not shown to increase wetland performance and became a problem since the biomass produced must have appropriated destination. We provide here an overview of the use and role of aquatic macrophytes in constructed wetland systems. The ability of plants to remove metals, pharmaceutical products, pesticides, cyanotoxins and nanoparticles in constructed wetlands were compared with the removal efficiency of non-planted systems, aiming to evaluate the capacity of plants to increase the removal efficiency of the systems. Moreover, this review also focuses on the management and destination of the biomass produced through natural processes of water filtration. The use of macrophytes in constructed wetlands represents a promising technology, mainly due to their efficiency of removal and the cost advantages of their implantation. However, the choice of plant species composing constructed wetlands should not be only based on the plant removal capacity since the introduction of invasive species can become an ecological problem.

Micro-aeration with hollow fiber membrane enhanced the nitrogen removal in constructed wetlands

The nitrogen removal efficiency in constructed wetlands (CWs) was largely affected by the dissolved oxygen (DO). In this study, micro-aeration with different numbers of hollow fiber membrane modules (HFMEs) was adopted to increase the oxygen availability and improve the nitrogen removal efficiency in CWs under different air temperatures and different hydraulic retention time (HRT). Compared to the plant oxygen release (ROL) of wetland plants and traditional mechanical aeration, HFME increased the oxygen availability and enhanced the nitrogen removal efficiency in CWs. The COD and NH₄⁺-N removal efficiencies increased with the increase of the HMFE. TN removal efficiency was increased by 8~16% after the application of HFME in CWs in the high-temperature stage. However, less HFME in CW-M1 realized the highest TN removal efficiency in low- and medium-temperature stages. At low temperature after 4-day HRT, the DO concentration respectively reached 6.25 mg L^-1 and 3.25 mg L^-1 in the upper zone and the bottom of CW-M1. The TN removal efficiencies in the upper zone of CW-M1 (60.69%) and the bottom of CW-M1 (64.98%) were all significantly higher than those in the upper zone of CK (35.98%) and the bottom of CK (39.9%). In addition, the microbial biomass and community analyses revealed that CW-M1 showed the most nitrifying bacteria and the best metabolic activity of bacteria. HEMF in CW-M1 also increased the nitrifying capacity from 0.12 to 0.46 mg kg^-1 h^-1. The application of HFME in CWs accelerated the nitrification process by enhancing nitrifying bacteria and less HFME realized the highest TN removal efficiency through nitrification-denitrification processes. Graphical abstract The application of hollow fiber membrane modules in CWs enhanced the pollutants (TN and COD) removal efficiency in the process of biological nitrification-denitrification and increased the number of nitrifying bacteria.

Can Constructed Wetlands be Wildlife Refuges? A Review of Their Potential Biodiversity Conservation Value

The degradation of wetland ecosystems is currently recognized as one of the main threats to global biodiversity. As a means of compensation, constructed wetlands (CWs), which are built to treat agricultural runoff and municipal wastewater, have become important for maintaining biodiversity. Here, we review studies on the relationships between CWs and their associated biodiversity published over the past three decades. In doing so, we provide an overview of how wildlife utilizes CWs, and the effects of biodiversity on pollutant transformation and removal. Beyond their primary aim (to purify various kinds of wastewater), CWs provide sub-optimal habitat for many species and, in turn, their purification function can be strongly influenced by the biodiversity that they support. However, there are some difficulties when using CWs to conserve biodiversity because some key characteristics of these engineered ecosystems vary from natural wetlands, including some fundamental ecological processes. Without proper management intervention, these features of CWs can promote biological invasion, as well as form an ‘ecological trap’ for native species. Management options, such as basin-wide integrative management and building in more natural wetland components, can partially offset these adverse impacts. Overall, the awareness of managers and the public regarding the potential value of CWs in biodiversity conservation remains superficial. More in-depth research, especially on how to balance different stakeholder values between wastewater managers and conservationists, is now required.

Enhancement of COD removal in constructed wetlands treating saline wastewater: Intertidal wetland sediment as a novel inoculation

This study investigated intertidal wetland sediment (IWS) as a novel inoculation source for saline wastewater treatment in constructed wetlands (CWs). Samples of IWS (5-20 cm subsurface sediment), which are highly productive and rich in halophilic and anaerobic bacteria, were collected from a high-salinity natural wetland and added to CW matrix. IWS-supplemented CW microcosms that are planted and unplanted Phragmites australis were investigated under salty (150 mM NaCl: PA+(S) and CT+(S)) and non-salty (0 mM NaCl: PA+ and CT+) conditions. The chemical oxygen demand (COD) removal potential of IWS-supplemented CWs was compared with that of conventional CWs without IWS (PA(S) and CT(S), PA, and CT). Results showed that the COD removal rate was higher in PA+(S) (51.80% ± 3.03%) and CT+(S) (29.20% ± 1.26%) than in PA(S) (27.40% ± 3.09%) and CT(S) (27.20% ± 3.06%) at 150 mM NaCl. The plants' chlorophyll content and antioxidant enzyme activity indicated that the addition of IWS enhanced the resistance of plants to salt. Microbial community analysis showed that the dominant microorganisms in PA+(S) and CT+(S), namely, Anaerolineae, Desulfobacterales, and Desulfuromonadales, enhanced the organic removal rates via anaerobic degradation. IWS-induced Dehalococcoides, which is a key participant in ethylene formation, improved the plants' stress tolerance. Several halophilic/tolerant microorganisms were also detected in the CW system with IWS. Thus, IWS is a promising inoculation source for CWs that treat saline wastewater.

Ecological aesthetic assessment of a rebuilt wetland restored from farmland and management implications for National Wetland Parks

While wetlands are usually used as a natural approach to remove biodegradable pollutants in surface water, their purification efficiencies coupled with their aesthetic features are of less concern. The water quality, plant landscape, acoustic environment and odour indicators were investigated in the surface water inlet and outlet of the Fujin National Wetland Park (FNWP), restored from farmlands in Northeast China. Major concentrations of pollutants in the inlet and the outlet subjected to surface flow wetland treatment were monitored, and the removal efficiencies were calculated based on 54 water samples (6 sites×3 seasons×3 replicates). The results showed that the total nitrogen (TN) and organic carbon in surface water decreased significantly after the wetland treatment, while the total phosphorus (TP) did not decrease significantly. The removal efficiencies for TN and BOD5 changed seasonally and reached 69.08% and 60.44%, respectively. An ecological aesthetic index (EAI) was developed based on the trophic state index coupled with plant landscape, acoustic and odour indicators, and the calculated EAI showed that the outlet delivered a more aesthetically harmonious appearance than the inlet in spring and autumn, but not in summer. Based on the current aquatic macrophyte species and documented purification efficiencies in FNWP, we recommend an improved ecological aesthetic management approach that utilizes and arranges diverse native plants from the surrounding wetlands (e.g. Scirpus validus) in addition to local Nelumbo nucifera, Nymphaea tetragona and Myriophyllum spicatum, and conserves the indicative and endangered species (Aldrovanda vesiculosa), from the visual appeal of the waterscape.

Efficiency of nitrogen and phosphorus compounds removal in hydroponic wastewater treatment plant

Wastewater treatment with the use of plants is one of the most promising treatment technologies for municipal, domestic and industrial wastewater. The aim of this study was to evaluate the reduction of biogenic compounds concentrations in municipal wastewater treated with the use of hydroponic technology as a tertiary treatment. Research was conducted to assess the suitability of hydroponic lagoon use for municipal wastewater treatment. Samples of sewage for its quality tests were taken from a wastewater treatment plant (WWTP) located in the south-western part of Poland. Average efficiencies of nutrients removal were calculated. Analyses show that the effectiveness of purification is similar for systems with and without the use of the third stage of purification and reached around 82-83% for total nitrogen and 77-79% for ammonium nitrogen. N-NO₂ and N-NO₃ removal efficiency was very low for both analysed systems where for total phosphorus and phosphates it oscillated around 83-84%.

Biochemical, Physiological and Anatomical Mechanisms of Adaptation of Callistemon citrinus and Viburnum lucidum to NaCl and CaCl2 Salinization

Callistemon citrinus and Viburnum lucidum are appreciated and widespread ornamental shrubs for their abundant flowering and/or brilliant foliage. The intrinsic tolerance to drought/salinity supports their use in urban areas and in xeriscaping. Despite adaptive responses of these ornamental species to sodium chloride (NaCl) have been extensively explored, little is known on the effects of other salt solution, yet iso-osmotic, on their growth, mineral composition and metabolism. The present research was aimed to assess responses at the biochemical, physiological and anatomical levels to iso-osmotic salt solutions of NaCl and CaCl2 to discriminate the effects of osmotic stress and ion toxicity. The two ornamental species developed different salt-tolerance mechanisms depending on the salinity sources. The growth parameters and biomass production decreased under salinization in both ornamental species, independently of the type of salt, with a detrimental effect of CaCl2 on C. citrinus. The adaptive mechanisms adopted by the two ornamental species to counteract the NaCl salinity were similar, and the decline in growth was mostly related to stomatal limitations of net CO2 assimilation rate, together with the reduction in leaf chlorophyll content (SPAD index). The stronger reduction of C. citrinus growth compared to V. lucidum, was due to an exacerbated reduction in net photosynthetic rate, driven by both stomatal and non stomatal limitations. In similar conditions, V. lucidum exhibited other additional adaptive response, such as modification in leaf functional anatomical traits, mostly related to the reduction in the stomata size allowing plants a better control of stomata opening than in C. citrinus. However, C. citrinus plants displayed an increased ability to retain higher Cl- levels in leaves than in roots under CaCl2 salinity compared to V. lucidum, thus, indicating a further attempt to counteract chloride toxicity through an increased vacuolar compartmentalization and to take advantages of them as chip osmotica.

Influence of Insolation on the Efficiency of NO3 Removal from Wastewater Treated in the Hydroponic System

The use of plants and natural processes for wastewater treatment is an issue that arouses interest among technologists and scientists around the world. The aim of the article was to analyze the influence of the air temperature and insolation on the removal of nitrate nitrogen from the wastewater treated in the hydroponic system, under greenhouse conditions. Samples of sewage for its quality tests were taken from a wastewater treatment plant (WWTP) located in the southwestern part of Poland. Data regarding daily sunshine duration and average daily air temperature values in selected periods of 2013-2016 come from a meteorological station located 30 km from WWTP. The conducted research and analyses of the results clearly indicate that under moderate climate conditions, the amount of solar radiation reaching the Earth's surface is insufficient to ensure the year-round, effective wastewater treatment process in the hydroponic system. In the case of air temperature, no correlation was found between the tested parameters, which indicates the lack of temperature influence on the efficiency of NO₃ removal from the wastewater by macrophytes growing in the lagoon.