Pollutant removal from landfill leachate employing two-stage constructed wetland mesocosms: co-treatment with municipal sewage

Constructed wetlands combined with microbial fuel cells (CW-MFCs) as a sustainable technology for leachate treatment and power generation

The physical and chemical treatment processes of leachate are not only costly but can also possibly produce harmful by products. Constructed wetlands (CW) has been considered a promising alternative technology for leachate treatment due to less demand for energy, economic, ecological benefits, and simplicity of operations. Various trends and approaches for the application of CW for leachate treatment have been discussed in this review along with offering an informatics peek of the recent innovative developments in CW technology and its perspectives. In addition, coupling CW with microbial fuel cells (MFCs) has proven to produce renewable energy (electricity) while treating contaminants in leachate wastewaters (CW-MFC). The combination of CW-MFC is a promising bio electrochemical that plays symbiotic among plant microorganisms in the rhizosphere of an aquatic plant that convert sun electricity is transformed into bioelectricity with the aid of using the formation of radical secretions, as endogenous substrates, and microbial activity. Several researchers study and try to find out the application of CW-MFC for leachate treatment, along with this system and performance. Several key elements for the advancement of CW-MFC technology such as bioelectricity, reactor configurations, plant species, and electrode materials, has been comprehensively discussed and future research directions were suggested for further improving the performance. Overall, CW-MFC may offer an eco-friendly approach to protecting the aquatic environment and come with built-in advantages for visual appeal and animal habitats using natural materials such as gravel, soil, electroactive bacteria, and plants under controlled condition.

Molecular transformation of dissolved organic matter in manganese ore-mediated constructed wetlands for fresh leachate treatment

The organic matter (OM) and nitrogen in Fresh leachate (FL) from waste compression sites pose environmental and health risks. Even though the constructed wetland (CW) can efficiently remove these pollutants, the molecular-level transformations of dissolved OM (DOM) in FL remain uncertain. This study reports the molecular dynamics of DOM and nitrogen removal during FL treatment in CWs. Two lab-scale vertical-flow CW systems were employed: one using only sand as substrates (act as a control, CW-C) and the other employing an equal mixture of manganese ore powder and sand (experimental, CW-M). Over 488 days of operation, CW-M exhibited significantly higher removal rates for chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), and dissolved organic matter (represented by dissolved organic carbon, DOC) at 98.2 ± 2.5%, 99.2 ± 1.4%, and 97.9 ± 1.9%, respectively, in contrast to CW-C (92.8 ± 6.8%, 77.1 ± 28.1%, and 74.7 ± 9.5%). The three-dimensional fluorescence excitation-emission matrix (3D-EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses unveiled that the influent DOM was predominantly composed of readily biodegradable protein-like substances with high carbon content and low unsaturation. Throughout treatment, it led to the degradation of low O/C and high H/C compounds, resulting in the formation of DOM with higher unsaturation and aromaticity, resembling humic-like substances. CW-M showcased a distinct DOM composition, characterized by lower carbon content yet higher unsaturation and aromaticity than CW-C. The study also identified the presence of Gammaproteobacteria, reported as Mn-oxidizing bacteria with significantly higher abundance in the upper and middle layers of CW-M, facilitating manganese cycling and improving DOM removal. Key pathways contributing to DOM removal encompassed adsorption, catalytic oxidation by manganese oxides, and microbial degradation. This study offers novel insights into DOM transformation and removal from FL during CW treatment, which will facilitate better design and enhanced performance.

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.

Anaerobic-aerobic treatment of wastewater and leachate: A review of process integration, system design, performance and associated energy revenue

Hybrid anaerobic-aerobic biological systems are an environmentally sustainable way of recovering bioenergy during the treatment of high-strength wastewaters and landfill leachate. This study provides a critical review of three major categories of anaerobic-aerobic processes such as conventional wetland, high-rate and integrated bioreactor systems applied for treatment of wastewaters and leachate. A comparative assessment of treatment mechanisms, critical operating parameters, bioreactor configurations, process control strategies, efficacies, and microbial dynamics of anaerobic-aerobic systems is provided. The review also explores the influence of wastewater composition on treatment performance, ammonium nitrogen removal efficacy, impact of mixing leachate, energy consumption, coupled bioenergy production and economic aspects of anaerobic-aerobic systems. Furthermore, the operational challenges, prospective modifications, and key future research directions are discussed. This review will provide in-depth understanding to develop sustainable engineering applications of anaerobic-aerobic processes for effective co-treatment of wastewaters and leachate.

Performance assessment of normal and electrode-assisted floating wetlands: influence of input pollutant loads, surface area, and positioning of anode electrodes

This study reports the design and development of microbial fuel cell (MFC) assisted floating wetlands and compares treatment removal performance with a normal (without electrodes) floating wetland. Both types of floating wetlands were planted with Phragmites plant and evaluated for real municipal wastewater treatment. The effective volume of each floating wetland was 0.5 m³. The floating wetlands were operated under variable hydraulic load rates, i.e., 20 and 60 mm/day. Mean 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), ammoniacal nitrogen (NH₄-N), total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), and coliform removal percentages ranged between 71 and 96%, 72 and 94%, 62 and 86%, 58 and 75%, 82 and 97%, 64 and 92%, and 72 and 93%, respectively within the normal and electrode-assisted MFC integrated floating wetlands. The electrode-integrated floating wetlands showed better pollutant removal performance than the normal system under unstable input pollutant loading conditions. Nitrogen and organic matter removals were achieved through both electrochemically active and inactive microbial removal routes. Physical separation processes, such as filtration and sedimentation, contributed to phosphorus, solids, and coliform removal. Plant uptake contributed to micro-scale nitrogen (≤ 1%) and phosphorus (≤ 0.1%) removal. Increment of hydraulic/pollutant load improved organic removal but decreased nutrient removal performance of the normal, electrode-integrated floating wetlands. The electrode-integrated floating wetlands produced power densities ranging between 0.7 and 1.4 mW/m³, and 0.2 and 2.3 mW/m³ during lower, upper input loading ranges, respectively. Bioenergy production of the electrode-integrated floating wetlands varied within the two operational periods due to a wider range of electrochemically inactive microbial populations in real wastewater that interfered with electrochemical organic matter oxidation. The positioning difference of the anode electrodes was a significant factor that improved pollutant removal within the electrode-integrated floating wetlands compared to the other variable, i.e., anode electrodes surface area.

Development of electrodes integrated hybrid constructed wetlands using organic, construction, and rejected materials as filter media: Landfill leachate treatment

This study developed microbial fuel cell (MFC)-based hybrid constructed wetland systems using different filter media, i.e., organic (biochar), construction (sand), and rejected (iron particle, concrete particle, and stone dust) materials, and evaluated the performance of the developed systems for treating landfill leachate. The mean ammonium nitrogen (NH₄-N), total nitrogen (TN), total phosphorus (TP), biochemical oxygen demand (BOD), chemical oxygen demand (COD) removal percentages within the hybrid systems ranged between 91 and 98%, 90 and 98%, 97 and 99%, 88 and 93%, 93 and 97%, respectively, despite higher pollutants concentration in leachate wastewater. The aerobic environment in the cathode compartment (due to intermittent load) and free-draining of wastewater (from cathode to anode compartment) supported electrochemically inactive, active pollutants removal in the electrodes integrated first stage vertical flow (VF) wetlands. The second stage electrodes integrated horizontal flow (HF) wetlands supported electrochemical-based organic removal and nitrification because of efficient organic removal in the previous VF wetland stages. Nitrogen, phosphorus accumulation percentages in plant tissues ranged between 0.3 and 7%, 0.4 and 14%, respectively. Nutrient removal was achieved through chemical and microbial routes. The biochar-packed VF wetland produced a maximum power density of 20.6 mW/m². The coexistence of unsaturated, saturated media in the partially saturated HF wetland maintained the required environmental gradient between the electrodes and improved operational performance.

Removal of selected pollutants from landfill leachate in constructed wetlands with different filling

Constructed wetlands (CW) with vertical flow were used to treat leachate from municipal landfills (active and closed, at different concentrations) using a combination of substrates, i.e., gravel, sand and an exchangeable layer, depending on a variant: organic substrate (pine bark) or mineral substrate (zeolite, expanded clay). The systems were planted with Phragmites australis. The aim of this study was to compare the efficiency of removal of selected pollutants from landfill leachate in CW using different types of filling. For most parameters, the best reductions were obtained on zeolite substrates. In the investigated CW, reductions were achieved at the levels of: AN 96%-99%; TKN 75%-88.5%; TN 62.5%-70%, TP 84-88% and heavy metals (Zn, Ni, Cu, Cr) 41%-56%.

Nitrogen and Phosphorus Removal Efficiency and Denitrification Kinetics of Different Substrates in Constructed Wetland

Constructed wetlands (CWs) are generally used for wastewater treatment and removing nitrogen and phosphorus. However, the treatment efficiency of CWs is limited due to the poor performance of various substrates. To find appropriate substrates of CWs for micro-polluted water treatment, zeolite, quartz sand, bio-ceramsite, porous filter, and palygorskite self-assembled composite material (PSM) were used as filtering media to treat slightly polluted water with the aid of autotrophic denitrifying bacteria. PSM exhibited the most remarkable nitrogen and phosphorus removal performance among these substrates. The average removal efficiencies of ammonia nitrogen, total nitrogen, and total phosphorus of PSM were 66.4%, 58.1%, and 85%, respectively. First-order continuous stirred-tank reactor (first-order-CSTR) and Monod continuous stirred-tank reactor (Monod-CSTR) models were established to investigate the kinetic behavior of denitrification nitrogen removal processes using different substrates. Monod-CSTR model was proven to be an accurate model that could simulate nitrate nitrogen removal performance in vertical flow constructed wetland (VFCWs). Moreover, PSM demonstrated significant pollutant removal capacity with the kinetics coefficient of 2.0021 g/m2 d. Hence, PSM can be considered as a promising new type of substrate for micro-polluted wastewater treatment, and Monod-CSTR model can be employed to simulate denitrification processes.

Efficiency and plant indication of nitrogen and phosphorus removal in constructed wetlands: A field-scale study in a frost-free area

Frost-free areas have suitable climate for wetland plant growth and constructed wetlands (CW) technology. Information on the quantification of plant biomass and uptake efficiency in field-scale CWs is limited in these climates. The removal efficiency of total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and total suspended solids (TSS) in wastewater from sewage plants, domestic sewage, and an industrial park in 15 rural and urban CWs in Guangdong Province, China, with an average temperature of 30 °C was evaluated. The effects of influent concentration, hydraulic load, the wastewater's physicochemical properties, operating conditions, and plant uptake were analysed. The mean removal rates were 40.0%, 45.2%, 41.1%, and 71.7% for TN, TP, COD, and TSS, respectively, which were higher than the removal load of the field-scale CWs in temperate regions. Removal loads of TN, TP, COD, and TSS were highest in CWs that have been operating for 5-6 years, treating wastewater volumes of over 1 m³/m²·d. The removal efficiency was mainly related to the inflow concentration and less affected by the type of CWs. Nutrient accumulation trends were primarily linked to influent concentrations (TN: r² = 0.89, P = 0.007; TP: r² = 0.96, P = 0.001) and plant biomass (TN: r² = 0.96, P = 0.001; TP: r² = 0.92, P = 0.004). Plant biomass contributed 2%-29% and 2%-70%, respectively, to removing N and P in CWs. The average uptake concentration of N and P in aboveground plant organs (15.66 ± 4.44 mg N/g, 2.15 ± 1.18 mg P/g) was generally higher than that of other temperate plants. A strong relationship between TN and TP in the biomass was also observed; however, the relationship is only restricted by the influent TP concentration. Arundo donax is well-adapted for nutrient accumulation and adaptation and is an ideal wetland plant to purify wastewater in frost-free climates.

Treatment of the actual landfill leachate in different constructed wetlands through intermittent and varied aeration mode

This study focused on the removal of organic matter and nitrogen and explored the feasible operation strategies to achieve short-cut nitrification and denitrification in two constructed wetlands (CWs), which were designed to treat the actual landfill leachate from a small county in parallel. The two CWs were horizontal sub-surface flow constructed wetlands (HFCW) with partial-area aeration and vertical sub-surface flow constructed wetlands (VFCW) with full-area aeration. The experimental results showed that both CWs could achieve an excellent organic matter and nitrogen removal performance under the conditions of intermittent aeration with high frequency and medium intensity (2 h of aeration and 4 h of rest). The removal efficiencies of COD and total nitrogen by HFCW were 89.08% and 73.22%, and the corresponding values of VFCW were 84.51% and 71.44%, respectively. Meanwhile, the inhibition kinetics model indicated that HFCW with partial-area aeration could enhance the free ammonium (FA) tolerance of ammonium-oxidizing bacteria (AOB) and reduce the conversion percentage of ammonia nitrogen. In addition, the intermittent aeration mode with high frequency and medium intensity could keep the DO concentration below under 0.60 mg L^-1 in HFCW, which helped to achieve stable short-cut nitrification and ensure the average nitrite accumulation rate (NAR) reach 50.96%. These results suggested that the intermittent aeration in partial-area could achieve successful short-cut nitrification in HFCW, thereby improving the removal efficiency of nitrogen in landfill leachate.

Unplanted wetland-type filter for co-treatment of landfill leachate and septic tank wastewater: Analysing gravel replacement by plastic and passive (filling-emptied) aeration effects at pilot scale

Nowadays, traditional residential and industrial wastewater treatment methods have been mainly developed as complex systems that consider costly infrastructure, which requires advanced control systems and highly qualified labour for their operation. The use of wetland-type infrastructure has been recognized both, by scientists and authorities, as an efficient and effective method to obtain good results in these processes. The most relevant elements in the design of horizontal subsurface filters are filtering media, as biofilm supporting material and flow control methods. Usually, these treatment systems use gravel as filling material. Despite the functionality of the stone material, its weight presents serious difficulties for its handling at source and on site. The use of a plastic support would lower transportation costs, improve manageability and reduce the probability to damage the underlying impermeable layer. In addition, it might extent the useful life of the reactor by cleaning it when clogged, and the potential use of recycled plastic would improve the sustainability of the process. To verify the possibility of using a lightweight plastic material to replace heavy gravel, an unplanted pilot scale treatment system, composed of four independent treatment units in parallel was implemented. The treatment units differed in the filtering media and the input flow regime. Two of the treatment units used gravel (Specific surface 305 m2/m3, 1475 kg/m3 apparent density) and two units used plastic material (Specific surface 750 m2/m3, 172 kg/m3 apparent density). To check the incidence level of passive aeration procedures, on the effectiveness of each material, two of the treatment lines used a continuous flow system and two of them used an automatic filling and emptying flow method that allows passive aeration of the support media. A mixture of landfill leachate and septic tank wastewater was treated, and the evolution of turbidity, chemical oxygen demand, nitrogen and phosphorus was monitored. Results showed that, in general, there are no significant differences regarding the performance of the materials tested, whereas passive aeration notably improves the abatement and solids retention performance of the pilot units. It is concluded that the plastic material tested can be used as a replacement for the stone material, without having appreciable losses in the efficiency of the system. Further research is needed to quantify the benefits associated with the use of this support in constructed wetlands-type technologies.

Effects of selection and compiling strategy of substrates in column-type vertical-flow constructed wetlands on the treatment of synthetic landfill leachate containing bisphenol A

A constructed wetland (CW) is a low-cost, eco-friendly, easy-to-maintain, and widely applicable technology for treating various pollutants in the waste landfill leachate. This study determined the effects of the selection and compiling strategy of substrates used in CWs on the treatment performance of a synthetic leachate containing bisphenol A (BPA) as a representative recalcitrant pollutant. We operated five types of lab-scale vertical-flow CWs using only gravel (CW1), a sandwich of gravel with activated carbon (CW2) or brick crumbs (CW3), and two-stage hybrid CWs using gravel in one column and activated carbon (CW4) or brick crumbs (CW5) in another to treat synthetic leachate containing BPA in a 7-d sequential batch mode for 5 weeks. CWs using activated carbon (CW2 and CW4) effectively removed ammonium nitrogen (NH₄-N) (99-100%), chemical oxygen demand (COD) (93-100%), and BPA (100%), indicating that the high adsorption capacity of activated carbon was the main mechanism involved in their removal. CW5 also exhibited higher pollutant removal efficiencies (NH₄-N: 94-99%, COD: 89-98%, BPA: 89-100%) than single-column CWs (CW1 and CW3) (NH₄-N: 76-100%, COD: 84-100%, BPA: 51-100%). This indicates the importance of the compiling strategy along with the selection of an appropriate substrate to improve the pollutant removal capability of CWs.

Cr, Ni, and Zn removal from landfill leachate using vertical flow wetlands planted with Typha domingensis and Canna indica

Chromium (Cr), Nickel (Ni), and zinc (Zn) removal from landfill leachate using mesocosm-scale vertical flow wetlands, the effect of recirculation, and the ability of macrophytes to retain metals were evaluated. Wetlands were filled with coarse sand and light expanded clay aggregates and planted with Typha domingensis or Canna indica. Wetlands were operated using intermittent loading, with and without recirculation. Raw leachate was diluted and spiked with metals to reach the following concentrations: 0.2 mg L^-1 Cr , 0.2 mg L^-1 Ni, and0.2 mg L^-1 Zn and 1.0 mg L^-1 Cr, 1.0 mg L^-1 Ni, and 1.0 mg L^-1 Zn. Wetlands planted with T. domingensis presented higher metal removal than those planted with C. indica. Recirculation enhanced metal removal efficiencies significantly, being for T. domingensis/C. indica: 60/54, 49/47, 61/47% for Cr, Ni, and Zn at 0.2 mg L^-1, and 80/71, 76/62, 73/59% for Cr, Ni, and Zn at 1.0 mg L^-1, respectively. Metals were efficiently retained by macrophytes. Plant biomass and metal concentrations in roots were significantly higher than in shoots. Scanning electron microscopy and X-ray microanalysis showed that metals were absorbed by internal root tissues. A hybrid wetland planted with T. domingensis may be implemented to improve not only metal but also chemical oxygen demand and total nitrogen removals.

Advances in Biological Nitrogen Removal of Landfill Leachate

With the development of economy and the improvement of people’s living standard, landfill leachate has been increasing year by year with the increase in municipal solid waste output. How to treat landfill leachate with high efficiency and low consumption has become a major problem, because of its high ammonia nitrogen and organic matter content, low carbon to nitrogen ratio and difficult degradation. In order to provide reference for future engineering application of landfill leachate treatment, this paper mainly reviews the biological treatment methods of landfill leachate, which focuses on the comparison of nitrogen removal processes combined with microorganisms, the biological nitrogen removal methods combined with ecology and the technology of direct application of microorganisms. In addition, the mechanism of biological nitrogen removal of landfill leachate and the factors affecting the microbial activity during the nitrogen removal process are also described. It is concluded that the treatment processes combined with microorganisms have higher nitrogen removal efficiency compared with the direct application of microorganisms. For example, the nitrogen removal efficiency of the combined process based on anaerobic ammonium oxidation (ANAMMOX) technology can reach more than 99%. Therefore, the treatment processes combined with microorganisms in the future engineering application of nitrogen removal in landfill leachate should be paid more attention to, and the efficiency of nitrogen removal should be improved from the aspects of microorganisms by considering factors affecting its activity.

Intensified constructed wetlands for the treatment of municipal wastewater: experimental investigation and kinetic modelling

This study reports organics and nutrient removal performances of the intensified constructed wetlands, i.e., tidal flow-based microbial fuel cell (MFC) and tidal flow wetlands that received municipal wastewater. The wetland systems were filled with organic (coco peat, biochar) or waste (Jhama brick, steel slag) materials, planted with Phragmites australis or Chrysopogon zizanioides (Vetiver) species, and operated under three flood periods: 8, 16, 24 h. Input ammonia nitrogen (NH3-N), total nitrogen (TN), phosphorus (P), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) load across the wetland systems ranged between 3-27, 12-78, 0.1-23, 36-1130, and 11-281 g/m2day, respectively; mean removal percentages were 60-83, 74-84, 95-100, 94-98, and 93-97%, respectively, throughout the experimental run. The wetland systems achieved similar organics and P removals; operational and media variation did not influence removal kinetics. All wetland systems achieved the highest TN removal (76-87%) when subjected to 24-h flood period. TN removal performances of waste material-based wetlands were comparable to organic media-based systems. Tidal flow-based MFC wetlands achieved better TN removal than tidal flow wetlands because of supplementary electron production through fuel cell-based organics degradation kinetics. Maximum power production rates across the tidal flow-based MFC wetlands ranged between 53 and 57 mW/m2. Monod kinetics-based continuous stirred tank reactor (CSTR) models predicted NH3-N, TN, and COD removals (in wetland systems) more accurately. Kinetic models confirmed the influence of substrate (i.e., pollutant) and environmental parameters on pollutant removal routes.