Influence of plants on microbial activity in a vertical-downflow wetland system treating waste activated sludge with high organic matter concentrations

Performance evaluation of three constructed wetland-microbial fuel cell systems: wastewater treatment efficiency and electricity generation potential

Constructed wetlands (CWs) have proven to be effective and environmentally friendly for removing pollutants, while microbial fuel cells (MFCs) offer the potential for electricity generation. Thus, this study evaluated the performance of three CW-MFC systems (zigzag, single-column, and triple-column continuous) for domestic wastewater treatment and electricity generation. Results showed that parallel connection of CW-MFCs significantly improved power generation compared to series connection. Additionally, using three copper wires to connect carbon fiber felt electrodes demonstrated superior pollutant capture capabilities compared to a single copper wire. During the 14-day testing period, the single-column system achieved the highest power density of 5.55 mW m-2, followed closely by the triple-column continuous system at 4.77 mW m-2. In contrast, the zigzag system exhibited a lower power density of 2.49 mW m-2. Interestingly, the implementation of facultative anaerobic conditions in the anode, along with the application of a plastic bag cover, facilitated the maintenance of anaerobic conditions in both the single-column and triple-column continuous systems. This resulted in increased power density and reduced internal resistance. In contrast, the zigzag system, with its larger surface area, aeration, and circulation, exhibited higher internal resistance and lower current dissipation. Despite its inferior electricity generation performance, the zigzag system demonstrated higher efficiency removal of chemical oxygen demand (COD), nitrate (NO3-), and phosphate (PO43-) than the single-column system. This can be attributed to the extended contact time, resulting in enhanced pollutant removal. Overall, the multi-column continuous system shows promise as a viable approach for simultaneous domestic wastewater treatment and electricity production, offering potential benefits for sustainable wastewater management.

Municipal Wastewater Treatment uses Vertical Flow Followed by Horizontal Flow in a Two-Stage Hybrid-Constructed Wetland Planted with Calibanus hookeri and Canna indica (Cannaceae)

The utilization of hybrid-constructed wetland systems has recently expanded due to more rigorous municipal wastewater discharge and also complex wastewaters treated in hybrid-constructed wetlands (HCWs). A lab-scale two-stage experimental setup of vertical flow followed by horizontal flow hybrid-constructed wetland (VFHCW-HFHCW) configuration was built. First-stage vertical flow hybrid-constructed wetland reactor with the surface area was 1963.49 cm² and second-stage horizontal flow hybrid-constructed wetland reactor with the surface area was 2025 cm². The HCW unit was planted with two type plants: Calibanus hookeri and Canna indica (Cannaceae). Influent Municipal wastewater flow rate 112.32 l/day, hydraulic loading rate (HLR) 0.55 m/day, and hydraulic retention time of 1 day. The efficiency was evaluated in municipal wastewater quality improvement and physico-chemical analysis in our laboratory. The removal rate after the second-stage horizontal flow of BOD₃ at 27 °C, COD, TSS, TP, NH₃-N, and NO₃-N reached 92.75%, 89.90%, 85.45%, 88.83%, 99.09%, and 96.05%, respectively. The results shown after both stage hybrid-constructed wetland VFHCW-HFHCW, treated effluent of Municipal wastewater produced high-quality effluent which may be reused in gardening, agriculture, and flushing in toilet purpose according to Bureau of Indian Standards (BIS) code for practices. However, in the future, hybrid-constructed wetlands could be standards design criteria developing and enhancing the performance standards and economic meets both to make more popular technology of the hybrid-constructed wetland (HCW).

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The online version contains supplementary material available at 10.1007/s11270-022-05984-0.

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

Constructed wetlands are low-cost, natural technologies that are often employed for the treatment of different types of wastewater. In this study, landfill leachate and municipal wastewater were co-treated by the three parallel two-stage Phragmites- or Vetiver-based constructed wetland mesocosms. Two-stage wetland mesocosms included vertical flow (VF) units as the first stage, followed by horizontal flow (HF)/surface flow (SF)/floating treatment (FT) units. VF and HF wetland mesocosms were filled with gravel, steel slag, concrete block, and intermittent carbon-saturated ceramic filters as substrates. Mean input nitrogen, organics, and phosphorus load across first stages were 75 g N/m² day, 283 g COD/m² day, 88 g BOD/m² day, and 10 g P/m² day, respectively. N and P accumulation rate was not substantial (< 10%) with respect to total removal in most wetland mesocosms. Gravel-based VF wetland mesocosm achieved better NH₄-N and BOD removal (55-59%) during landfill leachate treatment phase, when compared with co-treatment periods (12-52%). Slag-concrete- and ceramic filter-based VF wetland mesocosms maintained stable NH₄-N and BOD removals; the former wetland mesocosm was the most efficient VF unit (than other two wetland mesocosms) due to media characteristics. Media-based adsorption accelerated P removal (93%) in slag-concrete-based VF wetland mesocosm. Carbon scarcity limited denitrification in all VF wetland mesocosms; removal of TN was < 32%. Second stage wetland mesocosms achieved higher nitrogen (85-92%), organics (66-90%), and phosphorus (97-100%) removals regardless of operational variations; low input load, long retention time, media, and rhizosphere enhanced removal performances, particularly in HF and FT wetland mesocosms. In general, this study demonstrates potential application of two-stage wetland mesocosms for landfill leachate treatment or co-treatment with municipal sewage.

Nitrogen removal in response to plants harvesting in two kinds of enhanced hydroponic root mats treating secondary effluent

Hydroponic root mats (HRMs) are a green technology for various wastewaters purification. However, plants wilting will inevitably reduce the purification efficiency of HRMs. Harvesting as an important way of plant management for a better understanding of sustainability of HRMs has always been highly controversial. The goal of this study was to investigate the impacts and sustainability of harvesting on nitrogen removal and greenhouse gases (CH₄, N₂O) emissions of the two kinds of enhanced hydroponic root mats: autotrophic hydroponic root mat (AHRM) and heterotrophic hydroponic root mat (HHRM) for treating secondary effluent. The results showed that harvesting temporarily decreased nitrogen removal efficiency of the two systems, and removal efficiency recovered quickly because of the existence of external electron donors. The effects of harvesting are ordered as: HHRM > AHRM, NO₃^--N > NH₄⁺-N. Increasing C/N, S/N would reduce the impact of temperature on harvesting systems. Harvesting also increased the emission of greenhouse gases, and increasing C/N(=6), S/N(=1.1) could significantly reduce greenhouse gases emission of the harvesting systems at low temperature. In addition, composition analysis of the shoots of the harvested plants was also conducted, and the results showed that N contents of growing shoots were significantly higher than that of withered shoots. In order to make the hydroponic root mats sustainable, harvesting before the plants wilt is more effective in removing nitrogen from the system permanently and maintain a sustainable system.

Constructed wetland microcosms as sustainable technology for domestic wastewater treatment: an overview

Constructed wetland microcosms (CWMs) are artificially designed ecosystem which utilizes both complex and ordinary interactions between supporting media, macrophytes, and microorganisms to treat almost all types of wastewater. CWMs are considered as green and sustainable techniques which require lower energy input, less operational and maintenance cost and provide critical ecological benefits such as wildlife habitat, aquaculture, groundwater recharge, flood control, recreational uses, and add aesthetic value. They are good alternatives to conventional treatment systems particularly for smaller communities as well as distant and decentralized locations. The pH, dissolved oxygen (DO), and temperature are the key controlling factors while several other parameters such as hydraulic loading rates (HLR), hydraulic retention time (HRT), diversity of macrophytes, supporting media, and water depth are critical to achieving better performance. From the literature survey, it is evaluated that the removal performance of CWMs can be improved significantly through recirculation of effluent and artificial aeration (intermittent). This review paper presents an assessment of CWMs as a sustainable option for treatment of wastewater nutrients, organics, and heavy metals from domestic wastewater. Initially, a concise note on the CWMs and their components are presented, followed by a description of treatment mechanisms, major constituents involved in the treatment process, and overall efficiency. Finally, the effects of ecological factors and challenges for their long-term operations are highlighted.

Adsorption Behavior of Inorganic and Organic Phosphate by Iron Manganese Plaques on Reed Roots in Wetlands

Inorganic and organic phosphate adsorption by iron–manganese (Fe–Mn) plaques extracted from reed roots was investigated. Scanning electron microscopy indicated the roots had rough surfaces and fine particles attached. X-ray photoelectron spectra indicated that Fe and Mn in the Fe–Mn plaques were mainly in the +III and +IV oxidation states, respectively. The contact time, initial phosphate concentration, and temperature effects on inorganic and organic phosphate adsorption were investigated by performing batch tests. Pseudo-second-order model described inorganic and organic phosphate adsorption, indicating the chemisorption was the dominant adsorption process. Langmuir and Freundlich isotherm models were fitted to the equilibrium data, and the Langmuir model fitted best. The maximum inorganic and organic phosphate adsorption capacities at 298 K were 7.69 and 3.66 mg/g, respectively. The inorganic and organic phosphate adsorption processes were spontaneous and exothermic. The inorganic phosphate adsorption capacity was higher than the organic phosphate adsorption capacity, and the presence of organic phosphate did not negatively affect adsorption at inorganic to organic phosphate molar ratios between 1:1 and 3:1. Fourier-transform infrared spectra before and after adsorption showed abundant functional groups on Fe–Mn plaques and that phosphate was probably adsorbed via replacement of hydroxyl groups and inner-sphere surface complexation.

Nitrogen removal and power generation from treated municipal wastewater by its circulated irrigation for resource-saving rice cultivation

This study aims at improving the performance of the cultivating system of rice for animal feed with circulated irrigation of treated municipal wastewater by applying a larger amount of wastewater, as well as adding a microbial fuel cell (MFC) to the system. The results of bench-scale experiments indicate that this modification has increased the rice yield, achieving the target for the rice cultivar used in the experiment. In addition, an assessment of protein content of the harvested rice showed that the value of the rice as animal fodder has improved. Compared with normal one-way irrigation, circulated irrigation significantly enhanced the plant growth and rice production. The direction of the irrigation (bottom-to-top or top-to-bottom) in the soil layer had no significant effect. This modified system demonstrated >96% for nitrogen removal from the treated wastewater used for the irrigation, with approximately 40% of the nitrogen being used for rice plant growth. The MFC installed in the system facilitated power generation comparable with that reported for normal paddy fields. The power generation appeared to be enhanced by bottom-to-top irrigation, which could provide organic-rich treated wastewater directly to the bacterial community living on the anode of the MFC set in the soil layer.

Bacterial communities and enzymatic activities in the vegetation-activated sludge process (V-ASP) and related advantages by comparison with conventional constructed wetland

A new-developed vegetation-activated sludge process (V-ASP) was implemented for decentralized domestic wastewater treatment, and studied in lab-scale and full-scale. The main purpose of this work was the investigation of biomass activities and microbial communities in V-ASP by comparison with conventional constructed wetland (CW), to unveil the causations of its consistently higher pollutants removal efficiencies. Compared with CWs, V-ASP has greater vegetation nitrogen and phosphorus uptake rates, higher biomass and enzymatic activities, and more bacteria community diversity. The microbial community structure was comprehensively analyzed by using high-throughput sequencing. It was observed that Proteobacteria was dominated in both CWs and V-ASPs, while their subdivisions distribution was rather different. V-ASPs contained a higher nitrite-oxidizing bacteria (Nitrospira) abundances that resulted in a consistently better nitrogen removal efficiency. Hence, a long-term experiment of full-scale V-ASP displayed stably excellent capability in resistance of influent loading shocks and seasonal temperature effect.

Preliminary investigation on the effect of earthworm and vegetation for sludge treatment in sludge treatment reed beds system

Sewage sludge treatment is becoming one of the most significant challenges for domestic wastewater management. Optimization of sludge management for reducing sludge handling cost in wastewater treatment plant is highly demanded. Sludge treatment reed bed system (STRB) is an eco-environmentally friendly technology which has a low investment input and reduced costs for operation and maintenance. The objective of this study is to evaluate the effect of earthworm assistant STRB in terms of sludge dewatering and stabilization of surplus sludge. The results show that draining and evapotranspiration (ET) take the main role for sludge dewatering; with maximum of 77 and 43 % water was removed through draining and ET, respectively. Plants improved ET rate up to 13.1 % in the planted STRB compare with the unplanted STRB. The combination of plants and earthworms increased ET rate of 20.9 % more than the control STRB (unplanted without earthworms). The planted STRB with earthworm reached the lowest water content in accumulated sludge of 46 %. There was a systematic increase of total solids (TS) concentration from 0.5 % in the influent to 25-54 % in the accumulated sludge. Earthworms enhanced the sludge stabilization dramatic with the ratio of volatile solids (VS)/TS decreased from 49 % in the influent to 18 % in the accumulated sludge in the earthworm assistant STRB. The results demonstrated a good efficiency for sludge dewatering and stabilization with the assistant of earthworms in STRBs, which can be an alternative technology for sludge treatment in wastewater treatment plants.

Ibuprofen removal in horizontal subsurface flow constructed wetlands: treatment performance and fungal community dynamics

The treatment performance of ibuprofen (IBP)-enriched wastewater by horizontal subsurface flow constructed wetlands planted with cattail (Typha angustifolia) and unplanted control mesocosms was investigated. Removal efficiencies of IBP were significantly (p < .05) enhanced in the planted mesocosms (78.5%) compared to those in the unplanted beds (57.9%). An 18S rRNA gene high-throughput pyrosequencing approach was used to investigate the effects of IBP on the structure of the fungal community in these wetland systems. The overall diversity of the fungal community was reduced under the IBP exposure. Taxonomic analysis revealed that 62.2% of the fungal sequences were affiliated with Basidiomycota, followed by Ascomycota (37.4%) at the phylum level. Uncultured fungus (48.2%), Chaetomium sp. (14.2%), Aspergillus sp. (12.4%), Trichoderma sp. (5.7%), Cladosporium sp. (5.4%), and Emericellopsis sp. (5.2%) were identified as dominant genera. At the genus level, a distinct profile of the fungal community in the IBP-enriched mesocosms was observed as compared to the control beds, and as well specific fungal genera were enhanced in the planted beds, regardless of IBP enrichment. However, despite these differences, the composition of the fungal community (as measured by Bray-Curtis similarity) was mostly unaffected by the significant IBP enrichment. On the other hand, a consistent similarity pattern of fungal community structure in the planted mesocosms suggests that the presence of higher macrophytes in the wetland systems may well help shape the fungal community structure.

Effect of plant harvesting on the performance of constructed wetlands during winter: radial oxygen loss and microbial characteristics

The aboveground tissue of plants is important for providing roots with constant photosynthetic resources. However, the aboveground biomass is usually harvested before winter to maintain the permanent removal of nutrients. In this work, the effects of harvest on plants' involvement in oxygen input as well as in microbial abundance and activity were investigated in detail. Three series of constructed wetlands with integrated plants ("unharvested"), harvested plants ("harvested"), and fully cleared plants ("cleared") were set up. Better performance was found in the unharvested units, with the radial oxygen loss (ROL) rates ranging from 0.05 to 0.59 μmol O₂/h/plant, followed by the harvested units that had relatively lower ROL rates (0.01 to 0.52 μmol O₂/h/plant). The cleared units had the lowest removal efficiency, which had no rhizome resources from the plants. The microbial population and activity were highest in the unharvested units, followed by the harvested and cleared units. Results showed that bacterial abundances and enhanced microbial activity were ten times higher on root surfaces compared with sands. These results indicate that late autumn harvesting of the aboveground biomass exhibited negative effects on plant ROL as well as on the microbial population and activity during the following winter.

Numerical modeling analysis of VOC removal processes in different aerobic vertical flow systems for groundwater remediation

Vertical flow systems filled with porous medium have been shown to efficiently remove volatile organic contaminants (VOCs) from contaminated groundwater. To apply this semi-natural remediation strategy it is however necessary to distinguish between removal due to biodegradation and due to volatile losses to the atmosphere. Especially for (potentially) toxic VOCs, the latter needs to be minimized to limit atmospheric emissions. In this study, numerical simulation was used to investigate quantitatively the removal of volatile organic compounds in two pilot-scale water treatment systems: an unplanted vertical flow filter and a planted one, which could also be called a vertical flow constructed wetland, both used for the treatment of contaminated groundwater. These systems were intermittently loaded with contaminated water containing benzene and MTBE as main VOCs. The highly dynamic but permanently unsaturated conditions in the porous medium facilitated aerobic biodegradation but could lead to volatile emissions of the contaminants. Experimental data from porous material analyses, flow rate measurements, solute tracer and gas tracer test, as well as contaminant concentration measurements at the boundaries of the systems were used to constrain a numerical reactive transport modeling approach. Numerical simulations considered unsaturated water flow, transport of species in the aqueous and the gas phase as well as aerobic degradation processes, which made it possible to quantify the rates of biodegradation and volatile emissions and calculating their contribution to total contaminant removal. A range of degradation rates was determined using experimental results of both systems under two operation modes and validated by field data obtained at different operation modes applied to the filters. For both filters, simulations and experimental data point to high biodegradation rates, if the flow filters have had time to build up their removal capacity. For this case volatile emissions are negligible and total removal can be attributed to biodegradation, only. The simulation study thus supports the use of both of these vertical flow systems for the treatment of groundwater contaminated with VOCs and the use of reactive transport modeling for the assessment of VOCs removal and operation modes in these high performance treatment systems.

Influence of season and plant species on the abundance and diversity of sulfate reducing bacteria and ammonia oxidizing bacteria in constructed wetland microcosms

Constructed wetlands offer an effective means for treatment of wastewater from a variety of sources. An understanding of the microbial ecology controlling nitrogen, carbon and sulfur cycles in constructed wetlands has been identified as the greatest gap for optimizing performance of these promising treatment systems. It is suspected that operational factors such as plant types and hydraulic operation influence the subsurface wetland environment, especially redox, and that the observed variation in effluent quality is due to shifts in the microbial populations and/or their activity. This study investigated the biofilm associated sulfate reducing bacteria and ammonia oxidizing bacteria (using the dsrB and amoA genes, respectively) by examining a variety of surfaces within a model wetland (gravel, thick roots, fine roots, effluent), and the changes in activity (gene abundance) of these functional groups as influenced by plant species and season. Molecular techniques were used including quantitative PCR and denaturing gradient gel electrophoresis (DGGE), both with and without propidium monoazide (PMA) treatment. PMA treatment is a method for excluding from further analysis those cells with compromised membranes. Rigorous statistical analysis showed an interaction between the abundance of these two functional groups with the type of plant and season (p < 0.05). The richness of the sulfate reducing bacterial community, as indicated by DGGE profiles, increased in planted vs. unplanted microcosms. For ammonia oxidizing bacteria, season had the greatest impact on gene abundance and diversity (higher in summer than in winter). Overall, the primary influence of plant presence is believed to be related to root oxygen loss and its effect on rhizosphere redox.

A review on nitrogen and organics removal mechanisms in subsurface flow constructed wetlands: dependency on environmental parameters, operating conditions and supporting media

With the unique advantages of lower operational and maintenance cost, the applications of subsurface flow constructed wetlands for the treatment of wastewater have been increasing rapidly throughout the world. The removal of nitrogen and organics by such systems has gained substantial attention in recent years. In subsurface flow wetlands, the removal of pollutants often relies on a diverse range of co-existing physical, chemical and biological routes, which are vitally dependent on numerous environmental and operational parameters. This paper provides a comprehensive review of wetland structures, classic and novel nitrogen and organics removal mechanisms along with the key environmental parameters and operational conditions that enhance removal in subsurface flow wetland systems. The critical exploration identifies the major environmental parameters such as: pH, DO, and temperature, operational factors i.e. organic carbon availability, loading, feed mode, retention time, recirculation, harvesting, and the complex role (of both parameters) on classical nitrogen and organics removal pathways. Subsequently, the necessity of further extensive research on such factors, for promoting novel nitrogen removal routes in wetland systems has also been highlighted. The expansion of the review on the influence of the unconventional wetland matrix indicates that, the structural differences and inherent properties of these media can support substantial nitrogen and organics removal from wastewater, under optimal operating conditions. Overall, the critical review illustrates the necessity of a profound knowledge on the complicated inter-relationship between nitrogen and organics removal routes, governing environmental and operational parameters, and wetland matrix for improving the treatment performances of subsurface flow wetlands.

Stability and maturity of thickened wastewater sludge treated in pilot-scale sludge treatment wetlands

Thickened wastewater activated sludge was treated in 13 pilot-scale sludge treatment wetlands of various configurations that operated continuously for three years in North Greece. Sludge was loaded for approximately 2.5 years, and the beds were left to rest for the remaining period. Three different sludge loading rates were used that represented three different population equivalents. Residual sludge stability and maturity were monitored for the last year. Sludge was regularly sampled and microbial respiration activity indices were measured via a static respiration assay. The phytotoxicity of sludge was quantified via a seed germination bioassay. Measurements of total solids, organic matter, total coliforms, pH and electrical conductivity were also made. According to microbial respiration activity measurements, the sludge end-product was classified as stable. The germination index of the final product exceeded 100% in most wetland units, while final pH values were approximately 6.5. The presence of plants positively affected the stability and maturity of the residual sludge end-product. Passive aeration did not significantly affect the quality of the residual sludge, while the addition of chromium at high concentrations hindered the sludge decomposition process. Conclusively, sludge treatment wetlands can be successfully used, not only to dewater, but also to stabilize and mature wastewater sludge after approximately a four-month resting phase.