Nitrogen transformation processes and mass balance in deep constructed wetlands treating sewage, exploring the anammox contribution

Influence of Bed Depth on the Development of Tropical Ornamental Plants in Subsurface Flow Treatment Wetlands for Municipal Wastewater Treatment: A Pilot-Scale Case

The aim of this 2-year study was to evaluate the influence of bed depth (40 and 60 cm) on the development of tropical ornamental species (Alpinia purpurata, Heliconia latispatha and Strelitzia reginae) and on the removal of different contaminants such as chemical oxygen demand (COD), nitrate (N-NO3), ammonium (N-NH4), total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), total coliforms (TCs) and fecal coliforms (FCs), in horizontal subsurface flow constructed wetlands (HSSF-CWs) for municipal wastewater treatment. The results showed that the depth of 60 cm favored the removal of COD, with removal efficiencies of 94% for the three plant species. The depth of 40 cm was most effective for the removal of N-NH4 (80-90%). Regarding the removal of TN, the removals were similar for the different plants and depths (72-86%). The systems only achieved up to 60% removal of TCs and FCs. The depth of the CWs substrate and its saturation level influenced the development of ornamental vegetation, particularly flower production. For Heliconia latispatha, a bed depth level of 60 cm was more suitable, while for Alpinia purpurata 40 cm was better, and for Strelitzia reginae in both cases there was no flower production. The impact of bed depth on contaminant removal depends on the specific type of contaminant.

Nitrate and ammonium removal in constructed wetlands: Experimental insights and zero-dimensional numerical modeling

Constructed wetlands (CWs) have emerged as effective wastewater treatment systems, mimicked natural wetland processes but engineered for enhanced pollutant removal efficiency. Ammonium (NH4+) and nitrate (NO3-) are among common pollutants in wastewater, posing significant environmental and health risks. The primary objective of this study is to compares the performance of CWs using gravel and three sizes of natural pumice, along with phragmites australis, in horizontal and horizontal-vertical CWs for nitrate and ammonium removal in the complementary treatment of domestic wastewater. Additionally, the study aims to develop and validate a numerical model using MATLAB software to predict the removal efficiency of these pollutants, thereby contributing to the optimization of CW design and operation. The model operates as a zero-dimensional model based on the law of mass conservation, treating the wetland as a completely mixed reactor, thus avoiding complexities associated with solute movement in porous media. It accurately could predict removal efficiency of chemical, biochemical, and biological indicators while considering active and passive absorption mechanisms by plant uptake. Notably, the determination of coefficients in the model equation does not rely on potentially error-prone laboratory measurements due to sampling issues. Instead, optimization techniques alongside field data robustly estimate these coefficients, ensuring reliability and practicality. Results indicate that higher pollutant concentrations increase reaction rates, particularly enhancing CW efficiency in ammonium removal. Pumice, especially in larger sizes, exhibits superior absorption due to increased porosity and surface area. Overall, the model accurately predicts nitrates concentrations, demonstrating its potential for CW performance optimization and confirming the significance of effective pollutant removal strategies in wastewater treatment.

Economic and environmental benefits of natural treatment systems for sewage treatment: A life cycle perspective

Sewage treatment involves a trade-off of land vs. energy and the location of installing Sewage Treatment Plants (STPs) strongly impacts the decisions regarding treatment technologies. In the wake of rapid urbanization, deteriorating freshwater quality and water scarcity, it is crucial to plan adequate and low-cost sewerage infrastructure that can improve the quality of life in rural and urban areas. The present work involves a novel life cycle analysis through six scenarios generated from a holistic perspective that can aid urban planners and urban local bodies in planning the sewage treatment facilities in their cities, towns or villages. Instead of planning sewerage infrastructure for a long-term period of thirty years, it is suggested to create and operate the STPs only for the upcoming decade. Further, owing to the drawbacks of mechanized and natural treatment systems, adopting a mix of these treatment approaches in planning infrastructure is suggested and the benefits of implementing the same are quantified and discussed. Implementing these strategies results in almost 30 % cost savings and 40 % reduction in greenhouse gas emissions, hence, investing in land for natural treatment systems is suggested instead of incurring heavy electricity bills for mechanized treatment systems. The land cost significantly affects the decision-making regarding treatment technology selection; hence, the variation in the life cycle cost of different sewage treatment approaches is assessed for varying land rates in India.

New insights into the effects of wetland plants on nitrogen removal pathways in constructed wetlands with low C/N ratio wastewater: Contribution of partial denitrification-anammox

Nitrogen (N) removal in constructed wetlands (CWs) was often challenged by limited denitrification due to the lack of carbon source, and wetland plants would be more important in carbon (C) and N cycling in CWs with influent of low carbon to nitrogen (C/N) ratio. In this study, the underlying mechanisms of nitrate nitrogen (NO3--N) removal under different low C/N ratios were revealed by constructing microcosm CWs, and the unplanted group was set as the control to explore the role of plants in N removal. The results showed that plants and the concentration of influent carbon significantly affected NO3--N and total nitrogen (TN) removal (p < 0.05). The presence of plants significantly increased the concentration of DO and wetland plant-derived DOM (p < 0.05). The enhanced NO3--N and TN removal with increased C/N ratio attributed to high denitrification activity reflected in the abundance of denitrification microbes and genes. However, the contribution of partial denitrification-anammox (PDN/AMX) to N removal in CWs decreased from more than 75.3% at the C/N ratio of 0 to 70.4% and 22.3% with the C/N ratio increased to 1.5 and 3, respectively. Furthermore, the PDN/AMX process was negatively correlated with favorable oxygen environment in the planted group and plants roots carbon secretion, but the overall N removal efficiency of the CWs was enhanced by increased abundance of N removal-related functional genes in the presence of plants. Abovementioned results provided new insights to explain the mechanism of N removal in CWs under low C/N ratio.

Microbe-mediated simultaneous nitrogen reduction and sulfamethoxazole/N-acetylsulfamethoxazole removal in lab-scale constructed wetlands

Constructed wetlands (CWs) are increasingly used to treat complex pollution such as nitrogen and emerging organic micropollutants from anthropogenic sources. In this study, the denitrification, anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium, and nitrous oxide release rates following exposure to the frequently detected sulfonamides sulfamethoxazole (SMX) and its human metabolite, N-acetylsulfamethoxazole (N-SMX), were investigated in lab-scale CWs. Over a period of 190 d, the denitrification rates were noticeably inhibited in the SMX and N-SMX groups at week 5. Subsequently, the denitrification rates recovered, accompanied by an increase in the relevant nitrogen reduction and antibiotic resistance genes (ARGs). The composition of the microbial community also changed during this process. After the denitrification rates recovered, Burkholderia_Paraburkholderia and Gordonia exhibited a significant positive correlation with SMX exposure, which simultaneously reduced nitrate concentrations and degraded antibiotics. Burkholderia_Paraburkholderia is a key carrier of ARGs. Finally, nitrogen reduction (> 90%) and antibiotic removal (> 80%) also recovered in both SMX- and N-SMX-exposed lab-scale CWs during the operation, which revealed the interaction of SMX or N-SMX removal and nitrogen reduction.

Mainstream deammonification at ambient temperature treating real sewage by a plug-flow fixed-bed reactor based on zeolite/tourmaline-modified polyurethane carriers

A plug-flow fixed-bed reactor (PFBR) with zeolite/tourmaline-modified polyurethane (ZTP) carriers (PFBR_ZTP) was constructed to realize mainstream deammonification for real domestic sewage treatment. The PFBR_ZTP and PFBR were operated for 111 days treating aerobically pretreated sewage in parallel. A higher nitrogen removal rate of 0.12 kg N·(m³·d)^-1 was achieved in PFBR_ZTP despite lowering the temperature (16.8-19.7 ℃) and fluctuating water quality. Meanwhile, it was indicated that anaerobic ammonium oxidation dominated (64.0 ±13.2%) in PFBR_ZTP, by nitrogen removal pathway analysis and high anaerobic ammonium-oxidizing bacteria (AnAOB) activity (2.89 mg N·(g VSS·h)^-1). And, the lower protein/polysaccharides (PS) ratio further indicated a better biofilm structure in PFBR_ZTP owing to a higher abundance of microorganisms relevant to PS and cryoprotective EPS secretion. Furthermore, partial denitrification was an important nitrite supply process in PFBR_ZTP based on low AOB activity/AnAOB activity ratio, higher Thauera abundance and a remarkably positive correlation between Thauera abundance and AnAOB activity.

Optimization of depth of filler media in horizontal flow constructed wetlands for maximizing removal rate coefficients of targeted pollutant(s)

Varying the depth of HFCW media causes differences in the redox status within the system, and hence the community structure and diversity of bacteria, affecting removal rates of different pollutants. The key functional microorganisms of CWs that remove contaminants belong to the phyla Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes. Secondary data of 111 HFCWs (1232 datasets) were analyzed to deduce the relationship between volumetric removal rate coefficients (K_BOD, K_TN, K_TKN, and K_TP) and depth. Equations of depth were derived in terms of rate coefficients using machine learning approach (MLR and SVR) (R² = 0.85, 0.87 respectively). These equations were then used to find the optimum depth for pollutant(s) removal using Grey wolf optimization (GWO). The computed optimum depths were 1.48, 1.71, 1.91, 2.09, and 2.14 m for the removal of BOD, TKN, TN, TP, and combined nutrients, respectively, which were validated through primary data. This study would be helpful for optimal design of HFCWs.

Assessment of removal rate coefficient in vertical flow constructed wetland employing machine learning for low organic loaded systems

Secondary datasets of 42 low organic loading Vertical flow constructed wetlands (LOLVFCWs) were assessed to optimize their area requirements for N and P (nutrients) removal. Significant variations in removal rate coefficients (k₂₀) (0.002-0.464 md^-1) indicated scope for optimization. Data classification based on nitrogen loading rate, temperature and depth could reduce the relative standard deviations of the k₂₀ values only in some cases. As an alternative method of deriving k₂₀ values, the effluent concentrations of the targeted pollutants were predicted using two machine learning approaches, MLR and SVR. The latter was found to perform better (R² = 0.87-0.9; RMSE = 0.08-3.64) as validated using primary data of a lab-scale VFCW. The generated model equations for predicting effluent parameters and computing corresponding k₂₀ values can assist in a customized design for nutrient removal employing minimal surface area for such systems for attaining the desired standards.

Immobilization of chromium enhanced by arbuscular mycorrhizal fungi in semi-aquatic habitats with biochar addition

Arbuscular mycorrhizal fungi (AMF) exhibit great potential in heavy-metal immobilization in semi-aquatic habitats. Under high heavy-metal stress, however, the role of AMF is limited, and the detoxification mechanism of AMF in heavy metals' stabilization remains unclear. This study investigated the effects of AMF on a wetland plant (Iris pseudacorus) and chromium (Cr) immobilization at different water depths in semi-aquatic habitats with biochar addition. Results showed that AMF increased the physiological and photosynthetic functions in I. pseudacorus under Cr exposures. Besides, AMF alleviated the accumulation of reactive oxygen species and lipid peroxidation by enhancing the antioxidant enzyme activities. AMF and biochar significantly decreased Cr concentrations in outlet water and increased Cr accumulation in I. pseudacorus. Besides, biochar also vastly improved Cr accumulation in the substrate under the fluctuating water depth. AMF reduced Cr bioavailability in the substrate, with Cr (Ⅵ) concentrations and acid-soluble forms of Cr decreased by 0.3-64.5% and 19.0-40.8%, respectively. Micro-proton-induced X-ray emission was used to determine element localization and revealed that AMF improved the nutrients uptake by wetland plants and inhibited Cr translocation from roots to shoots. Overall, this study demonstrated that the interaction between AMF and biochar could significantly enhance the immobilization of high Cr concentrations in semi-aquatic habitats.

Performance of tidal and non-tidal mangrove constructed wetlands in treating maricultural wastewater

This study aims to evaluate the removal efficiency of nitrogen and phosphorus in the tidal and non-tidal constructed wetlands with typical mangrove (Aegiceras corniculatum) as a wetland plant model to treat simulated marine wastewater. The results showed that the average removals of NO₂^--N, NO₃^--N, NH₄⁺-N, TN and TP were 88.4, 80.5, 81.4, 79.7 and 40.8%, respectively, in the non-tidal subsurface flow (HF) mangrove wetland, and 65.3, 61.3, 90.6, 60.1 and 19.2% in the tidal (TF) mangrove wetland, and 11.4, 64.6, 68.7, 56.6 and 16.3% in the non-tidal free water surface (FWS) mangrove wetland, respectively. Moreover, it was observed that the composition of microbial communities in the HF mangrove wetland was beneficial to the nitrogen cycle and has more quantitative associations of N-metabolism genes. The results indicated that non-tidal HF mangrove wetland has a stable and an effective capacity for potential treatment of marine wastewater compared with the non-tidal FWS mangrove wetland and tidal TF mangrove wetland.

Designing the vertical flow constructed wetland based on targeted limiting pollutant

The requirement of large land area limits the adoption of constructed wetlands (CWs) in urban settings with limited land availability. The area calculations for CW design are commonly carried out following Kikuth approach where the removal rate constant (K) is derived from literature. Investigation of secondary data of 82 vertical flow CWs, performed in this study, yielded wide variations (0.0003 - 0.822 md^-1) in the calculated K values for different pollutants under different environmental and operational conditions indicating that it is important to incorporate the desired levels of pollutant removal to arrive at customized design of CWs. The results indicated that the relative standard deviation of K values could be narrowed by classifying the datasets based on design parameters like depth, hydraulic loading rates and substrate loading rates. These calculations can help arrive at more scientific design of CW to achieve the prevailing standards for the discharge or reuse of sewage.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes

Horizontal levees are a nature-based approach for removing nitrogen from municipal wastewater effluent while simultaneously providing additional benefits, such as flood control. To assess nitrogen removal mechanisms and the efficacy of a horizontal levee, we monitored an experimental system receiving nitrified municipal wastewater effluent for 2 years. Based on mass balances and microbial gene abundance data, we determined that much of the applied nitrogen was most likely removed by heterotrophic denitrifiers that consumed labile organic carbon from decaying plants and added wood chips. Fe(III) and sulfate reduction driven by decay of labile organic carbon also produced Fe(II) sulfide minerals. During winter months, when heterotrophic activity was lower, strong correlations between sulfate release and nitrogen removal suggested that autotrophic denitrifiers oxidized Fe(II) sulfides using nitrate as an electron acceptor. These trends were seasonal, with Fe(II) sulfide minerals formed during summer fueling denitrification during the subsequent winter. Overall, around 30% of gaseous nitrogen losses in the winter were attributable to autotrophic denitrifiers. To predict long-term nitrogen removal, we developed an electron-transfer model that accounted for the production and consumption of electron donors. The model indicated that the labile organic carbon released from wood chips may be capable of supporting nitrogen removal from wastewater effluent for several decades with sulfide minerals, decaying vegetation, and root exudates likely sustaining nitrogen removal over a longer timescale.

Quantifying the contribution rates of sulfonamide antibiotics removal mechanisms in constructed wetlands using multivariate statistical analysis

The removal of antibiotics in subsurface flow constructed wetlands is performed through various removal mechanisms, such as adsorption, hydrolysis, microbial degradation and plant uptake. However, the contribution rates of the removal mechanisms in constructed wetlands are still not well studied. This study conducted a series of experiments and used multivariate statistical analysis to determine contribution rates for substrate adsorption, hydrolysis, and microbial degradation. Multiple stepwise regression analysis indicated that specific surface area and salt content were the main factors influencing sulfonamide adsorption, while temperature and pH were the main factors influencing sulfonamide hydrolysis. Variance partitioning analysis showed that the influence of physical-chemical factors was greater than that of nutrients on the microbial community. Partial least squares path analysis showed that the path coefficients of microbial degradation, adsorption and hydrolysis for sulfonamides removal in vertical subsurface flow constructed wetlands were 0.6339, 0.3608 and 0.0351, respectively, while the corresponding path coefficient were 0.5658, 0.4707 and 0.1079 in horizontal subsurface flow constructed wetlands, respectively. This means that microbial degradation contributes the most to the removal of sulfonamides in subsurface flow constructed wetlands. Enhanced microbial degradation may be a powerful measure to improve the removal of sulfonamides. These results will be helpful for understanding the removal mechanism of antibiotics and will provide a definite direction for pertinently improving sulfonamide removal efficiency in constructed wetlands.

Bioelectrochemical systems for environmental remediation of estrogens: A review and way forward

Globally estrogenic pollutants are a cause of concern in wastewaters and water bodies because of their high endocrine disrupting activity leading to extremely negative impacts on humans and other organisms even at very low environmental concentrations. Bioremediation of estrogens has been studied extensively and one technology that has emerged with its promising capabilities is Bioelectrochemical Systems (BESs). Several studies in the past have investigated BESs applications for treatment of wastewaters containing toxic recalcitrant pollutants with a primary focus on improvement of performance of these systems for their deployment in real field applications. But the information is scattered and further the improvements are difficult to achieve for standalone BESs. This review critically examines the various existing treatment technologies for the effective estrogen degradation. The major focus of this paper is on the technological advancements for scaling up of these BESs for the real field applications along with their integration with the existing and conventional wastewater treatment systems. A detailed discussion on few selected microbial species having the unusual properties of heterotrophic nitrification and extraordinary stress response ability to toxic compounds and their degradation has been highlighted. Based on the in-depth study and analysis of BESs, microbes and possible benefits of various treatment methods for estrogen removal, we have proposed a sustainable Hybrid BES-centered treatment system for this purpose as a choice for wastewater treatment. We have also identified three pipeline tasks that reflect the vital parts of the life cycle of drugs and integrated treatment unit, as a way forward to foster bioeconomy along with an approach for sustainable wastewater treatment.

Microorganisms that participate in biochemical cycles in wetlands

Several biochemical cycles are performed in natural wetlands (NWs) and constructed wetlands (CWs). The knowledge of the microorganisms could be used to monitor the restoration of wetlands or the performance of the wastewater treatment. Regarding bacteria, Proteobacteria phylum is the most abundant in NWs and CWs, which possesses a role in N, P, and S cycles, and in the degradation of organic matter. Other phyla are present in lower abundance. Archaea participate in methanogenesis, methane oxidation, and the methanogenic N2 fixation. Sulfur and phosphorus cycles are also performed by other microorganisms, such as Chloroflexi or Nitrospirae phyla. In general, there is more information about the N cycle, especially nitrification and denitrification. Processes where archaea participate (e.g. methane oxidation, methanogenic N2 fixation) are still unclear their metabolic role and several of these microorganisms have not been isolated so far. The study can use 16S rDNA genes or functional genes. The use of functional genes gives information to monitor specific microbial populations and 16S rDNA is more suitable to perform the taxonomic classification. Also, there are several Candidatus microorganisms, which have not been isolated so far. However, it has been described their metabolic role in the biochemical cycles in wetlands.

Novel microbial nitrogen transformation processes in constructed wetlands treating municipal sewage: a mini-review

Traditionally nitrogen transformation in constructed wetlands (CWs) has been attributed to the activities of aerobic autotrophic nitrifiers followed by anoxic heterotrophic denitrifiers. However, the nitrogen balances in such systems are far from being explained as a large fraction of the losses remain unaccounted for. The classical nitrification-denitrification theory has been successfully employed in certain unit processes by culturing fast-growing bacteria, but the CWs offer an ideal environment for slow-growing bacteria that may be beneficially exploited to achieve enhanced nitrogen removal by manipulating the environmental conditions in their favor. In the last three decades, many novel microorganisms have been isolated from CWs that have led to the discovery of some other routes that have made researchers believe could play a significant role in nitrogen transformation processes. The increased understanding of novel discerned pathways like anaerobic ammonium oxidation (ANAMMOX), heterotrophic nitrification and aerobic denitrification, which are mediated by specialized bacteria has indicated that these microorganisms could be enriched by applying selection pressures within CWs for achieving high rates of nitrogen removal. Understanding these novel nitrogen transformation processes along with the associated microbial population can provide new dimensions to the design of CWs for enhanced nitrogen removal.

Achieving stable mainstream deammonification process by a novel combinatorial control strategy

In this study, a novel combinatorial control strategy was developed to guarantee a stable mainstream deammonification process, with three critical steps including (a) upflow airwater washing, (b) short-term increased nitrogen loading rate (NLR), and (c) low oxygen supply. Results showed that two upflow double-blanket filter (UDBF) reactors effectively performed the mainstream deammonification process with the nitrogen removal efficiency (NRE) 84.5 ± 2.2% and 84.6 ± 1.6%, respectively and nitrogen removal rate (NRR) 123.8 ± 2.9 and 125.5 ± 6.2 g N·(m³·d)^-1, respectively. Statistically, temperature and C/N were considered as two vital factors affecting the nitrogen removal pathways, which co-explained 80.9% and 78.4% of the maximum possible contribution of heterotrophic denitrification in both reactors. The deammonification process accounted for more than 59.8% of TN removal in R2 and 54.8% in R1, which cooperated well with heterotrophic denitrification for efficient performance in treating municipal sewage.