Kinetic modelling of nitrogen and organics removal in vertical and horizontal flow wetlands

Analysis of ammonia-nitrogen removal kinetics by stage in pilot scale vertical flow wetlands treating landfill leachate in series

This study explored the effect stage number and plant type have on ammonia-nitrogen (NH3-N) removal kinetics in a two-stage pilot-scale vertical flow constructed wetland (VFCW) system treating landfill leachate. Half of the VFCW columns were planted with Typha latifolia and the other half Scirpus californicus, and half of the columns were loaded with municipal solid waste landfill leachate (diluted to 1 part leachate to 2 parts total) with the effluent from these columns was collected in two separate barrels. The remaining columns were loaded with the effluent collected from the first columns, creating a two-stage VFCW system with four unique pairs to be tested. The leachate used here experienced no prior pre-treatment, and average influent concentrations of NH3-N for the first- and second-stage VFCWs were 508 and 321 mg L-1, respectively- much higher than many other VFCW treatment systems. Some reduction in chemical oxygen demand was observed, as well as generation of nitrate and nitrite, evidence of nitrification. No apparent correlation between aboveground biomass and removal of NH3-N was observed. Overall removal efficiency of NH3-N through two stages of VFCWs was 53.7% for columns planted with T. latifolia and 58.3% for those planted with S. Californicus. Average NH3-N removal efficiencies for the first stage VFCWs were 32.7% and 34.3%, while those in the second stage were 31.3% and 36.5%; no significant difference was observed between the first and second stage, suggesting that stage number does not have a significant effect on the removal efficiency of NH3-N in the primary treatment of landfill leachate via VFCWs. However, average mass removal rates of NH3-N in the first stage were 166 and 175 mg L-1 d-1; the second stage was significantly lower at 99.4 and 112 mg L-1 d-1, indicating that the first stage removed more pollutants overall.

Estimating ammonium changes in pilot and full-scale constructed wetlands using kinetic model, linear regression, and machine learning

Constructed wetlands (CWs) are a widely utilized nature-based wastewater treatment method for various effluents. However, their application has been more focused on pilot and full-scale CWs with substantial surface areas and extended operation times, which hold greater relevance in practical scenarios. This study used kinetics, linear regression (LR), and machine learning (ML) models to estimate effluent ammonium in pilot and full-scale CWs. From screening 1476 papers, 24 pilot and full-scale CW studies were selected to extract data containing 15 features and 975 data points. Nine models were fit to this data, revealing that linear models were less effective in capturing CW effluent compared to nonlinear ML algorithms. For training data, the Monod kinetic model predicted the poorest performance with an RMSE of 41.84 mg/L and R2 of 0.34, followed by simple LR (RMSE 24.29 mg/L and R2 0.77) and multiple LR (RMSE 22.63 mg/L and R2 0.80). In contrast, Cubist and Random Forest achieved high performances, with an average RMSE of 12.01 ± 5.38 and an average R2 of 0.93 ± 0.07 for Cubist, and an average RMSE of 15.94 ± 10.69 and an average R2 of 0.91 ± 0.08 for RF. The trained Random Forest performed the best for new data, with an R2 of 0.93 and RMSE of 13.48 mg/L. This ML-based model is a valuable tool for efficiently estimating effluent ammonium concentration in pilot and full-scale CWs, thereby facilitating the design of systems.

Robust control of water quality in horizontal sub-surface flow wetlands using decentralized quantitative feedback theory based controller

This paper addresses the regulation of water quality influencers such as Organic Nitrogen, Ammonia Nitrogen, Nitrate Nitrogen, Dissolved Organic Carbon and Dissolved Oxygen for a sub-surface flow horizontal wetland in a controlled environment. The plant uncertainty is quantified by using measurement data. The robust control of the process is achieved using a decentralized quantitative feedback theory based control strategy. The control structure also employs a feed-forward mechanism to minimize loop interactions and feedback control to ensure stability, tracking and disturbance rejection. The numerical simulations and hardware-in-loop implementation show that the performance specifications are robustly met.

Outflow risks of antibiotic-resistant bacteria in stormwater bioretention cells: understanding roles of adsorption and transmission

In this study, lab-scale bioretention cells were designed for the investigation of antibiotic-resistant bacteria (ARB) outflow profiles at different depths, effects of adsorption and transmission, as well as modelling evaluation of ARB outflow risks using the common decay models (e.g., first-order decay models). ARB outflow was first found in the upper layers (after 100 days of the operation) with the lowest transmission frequencies of antibiotic resistance. Although the adsorption of ARB onto the substrate and its surface biofilms was effective with the maximum amount of ARB adsorbed (Qmax) reaching 108 CFU/g of the substrate and 107 CFU/g of biofilms, ARB outflow was detected in the bottom outlets after over 4 months of operation, reflecting that there was still a risk of antibiotic resistance through the treatment of bioretention cells. ARB outflow for both upper and middle outlets could be well described by third-order polynomial equations with correlation coefficients 0.9067 (p = 0.0002) and 0.9780 (p < 0.0001), respectively, where there were both positive and negative relationships between outflow ARB and inflow ARB, confirming the combined action of mechanisms blocking ARB outflow (e.g., substrate adsorption) and promoting ARB outflow (like transmission). These suggested two potential controlling approaches for ARB outflow from stormwater bioretention cells.

Experimental checking and modeling of the influence of operation conditions on the first order kinetic constants in free water surface wetlands

The most commonly used model in constructed wetlands is the first-order removal model, and first order kinetic constants (k) are the key parameters. The presumption is often made that k are constants. However, it is possible that k are functions of operating conditions, but the influence of operation conditions on k is unclear. In this study, response surface methodology was used to explore the variation patterns of k_a (area rate constants) and k_V (volume rate constants) for the removal of total nitrogen (TN) and total phosphorus (TP) in free water surface (FWS) wetlands. The experimental variables included hydraulic loading rate (HLR), water depth, and inlet concentration (C_in). The results showed that k_V was more variable than k_a, and the area-based first-order model is more suitable for simulating TN and TP in FWS wetlands. Inlet concentration (C_in) was significant for k_a; C_in and water depth were significant for k_V; HLR and the interaction between factors were insignificant. The effects of C_in on k_a and k_V can be described by an upward convex quadratic curve, while the effect of water depth on k_V demonstrates a downward convex quadratic curve. The first-order area rate constant for TN removal was given by k = -47.66 + 22.01 C_in - 1.154 C_in²; the first-order area rate constant for TP removal was given by k = -27.75 + 95.88 C_in - 30.73 C_in². Based on the variation patterns, the traditional k-C model was modified to the k^ψ-C model. The k^ψ-C model produced the best results at simulating the outlet concentration and removal efficiency (RE).

Effectiveness of the tropical plants Rhynchospora corymbosa and Coix lacryma-jobi in vertical flow constructed wetlands for municipal primary sewage effluent treatment

The performance of two tropical plants, Rhynchospora corymbosa L. (RC) and Coix lacryma-jobi, L (CL) in treatment of primary sewage effluent in lab-scale vertical-flow constructed wetlands (VFCW) along with no plant control wetland was investigated. A batch-flow VFCWs were operated under batch fill and drain hydraulic loading system with hydraulic retention times (HRT) of 0.5, 1, and 2 days and fill rate of 8 L/day. Removal of solids, organics, nutrients, and pathogens were monitored. The volumetric contaminant removal rates were best described by 1st order kinetics except for ammonia and phosphate, which was best described by Stover-Kincannon kinetics. Influent TSS, PO43-, COD, BOD5, and total coliform concentration were low but high in NH4+ concentration. CL was better in nutrient removal as HRT increases compared to RC. RC was more efficient at TSS, turbidity, and organics removal. Pathogen removal was independent of plant type but HRT. Solids and organic removal were lower in CL planted CWs due to preferential flow paths created by their bulky root. CL planted CWs removed more nutrients followed by RC planted CWs and then no-plant control CWs. The results of these tests demonstrate that both CL and RC are suitable for the treatment of municipal wastewater in VFCW system.

Performance of wetland applied to ecological remediation of abandoned fish ponds: A case study in Gonghu Bay, Tai lake

To restore the abandoned fish ponds to "near natural" state, the wetland restoration was carried out in Gonghu Bay lakeside, and its long-term performance of controlling external load was studied for 5 years. The findings showed that water quality and biodiversity had been improved dramatically after the preliminary transformation. The concentrations of permanganate index (COD_Mn), total nitrogen (TN), and total phosphorus (TP) obviously decreased from 12.91 mg L^-1 to 4.32 mg L^-1, from 3.46 mg L^-1 to 1.42 mg L^-1, and from 0.27 mg L^-1 to 0.04 mg L^-1, respectively. The proportion of Cyanophyta was effectively reduced from 31.82% to 18.89%, and favored the growth of diatoms (31.82%-37.78%) to be the dominant algae species. Aquatic plant species and coverage gradually increased from 16 to 56 and from 5% to 60%, respectively. An in-deep monitoring done for 5 years (2013-2017) showed that the wetland achieved a satisfactory removal efficiency of 58.95% for TN, 64.60% for TP, and up to 77.83% for chlorophyll-a. Besides, three pollution scenarios, such as stormwater runoff, algal bloom, and continuous water transfer, were selected to explore the tolerance of the wetland to the suddenly increased pollution loads. The results dedicated that even if the inlet load was up to 1.0 × 10⁵ m³ d^-1, the removal rate coefficients of wetland for chlorophyll-a, TP, and TN were 0.135-0.239 d^-1, 0.041-0.112 d^-1, and 0.030-0.109 d^-1, respectively, which were equivalent to the well-running wetlands. This study confirmed that the wetland was not only a promising ecological remediation technique to contaminated abandoned fish ponds, but also could withstand high pollution load, which had the prospect of sustainable utilization.

Enhanced performance and microbial interactions of shallow wetland bed coupling with functional biocathode microbial electrochemical system (MES)

It is essential to remediate the polluted urban river, which endangers the aquatic creatures and affected human body's senses. The treatment wetland combined with microbial electrochemical system (MES) used for the remediation is becoming a new research focus due to its ideal pollutants removal efficiency and small footprint. Here this paper provided a kind of novel shallow wetland bed coupling with close-circuit microbial electrochemical system (WB-CMES) to remove pollutants in surface water. In contrast to the shallow wetland bed coupling with open-circuit MES (WB-OMES) and the shallow wetland bed without MES (WB), the enhancing effects and pollutants removal pathway were evaluated. After 62-day operation, average TN removal efficiency in WB-CMES was 87.7%, which was 19.7% and 13.8% higher than that of WB-OMES and WB respectively. The rate coefficient k of NO₃^--N degradation in WB-CMES was 1.6 and 1.8 times higher than that in WB-OMES and WB. The results of chlorophyll, protein and superoxide dismutase (SOD) in WB-CMES were 27.3%, 44.3% and 12.9% higher than those in WB. The microbial community structure analysis indicated that electroactive bacteria on anode like Desulfobulbus could oxidize organics and generate electrons to compensate cathode, meanwhile, cathode could enrich more species of functional bacteria like Rhodobacter, Pirellula, Hyphomicrobium, Thauera, which had a synergistic effect on oxygen reduction, nitrogen removal and plant growth. The results indicated that oxygen produced by submerged plants could be utilized by the oxygen-reducing functional biocathode of MES and the proper aerobic and anoxic environment might enhance nitrate removal mainly through simultaneous nitrification and denitrification (SND), aerobic denitrification and anammox. This research provided a novel technology with advantages of simple operation, flexible configuration, easy scale-up and low cost for application in remediation of highly polluted surface water.

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.

Assessment of the effectiveness of a field-scale combined ecological treatment system at removing water pollutants, after optimization using a system dynamic model: a case study of rural inland ponds in China

Field-scale combined ecological treatment systems (FCETS) are designed to remove nutrients from aquaculture wastewater in ponds according to the characteristics of the nutrients present. We designed and established a numerical model based on the system dynamic (SD) method, to optimize the parameters of FCETS. Results showed that the mean removal rates of TSS, TN, NO₃^--N, NH₄⁺-N, TP, DP, and COD_Mn ranged from 83.3 to 125.8%, 41.1 to 49.1%, 44.8 to 56.2%, 49.3 to 55.6%, 80.0 to 88.2%, 52.6 to 65.0%, and 52.0 to 61.5%, respectively. The SD model provided satisfactory estimates of water quality at the outlet throughout both the validation and calibration periods. In addition, we conducted a sensitivity analysis to determine the key parameters of the SD model. This also involved optimization of the N and P removal capacity of FCETS, and their corresponding discharge (Q), and concentration (C) at the inlet. This made it possible to use R and MATLAB to simulate seasonal differences in the removal of N and P. Our results indicate that a FCETS can be used to efficiently remove nutrients from rural wastewater in ponds. In addition, we demonstrated that the SD-based numerical model is a useful management support tool to ensure that decisions are made which result in the stable operation of a FCETS. This illustrates that contamination-free aquaculture from rural inland ponds is a feasible goal.

Wastewater Treatment Using Constructed Wetland: Current Trends and Future Potential

Constructed wetlands (CW) is an environmentally friendly technique for removing pollutants from wastewater and has been applied to municipal wastewater, petroleum refinery wastewater, agriculture drainage, acid mine drainage, etc. The past decade has seen a remarkable number of innovations in the exponentially growing field of microbiology. This manuscript covers a critical review of key aspects of CW, such as various types of CW, the contaminants and their removal mechanisms, degradation pathways, challenges and opportunities, materials, applications, and theory with a focus on recent advances in the last three decades. In addition, an attempt has been taken to project future advances in the field of CW and facilitate these advances by framing key unsolved problems in CW. Guidelines are prepared for the fast-growing CW field through the standardization of key design aspects. This review covers the evaluation of the current state-of-the-art of CW technology and provides definitions and performance metric nomenclature in an effort to unify the fast-growing CW community. It also contains an outlook on the emerging trends in CW and proposes future research and development directions.

Modeling the impacts of plants and internal organic carbon on remediation performance in the integrated vertical flow constructed wetland

The integrated vertical flow (IVF) constructed wetland consists of two or more chambers with heterogeneous flow patterns and strong aeration capability, possesses favorable remediation performance. The Constructed Wetland Model No.1 (CWM1) embedded in the OpenGeoSys # IPHREEQC was applied to investigate the wetland plant effects on treatment efficiency. Two fundamental functions of the plant roots (i) the radial oxygen loss (ROL) and (ii) exudation of internal organic carbon (IOC), are developed and implemented in the model to simulate the treating processes of planted laboratory-scale IVF wetlands fed by the synthetic wastewater. The good agreement between simulated results and measurements of the planted IVF wetland and the unplanted filters mimicking wetland demonstrates the combined effects of ROL and IOC and the model reliability. In summer the ammonia (NH₄-N) and total nitrogen (TN) removals are high as above 90% in both IVF wetlands, and in winter they decline significantly to around 55% and 45% in unplanted wetland, contrastively to about 85% and 78% in the planted wetland. The nitrogen removal - COD/N ratio relation curves of IVF wetlands are proposed and obtained by modeling to evaluate organic carbon loading status. Based on the curves, the COD/N ratios of unplanted and planted wetlands are about 3∼7 and 3∼10 gCOD/gN for high TN removal respectively. Planted wetlands can tolerate a wider range of COD/N ratio influents than unplanted ones. The ROL in the unplanted wetland promotes COD and NH₄-N removal, while may inhibit denitrification under low-temperature conditions. The single addition of IOC enhances the oxygen-consuming and restrains the nitrification under the full loaded COD condition. Summing up all organic carbon releases from substrate and roots as IOC, the quantification of IOC acts on nitrogen treatment was simulated and compared with the external organic carbon (EOC) loading from influent. IOC performs higher efficiency on TN removal than EOC at the same organic loading rates. The results provide the thoughts of the solution for low TN removal in the carbon deficient constructed wetlands.

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.

Improved solute transport and pollutant degradation model of free water surface constructed wetlands considering significant linear correlation between model parameters

To unify the structures of solute transport and pollutant degradation models and evaluate the wetland performance conveniently, a pollutant degradation model combining first-order kinetics with the hybrid solute transport model (plug flow with dispersion + continuous stirred-tank reactor, PFD + CSTR) was developed. Orthogonal tests revealed significant correlation between the model parameters, and the original models were optimized via linear substitution of parameters. The improved PFD + CSTR solute transport model exhibited a satisfactory fit with the original model, and the average relative errors of the determination coefficient (R²) and correlation coefficient were <5%. The multiple linear regressions between the hydraulic indicators and model parameters were reconstructed and exhibited consistent structures between different stages. The degradation constant k_aTN between the original and improved models exhibited high consistency (R² = 0.982). Conclusively, the improved models exhibited good consistency with the original models and allowed rapid and accurate performance evaluation.

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

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

Robust nitrate removal and bioenergy generation with elucidating functional microorganisms under carbon constraint in a novel multianode tidal constructed wetland coupled with microbial fuel cell

This study investigated synthetic wastewater treatment under low inflow C/N ratio and characterized NO₃^--N-transforming and electricity-producing bacteria in a multi-anode tidal constructed wetland-microbial fuel cell (TFCW-MFC). The optimal concurrent average removal rates of NH₄⁺-N and NO₃^--N were 73% and 78%, respectively, under a flood/rest/flood time of 4 h/2h/4h in "tide" mode accompanied by one recirculation. The lowest NO₃^--N concentration among all anodes was observed when the electrode gap was 45 cm. Similarly, the 45 cm anode exhibited selective enrichment of Variovorax and Azoarcus. Correction analysis showed that the high relative abundance of Azoarcus was crucial in enhancing NO₃^--N removal, and the internal resistance significantly decreased as the relative abundance of Acidovorax increased. These results suggest that NO₃^--N removal and bioelectricity generation can be promoted in a TFCW-MFC with limited carbon by improving the culture conditions for specific genera.

Polishing low-biodegradable and saline industrial effluent in a full-scale horizontal subsurface flow constructed wetland: evaluation of bio-treatability and predictive power of kinetic models

This study evaluates the bio-treatability performance and kinetic models of full-scale horizontal subsurface flow constructed wetland used for the tertiary treatment of composite industrial effluent characterized by high-salt content ranging from 5830 to 10,400 µS/cm and biochemical oxygen demand (BOD₅): chemical oxygen demand (COD) ratio below 0.2. The wetland vegetated with Phragmites australis was operated in a semi-arid climate under an average hydraulic loading rate of 63 mm/d. The results of a 4-year operation calculated based on the concentration of pollutants showed that the average removal efficiency of COD, BOD_5, and total suspended solids (TSS) were 17.5, 5.1, and 11.2%, respectively. The system reduced up to 6.5 ± 0.7% of electrical conductivity presenting poor phyto-desalination potential without considering the contribution of evapotranspiration in water balance in contrast to satisfying performance for heavy metals reduction. The comparison of the kinetics of organic matter removal obtained by the first-order and Monod models paired with continuous stirred-tank reactor and plug flow regime showed that Monod-plug flow model provided the best fit with the constants of 2.01 g COD/m²·d and 0.3014 g BOD₅/m²·d with the best correlation coefficient of 0.610 and 0.968 between the predicted and measured concentrations, respectively. The low kinetic rates indicate that the process is capable of effluent polishing instead of purification due to the presence of organic compounds recalcitrant to biodegradation and a high level of salinity.

Biotreatment of petroleum refinery wastewater in vertical surface-flow constructed wetland vegetated with Eichhornia crassipes: lab-scale experimental and kinetic modelling

The objectives of this study were to evaluate the potential of vertical surface flow constructed wetland (VSF-CW) vegetated with Eichhornia crassipes to treat petroleum refinery secondary wastewater under tropical conditions. Also, to provide a comparative evaluation of biotreatment kinetic models (traditional first order and other alternative kinetic models) proposed to describe the removal kinetics of organics (biochemical oxygen demand (BOD) and chemical oxygen demand (COD)), nitrate-nitrogen and total petroleum hydrocarbons (TPH) in wetland systems. The refinery secondary wastewater was characterized and treated in five VSF-CWs. Eichhornia crassipes were planted in three VSF-CWs and the remaining two VSF-CWs served as the unvegetated control. The wastewater relatively had high levels of turbidity (18.30 ± 3.88 NTU), BOD (20.40 ± 2.20 mg/L), COD (86 ± 6.0 mg/L), TPH (16.6 ± 1.76 mg/L), oil and grease (18.4 ± 2.00 mg/L), heavy metals (Cadmium (0.034 ± 0.01 mg/L), Lead (0.12 ± 0.05 mg/L), Chromium (0.47 ± 0.01 mg/L), Iron (1.54 ± 0.25 mg/L) and Nickel (0.09 ± 0.01 mg/L)) and Chloride (1412 ± 9.6 mg/L). The vegetated VSF-CWs significantly performed better than the unvegetated control and resulted in the removal efficiencies of 91.5% turbidity, 94.6% BOD₅, 80.2% COD, 92.6% TPH, 90.4% oil and grease, 94% cadmium, 92.5% lead, 93% chromium, 94.8% iron, 92.2% nickel, and 57.7% chloride. The results suggested that the proposed kinetic models were applicable for understanding the removal mechanisms of organics, nitrate-nitrogen and hydrocarbons in VSF-CW systems. Thus, Eichhornia crassipes planted VSF-CW has the potential of treating refinery secondary wastewater to discharge permissible limits.

Denitrification performance of Pseudomonas fluorescens Z03 immobilized by graphene oxide-modified polyvinyl-alcohol and sodium alginate gel beads at low temperature

Improving the effect of microbial denitrification under low-temperature conditions has been a popular focus of research in recent years. In this study, graphene oxide (GO)-modified polyvinyl-alcohol (PVA) and sodium alginate (SA) (GO/PVA-SA) gel beads were used as a heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria (Pseudomonas fluorescens Z03) carrier to enhance nitrogen removal efficiency levels at low temperatures (6-8°C). The removal efficiency of N H 4     + -N and N O 3       - -N and the variations in concentrations of extracellular polymeric substances (EPS) under different GO doses (0.03-0.15 g l-1) were studied. The results indicated that the addition of GO can improve the efficiency of nitrogen removal, and the highest removal efficiency level and highest carbohydrate, protein, and total EPS content levels (50.28 mg, 132.78 mg and 183.06 mg (g GO/PVA-SA gel)-1, respectively) were obtained with 0.15 g l-1 GO. The simplified Monod model accurately predicted the nitrogen removal efficiency level. These findings suggested that the application of GO serves as an effective means to enhance nitrogen removal by stimulating the activity of HN-AD bacteria.

Biofilm development dynamics and pollutant removal performance of ceramsite made from drinking-water treatment sludge

Alum-sludge ceramsite and denitrifying bacteria (XP-1, XP-2, CL-1, CL-3) were used as substrate and constructed biofilm for enhancing the removal of pollutants from wastewater. The results showed that, due to the large specific surface area, the maximum growth rate was 0.49 mg/(g·day) on the sludge ceramsite, and the mass of biofilm attached onto sludge ceramsite was 5.98 times higher than that when using commercial ceramsite as substrate. Better removal performance could be achieved with the combination of sludge ceramsite and bacteria, viz. 98.6%, 91.0%, and 85.8% reduction in total phosphorus (TP), total nitrogen (TN), and chemical oxygen demand (COD), respectively. Pseudo-first-order kinetics, pseudo-second-order kinetics, Monod kinetics, and multiple Monod kinetics combined with continuous-flow-stirred tank reactor (CFSTR) behavior were used to investigate the dynamics of the pollutant removal processes. The decrease in band brightness for bacteria attached onto sludge ceramsite was 11.5%, while it was more than 35.7% on commercial ceramsite during wastewater treatment according to results from denaturing gradient gel electrophoresis (DGGE). Sludge ceramsite played an important role in maintaining quantities and activities of denitrifying bacteria, and application of sludge ceramsite substrate and denitrifying bacteria was a reliable method to enhance the removals of phosphorus, nitrogen, and COD from domestic wastewater. PRACTITIONER POINTS: Alum-sludge ceramsite was a good substrate for phosphorus adsorption and denitrifying bacterial growth. There was 5.98 times more biofilm on sludge ceramsite than on commercial ceramsite The biofilm of denitrifying bacteria on sludge ceramsite was more stable. High removals of TP (98.6%), TN (90.1%) and COD (85.81%) were achieved.

Hydraulic conditions affect pollutant removal efficiency in distributed ditches and ponds in agricultural landscapes

Distributed ditches and ponds in agricultural landscapes can retain agricultural pollutants (such as nutrients and pesticides) like wetlands while facilitating crop field drainage. Their complex hydraulic conditions affect pollutant transport and degradation processes, but the existing lump-sum method for estimating pollutant removal treats the total area simply as one unit without considering their specific hydraulic conditions (HCs). In this paper we proposed an analytical method for evaluating pollutant removal efficiencies of distributed ditches and ponds by considering their different HCs explicitly. A realization factor (RF) was used to compare pollutant removal rates with and without considering specific HCs. Application of the method was demonstrated with a case study based on field investigations in an intensively farmed area in southeastern China. The total area of ditches and ponds accounts for 15% of drained crop fields; and the calculated RFs were 0.70-0.84% for various removal rate constants. The difference was mainly caused by the uneven distribution of ditches and ponds along different drainage paths. For pollutants with small values of removal rate constants, the calculated concentration reductions along different flow paths were proportional to their wetland sizes, making the pollutant removal as area limited. For pollutants with larger values of removal rate constant, however, the calculated pollutant removal became concentration limited when the wetland to farmland area ratio was high. Large ponds and ditches were major contributors (85-94%) of pollutant removal in the whole system, while the field ditches contributed to less than 10% of the total removal due to their small dimension and shallow water depth. The distributed nature of ditches and ponds poses some inherent limitations to their water quality functions due to variable hydraulic conditions; understanding such underlying constraints may help guide proper evaluation and conservation of the existing ditches and ponds in agricultural landscapes.

The use of biochar and crushed mortar in treatment wetlands to enhance the removal of nutrients from sewage

An experimental study was carried out using in pilot-scale constructed wetland systems, operated in parallel to treat raw sewage. Each system consisted of a vertical flow (VF) unit that was filled with biochar as the main media, followed by a horizontal flow (HF) unit filled with crushed cement mortar. Hydraulic loading (HL) ranged 340-680 mm/day was applied on the VF wetland units, where high total nitrogen (TN) mass removal rate (20-23 g N/m² d) was obtained, demonstrating that biochar media had a beneficial effect on the degradation of nitrogenous pollutants. Total phosphorus (TP) removal percentage (concentration based) was ≥ 86% in HF wetlands packed with mortar materials. In one system, the flow direction of the sewage was directed by the deployment of downflow pipes and vertical baffles, aiming to facilitate the formation of aerobic and anaerobic zones in the wetland matrices. The effects of such arrangement were analyzed by comparing pollutant removal efficiencies in the two systems. On average, 99, 96, 93, and 86 percentage removals were obtained for ammonia (NH₄-N), TN, biochemical oxygen demand (BOD), and TP, respectively, during the experiments. Biochar and crushed mortar proved to be a highly effective combination as media in subsurface flow constructed wetlands for wastewater treatment.

A hybrid constructed wetland for organic-material and nutrient removal from sewage: Process performance and multi-kinetic models

A pilot-scale hybrid constructed wetland with vertical flow and horizontal flow in series was constructed and used to investigate organic material and nutrient removal rate constants for wastewater treatment and establish a practical predictive model for use. For this purpose, the performance of multiple parameters was statistically evaluated during the process and predictive models were suggested. The measurement of the kinetic rate constant was based on the use of the first-order derivation and Monod kinetic derivation (Monod) paired with a plug flow reactor (PFR) and a continuously stirred tank reactor (CSTR). Both the Lindeman, Merenda, and Gold (LMG) analysis and Bayesian model averaging (BMA) method were employed for identifying the relative importance of variables and their optimal multiple regression (MR). The results showed that the first-order-PFR (M₂) model did not fit the data (P > 0.05, and R² < 0.5), whereas the first-order-CSTR (M₁) model for the chemical oxygen demand (COD_Cr) and Monod-CSTR (M₃) model for the COD_Cr and ammonium nitrogen (NH₄-N) showed a high correlation with the experimental data (R² > 0.5). The pollutant removal rates in the case of M₁ were 0.19 m/d (COD_Cr) and those for M₃ were 25.2 g/m²∙d for COD_Cr and 2.63 g/m²∙d for NH₄-N. By applying a multi-variable linear regression method, the optimal empirical models were established for predicting the final effluent concentration of five days' biochemical oxygen demand (BOD₅) and NH₄-N. In general, the hydraulic loading rate was considered an important variable having a high value of relative importance, which appeared in all the optimal predictive models.

Unraveling the active microbial populations involved in nitrogen utilization in a vertical subsurface flow constructed wetland treating urban wastewater

The dynamics of the active microbial populations involved in nitrogen transformation in a vertical subsurface flow constructed wetland (VF) treating urban wastewater was assessed. The wetland (1.5m²) operated under average loads of 130gCODm^-2d^-1 and 17gTNm^-2d^-1 in Period I, and 80gCODm^-2d^-1 and 19gTNm^-2d^-1 in Period II. The hydraulic loading rate (HLR) was 375mmd^-1 and C/N ratio was 2 in both periods. Samples for microbial characterization were collected from the filter medium (top and bottom layers) of the wetland, water influent and effluent at the end of Periods I (Jun-Oct) and II (Nov-Jan). The combination of qPCR and high-throughput sequencing (NGS, MiSeq) assessment at DNA and RNA level of 16S rRNA genes and nitrogen-based functional genes (amoA and nosZ-clade I) revealed that nitrification was associated both with ammonia-oxidizing bacteria (AOB) (Nitrosospira) and ammonia-oxidizing archaea (AOA) (Nitrososphaeraceae), and nitrite-oxidizing bacteria (NOB) such as Nitrobacter. Considering the active abundance (based in amoA transcripts), the AOA population revealed to be more stable than AOB in both periods and depths of the wetland, being less affected by the organic loading rate (OLR). Although denitrifying bacteria (nosZ copies and transcripts) were actively detected in all depths, the denitrification process was low (removal of 2gTNm^-2d^-1 for both periods) concomitant with NO_x-N accumulation in the effluent. Overall, AOA, AOB and denitrifying bacteria (nosZ) were observed to be more active in bottom than in top layer at lower OLR (Period II). A proper design of OLR and HLR seems to be crucial to control the activity of microbial biofilms in VF wetlands on the basis of oxygen, organic-carbon and NO_x-N forms, to improve their capacity for total nitrogen removal.

Nitrogen transforming bacteria within a full-scale partially saturated vertical subsurface flow constructed wetland treating urban wastewater

The aim of this study was to characterize the nitrogen transforming bacterial communities within a partially saturated vertical subsurface flow constructed wetland (VF) treating urban wastewater in southern Brazil. The VF had a surface area of 3144m², and was divided into four wetland cells, out of which two were operated while the other two rested, alternating cycles of 30days. The nitrifying and denitrifying bacterial communities were characterized in wetland cell 3 (764m² surface area) over a period of 12months by using the FISH technique. Samples were collected monthly (from Feb 2014 to Feb 2015) from different layers within the vertical profile, during operation and rest periods, comprising a total of 6 sampling campaigns while the cell was in operation and another 6 when the cell was at rest. This wetland cell operated with an average organic loading rate (OLR) of 4gCODm^-2d^-1 and a hydraulic loading rate of 24.5mmd^-1. The rest periods of the wetland cell presented influences on the abundance of ammonia-oxidizing bacteria (AOB) (8% and 3% for feed and rest periods, respectively), and nitrite-oxidizing bacteria (NOB) (5% and 2% for feed and rest periods, respectively). However, there was no influence of the rest periods on the denitrifying bacteria. AOB were only identified in the top layer (AOB β-proteobacteria) in both operational and rest periods. On the other hand, the NOB (Nistrospirae and Nitrospina gracilis) were identified in feed periods just in the top layer and during rest periods just in the intermediate layer. The denitrifying bacteria (Pseudomonas spp. and Thiobacillus denitrificans) were identified from the intermediate layer downwards, and remained stable in both periods. Based on the identified bacterial dynamics, the partially saturated VF wetland operated under low OLR enabled favorable conditions for simultaneous nitrification and denitrification.

Preliminary evidence of nutrients release from sediment in response to oxygen across benthic oxidation layer by a long-term field trial

In aquatic ecosystems, ecological processes such as organic matter mineralization and nutrient cycling are regulated by benthic O₂ in sediments, and application of in situ techniques in field environments has the potential to better define the links between O₂ dynamics and the unique biogeochemical phenomena occurring in these regions. The effects of benthic O₂ on sediment nutrients release were identified on the basis of field specific observations conducted over one and a half years at Taihu Lake. Sediment dredging (SD) practices have sharply reworked the benthic boundary oxidation layer, and the oxygen penetration depth (OPD) in the SD responded as expected to the new-born surface, increasing immediately (7.5 ± 0.8 - 10.5 ± 0.6 mm) after dredging, then further increasing with an unusually high heterogeneity when a significant submersed macrophytes (SM) coverage of about 40% was implemented. Multiple correlation analysis revealed that OPD was responsible for PO₄^3- and NH₄⁺ release. A lower benthic oxygen flux was immediately observed in dredging-related sediments in the case of dredging compared to SM or the control (CK), which suggested that oxygen demand is low in the uppermost sediments because of the degradable fresh organic carbon removal. SD and SDSM implementation was most successful at continuously reducing the size of PO₄^3- released from sediments over one and a half years, and a significant seasonal-dependent release was also observed. The direction of flux was consistent among SD and SDSM, suggesting the potential to reduce internal PO₄^3- release even further with the invasion of SM communities. Our results indicated that ecological engineering practices could alleviate internal nutrient loads from the contaminated bottom sediment, which was probably in positive response to benthic oxygen changes.

Modeling organic matter and nitrogen removal from domestic wastewater in a pilot-scale vertical subsurface flow constructed wetland

Constructed wetlands have become an attractive alternative for wastewater treatment. However, there is not a globally accepted mathematical model to predict their performance. In this study, the VS2DTI software was used to predict the effluent biochemical oxygen demand (BOD) and total nitrogen (TN) in a pilot-scale vertical flow constructed wetland (VFCW) treating domestic wastewater. After a 5-week adaptation period, the pilot system was monitored for another 6 weeks. Experiments were conducted at hydraulic retention times (HRTs) in the range of 2-4 days with Typha latifolia as the vegetation. The raw wastewater concentrations ranged between 144-430 and 122-283 mg L(-1) for BOD5 and TN, respectively. A first-order kinetic model coupled with the advection/dispersion and Richards' equations was proposed to predict the removal rates of BOD5 and TN from domestic wastewater. Two main physical processes were modeled in this study, porous material water flow and solute transport through the different layers of the VFCW to simulate the constructed wetland (CW) conditions. The model was calibrated based on the BOD5 and TN degradation constants. The model indicated that most of BOD and TN (88 and 92%, respectively) were removed through biological activity followed by adsorption. It was also observed that the evapotranspiration was seen to have a smaller impact. An additional data series of effluent BOD and TN was used for model validation. The residual analysis of the calibrated model showed a relatively random pattern, indicating a decent fit. Thus, the VS2DTI was found to be a useful tool for CW simulation.

Application of Pseudomonas flava WD-3 for sewage treatment in constructed wetland in winter

Recently, constructed wetland was applied for sewage treatment globally due to its high efficiency and relatively low investment. However, operation of many constructed wetlands in cold winter is quite difficult due to the inhibition effect of low temperature. The objective of this experiment is to study the sewage treatment efficiency of Pseudomonas flava WD-3 in the integrated vertical-flow constructed wetland (IVCW) during winter with different dosages (bacterial suspension concentration: 4.575×10(8 )mL(-1)). Two treatments were designed, inoculation of P. flava WD-3 with different dosages and the control without bacterium incubation. A simplified Monod model was applied to simulate and evaluate the pollutant removal efficiency of this bacterial strain with respect to its dosages. Results indicated that P. flava WD-3 could degrade organic pollutants, nitrogen, and phosphorus nutrients from wastewater effectively. The optimal dosage of this strain was 6.0%, and the removal rates of chemical oxygen demand (COD), ammonium nitrogen (NH4+-N), and total phosphorous (TP) were 85.82-87.00%, 73.91-84.18%, and 82.04-87.00%, respectively. Furthermore, the average removal efficiencies of COD, NH4+-N, and TP were 1.46, 1.49, and 1.76 times, respectively, than the control. The simplified Monod model accurately predicted the pollutant removal efficiency of P. flava WD-3 in the IVCW system in winter.

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.

Performance of pilot-scale constructed wetlands for secondary treatment of chromium-bearing tannery wastewaters

Tannery operations consist of converting raw animal skins into leather through a series of complex water- and chemically-intensive batch processes. Even when conventional primary treatment is supplemented with chemicals, the wastewater requires some form of biological treatment to enable the safe disposal to the natural environment. Thus, there is a need for the adoption of low cost, reliable, and easy-to-operate alternative secondary treatment processes. This paper reports the findings of two pilot-scale wetlands for the secondary treatment of primary effluents from a full tannery operation in terms of resilience (i.e., ability to produce consistent effluent quality in spite of variable influent loads) and reliability (i.e., ability to cope with sporadic shock loads) when treating this hazardous effluent. Areal mass removal rates of 77.1 g COD/m2/d, 11 g TSS/m2/d, and 53 mg Cr/m2/d were achieved with a simple gravity-flow horizontal subsurface flow unit operating at hydraulic loading rates of as much as 10 cm/d. Based on the findings, a full-scale wetland was sized to treat all the effluent from the tannery requiring 68% more land than would have been assumed based on literature values. Constructed wetlands can offer treatment plant resilience for minimum operational input and reliable effluent quality when biologically treating primary effluents from tannery operations.

Treatment of tannery wastewater in a pilot-scale hybrid constructed wetland system in Bangladesh

This paper reports the pollutant removal performances of a hybrid wetland system in Bangladesh for the treatment of a tannery wastewater. The system consisted of three treatment stages: a subsurface vertical flow (VF) wetland, followed by a horizontal flow (HF) and a VF wetland. The wetlands were planted with common reed (Phragmites australis), but employed different media, including organic coco-peat, cupola slag and pea gravel. In the first stage, experimental results demonstrated significant removal of ammonia (52%), nitrate (54%), BOD (78%), and COD (56%) under high organics loading rate (690 g COD m(-2)d(-1)); simultaneous nitrification, denitrification, and organics degradation were attributed to the unique characteristics of the coco-peat media, which allowed greater atmospheric oxygen transfer for nitrification and organic degradation, and supply of organic carbon for denitrification. The second stage HF wetland produced an average PO(4) removal of 61%, primarily due to adsorption by the iron-rich cupola slag media. In the third treatment stage, which was filled with gravel media, further BOD removal (78%) from the tannery wastewater depleted organic carbon, causing the accumulation of NO(3) in the wastewater. Overall, the average percentage removals of NH(3)-N, NO(3)-N, BOD, COD, and PO(4) were 86%, 50%, 98%, 98% and 87%, respectively, across the whole hybrid system. The results provided a strong evidence to support widespread research and application of the constructed wetland as a low-cost, energy-efficient, wastewater treatment technology in Bangladesh.