Kinetic modelling of organic matter removal in 80 horizontal flow reed beds for domestic sewage treatment

Efficiency of saturated vertical-flow filters planted with Panicum maximum reeds for passive wastewater treatment

A vertical-flow unit containing four filters filled with shale was used to study the removal of phosphorous, nitrogen and organic matter of an urban residual wastewater during a period of 90 days. The influence of both the shale granulometry and the plant density of Panicum Maximum were studied. The decrease of the shale granulometry led to a significant improvement of all the measured parameters, while the presence of plants did only influence the phosphate retention with a lower extent. By comparing the results to previous studies, we hypothesised that the effect of the root system of Panicum maximum would be different depending on the size and the depth of the reactors. For practical application, adjusting the material granulometry was proposed to be the most important parameter for improving the filtration efficiency. Concomitantly, adjusting the plant density helps to control the clogging percentage of the filters.

Kinetic models evaluation for chemical organic matter removal prediction in a full-scale primary facultative pond treating municipal wastewater

This study focuses on determining the bio-kinetic coefficients of chemical oxygen demand (COD) removal in full-scale primary facultative ponds (PFPs) system on the basis of 3-year continuous operation. The mean removal of chemical oxygen demand (COD), total suspended solid (TSS) and volatile suspended solid (VSS) were 80, 59 and 49%, respectively. The first-order model paired with continuous stirred-tank reactor (CSTR) and plug flow (PF) regimes, PF k-C*, Stover-Kincannon and Grau second-order models were applied to link COD concentrations at the inlet and outlet of the system and to compare the predictive power of models for the estimation of effluent COD concentrations. The Stover-Kincannon model showed the best adaptability (r² = 0.9294) with the maximum substrate utilization rate (U_max) of 79.14 g/L· d and saturation constant (K_B) of 80.65 g/L· d, whereas the Grau second-order model was the best model to predict outlet COD concentrations (r² = 0.6925). The computed constants, m and n, of the Grau second-order model were 0.6725 and 15.867 d^-1, respectively. While the Stover-Kincannon kinetic rates obtained in this study can be used to design the PFP systems in similar operational conditions, the appropriate prediction of pond behavior can be achieved using the Grau model.

Comparison of first-order kinetic models for sewage treatment in horizontal subsurface-flow constructed wetlands

There are several techniques for sizing horizontal subsurface-flow constructed wetlands systems (CWs), and mathematical models have been frequently used for this purpose because they more accurately represent the liquid behaviour in these reactors. The P-k-C* model has already been used in the prediction of organic matter removal in CWs, but it has been little explored in the literature. On other hand, the model proposed by Chan and Chu [Modeling the reaction kinetics of Fenton's process on the removal of atrazine. Chemosphere. 2003;51(4):305-311] was not known for prediction of organic matter removal in CW systems. In the present work, the kinetic data for chemical oxygen demand (COD) removal of 28 horizontal subsurface-flow constructed wetlands were used to compare the performance of two pseudo-first-order kinetic models (P-k-C* model and model from Chan and Chu. The comparisons of nonlinear regressions were performed considering Akaike information criterion (AIC), root-mean-square error (RMSE), and adjusted coefficient of determination (Radj2). In general, both models were able to provide good predictions of relative remaining concentration (C/C₀). However, the Chan and Chu model produced higher adjustment coefficients, showing the potential to be used in modelling and simulation of the degradation kinetics of organic matter in wetlands.

The removal of arsenic and metals from highly acidic water in horizontal subsurface flow constructed wetlands with alternative supporting media

A laboratory-scale horizontal subsurface flow constructed wetland system was used to quantify the arsenic removal capacity in the treatment of highly acidic, arsenic and metal-rich water: pH ≈ 2, Fe ≈ 57 mg/L, Pb ≈ 0.9 mg/L, Zn ≈ 12 mg/L. The system was operated in two stages, being As ≈ 2.1 mg/L in stage one, and ≈ 3.7 mg/L in stage 2. Limestone and zeolite were employed as main supporting media to build non-vegetated and vegetated cells with Phragmites australis. The system was very effective in the removal of arsenic and iron (> 96%), and lead (> 94%) throughout the whole experimental period, having the four treatment types a similar performance. The main effect of the media type was on the pH adjustment capacity: limestone cells were able to raise the pH to ≈ 7.1, whereas zeolite cells raised it to ≈ 3.8. The contribution of plant uptake to the overall removal of As, Fe and Zn was minor; accounting for less than 0.02%, 0.07% and 0.7% respectively. As such, pollutants were mainly retained in the wetland beds. Our results suggest that limestone is recommended over zeolite as wetland medium mainly due to its neutralization capacity.

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.

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.

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.

Kinetics of pollutants removal in vertical and horizontal flow constructed wetlands in temperate climate

This paper reports a comparative study on kinetics of organic matter expressed as BOD₅ and nitrogen removal in constructed wetlands operated in Poland. Analyzed data were collected at eight wetland systems, composed of subsurface flow beds: horizontal flow (HF) and vertical flow (VF), in different number and sequences. The analysis involved particularly mass removal rates (MRR) and first-order removal rate coefficients of BOD₅ and total nitrogen (k_A and k_v for VF and HF filters, respectively, and k₂₀ as a parameter averaged for a temperature of 20 °C). It was found that the higher the load of pollutants applied to the beds, the higher MRR values were obtained. The average k-rates in analyzed systems were mostly lower than those reported in the literature, especially in the case of total nitrogen. Its removal obtained in horizontal flow beds was k_v = 0.002-0.042 d^-1, while in vertical flow systems k_A varied from 0.007 m d^-1 to 0.0037 m d^-1. According to data given by previous studies, first-order reaction rates for nitrogen removal varied in range from k_v = 0.048 d^-1 to k_v = 0.19 d^-1 and k_A from 0.007 to 0.1 m d^-1 in HF and VF beds, respectively. Regarding BOD₅ shown in literature, removal rate k_v for HF beds varied from 0.071 to 6.11 d^-1, and k_A for VF beds varied from 0.019 to 1.0 m d^-1, while in this study lower k-rates were obtained: k_v = 0.005-0.085 d^-1 and k_A = 0.015-0.130 m d^-1. Relatively long monitoring period, for some of constructed wetland up to 16 years, resulted in good data set and enables creation of the graphs, which could be helpful in evaluation and designing of constructed wetlands for PE bigger than 50, in moderate climate conditions.

Presence and possible origin of positive Eu anomaly in shoot samples of Juncus effusus L

BACKGROUND

The rare earth elements (REE) are non-essential elements for plants. They stimulate plant growth at low doses, but at high levels are phytotoxic. There are differences in concentrations of REE in various organs of the same plant species, but the normalized REE patterns can be very similar in samples of the same species collected in different locations. Here we compare normalized REE curves in above-ground samples of Juncus effusus L. (common rush, soft rush) collected from sites with different land-use types.

METHODS

The concentrations of rare earth elements were measured in 55 shoot samples of J. effusus L. The samples were collected from 15 sampling sites located in the Holy Cross Mts., south-central Poland and analyzed with the use of inductively coupled plasma mass spectrometry (ICP-MS). The results were normalized to the North American Shale Composite and anomalies of different elements were calculated.

RESULTS

Total REE concentrations varied from 0.028 mg/kg to 2.7 mg/kg. The samples were enriched in the light REE (from La to Eu) with the highest concentrations of La and Ce. The North American Shale Composite (NASC)-normalized REE curves were roughly similar in all samples except for two samples collected in the acid mine drainageaffected areas.

CONCLUSION

All samples showed positive europium anomalies in NASC-normalized REE concentration patterns. The most probable explanation of this is that the uptake and translocation of Eu in J. effusus (and possibly in other wetland plants) is caused by a short-term decrease of the redox potential in a rhizosphere favoring reduction of Eu³⁺ to Eu²⁺ and thus enhancing Eu mobility in the soil-plant environment.

Two-stage constructed wetland systems for polluted surface water treatment

Two pilot scale wetland systems were studied for the removal of organics, nitrogen, phosphorus and coliform from polluted surface water. Each system consisted of two units: a vertical flow (VF) wetland packed with construction materials gravel, brick or organic sugarcane bagasse, followed by a surface flow (SF) or floating treatment (FT) wetland. All wetland units were planted with Phragmites. The wetland systems were operated under constant and shock hydraulic load (HL) periods. Input COD, N, P loadings ranged between 61 and 2181, 7-1040, 2-194 g/m²d, respectively across first stages of each system. Mean removal percentages ranged between 39 and 97, 11-83, 20-100% and 4-85, 16-86, 1.4-100% across first and second stage wetlands, respectively. Mass balance analyses revealed ≤7% N and ≤14% P accumulation in plants; as such, microbial and adsorption kinetics controlled removal dynamics. Nitrification was the limiting nitrogen removal factor in first stage wetlands; organic carbon was supplied by the employed media. Aerobic organics removal and nitrification were diminished during initial stage of shock load periods. In contrast, second stage SF and FT wetlands showed stable removal performances under similar conditions. Resuspension of settled particles decreased removal performance in second stage wetlands, as shock periods progressed toward final stage. Coliform mortality was increased in second stage wetlands. Physico-chemical properties of brick materials in construction material based VF wetland and hanging root volume inside the water column of FT wetland supplemented removal performance. In general, this study provides evidence on potential application of constructed wetlands for polluted surface water treatment.

Seasonal changes in concentrations of trace elements and rare earth elements in shoot samples of Juncus effusus L. collected from natural habitats in the Holy Cross Mountains, south-central Poland

Selected trace elements (Ag, As, Ba, Bi, Cd, Co, Cr, Mn, Cu, Fe, Ni, Pb, Tl, U, Zn) and rare earth elements were determined in 13 samples of Juncus effusus collected from three investigation sites in the Holy Cross Mts., south-central Poland. Sampling was carried out four times during a vegetative season of 2014. Almost all the elements examined showed different seasonal trends in their concentrations, except for Ag, Co and Ni. Maximum concentrations of Ag in samples of three investigation sites were found in May (0.068, 0.062, 0.047 mg/kg) whereas Co (0.124, 0.070, 0.079 mg/kg) and Ni (1.8, 0.998, 2.8 mg/kg) in July, respectively. Mean concentrations of Mn and Cd were higher in shoots (558 and 2.35 mg/kg) than in roots (435 and 1.7 mg/kg). Both these elements revealed much higher concentrations in J. effusus than their typical contents in plant samples. Principal component method allowed us to allocate Ni, Ba, Cd and Cu to one group with the highest positive loadings. The most probable explanation for this correlation is that bioavailability of these metals is increased by J. effusus through a release of oxygen to the rhizosphere. Light rare earth elements concentrations predominate over heavy rare earth elements in the samples examined. A fractionation of lanthanides occurs during their transport from roots to shoots, although this transport is rather limited. All shoot samples have a strong positive Eu anomaly.

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.

Achieving an extraordinary high organic and hydraulic loadings with good performance via an alternative operation strategy in a multi-stage constructed wetland system

In this study, a high organic loading rate of 58-146 g BOD₅/m² day with a hydraulic loading rate (HLR) of 1.63 m³/m² day and retention time (RT) of 16 h was achieved to maximize the treatment capacity of a four-stage alum sludge-based constructed wetland (CW) system. An alternative operation strategy, i.e., the first stage anaerobic up-flow and the remaining stage tidal flow with effluent recirculation, was investigated to achieve the goal with good treatment performance of 82% COD, 91% BOD₅, 92% SS, 94% NH₄-N, and 82% TN removal. Two kinetic models, i.e., first-order model and Monod plus continuous stirred-tank reactor (CSTR) flow model, were employed for predicting the removal dynamics. The results showed that the tidal flow strategy enhances oxygen transport and diffusion, thus improving reduction of organics and NH₄-N. Effluent recirculation could further increase elimination of organics by extending the interaction time and also benefit the denitrification process. In addition, denitrification could be further enhanced by anaerobic up-flow in the first stage.

Large-scale multi-stage constructed wetlands for secondary effluents treatment in northern China: Carbon dynamics

Multi-stage constructed wetlands (CWs) have been proved to be a cost-effective alternative in the treatment of various wastewaters for improving the treatment performance as compared with the conventional single-stage CWs. However, few long-term full-scale multi-stage CWs have been performed and evaluated for polishing effluents from domestic wastewater treatment plants (WWTP). This study investigated the seasonal and spatial dynamics of carbon and the effects of the key factors (input loading and temperature) in the large-scale seven-stage Wu River CW polishing domestic WWTP effluents in northern China. The results indicated a significant improvement in water quality. Significant seasonal and spatial variations of organics removal were observed in the Wu River CW with a higher COD removal efficiency of 64-66% in summer and fall. Obvious seasonal and spatial variations of CH₄ and CO₂ emissions were also found with the average CH₄ and CO₂ emission rates of 3.78-35.54 mg m^-2 d^-1 and 610.78-8992.71 mg m^-2 d^-1, respectively, while the higher CH₄ and CO₂ emission flux was obtained in spring and summer. Seasonal air temperatures and inflow COD loading rates significantly affected organics removal and CH₄ emission, but they appeared to have a weak influence on CO₂ emission. Overall, this study suggested that large-scale Wu River CW might be a potential source of GHG, but considering the sustainability of the multi-stage CW, the inflow COD loading rate of 1.8-2.0 g m^-2 d^-1 and temperature of 15-20 °C may be the suitable condition for achieving the higher organics removal efficiency and lower greenhouse gases (GHG) emission in polishing the domestic WWTP effluent. The obtained knowledge of the carbon dynamics in large-scale Wu River CW will be helpful for understanding the carbon cycles, but also can provide useful field experience for the design, operation and management of multi-stage CW treatments.

Water quantity and quality assessment on a tertiary treatment wetland in a tropical climate

This study aimed to assess the quantity and quality of water in a surface flow constructed wetland in Australia's far north Queensland. Owing to tropical climate in the region, the wetland provided dual functions: retention of a treated wastewater for zero discharge during the dry season and tertiary treatment prior to discharge during the wet season. Rainfall data, permeability of wetland soil, evaporation, inflow and outflow were analysed in a water balance analysis; the results showed that based on a 72-year-average rainfall pattern, daily wastewater inflow of 85 m(3)/d is the maximum this wetland can cope with without breaching its discharge certificate. In water quality analysis, the K-C* model was used to predict changes of biochemical oxygen demand (BOD, suspended solids (SS), total nitrogen (TN), total phosphorus (TP) and faecal coliforms (FC) in the wetland. Model predictions were compared with field sampling results. It was found that the wetland was effective in removing FC (>99.9%), TN (70.7%) and TP (68.2%), for which the predictions by the K-C* model were consistent with field testing results. However, significant disparities between the predictions and testing results were found for BOD and SS. A revised K-C* equation was proposed to account for the internal generation of organics in constructed wetlands with a long retention time.

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.

Removal processes for arsenic in constructed wetlands

Arsenic pollution in aquatic environments is a worldwide concern due to its toxicity and chronic effects on human health. This concern has generated increasing interest in the use of different treatment technologies to remove arsenic from contaminated water. Constructed wetlands are a cost-effective natural system successfully used for removing various pollutants, and they have shown capability for removing arsenic. This paper reviews current understanding of the removal processes for arsenic, discusses implications for treatment wetlands, and identifies critical knowledge gaps and areas worthy of future research. The reactivity of arsenic means that different arsenic species may be found in wetlands, influenced by vegetation, supporting medium and microorganisms. Despite the fact that sorption, precipitation and coprecipitation are the principal processes responsible for the removal of arsenic, bacteria can mediate these processes and can play a significant role under favourable environmental conditions. The most important factors affecting the speciation of arsenic are pH, alkalinity, temperature, dissolved oxygen, the presence of other chemical species--iron, sulphur, phosphate--,a source of carbon, and the wetland substrate. Studies of the microbial communities and the speciation of arsenic in the solid phase using advanced techniques could provide further insights on the removal of arsenic. Limited data and understanding of the interaction of the different processes involved in the removal of arsenic explain the rudimentary guidelines available for the design of wetlands systems.

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

This paper provides a comparative evaluation of the kinetic models that were developed to describe the biodegradation of nitrogen and organics removal in wetland systems. Reaction kinetics that were considered in the model development included first order kinetics, Monod and multiple Monod kinetics; these kinetics were combined with continuous-stirred tank reactor (CSTR) or plug flow pattern to produce equations to link inlet and outlet concentrations of each key pollutants across a single wetland. Using three statistical parameters, a critical evaluation of five potential models was made for vertical and horizontal flow wetlands. The results recommended the models that were developed based on Monod models, for predicting the removal of nitrogen and organics in a vertical and horizontal flow wetland system. No clear correlation was observed between influent BOD/COD values and kinetic coefficients of BOD(5) in VF and HF wetlands, illustrating that the removal of biodegradable organics was insensitive to the nature of organic matter. Higher effluent COD/TN values coincided with greater denitrification kinetic coefficients, signifying the dependency of denitrification on the availability of COD in VF wetland systems. In contrast, the trend was opposite in HF wetlands, indicating that availability of NO(3)-N was the main limiting step for nitrogen removal. Overall, the results suggested the possible application of the developed alternative predictive models, for understanding the complex biodegradation routes of nitrogen and organics removal in VF and HF wetland systems.

A review on numerous modeling approaches for effective, economical and ecological treatment wetlands

Constructed wetlands (CWs) for wastewater treatment have evolved substantially over the last decades and have been recognized as an effective means of "green technology" for wastewater treatment. This paper reviews the numerous modeling approaches ranging from simple first-order models to more complex dynamic models of treatment behaviour in CWs. The main objective of the modeling work is to better understand the process in CWs and optimize design criteria. A brief study in this review discusses the efforts taken to describe the process-based model for the efficient removal of pollutants in CWs. Obtaining better insights is essential to understand the hydraulic and biochemical processes in CWs. Currently, employed modeling approaches can be seen in two categories, i.e. "black-box models" and "process-based models". It is evident that future development in wetland technology will depend on improved scientific knowledge of internal treatment mechanisms.

The removal of nitrogen and organics in vertical flow wetland reactors: predictive models

Three kinetic models, for predicting the removal of nitrogen and organics in vertical flow wetlands, have been developed and evaluated. These models were established by combining first-order, Monod and multiple Monod kinetics with continuous stirred-tank reactor (CSTR) flow pattern. Critical evaluations of these models using three statistical parameters, coefficient of determination, relative root mean square error and model efficiency, indicated that when the Monod/multiple Monod kinetics was combined with CSTR flow pattern it allowed close match between theoretical prediction and experiment data of nitrogen and organics removal. The kinetic coefficients (derived from Monod/multiple Monod kinetics) was found to increase with pollutant loading, indicating that the coefficients may vary based on different factors, such as influent pollutant concentration, hydraulic loading, and water depth. Overall, this study demonstrated the validity of combining Monod and multiple Monod kinetics with CSTR flow pattern for the modelling and design of vertical flow wetland systems.