Effect of effluent recirculation on the performance of a reed bed system treating agricultural wastewater

Comparison of nitrogen removal performance and mechanism from low-polluted wastewater by constructed wetlands with two oxygen supply strategies: Tidal flow and intermittent aeration

Due to dissolved oxygen (DO) limited nitrogen removal efficiency in constructed wetlands (CWs), two representative oxygen-suppling CWs, i.e., tidal flow constructed wetlands (TFCWs) and intermittently aerated constructed wetlands (IACWs) were proposed to compare the effect of oxygen supply strategies on the nitrogen removal performance and mechanism. Results showed that the removal efficiencies of NH₄⁺-N and COD in IACWs were as high as 90.35-97.14% and 91.14-92.44%, respectively. In terms of TN, TFCWs (83.82%) showed a significantly higher removal efficiency than IACWs, and this result was derived with the flooded/drained phase (FP/DP) ratio of 21 h:3 h in TFCWs, because rhythmic FP and DP formed a high oxygen gradient at different depths of the system, which intensified the nitrification and denitrification simultaneously. The potential nitrifying and denitrifying bacteria (e.g., Nitrospira, Azospira, Haliangium, Bradyrhizobium and Arenimonas) were enriched more significantly in TFCWs compared with IACWs, as well as Bacillus for simultaneous nitrification and denitrification, which promoted nitrogen transformation together. Also, the results of molecular ecological network analysis showed that bacterial community structure in IACWs was more complex and robust than in TFCWs, because there were obviously more nodes and links as well as a higher proportion of negative interference. However, the relationship between genera in TFCWs was closer depending on shorter path distances, and the keystone genus (Nitrosomonas) in related to nitrification was considered to play an important role in nitrogen transformation performance.

Vermifiltration: Strategies and techniques to enhance the organic and nutrient removal performance from wastewater

The vermifiltration (VF) technology has gained significant attention as a green alternative for remediating domestic and industrial wastewater over the last few decades. Of late, the implementation of various modifications to the orthodox VF technology, including tweaks in the design and operation of the vermifilters, has been portrayed in the available literature. However, owing to the scatteredness of the available information, the knowledge regarding the execution of the modified vermifilters is still inadequate. Hence, an effort has been made to comprehensively overview the innovative strategies and techniques adopted to improve the organic and nutrient removal potential of the VFs from wastewater. In addition, future perspectives have been recognized to design more efficient and sustainable VFs. This review explores more of such novel tactics to improve the performance of the VF technology regarding organic and nutrient removal from wastewater. PRACTITIONER POINTS: Innovative strategies and techniques implemented to VF technology were comprehensively overviewed. Design modification and advantages of each innovation were highlighted. The pollutant removal performance of every modification was emphasized. Modified vermifilters were better than the conventional vermifilters in terms of organic and nutrient removal from the wastewater.

Enhanced nitrogen and phosphorus removal by natural pyrite-based constructed wetland with intermittent aeration

Four subsurface flow constructed wetlands (SFCWs) filled with different substrates including ceramsite, ceramsite+pyrite, ceramsite+ferrous sulfide, and ceramsite+pyrite+ferrous sulfide (labeled as SFCW-S1, SFCW-S2, SFCW-S3, and SFCW-S4) were constructed, and the removal of nitrogen and phosphorus by these SFCWs coupled with intermittent aeration in the front section was discussed. The key findings from different substrate analyses, including nitrification and denitrification rate, enzyme activity, microbial community structure, and the X-ray diffraction, revealed the nitrogen and phosphorus removal mechanism. The results showed that the nitrogen and phosphorus removal efficiency for SFCW-S1 always remained the lowest, and the phosphorus removal efficiency for SFCW-S4 was recorded as the highest one. However, after controlling the dissolved oxygen by intermittent aeration in the front section of SFCWs, the nitrogen and phosphorus removal efficiencies of SFCWs-S2 and S4 became higher than those of SFCW-S1, and SFCW-S3. It was noticed that the pollutants were removed mainly in the front section of the SFCWs. Both precipitation and adsorption on the substrate were the main mechanisms for phosphorus removal. A minute difference of nitrification rate and ammonia monooxygenase activity was observed in the SFCWs' aeration zone. The denitrification rates, nitrate reductase, nitrite reductase, and electron transport system activity for SFCW-S2 and SFCW-S4 were higher than those detected for SFCW-S1 and SFCW-S3 in the non-aerated zone. Proteobacteria was the largest phyla found in the SFCWs. Moreover, Thiobacillus occupied a large proportion found in SFCW-S2, and SFCW-S4, and it played a crucial role in pyrite-driven autotrophic denitrification.

Pharmaceutical compound removal efficiency by a small constructed wetland located in south Brazil

The fate of pharmaceuticals during the treatment of effluents is of major concern since they are not completely degraded and because of their persistence and mobility in environment. Indeed, even at low concentrations, they represent a risk to aquatic life and human health. In this work, fourteen pharmaceuticals were monitored in a constructed wetland wastewater treatment plants (WWTP) assessed in both influent and effluent samples. The basic water quality parameters were evaluated, and the removal efficiency of pharmaceutical, potential for bioaccumulation, and the impact of WWTP were assessed using Polar Organic Chemical Integrative Sampler (POCIS) and biofilms. The pharmaceutical compounds were quantified by High Performance Liquid chromatography coupled to mass spectrometry. The sampling campaign was carried out during winter (July/2018) and summer (January/2019). The WWTP performed well regarding the removal of TSS, COD, and BOD₅ and succeeded to eliminate a significant part of the organic and inorganic pollution present in domestic wastewater but has low efficiency regarding the removal of pharmaceutical compounds. Biofilms were shown to interact with pharmaceuticals and were reported to play a role in their capture from water. The antibiotics were reported to display a high risk for aquatic organisms.

Field-scale baffled and biorack hybrid constructed wetland: effect of fluctuating loading rates and recirculation for domestic wastewater treatment

The conventionally used constructed wetlands require modification/s to minimize clogging problems and space requirement. In this study, a field-scale baffled and biorack hybrid constructed wetland (BBHCW) was developed as a part of 42 KLD decentralized wastewater treatment (DWT) system at Walchand College of Engineering, Sangli (M.S.), India for domestic wastewater. Brickbats were used as support medium in the baffled portion and corrugated sheets in biorack. Mixed vegetation of Typha angustifolia and Canna indica was used in both baffled and biorack portions. BBHCW was operated under the dynamic conditions of flow (0.60-9.89 m³/m² day) and strength (0.12-2.12 kg COD/m² day) for 8 months. The performance was assessed for the removal of organic carbon and nitrogen with and without recirculation of treated effluent. Tracer studies showed that the hydraulic efficiency was satisfactory. COD, BOD₃, and TKN removal is possible to an extent of 26.30 ± 1.36, 29.08 ± 2.43, and 19.39 ± 2.27%, respectively, under dynamic conditions. Recirculation enhances the removal efficiency of COD by 5.00-10.00%. However, TKN removal was not significant with or without recirculation. Morphological study showed that vegetation growth was well supported in BBHCW. The discarded corrugated sheets in BR and brickbats in BSFW are the most appropriate low-cost options. The clogging problem is reduced significantly. BBHCW is sturdy enough to absorb shock loading and space requirement can be reduced by judicious choice of HLR and OLR. BBHCW is an alternative to conventionally used sub-surface constructed wetland as a part of DWT. Novelty statementDevelopment of newly configured baffled and biorack hybrid dual-species constructed wetland (BBHCW) for field scale application.Use of discarded brickbat and cement sheets as a new support medium and bioracks.Performance assessment of field-scale BBHCW for the removal of organic carbon (expressed as COD and BOD₃), and nitrogen (expressed as TKN) from domestic wastewater under highly dynamic conditions induced by fluctuating hydraulic loading rate (0.60-9.89 m³/m² day) and organic loading rate (0.12-2.12 kg COD/m² day).

Domestic wastewater treatment efficiency of the pilot-scale trickling biofilter system with variable flow rates and hydraulic retention times

In this study, a pilot-scale trickling biofilter (TBF) using pebbles and gravels media was evaluated for the treatment of domestic wastewater. The TBF system was installed in an open environment at residential area of Quaid-i-Azam University, Islamabad, Pakistan, and was operated at three different recirculation flow rates (Q), i.e. 0.04, 0.072 and 0.1 m³/day and under three different HRTs, i.e. 48, 72 and 96 h. It was observed that the efficiency of pilot-scale TBF system in terms of pathogens removal was significant, i.e. at flow rates of 0.04, 0.072 and 0.1 m³/day, an average reduction of 39.8-62.5% (p = 0.007), 35.9-48.6% (p = 0.01) and 25.8-57.3% (p = 0.009) respectively were attained in CFU/mL under different HRTs. Moreover, it has been observed that due to high void spaces up to 30%, pebbles and gravels filter media in co-ordination allowing good microbial growth and increased the diversity of bacterial species. Furthermore, it also facilitate the removal of different pollutant indicators, i.e. chemical oxygen demand (COD) (74.2-80.5%), total dissolved solids (TDS) (60.3-69.5%), electric conductivity (EC) (62.8-68.6%) and phosphates (PO₄) (45.3-60.3%). A significant reduction in total nitrogen (TN) (59-63.3%) was observed at flow rates of 0.04 and 0.072 m³/day (p = 0.005). The experimental data of this research study will be helpful for further modification in the TBF system using different filter media in association and selecting optimal HRTs and flow rates in future study to get maximum efficiency of TBF system while treating domestic wastewater.

Influence of Agapanthus africanus on nitrification in a vertical subsurface flow constructed wetland

The aim of this study is to evaluate the influence of Agapanthus africanus (A. africanus) on nitrification in a vertical subsurface flow constructed wetlands (VSSFs) system. Two lab-scale VSSFs were operated: a) one was planted with A. africanus (vertical flow planted, VFP), and b) the other was unplanted (vertical flow control, VFC). The operation strategy was divided into three phases and consisted of increasing the ammoniacal nitrogen loading rate (ALR) (Phase I: 1.4; Phase II: 2.4; Phase III: 4.4 g NH₄⁺-N·m^-2·d^-1). Nitrification was evaluated in the system at two different depths in the VSSFs (30.5 cm and 60.3 cm, from the top of the system). The removal efficiencies of COD, BOD₅, TP, and PO₄^-3-P were above 40% in the VFP and VFC during all operation. The mean removal efficiencies of NH₄⁺-N were above 70%. Nitrification was the principal NH₄⁺-N removal mechanism in both systems and transformed more than 50% of the NH₄⁺-N to NO₃^--N. In terms of the effect of A. africanus on NH₄⁺-N removal during the three operational phases, nonsignificant differences between the two VSSFs were noted (p > 0.05). Thus, A. africanus did not influence nitrification. Finally, the analysis at different depths showed that nitrification occurred in the upper 30.5 cm.

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.

The effects of different aeration strategies on the performance of constructed wetlands for phosphorus removal

The effects of different aeration methods such as tidal flow (TF), effluent recirculation (ER), and artificial aeration (AA) on the performance of vertical-flow constructed wetland (VFCW), horizontal-flow constructed wetland (HFCW), and hybrid constructed wetland (HCW) are extensively and critically evaluated in this review paper. Aerated constructed wetlands (CWs) demonstrate superior performance compared with non-aerated systems. The removal of total phosphorus (TP) showed substantial variation among different types of CWs and aeration strategies, with mean and standard deviation of 68 ± 20% estimated from all reviewed studies on aerated systems. The TF-VFCW designated the highest removal efficiency and removal rate of 88 ± 6% and 2.6 ± 2.5 g m^-2 day^-1, respectively, followed by the ER-HCW with values of 79 ± 18% and 1.3 ± 0.7 g m^-2 day^-1, respectively. The superior performance of TF-VFCW could be attributed to a positive effect of TF in rejuvenating the wetland with fresh air, thus enhancing dissolved oxygen (DO) in the system, and augmenting phosphorus precipitation and adsorption to the substrate. A positive correlation of TP and orthophosphate (PO₄^3--P) with DO indicates that the improvement in DO levels due to redox manipulation with aeration strategies facilitates the phosphorous removal processes (e.g., through precipitation and adsorption to the substrate). The conflicting results on the impact of AA and ER reported by many studies need the cautious interpretation of their impact and require further studies. Only few studies have examined the impact of oxidation-reduction potential on phosphorous removal, which requires more attention in future research, as it appears as an important factor in enhancing the phosphorus removal.

The performance of the intensified constructed wetlands for organic matter and nitrogen removal: A review

The effects of different aeration strategies including tidal flow (TF), effluent recirculation (ER) and artificial aeration (AA) on performance of vertical flow constructed wetland (VFCW), horizontal flow constructed wetland (HFCW) and hybrid constructed wetland (HCW) are comprehensively and critically reviewed in this paper. The removal efficiencies of nine types of intensified constructed wetlands (CWs) were examined in detail and their mean and standard deviation were estimated at 89 ± 11%, 84 ± 12%, 81 ± 17% and 63 ± 20% for total suspended solids (TSS), chemical oxygen demand (COD), ammonium-nitrogen (NH₄⁺N) and total nitrogen (TN), respectively. From the studied CWs, ER-HCW, TF-HCW, AA-VFCW and ER-VFCW emerged as the four best performing systems. The overall removal efficiency of TSS, COD, NH₄⁺N and TN by ER-HCW was 98 ± 2%, 85 ± 11%, 83 ± 15% and 73 ± 11%, respectively. Specifically, the ER enhances the interactions between pollutants and micro-organisms, consequently, the efficient removal of NH₄⁺N and TN has been achieved in ER-HCW. The TF has a positive effect in refreshing the wetland with fresh air to enhance the dissolved oxygen (DO) in the system. In case of AA, intermittent aeration is more effective than continuous aeration, as it facilitates the establishment of aerobic and anaerobic conditions suitable for nitrification and denitrification. Statistical analysis shows that DO, organic loading rate and specific surface area requirement are the most significant factors that influence the performance of intensified CWs.

Intensification of constructed wetlands for land area reduction: a review

The large land area requirement of constructed wetlands (CWs) is a major limitation of its application especially in densely populated and mountainous areas. This review paper provides insights on different strategies applied for the reduction of land area including stack design and intensification of CWs with different aeration methods. The impacts of different aeration methods on the performance and land area reduction were extensively and critically evaluated for nine wetland systems under three aeration strategies such as tidal flow (TF), effluent recirculation (ER), and artificial aeration (AA) applied on three types of CWs including vertical flow constructed wetland (VFCW), horizontal flow constructed wetland (HFCW), and hybrid constructed wetland (HCW). The area reduction and pollutant removal efficiency showed substantial variation among different types of CWs and aeration strategies. The ER-VFCW designated the smallest footprint of 1.1 ± 0.5 m² PE^-1 (population equivalent) followed by TF-VFCW with the footprint of 2.1 ± 1.8 m² PE^-1, and the large footprint was of AA-HFCW (7.8 ± 4.7 m² PE^-1). When footprint and removal efficiency both are the major indicators for the selection of wetland type, the best options for practical application could be TF-VFCW, ER-HCW, and AA-HCW. The data and results outlined in this review could be instructive for futures studies and practical applications of CWs for wastewater treatment, especially in land-limited regions.

Effects of tidal operation on pilot-scale horizontal subsurface flow constructed wetland treating sulfate rich wastewater contaminated by chlorinated hydrocarbons

Three different flow regimes were carried out in a pilot-scale horizontal subsurface flow constructed wetland-treating sulfate rich wastewater contaminated with monochlorobenzene (MCB) and perchloroethene (PCE). The three regimes were continuous flow, 7-day cycle discontinuous flow, and 2.5-day cycle discontinuous flow. The results show that intensifying the tidal regime (2.5-day cycle) significantly enhanced MCB removal before 2 m from the inlet and increasing PCE removal efficiency at 0.5 m. The PCE dechlorination process was promoted with tidal operation, especially under the 2.5-day cycle regime, with significant increases of cis-1,2- dichloroethenes (DCEs), vinyl chloride (VC), and ethene, but trans-1,2-DCE was significantly decreased after tidal operation. Due to the high sulfate concentration in the influent, sulfide was observed in pore water up to 20 and 23 mg L^-1 under continuous flow and 7-day cycle regime, respectively. However, sulfide concentrations decreased to less than 4 mg L^-1 under intensified tidal operation (2.5-day cycle). The increase of oxygen concentration in pore water through intensified tidal operation resulted in better MCB removal performance and the successful inhibition of sulfate reduction. In conclusion, intensifying tidal operation is an effective approach for the treatment of chlorinated hydrocarbons and inhibiting sulfide accumulation in horizontal subsurface flow constructed wetland.

The effect of aeration and recirculation on a sand-based hybrid constructed wetland treating low-strength domestic wastewater

The Duplex-constructed wetland (CW) is a hybrid system composed of a vertical flow (VF) CW on top of a horizontal flow filter (HFF). Each compartment is designed to play a different role: aerobic treatment in the VF CW due to intermittent feeding and anoxic treatment in the HFF due to saturated conditions. Three Duplex-CWs were used in this study: Control, Aerated and Recirculating. The role of each compartment was tested for pollutant removal and micro-invertebrate abundance. In all systems, the VF CW removed mainly organic matter, solids and NH4(+)-N. Pathogens were removed in both compartments. Likewise, total nitrogen removal occurred in both compartments, only the Recirculating HFF was not able to denitrify the nitrogen due to the slightly more oxic conditions as compared to the other systems. All systems met discharge guidelines for organic matter, but only the Control and Aerated systems met those for total nitrogen. At the applied loading rates, the pollutant removal was not significantly enhanced by the use of aeration and recirculation. Therefore, operation as in the Control system, without aeration or recirculation, is recommended for the tested Duplex-CWs. If artificial aeration will be used in CWs, the support material should be carefully selected to allow a proper air distribution.

Strategies and techniques to enhance constructed wetland performance for sustainable wastewater treatment

Constructed wetlands (CWs) have been used as an alternative to conventional technologies for wastewater treatment for more than five decades. Recently, the use of various modified CWs to improve treatment performance has also been reported in the literature. However, the available knowledge on various CW technologies considering the intensified and reliable removal of pollutants is still limited. Hence, this paper aims to provide an overview of the current development of CW strategies and techniques for enhanced wastewater treatment. Basic information on configurations and characteristics of different innovations was summarized. Then, overall treatment performance of those systems and their shortcomings were further discussed. Lastly, future perspectives were also identified for specialists to design more effective and sustainable CWs. This information is used to inspire some novel intensifying methodologies, and benefit the successful applications of potential CW technologies.

Effect of aeration on pollutants removal, biofilm activity and protozoan abundance in conventional and hybrid horizontal subsurface-flow constructed wetlands

The large area demand of constructed wetlands (CWs) is documented as a weak point that can be potentially reduced by applying active aeration. The aim of this study was, therefore, to understand the effects of aeration on the treatment performance, the biofilm activity, the protozoan population size and potential CW footprint reduction of different horizontal flow (HF) CW configurations. Two experimental periods were considered: a first period with low organic loading rate (OLR) and a second period with high OLR. Three HF CW configurations were compared: a conventional (control), an aerated and a hybrid CW (aerated followed by a non-aerated CW). The results obtained reinforced the competence of aerated CW for organic matter removal (81-89% of chemical oxygen demand) while for nitrogen elimination the control (19-24%) and hybrid (8-41%) systems performed better than the aerated system (-6% to 33%). Biofilm activity and protozoa abundance were distinctly higher at the inlet zones when compared with the outlet zones of all CWs, as well as in the aerated systems when compared with the non-aerated CWs. The protozoan abundance increased with an increase in the OLR and ciliates were found to be the dominant group. Overall, the active aeration highlighted the efficiency and stability of the CWs for organic matter removal and thus can be used as a promising tool to enhance microbial activity and grazing by protozoa; eventually reducing solid accumulation in the bed media. These beneficial effects contribute to reduce the CWs' area requirements.

Development of constructed wetlands in performance intensifications for wastewater treatment: a nitrogen and organic matter targeted review

The knowledge on the performance enhancement of nitrogen and organic matter in the expanded constructed wetlands (CWs) with various new designs, configurations, and technology combinations are still not sufficiently summarized. A comprehensive review is accordingly necessary for better understanding of this state-of-the-art-technology for optimum design and new ideas. Considering that the prevailing redox conditions in CWs have a strong effect on removal mechanisms and highly depend on wetland designs and operations, this paper reviews different operation strategies (recirculation, aeration, tidal operation, flow direction reciprocation, and earthworm integration), innovative designs, and configurations (circular-flow corridor wetlands, towery hybrid CWs, baffled subsurface CWs) for the intensifications of the performance. Some new combinations of CWs with technologies in other field for wastewater treatment, such as microbial fuel cell, are also discussed. To improve biofilm development, the selection and utilization of some specific substrates are summarized. Finally, we review the advances in electron donor supply to enhance low C/N wastewater treatment and in thermal insulation against low temperature to maintain CWs running in the cold areas. This paper aims to provide and inspire some new ideas in the development of intensified CWs mainly for the removal of nitrogen and organic matter. The stability and sustainability of these technologies should be further qualified.

Treatment performance and microorganism community structure of integrated vertical-flow constructed wetland plots for domestic wastewater

In order to investigate the treatment performance and microorganism mechanism of IVCW for domestic wastewater in central of China, two parallel pilot-scale IVCW systems were built to evaluate purification efficiencies, microbial community structure and enzyme activities. The results showed that mean removal efficiencies were 81.03 % for COD, 51.66 % for total nitrogen (TN), 42.50 % for NH4 (+)-N, and 68.01 % for TP. Significant positive correlations between nitrate reductase activities and TN and NH4 (+)-N removal efficiencies, along with a significant correlation between substrate enzyme activity and operation time, were observed. Redundancy analysis demonstrated gram-negative bacteria were mainly responsible for urease and phosphatase activities, and also played a major role in dehydrogenase and nitrate reductase activities. Meanwhile, anaerobic bacteria, gram-negative bacteria, and saturated FA groups, gram-positive bacteria exhibited good correlations with the removal of COD (p=0.388), N (p=0.236), and TP (p=0.074), respectively. The IVCW system can be used to treat domestic wastewater effectively.

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.

Influence of recirculation in a lab-scale vertical flow constructed wetland on the treatment efficiency of landfill leachate

Landfill leachate taken from a landfill situated in the north-western region of Bulgaria has been treated in a laboratory scale vertical flow constructed wetland (VF-CW) at different flow rates (40, 60 and 82 ml min(-1)) and recirculation ratios (time of water running through wetland to time of quiet water - 1:1; 1:2; 1:3). Young Phragmites australis was planted on the top layer of the reactor. The low flow rate (40 ml min(-1)) and recirculation ratio of 1:3 allowed removal efficiencies of 96% for COD (in 8 days), 92% for BOD(5) (in 3 days), 100% for ammonia (in 5 days) and 100% for total phosphorus (in 2 days). At the highest flow rate studied (82 ml min(-1)) and shorter quiet period (recirculation ratio 1:1) the water needs longer period of treatment (2 days more according to COD). The results of this study indicate that both flow rate and recirculation ratio should be taken into account for proper design of VF-CW.

Performance of a pilot-scale, three-stage constructed wetland system for domestic wastewater treatment

This study investigates the effects of season, organic matter loadings, hydraulic conditions, recycling, and rapid drainage on water quality in a pilot-scale, three-stage subsurface flow constructed wetland (SSF CW) system. The pilot CW system consisted of a vertical flow-gravel filtration (v-GF) wetland in the first stage, a horizontal-subsurface flow (h-SSF) bed planted with Iris in the second stage, and a vertical-subsurface flow (v-SSF) bed vegetated with Phragmites in the third stage. The objective of this study was to evaluate the potential of these CW systems to remove organic matter from domestic wastewater on a pilot-scale three-stage SSF CW system. Comparisons of average influent and effluent concentrations showed that the multistage system could effectively reduce total suspended solids (TSS), biological oxygen demand (BOD) and chemical oxygen demand (COD) levels in effluent by as much as 98% and total organic carbon (TOC) by as much as 79%. Contributions of the first, second and third stages to the overall treatment were approximately 10%, 45% and 45%, respectively. The average TSS, COD, and TOC concentrations were reduced in the entire CW system by 70%, 80% and 90%, respectively. The BOD and TOC removal efficiencies displayed seasonal variations with average removals generally increasing in warmer seasons. Our results also demonstrate that there were strong correlations between removal efficiencies and loading rates. Average removals decreased with an increase in the hydraulic retention time (HRT). The rapid drainage and recycling operation increased the efficiency of BOD removal only.

A statistical analysis on the removal of organic matter in subsurface flow constructed wetlands in the U.K

The performances of 98 subsurface flow constructed wetlands in the U.K. are analysed for the rates of organic matter removal from various wastewaters. The analyses of 78 subsurface horizontal flow wetlands are focused on deriving the value of rate constant K(BOD) in the Kickuth equation and evaluating the probability of meeting the target when the equation is used in design. The analyses of 20 vertical flow wetlands are focused on establishing the correlations between BOD (biochemical oxygen demand) removal rate and organic loading; an empirical relation has been developed to estimate the surface areas of the wetlands.

The effect of pre-aeration on the purification processes in the long-term performance of a horizontal subsurface flow constructed wetland

Different conditions (water level, oxygen supply) prevailing in both beds of the Kodijärve double-bed horizontal subsurface flow (HSSF) constructed wetland (CW) (Southern Estonia; constructed in 1996, total area 312.5 m(2), 40 pe) provide the opportunity to compare how different operational methods have altered the efficiency of the purification processes inside the HSSF CW. In summer 2002 a vertical subsurface flow (VSSF) CW (total area 37.4 m(2)) was added as the first stage of the system. Data from 18 sampling wells installed in Kodijärve HSSF CW from two periods is compared: 1st period -- January 2000-April 2002 (before the VSSF CW was built); 2nd period --October 2002-December 2004 (after the construction of the VSSF filter). The VSSF CW has remarkably improved aerobic conditions in both beds of the HSSF. Apart from total phosphorus concentrations in the right bed and nitrate nitrogen concentrations in the outflow of both beds, all of the water quality indicators (dissolved oxygen, total suspended solids, biological oxygen demand, ammonia nitrogen, nitrite nitrogen, total nitrogen and total iron) improved after the construction of the VSSF filter. Typically, purification processes in the HSSF CW were dependent on oxygen supply, which was partly influenced by the water level inside the filter beds.

Completely autotrophic nitrogen-removal over nitrite in lab-scale constructed wetlands: evidence from a mass balance study

A mass-balance study was carried out to investigate the transformation of nitrogenous pollutants in vertical flow wetlands. Landfill leachate containing low BOD, but a high concentration of ammonia, was treated in four wetland columns under predominately aerobic conditions. Influent total nitrogen in the leachate consisted mainly of ammonia with less than 1% nitrate and nitrite, and negligible organic nitrogen. There was a substantial loss of total nitrogen (52%) in one column, whereas other columns exhibited zero to minor losses (<12%). Net nitrogen loss under study conditions was unexpected. Correlations between pH, nitrite and nitrate concentrations indicated the removal of nitrogen under study conditions did not follow the conventional, simplistic, chemistry of autotrophic nitrification. Through mass-balance analysis, it was found that CANON (Completely Autotrophic Nitrogen-removal Over Nitrite) was responsible for the transformation of nitrogen into gaseous form, thereby causing the loss of nitrogen mass. The results show that CANON can be native to aerobic engineered wetland systems treating wastewater that contains high ammonia and low BOD.

Enhanced removal of organic matter and ammoniacal-nitrogen in a column experiment of tidal flow constructed wetland system

A tidal flow constructed wetland system was investigated for the removal of organic matter and ammoniacal-nitrogen from diluted piggery wastewater. The results demonstrated that the operation of tidal flow enhanced the transfer of oxygen into wetland matrices. The supply of oxygen by the operation (473 gO2/m2d) matched the demand for wastewater treatment. The overall oxygen consumption rate in the system was considerably higher than the typical rate obtainable in conventional wetlands; most oxygen being used for the decomposition of organic matter. Compared with conventional systems, the tidal flow system demonstrated greater efficiency in the removal of organic matter. Significant nitrification did not take place, although 27-48% ammonia was removed from the wastewater. Immobilization by microbial cells and adsorption were the likely routes to remove ammonia under the specific experiment conditions. Percentage removals of BOD5, NH4-N and SS increased after effluent recirculation at a ratio of 1:1 was employed.