Statistical modelling of organic matter and emerging pollutants removal in constructed wetlands

Multiple regression models, clustering tree diagrams, regression trees (CHAID) and redundancy analysis (RDA) were applied to the study of the removal of organic matter and pharmaceuticals and personal care products (PPCPs) from urban wastewater by means of constructed wetlands (CWs). These four statistical analyses pointed out the importance of physico-chemical parameters, plant presence and chemical structure in the elimination of most pollutants. Temperature, pH values, dissolved oxygen concentration, redox potential and conductivity were related to the removal of the studied substances. Plant presence (Typha angustifolia and Phragmites australis) enhanced the removal of organic matter and some PPCPs. Multiple regression equations and CHAID trees provided numerical estimations of pollutant removal efficiencies in CWs. These models were validated and they could be a useful and interesting tool for the quick estimation of removal efficiencies in already working CWs and for the design of new systems which must fulfil certain quality requirements.     

Influence of vegetation and substrate on the removal and transformation of dissolved organic matter in horizontal subsurface-flow constructed wetlands

The fate of dissolved organic matter (DOM) during horizontal subsurface-flow constructed wetlands (HSSF CWs) was examined. In several studies it had been demonstrated that factors such as vegetation and substrates type affected the treatment efficiency of DOM, while very few studies discerned their influence on the transformations of DOM. Thus three pilot-scale HSSF CWs, i.e. reed (Phragmites australis)/gravel bed (W1), hybrid vegetation{cattail (Typha latifolia), bulrush (Scirpus validus), reed}/gravel bed (W2) and reed/hybrid substrates bed (gravel, zeolite, slag) (W3), were designed, and were operated continuously to investigate soluble COD (SCOD) removal and DOM transformations affected by vegetation and substrate type, and to explore the correlation between SCOD and biodiversity. The results showed that cattail and bulrush contributed to higher SCOD removal than common reed, and that gravel, zeolite and slag did not show significant influence on SCOD removal. The composition of the dissolved organic carbon (DOC) could undergo a considerable shift in composition due to metabolism and senescence from plant and microorganism. Nonlabile aromatic hydrocarbons and alkyl hydrocarbons in the effluent were a significant portion compared with labile alcoholic and alkene in the influent. It was also observed that the type of vegetation and substrate had great influence on the structure of bacteria, and the Shannon-Wiener Index increased linearly with the decrease of SCOD concentration along water flow in W2 and W3 (R2=0.96).     

人工湿地对猪场废水有机物处理效果的研究

分别以香根草 (Vetiveriazizanioides)和风车草 (Cyperusalternifolius)为植被 ,按 1.0m× 0 .5m×0 .8m建立人工湿地 ,通过 4季测试 ,研究其对猪场废水有机物的净化功能及其随季节、进水浓度及水力停留时间变化的规律 .结果表明 ,4个季节香根草或风车草人工湿地对COD和BOD有较稳定的去除效果 ,两湿地抗有机负荷冲击能力强 .在春季 ,停留时间 1～ 2d ,COD和BOD去除率分别为 70 %和 80 %;在夏季 ,进水COD高达 10 0 0～ 140 0mg·L-1情况下 ,COD去除率接近 90 %;在秋季 ,停留时间 1～ 2d ,COD和BOD去除率分别为 5 0 %～ 6 0 %和 5 0 %;在冬季 ,进水COD达 10 0 3mg·L-1情况下 ,COD去除率在 70 %以上 .COD、BOD和SS的去除率在两湿地间没有显著差异 .人工湿地污染物 (Y)随水力停留时间 (t)延长的降解遵从指数方程规律Yt=Y0 ·e( -kt) .在相同停留时间时 ,随进水污染物浓度 (x)提高的出水污染物浓度 (y)的回归关系遵从直线方程规律 y =a+bx .     

Chromium removal in constructed wetlands: A review

The distribution, mobility and availability of metals in the environment depend not only on their total concentration but also on their formations and bounds with the soil. Hexavalent chromium is a very toxic, metal compound, frequently found in polluted industrial wastewaters, and causes serious environmental problems. The potential application of constructed wetlands in the treatment of chromium bearing wastewaters has been reported recently. This paper reviews research on constructed wetlands treating chromium polluted wastewaters, and focuses on several design and operational parameters. The review highlights the effect of vegetation type, hydraulic residence time and porous media type on wetland performance. Constructed wetlands have been proved to be rather efficient at treating chromium containing wastewaters.     

Linkages between organic matter mineralization and denitrification in eight riparian wetlands

Denitrification (N₂ production) and oxygen consumption rates were measured at ambient field nitrate concentrations during summer in sediments from eight wetlands (mixed hardwood swamps, cedar swamps, heath dominated shrub wetland, herbaceous peatland, and a wetland lacking live vegetation) and two streams. The study sites included wetlands in undisturbed watersheds and in watersheds with considerable agricultural and/or sewage treatment effluent input. Denitrification rates measured in intact cores of water-saturated sediment ranged from 20 to 260 mol N m^-2 h^-1 among the three undisturbed wetlands and were less variable (180 to 260 mol N M^-2 h^-1) among the four disturbed wetlands. Denitrification rates increased when nitrate concentrations in the overlying water were increased experimentally (1 up to 770 M), indicating that nitrate was an important factor controlling denitrification rates. However, rates of nitrate uptake from the overlying water were not a good predictor of denitrification rates because nitrification in the sediments also supplied nitrate for denitrification. Regardless of the dominant vegetation, pH, or degree of disturbance, denitrification rates were best correlated with sediment oxygen consumption rates (r ² = 0.912) indicating a relationship between denitrification and organic matter mineralization and/or sediment nitrification rates. Rates of denitrification in the wetland sediments were similar to those in adjacent stream sediments. Rates of denitrification in these wetlands were within the range of rates previously reported for water-saturated wetland sediments and flooded soils using whole core¹⁵N techniques that quantify coupled nitrification/denitrification, and were higher than rates reported from aerobic (non-saturated) wetland sediments using acetylene block methods.     

风车草和香根草在人工湿地中迁移养分能力的比较研究

为研究风车草（Cyperus alternifolius）和香根草（Vetiveria zizanioides）迁移养分的能力,建立17.0 m²风车草潜流式人工湿地和13.3 m²香根草潜流式人工湿地处理猪场废水,在四个季节末测定植物生物量和组织氮、磷、铜、锌含量.结果表明,香根草地下部生物量大于风车草,地上部生物量则是风车草大于香根草.风车草年地上部收获量为3406.47 g·m^-2,比香根草的1483.88 g·m^-2高2.3倍;风车草的氮含量为22.69 mg·g^-1,比香根草的15.44 mg·g^-1高7.25 mg·g^-1;风车草的磷含量为6.09 mg·g^-1,比香根草的5.47 mg·g^-1高0.62 mg·g^-1.植株含铜、锌量风车草略比香根草高.风车草每年迁移N 68.72 g·m^-2和P 18.49 g·m^-2,香根草迁移N 8.93 g·m^-2和P 3.69 g·m^-2.风车草人工湿地每年由植物迁移的氮、磷、铜、锌比香根草高4～7倍.     

Evaluation of relative importance of different microbial reactions on organic matter removal in horizontal subsurface-flow constructed wetlands using a 2D simulation model

In this study, we used a two-dimensional (2D) mechanistic mathematical model in order to evaluate the relative contribution of different microbial reactions to organic matter removal (in terms of COD) in horizontal subsurface-flow constructed wetlands that treated urban wastewater. We also used the model to analyse the effect of increasing or decreasing the organic loading rate (changing the hydraulic loading rate (HLR) at a constant influent organic matter concentration, or changing the organic matter concentration at a constant HLR) on both the removal efficiency and the relative importance of the microbial reactions. The model is based on the code RetrasoCodeBright, which we modified to include the main microbial processes related to organic matter and nitrogen transformations in the wetlands: hydrolysis, aerobic respiration, nitrification, denitrification, sulphate reduction and methanogenesis. The model was calibrated and validated with data from two wetlands (each with a surface area of 55 m²) located in a pilot plant near Barcelona (Spain). According to the simulations, anaerobic processes (methanogenesis and sulphate reduction) are more widespread in the wetlands and contribute to a higher COD removal rate (60–70%) than anoxic (denitrification) and aerobic reactions do. These model results are confirmed by experimental observations. In all the cases tested, the reaction that most contributed to COD removal was methanogenesis (33–52%). According to our simulations, decreasing the HLR (for example, from 40 to 25 mm/d) while maintaining a constant COD influent concentration has a clear positive impact on COD removal efficiency (which increases from 65% to 89%). Changing influent COD concentration (for example, from 290 to 190 mg/L) while maintaining a constant HLR has a smaller impact, causing efficiency to increase from 79% to 84%. Changes in influent COD concentration (at a constant HLR) affect the relative contribution of the microbial reactions to organic matter removal. However, this trend is not seen when the HLR changes and the COD influent concentration remains constant.     

Processes impacting on benzene removal in vertical-flow constructed wetlands

The overall aim of this research project was to reduce low molecular weight hydrocarbons such as benzene in produced wastewaters. Over 30 months of research was conducted to test the treatment performance in terms of benzene removal in vertical-flow constructed wetlands. Based on an influent concentration of 1 g L(-1) benzene, the results show mean benzene removal efficiencies between 88.71% and 89.77%, and 72.66% and 80.46% for indoor and outdoor constructed wetlands, respectively. A statistical analysis indicated that the five days at 20 degrees C N-allylthiourea biochemical oxygen demand (BOD(5)), chemical oxygen demand (COD), nitrate-nitrogen (NO(3)-N), dissolved oxygen (DO) and electric conductivity (EC) values of the effluent were positively correlated with the effluent benzene concentrations following the order COD>DO>EC>NO(3)-N>BOD(5), and negatively correlated according to the order pH>redox potential (redox)>temperature (T)>turbidity. No strong relationships between benzene and the variables ortho-phosphate-phosphorus (PO(4)(3-)) and ammonia-nitrogen (NH(4)-N) were recorded.     

人工湿地处理炼油废水的生态效益研究

来自茂名石化公司的炼油废水含有较高浓度的污染物 ,超过广东省规定的排放标准 ,未经处理不能直接排放。 4种草本植物 ,香根草、芦苇、宽叶香蒲和蒲草被用来测试建植人工湿地的效果以及所建植的人工湿地处理炼油废水的效果。在为期 2个月的处理过程中 ,人工湿地在前期对炼油废水的净化效率很高 ,它们对第一批高浓度废水中氨氮、COD、BOD和油的去除率分别是 97.7%、78.2 %、91 .4%和 95 .3% ,对第一批低浓度污水分别是 97.1 %、71 .5 %、73.7%和 89.8%。但是 ,随着时间的推移 ,湿地的净化效果会有一定程度的下降 ,然后逐渐趋于稳定。湿地对氨氮、COD、BOD和油类的去除效率始终表现为氨氮 >油类 >BOD>COD,但植物对它们的净去除量却是 COD>BOD>氨氮和油类。湿地建植之初 ,植物的净化功能很弱 ,但随植物的生长和生物量的增加而逐渐增强。然而 ,不同植物种对废水的净化率很接近 ,基本上无显著性差异。被测试的 4种植物在污水湿地中的生长表现都好过在清水湿地中的 ,但香根草、芦苇和宽叶香蒲在高浓度废水的分蘖数比在低浓度的废水少些 ,而蒲草相反 ,表明高浓度污水相对于低浓度污水而言已经对前面 3个种产生了伤害 ,却仍促进蒲草生长。在清水培养阶段 ,香根草产生分蘖的速度是 4个种中最低的 ;进入污水培养阶     

Plant photosynthesis and its influence on removal efficiencies in constructed wetlands

The photosynthetic characteristics of several wetland plants and their influence on oxygen-evolving activities and disposal efficiencies of horizontal flow subsurface constructed wetlands were compared. The results indicated that the photosynthetic rate of wetlands plants was highly correlated with light intensity and temperature. The photosynthetic characteristics of wetlands species can affect their ability to provide oxygen, and this ultimately influences their disposal efficiencies. Observations indicated that the ability of wetland plants to provide oxygen and remove pollutants decreases in the following order: Phragmites > Canna > Camellia > Dracaena.     

Metal removal by sulphate-reducing bacteria from natural and constructed wetlands

The use of wetlands is a promising technology to treat acid mine drainage, yet there is little understanding of the fundamental biological processes involved. They are considered to centre on the complex anaerobic ecology within sediments and involve the removal of metals by sulphate-reducing bacteria (SRB). These bacteria generate hydrogen sulphide and cause precipitation of metals from solution as the insoluble metal sulphide. Sulphate-reducing bacteria have been isolated from natural and constructed wetlands receiving acid mine drainage. Sulphide production by isolates and removal of the metals iron, manganese and zinc were measured, as well as utilization of a range of carbon sources. Marked ecological differences between the wetlands were reflected in population composition of SRB enrichments, and these consortia displayed significant differences in sulphide generation and rates of metal removal from solution. Rates of metal removal did not correlate with sulphide generation in all cultures, suggesting the involvement of other biological mechanisms of metal removal. Differences in substrate utilization have highlighted the need for further investigation of carbon flow and potential carbon sources within constructed wetlands.     

A comparison of bacterial removal efficiencies in constructed wetlands and algae-based systems

Constructed wetlands and algae-based systems have been compared regarding their efficiencies on faecal bacteria removal. Two types of constructed wetlands, sub-surface (SSF) and free water surface (FWS) flow systems, and two more types of algae-based systems, high rate algae ponds (HRAP) and maturation pond (MP) have been studied for two years. All systems treated the same wastewater from a rural locality in León (northwest of Spain). Hydraulic retention time was 3 days for both wetland systems, 20 days for the maturation pond and 10 days for the high rate algae pond. Total coliforms, faecal coliforms, faecal Streptococci, Clostridium perfringens, and Staphylococci were analyzed in the influent and effluents of each system. A comparison among the wetland systems showed that SSF were more efficient than FWS system when considering surface removal rates (cfu removed/m²/d). Nevertheless, differences were not statistically significant. Considering mean removal efficiencies (in log unit), results showed that higher reductions were observed in FWS for most of the groups except for clostridia and Staphylococci. Concerning algae-based systems, MP showed higher removal efficiencies than HRAP, getting higher surface removal rates in the HRAP. Generally constructed wetlands were more efficient than algae-based systems when considering both, efficiencies in % and surface removal rates.     

模拟人工湿地中植物多样性配置对硝态氮去除的影响

为检验植物多样性对人工湿地脱氮功能的影响,在模拟人工湿地试验系统中设置了植物单种和混种处理并定期供给氮形态仅为硝态氮的模拟污水。结果表明:混种系统的出水硝态氮浓度显著低于单种(P<0.05);混种与单种系统在基质氮含量和植物氮积累量上无统计差异;质量平衡分析表明混种促进系统反硝化强度;菩提子单种系统中的硝态氮移除能力显著高于香蒲、芦苇和菖蒲单种系统,后3种硝态氮移除能力则无显著差异。本研究可为人工湿地选择高效物种、多样性配置以提高氮去除率提供依据。     

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.     

Modelling pollutant removal in a pilot-scale two-stage subsurface flow constructed wetlands

The hydraulic behaviour and effluent pollutant concentrations in a pilot-scale two-stage subsurface flow constructed wetland for treatment of municipal wastewater have been simulated. The experimental pilot plant is located in San Michele di Ganzaria (Eastern Sicily) and consists of four lines of two-stage subsurface flow constructed wetlands for secondary or tertiary treatment of municipal wastewater. The first stage, for each line, consists of a horizontal flow bed, while in the second stage a vertical flow bed operates for two lines and a horizontal flow bed for the other two. Phragmites sp. was used as vegetation in two lines while the other two lines are without plants. The HYDRUS-2D software was applied to describe flow and single-solute transport, while the multi-component reactive transport module CW2D was used to model the transformation and elimination processes of organic matter, nitrogen and phosphorus. Tracer studies and chemical wastewater analyses were carried out to calibrate and validate the transport model. In general, the simulation results obtained show a good match with the measured data for water flow, tracer experiments and pollutant removal processes.     

Polishing domestic wastewater on a subsurface flow constructed wetland: organic matter removal and microbial monitoring

Microbial monitoring of constructed wetlands (CWs) treating domestic wastewater is generally scarce, despite the need of more knowledge about its biocenosis. The sanitation quality of a wastewater treated in a CW is a crucial aspect, mainly when the receiving water body is used as a swimming and/or recreation area. The present study was carried out in a horizontal subsurface flow CWplanted with Phragmites australis receiving pre-treated domestic wastewater (mean flow 50 m3 day(-1)), from a population of about 300 inhabitants. The monitoring programme undertaken during the first year operation, revealed removal efficiencies of 61% BOD5, 44% COD, and 65% TSS for inlet water with ca. 90 mg L(-1) BOD5, 157 mg L(-1) COD, and 17 mg L(-1) TSS. Total Coliform (TC) and Faecal Coliform (FC) bacteria were removed from wastewater (mean inlet values of 5 x 10(6) CFU 100 mL(-1) TC and of 9 x 10(5) CFU 100 mL(-1) FC), with efficiencies of 92 and 97%, respectively. The dynamics of microbial communities established in the system assessed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), had revealed a high bacterial diversity within the system, with no relevant differences in composition at the CW inlet and outlet but exhibiting temporal differences in bacterial communities.     

风车草和香根草在人工湿地中迁移养分能力的比较研究

为研究风车草(Cyperus alternifolius)和香根草(Vetiveria zizanioides)迁移养分的能力,建立17.0m²风车草潜流式人工湿地和13.3 m²香根草潜流式人工湿地处理猪场废水,在四个季节末测定植物生物量和组织氮、磷、铜、锌含量.结果表明,香根草地下部生物量大于风车草,地上部生物量则是风车草大于香根草.风车草年地上部收获量为3406.47 g·m^-2,比香根草的1483.88 g·m^-2高2.3倍;风车草的氮含量为22.69 mg·g^-1,比香根草的15.44 mg·g^-1高7.25 mg·g^-1;风车草的磷含量为6.09 mg·g^-1,比香根草的5.47 mg·g^-1高0.62 mg·g^-1.植株含铜、锌量风车草略比香根草高.风车草每年迁移N 68.72 g·m^-2和P18.49 g·m^-2,香根草迁移N 8.93 g·m^-2和P 3.69 g·m^-2.风车草人工湿地每年由植物迁移的氮、磷、铜、锌比香根草高4～7倍.     

人工湿地去污模型的统一结构特征

通过对典型人工湿地去污模型的分析比较,提出了基于各模型微分方程而建立的统一去污模型。该模型能够将典型去污模型作为特例而导出,并能很好地解释这些模型之间的过渡关系。以潜流湿地中NH4^＋和BOD5的降解为例,对统一去污模型的应用情况进行了简单探讨,表明利用统一去污模型结构有助于深入揭示去污机理,从而提出更为精确的去污模型。