Horizontal Subsurface Flow Wetland Research Articles

Performance evaluation of different horizontal subsurface flow wetland types by characterization of flow behavior, mass removal and depth-dependent contaminant load

For several pilot-scale constructed wetlands (CWs: a planted and unplanted gravel filter) and a hydroponic plant root mat (operating at two water levels), used for treating groundwater contaminated with BTEX, the fuel additive MTBE and ammonium, the hydrodynamic behavior was evaluated by means of temporal moment analysis of outlet tracer breakthrough curves (BTCs): hydraulic indices were related to contaminant mass removal. Detailed investigation of flow within the model gravel CWs allowed estimation of local flow rates and contaminant loads within the CWs. Best hydraulics were observed for the planted gravel filter (number of continuously stirred tank reactors N = 11.3, dispersion number = 0.04, Péclet number = 23). The hydroponic plant root mat revealed lower N and pronounced dispersion tendencies, whereby an elevated water table considerably impaired flow characteristics and treatment efficiencies. Highest mass removals were achieved by the plant root mat at low level: 98% (544 mg m−2 d−1), 78% (54 mg m−2 d−1) and 74% (893 mg m−2 d−1) for benzene, MTBE and ammonium–nitrogen, respectively. Within the CWs the flow behavior was depth-dependent, with the planting and the position of the outlet tube being key factors resulting in elevated flow rate and contaminant flux immediately below the densely rooted porous media zone in the planted CW, and fast bottom flow in the unplanted reference.

Variation of dissolved oxygen and redox potential and their correlation with microbial population along a novel horizontal subsurface flow wetland

Aims: This study was conducted to evaluate the performance of a novel horizontal subsurface flow wetland (HSFW) in naturally improving the dissolved oxygen (DO) and the impact on redox condition, microbial activity and the nitrogen removal in the HSFW bed. Materials and methods: The HSFW, equipped with cascaded natural aeration ditches (NADs), was the second stage of a hybrid constructed wetland (CW) after vertical-baffled flow wetland beds. The performances of the HSFW for organics and nitrogen removal in a full-scale hybrid CW system treating municipal wastewater for more than three years have been analysed. The spatial distributions of the oxidation-reduction potential (ORP), DO, microbial population density and specific oxygen uptake rate were determined, and their correlations were analysed in one selected section of the HSFW bed. Results: A 7-m-long shallow NAD increased the DO concentration from 0.28 mg O2 l−1 to 3.80 mg O2 l−1 and the ORP from+37.3 mV to+247.7 mV, creating an aerobic zone with a hydraulic retention time (HRT) of 0.5 h and an anoxic zone of another 0.5 h in series in the subsequent wetland bed. For the whole HSFW with three NADs, the macro aerobic and anoxic environment with a total HRT of 3 h can be created. Conclusions: The unique DO distribution in HSFW may contribute to an optimum environment for partial nitrification and anammox, and obtain a high performance for nitrogen removal. Correlation analysis showed that the microbial activity in the HSFW relied obviously on the redox condition.

Comparison of the removal of chlorpyrifos and dissolved organic carbon in horizontal sub-surface and surface flow wetlands

Chlorpyrifos is an organophosphorus pesticide widely used in Colombia for agricultural and domestic pest control. It is also commonly found in water sources in rural areas. Constructed wetlands are being used as viable alternatives for the treatment of domestic wastewater with large organic loads. For this research, three pilot-scale subsurface horizontal flow constructed wetlands and three horizontal surface flow wetlands were used for the treatment of synthetic wastewater with different concentrations of chlorpyrifos (0.0μgL−1, 478μgL−1, 589μgL−1 and 788μgL−1), 100mgL−1 of dissolved organic carbon and nutrients. The wetlands were constructed in equal dimensions and in the same size range as the gravel bed (3.18–6.35mm) and planted with Phragmites australis. The efficiencies of the removal of the pesticide and dissolved organic carbon were then determined. Additionally, other physicochemical parameters, as well as 3,5,6-trichloro-2-pyridinol, the main breakdown product of chlorpyrifos, were measured. The average removals of this agrochemical and dissolved organic carbon in the subsurface horizontal flow constructed wetlands were 93% and 92%, respectively, and in the horizontal surface flow wetlands, the average removal was 95% for both compounds. The removal is the result of the joint action of microorganisms and the adsorption and absorption of roots and rhizomes of plants found in wetlands. Both types of wetlands are very efficient at treating the domestic wastewater contaminated with pesticides and dissolved organic carbon, although the results were slightly better in the surface flow wetlands.

Evaluation of the giant reed (Arundo donax) in horizontal subsurface flow wetlands for the treatment of dairy processing factory wastewater

Two emergent macrophytes, Arundo donax and Phragmites australis, were established in experimental horizontal subsurface flow (HSSF), gravel-based constructed wetlands (CWs) and challenged by treated dairy processing factory wastewater with a median electrical conductivity of 8.9 mS cm(-1). The hydraulic loading rate was tested at 3.75 cm day(-1). In general, the plants grew well during the 7-month study period, with no obvious signs of salt stress. The major water quality parameters monitored (biological oxygen demand (BOD), suspended solids (SS) and total nitrogen (TN) but not total phosphorus) were generally improved after the effluent had passed through the CWs. There was no significance different in removal efficiencies between the planted beds and unplanted gravel beds (p > 0.007), nor was there any significant difference in removal efficiencies between the A. donax and P. australis beds for most parameters. BOD, SS and TN removal in the A. donax and P. australis CWs was 69, 95 and 26 % and 62, 97 and 26 %, respectively. Bacterial removal was observed but only to levels that would allow reuse of the effluent for use on non-food crops under Victorian state regulations. As expected, the A. donax CWs produced considerably more biomass (37 ± 7.2 kg wet weight) than the P. australis CWs (11 ± 1.4 kg wet weight). This standing crop equates to approximately 179 and 68 tonnes ha(-1) year(-1) biomass (dry weight) for A. donax and P. australis, respectively (assuming a 250-day growing season and single-cut harvest). The performance similarity of the A. donax and P. australis planted CWs indicates that either may be used in HSSF wetlands treating dairy factory wastewater, although the planting of A. donax provides additional opportunities for secondary income streams through utilisation of the biomass produced.

Experimental study of a novel hybrid constructed wetland for water reuse and its application in Southern China

A new type of hybrid constructed wetland (CW), consisting of both vertical-baffled flow wetland (VBFW) and horizontal subsurface flow wetland (HSFW), has been deployed in Southern China to naturally accelerate the removal of organic matter and nitrogen. The hybrid CW system is characterised by a combination of continuous baffled flow vertical wetland and 'S' pattern horizontal subsurface flow wetland with natural aeration ditches to increase the concentration of dissolved oxygen in the HSFW bed. An internal circulatory system from the HSFW effluent back to the VBFW may optionally be operated to enhance the biological denitrification effect. Cyperus alternifolius is the main macrophyte in the wetland bed. The performance of the hybrid CW was studied with a pilot-scale system and three full-scale systems for municipal sewage treatment in Southern China. The results suggest that this new hybrid CW can achieve removal efficiencies of chemical oxygen demand, suspended solids, ammonia nitrogen, total nitrogen, and total phosphorus of better than 83.6, 95.0, 71.7, 64.5 and 68.1% respectively, with a specific wetland bed area of 0.70-0.93 m(2) PE(-1). The mean effluent concentrations of these parameters would meet the regulatory discharge limits for wastewater treatment systems (GB18918, 2002) and reuse in the context of agricultural irrigation solutions in China.

Evaluation of the giant reed (Arundo donax) in horizontal subsurface flow wetlands for the treatment of recirculating aquaculture system effluent

Two emergent macrophytes, Arundo donax and Phragmites australis, were established in experimental subsurface flow, gravel-based constructed wetlands (CWs) receiving untreated recirculating aquaculture system wastewater. The hydraulic loading rate was 3.75 cm day(-1). Many of the monitored water quality parameters (biological oxygen demand [BOD], total suspended solids [TSS], total phosphorus [TP], total nitrogen [TN], total ammoniacal nitrogen [TAN], nitrate nitrogen [NO(3)], and Escherichia coli) were removed efficiently by the CWs, to the extent that the CW effluent was suitable for use on human food crops grown for raw produce consumption under Victorian state regulations and also suitable for reuse within aquaculture systems. The BOD, TSS, TP, TN, TAN, and E. coli removal in the A. donax and P. australis beds was 94%, 67%, 96%, 97%, 99.6%, and effectively 100% and 95%, 87%, 95%, 98%, 99.7%, and effectively 100%, respectively, with no significant difference (p > 0.007) in performance between the A. donax and P. australis CWs. In this study, as expected, the aboveground yield of A. donax top growth (stems + leaves) (15.0 ± 3.4 kg wet weight) was considerably more than the P. australis beds (7.4 ± 2.8 kg wet weight). The standing crop produced in this short (14-week) trial equates to an estimated 125 and 77 t ha(-1) year(-1) biomass (dry weight) for A. donax and P. australis, respectively (assuming that plant growth is similar across a 250-day (September-April) growing season and a single-cut, annual harvest). The similarity of the performance of the A. donax- and P. australis-planted beds indicates that either may be used in horizontal subsurface flow wetlands treating aquaculture wastewater, although the planting of A. donax provides additional opportunities for secondary income streams through utilization of the energy-rich biomass produced.

Nutrient removal from on-site wastewater in horizontal subsurface flow constructed wetlands in Ireland

Two horizontal subsurface flow wetlands were constructed on separate sites in Ireland - one to provide secondary treatment and the other to provide a tertiary treatment for single house domestic effluent. A comprehensive analysis over three years provided a robust characterisation of the internal dynamics of the systems with respect to N and P removal. The removal of Total N was only 29% and 30% in the secondary and tertiary treatment wetlands respectively and revealed a drop off in performance over time with a higher release of org-N during summer periods. 15N stable isotope studies confirmed that 35% of the ammonium from the septic tank was passing straight through the process without taking part in any biogeochemical processes. However, the study showed that influent N in both wetlands was being biologically assimilated into organic nitrogen (biomass or plants) and then released again as soluble ammonium – so-called nitrogen “spiraling”. Removal of Total P in the wetlands averaged 45% and 28% respectively. The results also showed that if the annual above ground stem matter was completely removed it would only account for 8.4% and 1.3% of the annual total P-load to the respective wetlands. Accordingly the effluent PO4-P concentrations were still found to be >5 mg/l on average.

Removal Efficiency of Horizontal Subsurface Flow Wetlands for Veterinary Pharmaceuticals

This study was undertaken to assess the removal of commonly used antibiotics on poultry and livestock farms, i.e., monensin, salinomycin, and narasin, in a horizontal subsurface flow (HSSF) treatment wetland. The tested wetland supported a stand of reed canary grass (Phalaris arundinaceae L.) and cattails (Typha latifolia L.) on sandy soil. Of the three compounds evaluated, monensin was found to be the most mobile, and the wetland exhibited an average removal efficiency of 40%. The removal rates for both salinomycin and narasin approached 50%. The effect of the applied concentration of antibiotics on wetland removal efficiency was found to be highly significant (p < 0.0001) only at the first two concentration levels (10 and 50 g L-1) versus the two higher levels (0.5 and 1.0 mg L-1). Even though raw data on temperature, dissolved oxygen (DO), oxidation reduction potential (ORP), and pH suggested a microbial contribution to overall antibiotics removal, the effect of these parameters was found to be non-significant (p < 0.05). However, the removal trends were found to be in line with our previously reported laboratory work on biodegradation of these pharmaceuticals.

Septic Tank Effluent Treatment by Subsurface Horizontal Flow Wetland

Domestic wastewater treatment in Indonesia, mostly treat by using septic tanks. The use of septic tanks preferred by the public because of the design, construction, and maintenance are easy and more economical for the system of domestic wastewater disposal. Effluent from the septic tank still contains BOD about 100 mg /l, COD about 250 mg/l and TSS about 100 mg /l. This effluent is still dangerous to public health and is not fulfill wastewater effluent standards yet. If there is open space available, extended treatment of septic tank effluent can be done by field infiltration. This open space must suitable for infiltration without contaminating the ground water or well water. In urban areas with ground water level is high such as Surabaya, water infiltrate into the ground causing the problem. Problems that arise are due to pollution of ground water and surface water contaminated by effluent from the septic tank. Because of these problems, it would require an alternative treatment of septic tank effluent. Appropriate technology would be a simple, economical, easy in operation and maintenance. One of wastewater processing that can be applied to the septic tank effluent is a constructed wetland system. Previous research carried out by using Cyperus papyrus plant with sand media in batch experiments showed satisfactory results. In this study, experiments conducted by using Cyperus papyrus plant with sand media with continuous flow in a horizontal subsurface flow wetland method. Wastewater flows with varying flow 90 ml /min and 180 ml /min. The objective of this research is to determine the ability of a horizontal subsurface flow wetland for treating septic tank effluent. The parameters analyzed were BOD and TSS. The results show the relation of organic loading rate with BOD removal efficiency. The highest organic loading rate of 0.051 kg BOD/m2.d on first day observations has BOD removal efficiency of 73.63% and the lowest organic loading rate of 0.029 kg BOD/m2.d with an efficiency of 70.82%. This shows that BOD removal efficiency is affected by organic loading rate. If BOD loading rate increase, it will decrease BOD removal efficiency. Organic loading rate recommended for domestic wastewater is 0.037-0.4 kg BOD5 /m2.d. Although in this study the organic loading rate exceeds the above recommendations, but efficiency is still good, above 70%.

Effects of intermittent loading on nitrogen removal in horizontal subsurface flow wetlands

Removal of CBOD(5) and nitrogen from septic tank effluent was evaluated in four horizontal subsurface flow (HSSF) wetlands. An intermittently loaded cell was compared to a continuously loaded control cell, with both treatments receiving the same weekly volume. The intermittent cell was rapidly drained and "rested" for 24-hr, then refilled in steps, twice weekly. Two media with different particle sizes but similar porosities were also compared. The two media, light weight expanded shale and gravel, were both continuously loaded. As hypothesized, the wetland cell that was intermittently loaded had higher dissolved oxygen, greater ammonia removal, and greater nitrate production than the continuously loaded cells. Areal NH(3)-N removal for the intermittently loaded cell was 0.90 g m(-2) d(-1) compared to 0.47 g m(-2) d(-1) for the control. Ammonia removal was also higher in continuously loaded gravel cells than in cells with expanded shale. Ammonia-N removal was an order of magnitude lower in a similar SSF wetland that had been in operation for 3 years. However, CBOD(5), total suspended solids, and total nitrogen did not vary substantially among the treatments.

Behavior and characteristics of DOM during a laboratory-scale horizontal subsurface flow wetland treatment: Effect of DOM derived from leaves and roots

In order to study the effect of dissolved organic matter (DOM) from the leaves and roots of wetland plants on the dissolved organic carbon (DOC) reduction during a laboratory-scale horizontal subsurface flow constructed wetland (HSFW) treatment, excitation–emission matrixces (EEMs) analysis was conducted as a tracer in an effort to understand the biological or physicochemical functioning of this complex environment. Using XAD-8/XAD-4 resins, the DOM in the secondary effluent, HSFW effluent, and the DOM derived from the leaves and roots (DOM L&R) were fractionated into five classes: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N) and hydrophilic fraction (HPI). DOC was removed by 37.2% during the HSFW treatment. On the other hand, hydrophobic/hydrophilic distribution of the HSFW effluent DOM changed significantly, with the effluent DOC becoming more hydrophobic due to a preferential removal of HPI (with a reduction of 64.8%). Furthermore, the trihalomethane formation potential (THMFP) reduction of the HSFW system was quite low (20.2%), partially being ascribed to the influence of the relatively higher specific THMFP (STHMFP) of the DOM L&R fractions (especially for HPO-A). The soluble microbial by-products (SMPs) and aromatic protein-like materials (polyphenols) from the leaves and roots could be the main reason for the increased STHMFP of the HSFW effluent DOM fractions. In addition, the peak of Region II ( λ ex/ λ em = (220–260)/(332–380)) might serve as a proxy for monitoring the DOM L&R (polyphenols) in the effluent from a matured wetland.

Phosphorus Removal Performance and Mechanisms of a Constructed Horizontal Subsurface Flow Wetlands Treating Reclaimed Water

Phosphorus (P) removal efficiency was compared between two types of wetlands: a blast furnace slag (BFS)-based constructed wetland (SCW) and a gravel-based constructed wetland (GCW). Both wetlands were 2.4 m long, 0.4 m wide, and 0.65 m high. Synthetic water to be reclaimed contained 1.1 mg·L−1 P. Hydraulic retention time (HRT) was set at 1 day in the first four phases and then increased to 2 days in the last two phases. Results showed that P removal with 1 day HRT application decreased from 80.4 to 56.4% in SCW and from 73.0 to 61.6% in GCW. With the increase of HRT, P removal decreased continually to 54.6% in SCW while it increased to 88.6% in GCW. The lower amount of P removed by SCW in the latter phases was mainly the result of substrates. P adsorption capacities did not show much difference between BFS and gravel in the batch experiment with a low initial P concentration less than 1.4 mg·L−1. However, chemical properties of substrates presented an obvious impact on P removal in the pilot experiment. Scanning electron microscopy (SEM) and electron dispersive spectrometry (EDS) analyses revealed that slag surfaces contained 1.07% P, which precipitated with CaCO3, whereas gravel surfaces contained 2.12% P, which was bound to Fe and Al oxides. P removal in SCW decreased synchronously with the pH decrease of 9.82 to 9.04. Decrease of pH and low P concentration was the basic reason for the formation of CaCO3, which led to the poor P removal. P accumulated by plants also influenced the P removal in SCW and GCW, which accounted for 9.63 and 16.17%, respectively. Besides, phosphatase activity in the SCW increased in the last two phases, which brought a large P release. This article shows that P removal by substrate varied with the initial P concentration and was influenced largely by pH in the wetlands. BFS used in this experiment is not a good substrate candidate for wetlands to treat reclaimed water.

Reduction of trihalomethane precursors of dissolved organic matter in the secondary effluent by advanced treatment processes

Wastewater effluent collected from the Wenchang Wastewater Treatment Plant (Harbin, China) was used as source water for advanced treatment and reclamation. Since dissolved organic matter (DOM) in the secondary effluent contains a high concentration of trihalomethanes (THMs) precursors, several processes of advanced treatments including granular activated carbon (GAC) adsorption, sand column biodegradation, horizontal subsurface flow wetland (HSFW) treatment, laboratory-scale soil aquifer treatment (SAT) and GAC + SAT were used in this study to compare and differentiate the removal mechanisms of DOM. DOM in the secondary effluent and the treated effluents was fractionated into five classes using XAD resins: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N), and hydrophilic fraction (HPI). Results showed that HPO-A and HPI were two main fractions of the DOM in the secondary effluent, accounting for 30.0% and 45.5% of the bulk DOM, respectively. HPO-A exhibited higher trihalomethane formation potential (THMFP) and specific THMFP (STHMFP) than HPI during the chlorination process. The order of the dissolved organic carbon (DOC) removal with respect to different advanced treatments was observed to be GAC + SAT > SAT > GAC > sand column > HSFW. As for the DOM removal mechanisms, the advanced treatment processes of GAC adsorption, SAT and GAC + SAT tended to adsorb more HPO-A, HPO-N and TPI-A and could reduce the aromaticity of those DOM fractions efficiently. Correspondingly, the advanced treatment processes of sand column, SAT, HSFW and GAC + SAT removed more HPI and TPI-N through biodegradation and each of the DOM fractions had an increased aromaticity. The removal order of the THMs precursor by the advanced treatment processes was GAC + SAT > GAC > SAT> sand column > HSFW. The adsorption reduced the STHMFP of the DOM fractions effectively, whereas the biodegradation mechanism of the treatments (sand column, SAT, GAC + SAT and HSFW) showed a converse trend. Moreover, the THMFP and STHMFP of the DOM in the HSFW effluent were obviously affected by the DOM derived from the leaves and roots.

Comparison of free water and horizontal subsurface treatment wetlands

The two most prevalent types of treatment wetland, especially during the early history of the technology, are free water surface (FWS) and horizontal subsurface flow (HSSF) wetlands. The several factors involved in the choice of which alternative to choose include size, cost, operability, together with health and nuisance issues and ancillary benefits. Contaminant removal performance differs by constituent, with the advantage to FWS for moderate to high biochemical oxygen demand (BOD), TSS, ammonia, total nitrogen and phosphorus. HSSF are more effective for tertiary BOD levels, nitrate and pathogens. Superpositions of the loading data show that the respective data clouds overlap virtually entirely for HSSF and FWS wetlands. There is little or no performance difference when they are compared on this areal basis. In general, there is little or no advantage of HSSF for space saving. In cold climates, HSSF systems are less cold sensitive, and easier to insulate for winter operation. The use of winter storage enables FWS to be used in freezing conditions, but the cost makes that option comparable to the more expensive HSSF. In general, economics do not favor the choice of HSSF wetlands. Factors other than reduction performance are also important in the selection process. Other principal reasons for selecting the HSSF option over the FWS option are prevention of human health contact problems, mosquito control and minimization of wildlife interactions.

Pilot-scale comparison of constructed wetlands operated under high hydraulic loading rates and attached biofilm reactors for domestic wastewater treatment

Four different pilot-scale treatment units were constructed to compare the feasibility of treating domestic wastewater in the City of Heraklio, Crete, Greece: (a) a free water surface (FWS) wetland system, (b) a horizontal subsurface flow (HSF) wetland system, (c) a rotating biological contactor (RBC), and (d) a packed bed filter (PBF). All units operated in parallel at various hydraulic loading rates (HLR) ranging from 50% to 175% of designed operating HLR. The study was conducted during an 8 month period and showed that COD removal efficiency of HSF was comparable (> 75%) to that of RBC and PBF, whereas that of the FWS system was only 57%. Average nutrient removal efficiencies for FWS, HSF, RBC and PBF were 6%, 21%, 40% and 43%, respectively for total nitrogen and 21%, 39%, 41% and 42%, respectively for total phosphorus. Removals of total coliforms were lowest in FWS and PBF (1.3 log units) and higher in HSF and RBC (2.3 to 2.6 log units). HSF showed slightly lower but comparable effluent quality to that of RBC and PBF systems, but the construction cost and energy requirements for this system are significantly lower. Overall the final decision for the best non-conventional wastewater treatment system depends on the construction and operation cost, the area demand and the required quality of effluent.

Wastewater Treatment Using Horizontal Subsurface Flow Constructed Wetland

The last few decades witnessed sharp focus on environment pollution and its impact on life in nature. Wetlands can be used for biological treatment of wastewater. Problem statement: Scarcity of water is considered as a global problem and Iran is one the countries which is facing water shortage problem. Pollution of water bodies restrict the availability of water for various uses. Treatment of waste water before disposal contributes to water conservation efforts. Constructed wetlands are techniques aim to polish water quality and reduce the harmful effect of effluent. Approach: In this study, four horizontal subsurface flow wetlands (HSSF) were constructed at the Research Station of Tehran University, located in Karaj, Iran. The study was carried out from April to September, 2007. Gravel and zeoilte were used in this study as substrate. Gravel-beds with and without plants (called GP and G) and gravel-beds mixed with (10%) zeolite, with and without plants (called ZP and Z) were examined to investigate the feasibility of treating synthetic wastewater which was specifically produced and modified to imitate agricultural wastewater. Results: The results of this study indicated that the system had acceptable pollutant removal efficiency and that both plants were found to be tolerant under the tested conditions. The wetland system could achieve the NO3-N removal of (79%) in ZP, (86%) in Z, (82%) in GP and finally (87.94%) in G. As for the P removal, the efficiencies of 93, 89, 81 and 76% were respectively achieved for ZP, GP, Z and G. The outflow concentrations of Pb and Cd were found to be under the detection limit; however, as for Zn, the removal efficiencies of 99.9, 99.76, 99.71 and 99.52% were concluded for ZP, Z, GP and G respectively. Conclusions/Recommendations: It can be concluded that constructed wetlands are efficient in removing Zn, Pb and Cd from agricultural wastewater. Plants types such as Phragmites Australis and Juncus Inflexus can contribute in treating wastewater, while Zeolite and gravel materials provide a suitable plant growth medium to replace conventional sand and gravel substrates. So it is highly recommended to use Constructed wetland for treating wastewater before disposal.

Potential of constructed wetlands in treating the eutrophic water: Evidence from Taihu Lake of China

Three parallel units of pilot-scale constructed wetlands (CWs), i.e., vertical subsurface flow (VSF), horizontal subsurface flow (HSF) and free water surface flow (FWS) wetland were experimented to assess their capabilities in purifying eutrophic water of Taihu Lake, China. Lake water was continuously pumped into the CWs at a hydraulic loading rate of 0.64 m d −1 for each treatment. One year’s performance displayed that average removal rates of chemical oxygen demand (COD), ammonia nitrogen ( NH 4 + – N ) , nitrate nitrogen ( NO 3 - – N ) , total nitrogen (TN) and total phosphorous (TP) were 17–40%, 23–46%, 34–65%, 20–52% and 35–66%, respectively. The VSF and HSF showed statistically similar high potential for nutrients removal except NH 4 + – N , with the former being 14% higher than that of the latter. However, the FWS wetland showed the least effect compared to the VSF and HSF at the high hydraulic loading rate. Mean effluent TP concentrations in VSF (0.056 mg L −1) and HSF (0.052 mg L −1) nearly reached Grade III (⩽0.05 mg L −1 for lakes and reserviors) water quality standard of China. Wetland plants ( Typha angustifolia) grew well in the three CWs. We noted that plant uptake and storage were both important factors responsible for nitrogen and phosphorous removal in the three CWs. However, harvesting of the above ground biomass contributed 20% N and 57% P of the total N and P removed in FWS wetland, whereas it accounted for only 5% and 7% N, and 14% and 17% P of the total N and P removed in VSF and HSF CWs, respectively. Our findings suggest that the constructed wetlands could well treat the eutrophic lake waters in Taihu. If land limiting is considered, VSF and HSF are more appropriate than FWS under higher hydraulic loading rate.

Constructed wetlands for grey water treatment

A vertical flow and horizontal sub-surface flow wetland were compared with a novel system (the GROW green roof water recycling system) for treatment of low-organic strength domestic grey water for reuse. The vertical and horizontal flow wetlands were planted with Phragmites australis in a sand:soil:compost medium. The GROW system was planted with a variety of marginal plants selected for treatment and aesthetic characteristics. BOD removal did not differ significantly between the three systems which all consistently met USEPA standards for grey water for reuse. However, the GROW system was most effective at removal of suspended solids and turbidity (mean removal 91.2% and 98.2% respectively) and both GROW and VF were more effective at removing pathogens (4.2 and 4.8 log reduction of total coli-forms respectively) than the HF (2.7 log reduction).

Phosphorous Sorption by Filtralite P—Small Scale Box Experiment

Phosphorus (P) sorption of light weight aggregate, Filtralite P has been examined through a box experiment which imitates a horizontal subsurface flow wetland system. The results showed that after the P breakthrough, the outlet P concentration increased with time according to the amount of P applied. Small scale boxes with a high inlet P concentration (15 ppm) and high loading rate (5–2.5 L d−1) reached 90% saturation level relatively quickly (after about 150 days of operation), while the boxes with low hydraulic loading rate (1.25 L day−1) were 70–90% saturated after 18 months of operation. The total P removal was dependent on pH, Ca, and the inlet P concentrations, but was independent of the hydraulic loading rate. Extraction of total P from the saturated filter material showed that the sorbed P accumulated within the inlet section of the box and decreased gradually towards the outlet as well as towards the bottom layer. Even after large amounts of Ca had leached out of the system, Filtralite P still had a very high P removal capacity. After resting periods the P sorption capacity of the material was regenerated, the P concentration in the effluent decreased by 22–53%.

Nitrogen removal from domestic effluent using subsurface flow constructed wetlands: influence of depth, hydraulic residence time and pre-nitrification

This paper describes two studies into the BOD and TN removal performance of horizontal subsurface flow wetlands (reed beds) in subtropical Australia. The aim of the first study was to determine the influence of HRT and vertical position on BOD and TN concentration and removal performance in a 0.5 m deep reed bed (System 1) by taking samples from three levels (or layers) in the water column at five points along the length of the bed. The aim of the second study was to investigate the TN removal performance of a treatment train consisting of a vertical flow intermittently dosed sand filter preceding a reed bed (System 2). Both systems were dosed with primary settled municipal wastewater (BOD 194 mg L(-1); TN 49 mg L(-1)). System 1 achieved a TN load removal of 58% under a HLR of 22 mm day(-1) (HRT 10.5 days), producing effluent BOD concentrations consistently less than 8 mg L(-1). There was no significant difference in BOD attenuation rate between the three layers. While there were differences in both the nitrification and denitrification rates between the three layers, the TN concentration was found to decline steadily in all layers up to an HRT of 8.7 days. System 2 reduced TN influent load by 33%, less than half of which was removed by the reed bed. The lack of substantial TN removal within this reed bed was attributed to the low concentrations of BOD and consequent lack of dissolved organic carbon to drive the denitrification process.