Hydraulic behaviour and removal efficiencies of two H-SSF constructed wetlands for wastewater reuse with different operational life

This work focuses on the performance evaluation of two full-scale horizontal suburface flow constructed wetlands (H-SSF CWs) working in parallel, which have an almost equal surface area (about 2,000 m2) but with different operational lives: 8 and 3 years. Both H-SSF CWs, located in Southern Italy (Sicily), are used for tertiary treatment of the effluent of a conventional wastewater treatment plant. This study evaluates and compares H-SSF CW efficiency both in terms of water quality improvement (removal percentage) and achievement of Italian wastewater discharge and irrigation reuse limits. The mean removal percentage, for the overall operational life, of TSS, COD and BOD (80%, 63%, 58% obtained for H-SSF1 and 67%, 38%, 41% for H-SSF2), confirm the high reliability of CWs for wastewater treatment. However, despite the satisfactory removal of microbial indicators (the mean E. coli removal was up to 2.5 log unit for both beds), CWs didn't achieve the Italian limits for wastewater reuse. Information on hydraulic properties of the CWs were extracted from breakthrough curves of a non-reactive tracer (NaCl). By comparing the nominal (tau(n)) and actual residence time (tau), hydraulic behaviour was revealed.     

Comparison of nitrogen elimination rates of different constructed wetland designs

In this paper the nitrogen elimination rates of different constructed wetland (CW) designs reported in literature are compared with those obtained for outdoor and indoor 2-stage vertical flow (VF) systems. The outdoor system is located about 150 km west of Vienna. Both stages are planted with Phragmites australis and the system has been operated for 4 years continuously. During this period the average value of the nitrogen elimination rate was 3.30 g N m(-2) d(-1). The indoor system comprises three parallel operated 2-stage VF systems and is located in the technical lab hall at BOKU University. The design of the indoor system resembles the outdoor system. However, there are a few differences: (1) the indoor systems are not planted, and (2) different filter media have been used for the main layer of the first stages. With the indoor system the highest nitrogen elimination rate achieved was 2.24 g N m(-2) d(-1) for the system with zeolite and impounded drainage layer. Similar results have been found in France for treating raw wastewater with VF and horizontal flow (HF) beds in series with nitrogen elimination rates of 1.89 and 2.82 g N m(-2) d(-1) for differently designed HF beds. The highest nitrogen elimination rates of 15.9 g N m(-2) d(-1) reported were for pilot-scale VF CWs treating high-strength synthetic wastewater (total nitrogen of 305 mg L(-1) in the influent) in Thailand. It has been shown that the outdoor two-stage VF CW system has one of the highest nitrogen elimination rates of CWs treating domestic wastewater.     

Simulation of a subsurface vertical flow constructed wetland for CSO treatment.

Constructed wetlands (CWs) have proved to be a highly effective measure to reduce the ecological impact of combined sewer overflows (CSOs) on receiving waters. Due to the stochastic nature of the loading regime and the multitude of environmental influences, assessment of the performance of such plants requires detailed mathematical modelling. A multi-component reactive transport module (CW2D) was applied to simulate the flow, transport and degradation processes occurring in a CW for CSO treatment. CW2D was originally developed to simulate the treatment of municipal wastewater in subsurface flow CWs. Loading and operational conditions in CSO treatment differ fundamentally from the conditions occurring for wastewater treatment. Despite these differences, first results from the simulation of lab-scale experiments show, that the model is generally applicable to this type of plant. Modelling of adsorption, degradation processes, and influent fractionation, however, require further research.     

The role of SSVF and SSHF beds in concentrated wastewater treatment, design recommendation

The return flows of reject water from sewage sludge dewatering alter the activated sludge process in a conventional WWTP and increase TN concentration in the final effluent from WWTP. The objective of the investigation carried out was to consider the application of multistage treatment wetland (MTW) for the treatment of reject water from sewage sludge dewatering in a centrifuge (RWC). This paper aims to present the design and performance of each stage of the treatment as well as the efficiency of total MTW. The full scale pilot plant for RWC, consisting of two vertical flow beds (SS VF) working in series, followed by an horizontal flow bed (SS HF), was built in 2008. The applied configuration ensured a very high removal efficiency of principal pollutant (COD - 76.0% and NH4+-N - 93.6%). In the investigated facilities, the SS VF beds ensured an effective removal of nitrogen compounds, especially NH4+-N, whereas the decomposition of hardly degradable Org-N and COD took place in SS HF. This research illustrates that the MTW could be successfully applied for the treatment of RWC.     

Influence of high organic loads during the summer period on the performance of hybrid constructed wetlands (VSSF + HSSF) treating domestic wastewater in the Alps region

One of the limits for the application of constructed wetlands (CWs) in mountain regions (such as the Alps) is associated with the considerable land area requirements. In some mountain areas, the treatment of domestic wastewater at popular tourist destinations is particularly difficult during the summer, when the presence of visitors increases hydraulic and organic loads. This paper aims to evaluate whether a hybrid CW plant designed on the basis of the resident population only, can treat also the additional load produced by the floating population during the tourist period (summer, when temperatures are favourable for biological treatment), without a drastic decrease of efficiency and without clogging problems. The research was carried out by considering two operational periods: the first one was based on literature indications (3.2 m(2)/PE in the VSSF unit) and the second one assumed higher hydraulic and organic loads (1.3 m(2)/PE in the VSSF unit). The removal efficiency in the hybrid CW system decreased slightly from 94 to 88% for COD removal and from 78 to 75% for total N removal, even after applying a double hydraulic (from 55 to 123 L m(-2) d(-1)) and organic load (from 37 to 87 g COD m(-2) d(-1) and from 4.4 to 10.3 g TKN m(-2) d(-1)). The results showed that in the summer period the application of high loads did not affect the efficiency of the hybrid CW plant significantly, suggesting that it is possible to refer the CW design to the resident population only, with subsequent considerable savings in superficial area.     

Slag columns for upgrading phosphorus removal from constructed wetland effluents.

The current best option to upgrade constructed wetlands (CWs) for phosphorus (P) retention, in terms of efficiency, cost and simplicity, consists in using media having a strong P affinity. The media can be used either in the planted beds or in a filtration system downstream of the beds. The use of slag filters was shown to be efficient for removing P from wastewater as it represented a slow release source of calcium and hydroxide, favouring the formation of hydroxyapatite. Our study aimed at maximising the P retention capacity of slag filters located at the outlet of CWs since electric arc furnace slag has been shown to inhibit the growth of macrophytes when used in the filtration matrix. Bench-scale columns (Vtot = 6.2 L) filled with various combinations of filter media (slag, granite, limestone) of different sizes (2-5, 5-10, 10-20 mm) were fed on-site during four months with a CW effluent (in mg/L: 30 COD, 30 TSS, 10 Pt). Results showed that the best media combination enabling the maximum o-PO4 retention (more than 80% removal without clogging) consisted in a series of a ternary mix column (slag 5-10 mm, granite 2-5 mm, limestone 5-10 mm) followed by a slag column (slag 5-10 mm). Pilot scale columns (Vtot = 300 L), filled with the best media combination, were installed at the outlet of a 28 m2 CW. These columns showed more than 75% removal efficiency during one year and were designed to be easily replaced each year.     

Optimization of subsurface vertical flow constructed wetlands for wastewater treatment.

Constructed wetlands (CWs) use the same processes that occur in natural wetlands to improve water quality and are used worldwide to treat different qualities of water. This paper shows the results of an Austrian research project having the main goals to optimize vertical flow beds in terms of surface area requirement and nutrient removal, respectively. It could be shown that a subsurface vertical flow constructed wetland (SSVFCW) operated with an organic load of 20 g COD x m(-2) x d(-1) (corresponding to a specific surface area demand of 4 m2 per person) can fulfil the requirements of the Austrian standard regarding effluent concentrations and removal efficiencies. During the warmer months (May - October), when the temperature of the effluent is higher than 12 degrees C, the specific surface area might be further reduced. Even 2 m2 per person have been proven to be adequate. Enhanced nitrogen removal of 58% could be achieved with a two-stage system (first stage: grain size for main layer 1-4 mm, saturated drainage layer; and second stage: grain size for main layer 0.06-4 mm, free drainage) that was operated with an organic load of 80 g COD x m(-2) x d(-1) for the first stage (1 m2 per person), i.e. 40 g COD x m(-2) x d(-1) for the two-stage system (2 m2 per person). Although the two-stage system was operated with higher organic loads a higher effluent quality compared to a single-stage SSVFCW (grain size for main layer 0.06-4 mm, free drainage, organic load 20 g COD x m(-2) x d(-1)) could be reached.     

Numerical modelling: a tool for better constructed wetland design?

There is a need for a simplified computer-based design tool for subsurface flow constructed wetlands (CWs) which is based on process-based numerical models. Parameters of existing design guidelines and rules have been derived from experiments under specific conditions. Therefore designing CWs using these parameters is limited to these conditions (i.e., temperature, wastewater composition, filter material, etc.). Process-based numerical models describe the main processes in CWs in detail. If the design of CWs is based on these models it will be possible to design CWs for a variety of different boundary conditions and therefore the main limitation of existing design guidelines and rules could be overcome. The use of process-based models is currently limited mainly due to their complexity in structure and use. To make numerical modelling a useful and applicable tool for design, a simplified computer-based design tool that does not require special knowledge of numerical modelling is needed. Additionally, simple models for pre- and post-treatments are also required. Besides allowing designs for various boundary conditions, design tools based on process-based models can also predict the dynamic behaviour of the designed system thus showing e.g., the higher robustness of CWs against fluctuating inflows and peak loads compared to other treatment solutions. Such a tool could increase the quality of CW design and the acceptance and use of CW simulation in practice.     

水平潜流人工湿地脱氮除磷研究进展

总结了水平潜流人工湿地的脱氮除磷机理,分析了进水水质、植物、基质、温度和水力停留时间这5种影响处理效果的主要因素,从不同角度论述了提高脱氮除磷效果所采取的措施,并探讨了水平潜流人工湿地存在的局限性和发展趋势。     

Safety of treated water for re-use purposes--comparison of filtration and disinfection processes.

A study was conducted on the distribution of pollutants in treated wastewater and the its safety for re-use purposes. Based on the results of a series of tertiary treatment experiments, the effects of three filtration processes, i.e. coagulation-filtration, ozonation-biological activated carbon filtration (O3-BAC) and ultrafiltration (UF), and two chemical disinfection processes, i.e. chlorination and ozonation, on the safety of water re-use were evaluated. It was found that the concentrations of the main pollutants in the secondary effluent and further filtered water follow a log-normal distribution and, therefore, a log-normal probabilistic function can be used to evaluate the suitability of the treated water for re-use purposes. Among the three filtration processes evaluated, UF is the most effective in turbidity removal but less effective in colour and COD removal, while coagulation-filtration and O3-BAC can ensure a good removal of all these pollutants. Regarding chemical disinfection, although chlorine is very effective in inactivation of coliform bacteria, it can not achieve a substantial decrease in viruses. As ozone is applied, effective virus removal can be achieved.     

Kinetics of heterotrophic biomass and storage mechanism in wetland cores measured by respirometry

Although oxygen uptake rate has been widely used in activated sludge for measuring kinetic and stoichiometric parameters or for wastewater characterization, its application in constructed wetlands (CWs) cores has been recently proposed. The aim of this research is to estimate the kinetic and stoichiometric parameters of the heterotrophic biomass in CW cores. Respirometric tests were carried out with pure carbonaceous substrate and real wastewater. Endogenous respiration was about 2 gO2 m(-3) h(-1) (per unit of bed volume), while the kinetic parameters obtained for COD oxidation were very high (maximum rate per unit of bed volume of 10.7-26.8 gCOD m(-3) h(-1)) which indicates high biodegradation potential in fully aerobic environment. Regarding to stoichiometric parameter, the maximum growth yield, Y(H), was 0.56-0.59 mgCOD/mgCOD, while the storage yield, Y(STO), was 0.75-0.77 mgCOD/mgCOD. The storage mechanism was observed in CW cores during COD oxidation, which leads to the transformation of the external soluble substrate in internal storage products, probably as response to intermittent loads applied in CW systems, transient concentrations of readily biodegradable substrate and alternance of feast/famine periods.     

Treatment of septage in constructed wetlands in tropical climate: lessons learnt from seven years of operation.

In tropical regions, where most of the developing countries are located, septic tanks and other onsite sanitation systems are the predominant form of storage and pre-treatment of excreta and wastewater, generating septage and other types of sludges. The septage is disposed of untreated, mainly due to lack of affordable treatment options. This study presents lessons that have been learned from the operation of pilotscale constructed wetlands (CWs) for septage treatment since 1997. The experiments have been conducted by using three CW units planted with narrow-leave cattails (Typha augustifolia) and operating in a vertical-flow mode. Based on the experimental results, it can be suggested that the optimum solids loading rate be 250 kg TS/m2 yr and 6-day percolate impoundment. At these operational conditions, the removal efficiencies of CW units treating septage at the range of 80-96% for COD, TS and TKN were achieved. The biosolid accumulated on the CW units to a depth of 80 cm has never been removed during 7 years of operation, but bed permeability remained unimpaired. The biosolid contains viable helminth eggs below critical limit of sludge quality standards for agricultural use. Subject to local conditions, the suggested operational criteria should be reassessed at the full-scale implementation.     

Grey water characterization and treatment for reuse in an arid environment

Grey water from a university facilities building in Cairo, Egypt was analysed for basic wastewater parameters. Mean concentrations were calculated based on grab samples over a 16-month period. Values for chemical oxygen demand (COD) and nutrients exceeded values reported in a number of other studies of grey water, while coliform counts were also high. A submerged membrane bioreactor (SMBR) system using a hollow fibre ultrafiltration membrane was used to treat the grey water with the aim of producing effluent that meets reuse guidelines for agriculture. A test run for 50 days at constant transmembrane pressure resulted in very good removal for key parameters including COD, total suspended solids (TSS), colour, turbidity, ammonia nitrogen, anionic surfactants, and coliform bacteria. High standard deviations were observed for COD and coliform concentrations for both monthly grab samples and influent values from the 50-day SMBR experiment. SMBR effluent meets international and local guidelines for at least restricted irrigation, particularly as pertains to COD, TSS, and faecal coliforms which were reduced to mean treated values of 50 mg/L, 0 mg/L (i.e., not detected), and <50 cfu/100 mL, respectively.     

Microbial diversity and community composition in recirculating vertical flow constructed wetlands

Microorganisms constitute a central component of constructed wetlands (CWs), playing a major role in these systems' capacity for treating wastewater. The aim of this study was to determine the diversity and composition of the microbial community found in a recirculating vertical flow CW (RVFCW) bed fed with primarily settled domestic wastewater and its response to the presence of plants, season and location in the bed. The RVFCW removed 90-95% of TSS and BOD(5) to below 10 mg L(-1). The effluent quality was not significantly affected by seasonal temperature or the existence of plants in the bed. None of these factors had discernible effects on bacterial diversity, e.g. in the planted RVFCW, the richness (S') and Shannon-Weiner diversity (H') indices were 18.3 (±3.5) and 2.49 (±0.15), respectively, which are similar to the values of 19.4 (±3.5) and 2.57 (±0.18) in the unplanted RVFCW. However, there were indications that the structure of the microbial community underwent changes that were uncorrelated with the environmental factors tested and that did not affect the overall performance. The consistency in diversity and composition/structure of the bacterial community in the face of temporal and environmental influences possibly contributes to the robustness and high treatment capacity of the RVFCW system.     

Comparison of maturation ponds and constructed wetlands as the final stage of an advanced pond system.

The treatment performance of a maturation pond (MP), the typical final polishing stage of an Advanced Pond System (APS), is compared with that of a surface-flow constructed wetland (CW) over 19 months. Both received approximately 67 mm d-1 of wastewater after passage through upstream stages of the APS. The MP, with greater sunlight exposure, had higher algal biomass (and associated suspended solids) than the CW, showed higher dissolved oxygen (DO) concentrations and greater diurnal variation in DO and pH. Neither polishing stages reduced nutrients markedly, with the CW exporting slightly more NH(3)-N and DRP, and less NO(3)-N than the MP. Disinfection was more efficient in the MP (geometric mean 1 log load removal, 12 MPN (100ml)-1) compared to the CW (0.47 log load removal, 53 MPN (100ml)-1). Incorporation of a final rock filter (28% of area) reduced median solids levels to < 10 g m(-3) in both the MP and CW. A hybrid between MPs and CWs with alternating zones of open-water (for enhanced disinfection and zooplankton grazing of algal solids) and wetland vegetation (promoting sedimentation and denitrification, and providing refugia for zooplankton) may provide more consistent effluent quality that either stage alone.     

Pumped flow biofilm reactors (PFBR) for treating municipal wastewater

A novel laboratory bench-scale sequencing batch biofilm reactor (SBBR) system was developed for the treatment of synthetic domestic strength wastewater, comprising two side-by-side 18 l reactor tanks, each containing a plastic biofilm media module. Aerobic and anoxic conditions in the biofilms were effected by intermittent alternate pumping of wastewater between the two reactors. With a media surface area loading rate of 4.2 g chemical oxygen demand (COD)/m2.d, the average influent COD, total nitrogen (TN) and ammonium-nitrogen (NH4-N) concentrations of 1021 mg/l, 97 mg/l and 54 mg/l, respectively, reduced to average effluent concentrations of 72 mg COD/l, 17.8 mg TN/l, and 5.5 mg NH4-N /l. Using a similar alternating biofilm exposure arrangement, a 16 person equivalent pilot (PE) plant was constructed at a local village treatment works to remove organic carbon from highly variable settled municipal wastewater and comprised two reactors, one positioned above the other, each containing a module of cross-flow plastic media with a surface area of 100 m2. Two different pumping sequences (PS) in the aerobic phase were examined where the average influent COD concentrations were 220 and 237 mg/l for PS1 and PS2, respectively, and the final average effluent COD was consistently less than 125 mg/l--the European Urban Wastewater Treatment Directive limit--with the best performance occurring in PS1. Nitrification was evident during both PS1 and PS2 studies. A 300 PE package treatment plant was designed based on the bench-scale and pilot-scale studies, located at a local wastewater treatment works and treated municipal influent with average COD, suspended solids (SS) and TN concentrations of 295, 183 and 15 mg/l, respectively resulting in average effluent concentrations of 67 mg COD/l, 17 mg SS/l and 9 mg TN/l. The SBBR systems performed well, and were simple to construct and operate.     

Comparison of nutrient cycling in a surface-flow constructed wetland and in a facultative pond treating secondary effluent.

There is a growing interest in the possibilities offered by combinations of waste stabilisation ponds (WSP) and constructed wetlands (CW). The purpose of our study was to compare treatment performances and nutrient cycling in a surface-flow wetland (SFW) and in a WSP treating secondary effluent. In the period between 2000 and 2003, a pilot SFW and a pilot WSP were constructed at the outlet of the wastewater treatment plant and their performance monitored while both were active under the same conditions. The SFW was planted with Phragmites australis and Eichhornia crassipes, while in the WSP development of algae was spontaneous. Performance efficiency was monitored by means of evaluation of physical and chemical parameters in water, by measurement of plant productivity and by analysis of N and P contents in biomass. The SFW with macrophytes proved more efficient in decreasing the suspended solids (64.6%), settleable solids (91.8%), organic N (59.3%), total N (38%), COD (67.2%) and BOD5 (72.1%) than the WSP. The WSP with algae was more efficient in treatment of ammonia nitrogen (48.9%) and ortho-phosphate (43.9%). The results of this study provide data that are of help in optimising combinations of SFW and WSP.     

Performance of a constructed wetland for treating farm-yard dirty water

Constructed wetlands (CWs) have been used to treat agricultural effluents with varying success especially with respect to their operational efficiency in winter and ability to retain phosphorus. Dirty water (DW) from dairy farms is a mixture of manure contaminated runoff and milk parlour washings with a highly polluting biochemical oxygen demand (BOD) < or =3000 mg/L. The initial performance a CW of a 1.2 ha horizontal flow CW consisting of five ponds in series designed to treat DW from a dairy unit was assessed over four years. Ponds were earth-lined and shallow (0.3 m) with a water residence time of 100 days and planted with five species of emergent macrophytes. In comparison to CW inflow, annual reductions were as follows: BOD 99%, P 95% and N 92.8%. Coliforms were reduced by a 10(-5) factor to natural levels. From May to October there was little CW discharge due to evaporative losses. Final effluent quality was poorest in February but remained within a regulatory effluent standard for BOD of 40 mg/L. If the CW had only four ponds (25% less surface area) effluent would have failed the BOD standard in three years.     

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.     

Treatment of winery wastewaters in a membrane submerged bioreactor.

Wine production is seasonal, and thus the wastewater flow and its chemical oxygen demand (COD) concentrations greatly vary during the vintage and non-vintage periods, as well as being dependant on the winemaking technologies used, e.g. red, white or special wines production. Due to this seasonal high variability in terms of organic matter load, the use of membrane biological reactors (MBR) could be suitable for the treatment of such wastewaters. MBR offers several benefits, such as rapid start up, good effluent quality, low footprint area, absence of voluminous secondary settler and its operation is not affected by the settling properties of the sludge. A pilot scale hollow fibre MBR system of 220 L was fed by adequately diluting white wine with tap water, simulating wastewaters generated in wineries. The COD in the influent ranged between 1,000 and 4,000 mg/L. In less than 10 days after the start up, the system showed a good COD removal efficiency. The COD elimination percentage was always higher than 97% regardless of the organic loading rate (OLR) applied (0.5-2.2 kg COD/m3 d), with COD concentrations in the effluent ranging between 20 and 100 mg/L. Although the biomass concentration in the reactor increased from 0.5 to 8.6 g VSS/L, the suspended solids concentration in the effluent was negligible. Apparent biomass yield was estimated in 0.14 g VSS/g COD.