Performance of lagoon and constructed wetland systems for tertiary wastewater treatment and potential of reclaimed water in agricultural irrigation

Climate change poses challenges to agricultural water resources, both in terms of quantity and quality. As an adaptation measure, the new European Regulation (EU) 2020/741 establishes different water quality classes for the use of reclaimed water in agricultural irrigation. Italy is also working on the definition of a new regulation on reclaimed water reuse for agricultural irrigation (in substitution of the current one) that will also include the specific requirements imposed by the European one. Nature-based Solutions (NBS) can be a cost-effective and environmentally friendly way to facilitate water reclamation and reuse. The present study reports the outcomes of a long-term monitoring campaign of two NBS (e.g., a constructed wetland (CW) and a lagoon system (LS)) comparing influent and effluent concentrations of different contaminants (e.g., E. coli, BOD5, TSS, TN and TP) with the threshold values imposed by the new regulations. The results showed that in both the case studies, E. coli (about 100 CFU 100 mL-1) and BOD5 (lower than 25 mg L-1) mean effluent concentration need to be further reduced in reclaimed water to be suitable for unlimited reuse. As a negative aspect, in both the monitored NBS, an increase in TSS mean concentration in the effluent was observed, up to 40 mg L-1 in the case of the LS, making reclaimed water unsuitable for agricultural reuse. The CW has proven to be more effective in nitrogen removal (the effluent mean concentration was 3.4 mg L-1), whereas the LS was better at phosphorus removal (with an effluent mean concentration of 0.4 mg L-1). Based on the results, recommendations were made to further improve the performance of both systems in order to have adequate water quality, even for class A. Furthermore, the capacity of reclaimed water to meet crop water and nutrient needs was analyzed, and total nitrogen removal rate coefficients were calculated for the design of future LSs.

Comparison of simple models for total nitrogen removal from agricultural runoff in FWS wetlands

Free water surface (FWS) wetlands can be used to treat agricultural runoff, thereby reducing diffuse pollution. However, as these are highly dynamic systems, their design is still challenging. Complex models tend to require detailed information for calibration, which can only be obtained when the wetland is constructed. Hence simplified models are widely used for FWS wetlands design. The limitations of these models in full-scale FWS wetlands is that these systems often cope with stochastic events with different input concentrations. In our study, we compared different simple transport and degradation models for total nitrogen under steady- and unsteady-state conditions using information collected from a tracer experiment and data from two precipitation events from a full-scale FWS wetland. The tanks-in-series model proved to be robust for simulating solute transport, and the first-order degradation model with non-zero background concentration performed best for total nitrogen concentrations. However, the optimal background concentration changed from event to event. Thus, to use the model as a design tool, it is advisable to include an upper and lower background concentration to determine a range of wetland performance under different events. Models under steady- and unsteady-state conditions with simulated data showed good performance, demonstrating their potential for wetland design.

Nature-based solutions in the urban context: terminology, classification and scoring for urban challenges and ecosystem services

The concept of Nature-Based Solutions (NBS) has emerged to foster sustainable development by transversally addressing social, economic, and environmental urban challenges. However, there is still a considerable lack of agreement on the conceptualization of NBS, especially concerning typologies, nomenclature, and performance assessments in terms of ecosystem services (ES) and urban challenges (UC). Therefore, this article consolidates the knowledge from 4 European projects to set a path for a common understanding of NBS and thus, facilitate their mainstreaming. To do so, firstly, we performed elicitation workshops to develop an integrative list of NBS, based on the identification of overlaps among NBS from different projects. The terminologies were formalized via web-based surveys. Secondly, the NBS were clustered, following a conceptual hierarchical classification. Thirdly, we developed an integrative assessment of NBS performance (ES and UC) based on the qualitative evaluations from each project. Afterwards, we run a PCA and calculated the evenness index to explore patterns among NBS. The main conceptual advancement resides in providing a list of 32 NBS and putting forward two novel NBS categories: NBS units (NBS_u) that are stand-alone green technologies or green urban spaces, which can be combined with other solutions (nature-based or not); NBS interventions (NBS_i) that refer to the act of intervening in existing ecosystems and in NBS_u, by applying techniques to support natural processes. The statistical analysis suggests that NBS_u are more versatile than NBS_i in terms of UC and ES. Moreover, the results of the integrative assessment of NBS performance suggest a greater agreement concerning the role of NBS in addressing environmental UC, cultural and regulating ES than regarding socio-economic UC and supporting and provision ES. Finally, the 'green factor' and the replication of non-intensive practices occurring in nature seem to be key criteria for practitioners to identify a particular solution as an NBS.

Developing sanitation planning options: A tool for systematic consideration of novel technologies and systems

To provide access to sustainable sanitation for the entire world population, novel technologies and systems have been developed. These options are often independent of sewers, water, and energy and therefore promise to be more appropriate for fast-growing urban areas. They also allow for resource recovery and and are adaptable to changing environmental and demographic conditions what makes them more sustainable. More options, however, also enhance planning complexity. Structured decision making (SDM) can help balance opposing interests. Yet, most of the current research focuses on the selection of a preferred option, assuming that a set of appropriate options is available. There is a lack of reproducible methods for the identification of sanitation system planning options that can consider the growing number of available technology and the many possible system configurations. Additionally, there is a lack of data, particularly for novel options, to evaluate the various sustainability criteria for sanitation.To overcome this limitation, we present a novel software supported approach: the SANitation sysTem Alternative GeneratOr (Santiago). To be optimally effective, Santiago is required to be integrated into an SDM approach. In this paper, we present all the elements that such an integration requires and illustrate these methods at the case of Arba Minch, a fast growing town in Ethiopia. Based on this example and experiences from other cases, we discuss the lessons learnt and present the advantages potentially brought by Santiago for sanitation planning The integration requires four elements: a set of technologies to be looked at, decision objectives for sustainable sanitation, screening criteria to evalute technology appropriateness, and about the technologies and the casea. The main output is a set of sanitation system options that is locally appropriate, diverse in order to reveal trade-offs, and of a manageable size. To support the definition of decision objectives, we developed a generic objective hierarchy for sustainable sanitation. Because one of the main challenges lies in the quantification of screening criteria, we established the data for 27 criteria and 41 technologies in a library.The case studies showed, that if the integration is successful, then Santiago can provide substantial benefits: (i) it is systematic and reproducible; (ii) it opens up the decision space with novel and potentially more appropriate solutions; (iii) it makes international data accessible for more empirical decision making; (iv) it enables decisions based on strategic objectives in line with the sustainable development goals; (v) it allows to prioritise appropriate and resource efficient systems right from the beginning (vi) and it contributes to a more citywide inclusive approach by birding strategic objectives with an area-based appropriateness assessment. The here presented approach enables the prioritisation of appropriate and resource efficient sanitation technologies and systems in strategic planning. Thereby this approach contributes to SDG 6.2, 6.3, and 11, sustainable sanitation for all.

Aeration intensity simulation in a saturated vertical up-flow constructed wetland

Simulation and performance results of a saturated vertical up-flow constructed wetland (SVU CW) operated under different operational conditions are presented. The SVU CW consists of two different systems planted with Cyperus alternifolius and Iris pseudacorus, and each system consists of three SVU beds operated in series. The SVU CW operates in continuous aeration (CA) mode using different air-water ratios from 0.5:1 to 4:1. The aerated SVU CW achieves a high (more than 85%) removal of chemical oxygen demand (COD), ammonium (NH₄⁺-N), total nitrogen (TN) and total phosphorus (TP). Furthermore, we simulate the SVU CW using the HYDRUS Wetland Module using the CWM1 biokinetic model under CA mode. According to the simulation results, aeration intensity controls the substrate distribution and growth of bacteria with depth in the SVU CW. Organic matter (OM) and nitrogen are removed in the top region (0-30 cm) of the SVU CW. The root mean square error for COD and NH₄⁺-N is >1.5, whereas R² is >0.99. A good match between observed and simulated data suggests that the CWM1 model is a suitable tool for simulating various processes and bacterial dynamics in aerated SVU CWs.

Influence of design parameters on the treatment performance of VF wetlands - a simulation study

The main approach for designing vertical flow (VF) treatment wetlands is based on areal requirements ranging from 2 to 4 m² per person equivalent (PE). Other design parameters are the granularity of the filter material, filter depth, hydraulic and organic loading rates, loading intervals, amount of single doses as well as the number of openings in the distribution pipes. The influence of these parameters is investigated by running simulations using the HYDRUS Wetland Module for three VF wetlands with different granularity of the filter material (0.06-4 mm, 1-4 mm, and 4-8 mm, respectively). For each VF wetland, simulations are carried out at different temperatures for different organic loading rates, loading intervals and number of distribution points. Using coarser filter material results in reduced removal of pollutants and higher effluent concentrations if VF wetlands are operated under the same conditions. However, the treatment efficiency can be increased by applying more loadings and/or a higher density of the distribution network. For finer filter material, longer loading intervals are suggested to guarantee sufficient aeration of the VF filter between successive loadings.

Treatment of de-icing contaminated surface water runoff along an airport runway using in-situ soil enriched with structural filter materials

Airport surface runoff during wintertime contains high concentrations of pavement de-icing fluids (PDFs). Uncontrolled discharge of this runoff poses a potential environmental hazard for the terrestrial and aquatic ecosystem. Several technologies for collection, transportation and treatment of contaminated runoff water are available, mainly technical systems, which require high operation and maintenance efforts. For moderately contaminated runoff, the discharge to a wastewater treatment plant is usually applied. In this study, a passive soil-based filter is proposed to treat the contaminated surface water runoff. The degradation of two PDFs was under investigation, namely Safeway® KA-Hot based on potassium acetate, and urea. The main research objective was to determine the capability of the in-situ soil and a soil based filter using zeolite and perlite as additional filter media to degrade the organic pollutants in the runoff. Column experiments at temperatures between 3 °C and 5 °C were carried out to determine the degradation potential when using 50% in-situ soil mixed with zeolite and perlite. Besides TOC, the nitrogen degradation was also under investigation. Due to the low temperatures, available nutrients are a key factor for the TOC degrading microorganisms. Overall TOC reduction rates were found from 76% up to 98%, with TOC effluent concentrations in the range of 18 to 870 mg·L^-1, depending on the influent concentration. Based on the results, the use of a soil-based filter is a promising, passive, natural based solution for the treatment of de-icing runoff.

Numerical simulation of vertical flow wetlands with special emphasis on treatment performance during winter

In Austria, single-stage vertical flow (VF) wetlands with intermittent loading are a state-of-the-art technology for treating domestic wastewater. They are designed according to the Austrian design standard with a specific surface area of 4 m² per person (i.e. 20 g COD/(m²·d)) and thus demand a bigger amount of land to treat the same amount of wastewater compared to intensified technical treatment systems. In order to reduce the amount of land needed, a modified design for VF wetlands has been proposed. The modified design has a specific surface area of 2.5 m² per person (i.e. 32 g COD/(m²·d)) and it has been shown to be able to meet the Austrian effluent requirements. To allow higher organic loading, more loadings per day but lower volume of a single loading, a constant loading interval, and increased number of openings per m² are applied. A simulation study using the HYDRUS Wetland Module was carried out to compare the treatment efficiencies of single-stage VF wetlands with classical and modified design. Data from a classical Austrian single-stage VF wetland was used for calibration of the model using the standard parameter set for the CW2D biokinetic model. The influent COD fractionation was calibrated to adapt to the wastewater. The simulations showed a good performance of the modified design compared to a classical VF wetland for COD removal with COD effluent concentrations in winter (effluent water temperature of 4.5 °C) of 35 and 29 mg/L, respectively. The simulation study showed that during high-loading events the VF wetland with modified design has lower maximum NH₄-N effluent concentrations. Single-stage VF wetlands with modified design seem to be very effective and allow application of higher organic loads compared to single-stage VF wetlands with classical design.

The new German standard on constructed wetland systems for treatment of domestic and municipal wastewater

The German Association for Water, Wastewater and Waste e.V. (DWA) has published a new standard for the dimensioning, construction, and operation of constructed wetlands for treatment of domestic and municipal wastewater. The changes to the standard are based on a wide range of experience gained in recent years in Germany and Europe. For the first time ever, the standard has been officially translated and published in English. This paper summarizes the new standard for secondary treatment of domestic wastewater with classical one-stage unsaturated vertical flow (VF) wetlands, VF wetlands with lava sand for treatment of wastewater from combined sewer systems, and actively aerated VF and horizontal flow (HF) flow wetlands. Two-stage unsaturated VF wetlands treating raw wastewater (French VF wetlands), are also included in the new standard. HF wetlands are no longer described in the standard for secondary treatment of domestic wastewater. This does not exclude their application. Existing HF wetland systems in Germany may continue to be operated so long as effluent parameters are met and proper operations and maintenance is ensured. This paper gives an overview of the new design standard, including key information on wastewater type and loading, as well as primary attributes of each wetland design.

Treatment wetlands in decentralised approaches for linking sanitation to energy and food security

Treatment wetlands (TWs) are engineered systems that mimic the processes in natural wetlands with the purpose of treating contaminated water. Being a simple and robust technology, TWs are applied worldwide to treat various types of water. Besides treated water for reuse, TWs can be used in resources-oriented sanitation systems for recovering nutrients and carbon, as well as for growing biomass for energy production. Additionally, TWs provide a large number of ecosystem services. Integrating green infrastructure into urban developments can thus facilitate circular economy approaches and has positive impacts on environment, economy and health.

Non-equilibrium model for solute transport in differently designed biofilters targeting agricultural drainage water

Biogeochemical processes in subsurface flow constructed wetlands are influenced by flow direction, degree of saturation and influent loading position. This study presents a simulation tool, which aims to predict the performance of the unit and improve the design. The model was developed using the HYDRUS program, calibrated and verified on previously measured bromide (Br^-) pulse tracer tests. Three different hydraulic designs (Horizontal (HF), Vertical upward (VF-up), Vertical downward (VF-down) and two different flow rates: Low (L), and High (H)) were investigated. The model simulated well the Br^- transport behaviour and the results underline the importance of the hydraulic design. Calibrated model parameters (longitudinal dispersivity, immobile liquid phase, mass transfer coefficient) showed a common trend for all the designs, for increasing flow rates within the investigated range. The VF-down performed best, i.e. had the highest hydraulic retention time.

Survey on number and size distribution of treatment wetlands in Austria

In Austria, 1,840 wastewater treatment plants (WWTPs) with design size >50 population equivalent (PE) serve about 95% of the population. The remaining 5% of the population live in single houses and small settlements that require on site and decentralized wastewater treatment technologies. There is no common database on small WWTPs with design size <50 PE; thus data had to be collected from the Austrian federal states and compiled in a database. The total number of small WWTPs in Austria is about 28,700 comprising 1,300 WWTPs with design size 51-500 PE and 27,400 with design size <50 PE, respectively. The total number of treatment wetlands implemented in Austria is 5,450. Due to legal requirements (nitrification), only vertical flow wetlands are implemented in Austria. From the 5,450 treatment wetlands, about 100 are of design size larger than 50 PE and about 2,800 treatment wetlands have a design size of 5-10 PE. The peak of wetland implementation was in the years 2007-2011 with 2,200 implemented systems in 5 years. Since about 2000, about 30-40% of the new implemented small WWTPs are treatment wetlands.

Using numerical simulation of a one stage vertical flow wetland to optimize the depth of a zeolite layer

This simulation study investigates the treatment performance of a compact French vertical flow wetland using a zeolite layer in order to increase ammonium nitrogen removal. For the modelling exercise, the biokinetic model CW2D of the HYDRUS Wetland Module is used. The calibrated model is able to predict the effect of different depths of the zeolite layer on ammonium nitrogen removal in order to optimize the design of the system. For the model calibration, the hydraulic effluent flow rates as well as influent and effluent concentrations of chemical oxygen demand (COD) and NH₄-N have been measured. To model the adsorption capacity of zeolite, Freundlich isotherms have been used. The results present the simulated treatment performance with three different depths of the zeolite layer, 10 cm (default), 15 cm and 20 cm, respectively. The increase of the zeolite layer leads to a significant decrease of the simulated NH₄-N effluent concentration.

Performance of subsurface flow constructed wetland mesocosms in enhancing nutrient removal from municipal wastewater in warm tropical environments

Nutrient-rich effluents from municipal wastewater treatment plants (WWTPs) have significantly contributed to eutrophication of surface waters in East Africa. We used vertical (VF, 0.2 m(2)) and horizontal (HF, 0.45 m(2)) subsurface flow (SSF) constructed wetland (CW) configurations to design single-stage mesocosms planted with Cyperus papyrus, and operating under batch hydraulic loading regime (at a mean organic loading rate of 20 g COD m(-2) d(-1) for HF and 77 g COD m(-2) d(-1) for VF beds). The aim of the investigation was to assess the performance of SSF CWs as hotspots of nutrient transformation and removal processes between the WWTP and the receiving natural urban wetland environment in Kampala, Uganda. C. papyrus coupled with batch loading enhanced aerobic conditions and high efficiency regarding the elimination of suspended solids, organic matter, and nutrients with significant performance (P < .05) in VF mesocosms. The mean N and P elimination rates (g m(-2) d(-1)) were 9.16 N and 5.41 P in planted VF, and 1.97 N and 1.02 P in planted HF mesocosms, respectively. The lowest mean nutrient elimination rate (g m(-2) d(-1)) was 1.10 N and 0.62 P found in unplanted HF controls. Nutrient accumulation in plants and sediment retention were found to be essential processes. It can be concluded that whereas the SSF CWs may not function as independent treatment systems, they could be easily adopted as flexible and technologically less intensive options at a local scale, to increase the resilience of receiving environments by buffering peak loads from WWTPs.

Sensitivity analysis of the CLARA Simplified Planning Tool using the Morris screening method

The Morris screening sensitivity analysis (SA) has been used to assess how the uncertainty of input parameters influences the output of the CLARA Simplified Planning Tool (CLARA-SPT). To assess the sensitivity of the tool, four hypothetical waste collection and treatment alternatives, which planned to serve 10,000 people, have been proposed and analysed. These alternatives are (A1) dry sanitation with urine diversion dry toilets (UDDTs), (A2) water-aided sanitation with decentralised treatment units, (A3) water-aided sanitation with central technical treatment and (A4) water-aided sanitation with cesspits. The SA was used to identify the influence of two global and 29 technological input parameters on lifetime costs and residual values of sanitation alternatives. The top two important parameters identified for each alternative are: 'type of urine transport' and 'persons using one UDDT' for alternative A1, 'persons served per septic tank' and 'required surface area for vertical flow constructed wetland' for alternative A2, 'daily diesel generator working hours' and 'expected annual growth' for alternative A3 and 'cesspit volume' and 'expected annual growth' for alternative A4. Additionally, the Morris SA identified non-linearity and/or parameter interaction response. The SA of the specified alternatives shows that from the 29 technological parameters investigated, a subset of 14 important parameters need estimates that are more accurate, whereas a subset of 15 non-influential parameters can be fixed to a certain value. In particular, two parameters (i.e. cesspit volume and persons using one UDDT) that have been internally fixed in the SPT were found to be important and thus should be made available as input parameters to the user. Overall, the study provides guidance for further modification and simplification of the CLARA-SPT.

Simulation and verification of hydraulic properties and organic matter degradation in sand filters for greywater treatment

To evaluate the treatment performance of vertical flow sand filters, the HYDRUS wetland module was used to simulate treatment in an experimental set-up. The laboratory filters were intermittently dosed with artificial greywater at a hydraulic loading rate of 0.032 m³ m⁻² day⁻¹ and an organic loading rate of 0.014 kg BOD5 m⁻² day⁻¹. The hydraulic properties of the filter were characterised, as were inflow and outflow concentrations of chemical oxygen demand (COD), biochemical oxygen demand (BOD), ammonia, nitrate and total nitrogen. The inverse simulation function of the HYDRUS software was used to calibrate the water flow model. The observed effect of water flowing faster along the column wall was included in the inverse simulations. The biokinetic model was calibrated by fitting heterotrophic biomass growth to measurements of potential respiration rate. Emphasis was put on simulating outflow concentrations of organic pollutants. The simulations were conducted using three models of varying degree of calibration effort and output accuracy. The effluent concentration was 245 mg COD L⁻¹ for the laboratory filters, 134 mg COD mg L⁻¹ for the model excluding wall flow effects and 338 mg COD mg L⁻¹ for the model including wall flow effects.

Experiences from the full-scale implementation of a new two-stage vertical flow constructed wetland design

This paper describes the results of the first full-scale implementation of a two-stage vertical flow constructed wetland (CW) system developed to increase nitrogen removal. The full-scale system was constructed for the Bärenkogelhaus, which is located in Styria at the top of a mountain, 1,168 m above sea level. The Bärenkogelhaus has a restaurant with 70 seats, 16 rooms for overnight guests and is a popular site for day visits, especially during weekends and public holidays. The CW treatment system was designed for a hydraulic load of 2,500 L.d(-1) with a specific surface area requirement of 2.7 m(2) per person equivalent (PE). It was built in fall 2009 and started operation in April 2010 when the restaurant was re-opened. Samples were taken between July 2010 and June 2013 and were analysed in the laboratory of the Institute of Sanitary Engineering at BOKU University using standard methods. During 2010 the restaurant at Bärenkogelhaus was open 5 days a week whereas from 2011 the Bärenkogelhaus was open only on demand for events. This resulted in decreased organic loads of the system in the later period. In general, the measured effluent concentrations were low and the removal efficiencies high. During the whole period the ammonia nitrogen effluent concentration was below 1 mg/L even at effluent water temperatures below 3 °C. Investigations during high-load periods, i.e. events like weddings and festivals at weekends, with more than 100 visitors, showed a very robust treatment performance of the two-stage CW system. Effluent concentrations of chemical oxygen demand and NH4-N were not affected by these events with high hydraulic loads.

Experiences with pre-precipitation of phosphorus in a vertical flow constructed wetland in Austria

Using constructed wetlands (CWs) with vertical flow and intermittent loading, high organic matter and ammonium removal can be achieved. In the case of additional requirements for phosphorus removal, which in Austria often occurs if the treated wastewater is discharged into small sensitive receiving waters, additional measures have to be taken. The objective of this work was to investigate the applicability of conventional phosphorus pre-precipitation with sodium aluminate for a CW system. The experiment was carried out at a full-scale CW in Oberwindhag in Lower Austria, a two-stage vertical flow CW with intermittent loading designed for a size of 60 person equivalents (PE). The goal was to reach the required value of 1.6 mg/L PO4-P for the effluent of the system. Prior to the experiments the plant was in operation for 3 years without measures for phosphorus removal. After pre-precipitation with sodium aluminate was activated, three different dosages were investigated. Satisfying results in the preliminary treatment chambers were not obtained until a high dosage (ß = 3.5, i.e. 3.5 times the dose required from stoichiometry) was applied. After an adaptation time of several months the required effluent concentration of 1.6 mg PO4-P/L could be reached and maintained. However, the additional phosphorus pre-precipitation increases the yearly operating costs of a vertical flow CW system significantly, e.g. for 60 and 25 PE, by 15 and 38%, respectively, thus indicating the need for optimizing the dosing of the chemical.

Are constructed treatment wetlands sustainable sanitation solutions?

The main objective of sanitation systems is to protect and promote human health by providing a clean environment and breaking the cycle of disease. In order to be sustainable, a sanitation system has to be not only economically viable, socially acceptable and technically and institutionally appropriate, but it should also protect the environment and the natural resources. 'Resources-oriented sanitation' describes the approach in which human excreta and water from households are recognized as resource made available for reuse. Nowadays, 'resources-oriented sanitation' is understood in the same way as 'ecological sanitation'. For resources-oriented sanitation systems to be truly sustainable they have to comply with the definition of sustainable sanitation as given by the Sustainable Sanitation Alliance (SuSanA, www.susana.org). Constructed treatment wetlands meet the basic criteria of sustainable sanitation systems by preventing diseases, protecting the environment, and being an affordable, acceptable, and simple technology. Additionally, constructed treatment wetlands produce treated wastewater of high quality, which is fostering reuse, which in turn makes them applicable in resources-oriented sanitation systems. The paper discusses the features that make constructed treatment wetlands a suitable solution in sustainable resources-oriented sanitation systems, the importance of system thinking for sustainability, as well as key factors for sustainable implementation of constructed wetland systems.

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.

Long-term behaviour of a two-stage CW system regarding nitrogen removal

In the first two years of operation a nitrogen removal efficiency of 53% and a high average elimination rate of 1,000 g N m(-2) yr(-1) could be observed for a two-stage vertical flow (VF) constructed wetland (CW) system. The two-stage system consists of two VF beds with intermittent loading operated in series, each stage having a surface area of 10 m2. The first stage uses sand with a grain size of 2-3.2 mm for the 50 cm main layer and has a drainage layer that is impounded; the second stage sand with a grain size of 0.06-4 mm and a conventional drainage layer (with free drainage). The two-stage VF system was designed for and operated with an organic load of 40 g COD m(-2) d(-1) (i.e. 2 m2 per person equivalent). Data from the following years of operation showed that from the third year nitrogen elimination increased and stabilized. The median values of the nitrogen elimination rate in the first five years of operation have been 3.51, 2.76, 4.20, 3.84 and 4.07 g N m(-2) d(-1), the median value of the last three years being 3.8 g N m(-2) d(-1) and 1,380 g N m(-2) yr(-1), respectively, and the nitrogen removal > 60%. It can be assumed that the vegetation as well as the biofilm development in the two-stage VF CW system plays the major role for the enhancement of the nitrogen elimination rate.

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.

Comparison of single-stage and a two-stage vertical flow constructed wetland systems for different load scenarios

Constructed wetlands (CWs) are known to be robust wastewater treatment systems and are therefore very suitable for small villages and single households. When nitrification is required, vertical flow (VF) CWs are widely used. This contribution compares the behaviour and treatment efficiencies of a single-stage VF CW and a two-stage VF CW system under varying operating and loading conditions according to standardized testing procedures for small wastewater treatment plants as described in the European standard EN 12566-3. The single-stage VF CW is designed and operated according to the Austrian design standards with an organic load of 20 g COD m(-2) d(-1) (i.e. 4 m(2) per person equivalent (PE)) The two-stage VF CW system is operated with 40 g COD m(-2) d(-1) (i.e. 2 m(2) per PE). During the 48 week testing period the Austrian threshold effluent concentrations have not been exceeded in either system. The two-stage VF CW system showed to be more robust as compared to the single-stage VF CW especially during highly fluctuating loads at low temperatures.

CWM1: a general model to describe biokinetic processes in subsurface flow constructed wetlands

This paper presents the Constructed Wetland Model No1 (CWM1), a general model to describe biochemical transformation and degradation processes for organic matter, nitrogen and sulphur in subsurface flow constructed wetlands. The main objective of CWM1 is to predict effluent concentrations from constructed wetlands without predicting gaseous emissions. CWM1 describes aerobic, anoxic and anaerobic processes and is therefore applicable to both horizontal and vertical flow systems. 17 processes and 16 components (8 soluble and 8 particulate) are considered. CWM1 is based on the mathematical formulation as introduced by the IWA Activated Sludge Models (ASMs). It is important to note that besides the biokinetic model a number of other processes including porous media hydrodynamics, the influence of plants, the transport of particles/suspended matter to describe clogging processes, adsorption and desorption processes and physical re-aeration must be considered for the formulation of a full model for constructed wetlands.

Experiences with a top layer of gravel to enhance the performance of vertical flow constructed wetlands at cold temperatures

In a first phase of this study it was shown that the Austrian effluent standards for organic matter could not be met in winter for vertical flow (VF) beds designed for and loaded with 27 g COD.m(-2).d(-1) (3 m2 per person equivalent). The aim of this second phase of the study was to investigate, if the performance of a constructed wetland can be enhanced, i.e. if the effluent requirements can be met, when an additional gravel layer (15 cm, 4-8 mm) is added on top of the main layer of the VF bed. The hypothesis was that this top layer would increase the thermal insulation and consequently the temperatures in the filter bed during cold periods, thus resulting in higher removal efficiencies during winter. Two VF beds were operated in parallel; one bed with such a 15 cm top layer, one without. Otherwise the construction of both beds was identical: surface area of about 20 m2, 50 cm main layer (grain size 0.06-4 mm, d10=0.2 mm; d60=0.8 mm), planted with common reed (Phragmites australis). The beds were intermittently loaded 4 times per day with mechanically pre-treated wastewater (hydraulic loading: 47 mm.d(-1); median value of the influent concentration: 505 mg COD.L(-1)). Despite a better performance during the first winter, the bed with additional top layer showed in general a very unstable performance. It is assumed that the main reason for this was that the oxygen transfer was reduced by the additional top layer so far that suspended organic matter could not any longer be degraded in between loadings. Therefore clogging of the filter occurred.

A two-stage subsurface vertical flow constructed wetland for high-rate nitrogen removal

By using a two-stage constructed wetland (CW) system operated with an organic load of 40 gCOD.m(-2).d(-1) (2 m2 per person equivalent) average nitrogen removal efficiencies of about 50% and average nitrogen elimination rates of 980 g N.m(-2).yr(-1) could be achieved. Two vertical flow beds with intermittent loading have been operated in series. The first stage uses sand with a grain size of 2-3.2 mm for the main layer and has a drainage layer that is impounded; the second stage sand with a grain size of 0.06-4 mm and a drainage layer with free drainage. The high nitrogen removal can be achieved without recirculation thus it is possible to operate the two-stage CW system without energy input. The paper shows performance data for the two-stage CW system regarding removal of organic matter and nitrogen for the two year operating period of the system. Additionally, its efficiency is compared with the efficiency of a single-stage vertical flow CW system designed and operated according to the Austrian design standards with 4 m2 per person equivalent. The comparison shows that a higher effluent quality could be reached with the two-stage system although the two-stage CW system is operated with the double organic load or half the specific surface area requirement, respectively. Another advantage is that the specific investment costs of the two-stage CW system amount to 1,200 EUR per person (without mechanical pre-treatment) and are only about 60% of the specific investment costs of the singe-stage CW system.

Long-term evaluation of a spectral sensor for nitrite and nitrate

A spectral in-situ UV sensor was investigated to measure nitrite and nitrate concentrations in the effluent of the EAWAG pilot-scale plant. The sensor was used with a calibration that was based on data from another WWTP and was operated over a period of 1.5 years. The results showed constant accuracy although the sensor was operated with minimal maintenance (manual cleaning once a month). It could be shown that the sensor was able to accurately predict the nitrite and nitrate concentration with a precision of 0.32 mg N/l (95% prediction interval at mean lab value of 1.15 mg N/l) and 1.08 mg N/l (at 5.55 mg N/l) for nitrite and nitrate, respectively. The UV sensor showed good results for nitrite in the low concentration range and very accurate results for higher concentrations (up to 10 mg N/l). This allows using the sensor for alarm systems as well as for control concepts at WWTPs.

Generation of diurnal variation for influent data for dynamic simulation

When using dynamic simulation for fine tuning of the design of activated sludge (AS) plants diurnal variations of influent data are required. For this application usually only data from the design process and no measured data are available. In this paper a simple method to generate diurnal variations of wastewater flow and concentrations is described. The aim is to generate realistic influent data in terms of flow, concentrations and TKN/COD ratios and not to predict the influent of the AS plant in detail. The work has been prepared within the framework of HSG-Sim (Hochschulgruppe Simulation, http://www.hsgsim.org), a group of researchers from Germany, Austria, Luxembourg, Poland, the Netherlands and Switzerland.

Removal efficiency of subsurface vertical flow constructed wetlands for different organic loads

Using subsurface vertical flow constructed wetlands (SSVFCWs) with intermittent loading it is possible to fulfil the stringent Austrian effluent standards regarding nitrification. For small plants (less than 500 persons) standards for ammonia nitrogen concentration have to be met at water temperatures higher than 12 degrees C, effluent concentrations and treatment efficiencies for organic matter have to be met the whole year around. According to the Austrian design standards the required surface area for SSVFCWs treating wastewater was 5 m2 per person. Within the first part of an Austrian research project the goal was to optimise, i.e. minimise the surface area requirement of vertical flow beds. Therefore, three SSVFCWs with a surface area of 20 m2 each have been operated in parallel. The organic loads applied were 20, 27 and 40 g COD/m2/d, which corresponds to a specific surface area requirement of 4, 3 and 2 m2 per PE, respectively. The paper compares the effluent concentrations and elimination efficiencies of the three parallel operated beds. It could be shown that a specific area demand of 4 m2 per person is suitable to be included in the revision of the Austrian design standard. Additionally it could be shown that during the warmer seasons (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 has been proven to be adequate.

Diversity of ammonia oxidising bacteria in a vertical flow constructed wetland

Vertical flow constructed wetlands (VFCWs) with intermittent loading are very suitable for nitrification. Ammonia oxidising bacteria (AOB) are the limiting step of nitration. Therefore the AOB community of a full-scale VFCW, receiving municipal wastewater, was investigated within this study. The diversity of the functional gene encoding the alpha-subunit of the ammonia monooxygenase (amoA), present only in AOB, was assessed by denaturing gradient gel electrophoresis (DGGE). Only very few amoA sequence types dominated the wetland filter substrate; nevertheless a stable nitrification performance could be observed. During the cold season the nitrification was slightly reduced, but it has been shown that the same AOB could be identified. No spatial AOB pattern could be observed within the filter body of the VFCW. The most prominent bands were excised from DGGE gels and sequenced. Sequence analyses revealed two dominant AOB lineages: Nitrosomonas europaea/"Nitrosococcus mobilis" and Nitrosospira. Species of the Nitrosomonas lineage are commonly found in conventional wastewater treatment plants (WWTPs). In contrast, members of the Nitrosospira lineage are rarely present in WWTPs. Our observations indicate that the AOB community in this VFCW is similar to that found in horizontal flow constructed wetlands, but differs from common WWTPs regarding the presence of Nitrosospira.

Comparison of measured and simulated distribution of microbial biomass in subsurface vertical flow constructed wetlands

The multi-component reactive transport module CW2D has been developed to model transport and reactions of the main constituents of municipal wastewater in subsurface flow constructed wetlands and is able to describe the biochemical elimination and transformation processes for organic matter, nitrogen and phosphorus. It has been shown that simulation results match the measured data when the flow model can be calibrated well. However, there is a need to develop experimental techniques for the measurement of CW2D model parameters to increase the quality of the simulation results. Over the last years methods to characterise the microbial biocoenosis in vertical subsurface flow constructed wetlands have been developed. The paper shows measured data for microbial biomass and their comparison with simulation results using different heterotrophic lysis rate constants.

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.

Uncertainties of spectral in situ measurements in wastewater using different calibration approaches

Three calibration methods were applied to UV/VIS spectra recorded in the influent of six wastewater treatment plants (WWTPs) to measure total COD (CODtot), filtered COD (CODfil), nitrate and nitrite nitrogen (NO(x)-N) and total suspended solids (TSS). It could be shown that a calibration of the sensor using data sets from four Swiss WWTPs leads to an improvement of the precision in comparison to the global calibration provided by the manufacturer. A calibration to the specific wastewater matrix always improves the results and gives the highest accuracy. For CODtot a mean coefficient of variation CVx of 12.5% could be reached, whereas for NOx-N only weak results were achieved (average CVx = 36%).

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 plant uptake on the removal of organic matter and nutrients in subsurface flow constructed wetlands: a simulation study

Plants in constructed wetlands have several functions related to the treatment processes. It is generally agreed that nutrient uptake is a minor factor in constructed wetlands treating wastewater compared to the loadings applied. For low loaded systems plant uptake can contribute a significant amount to nutrient removal. The contribution of plant uptake is simulated for different qualities of water to be treated using the multi-component reactive transport module CW2D. CW2D is able to describe the biochemical elimination and transformation processes for organic matter, nitrogen and phosphorus in subsurface flow constructed wetlands. The model for plant uptake implemented describes nutrient uptake coupled to water uptake. Literature values are used to calculate potential water and nutrient uptake rates. For a constructed wetland treating municipal wastewater a potential nutrient uptake of about 1.9% of the influent nitrogen and phosphorus load can be expected. For lower loaded systems the potential uptake is significantly higher, e.g. 46% of the nitrogen load for treatment of greywater. The potential uptake rates could only be simulated for high loaded systems i.e. constructed wetlands treating wastewater. For low loaded systems the nutrient concentrations in the liquid phase were too low to simulate the potential uptake rates using the implemented model for plant uptake.

Time-resolved delta spectrometry: a method to define alarm parameters from spectral data

Alarm parameters are in many ways different from measurements of well defined chemical substances. Being confronted with an increasing number of potentially harmful compounds as well as financial and logistic constraints, new variables (such as alarm parameters) that allow for an integrated assessment or for a first screening can be a solution. To monitor for surrogate or aggregate variables can be a useful strategy to overcome some of the constraints. It must be conceived that this can go along with losses in terms of comparability of results and even in tailor-made variables. Spectral data and their evolution over time are rich in information and compensate for losses due to aggregation and generalisation. Therefore it can be expected that alarm parameters developed from spectral data are transferable, accurate and selective to an extent which is beyond the state-of-the-art. The paper introduces time-resolved delta spectrometry, a method that was developed to generate alarm parameters from spectral data.

Spectral in-situ analysis of NO2, NO3, COD, DOC and TSS in the effluent of a WWTP

An in-situ UV spectrometer was applied to the effluent of a WWTP in Switzerland and calibrated using a multivariate calibration algorithm based on PLS regression. Except for nitrite, the calibration was based on comparative measurements of the effluent in the plant laboratory. Samples made of stock solution added to three different matrices prepared in the EAWAG laboratory were used for the nitrite calibration because the effluent concentrations were always in the range of 0.06-0.26 mg/l. The results show very good precision for nitrite and nitrate. The measuring range for COD and DOC was not completely covered by the measurements, so the meaningfulness of the results is limited. Nevertheless the precision obtained for soluble COD is high enough for most applications at WWTPs. The accuracy of the TSS measurement is unsatisfactory as regards effluent limits since the spectrometer used does not cover the wavelength region up to 700 nm, which gives better signals for TSS calibration due to its strong correlation with turbidity.

Monitoring of a paper mill wastewater treatment plant using UV/VIS spectroscopy

A submersible UV/VIS spectrometer was used to monitor a paper mill wastewater treatment plant. It utilises the UV/VIS range (200-750 nm) for simultaneous measurement of COD, filtered COD, TSS and nitrate with just a single instrument. The instrument measures in-situ, directly in the process. Paper mill wastewater shows typical and reproducible spectra at various process measuring points. There is a relative maximum at 280 mm due to the absorbance by dissolved organic substances, mainly ligninic acids. Comparison of absorbance spectra distinctly shows the decrease of this peak, indicating biological degradation throughout the treatment process. Summarising, one can say that paper mill wastewater cannot be monitored by a simple UV probe measuring only the absorbance at a single wavelength. The required information can only be gained from the whole spectra. Regarding plant control it is suggested that only the overall spectral information is used. Calibrations to conventional parameters are now merely carried out for purposes of reference-checking.

On-line monitoring for control of a pilot-scale sequencing batch reactor using a submersible UV/VIS spectrometer

A submersible UV/VIS spectrometer was used to monitor a pilot-scale sequencing batch reactor (SBR). The instrument utilises the whole UV/VIS range between 200 and 750 nm. With just one single instrument nitrate, organic matter and suspended solids can be measured simultaneously. The spectrometer is installed directly in the reactor, measures in real-time, and is equipped with an auto-cleaning system using pressured air. The paper shows the calibration results for measurements in the SBR tank, time series for typical SBR cycles, and proposes possible ways for optimisation of the operation by using these measurements.

A guideline for simulation studies of wastewater treatment plants

A guideline for simulation studies of wastewater treatment plants is proposed. The aim of the HSG-guideline is to define a reference quality level that helps to make the results of simulation studies comprehensible and comparable and therefore increases the quality and reliability of mathematical modelling in wastewater treatment. The paper gives a summary of the HSG-guideline, written by a group of university members from Germany, Austria and Switzerland.

Evaluation of substrate clogging processes in vertical flow constructed wetlands

Substrate clogging is by far the biggest operational problem of vertical flow constructed wetlands. The term "substrate clogging" summarises several processes which lead to reduction of the infiltration capacity at the substrate surface. The lower infiltration rate causes a reduced oxygen supply and further leads to a rapid failure of the treatment performance. Reasons for substrate clogging include accumulation of suspended solids, surplus sludge production, chemical precipitation and deposition in the pores, growth of plant-rhizomes and roots, generation of gas and compaction of the clogging layer. However, it is not clear how much each process contributes to the clogging process. Detailed investigations were carried out at pilot-scale constructed wetlands (PSCWs) using a variety of methods: e.g. soil physical investigations, microbial methods, and various analysis methods of drinking water and wastewater. The paper shows the results of these investigations and presents an equation to calculate the theoretical clogging time.

Rapid automated detection of nitrification kinetics using respirometry

There is no doubt that respirometry is a useful measurement principle in the field of wastewater treatment. Although a large variety of methods and case studies have been published, respirometry has become neither a standard tool for control nor for assessment and optimisation of treatment plants. drawback of the conventional method for determining nitrification kinetics is the long experimental time. This disadvantage can be avoided by "turning over" the experiment. Starting with low ammonia concentrations the steep slope of the Monod curve is measured first. The low concentration branch of the Monod curve is also the part where e.g. inhibition can be detected. Therefore the proposed procedure allows us to speed up the measurement of nitrification kinetics and to measure nitrification inhibition on-line.

A multivariate calibration procedure for UV/VIS spectrometric quantification of organic matter and nitrate in wastewater

A submersible UV/VIS spectrometer for in-situ real-time measurements is presented. It utilises the UV/VIS range (200-750 nm) for simultaneous measurement of COD, filtered COD, TSS and nitrate with just a single instrument. A global calibration is provided that is valid for typical municipal wastewater compositions. Usually high correlation coefficients can be achieved using this standard setting. By running a local calibration improvements concerning trueness, precision and long term stability of the results can be achieved. The calibration model is built by means of PLS, various validation procedures and outlier tests to reach both high correlation quality and robustness. This paper describes the UV/VIS spectrometer and the calibration procedure.

Simulation of subsurface flow constructed wetlands--results and further research needs

Simulation of constructed wetlands has two main tasks: to obtain a better understanding of the processes in constructed wetlands, and to check and optimise existing design criteria. This paper shows simulation results for two indoor pilot-scale constructed wetlands for wastewater and surface water treatment respectively. The results presented and discussed are mainly focussed on the hydraulic behaviour of the constructed wetland systems. In addition results of reactive transport simulations with CW2D are shown. The multi-component reactive transport model CW2D (Constructed Wetlands 2 Dimensional) was developed to model transport and reactions of the main constituents of wastewater (organic matter, nitrogen, and phosphorus) in subsurface flow constructed wetlands. For the pilot-scale constructed wetlands a calibration of the flow model was possible and therefore the results of the reactive transport simulations with CW2D fit the measured data well. The further research needs regarding the simulation of subsurface flow constructed wetlands are discussed.

Management of sensible water uses with real-time measurements

For the protection of bankside wells and a groundwater recharge an early warning system had to be developed. The monitoring network design is based on sensor measurements only. For this purpose a new submersible spectrometer has been successfully tested for multi-parameter measurements directly in the medium. The developed system can easily be upgraded with other new sensors. Only calibration and validation data are supplied by conventional grab sampling and laboratory analysis.. A conventional testfilter improved by on-line monitoring at 5 sampling sites serves as a reference system. The whole system is equipped with remote control and the internet serves as the control centre of the network. All measurement data from all 9 sites are available in real time on the internet.