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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.