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 (NBSu) that are stand-alone green technologies or green urban spaces, which can be combined with other solutions (nature-based or not); NBS interventions (NBSi) that refer to the act of intervening in existing ecosystems and in NBSu, by applying techniques to support natural processes. The statistical analysis suggests that NBSu are more versatile than NBSi 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.

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.

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.

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.

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.

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.

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%).

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.

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.

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.

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.

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.

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.

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.

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.