Additional ecological services of CSO-CW besides water treatment: modelling CSO-CW behaviour for urban runoff management

INTRODUCTION Combined sewer overflows (CSOs) have been recognized as a dangerous pollutant source for receiving water bodies, and CSO treatment is hence very important to promote a sustainable development. Constructed wetlands (CWs) are starting to be considered as a viable and eco-sustainable technology to treat CSOs (Meyer et al., 2013). However, CSO-CW provides other ecological services beside to water treatment: (i) urban runoff management, (ii) biodiversity increase, (iii) social services (e.g., recreation). Here we have developed a mathematical model of a real case study to highlight the functioning of CSO-CW as also a flood mitigation system, which promotes an urban runoff management from a post-development (high peak, short duration) back again to a pre-development (low peak, high duration) hydrograph influent to the river (Fletcher et al., 2013). METHODS The experimental case study is located in Gorla Maggiore, Italy (46°N, 9°E). The CSO-CW is composed of: (i) grid and sedimentation tank as first flush primary treatment; (ii) four French-type vertical subsurface flow (VF) CW beds as secondary stage (3840 m2) designed to treat the first flush (up to 640 l s-1); (iii) a free water surface flow (FWS) wetland with multiple roles of tertiary treatment of first flush and also second flush treatment (3174 m2), biodiversity increasing, recreational area, and hydraulic buffer (with a floodable surface area up to 7200 m2). The theoretical hydraulic retention time (HRT) is equal to 36 h. A sampling campaign has been done in 2014 in order to characterise temporal variations of CSO quality and quantity and to assess CW removal performances. The data about water quantity (CSO flow rates continuously registered by an automatic sensor with a sampling frequency of 15 minutes) are here used as input of the mathematical model. The mathematical model simulates the unsaturated water flow in VF beds (Richards equation) and the depth of the ponding layer above the VF surface and in the FWS (mass balance equations). In this way, water outflows from each stage of the CW plant are estimated, and the flood mitigation efficiency of the CW is evaluated for different type of CSO events (i.e., single or multiple average CSO events, high return time CSO event). RESULTS AND DISCUSSION The model results show the good performance of the CSO-CW as flood mitigation system. The single CSO average event (883 m3 over 2.4 hours, with a maximum flow rate of 250 l/s) is satisfactorily laminated: (i) the peak flow is reduced by 95%; (ii) the outflow duration is 21 times longer than the one of the CSO event; (iii) the CW is able to store 95% of the influent volume during the CSO event.The CSO-CW exhibits also performs well for CSO mitigation when a sequence of consecutive CSO average events (up to 5, i.e. the maximum number of consecutive CSO events registered) is considered as shown in Figure 1. In this case, the peak flow is reduced by 53%, the outflow is prolonged 5.7 times compared to the CSO event duration, and 38% of the influent volume is stored during the CSO event. Flood mitigation performances remain high also for events with high return time (equal to 10 years – maximum flow rate: 3.4 m3 s−1, volume: 11497 m3, duration: 4.8 h), for which the FWS behaves as a buffer system storing 71% of the influent volume, in addition to the lower (11%) but not negligible mitigation effect provided by the VF beds. Moreover, the peak flow (86% reduction) and the outflow duration (27 times longer than the CSO event duration) are satisfactorily improved for such 10 year return time events. Fig. 1. Influent and simulated effluent flow rate from CSO-CW treatment for a sequence of 5 consecutive CSO mean events: influent CSO (gray line), VF outflow (dotted line), VF overflow (dashed line), and FWS outflow (continuous line). CONCLUSIONS The results of this modelling study confirm the potential of CWs to behave as flood mitigation systems providing the additional ecological service of sustainable urban runoff management. The selected case study demonstrates how CSO-CW promotes a shift from a post-development (high peak, short duration) to a pre-development (low peak, high duration) hydrograph influent to the river water body

Evaluation of the influence of filter medium composition on treatment performances in an open-air green wall fed with greywater.

Abstract According to the European Research and Innovation Policy Agenda, nature-based solutions (NBSs) are key technologies to improve the sustainability of urban areas. Among NBSs, green walls have been recently studied for several applications, among the others the treatment of lowly polluted wastewater flows as greywater (GW, e.g. domestic wastewater excluding toilet flushes). This work is aimed at the evaluation of the influence of four additives (compost, biochar, granular activated carbon, polyacrylate) mixed with a base filter medium made of coconut fibre and perlite, on the performances of a green wall fed in batch mode with synthetic GW. The green wall was operated with a high hydraulic loading rate of GW (740.8 L/m2/day) in open-air winter conditions (3.5–15 °C measured for GW) between January and April. The performances of the green wall have been assessed though the monitoring every 1–2 weeks of physicochemical and biological parameters (pH, electric conductivity, total suspended solids, dissolved oxygen, BOD5 and COD, nitrogen and phosporus compounds, chlorides and sulphates, anionic surfactants and E. coli). Removal performances were excellent for BOD5 (>95%) and E.coli (>98%) for all additives; compared to the base medium, biochar was the best performing additive over the highest number of parameters, achieving removals equal to 51% for COD, 47% for TKN and nitric nitrogen and 71% for anionic surfactants. Compost also achieved high removal performances, but the frequent clogging events occurred during the monitoring period do not make its use recommendable. Granular activated carbon and the combination of biochar and polyacrylate performed better than the base medium, but only about the removal of nitric nitrogen. These results demonstrated that, in the considered experimental boundaries, biochar could improve the overall treatment performances of a green wall fed by GW and operated in challenging conditions

A review of nature-based solutions for greywater treatment: Applications, hydraulic design, and environmental benefits

Abstract Recognizing greywater as a relevant secondary source of water and nutrients represents an important chance for the sustainable management of water resource. In the last two decades, many studies analysed the environmental, economic, and energetic benefits of the reuse of greywater treated by nature-based solutions (NBS). This work reviews existing case studies of traditional constructed wetlands and new integrated technologies (e.g., green roofs and green walls) for greywater treatment and reuse, with a specific focus on their treatment performance as a function of hydraulic operating parameters. The aim of this work is to understand if the application of NBS can represent a valid alternative to conventional treatment technologies, providing quantitative indications for their design. Specifically, indications concerning threshold values of hydraulic design parameters to guarantee high removal performance are suggested. Finally, the existing literature on life cycle analysis of NBS for greywater treatment has been examined, confirming the provided environmental benefits

A review of nature-based solutions for urban water management in European circular cities: a critical assessment based on case studies and literature

Abstract Nature-based solutions (NBS) can protect, manage and restore natural or modified ecosystems. They are a multidisciplinary, integrated approach to address societal challenges and some natural hazards effectively and adaptively, simultaneously providing human well-being and biodiversity benefits. NBS applications can be easily noticed in circular cities, establishing an urban system that is regenerative and accessible. This paper aims to offer a review on NBS for urban water management from the literature and some relevant projects running within the COST Action 'Implementing nature-based solutions for creating a resourceful circular city'. The method used in the study is based on a detailed tracking of specific keywords in the literature using Google Scholar, ResearchGate, Academia.edu, ScienceDirect and Scopus. Based on this review, three main applications were identified: (i) flood and drought protection; (ii) the water-food-energy nexus; and (iii) water purification. The paper shows that NBS provide additional benefits, such as improving water quality, increasing biodiversity, obtaining social co-benefits, improving urban microclimate, and the reduction of energy consumption by improving indoor climate. The paper concludes that a systemic change to NBS should be given a higher priority and be preferred over conventional water infrastructure

Modelling human-made wetlands for a sustainable development

Subtitle: Investigation of paddy fields and constructed wetlands. The PhD thesis discusses the investigation of methane emissions from paddy fields and the stochastic behaviour of constructed wetland via a modelling approac

French Reed Bed as a Solution to Minimize the Operational and Maintenance Costs of Wastewater Treatment from a Small Settlement: An Italian Example

French Reed Bed (FRB) is a particular constructed wetland (CW) solution which receives raw wastewater. Data from the full-scale FRB wastewater treatment plant of Castelluccio di Norcia (center of Italy) were collected to show the FRB capability to minimize the operational and management (O&M) costs. The system was designed to treat wastewater variable from 200 person equivalent (PE) in off-season up to 1000 PE. Data from 2014 up to 2016 showed high removal efficiency in line with French experiences with FRBs. An interview was conducted with the Water Utility to estimate the operational and maintenance (O&M) costs faced by the WWTP, which allowed us to detail the O&M costs for energy consumption, water quality samples, and personnel for inspection. Other O&M expenditure items were estimated on the basis of parametric costs from the executive design. The FRB O&M costs in euro for 500–1000 PE (6–11 € PE−1 year−1) resulted from 5 to 13 lower in comparison to those reported for classical activated sludge systems in an Italian context (45–90 € year−1). The low O&M costs are mainly due to the limited energy consumed and to the minimized costs of sludge management

Can microbial fuel cells be an effective mitigation strategy for methane emissions from paddy fields?

Microbial fuel cells (MFCs) are bioelectrochemical systems able to generate electricity from wetland soils, including paddies, exploiting the microbial decomposition of organic matter. Additionally, MFCs can also be applied as a novel mitigation strategy for emissions of methane (CH4) from paddy fields. The MFC efficiency in reducing CH4 fluxes is still poorly understood, and it is here investigated via a one-dimensional process-based model that simulates the vertical and temporal dynamics of the chemical compounds affecting CH4 fate within paddy soil. The DOC oxidation rate of MFC is modeled assuming a zero-order kinetics proportional to the generated electricity and different anode depths and current densities are tested. By assuming current densities presently achieved in paddies, MFCs are able to reduce up to −27.9%, −16.7%, and −22.0% of daily minimum, daily maximum, and total CH4 emissions, respectively. Moreover, CH4 reductions are even higher (up to −28.1%, −24.1%, and −26.5%) if we assume 5% of the current density developed on laboratory acetate-fed MFC. The system shows a limiting effect of transport processes on the mitigation of CH4 emissions at high current density. In order to maximize the reduction of CH4 emissions, simulation results suggest to place the anode in the middle portion of the superficial layer of paddy soil. The findings demonstrate the need to further investigate and develop this new technology for field-scale application