To simultaneously consider the complexity of hydrology (that is, the impact of rainfall intensity and local topography, which influence flooding) and water quality, urban runoff storage and treatment processes should be more common, especially for densely populated cities where natural landscape is insufficiently available to process, infiltrate and treat stormwater. New and strategically geolocalized infiltration areas, collection systems and/or modular treatment processes that provide certain flexibility for expansion can help mitigate floods and the load of contaminants during peak rainfall or snowmelt events. Large-scale viable and sustainable solutions are needed to store and passively treat urban runoff and deal with intense rainfall events that cannot be hydraulically supported by existing wastewater treatment plants designed to treat lower flow rates. Examples of such existing solutions, as well as more sustainable solutions to be adapted for runoff treatment, include retention ponds, bioretention cells or raingardens (~95% particle removal), coarse sand filters, bio-assisted aggregation and filtration systems, aerated ponds, underground tanks in dense urban areas, adsorption via functionalized media in a granular filter, passive aggregation and settling tanks and passive O2/ultraviolet (photo)oxidation. Such retention processes could act as onsite surge tanks while also removing several contaminants from runoff, combined sewer overflow, or cross-connected sewers before discharge into natural waters.
Examples of existing and new promising solutions are presented in Fig. 2 and include hydraulic buffers (solutions 2, 4, 5, 7, 8, 9 and 10), physicochemical filtration and adsorption systems (solution 6, for soluble and particulate matters), bioretention and biodegradation processes (solutions 4, 7, 9 and 10), underground separation units based on centripetal or gravitational force (solutions 3 and 5, for particulates), and (bio)flocculant-assisted bioretention and settling tank (solution 2; partially buried, for soluble and particulate matters). Simple process units can be implemented directly in stormwater sewers or manholes; for instance, vortex separators (solution 3) to remove denser particles from water, screens to trap larger debris (>10 mm), modular biofilters to remove nutrients, heavy metals and oils, and porous granular filters to trap smaller particles (<1 mm; solution 6). On a domestic scale, green roofs (solution 7, which can lead to considerable runoff reduction with only 10% of buildings having green roofs)14, infiltration areas (for example, grass and gardens, mulches and sand-capped lawns rather than concrete pavement; solutions 7 and 8) and small (underground) reservoirs (solution 5) — all acting as surge ‘tanks’ or a hydraulic buffer — could also be considered to reduce the load on larger municipal infrastructure. All of these solutions could be designed with a bypass when the system is at capacity, which is expected to occur during intense rainfall events and to be exacerbated due to climate change. Moreover, to reduce cost and facilitate integration of such solutions in dense cities, some systems could be designed to deal with the runoff ‘first flush’, as the initial rainfall usually releases higher contaminant loads11. Ideally, the proposed processes must be designed to require minimal maintenance between rainfall events.
Audrey Desaulniers (Orceine).
Each number represents a different solution for runoff treatment, as shown in the corresponding key. In solution 2, contaminant aggregation or settling is improved with (bio)flocculants. Valve 2 is open to send settled sludge to 1 and clean water is sent back to the river. Valve 1 is open when turbidity is acceptable, or closed for longer settling. In solutions 3 and 5, centripetal and gravitational forces, respectively, separate large/dense solids from water. In solution 4, no chemical is added, colloids are removed via passive settling and bioretention. Longer residence times enable biofilm formation and biodegradation. In solution 6, recycled crushed glass grafted with metals improves colloid retention and adsorption of soluble contaminants via electrostatic interactions. In solutions 7–10, colloids and soluble contaminants are removed and the processes also act as hydraulic buffers. Arrows in dark and light blue indicate raw and treated waters, respectively.
Besides the positive impact on water quality and helping to preserve biodiversity and mitigate urban heat island effects, the amount of green space in dense urban areas has been correlated with human health and socioeconomic benefits15. As successfully reported in some cities (for example, Philadelphia, Singapore and Hong Kong), green (treatment) infrastructure could reduce runoff flows and floods, and recharge and maintain the quality of aquifer and groundwater to secure water supply in some developing and/or arid countries16,17. Green treatment infrastructure in the United States currently represents <10% (US$4.2 billion) of the total capital investments used (US$48.0 billion) to address combined sewer overflows and meet water-quality objectives of the Clean Water Act18. Yet, several cities report that green infrastructure itself is more cost-effective than conventional ‘grey infrastructure’ sewer systems (for example, Philadelphia and Milwaukee)5, in addition to reducing the load directed to wastewater treatment plants (that is, smaller sewer systems and plants are required). Moreover, with climate change and rapid urbanization, increasing green space in cities dedicated to water infiltration would reduce the risk of flooding — and its associated economic burden — caused by the growth of impervious surfaces in dense urban areas19. Existing green infrastructures are currently geolocalized and designed to manage floods and water accumulation. If cities are aiming for more versatile green infrastructure, the design should consider requirements for both water storage and treatment. Besides precipitation rates and intensities, the climate would also impact the design. For example, lower temperatures are known to impact adsorption kinetics in porous granular filters and increase water viscosity, which also impacts particle separation via settling. Hence, the required contact time in cold water during filtration and settling could also govern the size of the system. Moreover, the type of technology implemented (for example, granular filter versus adsorbent) will be largely influenced by local contamination patterns and water characteristics. For example, runoff with high concentrations of suspended solids (such as sand and tyre-wear particles) may require different technologies than runoffs with high levels of soluble phosphorus.
Cities have limited resources available for stormwater management. Hence, to maximize the cost-effectiveness of existing and future green infrastructures, and to reduce the risk of acute toxicity in natural waters, the proposed solutions could be coupled with more advanced process control or with data-driven machine-learning techniques20. Rainfall intensity–duration–frequency curves, storm water models, weather forecasts, sudden and planned events (for example, hydrocarbon spills and salts applied in winter), novel qualitative and quantitative tools, and river flows could all be included in the data stream. For example, by using such predictive analytics, the retention tanks proposed in Fig. 2 (solutions 2 and 4) could be deliberately purged to prioritize expected incoming acute contaminations; for instance, combined sewer overflows and perfluoroalkyl-substances-based flame retardants released during fire controls.
As few policies constrain the design of solutions, cities should benefit from a certain flexibility and be able to implement locally adapted, realistic, sustainable and low-cost processes. Despite the challenges to the implementation of new processes for runoff, we believe that such holistic solutions should be considered globally by cities when opportunities for infrastructure changes arise. This could mitigate and prevent the influx of contaminated runoff into aquatic ecosystems and protect animals, people and resources that are imperative to our global communities.
Source: Resources - nature.com
