How Green Infrastructure is Transforming Sustainable Wastewater Treatment

Sustainable solutions for managing wastewater are becoming critical as urban and suburban areas expand and climate shift projections are anticipating higher rain intensities and durations. While traditional systems were effective, they’re often costly to update and can put undue pressure on the natural environment. Green infrastructure, an up-and-coming strategy, uses nature-based solutions to manage stormwater, bolster resilience, and help municipalities and new development meet goals for environmental stewardship and protection.

What is Green Infrastructure?

In 2019, the United States Congress enacted the Water Infrastructure Improvement Act, which defines green infrastructure as "the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters.” By mimicking how nature manages rain, green infrastructure can help reduce the impact of that water. 

Constructed Treatment Wetlands

Constructed treatment wetlands (CTW’s) use plants, soil, and microbes to slow down water so that pollutants can be filtered out and/or broken down in stormwater. As water flows into the system, nutrients, heavy metals, and other contaminants are naturally absorbed by plants or broken down by microbes that grow on their roots. In addition to cleaning the water and managing flow, CTW’s create habitat for wildlife to settle into.

Conventional treatment plants are becoming antiquated and overburdened by continued development. To upgrade them can be extremely costly and oftentimes impossible due to surrounding land use.  Constructed treatment wetlands, by contrast, can be built into the landscape itself, providing green space for pollinators, habitat for numerous species, and permanent open space for municipalities. 

Green Roofs 

Green roofs are typically incorporated into new construction due to increased loads needed to carry their weight. Green roofs can be categorized by the thickness of soil – Extensive, 2” to 4” of soil media; Semi-Intensive, 4” to 8” of soil media; and Intensive, anything above 8” of soil media.  The deeper the soil media, the heavier, more costly, but greater diversity of plants that can be grown.  The greatest benefit of a green roof may be its longevity since ultraviolet rays are absorbed by plants and soil while freeze thaw on a roof deck is reduced by plants and soil.  Green roofs also intercept airborne pollutants, hold stormwater, and provide much needed wildlife habitat, especially in urban areas.   

Permeable Pavements

Impermeable surfaces such as parking lots and sidewalks shed nearly 100% of the rain that falls on them, also taking with the runoff any pollutants, grease, oil, gasoline, and sediment that is sitting on the surface. Permeable pavements, which include pavers and monolithic pavements (concrete and asphalt), infiltrate nearly 100% of the rain that falls on them, taking the water into stratified layers of open-graded rock that help filter, slow down, and cleanse stormwater.  In these systems, water can be infiltrated into the soil below or evaporated back through the surface as evapotranspiration. Permeable systems also help reduce the need for de-icer in the winter and resurfacing over many years, which is detrimental to water quality and thus plants and animals.

Rain Gardens and Bioswales

Rain gardens and bioswales are considered green infrastructure and can be effective tools at many scales – residential, business/commercial, industrial.  Both rely on native plants and soil to capture, cleanse, and infiltrate rainwater from impermeable surfaces such as rooftops, parking lots, and driveways.  Rain gardens are typically associated with residential properties and downspouts. They are depressed areas in the landscape that collect rain water and are often planted with native grasses and flowering perennials.  Rain gardens can be a cost effective and beautiful way to reduce runoff from a property, absorb and filter pollutants, reduce contaminants, and provide food and shelter for butterflies, songbirds, insects, and other wildlife.

Bioswales are most often associated with parking lots and perform similarly to rain gardens. Within a parking lot, they become linear landscape features that are depressed to collect runoff from adjacent impermeable surfaces.  Rain that enters a bioswale is slowed, cleansed, and infiltrated using native plants, amended soils, and quite often underdrains to ensure water does not back up into a parking lot. 

An important component of both systems is maintenance. Because they collect runoff from rooftops and parking lots that can have high concentrations of sediment, nutrients, and solids, they need to have monthly cycles of plant care and removal of garbage.

Other Benefits of Green Infrastructure

While the primary goal is to prevent traditional systems from being overwhelmed, green infrastructure systems have other benefits as well.

• Beautify a neighborhood with increased landscape, including trees that help mitigate the “urban heat island” effect, where pavement and concrete absorb more heat and increase air temperatures.

• Help protect streams and rivers from receiving pollution by reducing stormwater runoff.

• Reduce the risk of flooding and drainage problems.

• Native plants assist with air quality by absorbing carbon dioxide, airborne pollutants, and other emissions.

• As “passive” systems, they limit energy consumption, noise and light pollution, and other issues around traditional systems.

• They provide a low-cost way to add green space to urban areas, and can be used in a variety of brownfields and other land that would otherwise detract from urban health.

By making use of nature-based solutions, and integrating them into urban design, we can bolster both the resilience and beauty of the spaces we live, play, and work in.