Special Section on Natural Treatment Systems: More Than Just a Solution to Pollution

The field of environmental engineering is focused on prohibiting the release of pollutants and mitigating anthropogenic impacts. We often consider the mantra “protect human health and the environment” as our mission. Without question, our top priority is protecting human health and welfare, but in many cases, we do not consider the life-cycle impacts of our approach in providing that protection or consider potential life-cycle benefits that are possible. In some cases, natural treatment systems can provide both mitigation of anthropogenic impacts and benefits to ecology and community.

Our field has greatly evolved and expanded from the sanitary engineering days of treating drinking water we put into a pipe system and treating wastewater at the end of the pipe. In many cases, we have clearly identified the valuation of far more than the $\mathrm{mg}/\mathrm{L}$ of a pollutant in a stream of air or water. As a profession and society, we even routinely bank wetlands, under section 404 of the Clean Water Act. As a society, we have chosen to invest billions in wetlands mitigation and restoration. Wetlands have been assessed to have specific value such that we will demand an ecosystem be built to replace what has been consumed in infrastructure or urban development. Although certainly not entirely novel, many of the articles of this special section look at natural treatment systems that use wetlands, plants, and integrated approaches that use plants as the central piece in directing energy and influencing chemistry and biology to serve a treatment purpose, i.e., as the solution to pollution. At the same time, these systems do provide an ecological benefit. The broad term green infrastructure has been applied to many of the systems discussed in this issue. The EPA has invested considerable effort in promoting green infrastructure approaches and not only notes water quality, but incorporates community, energy, and habitat in the discussion. Effective natural treatment systems inherently offer benefits to people and the planet, but the remaining question is often the cost (i.e., prosperity) and time frames to meet the trifecta of ${\text{P}}^3$ outcomes.

In considering green infrastructure and many other natural treatment system approaches as options, new time line considerations are necessary when addressing large-scale infrastructure and environmental issues. Groundwater pollutants have often taken decades to reach their current state, because entropy happens. Either great energy or time is required to remedy the current state of the problem. In other cases, decades or centuries of development lead to current environmental/infrastructure quandaries. Because many cities are now facing storm-water issues, municipalities are addressing more than a hundred years of infrastructure growth and development that were intended to serve a purpose, and that purpose has now changed. The River Des Peres in St. Louis is a prime example. At the beginning of the twentieth century, St. Louis was marveled for having a state-of-the-art sewer system when hosting the 1904 World’s Fair, and today some of that same infrastructure is still in place. The River Des Peres was essentially used as an open sewer system in the 1800s and later was buried and channelized to conceal and expedite the flow of storm water and sewage to the Mississippi River. The project was a huge success at the time, and cholera outbreaks that decimated as much as 10% of the St. Louis population in a single outbreak and flooding issues that killed hundreds were both mitigated. In 1988, ASCE declared the River Des Peres project a national civil engineering landmark. This landmark project is now being changed again. Efforts are underway to drastically improve the water quality and return both the natural and community function of the River Des Peres, St. Louis, and the Mississippi River, in part because of a consent decree to reduce combined sewer overflows. The St. Louis Metropolitan Sewer District will spend $100 million on green infrastructure to reduce combined sewer overflows that reach the Mississippi River and in many cases were part of the River Des Peres design that started over 150 years ago. The projects will take time, but they are integrated to solve the environmental problem and provide ecological and community benefits.

As environmental engineers and scientists, we must look to protect human health and do so expeditiously, but as problem solvers, we can also consider longer time frames to grow our way out of a problem and into a natural, integrated solution when imminent health impacts are not apparent. By better understanding, designing, and applying natural treatment system approaches, we can minimize the energy needs of mitigating a pollutant or an impact, and we can concurrently provide or enhance ecosystem and community services. Opportunities for many green infrastructure approaches must be integrated with larger projects and infrastructure rehabilitations as needed. The cost to tear out functioning infrastructure is too great, so more time is needed. Because we just passed the 40th anniversary of the Clean Water Act, the idea of long-term approaches and the impacts on water quality should be evident. We have grown our way into many of our current problems through centuries of industrial and infrastructure development, and by using plant-based natural systems, we can work to grow our way into some new approaches, solutions, and benefits along the way.

This issue is also my last as an associate editor of the Journal of Environmental Engineering. Thank you to the journal production staff and the editorial staff. Particularly, I offer a whole-hearted thank you to Ray Ferrara for his work to continually improve this journal as editor through great leadership and tireless efforts. I would like to offer another thank you to Dion Dionysiou for his work on special issues, including this special section. I also appreciate the effort and patience of the authors that made this issue possible. I was honored to serve our great profession as an associate editor for the Journal of Environmental Engineering.