By Brooke K. Mayer, PhD, PE
Benjamin Franklin once opined, “In this world nothing can be said to be certain, except death and taxes.” As one of the United States’ Founding Fathers, Franklin may have been reticent to add a third item to the list, one that we nonetheless know to be true in the field of water/wastewater: people will keep on ‘going.’ Of course, in this context, going means that people invariably produce human waste; in fact, they produce it in large quantities, at a rate of approximately 34 billion gallons of wastewater per day in the US. The majority of the nation’s domestic wastewater is treated by one of the ~16,500 publicly owned wastewater treatment works, with the remaining ~20 percent of the population being served by onsite wastewater systems.,
Throughout history, the primary goal of wastewater treatment has been protecting human health, which couples closely with protecting environmental health. The more recent paradigm shift from a sole focus on wastewater treatment toward also embracing opportunities for resource recovery has helped to advance sustainability targets.[3–5] Specifically, wastewater contains many materials with embedded chemical or energy value, as illustrated in Figure 1. As such, wastewater is increasingly recognized for its resource recovery potential, including the water itself, energy, nutrients and other valuable materials. Turning the pollutants in used water into profits can potentially account for millions of dollars per year. Verstraete et al. (2009) estimated relative contributions to the value proposition, as shown in Figure 2, highlighting water as the most valuable recoverable resource. In addition to the recovered products, resource recovery facilities provide tremendous value in the form of more-difficult-to-monetize services, e.g., improving water quality, improving operation and performance at wastewater treatment facilities, and improving food and security as well as social equity.
In addition to the water, energy and nutrient products shown in Figure 2a, many metals in wastewater associate with biomass, which is separated as wastewater sludge during treatment. This sludge can be treated for beneficial reuse, at which point it is referred to as biosolids. In 2019, the US produced ~4.75 million dry metric tons of biosolids, roughly half of which were land applied. For a community of one million people, the value of the metals present in biosolids was estimated at up to $13 million/year, with the 13 most lucrative elements accounting for $280/ton of sludge (Figure 3a). Gold and silver alone account for approximately 20 percent of the potential economic value of metals recovery. Thus, to adapt Mark Twain’s adage attributed to prospectors of old, “There’s gold in them thar waters” (…and silver, copper, aluminum…). While typically present at lower levels compared to macro-constituents such as organics and nutrients (critical elements for food systems), precious metals and energy-critical elements (Ga, Pd, Ag and Ir) can boost the value proposition of resource recovery from wastewater.,
The economics of resource recovery are further impacted by the availability of virgin resources. This is indicated in Figures 3b and 3c, which show relatively limited supply of some of the metals that are potentially recoverable from wastewater. This is compounded by limited geographic distribution, thereby impacting global trade. For instance, in 2020, the USGS identified commodities including aluminum, gallium, platinum-group metals and titanium as some of the greatest supply risks for the US manufacturing sector. While reserves are not about to run out any time soon, the constraints of non-renewable resources dictate that at some time in the future, the costs of extracting depleting reserves will exceed practical limits and production will decline. Hence, wastewater contains resources worth recovering as part of a circular economy and continued development of technologies, practices and policies in support of these efforts has broad geopolitical implications.
Given the potential benefits of resource recovery, the question arises, How can we most effectively and efficiently extract value-added resources from heterogenous wastewater flows? Identifying appropriate techniques that are both efficient (in terms of performance and cost) and scalable to location-specific needs can pose a substantial hurdle. Diaz-Elsayed et al. (2019) conducted a comprehensive analysis of mature resource recovery options appropriate for implementation at small-scales (onsite systems), medium-scales (satellite systems) and large-scales (centralized systems), results of which are shown in Figure 4. They reported that water reuse was prevalent at all scales for non-potable aims, whereas potable reuse (both direct and indirect) was typically practicable at large scales. Likewise, energy recovery was primarily implemented at large scales for biogas and electricity production. Alternately, nutrient recovery had the greatest potential for resource recovery at small scales, via urine source separation, whereas current implementation is typically at medium- and large-scales.
As shown, the degree of waste valorization depends on the scale of installation. Centralized treatment with resource recovery often benefits from economies of scale, as well as embodied energy and carbon footprint. For example, Cornejo et al. (2016) found that the embodied energy (including collection, treatment and water reuse) was lowest for centralized treatment, increased by approximately double for community-level treatment and increased by about 2.5-fold for household-scale treatment. Similarly, the carbon footprint doubled for community treatment versus centralized treatment and tripled for the household-scale scenario  Centralized treatment, however, may not be practical in rural communities, peri-urban areas (particularly in developing areas), or land-scarce urban settings due for infrastructure rehabilitation/refurbishment. Decentralized treatment may help to fulfill these needs. For example, a hybrid approach wherein communities close to a centralized facility use centralized resource recovery while communities farther from the centralized facility use decentralized satellite resource recovery was found to be most competitive for several different decision-making strategies in a California case study.
Additionally, advances in less-mature treatment strategies (e.g., membrane bioreactors, upflow anaerobic sludge blanket reactors, selective adsorption/recovery materials, thermo-chemical extraction processes, etc.) are likely to enable increasingly cost-effective organic removal at increasingly smaller scales, thereby helping
to overcome limitations in the economy of scale while facilitating energy savings and/or nutrient recovery. Accordingly, resource recovery will not be a one-size-fits-all endeavor and a variety of locale-specific solutions must be considered to maximize the full value of this approach.
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2. US Department of Homeland Security. “Water and Wastewater Systems Sector-Specific Plan.” In: National Infrastructure Protection Plan (NIPP). ; 2015:1-56. https://www.cisa.gov/sites/default/ files/publications/nipp-ssp-water-2015-508.pdf
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About the author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds Bachelors, Masters and doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.