Wastewater alkalinity addition as a novel approach for ocean negative carbon emissions.

Anthropogenic CO2 emissions have greatly increased atmospheric CO2 contributing to global warming and leading to ocean acidification (Figure 1). As reflected in the recent IPCC report, the scientific community's consensus is that emissions reductions alone are not sufficient or timely enough to avoid a global warming catastrophe. Thus, negative-carbon-emission technologies are needed to avoid atmospheric CO2 overshoot scenarios and limit global warming to less than 2°C by the end of this century per the Paris Agreement. Due to the urgency and scale of the issue, multiple negative-emission technologies should be evaluated and adopted with broad community involvement to address our society's pressing climate crisis. The goal is to remove at least 10 Gt-CO2/year from the atmosphere by the mid to-late century, which is more than the current annual anthropogenic CO2 uptake by the global ocean (Figure 1). Among various ocean negative-carbon-emission approaches or ocean-based carbon dioxide removal (CDR) technologies, ocean alkalinity enhancement (OAE) is an approach that will decrease sea surface pCO2 via the addition of alkaline materials and promote CO2 uptake from the atmosphere. Additionally, as the oceanic dissolved inorganic carbon (DIC) reservoir is nearly 50 times the atmospheric CO2 content, the sequestered CO2 can remain in the ocean DIC pool as bicarbonate (HCO3−) for centuries. OAE is viewed with high confidence under the efficacy criterion and medium on environmental risk in the recent report by the National Academies of Sciences, Engineering, and Medicine.

Figure 1

Conceptualization of adding alkaline materials to the effluent of wastewater treatment plants to achieve ocean alkalinity enhancement and ocean carbonate chemistry principles (the inserted part)

R is the ratio of total alkalinity (TA = [HCO3−] + 2[CO32−] + [OH−]) to dissolved inorganic carbon, the sum of all dissolved inorganic carbon species. Rivers usually have Rr ≈ 1.0. Sewage has RS < 1.0, but after adding alkaline materials, sewage can have RS-OAE >> 1.2. Increasing the oceanic TA to dissolved inorganic carbon ratio (Ro) by 2.5% from 1.17 to 1.2 can achieve the ocean-based carbon-dioxide-removal goal with an initial flux of 3 and a final flux of 16 Gt-CO2/year in addition to the current 9 Gt-CO2/year (i.e., a total flux of 12–25 Gt-CO2/year). The targeted global ocean CO2 uptake flux of 12–25 Gt-CO2/year can be achieved by raising the ocean surface TA by 0.75%–1.5% and reducing pCO2 by 25–50 ppm. The current CO2 emission and uptake fluxes are based on National Academies of Sciences, Engineering, and Medicine. OA, ocean acidification. In the inserted part, red dotted lines (from points 1 to 2 and to 3) represent OAE schemes via the addition of NaOH followed by CO2 uptake from the atmosphere. The purple lines represent the addition of CaCO3. A maximal increase of TA by 4.7% via NaOH addition would decrease sea-surface pCO2 to 280 ppm and increase pH to 8.17 (year 1750 or preindustrial conditions). 0.75%, 1.5%, and 3.0% TA enhancements will bring ΔpCO2 (sea-air) to −25, −50, and −100 ppm, respectively, and the air-sea flux to −1.68, −4.19, and −8.39 mmol/m2/d, respectively. These fluxes are scaled up to derive global ocean CO2 uptake fluxes, on which a 2.57 Gt-CO2/year preindustrial river flux is added to the net flux to derive the final global ocean anthropogenic CO2 uptake fluxes.

Several fundamental questions have not yet been answered regarding OAE. These include “how realistically and effectively can highly alkaline source materials such as NaOH and Ca(OH)2 mix into natural seawater without forming substantial secondary mineral (CaCO3) precipitation” and “how realistically will readily available carbonate and silicate minerals dissolve in seawater?” This is because ocean-surface water conditions are supersaturated with respect to CaCO3 minerals and unfavorable to the dissolution of silicate minerals such as olivine. Thus, how to appropriately and efficiently add alkaline materials to the ocean to achieve OAE and CDR is a tremendous challenge before us.

Sewage alkalinity addition

We postulate here the novel idea that OAE can be applied to safely and permanently and cost-effectively sequester atmospheric CO2 using wastewater-treatment-plant effluent (Figure 1). As wastewater (sewage) has low pH, high pCO2, and high concentrations of organic acids, it can be used to deliver strong bases without substantial secondary precipitation (i.e., CaCO3) and can dissolve abundant and inexpensive olivine and CaCO3 minerals in dissolution-favorable conditions. More importantly, the added alkalinity can be exported offshore by river plumes and ocean currents to achieve OAE and CDR at scale.

Each year, nearly 1000 km3 of wastewater is generated around the world with about 300 km3 discharged as municipal wastewater and over 600 km3 as industrial wastewater. Sewage has a high DIC concentration of 3000–5000 μmol C/kg and a pCO2 value of over 10 000 parts per million (ppm), the release of which may lead to CO2 degassing from and acidification of coastal waters. While opportunities for carbon capture and utilization during wastewater treatment have received extensive attention, the potential for using wastewater as an effective means for the production and delivery of alkalinity to the ocean (i.e., OAE) for CDR has not been explored and is the focus of this paper.

Ocean carbonate chemistry

With the atmospheric CO2 increases from the preindustrial level of 280 ppm to nearly 420 ppm at present, CO2 entered the ocean, leading to ocean acidification (Figure 1). As a result, ocean DIC concentrations increased, but total alkalinity (TA) did not change. In contrast, under the OAE strategy, TA and DIC increase at a ratio of 2:1 for the case of carbonate mineral addition (CaCO3 + CO2 → 2HCO3−), and when NaOH or Ca(OH)2 is added (CO2 + OH− → HCO3−), TA increases with no DIC change, which subsequently promotes a strong CO2 uptake from the atmosphere. Thus, the latter is a more efficient pathway for ocean-based CDR than CaCO3 dissolution (Figure 1).

An important distinction between OAE and ocean acidification lies in the levels of pCO2 and pH change. In the ocean-acidification consideration, the change or modification is from 280 ppm to about 1000 ppm (early next century business-as-usual prediction), corresponding to a pH decrease from 8.2 to 7.7. In contrast, in the OAE consideration, the need to reduce sea-surface pCO2 in the open ocean is small, well within recent historical levels. For example, on a global ocean scale, and assuming an equilibrium between surface ocean and the atmosphere, if we can reduce, on average, sea-surface pCO2 by a total of 25, 50, and 100 ppm to 395 (mid 2010s level), 370 (early 2000s), and 320 ppm (late 1960s), we can achieve a CO2 uptake rate of 13.5, 24.4, and 46.2 Gt-CO2/year, respectively, assuming that atmospheric pCO2 will be stabilized at 420 ppm (mid 2020s) (Figure 1). This is equal to or higher than the global CDR goal of removing at least 10 Gt-CO2/year. Though it is unlikely that pCO2 can (or needs to) be reduced by 50–100 ppm everywhere in the global surface ocean, and adding alkalinity may take decades, we clearly do not need to reduce sea-surface pCO2 or modify seawater pH greatly to reach the CDR goal. As illustrated in Figure 1, this pCO2 reduction can be induced by raising sea-surface TA by <1%–3%. Thus, in the ocean, where most CO2 uptake occurs due to its vast area, the modification of carbonate chemistry will be minimal, and the likely environmental, biological, and ecosystem impacts are expected to be small. However, numerical models at both regional and global scales should be carried out to simulate the amount, rate, location, and duration of TA additions and assess the efficacy and impacts of this proposed CDR strategy.

Advantages of our approach

A sewage-delivered OAE approach is a novel concept in the ocean-based CDR family and has the following advantages over other OAE approaches. (1) Direct application of NaOH and Ca(OH)2 into the open ocean water will unavoidably induce CaCO3 or even Mg(OH)2 precipitation and alkalinity loss at the release points, while low pH wastewater may allow us to avoid this problem. (2) Most ocean waters are supersaturated with respect to CaCO3 minerals and are dissolution unfavorable to silicate minerals no matter how finely they are pulverized. Low pH and organic-acid-rich wastewaters provide an ideal site where these minerals may dissolve at meaningful rates for CO2 sequestration purposes. (3) Coastal waters are often CO2 sources to the atmosphere and have suffered from severe seasonal ocean acidification due to the synergistic effects of anthropogenic and respiration-induced CO2. Application of alkaline materials in wastewater treatment plants and their effluents will reduce CO2 release and ameliorate acidification in estuaries, providing co-benefits for environmental health and fisheries. (4) Applying mineral alkaline sources to coastal waters is economically more viable than transporting them to the open ocean because transportation of minerals using ∼1000 ships is a major factor in the cost and energy-consumption estimation of the known OAE approaches. While we advocate the sewage-delivered- OAE approach, we strongly recommend that impacts on environments, organisms, and ecosystems be carefully evaluated because coastal zones have already been facing multiple environmental and climate-related stressors and are more vulnerable than the open ocean.

Concluding remarks

Using sewage and other low pH coastal waters as a means to add alkalinity to the ocean is novel and has many advantages for achieving ocean alkalinity enhancement and CO2 removal with the co-benefit of ameliorating ocean acidification in both the coastal and open ocean. Regional- and global-scale numerical model simulations should be used to more fully evaluate the efficacy of this proposed approach in ocean negative carbon emissions or CDR, while water quality and biological impacts in coastal environments should be monitored and studied.