Article Cite This: Environ. Sci. Technol. 2018, 52, 13995−14005
pubs.acs.org/est
Life Cycle Impact and Benefit Trade-Offs of a Produced Water and Abandoned Mine Drainage Cotreatment Process Yan Wang,† Sakineh Tavakkoli,† Vikas Khanna,†,‡ Radisav D. Vidic,†,‡ and Leanne M. Gilbertson*,† †
Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
‡
Downloaded via EAST CAROLINA UNIV on January 14, 2019 at 23:16:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: A cotreatment process for produced water and abandoned mine drainage (AMD) has been established and demonstrated at the pilot-scale. The present study evaluates the potential of the proposed process to aid in management of two high volume wastewater resources in Pennsylvania. A systems-level approach is established to evaluate the primary trade-offs, including cotreatment process environmental impacts, transportation impacts, and environmental benefits realized from precluding direct AMD release to the environment. Life cycle impact assessment was used to quantify the environmental and human health impacts as well as to identify “hot spots” of the cotreatment process. Electricity use was found to be the dominant contributor to all impact categories. Extending the system boundary to include transportation of the two wastewaters to a to-be-determined cotreatment site revealed the important impact of transportation. An optimization approach was employed (using the region of Southwest Pennsylvania) to evaluate minimization of transportation distance considering the location and number of treatment sites. Finally, a quantitative analysis of environmental benefits realized by precluding direct AMD release to the environment was performed. The results suggest that the magnitude of benefit realized in treating a highly polluted AMD is greater than the magnitude of impacts from the cotreatment process.
■
INTRODUCTION Pennsylvania’s unique geology and geography enabled the state to become the second-largest natural gas producer (primarily produced from the Marcellus Shale) and the third-largest coal producer in the nation in 2017.1 The extraction of these two primary energy resources introduces environmental and economic burdens, including significant volumes of wastewater. Natural gas extraction using hydraulic fracturing generates large quantities of produced water (around 1 million gallons per gas well on average in Marcellus Shale region),2,3 which is most notably characterized by extremely high salinity and the presence of naturally occurring radioactive materials. (Note: Produced water is defined herein to include both the flowback, first 2−3 weeks, and in-production periods.) Another prominent polluted aqueous waste stream in Pennsylvania is abandoned mine drainage (AMD), which is produced when water fills abandoned coal mines and is released directly to the environment. The estimated rate of AMD production is 700− 2000 gallons per minute throughout Pennsylvania’s western and central regions.4 More than 3000 miles of contaminated surface and ground waters are said to be caused by AMD leading to the destruction of local ecosystems.5,6 Potential solutions to technical, economic, and regulatory issues related to the use of AMD for hydraulic fracturing operations were proposed in a roundtable conference hosted by the RAND Corporation in December 2011 4 and © 2018 American Chemical Society
disseminated in the White Paper released by the Pennsylvania Department of Environmental Protection (PADEP) in January 2013.7 The feasibility of using AMD for hydraulic fracturing was agreed to be technically viable, but the suitability of direct use was questioned due to large variation in the chemical composition (e.g., sulfate, pH, iron). Despite these proposed action plans,4,7 there remains an opportunity to implement an effective and lasting solution. One potential solution has been demonstrated at the lab- and pilot-scale by Vidic et al.6,8,9 and involves leveraging the complementary chemistries of produced water and AMD to remediate both waste streams through a straightforward mixing process. The treated water is proposed for use in hydraulic fracturing operations, offering the benefit of offsetting freshwater demand. In the Marcellus Shale, only 10−30% of the injected fracturing fluid returns to the surface as produced water during the flowback period.10−12 Thus, freshwatertermed “makeup” watermust be added to the produced water for use in subsequent injections. The estimated volume of makeup water ranges from 3 to 8 million gallons per well.2,4,12,13 The volume of AMD in Pennsylvania is abundant for the implementation of the proposed cotreatment Received: Revised: Accepted: Published: 13995
July 9, 2018 November 3, 2018 November 7, 2018 November 7, 2018 DOI: 10.1021/acs.est.8b03773 Environ. Sci. Technol. 2018, 52, 13995−14005
Article
Environmental Science & Technology
Figure 1. System boundary for the life cycle impact assessment of the produced water and AMD cotreatment process. The numbers 1 to 6 refer to the stages in the system. 1-produced water storage; 2-AMD storage and pH adjustment; 3-rapid mixing of produced water and AMD; 4flocculation; 5-sedimentaion; and 6-sludge dewatering. Bold arrows represent the flows linking the different activities in the system, and thin arrows represent the primary material and energy flows at each stage. The expanded system boundary (indicated by the dashed blue line) includes the transportation of produced water and AMD to the cotreatment site.
The primary focus of this study is to evaluate the life cycle impact and benefit trade-offs of the cotreatment process to inform sustainable implementation of the cotreatment design. Approximately 90% of the produced water in Pennsylvania is currently reused for subsequent hydraulic fracturing and approximately 10% of that is transported to other states for subsurface injection.9,18,19 Depending on the produced water chemistry (e.g., the concentrations of total dissolved solids, sulfate, and other metal ions), the produced water can be directly reused when blended with freshwater or reused after treatment, with the former being the dominant strategy that industries adopt. A range of treatment technologies are employed, such as solid removal (e.g., sedimentation, flotation, filtration), chemical precipitation, reverse osmosis, distillation, and crystallization.20,21 The cotreatment process evaluated in this study is unique compared to other treatment processes in that it mitigates ecosystem impacts associated with AMD discharge. Results presented herein identify opportunities to reduce impacts by considering modifications to specific unit processes. Expansion of the system boundary was pursued to consider impacts associated with transport of the two source waters to a to-be-determined cotreatment site. Finally, while it is challenging to capture and quantify all potential benefits from implementing the proposed water management strategy, environmental gains realized through capture and use of AMD were estimated and compared to the cotreatment process impacts to benchmark the potential magnitude of this environmental credit. While the focus here is on two specific waste streams in Pennsylvania, the approach and study findings have broader implications applicable to treatment and management of impaired and waste waters.
approach. Approximately 600 billion gallons of AMD is discharged annually, which is more than 10 times the estimated water demand annually for hydraulic fracturing water demand annually for hydraulic fracturing.4,14 The proximity of AMD discharge sites to shale gas wells further supports the opportunity to utilize this cotreatment process. Sulfate concentration is the most important water quality requirement for use in Marcellus Shale region and is restricted to
