Grantee Research Project Results
Final Report: Electro-Assisted Wastewater Nutrient Recovery
EPA Grant Number: SU840151Title: Electro-Assisted Wastewater Nutrient Recovery
Investigators: Tarpeh, William A , Dong, Hang , Kogler, Anna , Clark, Brandon , Chow, William
Institution: Stanford University
EPA Project Officer: Page, Angela
Phase: I
Project Period: December 1, 2020 through November 30, 2021
Project Amount: $25,000
RFA: P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2020) RFA Text | Recipients Lists
Research Category: P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources
Objective:
The overarching goal of the proposed work is to develop and demonstrate a self-sustaining selective nutrient capture and recovery unit. This project addresses a critical barrier to sustainable water treatment: a lack of cost-effective, flexible-scale options for preventing
nutrient-induced algal blooms. This barrier stems from a lack of scalable unit processes capable of both removal and recovery of nitrogen and phosphorus. Recovered products must be high-purity, which demands selective capture of nitrogen and phosphorus as fertilizers and other commodity chemicals. We address this technical challenge using selective nano-adsorbents because they are tunable to different pollutants and amenable to regeneration to extract valueadded products. We also advance electrochemical regeneration, which can be applied for on-site treatment in remote, rural, and disadvantaged communities because it replaces chemical inputs with electricity.
The objectives of this proposed work are (1) to design and experimentally demonstrate deployable, durable and nutrient-selective nano-adsorbents to enable selective nutrient capture; (2) to develop novel on-site electrochemical regeneration and recovery process for the nano-adsorbents to enable pure fertilizer production with minimal chemical inputs and transportation; and (3) to validate the field performance of electro-assisted nutrient recovery at various scales at the Codiga Resource Recovery Center. Nano-adsorbents enable selective adsorption of N and P and efficient desorption via bulk pH shifts, which can be achieved through on-site electrochemical water-splitting. Our proposal synergistically combines molecular-level design, lab-scale development, and field demonstration to advance nutrient management benefits for people, prosperity and planet. Selective nutrient capture benefits people by preventing harmful algal blooms in drinking water sources and recreational areas to improve the quality of people’s lives; Simultaneous recovery benefits prosperity by producing valuable agricultural fertilizers. The combination of capture and recovery benefits the planet by reimagining nutrient cycles from linear extract-and-emit schemes to circular economies that reduce emissions and conserve natural resources. Educationally, we partner with the Codiga Resource Recovery Center to engage diverse tour groups (e.g., local community, K-12 students, campus staff and students) to increase awareness of resource-efficient water treatment. Students in Introduction to Chemical Engineering and Chemical Engineering Plant Design courses will also participate in hands-on laboratory and scale-up studies to evaluate adsorption capacity, regeneration efficiency, and selectivity.
Summary/Accomplishments (Outputs/Outcomes):
In Phase I, we have developed and demonstrated a N-selective adsorbent using transition metal-loaded polymeric cation exchange resins (O1); achieved electro-assisted regeneration for P-selective adsorbents and established proof-of-concept for electro-regenerating the new N-selective adsorbent (O2); and validated the adsorbent and electro-assisted regeneration in a scaled-up system using real wastewater from Codiga Resource Recovery Center (CR2C, O3). The Phase I progress has demonstrated the feasibility of N and P recovery using selective adsorbents and electro-assisted regeneration, which laid the foundation to advance them into a mature technology in Phase II.
Nitrogen-selective adsorbent. To achieve both N and P recovery, we developed a N-selective adsorbent to supplement commercially available P-selective adsorbents. Together, these two materials can enable a combined system that captures and recovers both nutrients. Commercial P-selective adsorbents utilize an anion exchange resin doped with ferric oxide nanoparticles to enable specific inner-sphere complexation between phosphate and metal oxides. Using a similar metal-polymer strategy, we prepared the N-selective adsorbent by loading metal cations into a cation exchange resin, in which ammonia-metal binding enabled ammonia-selective capture. Comprehensive evaluations were performed using the N-selective adsorbent to capture total ammonia nitrogen (TAN, sum of aqueous NH4+ and NH3). We surveyed varying initial TAN concentrations, metals (e.g., Cu, Zn), and functional groups of the cation exchange resin (e.g., carboxylate, iminodiacetate) to maximize N selectivity and capacity. We also considered the effects of these components on key concerns such as adsorbent regeneration and metal elution.
Electro-assisted regeneration. The developed N-selective adsorbent can be combined with a commercial P-selective adsorbent (hybrid anion exchanger, HAIX) to enable both N and P capture. However, selective capture is only one part of our goal; complete recovery of high-purity nutrients requires improved regeneration that minimizes chemical and energy inputs. We developed an electro-assisted regeneration approach for both adsorbents via on-site acid and base regenerant production. We identified the pH ranges that can be efficiently achieved in a two-chamber electrochemical cell (an anode and cathode chamber separated by a cation exchange membrane) with minimal chemical and energy requirements. This two-chamber electrochemical cell represents the simplest cell design that can achieve simultaneous production of acid (in the anolyte due the oxygen evolution reaction) and base (in the catholyte due to the hydrogen evolution reaction). With the electro-produced acid and base, we evaluated the regeneration efficiency of both the P-selective adsorbent (HAIX), and the host resin of the N-selective adsorbents (WACG) without copper loading. Due to the complexity of Cu-WACG regeneration that requires minimizing the copper elution, we primarily investigated an unconventional DI regeneration approach, and identified the strategies for adopting the electro-assisted approach with the DI regeneration for maximal N recovery and minimal copper elution.
Preliminary pilot tests. Because we have fully tested electro-assisted regeneration in lab-scale for HAIX but not for Cu-WACG, we scaled-up the HAIX system first for primary pilot tests with real wastewater from the Codiga Resource Recovery Center (CR2C at Stanford University). CR2C is a pilot-scale wastewater treatment plant at Stanford that collects a portion of campus and Palo Alto wastewater for the express purpose of piloting novel resource recovery processes at a demonstration scale. The scaled-up system contained 50 mL HAIX (>3 times more adsorbent than lab scale tests, 15 mL), and was tested with the secondary effluent from an anaerobic reactor in CR2C (~ 7 mg P/L compared to 3 mg P/L in the synthetic wastewater during lab tests). The key finding was that HAIX removed over 90% P from real wastewater, which demonstrated the feasibility of using HAIX and electro-assisted regeneration in bigger scales and with real wastewaters.
Conclusions:
Through our Phase I investigation, we have successfully developed a novel N-selective adsorbent (Cu-WACG) exhibiting the highest selectivity and capacity to date. While achieving electro-assisted regeneration of Cu-WACG is the goal, we accelerated the demonstration of the novel regeneration approach by first testing with the commercial P-selective adsorbent (HAIX) and the host resin of Cu-WACG (WACG). The results collected from HAIX and WACG regeneration enabled us to design a novel electro-assisted regeneration strategy for Cu-WACG only with input of water and electricity. We gained both molecular level understanding on the selectivity and regenerability of Cu-WACG, and process level design parameters for combing Cu-WACG with HAIX as a unit. At molecular level, we identified that selective regeneration was needed for Cu-WACG, which is a new concept supplementing selective adsorption. Selective regeneration enables maximal N recovery from Cu-WACG with minimal metal elution, during which the regenerant pH needs to be tuned to below 9 (ammonia pKa) for ammonia protonation and elution, and above 5 (carboxylate group pKa) for preventing metal exchange by protons. This molecular level understanding informed the feasibility of electro-assisted regeneration of Cu-WACG as we have demonstrated electrochemical pH adjustment for regenerating both HAIX and WACG. Additionally, Cu-WACG and HAIX regeneration can be paired together using a two-chamber electrochemical cell because Cu-WACG requires decreasing the regenerant pH (anode chamber) while HAIX requires increasing the regenerant pH (cathode chamber). The similar regeneration enables intensified device design by combining the two selective adsorbents to enable both N and P capture and recovery. The studies on nutrient-selective adsorbents in Phase I have demonstrated high N and P selectivity for nutrient removal from wastewaters. Excessive nutrient discharge contributes to harmful algal blooms in drinking water sources or recreational waters. Thus, nutrient-selective adsorbents provide an innovative, cost-effective technological solution for preventing, pretreating, or mitigating harmful algal blooms for both environmental and human benefits. In addition to nutrient removal, the developed electro-assisted regeneration enables nutrient recovery as valuable products with minimal inputs. The removal and recovery practice enables a circular economy that benefits prosperity. Our Phase I results have demonstrated the feasibility of using the nutrient-selective adsorbents and the electro-assisted regeneration in different scales, which will inform future technology implementation to benefit both the environment and human health.
While more pilot scale tests are needed to provide a comprehensive evaluation for N recovery, we first evaluated P recovery and its associated energy consumption based on the more available lab scale results. We designed a P recovery strategy by combining HAIX (capturing phosphate) with WACG (capturing calcium).10 The captured phosphate and calcium were eluted from HAIX and WACG using pH 11 catholyte and pH 3 anolyte, respectively, and concentrated into the spent regenerants. We achieved more than 95% phosphate recovery as hydroxyapatite with an energy consumption of 20 kWh/kg P by mixing the spent catholyte and anolyte regenerant to precipitate phosphate. This energy consumption was comparable to other reported electrochemical P recovery systems, but with significantly improved system selectivity, scalability, and reduced chemical inputs. Our previous LCA demonstrated that chemical consumption during adsorbent regeneration contributes over 70% energy consumption and GHG emission for N recovery. Thus, electro-assisted regeneration that replaces chemicals with electricity could reduce both energy consumption and emissions. Based on real chemical pretreatment data from a 18 MGD full-scale wastewater reuse facility, we estimated 55% cost reduction would be achieved using P-selective adsorbents and the electro-assisted regeneration (compared to conventional chemical dosing pretreatment) due to the high selectivity and capacity of the adsorbents and low prices of electricity compared to chemicals (strong acids and antiscalants). During Phase II we will perform more detailed cost evaluations based on future comprehensive pilot studies focused on N and P.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 3 publications | 2 publications in selected types | All 2 journal articles |
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Dong H, Wei L, Tarpeh WA. Electro-assisted regeneration of pH-sensitive ion exchangers for sustainable phosphate removal and recovery. Water Research 2020;184:116167.. |
SU840151 (Final) |
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Dong H, Wu Z, Liu MJ, Tarpeh WA. The role of intraparticle diffusion path length during electro-assisted regeneration of ion exchange resins:Implications for selective adsorbent design and reverse osmosis pretreatment. Chemical Engineering Journal 2021;407:127821. |
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Supplemental Keywords:
Selective adsorbents, nutrient recovery, electro-assisted regenerationRelevant Websites:
P3 Phase II:
Electro-Assisted Wastewater Nutrient RecoveryThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.