Date of Award
4-3-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Phosphorus (P) is life’s bottleneck element and an essential nutrient for food production. However, phosphate rock is a non-renewable resource and its geological distribution is highly uneven, making P recovery a crucial step for a circular economy. Approximately 55% of mined phosphorus ends up in centralized wastewater treatment plants (WWTPs), where up to 90% is captured in the sludge. This sludge, treated in anaerobic digesters to produce anaerobically digested sludge (ADS), emerges as a viable secondary source for phosphorus recovery. Recovering phosphorus from ADS typically involves leaching and separation. Leaching mobilizes phosphates from sludge solid phase into a liquid phase, often through acidification, followed by separation of the mobilized phosphate. Traditional methods for these steps are energy and chemically intensive. However, electrochemical systems present a promising alternative with their modular design, operational flexibility, and minimal chemical requirements. Despite these benefits, research into electrochemical P recovery from ADS is still in its early stages, with much to explore in process mechanisms, design optimization, recovery efficiency, product reuse, and solid waste management. This dissertation examines the efficacy of electrochemical P recovery from liquid streams and the development of three electrochemical systems for P recovery from solid stream ADS. It aims to minimize chemical inputs and enhance the quality of recovered P products. The study demonstrates the application of recovered P products in microalgae cultivation and investigates the underlying mechanisms for sludge dewaterability improvement after P recovery. To recover P from liquid streams for reuse, I developed a coupled system consisting of a microbial nutrient recovery cell (MNRC) and a photobioreactor (PBR). The MNRC-PBR system, tested on both synthetic and real food wastewater, significantly enhanced microalgal biomass density and purity compared to direct cultivation in food wastewater. Operational parameters, like the liquid exchange ratio between MNRC and PBR and the water recycling ratio in the PBR, were found to be optimal for microalgae growth at 30% and 50%, respectively. A systematic literature analysis on electrochemical sludge P recovery revealed that the most common P leaching approach for ADS is anodic leaching with a median leaching rate of 92.4 mg d-1. However, few studies report energy consumption data and there was a scarce of studies on chemical-free P recovery methods. This gap led to the development of the electrochemical nutrient recovery cell (ENRC) and the electrochemical phosphorus recovery cell (EPRC). The ENRC, achieving P leaching efficiency similar to direct acid adjustment, separated P via the selective anion exchange membrane. Catholyte acidification was found important to improve P recovery efficiency from 42% to 90%. Conversely, EPRC used electrolysis-assisted precipitation for direct P precipitation in ADS liquid phase, significantly lowering energy consumption from 229 to 83 kWh kg-1 P and reducing chemical input by over 80% as compared to ENRC. The long-term performance, maintenance strategies, and product contamination were discussed and characterized. To lower product contamination, a two-step electrochemical phosphorus recovery system (EPRS) was developed, featuring a leaching unit for P leaching and a recovery unit for its purification and concentration. The EPRS was operated over long-term to reach a steady-state PO43--P concentration of 4040 mg L-1, which can be increased to 5900 mg L-1 by adjusting the anolyte pH to 3-4 according to a mechanistic model. EPRS effectively separated P from heavy metals (Ni, Cd, Cr, Pb, Cu) but showed limited effectiveness against As. EPRS emerged as an energy-intensive process with 441 kWh kg-1 P energy input but produced struvite and magnesium phosphate with minimal impurities. The leaching unit also produced biosolids meeting USEPA Class A pathogen standards. In the dewaterability test, electrochemical P leaching significantly improved sludge dewaterability, reducing the specific resistance to filtration by over 97% for raw sludge before anaerobic digestion, ADS, and primary sludge. The acidification, cation removal, and electrooxidation showed a synergistic effect in releasing bound water. The dissertation concludes with identifying future research opportunities in integrating nutrient resource recovery with sludge pretreatment, evaluating P product reuse, improving electrochemical P recovery efficiency, and scaling up and deploying the technology.
Language
English (en)
Chair
Zhen He
Committee Members
Peng Bai, Arpita Bose, Daniel E. Giammar, Young-Shin Jun
Comments
File updated 10/3/2024 with corrections to figure link formatting errors on pages 74-77.