Date of Award
5-7-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Nitrogen plays an indispensable role in living organisms and thus is a key element for agricultural production. Ammonia (NH3), as an important part in nitrogen cycle, can be synthesized by nitrogen fixing bacteria and the Haber-Bosch process, the latter of which requires an extensive energy input. Wastewaters can contain a large amount of nitrogen compounds because of kitchen and toilet wastes being discharged to sewage. The concentration of ammonia nitrogen (NH4+-N) can be hundreds mg per liter in some wastewaters like anaerobic digester (AD) centrate. However, both Haber-Bosch process and conventional wastewater ammonia treatment process are energy intensive, which making direct ammonia recovery from wastewater streams become increasingly beneficial in order to achieve sustainable nitrogen management. Electrochemical and membrane technology are emerging technologies that can achieve ammonia recovery with high recovery efficiency, while many of the those are still energy- and chemical-intensive. These motivate us to develop novel integrated membrane and electrochemical systems for ammonia recovery from wastewater. Meanwhile, the mechanisms and performance of the new systems should be investigated systematically through experiment and modeling, and the application of the recovery products also needs to be evaluated. To start with, a microbial electrochemical system (MES) was used to recover ammonia from a mixture of AD centrate and food wastewater at an optimal volume ratio of 3:1. The catholyte of the MES, which contained the recovered ammonia, was used to prepare fertilizers to support the growth of a model plant Arabidopsis thaliana. It was observed that A. thaliana grown on the MES generated fertilizer amended with extra potassium, phosphorus, and trace elements showed similar appearance to the control group that was added with a commercial fertilizer. RNA-Seq analyses were used to examine A. thaliana genetic responses to the MES generated fertilizers or the commercial counterpart. The comparative study offered metabolic insights into A. thaliana physiologies subject to the recovered nitrogen fertilizers. The results of this study have demonstrated the potential application of using the recovered ammonia from AD centrate as a nitrogen source in fertilizer. Next, a novel electrochemical membrane system (EMS) was developed to recover nitrogen from real AD centrate. The EMS synergistically coupled electrodialysis with membrane contactor to facilitate the selective recovery of target nutrient. Under a constant current of 10 mA cm-2, the EMS recovered more 80% of NH4+-N. The results of this study have demonstrated the feasibility of the proposed EMS and encouraged further investigation to reduce its energy consumption and improve nutrient recovery. Afterwards, a four-chamber electrochemical membrane system was developed to reduce the energy consumption and nutrient recovery cost. The lowest specific energy consumption of 8.2 ± 0.2 kWh kg-1 N was achieved under 1.25 mA cm-2. A preliminary cost analysis examined the relationship between acid price and dosage and estimated the operating cost of $0.58~0.83 kg-1N; simultaneously recovered phosphorus would further increase the benefit of this system. This work further studied the kinetics in different electrochemical systems to provide insights towards the operation and design of the system through mathematical modeling. Herein, we have performed Tafel analysis with Marcus-Hush-Chidsey (MHC) model for the first time to understand the anodic reaction kinetics in MES. After fitting the data over five months of cultivation, our results showed that MHC fitting curves can match better with a multi-electron transfer mechanism than with a one-electron transfer mechanism. To continue with, the kinetics of acid/base production, two important contributors to recover ammonia in the MES, was modeled with an empirical model by coupling MHC model and polynomial regression. When the EMS was fed with synthetic AD centrate, good fitting performance was achieved for both the anode and cathode half reactions. Moreover, the coupled model also showed decent prediction values when real AD centrate was fed into EMS if the bicarbonate concentration was included to modify the model. Challenges and opportunities were identified for using electrochemical and membrane technologies for ammonia recovery. It is recommended that long-term operation of the EMS to be conducted in order to evaluate the performance of the system after a few months, this would help to get a more comprehensive economic analysis of the cost to recover nutrients from AD centrate in EMS. Also, developing versatile functions of EMS together with ammonia recovery will uplevel its value, such as volatile fatty acid recovery, carbon capture and storage, heavy metals removal, etc. It is still challenging to enlarge the EMS to pilot-scale, but with the experiment and modeling work conducted in this dissertation, the path to a more sustainable cycle of nitrogen resources is smoother.
Language
English (en)
Chair
Zhen He