Semester

Summer

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Lian-Shin Lin

Committee Member

Leslie Hopkinson

Committee Member

Antarpreet Jutla

Committee Member

Nianqiang (Nick) Wu

Committee Member

Michael J. Wilkins

Abstract

In moving toward more sustainable wastewater management, anaerobic treatment is gaining increasing popularity due to its simplicity, low energy requirement, low sludge production and less emission of greenhouse gases compared to typical aerobic wastewater treatment systems. Electron acceptors such as nitrate, sulfate, and CO2 have been used in various anaerobic processes for removal of organic matters from wastewater under anoxic or anaerobic environments. In energy producing regions, ferric iron, Fe(III), is a predominant element in iron containing wastes such as acid mine drainage (AMD) and coal ash, which can potentially be used as a source of iron in novel anaerobic wastewater treatment. Such an iron-based treatment approach can offer multi-faceted benefits over existing treatment methods including use of iron-containing wastes, no aeration, unique reaction mechanisms for coagulation, sulfide control, organic micropollutant removal, and useful sludge byproducts. The overall goal of this research was to develop an innovative Fe(III)-dosed anaerobic wastewater treatment process through incorporating known and novel biogeochemical reactions of iron in an engineered biological system.

The major research objectives include (1) identifying the critical factors and investigating their effects on the treatment performance of Fe(III)-dosed wastewater treatment; (2) developing a continuous Fe(III)-dosed anaerobic biological treatment system and examining its technical feasibility and potential issues in long-term operations; (3) developing a method for transforming the sludge materials from the Fe(III)-dosed bioreactor into magnetic byproducts; and (4) exploring the applicability of this Fe(III)-dosed treatment method for nutrient removal and recovery.

A detail literature review was first conducted to evaluate the suitability of using iron reduction for wastewater treatment and identify critical factors affecting the treatment. Several factors were identified that affect organics oxidation coupled to iron reduction, including the types of the ferric compound, microorganisms, ferric bioavailability and availability of substrate. Amorphous iron materials (e.g. iron sludge from AMD) with large surface areas and high ferric dissolution rates have great potential to be used in Fe(III)-dosed wastewater treatment process to enhance ferric bioavailability to iron reducers. Given the significant levels of sulfate (SO42-) in wastewater, sulfate reduction is expected to be co-occurring with iron reduction in the iron-dosed anaerobic treatment. Shift in microbial composition in relation to ferric and sulfate concentrations (expressed as Fe/S ratio) and their effects on organics removal are important knowledge gaps for developing such treatment technology. In particular, there is a need to understand the nature of the relationships between iron reducing bacteria (IRB) and sulfate reducing bacteria (SRB) (i.e., symbiotic or competitive) to identify optimal operating conditions for this type of wastewater treatment.

Batch experiments on iron-dosed anaerobic biological treatment of wastewater under three different molar Fe/S ratios (1, 2 and 3) showed positive correlation between organics (chemical oxygen demand, COD) oxidation rate and Fe/S ratio. Microbiological analysis suggested that both iron reducers and sulfate reducers contributed to this organic oxidation. Maximum COD oxidation rate, Vmax estimated from Michaelis-Menten model ranged from 0.47 mg/L×min to 1.09 mg/L×min as Fe/S ratio increased from 1 to 3. A positive correlation was also observed between COD oxidation rate and the relative abundance of iron reducers, and both increased with the Fe/S ratio.

Long-term continuous wastewater treatment using an anaerobic bioreactor dosed with ferric iron showed satisfactory COD removal of 84 ± 4%, 86 ± 4% and 89 ± 2% under Fe/S molar ratio 0.5, 1 and 2 respectively. Fe/S ratio was also observed to regulate the effluent quality by removing excess sulfide from aqueous phase with increasing quantity of ferrous through ferrous sulfide precipitation. The sludge materials contained both biomass (20-40 w/w%) and inorganic precipitates (80-60 w/w%) with the inorganic fraction increasing with Fe/S ratio. Spectroscopic and chemical elemental analyses indicated that the inorganic fraction of the sludge materials mainly contained FeS and FeS2. Microbiological analyses of the sludge materials identified Geobacter sp., Geothrix sp. and Ignavibacteria sp. as putative iron reducers, and Desulfovibrio sp., Desulfobulbus sp., Desulfatirhabdium sp., Desulforhabdus sp. and Desulfomonile sp. as putative sulfate reducers.

A simple thermal treatment method was applied to transform the iron sulfide sludge from the bioreactor into magnetic particles. Sludge samples were treated at five different temperatures (300, 350, 400, 450, and 500°C) to evaluate the transformation of iron sulfide sludge into different magnetic phases of iron oxide particles. X-ray Diffraction (XRD) analysis and magnetization measurements showed successful transformation of the sludge to magnetic byproducts and indicated the presence of ferromagnetic magnetite and maghemite phases at different temperatures. The magnetic sludge byproducts have potential applications in biomedicine sector and wastewater treatment (e.g. coagulant, adsorbent). Crystallinity and crystallite size of the thermally derived particles were observed to play a noteworthy role in regulating the magnetization of the byproducts. Adsorption study revealed that both samples baked at 350°C and 500°C had high adsorption capacities to remove phosphate from aqueous solutions.

A study to explore applicability of this Fe(III)-dosed treatment process for nutrient removal and recovery was conducted with synthetic wastewater containing typical concentrations of COD (420 mg/L), phosphate (10 mg/L), SO42- (50 mg/L) and ammonium (50 mg/L). Average removal efficiencies of COD, phosphate, SO42- and ammonium were 97 ± 2%, 99.7 ± 0.5%, 87.1 ± 3% and 20.3 ± 9% respectively. The results showed in addition to organics oxidation, significant phosphate and ammonium removals were achieved in the bioreactor. Potential removal mechanisms include chemical precipitation as ferric phosphate (FePO4) or ferrous ammonium phosphate (FAP). SEM-EDS and XPS analysis suggested the presence of FAP in the sludge materials.

This innovative treatment process has shown consistent treatment performance and long-term stability under different operating conditions, suggesting its potential for large scale applications. Pilot-scale applications of this treatment approach using iron-containing wastes will give better understanding on the functionality of this process in a field scale environment. Utilizing iron wastes in this novel wastewater treatment process along with recovery of useful sludge byproducts not only can create new avenues to alleviate iron waste disposal, but also improve the sustainability of wastewater treatment.

Embargo Reason

Publication Pending

Available for download on Thursday, July 15, 2021

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