Joseph R.V. Flora

Hydrothermal Carbonization of Municipal Waste Streams

Hydrothermal carbonization (HTC) is a novel thermal conversion process that can be used to convert municipal waste streams into sterilized, value-added hydrochar. HTC has been mostly applied and studied on a limited number of feedstocks, ranging from pure substances to slightly more complex biomass such as wood, with an emphasis on nanostructure generation. There has been little work exploring the carbonization of complex waste streams or of utilizing HTC as a sustainable waste management technique. The objectives of this study were to evaluate the environmental implications associated with the carbonization of representative municipal waste streams (including gas and liquid products), to evaluate the physical, chemical, and thermal properties of the produced hydrochar, and to determine carbonization energetics associated with each waste stream. Results from batch carbonization experiments indicate 49-75% of the initially present carbon is retained within the char, while 20-37% and 2-11% of the carbon is transferred to the liquid- and gas-phases, respectively. The composition of the produced hydrochar suggests both dehydration and decarboxylation occur during carbonization, resulting in structures with high aromaticities. Process energetics suggest feedstock carbonization is exothermic.

Influence of reaction time and temperature on product formation and characteristics associated with the hydrothermal carbonization of cellulose

Studies have demonstrated that hydrothermal carbonization of biomass and waste streams results in the formation of beneficial materials/resources with minimal greenhouse gas production. Data necessary to understand how critical process conditions influence carbonization mechanisms, product formation, and associated environmental implications are currently lacking. The purpose of this work is to hydrothermally carbonize cellulose at different temperatures and to systematically sample over a 96-h period to determine how changes in reaction temperature influence product evolution. Understanding cellulose carbonization will provide insight to carbonization of cellulosic biomass and waste materials. Results from batch experiments indicate that the majority of cellulose conversion occurs between the first 0.5-4h, and faster conversion occurs at higher temperatures. Data collected over time suggest cellulose solubilization occurs prior to conversion. The composition of solids recovered after 96h is similar at all temperatures, consisting primarily of sp(2) carbons (furanic and aromatic groups) and alkyl groups.

Modeling of electrochemically-activated denitrifying biofilms

The ability of an electrode-biofilm system to reduce nitrate has been experimentally demonstrated. In this system, a denitrifying biofilm is attached to an electrode and the H2 produced by the electrolysis of water is used for the reduction of NO3− to N2 gas. Complete denitrification was achieved in the presence of an electric current. A steady-state biofilm model describing the system is presented. The model takes into account the mass transfer rates, biological reaction rates and electrochemical reaction rates of H+, OH−, H2, NO3− and supporting electrolytes. The denitrification rates calculated using the model are in fairly good agreement with experimental results. The net denitrification rate is affected significantly by the electric current and its response can be divided into three regions, namely, the “linear”, “plateau” and “decline” regions. The overall feasibility of the electrode-biofilm system is theoretically demonstrated.

Methanogenesis and sulfate reduction in chemostats—I. Kinetic studies and experiments

Abstract Six anaerobic chemostats containing mixed microbial cultures were used to investigate the interactions between sulfate reduction and methanogenesis for three substrates: acetic acid, methanol and formic acid. Sulfate reducers outcompeted methanogens in acetate-fed chemostats while methanol was not utilized by sulfate reducers. In the formic acid-fed chemostats, competition was observed between methanogens and sulfate reducers with 62 and 24% of the substrate utilized through sulfate reduction and methanogenesis, respectively. Iron was added to the sulfate-reducing chemostats to precipitate the hydrogen sulfide produced, thus eliminating sulfide inhibition and ensuring stable chemostat operation. This study involved the measurement of the oxidation-reduction potential (ORP) of the chemostats using a novel technique. Batch spike tests were also conducted to evaluate kinetic parameters for the degradation of different substrates. Although both methanol-fed chemostats were exclusively methanogenic, scanning electron microscopy (SEM) analysis revealed the presence of two different strains of methanogens. This difference was also manifested by the ORP values and the kinetic parameters.

Hydrothermal carbonization of food waste and associated packaging materials for energy source generation

Hydrothermal carbonization (HTC) is a thermal conversion technique that converts food wastes and associated packaging materials to a valuable, energy-rich resource. Food waste collected from local restaurants was carbonized over time at different temperatures (225, 250 and 275°C) and solids concentrations to determine how process conditions influence carbonization product properties and composition. Experiments were also conducted to determine the influence of packaging material on food waste carbonization. Results indicate the majority of initial carbon remains integrated within the solid-phase at the solids concentrations and reaction temperatures evaluated. Initial solids concentration influences carbon distribution because of increased compound solubilization, while changes in reaction temperature imparted little change on carbon distribution. The presence of packaging materials significantly influences the energy content of the recovered solids. As the proportion of packaging materials increase, the energy content of recovered solids decreases because of the low energetic retention associated with the packaging materials. HTC results in net positive energy balances at all conditions, except at a 5% (dry wt.) solids concentration. Carbonization of food waste and associated packaging materials also results in net positive balances, but energy needs for solids post-processing are significant. Advantages associated with carbonization are not fully realized when only evaluating process energetics. A more detailed life cycle assessment is needed for a more complete comparison of processes.

Chirality affects aggregation kinetics of single-walled carbon nanotubes.

Aggregation kinetics of chiral-specific semiconducting single-walled carbon nanotubes (SWNTs) was systematically studied through time-resolved dynamic light scattering. Varied monovalent (NaCl) and divalent (CaCl(2)) electrolyte composition was used as background solution chemistry. Suwannee River humic acid (SRHA) was used to study the effects of natural organic matter on chirally separated SWNT aggregation. Increasing salt concentration and introduction of divalent cations caused aggregation of SWNT clusters by suppressing the electrostatic repulsive interaction from the oxidized surfaces. The (6,5) SWNTs, i.e., SG65, with relatively lower diameter tubes compared to (7,6), i.e., SG76, showed substantially higher stability (7- and 5-fold for NaCl and CaCl(2), respectively). The critical coagulation concentration (CCC) values were 96 and 13 mM NaCl in the case of NaCl and 2.8 and 0.6 mM CaCl(2) for SG65 and SG76, respectively. The increased tube diameter for (7,6) armchair SWNTs likely presented with higher van der Waals interaction and thus increased the aggregation propensity substantially. The presence of SRHA enhanced SWNT stability in divalent CaCl(2) environment through steric interaction from adsorbed humic molecules; however showed little or no effects for monovalent NaCl. The mechanism of aggregation-describing favorable interaction tendencies for (7,6) SWNTs-is probed through ab initio molecular modeling. The results suggest that SWNT stability can be chirality dependent in typical aquatic environment.

Mechanisms for naphthalene removal during electrolytic aeration.

Batch tests were performed to investigate chemical and physical processes that may result during electrolytic aeration of a contaminated aquifer using naphthalene as a model contaminant. Naphthalene degradation of 58-66% took place electrolytically and occurred at the same rates at a pH of 4 and 7. 1,4-naphthoquinone was identified as a product of the electrolysis. Stripping due to gases produced at the electrodes did not result in any naphthalene loss. Hydrogen peroxide (which may be produced at the cathode) did not have any effect on naphthalene, but the addition of ferrous iron (which may be present in aquifers) resulted in 67-99% disappearance of naphthalene. Chlorine (which may be produced from the anodic oxidation of chloride) can effectively degrade naphthalene at pH of 4, but not at a pH of 7. Mono-, di- and poly chloronaphthalenes were identified as oxidation products. Ferric iron coagulation (due to the oxidation of ferrous iron) did not significantly contribute to naphthalene loss. Overall, electrolytic oxidation and chemical oxidation due to the electrolytic by-products formed are significant abiotic processes that could occur and should be accounted for if bioremediation of PAH-contaminated sites via electrolytic aeration is considered. Possible undesirable products such as chlorinated compounds may be formed when significant amounts of chlorides are present.

Anaerobic dechlorination using a fluidized-bed GAC reactor

A simulated high-strength industrial wastewater containing acetate, phenol, and orthochlorophenol (2-CP) was treated using an anaerobic fluidized-bed granular activated carbon (GAC) reactor. Although carbon replacement was initiated to maintain the effluent quality from the reactor and to minimize the inhibitory effects of 2-CP, the latter phase of this study demonstrated that carbon replacement was not necessary. A large pulse input of organic feed was introduced to investigate the resilience of the reactor to severe and sudden increases in influent organic loading. With appropriate pH adjustment, the reactor stabilized within less than two days. A mathematical model was developed to investigate the interaction between the adsorption and biodegradation of compounds in the reactor during the large pulse input. The model confirmed that the GAC was instrumental in controlling the buildup of 2-CP in the reactor.

Assessing the environmental impact of energy production from hydrochar generated via hydrothermal carbonization of food wastes.

Although there are numerous studies suggesting hydrothermal carbonization is an environmentally advantageous process for transformation of wastes to value-added products, a systems level evaluation of the environmental impacts associated with hydrothermal carbonization and subsequent hydrochar combustion has not been conducted. The specific objectives of this work are to use a life cycle assessment approach to evaluate the environmental impacts associated with the HTC of food wastes and the subsequent combustion of the generated solid product (hydrochar) for energy production, and to understand how parameters and/or components associated with food waste carbonization and subsequent hydrochar combustion influence system environmental impact. Results from this analysis indicate that HTC process water emissions and hydrochar combustion most significantly influence system environmental impact, with a net negative GWP impact resulting for all evaluated substituted energy-sources except biomass. These results illustrate the importance of electricity production from hydrochar particularly when it is used to offset coal-based energy sources. HTC process water emissions result in a net impact to the environment, indicating a need for developing appropriate management strategies. Results from this analysis also highlight a need for additional exploration of liquid and gas-phase composition, a better understanding of how changes in carbonization conditions (e.g., reaction time and temperature) influence metal and nutrient fate, and the exploration of liquid-phase treatment.

Ultrasonic degradation of acetaminophen and naproxen in the presence of single-walled carbon nanotubes

Ultrasonic (US) and single-walled carbon nanotube (SWNT)-catalyzed ultrasonic (US/SWNT) degradation of a pharmaceutical (PhAC) mixture of acetaminophen (AAP) and naproxen (NPX) used as analgesics was carried out in water. In the absence of SWNTs, maximum degradations of AAP and NPX occurred at a high frequency (1000 kHz) and under acidic conditions (pH 3) and different solution temperatures (25 °C at 28 kHz and 35 °C at 1000 kHz) during US reactions. Rapid degradation of PhACs occurred within 10 min at 28 kHz (44.5% for AAP; 90.3% for NPX) and 1000 kHz (39.2% for AAP; 74.8% for NPX) at a SWNT concentration of 45 mgL(-1) under US/SWNT process, compared with 28 kHz (5.2% for AAP; 10.6% for NPX) and 1000 kHz (29.1% for AAP; 46.2% for NPX) under US process. Degradation was associated with the dispersion of SWNTs; small particles acted as nuclei during US reactions, enhancing the H2O2 production yield. NPX removal was greater than AAP removal under all US-induced reaction and SWNT adsorption conditions, which is governed by the chemical properties of PhACs. Based on the results, the optimal treatment performance was observed at 28 kHz with 45 mgL(-1) SWNTs (US/SWNT) within 10 min.