Hydrothermal conversion of toilet waste: effect of processing conditions on gas phase emissions.

Globally, many populations suffer from a lack of access to basic sanitation facilities. This is partly caused by a combination of water resource shortages and the high cost of conventional centralised treatment systems. A novel decentralised treatment technology based on sub-critical hydrothermal processing of organic wastes at toilet-scale, contributes to addressing these economic and resource limitations. To be effective, this technology needs to satisfy a broad range of environmental and safety considerations, including the nature and quantity of formed gas products. We investigated the impact of four process parameters (temperature; O2: COD ratio (λ); time; feed solids content) on off-gas composition by quantifying volatile organic compounds (VOCs), CO, H2 and CO2 in factorial experiments. Temperature and λ influenced VOCs generation greatly. The lowest VOC emissions occurred at 200% λ and 300 °C. Aldehydes and ketones were mostly generated at 200% λ and intermediate temperatures, sulphur compounds in the absence of oxygen, and aromatics, furans, and pyrroles at intermediate oxygen levels and elevated temperatures. Most CO was created at 300 °C but its concentration decreased at longer processing times. Processing conditions have complex impacts and require careful consideration when designing for real world deployment.

An improved model for the kinetics of non-oxidative hydrothermal process.

Hydrothermal processing as a post-treatment technology for sludge has attracted great interest globally as it could reduce the amount of sludge considerably. This experimental study developed a comprehensive kinetic model of cellulose degradation via non-oxidative hydrothermal processing at various temperatures (ranges 180-260 °C). Values of activation energies and pre-exponential factors were determined using chemical oxygen demand (COD)-based lumped concentrations. In this study, a new reaction pathway between solid, soluble matter and gaseous products was proposed which not only enables prediction of solid phase degradation but also can predict the formation of various types of products (in liquid and gas phase) during the reaction time. The results show that the reaction rate of cellulose to liquid products (k1=2.7×109exp(-102810RT)) were fast compared to that of for liquid products to gaseous products (k2=4.4×103exp(-64629RT)). Moreover, the model infers that the major part of solid degradation leads to the formation of the gaseous product with the reaction rate constant of k3=5.7exp(-12905RT). The proposed model can provide an opportunity to predict the performance of the non-oxidative hydrothermal processing of organic solid waste.

Rheological Behavior of High Cell Density Pseudomonas putida LS46 Cultures during Production of Medium Chain Length Polyhydroxyalkanoate (PHA) Polymers.

The rheology of high-cell density (HCD) cultures is an important parameter for its impact on mixing and sparging, process scale-up, and downstream unit operations in bioprocess development. In this work, time-dependent rheological properties of HCD Pseudomonas putida LS46 cultures were monitored for microbial polyhydroxyalkanoate (PHA) production. As the cell density of the fed-batch cultivation increased (0 to 25 g·L-1 cell dry mass, CDM), the apparent viscosity increased nearly nine-fold throughout the fed-batch process. The medium behaved as a nearly Newtonian fluid at lower cell densities, and became increasingly shear-thinning as the cell density increased. However, shear-thickening behavior was observed at shearing rates of approximately 75 rad·s-1 or higher, and its onset increased with viscosity of the sample. The supernatant, which contained up to 9 g·L-1 soluble organic material, contributed more to the observed viscosity effect than did the presence of cells. Owing to this behavior, the oxygen transfer performance of the bioreactor, for otherwise constant operating conditions, was reduced by 50% over the cultivation time. This study has shown that the dynamic rheology of HCD cultures is an important engineering parameter that may impact the final outcome in PHA cultivations. Understanding and anticipating this behavior and its biochemical origins could be important for improving overall productivity, yield, process scalability, and the efficacy of downstream processing unit operations.

Development of High Cell Density Cultivation Strategies for Improved Medium Chain Length Polyhydroxyalkanoate Productivity Using Pseudomonas putida LS46.

High cell density (HCD) fed-batch cultures are widely perceived as a requisite for high-productivity polyhydroxyalkanoate (PHA) cultivation processes. In this work, a reactive pulse feed strategy (based on real-time CO2 or dissolved oxygen (DO) measurements as feedback variables) was used to control an oxygen-limited fed-batch process for improved productivity of medium chain length (mcl-) PHAs synthesized by Pseudomonas putida LS46. Despite the onset of oxygen limitation half-way through the process (14 h post inoculation), 28.8 ± 3.9 g L-1 total biomass (with PHA content up to 61 ± 8% cell dry mass) was reliably achieved within 27 h using octanoic acid as the carbon source in a bench-scale (7 L) bioreactor operated under atmospheric conditions. This resulted in a final volumetric productivity of 0.66 ± 0.14 g L-1 h-1. Delivering carbon to the bioreactor as a continuous drip feed process (a proactive feeding strategy compared to pulse feeding) made little difference on the final volumetric productivity of 0.60 ± 0.04 g L-1 h-1. However, the drip feed strategy favored production of non-PHA residual biomass during the growth phase, while pulse feeding favored a higher rate of mcl-PHA synthesis and yield during the storage phase. Overall, it was shown that the inherent O2-limitation brought about by HCD cultures can be used as a simple and effective control strategy for mcl-PHA synthesis from fatty acids. Furthermore, the pulse feed strategy appears to be a relatively easy and reliable method for rapid optimization of fed-batch processes, particularly when using toxic substrates like octanoic acid.

Hydrogen Oxidation Influences Glycogen Accumulation in a Verrucomicrobial Methanotroph.

Metabolic flexibility in aerobic methane oxidizing bacteria (methanotrophs) enhances cell growth and survival in instances where resources are variable or limiting. Examples include the production of intracellular compounds (such as glycogen or polyhydroxyalkanoates) in response to unbalanced growth conditions and the use of some energy substrates, besides methane, when available. Indeed, recent studies show that verrucomicrobial methanotrophs can grow mixotrophically through oxidation of hydrogen and methane gases via respiratory membrane-bound group 1d [NiFe] hydrogenases and methane monooxygenases, respectively. Hydrogen metabolism is particularly important for adaptation to methane and oxygen limitation, suggesting this metabolic flexibility may confer growth and survival advantages. In this work, we provide evidence that, in adopting a mixotrophic growth strategy, the thermoacidophilic methanotroph, Methylacidiphilum sp. RTK17.1 changes its growth rate, biomass yields and the production of intracellular glycogen reservoirs. Under nitrogen-fixing conditions, removal of hydrogen from the feed-gas resulted in a 14% reduction in observed growth rates and a 144% increase in cellular glycogen content. Concomitant with increases in glycogen content, the total protein content of biomass decreased following the removal of hydrogen. Transcriptome analysis of Methylacidiphilum sp. RTK17.1 revealed a 3.5-fold upregulation of the Group 1d [NiFe] hydrogenase in response to oxygen limitation and a 4-fold upregulation of nitrogenase encoding genes (nifHDKENX) in response to nitrogen limitation. Genes associated with glycogen synthesis and degradation were expressed constitutively and did not display evidence of transcriptional regulation. Collectively these data further challenge the belief that hydrogen metabolism in methanotrophic bacteria is primarily associated with energy conservation during nitrogen fixation and suggests its utilization provides a competitive growth advantage within hypoxic habitats.

The impact of environmental parameters on the conversion of toluene to CO2 and extracellular polymeric substances in a differential soil biofilter.

The fraction of pollutant converted to CO2 versus biomass in biofiltration influences the process efficacy and the lifetime of the bed due to pressure drop increases. This work determined the relative quantitative importance and potential interactions between three critical environmental parameters: toluene concentration (Tol), matric potential (ψ) and temperature (T) on % CO2, elimination capacity (EC) and the production rate of non-CO2 products. These parameters are the most variable in typical biofilter operation. The data was fit to a non-linear model of the form y=a(Tol)bTcψd. A rigorous carbon balance (100.5 ±â€¯7.0%) tracked the fate of degraded toluene as CO2 and non-CO2 carbon endpoints. The % CO2 mineralization varied from (34-91%) with environmental parameters: temperature (20-40 °C), matric potential, (-10 to -100 cmH2O) and residual toluene, (20-180 ppm). The highest conversion to CO2 was at the wettest conditions (-10 cmH2O) and lowest residual toluene concentration (18 ppm). Matric potential had twice the impact of toluene concentration on % CO2, while temperature had less impact. The elimination capacity varied from 11 to 50 gC⋅m-3h-1 and was highest at 40 °C, the wettest conditions with limited impact by toluene concentrations. Temperature increased the EC and non-CO2 production rates strongly while matric potential and toluene concentration had less influence (4x - 10x less). This study illustrated the quantitative significance and simultaneous interaction between critical environmental parameters on carbon endpoints and biofilter performance. This kind of multivariable parameter study provides valuable insights which can address performance and clogging issues in biofilters.

The role of dissolved oxygen content as a modulator of microbial polyhydroxyalkanoate synthesis.

Polyhydroxyalkanoates (PHAs) are a diverse class of bio-polymers synthesized by bacteria, usually during imbalanced growth conditions. Optimizing PHA productivity is highly dependent on the bioreactor oxygen transfer rate (OTR), which is an important consideration for process performance and economics, particularly with increasing scale. Relatively few in-depth studies are available regarding the effect of OTR and dissolved oxygen content (DOC) on PHA formation, synthesis rates, composition, and characteristics. This review examines past research studies on the effect of low DOC environments on production of short-chain length (scl-) PHAs, synthesized by both pure and mixed cultures, in order to identify opportunities and gaps concerning the effect of DOC on production of medium-chain length (mcl-) PHAs, an area that has not been studied in detail. The literature indicates that production of scl-PHA (a reductive process) acts as an electron sink allowing cells to maintain balanced redox state at low DOC. Conversely, production of mcl-PHA via fatty acid de novo synthesis (also a reductive process) does not occur to any significant extent in low DOC environments, while mcl-PHA synthesis from fatty acids (an oxidative process) can be promoted in low DOC environments. The monomer composition, molecular mass, as well as physical and thermal properties of the polymer can change in response to OTR, but further research in this area is required for both scl- and mcl-PHAs. Process design and management of bioreactor OTR in PHA production might therefore be directed by the final application of the polymer rather than cost considerations.

Effect of hydrodynamic mixing conditions on wet oxidation reactions in a stirred vessel reactor.

The aim of this study was to investigate the impact of mixing intensity and mixing flow patterns on solid waste degradation, and production of valuable intermediate by-products such as acetic acid. Total suspended solids generally decreased, soluble chemical oxygen demand, dissolved organic carbon, and acetic acid concentration generally increased with the progress of the reaction and increase in the mixing intensity. The results showed that axial-radial flow pattern (using pitch blade impeller) and medium impeller speed (500 rpm) resulted in a higher degree of solid degradation and production of acetic acid.

Mixotrophy drives niche expansion of verrucomicrobial methanotrophs.

Aerobic methanotrophic bacteria have evolved a specialist lifestyle dependent on consumption of methane and other short-chain carbon compounds. However, their apparent substrate specialism runs contrary to the high relative abundance of these microorganisms in dynamic environments, where the availability of methane and oxygen fluctuates. In this work, we provide in situ and ex situ evidence that verrucomicrobial methanotrophs are mixotrophs. Verrucomicrobia-dominated soil communities from an acidic geothermal field in Rotokawa, New Zealand rapidly oxidised methane and hydrogen simultaneously. We isolated and characterised a verrucomicrobial strain from these soils, Methylacidiphilum sp. RTK17.1, and showed that it constitutively oxidises molecular hydrogen. Genomic analysis confirmed that this strain encoded two [NiFe]-hydrogenases (group 1d and 3b), and biochemical assays revealed that it used hydrogen as an electron donor for aerobic respiration and carbon fixation. While the strain could grow heterotrophically on methane or autotrophically on hydrogen, it grew optimally by combining these metabolic strategies. Hydrogen oxidation was particularly important for adaptation to methane and oxygen limitation. Complementary to recent findings of hydrogenotrophic growth by Methylacidiphilum fumariolicum SolV, our findings illustrate that verrucomicrobial methanotrophs have evolved to simultaneously utilise hydrogen and methane from geothermal sources to meet energy and carbon demands where nutrient flux is dynamic. This mixotrophic lifestyle is likely to have facilitated expansion of the niche space occupied by these microorganisms, allowing them to become dominant in geothermally influenced surface soils. Genes encoding putative oxygen-tolerant uptake [NiFe]-hydrogenases were identified in all publicly available methanotroph genomes, suggesting hydrogen oxidation is a general metabolic strategy in this guild.

Fundamental mechanisms and reactions in non-catalytic subcritical hydrothermal processes: A review.

The management and disposal of solid waste is of increasing concern across the globe. Hydrothermal processing of sludge has been suggested as a promising solution to deal with the considerable amounts of sludge produced worldwide. Such a process not only degrades organic compounds and reduces waste volume, but also provides an opportunity to recover valuable substances. Hydrothermal processing comprises two main sub-processes: wet oxidation (WO) and thermal hydrolysis (TH), in which the formation of various free radicals results in the production of different intermediates. Volatile fatty acids (VFAs), especially acetic acid, are usually the main intermediates which remain as a by-product of the process. This paper aims to review the fundamental mechanism for hydrothermal processing of sludge, and the formation of different free radicals and intermediates therein. In addition, the proposed kinetic models for the two processes (WO and TH) from the literature are reviewed and the advantages and disadvantages of each model are outlined. The effect of mass transfer as a critical component of the design and development of the processes, which has been neglected in most of these proposed models, is also reviewed, and the effect of influencing parameters on the processes' controlling step (reaction or mass transfer) is discussed.

Rheological measurements as a tool for monitoring the performance of high pressure and high temperature treatment of sewage sludge.

Hydrothermal processing plays a significant role in sewage sludge treatment. However, the rheological behaviour of sludge during these processes is not fully understood. A better understanding of the sludge rheology under hydrothermal processing conditions can help improve process efficiency. Moreover, sludge rheology is easier to measure than chemical analyses. If a relationship could be established, it provides a possibility of using rheological measurement as a basis for monitoring the performance of hydrothermal processing. The rheological changes in thickened waste activated sludge (7 wt%) was investigated using a pressure cell-equipped rheometer during 60-min thermal hydrolysis (TH) at various temperatures (80-145 °C) and constant pressure (5 bar). Changes in the soluble chemical oxygen demand (COD) were measured using a separate reactor with a similar operating condition. The sludge behaved as a shear-thinning fluid and could be described by the Herschel-Bulkley model. At constant temperature, the yield stress and high-shear (600 s-1) viscosity of sludge decreased logarithmically over 60 min. At constant time, the yield stress and the high-shear viscosity decreased linearly with increasing TH temperature and these values was much less than corresponding properties after treatment and cooling down to 25 °C. The soluble COD of sludge also increased logarithmically over 60 min at constant temperature, and increased linearly with increasing temperature at constant time. Furthermore, the yield stress and high-shear viscosity reduction showed a linear correlation with the increase in soluble COD.

Hydrothermal processing of cellulose: A comparison between oxidative and non-oxidative processes.

This study investigates oxidative and non-oxidative hydrothermal processing of cellulose at five different temperatures (180-260°C). Volatile fatty acids (VFAs) concentration, total suspended solid (TSS) degradation, dissolved organic carbon (DOC) and chemical oxygen demand (COD) were measured and compared in both processes. Moreover, the existence of hydrogen peroxide in both oxidative and non-oxidative processes was confirmed experimentally for the first time in literature. At temperatures ⩽220°C the amount of H2O2 produced in the oxidative process was higher (50 fold) than that of in the non-oxidative while at higher temperatures (⩾240°C) it was more for non-oxidative (3.5-5 fold). The concentration of VFAs in the non-oxidative process was lower than 10% of that in oxidative process. In both processes soluble COD increased with time and temperature, however at 260°C after reaching a maximum, it decreased with time due to conversion of some soluble intermediates to CO2 and water.

Formation and degradation of valuable intermediate products during wet oxidation of municipal sludge.

The current study investigated the formation of organic acids and alcohols as major intermediate products of wet oxidation of municipal sludge. Municipal sludge was subjected to 60-min wet oxidation at temperatures ranging from 220 to 240°C, with 20bar oxygen partial pressure. Acetic acid was the main intermediate compound produced in this study, followed by propionic, n-butyric, iso-butyric and pentanoic acids and methanol. It was found that the process severity has a significant influence on the formation and degradation of these intermediate products.

A kinetic model of municipal sludge degradation during non-catalytic wet oxidation.

Wet oxidation is a successful process for the treatment of municipal sludge. In addition, the resulting effluent from wet oxidation is a useful carbon source for subsequent biological nutrient removal processes in wastewater treatment. Owing to limitations with current kinetic models, this study produced a kinetic model which predicts the concentrations of key intermediate components during wet oxidation. The model was regressed from lab-scale experiments and then subsequently validated using data from a wet oxidation pilot plant. The model was shown to be accurate in predicting the concentrations of each component, and produced good results when applied to a plant 500 times larger in size. A statistical study was undertaken to investigate the validity of the regressed model parameters. Finally the usefulness of the model was demonstrated by suggesting optimum operating conditions such that volatile fatty acids were maximised.

A review of wet air oxidation and Thermal Hydrolysis technologies in sludge treatment.

With rapid world population growth and strict environmental regulations, increasingly large volumes of sludge are being produced in today's wastewater treatment plants (WWTP) with limited disposal routes. Sludge treatment has become an essential process in WWTP, representing 50% of operational costs. Sludge destruction and resource recovery technologies are therefore of great ongoing interest. Hydrothermal processing uses unique characteristics of water at elevated temperatures and pressures to deconstruct organic and inorganic components of sludge. It can be broadly categorized into wet oxidation (oxidative) and thermal hydrolysis (non-oxidative). While wet air oxidation (WAO) can be used for the final sludge destruction and also potentially producing industrially useful by-products such as acetic acid, thermal hydrolysis (TH) is mainly used as a pre-treatment method to improve the efficiency of anaerobic digestion. This paper reviews current hydrothermal technologies, roles of wet air oxidation and thermal hydrolysis in sludge treatment, and challenges faced by these technologies.

Rheology of a primary and secondary sewage sludge mixture: dependency on temperature and solid concentration.

The main objective of this study was to investigate the rheology of mixed primary and secondary sludge and its dependency on solid content and temperature. Results of this study showed that the temperature and solid concentration are critical parameters affecting the mixed sludge rheology. It was found that the yield stress increases with an increase in the sludge solid content and decreases with increasing temperature. The rheological behaviour of sludges was modelled using the Herschel-Bulkley model. The results of the model showed a good agreement with experimental data. Depending on the total solid content, the average error varied between 3.25% and 6.22%.

Physico-chemical properties of polyhydroxyalkanoate produced by mixed-culture nitrogen-fixing bacteria.

Ultra-high molecular weight polyhydroxyalkanoates (PHAs) with low polydispersity index (PDI = 1.3) were produced in a novel, pilot scale application of mixed cultures of nitrogen-fixing bacteria. The number average molecular weight (M (n)) of the poly(3-hydroxybutyrate) (P(3HB)) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) was determined to be 2.4 x 10(6) and 2.5 x 10(6) g mol(-1), respectively. Using two types of carbon sources, biomass contents of the P(3HB) and P(3HB-co-3HV) were 18% and 30% (PHA in dry biomass), respectively. The extracted polymers were analysed for their physical properties using analytical techniques such as nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). NMR confirmed the formation of homopolymer and copolymer. DSC showed a single melting endotherm peak for both polymers, with enthalpies that indicated crystallinity indices of 44% and 37% for P(3HB) and P(3HB-co-3HV), respectively. GPC showed a sharp unimodal trace for both polymers, reflecting the homogeneity of the polymer chains. The work described here emphasises the potential of mixed colony nitrogen-fixing bacteria cultures for producing biodegradable polymers which have properties that are very similar to those from their pure-culture counterparts and therefore making a more economically viable route for obtaining biopolyesters.