Yes / To date, mixing design practice in anaerobic digestion has focussed on biogas production, but no adequate consideration has been given to energy efficiency. A coherent, comprehensive and generalized strategy based on computational fluid dynamics (CFD) modelling is proposed to improve mixing efficiency of a full-scale, unconfined gas-mixed digester for wastewater treatment. The model consists of an Euler-Lagrange (EL) model where biogas bubbles are modelled as the Eulerian dispersed phase, and non-Newtonian sludge as the Lagrangian continuous phase. Robustness tests show that mixing predictions are independent of bubble size. The CFD strategy comprises the assessment of different mixing geometries and a range of input gas flow rates. Quantitative results show that simple retrofitting measures are able to achieve a significant improvement in the degree of mixing with reduced mixing times, and consequently recommendations for best mixing geometry and gas flow rate are given. A generalization to a generic digester is discussed in a form that is readily usable by professionals and consultants.

The performance and stability of a thermophilic anaerobic digestion system are inherently dependent on the engineered environment within each reactor. While the selection of operational parameters such as mixing, solids retention time, and digestion temperature are often selected on the basis of certain desirable outcomes such as the deactivation of human pathogens, these parameters have been shown to have a broad impact on the overall sludge digestion process. Since the current time-temperature requirements for biosolids pathogen reduction are most easily met at elevated digestion temperatures within the thermophilic range, it is certainly worth examining the effect of specific digestion temperatures on ancillary factors such as operational stability and the aesthetic quality of biosolids. A series of experiments were carried out in which wastewater sludge was digested at a range of temperatures (35°C, 49°C, 51°C, 53°C, 55°C, 57.5°C). Each reactor was operated for a period at steady state in order to make observations of microbial activity, digestion performance, and biosolids aesthetics as affected solely by digestion temperature. Results of this study show that poor operational stability arises in reactors operated at 57.5°C. Elevated concentrations of hydrogen and short-chain fatty acids in the 57.5°C digesters are evidence that the observed temperature-induced digester failures are related to the temperature sensitivity of hydrogenotrophic (CO₂-reducing) methanogens. Reactors operated at other temperatures performed equally well with respect to solids removal and operational stability. In addition, peak volatile organic sulfur compound (VOSC) production from biosolids treated at 51°C and above was greatly reduced in comparison with mesophilic anaerobic digestion and a lower temperature (49°C) thermophilic system. Since the biosolids methanogenic community appeared to be equally capable of degrading VOSC over the range of thermophilic temperatures, the conclusion is that the activity of VOSC producing organisms in digested and dewatered biosolids is greatly reduced when operating temperature in excess of 51°C are used. This study shows that small changes in an operationally defined parameter such as digestion temperature can have a large impact on the performance and stability of a digestion process. Single minded selection of digestion temperature in order to achieve effective pathogen reduction can result in poor digester performance and the production of an aesthetically unacceptable product. Careful selection, however, of an appropriate digestion temperature can not only ensure successful pathogen reduction in compliance with current regulations, but can also improve the performance, stability, and aesthetic quality of digestion systems employing thermophilic anaerobic digestion. / Master of Science

This study aims to provide the latest understanding of cutting-edge technologies that enable wastewater organic carbon diversion and recovery through the enhancement of sludge production and blending, digestibility, dewaterability, and dewatered cake odor emission control. A comprehensive literature review showed that iron-based coagulants tend to show less negative impact than aluminum-based coagulants. This can be attributed to the reduction of ferric to ferrous ions in the course of anaerobic digestion (AD), which leads to a suite of changes in protein bioavailability, alkalinity, and hydrogen sulfide levels, and in turn the sludge dewaterability and odor potential. In terms of the roles of thermal hydrolysis pretreatment (THP), the mechanism review indicated that the improvement of sludge dewaterability and anaerobic digestibility as a result of THP was because of the destruction of extracellular polymeric substances and increase of hydrolysis rate. However, THP also brings side effects such as high free residual ammonia and recalcitrant dissolved organic nitrogen (rDON) in the effluent. Besides, a comprehensive understanding of the formation of the odorous compounds in the sludge treatment processes indicated that sulfurous and nitrogenous compounds are usually regarded as the major odor-causing substances. A Pilot THP-AD study indicated that adding aluminum to produce primary sludge can improve overall plant sludge digestibility, dewaterability, and well as the rDON reduction. Moreover, results from a pilot THP-AD and biochemical methane potential (BMP) test study indicated that adding a secondary thermal hydrolysis after a primary thermal hydrolysis-AD system can still create new BMP. Finally, a pilot study was conducted to evaluate the effect of aeration in the sludge holding tank on biosolids odor emission. The two rounds of bench-scale aeration studies indicated that aerating the sludge in holding tanks reduced peak emission concentrations of sulfurous odorous compounds. Further full-scale validation confirmed that aeration can be used by utilities as a simple means for biosolids odor control. / Doctor of Philosophy / Public wastewater treatment annually consumed 3-4% energy production and contributed 1% greenhouse gas emission in the U.S. Meanwhile, the chemical energy contained in wastewater was estimated to be 9.3 times the energy it takes to treat it. Therefore, harvesting wastewater energy is proposed as a viable means for achieving energy and carbon neutral wastewater treatment. The approach to sending wastewater energy as much as possible to anaerobic digesters in which microorganisms help harvest useful energy in the form of flammable methane was evaluated in this study. From literature, we learned that chemicals used for upstream wastewater energy capture and nutrient removal may make the downstream energy recovery difficult. While, thermal hydrolysis pretreatment, an industrial-scale pressure cooker, can be used to improve the ease of microbial bioenergy harvesting by making organics more biodegradable. However, thermal hydrolysis may also bring side effect in terms of recalcitrant organic formation. Also, in the course of energy recovery, the production and emission of nuisance odor may occur but can be controlled. Building on this existing knowledge, this study evaluated the pros and cons of the approach to using chemicals to capture and recover energy from wastewater. The results showed that the extents of energy recovery and savings was greater than the compromised solids reduction from the process. Moreover, results from a biochemical methane potential test study indicated that adding a secondary thermal hydrolysis can recover even more chemical energy from wastewater. In the end, a pilot study was conducted to develop a simple and economical approach to mitigating the odor emission issue during sludge handling. Results showed that pumping air into the sludge holding tank can substantially reduce peak odor emission. This approach was later verified in a full-scale test and recommended to utilities as a simple means for biosolids odor control.

Esta tesis doctoral se centra en la dinámica de la inhibición y la recuperación del proceso de digestión anaerobia de purines de cerdo para encontrar indicadores, predecir fallos del proceso, minimizar las pérdidas de metano (CH4) y evaluar las mejores prácticas de gestión a nivel de la planta de biogás. Para cumplir con este objetivo, cinco ensayos fueron diseñados y ejecutados. En primer lugar, se diseñó un experimento para controlar los cambios físico-químicos y de emisión de gas de dos tipos de purines envejecidos durante 15 semanas de almacenamiento en condiciones de verano. En segundo lugar, diferentes concentraciones de sulfato (SO42-) fueron evaluadas en digestión anaerobia termofílica de purines de cerdo y vacuno controlando la producción de CH4 y los cambios fisicoquímicos en un ensayo en discontinuo. Así mismo, la degradación anaeróbica de la materia orgánica (MO) y el límite de inhibición de SO42- fueron investigados. En tercer lugar, los efectos de incluir purines de cerdo acidificados con SO42- en un codigestión anaerobia con purines de cerdo convencionales se estudió a dos escalas (a escala laboratorio y a gran escala), donde se evaluó el rendimiento del proceso. Así mismo, los indicadores clave del proceso fueron identificados. En cuarto lugar, una combinación de dos métodos, reacción en cadena de la polimerasa cuantitativa en tiempo real (qPCR) y microscopía electrónica de barrido cualitativa (SEM) fueron utilizados para evaluar los cambios en la población microbiana de los digestores anaerobios durante la adición de los purines. Finalmente, el rendimiento de CH4, la composición físico-química y la estructura y dinámica de la comunidad microbiológica fueron evaluadas durante la puesta en marcha de la digestión anaerobia de purines de cerdo a escala de laboratorio. Se evaluaron cuatro estrategias de puesta en marcha: inanición y no inanición, seguida de una adición gradual o brusca de los purines. Los resultados presentados en esta tesis doctoral per / Moset Hernández, V. (2012). Monitoring anaerobic digestion of animal slurry during inhibition and recovery phases [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/18069

Waste sludge is frequently treated by anaerobic digestion to kill pathogens, generate methane gas and reduce odors so the sludge can be safely land applied. In an attempt to reduce sludge volumes and improve sludge dewatering properties, the use of thermal hydrolysis (TH), a sludge pretreatment method, has been adopted by numerous wastewater treatment plants, among them being the District of Columbia Water and Sewage Authority (DC WASA). The use of anaerobic digestion in collaboration with thermal hydrolysis has been shown to increase VS removal, COD removal and biogas production. The sludge generated also dewaters to a higher cake solids than from conventional anaerobic digestion. Unfortunately, DC WASA has found that the use of thermal hydrolysis had brought about two major issues. These are: (a) does thermal hydrolysis increase destruction of fats, oils and greases compared to conventional digestion? and (b) is the mixing method used at Virginia Tech (recirculating gas mixing) capable of stripping ammonia from the digester? Therefore the main purpose of this study is to evaluate these issues which occur with the use of the thermal hydrolysis process. Experiments were conducted in two phases. The first phase was to assess the performance of anaerobic digesters via their biogas production with and without long chain fatty acid addition and with or without thermal hydrolysis. This research was further carried out in two stages. First a mixture of unsaturated long chain fatty acids (hydrolyzed and unhydrolyzed) was used. The fatty acid mixture included oleic, linoleic and linolenic acids, which contain one, two and three double bonds, respectively. In the second stage, the effect of a single unsaturated fatty acid (hydrolyzed and unhydrolyzed) was analyzed. If extra gas is generated, grease addition to the digesters will be implemented. If thermal hydrolysis produces more gas, the greases will be added through the thermal hydrolysis unit rather than being added directly to the digester. The results showed that addition of long chain fatty acids greatly increased gas production and the long chain fatty acids that were thermally hydrolyzed generated more gas than the untreated long chain fatty acids, although the gain was not large. The second phase of the study was carried out by alternating the type of recirculating gas mixing (partial and continuous) in the anaerobic bioreactor. To achieve this goal, short-term anaerobic bioreactor studies were conducted by varying the frequency of the gas. The result showed that continuous gas recirculation at the bottom of the digester was responsible for stripping ammonia from the system. It appeared that up to 500 mg/L of ammonia was being stripped from the digester operating at 20 day solids retention time. This suggests that ammonia can be stripped if a reduction of ammonia in the digester was desired. / Master of Science

Dairy farms can apply anaerobic digestion (AD) as a manure management system, while producing renewable energy. Ultimately, this can reduce greenhouse gas (GHG) emissions. There is much research work that has quantified the changes in emissions due to AD. However, important methodologies such as the Intergovernmental Panel on Climate Change (IPCC) Guidelines, rely only on a small sample of the accumulated scientific findings in the field. This thesis improves the robustness of these methodologies by applying data-driven techniques to estimate values of the energy output of AD systems and their consequent effect in GHG emissions. For this purpose, meta-analyses techniques are applied to mathematically combine metrics from 155 non-standardised research publications (i.e. with different boundaries, scopes and functional units). As a first step, a novel database is created by systematically searching for relevant articles and assessing them against defined criteria. The database is divided in two parts. Given that the offset of GHGs is highly dependent on the energy output of AD system, empirical methane yields (i.e. key metric of AD performance) are collected in Part I of the database. GHG released from different farm activities are input to Part II of the database. To quantify the change in emissions from these activities, standard baseline and AD scenarios are defined. The second step of the meta-analysis consists of applying uni- and multi-variate tests to the database. For Part I, methane yields are analysed in terms of type of digesters. From the batch digesters, new maximum methane yields are proposed based on the combined results of 42 peer reviewed articles. These results offer better estimates than default values of methane yields from the 2007 Guidelines of the IPCC, which only consider two studies. For continuously stirred tank digesters and semi-continuous digesters lower methane yields are revealed. Multi-variate analysis of methane yields together with operating conditions and manure composition, enable the identification of clusters. These groups of variables can be useful to build potential AD scenarios in dairy farms. For Part II of the database, relative changes in emissions between the activities in the standardised baseline and AD scenarios, are examined. It is found, through meta-analysis, that replacing raw manure by anaerobically-treated manure (i.e. digestate) in storage tanks and for field- application, mitigates baseline emissions by 38.7% and 6.9%, respectively. These relative changes can be used to estimate emissions from digestate, being more specific and evidence-based than the current methodology from the IPCC. In addition, relative changes found for offset of fossil fuels by biogas generated in the AD scenarios indicate a reduction of baseline emissions by 9.0%. Only methane leaks from digesters significantly increase the baseline emissions (by 7.4%). Finally, results found by meta-analyses of methane yields and changes in emissions are applied to four dairy farm case studies. The work presented in the case studies demonstrates the benefits of enhancing the robustness of methods to estimate the effect of AD on GHG emissions from dairy farms.

This study was conducted to address the controlling factors of biosolids stability as they relate to mesophilic anaerobic digestion, dewatering processes and digestion enhancement by wet sludge disintegration technologies. The working hypothesis of this study is that digestion performance; nuisance odor generation and the degree of digestion enhancement by wet sludge disintegration are directly related to anaerobic floc structure and its interaction with shearing forces. Mesophilic digestion was studied in two modes of operation, convention high rate and internal recycle mode to enhanced digestion using a wet sludge disintegration device. The internal recycle system operated on the premise that stabilized sludge would be removed from the digester disintegrated, either by mechanical shear or ultrasonic disintegration for this study, and returned it for to the digester further for further stabilization. Both benchscale and full-scale demonstrations found this mode of digestion enhancement to be effective for mechanical shear and ultrasonic disintegration. It was also determined that volatile solids destruction in both conventional and enhanced mesophilic anaerobic digesters can be reasonably predicted by the concentration of cations in the sludge being treated. It was found that depending on the disintegration device used to enhance digestion performance was influenced by different cation associated fractions of the sludge floc. Along with the improvement of digester performance, overall biosolids stability was investigated through of volatile organic sulfur emissions from dewatered biosolids. In doing so, a method to mimic high solids centrifugation in the laboratory was developed. The centrifugation method identified three major factors that contribute to the generation of odors from biosolids: shear, polymer dose, and cake dryness. The inclusion of shearings suggest that one means of reducing odors from biosolids generated by centrifugation is to use a shear enhanced digestion technology to degrade odor precursors, such as amino acids, within the digester prior to dewatering. Furthermore, the mechanical shearing within a digester is thought to be similar to that of mechanical shear enhanced digestion; therefore, the floc properties that control the digestion process would control observed odor generation. / Ph. D.

Anaerobic Digestion (AD) is an essential component in wastewater treatment to recover energy from waste and deals with sludge management issues effectively. AD is a treatment process that converts organic matter to methane and carbon dioxide with multi-step biological reactions. Methanogenesis, the subprocess of AD that produces methane, is an important indicator of the stability of AD and is influenced by pH, temperature, ammonia, volatile fatty acids (VFAs), and solids concentrations among other factors. Ammonia is an essential nutrient for methanogenic bacteria but at certain ammonia concentrations and pH levels, ammonia is said to be a toxicant for methanogenic archaea. Substrates that are high in ammonia content can include those high in protein, such as food waste, which can be inhibitory to methanogens in the digestion process. Thickened waste activated sludge (TWAS) also contains a large amount of nitrogen with its higher solids concentration, promoting methane production. VFAs are produced during acidogenesis and they can negatively affect methanogenic archaea. High organic loading rates into AD can lead to an accumulation of VFAs and thus inhibition of methanogenic activity. Even with well-known inhibitory effects of ammonia and VFAs on methanogenesis, there are limited tools available for modelling these inhibitions, especially when evaluating diverse compositions of substrate. The objectives of this research work are to experiment for various pairings of pH, ammonia, and acetate levels using batch reactors and to quantify the inhibition on the overall methane production using an AD-based model focused on biological reactions. / Thesis / Master of Applied Science (MASc)

In municipal wastewater treatment, anaerobic digestion is the slowest process requiring at least 15 day solids retention time (SRT). Treating only a small fraction of the total wastewater stream, anaerobic digesters require large reactor volumes and consistent heating (40°C). Thus, there is a growing need to investigate techniques to improve digestion efficiency. The long SRT requirement is a result of the time required for biological reactions such as hydrolysis and acetoclastic methanogenesis. There are numerous pretreatment methods which have so far been developed to particularly enhance hydrolysis. These pretreatment methods include thermalization, mechanical treatments, and chemical treatments. These methods aim to increase the degradability of the influent waste sludge which in turn will increase the efficiency of the digestion process. The goal of the research presented in this thesis is to enhance another limiting biological reaction: acetoclastic methanogenesis. Microbial electrolysis cell (MEC) technology was integrated into lab-scale anaerobic digesters in order to accelerate biosolids destruction under various SRT and temperature conditions. Various mathematical simulations were conducted using a developed steady-state ADM1 (Anaerobic Digestion Model No.1) model to further evaluate the performance of the digesters. The results of the research indicate that the proposed method is effective at shortened SRTs (e.g., 6 days) and can enhance the stability of anaerobic digestion when exposed to variations in temperature and influent composition. / Thesis / Master of Applied Science (MASc)

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