1,721,207 research outputs found
Assessing the aerobic/anoxic enrichment efficiency at different C/N ratios: pilot scale polyhydroxyalkanoates production from waste activated sludge
No full textPolyhydroxyalkanoates (PHA) can be produced using fermentation products of an excess sewage sludge fermentation process. An efficient method to enrich a PHA-producing community is an aerobic-feast/anoxic-famine enrichment strategy. The effect of different carbon to nitrogen (C/N) feed ratios of 1, 2 and 3.5 g COD/g N on the process performance was studied. The study was executed on a pilot plant scale using fermented waste activated sludge as the organic carbon source. The system's performance was monitored in terms of removing contaminants, producing PHA, and reducing N2O emissions. The results indicated that a lower C/N ratio results in lower PHA production, with PHA content in the sludge of 20, 24 and 36 % w/w for C/N ratios of 1, 2 and 3.5 g COD/g N, respectively. At the lowest C/N ratio, the highest nitrite accumulation rate (77 %), nitrification efficiency (89 %) and denitrification efficiency (89 %) were observed, but the N2O production was also the highest (0.77 mg N2O-N/L). The long-term comprehensive monitoring carried out in this study revealed high carbon and ammonia removal efficiencies (never below 80 %) despite the C/N shifts and high COD and ammonia concentrations. At the same time, the system showed relatively low PHA production and high environmental impact in terms of high gaseous N2O emission. These findings question the sustainability of the aerobic-feast/anoxic-famine enrichment strategy for PHA production in full-scale plants
From waste activated sludge to polyhydroxyalkanoate: Insights from a membrane-based enrichment process
Get PDFPolyhydroxyalkanoate (PHA) production is a promising technology fostering the spread of the circular bio-economy approach. However, the environmental implication of the process is usually neglected. This paper shows the results of a membrane-based PHA production pilot plant fed with no-pretreated waste activated sludge (WAS). The system was monitored for effluent water quality, nitrous oxide (N2O), and PHA production by dynamic accumulation over a long-term period to assess the consistency of the results over several fluctuations. The experimental study was characterized by three C/N ratios of 9, 4.5, and 4 g COD/g N. The system achieved a stable and high removal efficiency for carbon and nitrogen (96.3 ± 2.6 % and 89.9 ± 6.7 %, respectively), despite the only legislation limit respected being the biological oxygen demand concentration discharge limits imposed by 2020/741/EU. Low N2O gaseous and liquid concentrations were achieved over the 200-day experimental period, never exceeding 0.52 mg N2O-N/L. Despite the high concentration, the N2O emission factor accounted for only 0.21 ± 0.14 % of the influent nitrogen. Finally, the system produced an average of 36.3 ± 1.8 % g PHA/g VSS with a storage yield of up to 0.42 g CODPHA/g CODVFA. The system revealed a high stability over a long-term experimental period, achieving a considerable amount of PHA while maintaining a low N2O emission. Promising effluent water quality was achieved, highlighting the potential of applying the water reuse practices
Comparing two advanced selection strategies for polyhydroxyalkanoate production from domestic waste activated sludge
Get PDFThis study compares two membrane bioreactor-based enrichment strategies to produce polyhydroxyalkanoates (PHAs) from domestic waste-activated sludge. The aerobic dynamic feeding was implemented in layout 1 while layout 2 employs an aerobic/anoxic enrichment adopting an additional nitritation reactor. Both systems achieved around 38 % w/w of PHA with storage yields of 0.28–0.42 and 0.35–0.53 gCODPHA/gCODVFA for layouts 1 and 2, respectively. Layout 2 demonstrated an average N removal efficiency of 88.8 ± 3.9 %, slightly higher than layout 1 (82.7 ± 9.9 %). However, layout 2 showed greater nitrous oxide (N2O) emissions, averaging 0.7 ± 0.2 mg N2O-N/L almost doubling layout 1 (0.4 ± 0.1 mg N2O-N/L). Additionally, layout 2 exhibited a 42 % increase in carbon footprint compared to layout 1, reaching 10.2 kg CO2/day. This research highlights the high potential and drawbacks of the AE/AN enrichment strategy for integrating PHA production into wastewater treatment plant operations
Greenhouse gas emissions from membrane bioreactors: Analysis of a two-year survey on different MBR configurations
Get PDFThis study aimed at evaluating the nitrous oxide (N2O) emissions from membrane bioreactors (MBRs) for wastewater treatment. The study investigated the N2O emissions considering multiple influential factors over a two-year period: (i) different MBR based process configurations; (ii) wastewater composition (municipal or industrial); (iii) operational conditions (i.e. sludge retention time, carbon-to-nitrogen ratio, C/N, hydraulic retention time); (iv) membrane modules. Among the overall analysed configurations, the highest N2O emission occurred from the aerated reactors. The treatment of industrial wastewater, contaminated with salt and hydrocarbons, provided the highest N2O emission factor (EF): 16% of the influent nitrogen for the denitrification/nitrification-MBR plant. The lowest N2O emission (EF 1⁄4 0.5% of the influent nitrogen) was obtained in the biological phosphorus removal-moving bed-MBR plant likely due to an improvement in biological performances exerted by the co-presence of both suspended and attached biomass. The influent C/N ratio has been identified as a key factor affecting the N2O production. Indeed, a decrease of the C/N ratio (from 10 to 2) promoted the increase of N2O emissions in both gaseous and dissolved phases, mainly related to a decreased efficiency of the denitrification processes
On the ecology and applications of glucose and xylose fermentations
No full textMicrobial fermentations are a key process in naturally and man-made ecosystems. Microbial fermentations play a key role in creating and digesting our food and they are useful in designing bioprocesses that can produce biogas, biofuels, bioplastics, and many other functional molecules (Chapter 1). Furthermore, studying the competition and cooperation in microbial fermentative ecosystems can help to solve the question how microbial diversity is shaped. Glucose is a molecule central to most forms of life, therefore glucose was chosen as a model substrate to perform fermentative enrichment studies. Xylose is an important monomer in many types of hemicellulose and was therefore chosen as second model substrate. Glucose and xylose can be fermented to volatile fatty acids, alcohols or lactic acid. The biomass specific uptake and production rates at which microbial fermentations are performed are high compared to other biological anaerobic carbon conversions. This rate difference is useful when studying fermentation using an enrichment culture approach. Such fermentative enrichment cultures can be used to develop mixed culture fermentation technologies, which offer alternative technological possibilities for processing feedstocks and residual streams containing carbohydrates (Chapter 1). Biogas production is a relatively well-established industry, but remains to be economically outcompeted by natural gas. The market for (bio)hydrogen production is relatively big, as the hydrogen economy stood for 130 billion USD in 2017. Actual large-scale hydrogen production and capture using biological systems has yet to prove itself. Lactate and ethanol can both be produced using mixed culture fermentation, where ethanol production remains to be a challenging business case due to small profit margins. Medium chain fatty acids are also a potential product. These molecules are expected to have many applications, with a likely higher value than biogas or biofuel, thus promising a healthy business case. Producing polyhydroxyalkanoates from volatile fatty acids produced by mixed culture fermentation promises a healthy industrial feasibility. When assuming solely competition on substrates to occur, limiting a single substrate in a microbial ecosystem is expected to result in one dominant species. The results of Chapter 2 confirm this hypothesis, to the extent of >85% of the observed cell surface belonging to a single species for three out of the four enrichment cultures. A population of Enterobacter cloacae and Citrobacter freundii dominated the glucose and xylose limited sequencing batch cultures respectively. Continuous glucose limitation showed the dominance of Clostridium intestinale. A xylose limited continuous enrichment culture resulted in the coexistence of Citrobacter freundii, and a Lachnospiraceae and Muricomes population. Chapter 3 aims to answer the question how dual substrate limitation influences a fermentative microbial community. Dual xylose and glucose limitation led to a generalist population of Clostridium intestinale in continuous feeding, and a generalist population of Citrobacter freundii in sequencing batch culturing. No apparent carbon catabolite repression was observed when analysing a batch cycle or when performing a batch experiment in the continuous dual limited enrichment culture. This response is of value when designing large scale fermentative bioprocesses, as in industry, typically microorganisms are used which show carbon catabolite repression in mixtures of glucose and xylose. The kinetic, stoichiometric and bioenergetic analysis of enrichment cultures in continuously limited or sequencing batch environments showed that sequencing batch enrichments select for rate, while continuous limited enrichments select for efficiency (Chapter 2). Rate is considered as the biomass-specific substrate uptake rate (qsmax) and efficiency is considered as yield of biomass on ATP harvested in catabolism (Yx,ATP). These findings fit within the r- and K-selection theory. Furthermore, it was found that butyrate production is linked to a lower uptake rate than combined acetate and ethanol production. Potentially, more energy is harvested in butyrate production than in combined acetate and ethanol production, through electron bifurcation. More microbial diversity (i.e. more than one species) was observed than what was expected from a competitive point of view in all six enrichments performed in Chapter 2 and 3. Therefore, in Chapter 5 a complementary approach of metabolomics, metagenomics and isolation studies where performed to generate an evidence based hypothesis on how the Enterobacteriaceae and Clostridiales populations in the continuous xylose limited enrichment culture interacted. The metagenomic evaluation resulted in three dominant bins, one for Citrobacter freundii, one for “Ca. Galacturonibacter soehngenii” and one for a Ruminococcus sp. The interaction between Citrobacter freundii and “Ca. Galacturonibacter soehngenii” is proposed to be a sharing of biotin, pyridoxine and alanine by Citrobacter freundii with “Ca. Galacturonibacter soehngenii”. A differential enrichment study showed that indeed the fraction of “Ca. Galacturonibacter soehngenii” increased and Enterobacteriaceae decreased, when these three metabolites were directly supplemented to the enrichment culture. Thus, commensalism and competition were likely to driving microbial diversity in this culture. Chapter 4 aimed to study the ecology of lactic acid bacteria. Bacteria can produce lactic acid from glucose, which is a different metabolism than producing acetate and butyrate. Sequencing batch reactors were used to enrich, comparing a mineral and complex medium. The media were identical, except for the addition of peptides and 9 B vitamins in the complex medium. Glucose was fermented to a mixture of lactic acid and ethanol when using the complex medium, thereby a heterofermentation. Using the mineral medium, glucose was fermented to a mixture of acetate, butyrate and hydrogen, with smaller amounts of lactic acid and ethanol. A population of Lactobacillus, Lactococcus and Megasphaera was enriched on complex medium. On mineral medium, a population of Ethanoligenens dominated the enrichment with a small fraction of Clostridium. Lactic acid producing bacteria are hypothesised to have taken over the fermentation, due to a 94% increase in biomass-specific substrate uptake rate, leading to a higher growth rate. The increase in growth rate is argued to be caused due to resource allocation, whereby lactic acid bacteria optimise their enzyme levels in anabolism and catabolism, attaining a higher growth rate than mineral-type fermenters such as Ethanoligenens. Chapter 6 aims to direct further research, which lies in studying the effect of different parameters on fermentative ecosystems. These parameters are concentrations of: gaseous compounds (I), cations used to neutralise (II), nutrients, such as B vitamins (III). Also, very low pH environments (pH<3.5) are considered an opportunity (IV). Finally, analysing the composition of “real” fermentable streams and their effect on the arising product spectra is of interest (V). Kinetics and bioenergetics are discussed using enzymatic Michaelis-Menten kinetics and the concept of resource allocation. In this way, efforts can be directed into the ability to predict product formation a priori in fermentative ecosystems. Future experimentation is guided to take place on four distinct levels, and useful experiments to verify concepts in this thesis are outlined. Finally, commensalism and/or mutualism might both be relevant in open microbial ecosystems which remains to be settled by future work.BT/Environmental Biotechnolog
Aerobic Granular Sludge: Effect of Substrate on Granule Formation
No full textDischarging untreated wastewater will contaminate the surface waters and can lead to spread of diseases and long term ecological damage. The most common method for treatment is by the activated sludge process. In this process, nutrients like nitrogen, phosphorus and COD are removed by bacteria grown in flocs. These bacterial flocs are separated from the treated water by settling. Due to the slow settling velocities of these flocs large settling tanks are needed. Settling tanks take up most of the required space for a wastewater treatment plant. Aerobic granular sludge is a compact technology designed to reduce area requirements, save energy while providing excellent effluent quality. Bacteria are grown in granules instead of flocs and have therefore a much higher settling velocity. This reduces the area requirement significantly. So much even so, that external settling tanks are completely omitted. To grow aerobic granules a few selection principles are needed. First, the influent is brought in contact with the biomass in an anaerobic environment. Here COD is converted by the bacteria into storage polymers. These storage polymers are then used for growth in the presence of oxygen hereby removing phosphorus and nitrogen from the bulk liquid. Secondly, a settling pressure is applied by which slow settling biomass is removed from the reactor, thus leading to the formation of granules. In previous research by the PhD students Janneke Beun (principle of aerobic granulation), Merle de Kreuk (basic process technology for granular sludge nutrient removal) and Mari Winkler and Joao Bassin (Microbiology and process engineering aspects of granular sludge) the basic concepts of the granular sludge technology were worked out. In this thesis the effects of several operational conditions on the conversion processes, formation and stability of aerobic granular sludge was studied. The quick implementation of the technology in practice also meant that several important subjects still needed further investigation. To ensure a well-functioning technology in domestic and industrial applications these subjects were studied in more detail (i.e. adsorption, effect of salinity, higher temperature and other substrates). Besides laboratory work also the start-up and performance of one of the first full-scale aerobic granular sludge reactors treating domestic wastewater is described. The ammonium adsorption properties of aerobic granular sludge, activated sludge and anammox granules have been investigated in Chapter 2. During operation of a pilot-scale aerobic granular sludge reactor, a positive relation between the ammonium influent concentration and the ammonium adsorbed was observed. Aerobic granular sludge exhibited much higher adsorption capacity compared to activated sludge and anammox granules. At an ammonium concentration of 30 mg N L-1, adsorption obtained with activated sludge and anammox granules was around 0.2 mg NH4+-N gVSS-1, while aerobic granular sludge from lab- and pilot scale exhibited an adsorption of 1.7 and 0.9 mg NH4+-N gVSS-1, respectively. No difference in the ammonium adsorption was observed in lab-scale reactors operated at different temperatures (20 and 30 ºC). In a lab-scale reactor fed with saline wastewater, we observed that the amount of ammonium adsorbed, decreased considerably when the salt concentration increased. The results indicate that adsorption or better: ion-exchange of ammonium should be incorporated into models for nitrification/denitrification, certainly when aerobic granular sludge is used. Salinity can adversely affect the performance of most biological processes involved in wastewater treatment (Chapter 3). The effect of salt (NaCl) on the main conversion processes in an aerobic granular sludge (AGS) process accomplishing simultaneous organic matter, nitrogen, and phosphate removal was evaluated in this chapter. Hereto an AGS sequencing batch reactor was subjected to different salt concentrations (0.2 to 20 g Cl- L-1). Granular structure was stable throughout the whole experimental period, although granule size decreased and a significant effluent turbidity was observed at the highest salinity tested. A weaker gel structure at higher salt concentrations was hypothesized to be the cause of such turbidity. Ammonium oxidation was not affected at any of the salt concentrations applied. However, nitrite oxidation was severely affected, especially at 20 g Cl- L-1, in which a complete inhibition was observed. Consequently, high nitrite accumulation occurred. Phosphate removal was also found to be inhibited at the highest salt concentration tested. Complementary experiments have shown that a cascade inhibition effect took place: first, the deterioration of nitrite oxidation resulted in high nitrite concentrations and this in turn resulted in a detrimental effect to polyphosphate-accumulating organisms (PAOs). By preventing the occurrence of the nitrification process and therefore avoiding the nitrite accumulation, the effect of salt concentrations on the bio-P removal process was shown to be negligible up to 13 g Cl- L-1. Salt concentrations equal to 20 g Cl- L-1 or higher in absence of nitrite also significantly reduced phosphate removal efficiency in the system. When aerobic granular sludge is applied for industrial wastewater treatment different soluble substrates can be present. For stable granular sludge formation on volatile fatty acids (e.g. acetate), production of storage polymers under anaerobic feeding conditions has been shown to be important. This prevents direct aerobic growth on readily available COD, which is thought to result in unstable granule formation. In Chapter 4 we investigate the impact of acetate, methanol, butanol, propanol, propionaldehyde and valeraldehyde on granular sludge formation at 35 °C. Methanogenic archaea, growing on methanol, were present in the aerobic granular sludge system. Methanol was completely converted to methane and carbon dioxide by the methanogenic archaeum Methanomethylovorans uponensis during the one-hour anaerobic feeding period, despite the relative high dissolved oxygen concentration (3.5 mg O2 L-1) during the subsequent two-hour aeration period. Propionaldehyde and valeraldehyde were fully disproportionated anaerobically into their corresponding carboxylic acids and alcohols. The organic acids produced were converted to storage polymers, while the alcohols (produced and from influent) were absorbed onto the granular sludge matrix and converted aerobically. Our observations show that easy biodegradable substrates not converted anaerobically into storage polymers could lead to unstable granular sludge formation. However, when the easy biodegradable COD is absorbed in the granules and/or when the substrate is converted by relatively slow growing bacteria in the aerobic period stable granulation can occur. The influence of sludge age on granular sludge formation and microbial population dynamics in a methanol- and acetate-fed aerobic granular sludge system operated at 35 °C is investigated in Chapter 5. During anaerobic feeding of the reactor, methanol was initially converted to methane by methylotrophic methanogens. These methanogens were able to withstand the relatively long aeration periods. Lowering the anaerobic solid retention time (SRT) from 17 to 8 days enabled selective removal of the methanogens and prevented unwanted methane formation. In absence of methanogens, methanol was converted aerobically, while granule formation remained stable. At high SRT-values (51 days) γ-Proteobacteria were responsible for acetate removal through anaerobic uptake and subsequent aerobic growth on storage polymers formed (so called metabolism of glycogen accumulating organisms). When lowering the SRT (24 days), Defluviicoccus-related organisms (cluster II) belonging to the α-Proteobacteria outcompeted acetate consuming γ-Proteobacteria at 35 ºC. DNA from the Defluviicoccus-related organisms in cluster II was not extracted by the standard DNA extraction method but with liquid nitrogen, which showed to be more effective. Remarkably, the two glycogen accumulating organisms (GAO) types of organisms grew separately in two clearly different types of granules. This work further highlights the potential of aerobic granular sludge systems to effectively influence the microbial communities through sludge age control in order to optimize the wastewater treatment processes. Recently, aerobic granular sludge technology has been scaled-up and implemented for industrial and municipal wastewater treatment under the trade name Nereda®. With full-scale references for industrial treatment application since 2006 and domestic sewage since 2009 only limited operating data have been presented in scientific literature so far. In this study performance, granulation and design considerations of an aerobic granular sludge plant on domestic wastewater at the WWTP Garmerwolde, the Netherlands were analysed (Chapter 6). After a start-up period of approximately 5 months, a robust and stable granule bed (> 8 g L-1) was formed and could be maintained thereafter, with a sludge volume index after 5 minutes settling of 45 mL g-1. The granular sludge consisted for more than 80 % of granules larger than 0.2 mm and more than 60 % larger than 1 mm. Effluent requirements (7 mg N L-1 and 1 mg P L-1) were easily met during summer and winter. Maximum volumetric conversion rates for nitrogen and phosphorus were respectively 0.17 and 0.24 kg (m3 d)-1. The energy usage was 13.9 kWh (PE150∙year)-1 that is 58 – 63 % lower than the average conventional activated sludge treatment plant in the Netherlands. Finally, this study demonstrated that aerobic granular sludge technology can effectively be implemented for the treatment of domestic wastewater
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