Anaerobic biodegradation of solvents from the packaging industry: study and enhancement
- Autores: Nadine Vermorel
- Directores de la Tesis: J. M. Penya-Roja (dir. tes.), Marta Izquierdo Sanchis (codir. tes.)
- Lectura: En la Universitat de València ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Paula Marzal Doménech (presid.), Miguel Martín Monerris (secret.), Pierre Le Cloirec (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Química, Ambiental y de Procesos por la Universitat de València (Estudi General)
- Materias:
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- Resumen
In order to preserve the environment and to avoid harming human health, industrial emissions of solvents are strictly regulated by laws, such as the European Industrial Emissions Directive (2010/75/EU). The packaging industry is one of the important sectors using solvents. The organic solvents mainly found in this type of industry are ethanol – usually the main compound (50 to 60%) – and secondary solvents each representing less than 15%: ethoxypropanol (EP), methoxypropanol (MP), isopropanol (IPA) and ethylacetate (EA). Using biological treatment methods to treat emissions containing these solvents is demonstrated to be both environmentally sustainable and economical feasible. Up to now, the developed biotechnologies such as biofilter, biotrickling filters and bioscrubber were using aerobic treatment.
The novel technique developed by the research group GI2AM and the company Pure Air Solutions in the framework of the European project TrainonSEC to which this thesis project belongs, is based on an anaerobic bioscrubber (patent P201430125). It allows overcoming some of the limitations of existing technologies and offer many advantages in comparison with currently used traditional aerobic biotechnologies, such as presenting a lesser footprint and the conversion of the pollutant into biogas—a readily source of energy. However, there was a clear necessity to carry out additional research before the industrial implementation of the biotechnology, especially regarding the biodegradability of the solvents of interest.
Therefore, the general objectives of this PhD thesis were to enhance the removal of solvents from the packaging industry and the biogas associated to their degradation through the study of the anaerobic degradation of the compounds of interest. In order to achieve these goals, three main lines of study were pursued: on the biodegradability of the solvents (chapter 5), on their degradation rates (chapter 6) and on the enhancement of their degradation via the supplementation of specific micronutrients (chapter 7).
The study on the biodegradability of all the solvents of interest was carried out at laboratory-scale, in batch and continuous mode, at 25 °C. First of all, the influence of factors such as the inoculum to substrate ratio, the source of inoculum (with or without previous exposure to the solvents) or long periods of inactivity of the sludge were determined in batch bioassays, with binary mixtures of solvents. These experiments demonstrated that granular anaerobic sludge from high-rate bioreactors treating brewery wastewaters is a suitable source of sludge for the treatment of the mixture of solvents studied, even though some acclimation time might be needed to treat the secondary solvents. Indeed, previous exposure to the secondary solvents induced better performances for their treatment (reduced lag time and higher Specific Methanogenic Activity (SMA)).
Then, the anaerobic biodegradability of the secondary solvents found in the effluents of the packaging industry (ethyl acetate, 1-ethoxy-2-propanol, 2-propanol and 1-methoxy-2-propanol) was investigated in batch reactors, in a binary mixture with ethanol. Results demonstrated that they can all be degraded at concentrations up to 25 g COD L-1. Most of them (except ethyl acetate) had slower degradation rates than ethanol (150-200 mL CH4 g VSS-1 d-1 for ethanol and around 34-36 mL CH4 g VSS-1 d-1 for the other solvents at initial concentrations between 1-2 g COD L-1) explaining their observed accumulation in the pilot-scale anaerobic bioscrubber. Moreover, these solvents did not inhibit the degradation of ethanol, at any of the tested concentration. 1-methoxy-2-propanol and 1-ethoxy-2-propanol could be degraded after some enzymatic development, with 1-methoxy-2-propanol as the slowest degraded solvent and with a lag of 11-14 days before its degradation at concentrations from 1 to 25 g COD L-1. Ethyl acetate could also be degraded anaerobically, given that sufficient alkalinity was provided, to prevent the acidification of the bioreactor due to the fast hydrolysis of the compound into acetic acid.
The degradation of 5 g COD L-1 of 1-ethoxy-2-propanol, 2-propanol and acetone (expected to be an intermediate of the degradation of the other two solvents) as sole substrate indicated similar SMA around 34-36 mL CH4 g VSS-1 d-1—supporting the assumption that the degradation of acetone is the limiting step for 1-ethoxy-2-propanol and 2-propanol.
Given that 2-propanol can be the main solvent in some packaging industries, instead of ethanol, its degradation was further studied in a continuous mode in presence of ethanol or alone, using a laboratory-scale CSTR. Stable and high removal efficiencies were achieved using 3 kg COD m-3 d-1 of 2-propanol, the equivalent of a Sludge Loading Rate (SLR) of 0.17 kg COD kg VS-1 d-1. This value matches with the one observed in batch assays for 1.2 g COD L-1. The experiments supported the assumption that 2-propanol requires very specific microorganisms for its degradation, which have to be either present or developed in the seeded sludge.
The second line of study of this thesis aimed at acquiring a better understanding of the degradation kinetics of the solvents of interest, for the industrial implementation of the process. Experiments were undertaken in the 8.7 m3 pilot-scale EGSB reactor of the industrial prototype of the anaerobic bioscrubber, seeded with 3 m3 of granular anaerobic sludge originating from a brewery wastewater treatment plant (S-B1), and coupled with a recirculation tank. The water volume of the entire system, operated in a closed loop with a purge, was of 12 m3. This EGSB reactor, located in a packaging factory in the Netherlands, had been previously treating a fraction of the industrial emissions of the factory, during an 18-month period for another research work.
First, experiments were undertaken with 1-ethoxy-2-propanol, which was the main secondary solvent found in the water of the anaerobic bioscrubber. The results of this trial indicated the feasibility of anaerobic treatment of typical effluents from flexographic packaging factories, with ethanol as the main solvent and 10% of 1-ethoxy-2-propanol, at an organic loading rate (OLR) of 3.3 kg COD d-1 m-3, that is a SLR of 0.23 kg COD d-1 kg VSS-1 (considering a sludge volume of 3 m3).
Then, the degradation of 2-propanol as a secondary solvent, alongside ethanol, was studied. These studies have demonstrated that 2-propanol can be effectively degraded as a minoritory solvent or as the main substrate in the pilot-scale EGSB reactor, expanding the applicability of the anaerobic bioscrubber to industries emitting effluents with this solvent as the major compound. Anaerobic granular sludge from brewery wastewater treatment plant was found to be able to remove 2-propanol loads up 0.29 kg COD kg VSS-1 d-1 at 26 °C (corresponding to a punctual OLR of 3.9 kg COD m-3 d-1), when a smooth and progressive exposure to 2-propanol was used (steps of 0.6–0.7 kg COD m-3 d-1). On the other hand, high degradation and methane yields could not be achieved for temperature under 20 °C, indicating that psychrophilic conditions are not adequate for 2-propanol anaerobic treatment, at least at such SLR. Additionally, experiments were carried out to evaluate the tolerance of the sludge to pshychrophilic or sub-mesophilic temperatures, this time using ethanol as the main substrate. Findings highlighted decreasing reactor performances with decreasing temperatures, with a minimum recommended temperature slightly lower than for 2-propanol: around 18 °C for OL of 15-30 kg COD d-1 (OLR of 1.7-3.4 kg COD m-3 d-1). Higher temperatures (26 °C) would allow treating higher organic loads, even with peaks at 80 kg COD d-1 (OLR of 9.2 kg COD m-3 d-1). Some of these results related to the anaerobic degradation of 2-propanol at pilot-scale, together with results from studies at laboratory-scale were recently published in:
Vermorel, N., San Valero, P., Izquierdo, M., Gabaldon, C., Penya-Roja, J.M., Anaerobic degradation of 2-propanol: laboratory and pilot-scale studies. Chemical Engineering Science 172 (2017), 42-51.
Side-studies on the optimisation of the composition of the nutrients (refining the N/P and N/S ratios) permitted minimizing the volume of nutrients to be fed in the pilot-scale EGSB reactor. This fine-tuning of the original formula of macronutrients should allow reducing operational cost at industrial scale and minimizing the H2S content.
Finally, these experiments at pilot-scale give some insight on important restrictions and key parameters to achieve good performances at full scale. Especially if the full-scale installation should run with 2-propanol as the main solvent and a sludge from a brewery wastewater treatment plant, the period of acclimation to 2-propanol should be monitored with the COD and VFA concentrations, as well as the pH and biogas production. A transitory start-up period of around 3 weeks-1 month is expected for the anaerobic bioscrubber, during which step-wise increases in the organic load (OL) should be applied, through the control of the airflow containing the solvents and entering the scrubber. Some addition of an easily biodegradable substrate such as ethanol might be necessary, during the first phases with low OL of 2-propanol, in order to keep a minimum total OL for the consortium of bacteria. If possible, subsequent full-scale installations running with 2-propanol should then be (partially) seeded with the adapted sludge from the first full-scale plant. Moreover, whatever solvent is the main substrate (ethanol or 2-propanol), it is advisable to have the temperature of the anaerobic reactor kept above 20 °C, and thus this parameter should also be monitored.
The third line of research aimed at improving the anaerobic degradation of the solvents through the optimisation of the nutrients dosing. As macronutrients supplementation is quite abundantly referenced, the study was focused on micronutrients. These studies highlighted the importance and complexity of determining a proper dosing strategy for micronutrients in anaerobic bioreactors.
First experiments extracted the total metal contents (Fe, Zn, Cu, Mo, Ni, Co, Mn and Se) of two types of sludge samples (from a brewery wastewater treatment plant (S-B1)) or the same type of sample seeded in the pilot-scale anaerobic bioscrubber and treating effluents from the packaging industry for more than a year (S-FP) with microwave-assisted digestion. Comparison of results with similar sludges from the literature highlighted the importance of the source of inoculum chosen, as different types of sludge had different initial metals contents, which can have a great impact on the response of the system to micronutrient dosing.
Then, a screening of metals influence and interactions was undertaken, considering the following trace metals: Fe, Co, Mn, Mo, Ni and Zn and using a fractional factorial experimental plan (26-2). The experiments were carried out at laboratory scale, using batch reactor at 25 °C, and evaluating the influence of different dosing on the degradation of a binary mixture of ethanol (used as a control) and 1-methoxy-2-propanol. Results from this study on micronutrients dosing pointed out that optimal dosing is not only important for increasing the SMA or degradation rate, but also play a significant role in reducing the lag phase observed with 1-methoxy-2-propanol. Thus, this study seemed to confirm that micronutrients can impact the onset of the activity or growth of the bacterial population through the onset of particular enzymes. Iron higher dosing had the most positive effect on the methane production rate associated with the degradation of 1-methoxy-2-propanol. Simultaneous higher dosing of Fe and Zn as well as Fe and Mn also significantly enhanced the methane production rate- indicating a positive interaction between these elements. These responses could confirm that there was a limitation of iron and zinc in the sludge used in the EGSB reactor, as suggested by the study of the total metal content, where these metals were found in lower concentrations than expected.
Furthermore, the metals allocation in the sludges S-B1 and S-FP was determined through a sequential extraction method, which is essential for a good understanding and interpretation of the results for the micronutrients dosing. In both sludge samples, most of the metals were mainly present in the most strongly bound fractions, i.e. the organic/sulphide-bound fraction and the residual fraction. These findings might indicate the necessity to have freshly-dosed trace metals in the bioreactor, in order to have micronutrients more bioavailable to the sludge.
Finally, the evolution of the micronutrients contents at pilot-scale is presented, for different dosing strategies which were based on the laboratory-scale results. The adjustment in the dosing seemed to quickly (in a month or less) increase the metal content in the sludge used to seed the EGSB reactor (S-B1). Analyses showed that the dosing strategy should be reduced once the metal content in the sludge and water have increased to compensate some detected initial limitations in the sludge, in order to keep low residual trace metals concentrations in the water. Guidelines were issued for an adjustment in the micronutrients, recommending a reduction in their dosing for normal operations (after the start-up period).
