Potential assessment and kinetic modeling of carboxylic acids production using dairy wastewater as substrate

doi:10.1016/j.bej.2020.107502

Biochemical Engineering Journal

Volume 156, 15 April 2020, 107502

https://doi.org/10.1016/j.bej.2020.107502 Get rights and content

Highlights

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Dairy wastewater (DW) is a potential substrate to produce carboxylic acids (CA).
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Kinetic modeling is a useful tool to elucidate acidogenic fermentation kinetics.
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High yield of CA (0.66 mgCA mgCOD_A⁻¹) was obtained from DW acidogenic fermentation.
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Models that describe an exponential phase are suitable to simulate CA production.
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Low concentrations of a medium-chain CA were achieved without e-donors addition.

Abstract

Carboxylic acids (CA) are high added-value compounds that can be produced via anaerobic fermentation by using agroindustrial residues as substrates. However, different compounds in wastewater impose uncontrolled metabolic pathways, in which the acidogenic fermentation kinetics need to be elucidated. This work aimed to assess the potential production of CA from dairy wastewater (DW) and to perform the kinetic modeling of the process. The experiments were conducted in quadruplicate batch reactors (250 mL of working volume) with inoculum from a brewery UASB at 0.61 ± 0.04 gCOD gVSS⁻¹. To inhibit methanogenesis, 0.05 % (v/v) chloroform was added to the reactors. The tests showed that DW is a readily fermentable substrate to acidogenic microorganisms because it presents high rates of short-chain CA formation in the first two days of the experiment. The low concentrations of medium-chain CA found indicate that fats and proteins did not function as the main carbon source for DW fermentation. The yield obtained was 0.66 mgCA mgCOD_A⁻¹, which corresponds to 0.83 mgCOD_CA mgCOD_A⁻¹. Kinetic modeling studies have shown that mathematical models that can describe an exponential phase, such as First-order and Fitzhugh models, are suitable for simulating the production of carboxylic acids. Finally, DW seems to be a promising substrate to be investigated in the carboxylic platform.

Graphical abstract

Introduction

The recovery of high added-value products from agroindustrial wastes can be a sustainable and economically viable alternative to decrease the dependence on fossil fuels and implement environmentally friendly chemical processes. Carboxylic acids (CA) are among these added-value products, which can be prospected anaerobically from macromolecules fermentation by microbial consortium, usually requiring inhibition of methanogenesis and sulfetogenesis [[1], [2], [3]].

During acidogenic fermentation, the major CAs produced are short-chain carboxylic acids, which have up to five carbon atoms. These compounds can be used as carbon source for further processing or in the synthesis of building block chemicals, pharmaceuticals, cosmetics, materials, bioplastics, such as polyhydroxyalkanoates (PHA), and biofuels [[4], [5], [6]]. Compared to methane (CH₄), they are easier and safer to store and transport, besides having a higher market price [7].

In the context of agroindustry, dairy production is one of the most important components of the trade balance in developed and developing countries. World milk production reached 811 million tons in 2017 [8]. The dairy industry consumes high amounts of water and generates large volumes of wastewater, about 0.2−10 L of wastewater per liter of processed milk, with about 4–11 million tons of wastewater discharged annually worldwide [9,10]. Dairy wastewater (DW) is composed of suspended and dissolved solids, soluble organic components, lactose, nutrients, fats, detergent residues, and disinfectants [11,12].

Due to its high chemical oxygen demand (COD) and a high volume of wastewater generated, they may represent an interesting substrate to be investigated either on the biorefinery concept or on the carboxylic platform [11,13]. According to Atasoy et al. [14], it is estimated that, on a global scale, approximately 9.15 Mt acetic acid, 6.47 Mt propionic acid, and 5.39 Mt butyric acid could be recovered from DW, evidencing the potential of using this residue in the production of CA.

Mathematical modeling of fermentation processes is an attractive strategy for its scaling and optimization, as simulations can be applied to resource recovery treatment plants for engineering design, operation and prediction, where small process improvements can generate significant economic [15,16]. These tools are already widely studied for the complete anaerobic digestion [[17], [18], [19]], especially to optimize biogas production from organic wastes. However, as far as we are concerned, there are just a few studies of kinetic modeling assessing acidogenic fermentation of agroindustrial wastes, especially DW, in the perspective of the carboxylic platform.

Thus, the objective of this work was to assess the potential of CA production, perform mathematical modeling and estimate kinetic parameters that describe the hydrolysis of particulate organic matter, soluble substrate consumption and CA production under acidogenic anaerobic conditions using DW as substrate and microbial consortium as inoculum.

Section snippets

Substrate and inoculum

DW was collected from a dairy industry located in the municipality of Maranguape, Ceará, Brazil. After collection, the wastewater was stored in a refrigerator at approximately 3 °C to avoid its degradation. The physicochemical characterization is shown in Table 1, whose values are in agreement with the literature [11,20].

The batch reactors used in the assays were inoculated with a sludge collected from an upflow anaerobic sludge blanket (UASB) reactor used in the treatment of brewery

Mass balance

The mass balance showed that most of the initial substrate COD that was available to the microorganisms (691.97 ± 0.00 mgCOD −COD_A) was converted to CA (83 ± 1 %), while the remaining COD was either directed to new cell formation (15 ± 1 %) or remained available for microbial conversion or other non-detected products (1 ± 1 %) (Table 4). Therefore, 99 ± 1 % of the total applied COD was biologically transformed in CA and new cells, representing a mass of 684.5 ± 4.9 mgCOD (COD_C), and

Conclusions

The tests showed that DW is a readily fermentable substrate to acidogenic microorganisms because it presents high rates of short-chain CA formation in the first two days of the experiment. The low concentrations of medium-chain CA found indicate that fats and proteins did not function as the main carbon source for DW fermentation. However, HCa was produced without the external addition of electron donors. The yield obtained was 0.66 mgCA mgCOD_A⁻¹, which corresponds to 0.83 mgCOD_CA mgCOD_A⁻¹.

Declaration of Competing Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author contribution statement

Milena Maciel Holanda Coelho: helped the experiments during the undergraduate studies on the Biotechnology Major at Federal University of Ceará.

Naassom Wagner Sales Morais: performed the experiments during his master´s study at the Civil Enginnering Program at the Federal University of Ceará.

Erlon Lopes Pereira: Assistant professor Federal University of Ceará and co-supervisor of the students.

Renato Carrhá Leitão: part of the analysis was performed at Embrapa and he helped in the experiment

Acknowledgments

The authors would like to thank the support given by the following Brazilian institutions: National Council for Scientific and Technological Development – CNPq; Higher Education Personnel Improvement Coordination – CAPES; Minas Gerais State Research Support Foundation – FAPEMIG; National Institute of Science and Technology in Sustainable Sewage Treatment Stations – INCT Sustainable ETEs and Brazilian Agricultural Research Corporation -– EMBRAPA.

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Regular articlePotential assessment and kinetic modeling of carboxylic acids production using dairy wastewater as substrate

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Substrate and inoculum

Mass balance

Conclusions

Declaration of Competing Interests

Author contribution statement

Acknowledgments

J. Environ. Chem. Eng.

Biochem. Eng. J.

J. Environ. Sci. (China).

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Trends Food Sci. Technol.

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Photobiol. A Chem.

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Regular article
Potential assessment and kinetic modeling of carboxylic acids production using dairy wastewater as substrate