Differential Impact of the Prior Mix by Stirring in the Biodegradation of Sunflower Oil

*Pedro Eulogio Cisterna Osorio and Barbara Faundez-Miño*

#### **Abstract**

Fats and oils present in wastewater are usually eliminated by physical and biological processes. In this experience, the fatty wastewaters are treated biologically, and it assesses the impact of the mix in the fats and oils biodegradation and carried out the experiments in a laboratory scale unit. The biodegradation of fats and oils was analysed in two sceneries, with mix previous by mechanical agitation and without mix. Key parameters were monitored, such as the concentration of fats and oils in the influents and effluents, mass loading, and the efficiency of biodegradation. The mass loading range was similar in both sceneries. In the experimental activated sludge plant without mix, the biodegradation of fats and oils reached levels in the range of 28 to 42.5%. For the wastewater treatment plant with a previous mix by mechanical agitation, the levels of biodegradation of fats and oils ranged from 64 to 75%. Therefore, considering the efficiency of the biodegradation of fats and oils in both sceneries, the results indicated that the level mix is a high incidence.

**Keywords:** biodegradation, fats and oils, activated sludge

#### **1. Introduction**

Nowadays, the growing sensitivity with respect to environmental protection has led to increasing regulatory pressure on wastewater treatment, imposing severe limitations on pollutant concentrations before discharging them to the environment. In this context, one of the major challenges is represented by the biological treatment of oily wastewater [1]. Domestic and industrial wastewaters contain fats and oils. The fraction of lipids in urban wastewater is 30–40% of the chemical oxygen demand (COD) [2].

The biodegradation of lipids in activated sludge processes is not well known. The literature states that these can be treated by biological treatment, which eventually causes foam formation composed of filamentous bacteria and flocs that affect biodegradation [3]. Considering generic information, fats and oils are classified as slowly biodegradable substances. As for the biodegradation process, bacteria initially save these substances in their cytoplasm and later through the enzymatic process; it starts hydrolysis to produce an assimilable substrate that can be biodegraded [4].

There are three types of reactions catalysed by microbial enzymes: oxidative, hydrolytic, and synthesis. Hydrolytic enzymes are used to hydrolyse insoluble

complex compounds, such as fats and oils, on simple components that pass through the cellular membrane by diffusion. These enzymes (i.e., oxidoreductases) act outside the cell wall [5]. The extracellular enzyme called lipases is the most common, releasing fatty acids as a consequence of enzymatic action [6]. Lipases break down molecules into simpler components, which appear as end products or intermediates that are consumed by microorganisms. If the solid substrate is sufficiently porous, the enzyme can diffuse and biodegradation can take place inside it; for low porosity material, such as oils where enzymes cannot diffuse, the reaction takes place on the outer surface of the particle [7].

Furthermore, a wider range of microorganisms biodegrades fatty acids from other microorganisms that do not produce extracellular lipolytic enzymes [8]. Wastewater with high lipid concentration inhibits the activity of microorganisms in biological treatment systems, such as active sludge and methane fermentation. To reduce such inhibitory effects, microorganisms capable of effectively degrading edible oils can be selected from different environmental sources [9]. There are many researches that study about elimination and biodegradation of lipids by biological treatment [10]. Biodegradation of fats and oils and other substrates, not soluble in water, is one of the greatest problems for the biological treatment of wastewater [11].

A wide variety of organic compounds such as carbon and energy are used as a source by microorganisms. When substrates possess low or zero solubility, the use of biosurfactants is recommended [12]. One of the main characteristics of emulsified mixtures is the presence of at least one hydrophilic polar liquid and at least one lipophilic; the simplest and most frequent case is when liquids are oil and water [13]. Depending on the emulsification process, the diameter of droplets in the internal phase ranges from 0.1 μm to 0.1 mm. Usually, there is a wide size distribution of bubbles; a narrow distribution can be considered when the ratio between the smallest and largest size droplets is 1:10. Emulsions of this class are normally thermodynamically unstable, so it is known the tendency to reduce the interface area between aqueous and oily phase, causing the oil droplets to coalesce. Coalescence of oil droplets can be reduced or even eliminated through stabilization mechanisms [13].

The unstable nature of an emulsion is because contact between oil and water molecules is not energetically favourable. Therefore, surfactants (emulsifiers) are added to the emulsion to improve the system stability; molecules inside are adsorbed to the surface of oil droplets during homogenization, providing a protective membrane that prevents flocculation and coalescence [14].

In a reactor, energy is delivered to the system, by mechanical agitation or electric fields, and the efficient contact of the phases increases the interfacial area and the transfer of matter. Mechanical agitation is simpler and of greater operational variety. Agitation speed is an important factor in the industrial application for the mixing efficiency to increase productivity [15]. The use of electric fields is more energetically efficient since the electric forces are applied on the interface of the fluids, unlike the mechanical agitation that delivers the energy to the bosom of the liquid and only a part of it is transferred to the interface [16]. A third technique to achieve an adequate mixture is ultrasound, which has mechanical and/or chemical effects, the first is due to the implosion of microbubbles, generating highly reactive free radicals, and the mechanical is caused by wave shocks during symmetric cavitation [17].

In biological treatment by activated sludge, the magnitude of the contact area of phases, water and oil is very important; therefore, a significant interfacial area is required. The interfacial area can be expanded by delivering energy to the system through mechanical stirring or an electric field. The increment of interfacial surface between the aqueous phase and the oily phase is often implemented by mechanical stirring [18].

#### *Differential Impact of the Prior Mix by Stirring in the Biodegradation of Sunflower Oil DOI: http://dx.doi.org/10.5772/intechopen.100480*

It is known that the smaller the size of the emulsions, the more the biodegradation process is favoured, since the interface area increases and, therefore, the possibility that lipases in an aqueous medium can carry out the oil hydrolysis. As the stirring speed increases, the average droplet size decreases steadily [19]. It was observed that the enzymatic pre-hydrolysis under the influence of ultrasound drastically reduces the reaction time from 24 h to 40 min as compared to conventional stirring with improved yield [20].

Electrical potentials are also applied to reduce emulsion sizes and thereby increase the homogeneity of the oil and water system. There are studies, in which the following bubble sizes between 1 and 36 μm were obtained for a stirred system with two electrodes [21]. When electric fields are applied to increase the degree of dispersion of the discontinuous phase in the continuous one, the bubbles acquire a surface charge, which generates a self-rejection that leads to a reduction in the sizes of the bubbles [22].

The fatty acids produced during the course of the reaction act as surfactants, stabilizing the emulsions [18]. Biosurfactants are surface-active molecules that are produced by a wide range of microbes including bacteria, fungi, and yeast. They have several advantages over the chemical surfactants such as higher biodegradability, lower toxicity, better environmental compatibility, high selectivity, higher foaming, and specific activity under extreme conditions such as temperature, pH, and salinity [23].

Synthetic surfactants increase the desorption and solubilization of nonpolar compounds in soils and aquifer materials, but they cause environmental problems during their production, and they are resistant to biodegradation and can be toxic when they accumulate in the ecosystem [24].

Mechanical agitation is susceptible to improvement through the application of a non-stationary behaviour that consists of the displacement of the rotating propeller from top to bottom, which cancels the existence of segregated regions that are not affected by stable agitation due to the position of the propeller in the pond, and another mechanism is counter-rotation [25].

Mixing and corresponding dispersion is achieved in the aeration tank used; there are two important factors that largely determine the emulsion level: bubble size and distribution, and the fraction of the dispersed phase. The average bubble size is between 150 μm and250 μm [16], which can be obtained by means of a suitable booster and fine bubble diffusers. If there is a suitable enzyme concentration and the optimal interfacial area between phases: aqueous and oily, the mass transfer is solved and the hydrolysis stage starts [26]. Further studies on the effects of agitation speed from 0 rpm (static) to 200 rpm on tannase production showed that increasing agitation speed caused the fungal pellets to decrease in size but to increase in number per unit volume, increasing the interface area [27].

This work analyses and evaluates the differential impact caused by mixing by stirring on the behaviour of the biological treatment system for activated sludge on a laboratory scale when treating an influent containing fats and oils. In this experience, we work with vegetable oil.
