**4. Mass balance and biodegradability analysis**

In this case, mass balance presents a difference with respect to the balance made for the case where the oil is directly added to the aeration tank, since in this experience the mixture of water and oil is previously prepared in the feed tank; part of the added oil remains accumulated in tank, so it is pertinent to estimate this fraction of oil does not enter the treatment system, as shown in **Figure 2**. In the following experiment, the mixture of water with oil is subjected to the previous stirring to evaluate the mixing effect on biodegradation and oil elimination, since a greater mixture stimulates the development of smaller emulsions and precisely treats to evaluate how this variable affects the biodegradation of fats and oils.

This experience worked with an influent that is subjected to previous mechanical stirring to evaluate the influence of and stirring on biodegradation of oil, since when stirring is carried out before entering the aerator tank, the level of emulsification of oil particles in the water is increased, and with this, the contact area between water-oil and oil-microorganisms is increased, which is closely related to the mass transfer phenomena that involve biodegradation of sunflower oil in active sludge treatment system.

Influents with concentrations of 300, 600, and 900 mg/l are prepared in a feeding tank, for a period of approximately one week, to measure the biodegradability levels achieved, and therefore compare the behaviour of system with results obtained where influent is not previously agitated. The operating conditions regarding airflow, residence time, recirculation ratio, and COD: N: P ratio at the feed level remain unchanged.

Biodegradation levels obtained improve considerably when a mixture of water and oil is subjected to mechanical stirring before being fed to an aeration tank; the agitation carried out is with a propeller-type mechanical stirrer. It should be mentioned that this type of stirring is limited which is significantly reflected in the oil that does not enter the system since it is not emulsified, which gives rise to the fraction of the oil that is retained.

Biodegradation values between 64 and 75% obtained with stirring are similar to those corresponding to aerobic biodegradation experience that eliminates fats and oils from the dairy industry, which uses a mixture of isolated and selected native bacteria as biomass, reaching 72% biodegradation efficiency [38].

Since the only substrate is oil, this system can be assimilated to the proposal for the treatment of fats and oils that is contemplated in the wastewater treatment project of the Los Angeles commune developed by DEGREMONT, which consists of treating the fats and oils together with the sludge in an aerobic digester; therefore, the only available substrate is the fats and oils collected in the primary treatment and that was dosed based on criteria of optimal distribution and mixing [39].

Recent researches have studied the behaviour and performance of aerobic thermophile bacteria for wastewater with a high oily organic content, verifying that between 55 and 58°C, the maximum growth rate is achieved [40]. Now, this is important given that at higher temperatures an increase in the dissolution of fats and oils is achieved, and as it is an aerobic digester, it is more feasible to reach temperatures in this range.

However, analysis of the first-order kinetic model constants showed that alteration in rotor speed resulted in an increase in the values of the kinetic constants (for instance, from 0.57 h<sup>1</sup> at 50 rpm to 0.84 h<sup>1</sup> at 75 rpm) [41].

It is important to note that the oil that enters the system is the one that has been emulsified and therefore the size of the oil droplets that enter the system reaches a comparatively much smaller size than in the previous case (without mixing), which allows a considerably greater interfacial area between the mixed liquor and the oil.

The bacterial mass is suspended in water, and therefore, contact level between microorganisms and oil droplets is considerably increased, which stimulates the production of the lipase by bacteria. Then, increasing the oil-water interfacial area, where occurs oil hydrolysis, the one that is increased and therefore the amount of fatty acids and glycerol, compounds that bacterial mass will biodegrade them; therefore, the increase of interfacial area allows the substantial increase in biodegradation levels.

#### **4.1 COD elimination**

According to observed, it is confirmed that an important part of organic matter corresponding to vegetable oil is accumulated in secondary settler and that the accumulated oil mass is clearly correlated with oil concentration in influent.

The COD values inform us that the sum of oil biodegradation and flotation phenomena makes it possible to achieve global elimination of vegetable oil, which is considerable. On the other hand, it is observed the biodegradation of oil does not depend on the mass load.

Considering the mass load values, it means an initial work with extended aeration and final stage in conventional type regime, which is consistent with theory and experience of wastewater treatment, since differences between both regimes are not observed in effluent quality.

In any case, the oil removal is mainly due to biodegradation of vegetable oil around 70%, while 20% corresponds to flotation (**Figure 7**).

**Figure 7.** *Performance of COD removal efficiency and mass loading for sunflower oil feed with the previous stirring.*

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

**Figure 8.**

*The behaviour of sunflower oil concentration in influent, effluent, and effluent plus accumulated effluent and oil removal efficiency with prior agitation.*

#### **4.2 Removal of vegetable oil**

The elimination of oil by biodegradation has a huge advantage over the physical and physicochemical processes currently used in the elimination of this substrate, since they generate a greasy residue that requires a final disposal and tends to accumulate, unlike the elimination by aerobic biodegradation that biologically oxidizes fats and oils to CO2, incorporate them into the carbon cycle.

From **Figure 8**, it can be seen that the concentration of sunflower oil of the influent is practically constant during a determined amount of approximately 8 days, three periods are distinguished, since the feeding during each period has different concentrations of vegetable oil in the influent. Initially, the oil concentration is 220 mg/l, later it increases to a value close to 470 mg/l and for the last days of operation, which corresponds to a concentration of vegetable oil in the influent close to 500 mg/l, this corresponds to a COD concentration above 1100 mg/l, and there is a slight increase in the COD of the effluent.

Regarding the elimination of oil, elimination levels that are around 90% are reached, where a considerable percentage of this elimination corresponds to oil that is biodegraded and the remainder is separated by flotation, as shown in **Figure 8**.

The amount of accumulated oil increases with the concentration of oil in the influent. It should be noted that the increase in oil concentration in the influent causes a decrease in the biodegradation performance of the activated sludge system, which is corroborated with the results of the material balance, such that the biodegradability of the oil decreases by 75% for vegetable oil concentrations of 220 mg/l and up to 64% when the concentration increases to 500 mg/l.

#### **5. Conclusions**

For influents with concentrations of fats and oils that range between 200 and 800 mg/l and that are not subjected to a previous mixing, the elimination by

## *Biodegradation Technology of Organic and Inorganic Pollutants*

biodegradation of the same reaches 42.5 for the concentrations of smaller magnitude and for the concentrations of the highest rank decreases to 28%.

For influents with the concentrations of fats and oils ranging between 330 and 465 mg/l and that are subjected to a previous mixing, their elimination by biodegradation ranges from 64 to 75%, which has as a consequence a considerable reduction of greasy residues that accumulate and take up space to be disposed of.

From the results, it is concluded that the previous mixing is a relevant factor to increase the elimination by biodegradation of fats and oils in an oily influent.

The global elimination that includes biodegradation and flotation exceeds 80% at all events.

The biodegradation efficiency of sunflower oil increases through greater agitation, which is a contribution from the environmental point of view, since fats and oils are eliminated, transforming them into CO2 by the biological route and thus incorporating these residues into the cycle of carbon.
