Brazilian Journal of Chemical Engineering ASSESSMENT OF OZONE AS A PRETREATMENT TO IMPROVE ANAEROBIC DIGESTION OF VINASSE

This paper presents an assessment of ozone oxidative effects on the biodegradability of sugar cane vinasse, aiming at increasing the methane yield by anaerobic digestion of this effluent. Furthermore, as a new approach, an economic balance of this process was made. Using a bench scale reactor, ozone was applied at 60, 120, 180, 240 mgO3.gCOD doses in raw vinasse and at three initial pH values (4.8, 7 and 9). Applying 60 mgO3.gCOD, the biodegradability of vinasse was increased by 22.7% at the initial pH value of 4.8. The application of the two-way ANOVA test indicated a significant statistical interaction between the pH value and ozone. However, a preliminary energy assessment showed that the amount of electricity consumed in a full-scale ozonation plant would be almost 6 times higher than the energy recovered from the combustion of the additional methane produced (13.6%). These results indicate that ozonation of raw vinasse to increase the methane production in a subsequent anaerobic process is economically unfeasible.


INTRODUCTION
It is widely known that, due to the high organic content of vinasse, its anaerobic processing to obtain methane is very interesting from an economical point of view.In this case, there is little or no power consumption, the sludge production is minimal and the nutrient demand is low.Energy from methane combustion could be an additional source of income for the distillery (Jiménez et al., 2003).
However, research indicates that the presence of certain recalcitrant and inhibitory substances in the vinasse severely hinders the anaerobic process.These substances are mainly phenolic compounds.In addition, heavy metals, melanoidins, glycerol, antibiotics and other organic xenobiotic pollutants are also present (Wilkie et al., 2000).According to several authors, these substances may decrease the efficiency of the anaerobic process in terms of its kinetics, COD removal and methane production (Pearson et al., 1980;Borja et al., 1993;Benitez et al. 1997;Jiménez et al., 2006;Chen et al., 2008).A way to overcome the toxic effect of some of these substances is to pretreat them with ozone (O 3 ), which can transform complex recalcitrant compounds into biodegradable molecules, improving the biodegradability ratio (BOD 5 /COD) of the vinasse (Amat et al., 2003).As simpler molecules are more assimilable by anaerobic microorganisms than complex ones, ozonation of vinasse can lead to improvements in the anaerobic process in terms of greater stability and methane yield.Therefore, the objective of ozonation Brazilian Journal of Chemical Engineering prior to a biological treatment should be the partial oxidation of refractory matter, not its complete mineralization, so that successful ozonation will result in an increase in the BOD 5 /COD ratio.
Nevertheless, vinasses may differ in characteristics and composition due to differences in the raw material used to produce ethanol (cassava, corn, beets, grapes, cherry, wine and sugar cane) and specific practices in industrial processing in the soil and climate of plantations (Wilkie et al., 2000).Consequently, it is difficult to extrapolate data from ozonation of one kind of vinasse to another one.In addition, the literature does not present an energy balance to justify the use of this oxidant.
Taking all this into account, the aim of this study was to evaluate the effects of ozone on the biodegradability of a vinasse from a Brazilian ethanol distillery processing sugar cane as a raw material.In addition, a preliminary energy balance completed this assessment.

Vinasse
The vinasse used in this study consisted of samples collected from the effluent of the ethanol distillery plant of Usina São Martinho located in the municipality of Pradópolis, the state of São Paulo, Brazil.The plant processes approximately 8.500,000 tons of sugarcane per harvesting season.The effluent was fully analyzed upon receipt and stored at 4 ºC until use.Table 1 shows the effluent properties of interest to this work.

Ozone Generation and Experimental Set-Up
An Eaglesat PXZ3507 generator (Brazil) produced the ozone for this experiment.This equipment consists of a PSA (Pressure Swing Adsorption) unit for oxygen enrichment and a corona discharge ozonator.The system was designed to have a maximum production capacity of 7 gO 3 .h - from the flow of oxygen produced by the oxygen generator (PXZ3507).However, according to the calibration tests (data not shown), as well as the environmental conditions in which the tests were performed, and mainly, by the ozone flow (1 Lpm), the actual production capacity used in this work was 0.65 gO 3 .h - .

Oxidation System
In addition to the ozone generator, the oxidation system consisted of a reactor, the foam trap and the off-gas scrubber (Figure 1).The reactor used for applying the ozone to the vinasse was built using a PVC (polyvinyl chloride) pipe with the following dimensions: total height of 850 mm, of which 450 mm was the useful height.The internal diameter was 40 mm, giving a useful volume of 1 L. The experiments showed that there was great efficiency in the ozone transfer to the liquid column.This was known from the mass balance made between the amount of ozone applied to the system and that lost by the system (Table 2).

Analytical Methods
Calibration of the ozonator and the amount of ozone measurements lost from the system were measured by the titration method, using potassium iodide solution (KI) at 2%, which filled the gas scrubber.Data were analyzed via factorial two-way ANOVA (Table 5).It can be observed that the effect of the interaction between the pH and ozone was significant in the biodegradability increase of vinasse (pvalue 0.01712).Using the same statistical analysis for the absolute values of BOD 5 and COD, it was found that this interaction improved due to the effect caused on the BOD 5 (p-value = 0.003407) since the effects on COD were not significant (p-value = 0.149749) (data not shown).Therefore, when the BOD 5 /COD ratio increased, it was due more to a result of increasing the BOD 5 values rather than decreasing the COD values (which remained almost unchanged).As a direct consequence of this fact, the BOD 5 /COD ratio increased.As the results from this work and from other researchers show, the increase in vinasse biodegradability and methane production is not very high.Thus, the economic feasibility of ozonation becomes an important issue that is seldom considered in vinasse oxidation studies.

Preliminary Economic Assessment
Using the results from this work and from the literature, a preliminary energy balance evaluation was prepared as a surrogate for an economic feasibility study.The evaluation is presented for a model Brazilian distillery to better represent the order of magnitude of ozone and energy consumption.This model is based on data from Coopcana (Cooperativa Agrícola Regional de Produtores de Cana), which indicate that an average size Brazilian distillery produces around 800 m 3 of alcohol daily.Taking into account the production of 12 litres of vinasse per litre of ethanol, the model distillery flow rate (Q) of vinasse to be ozonised would be 9.600 m 3 .d - with a COD concentration of 40 gO 2 .L -1 (concentration and flow rate held constant for calculations).
According to the results presented here, it was necessary to apply 60 mgO 3 .gCOD - to increase the biodegradability of vinasse by 22.7%.Therefore, this plant would require 23.040 kgO 3 .d - .
Data from industrial ozonator manufacturers indicate that the electrical energy required to produce one kilogram of ozone from air is 14 kWh.kgO 3 -1 or 7 to 8 kWh.kgO 3 -1 using pure oxygen (Qingdao Guolin Industry Co., Ltd.-Guolin).
Therefore, it would be required to have 322.560kWh to pre-treat the vinasse of the model distillery.For comparison purposes, the reader should be aware that this amount of ozone is more than enough for a very large bleach plant in a cellulose pulp mill, perhaps the industrial operation with the highest consumption of ozone (Germer et al., 2011).
Concerning the calculations, data published by Siles et al. (2011) were used.Vinasse anaerobic digestion previously treated with ozone would yield 13.6% more methane than vinasse digestion that was not previously treated with ozone.This increase in production is due to the conversion of non-biodegradable matter into a biodegradable one, and can be expressed as an increase in the organic matter conversion efficiency of the reactor.Assuming for the calculation that 70% of the total organic load (384.000kg COD.day -1 ) would be removed, 268.800 kg of methane would be produced in one day.As a result, considering that stoichiometrically 400 kg of COD removed from a reactor yields 100 kg of methane, an extra 9.140 kg of CH 4 (M CH 4 ) could be produced by processing the ozonated vinasse.Thus, one kilogram of ozone added to the vinasse would produce only 0.4 kg of additional methane.
The electrical energy (E el ) produced using a given mass of methane (M CH 4 ) can be calculated considering its calorific power (H c ) and the overall efficiency of chemical energy conversion of methane into electrical energy using an electrical generator coupled to an internal combustion engine (η conv ).Based on manufacturer's data, this electrical efficiency was assumed to be 44% (Caterpillar, 2014).The calorific power of methane is equal to 50.4 MJ per kgCH 4 (van Haandel, 2005).Equation ( 1) is used to evaluate the energy produced: where 0.2778 is the conversion factor from MJ to kWh.Therefore, with ozonization, it can be concluded that an extra 56.440 kWh of electricity would be obtained per day.This represents 2.450 kWh.kgO 3 -1 .Nevertheless, every day, the plant would also be spending 322.560kWh to generate the needed 23.040 kg of ozone.Therefore, it is clear that pre-Brazilian Journal of Chemical Engineering ozonation of raw vinasse aimed at increasing electrical power generation by using the additional methane is still unfeasible from an economical point of view.Approximately 5.7 times more energy would be necessary for ozone production than the energy that can be recovered by burning the additional methane.
To complete the energy balance, an evaluation of the lowest possible energy expenditure to produce ozone was compared with the energy gain from ozonation.
The lowest theoretical energy required to produce ozone is the standard enthalpy of ozone formation, represented in Equation (2) (Atkins, 2002).
The enthalpy of ozone formation from its elemental species Δ f H o (O 3 ) is +142.3kJ.mol. - (Atkins, 2002), and results in a minimum theoretical power consumption of 0.825 kWh.kgO 3 -1 .In this case, 2.450 kWh generated from each kilogram of O 3 injected into the vinasse would pay this cost, resulting in a positive final balance of 1.625 kWh.Although progress has been observed with respect to ozonator efficiencies, there is still a long way to go to reach values that will make vinasse pre-ozonation an economically feasible process (where the specific objective is to increase the methane yield in its anaerobic digestion).Simply to balance the required energy with the energy produced by the additional methane combustion, it would be necessary to increase the overall efficiency of an ozonator using air as an oxygen source from its current value of 6% to 34%.

CONCLUSIONS
By adjusting operational parameters, ozonation can be used to increase the biodegradability of raw vinasse.
The increase in the BOD 5 /COD ratio of vinasse was not associated with reduced phenol content in this study, as phenol was not found in the raw vinasse.
Economically, the application of ozone to raw vinasse at its natural acidic pH (4.8) is more appropriate for increasing the biodegradability of vinasse, as alkali substances (such as NaOH) would not be needed to adjust the pH to 7 or 9.
In order to make a profit from the ozonation of vinasse, ozonators would be required to spend less than 1.645 kWh.kgO 3 -1 , instead of the current 14 kWh.kgO 3 -1 .
Using these results, it can be concluded that, while it may be of scientific interest to study the mechanisms of biodegradability improvement and increase of methane production by ozonation of vinasse, the practical applicability of this process is still not justified.

Table 5 : ANOVA analysis of the interaction be- tween pH and ozone on the biodegradability of vinasse.
*p<0.05 SS: total sum of squares; df: degrees of freedom; MS: mean square This study was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Bra- zil (CAPES) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), grant n. 304.394/2009-2.Δ f H o (O 3 ) standard enthalpy of ozone formation E el equivalent amount of electrical energy η conv overall efficiency (of the motor-generator) to convert methane chemical energy into electricity.