Acidogenic fermentation of organic waste (sewage sludge, food waste, animal manure, etc.) can also provide a desirable choice for sustainable, eco-friendly and economic production of volatile fatty acids (VFAs), which have many industrial applications (bioplastics, biofuels, biochemicals). This study investigated the simultaneous effects of hydraulic retention time (HRT) and pH on the fermentative production of VFA from food waste leachate using response surface analysis. 1 In South Korea organic waste treatment facility (unit: number of anaerobic digestion facility, Source: Ministry of Environment in Korea).
Introduction
- Trend of organic waste treatment
- Biological production of volatile fatty acid
- Biological nitrogen removal
- Objectives of the study
The anaerobic digestion process is shown in the following Figure 1 and its schematic chemical reaction (in the case of glucose) is described in equation (1). Successive processes are generated according to the interaction of different microorganisms, and in the first step, the materials with complicated structures (carbohydrates, proteins, fats, and so on) are converted into materials with simpler types (glucose, amino acids and so on) by hydrolysis of microorganisms. At this time, membrane-bound ammonia monooxygenase (AMO) and hydroclamnine oxidoreductase (HAO) are included in the reaction.
Material and Methods
- Response surface analysis (RSA) design
- Continuous fermentation reactor operation
- Denitrification test
- Analytical methods
Response surface analysis (RSA) is a series of procedures of collecting data, expressing it with a quadratic equation, judging the adequacy of the model, improving the test design or model when it is inadequate, and adding the required test and if there is t almost all curved surfaces in the area that is to be tested, is expressed by a first-order equation, and then when the region where the optimum point is found, more than a second-order equation is used because the response surface has curvature. Since the central composite design can often be constructed by adding the pivot point and the central point to the previous factorial test, it is particularly useful for the sequential test, but this has the disadvantage that there may be an error in the test result if the test cannot be carried out in a steady state due to too many tests that had to be done. For the central point among the 9 test points previously designed by RSA, 3-fold repeat tests were performed.
As for the test conditions, the device was operated with a response capacity of 1ℓ, and the temperature was set as average temperature (35 ± 2℃), and the pH range set for the test was maintained through a pH pump with NaOH solution with a concentration of 10M because the pH decreases due to the formation of organic acid. In the case of the food waste leachate used as substrate in this test, the water flowing to the food waste storage hopper in the anaerobic biogas digestion plant processing 100 tons of food waste and 50 tons of animal manure daily in Ulsan District , used. and this is shown in Figure 5 and FWL Physicochemical properties shown in Table 2. Therefore, this test was continued by filling 100% of the food wastewater into the reaction device from the beginning without performing the vaccination.
For the analysis and collection of data, normal state data was ensured by analyzing the samples collected for more than 3 times at each of the different times after the 3 turns, that is, when 3 rounds of HRT had passed. In addition, for the comparison of the denitrification efficiency of fermentation product obtained from the optimum fermentation condition through this test, acetic acid, that is, the substrate that is easily used by microorganisms, methanol that is used from the general denitrification process as external carbon source, and commercial carbon source (OC) used by the facility whose sludge is collected for this test was used to advance the test. Accordingly, the concentration of nitrogen wastewater was set as 50 mg NO3—N/L by filling the artificially produced nitrate wastewater (10mL), denitrification sludge (2mL) and carbon source (50mL) into each of the test bottles, and final concentration of carbon source was determined as 350mg COD/L and total reaction capacity of 100mL was achieved using distilled water.
The biomass amount of the inoculated sludge was 172 mg volatile suspended solids (VSS)/L and the C/N ratio for the test was set as 7 based on the reference examination (Xie et al. 2012).
Results and Discussion
Explored optimal condition of fermentative FWL
- Effect of pH and HRT on VFAs production
- Response surface modeling
- Experimental validation of optimal point
From the pH 4.5 condition, acetic acid and ethanol could be observed as predominant and very low VFA production was observed compared to different pH conditions. Finally, the highest level of total volatile fatty acid was produced from the pH 6.5 condition, and large amounts of long-chain fatty acid were generated compared to other conditions. From a pH value of 5.5, a slightly different aspect could be observed from the previous test, and for the 3 test times the highest part was occupied by butyric acid and the generation amount of the fatty acids (valeric acid, caporic acid) with a longer chain structure became conspicuous increased.
Finally, butyric acid took the largest share already from the state of pH 6.5, followed by the massive formation of propionate. At higher pH, butyric acid was the largest amount, and from the pH 6.5 condition, as in the case of the previous result, propionate was next. In addition, it was possible to confirm that net production was not shown by ethanol by comparing the amount contained in the substrate used for this test (20.8 g/L) and the amount of 13.6.
Based on this figure, the lowest production amount of total volatile fatty acid (8.6-12.0 COD g/L) could be identified from the test with pH 4.5, and at this moment it could be confirmed that the portion of acetic acid was more than 93%. In addition, it can be understood that this acetic acid tends to decrease (10-47%) as pH is increased, and the portion of long-chain VFAs was increased and the increasing amount of butyric acid was noticeable. This result agrees with the result of the previous research on acid fermentation that acetic acid and butyric acid prevail over the product of volatile fatty acid.
From the test conducted under pH 6.5, the highest level of VFA amount of COD (g/l) could be observed, and this was by far the higher value compared to other conditions. Moreover, it could be confirmed that there was no major difference between the estimation value and the test result of the model. Looking at Figure 11, it can be confirmed that the response value increases rapidly along with the increase in pH, and the two are proportionally related.
Batch denitrification test
- Denitrification profiles
- Nitrogen removal kinetics
Interestingly, in terms of nitrate nitrogen removal rate, the filtered fermentation product material was much faster than acetic acid, but in the case of nitrate nitrogen removal rate, it took 33 hours for complete removal from the two carbon sources. This result suggests that the fermentation filtrate is more effective for the removal of nitrite nitrogen than acetic acid. The amount of COD consumed during the denitrification process ranged from 4.4 to 5.8 g COD/g N.
In the case of fermentation filtrate and acetic acid, it could be confirmed that the removal rate was more than 3.6 times higher for NO3--N and more than 6.1 times higher, based on Rm values compared to methanol and OC. faster for NOX-N. At this time, the Rm value of the fermentation filtrate was 1.6 times higher than that of acetic acid, and in the case of NO3--N, the rates of both carbon sources were similar. This is complemented by the observation result of the previous figure 14 that in the case of NO2-N removal rate, fermentation filtrate was faster than acetic acid.
Furthermore, the fermentation filtrate maintained the shortest phase lag time for NO2-N and NO3-N. Based on the denitrification test, it can be confirmed that fermentation filtrate is generally the most efficient item compared to other carbon sources in terms of reaction time and lag phase length. Although acetic acid has been widely reported to cause the fastest nitrogen removal among synthetic chemicals, some have shown that waste-derived VFAs have better denitrification potential (Lee et al 2014), which is consistent with our observations.
This is likely due to the synergistic effects of substances other than VFAs present in the fermentation mixture (Lee et al 2014).
Conclusions
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