Pengaruh Ph Pd Hidrolisis

S. Dinamarca*, G. Aroca** , R. Chamy** and L. Guerrero* * The Technical University Federico Santa Maria, Av. España 1680, Valparaiso, Chile (E-mail: [email protected]) ** The Catholic University of Valparaíso, Av. Brasil 2147, Valparaiso, Chile (E-mail: [email protected]) Abstract The influence of the pH in the first stage, the hydrolytic stage, of the anaerobic digestion of the organic fraction of urban solid waste in a two phase anaerobic reactor was studied. The reactor was fed with a solution of the organic fraction of urban solid residues containing 5 to 7% solids. Four reactors with a working volume of 3 L were used, the experiments were done at three controlled pHs; 6, 7, and 8, and one with free pH, the temperature was keep at 37°C in all the experiments. The higher degradation of TSS and VSS was obtained in the reactors operated at pH 7 and 8; 75% degradation of TSS and 85% degradation of VSS. The volatile fatty acids were determined at the different pH conditions, no significant differences were found, and as was expected, the acetic acid was found at the higher value among them (from 25 to 29 g/L). According to the results obtained it is possible to conclude that in the case of the hydrolytic stage of the anaerobic digestion of the organic fraction of urban solid waste it is not necessary to control the pH, the pH is kept stable by the buffer effect of the protein residues and other macromolecules present in the residue. Keywords Anaerobic digestion; hydrolytic stage; two phase anaerobic reactor; urban solid waste

Introduction

Urban solid waste with high content of organic material is frequently treated through composting or anaerobic digestion. This last process has the advantage of obtaining biogas, which may be used as fuel, as well as a stable residue which may be used as a soil improver. The anaerobic digestion of solid residues is a complex process due to the high percentage of total solids, so it is convenient to analyze the system’s operating conditions for its optimization. Conventional systems for sludge digestion are used when the total solid concentration is not greater than 15%; these systems are not effective for residues with higher concentrations (Ten Brummeler et al., 1991). It has been reported that at concentrations between 30 to 40% of total solids, the maximum degradation rate is obtained, variables like temperature or mixing do not have a significant effect in those reports (Rivard, 1993; Rivard et al., 1993; Ten Brummeler et al., 1991; Six and de Baere, 1992). The separation of the anaerobic process into two phases has been proposed, in the first phase the hydrolytic-fermentative process occurs, and the methanogenic process in the other, giving adequate environmental conditions for the growth of the particular microbial populations in each reactor (Jimeno et al., 1990). The anaerobic digestion of the organic fraction of the solid residues in a two phase reactor allows for: the reduction in the time required to get the system stable and the area required for the operation, and the production of recyclable end products. The results obtained in determining the influence of pH in the hydrolytic stage of a two phase anaerobic reactor is reported here, the reactors were fed with a solution of the organic fraction of urban solid residues with concentrations in the range of 5 to 7% of total solids; with this information it is possible to determine whether it is necessary or not to adjust the pH in the reactor for a stable and efficient operation.

Water Science and Technology Vol 48 No 6 pp 249–254 © IWA Publishing 2003

The influence of pH in the hydrolytic stage of anaerobic digestion of the organic fraction of urban solid waste

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Methods and materials Experimental set up

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Each reactor was built with a glass cylinder giving a working volume of 3 L, the temperature in the reactor was keep at 37°C, the mixing was provided by a mixer with adjustable speed. Each reactor is continuously feed, the inlet is at the bottom of the reactor and the effluent is evacuated by a spillway. A diagram of the experimental set-up is shown in Figure 1. Four identical reactors were installed, one without pH control, and in the other three the pH was controlled at 6, 7, and 8, to determine the effect of the pH in the formation of Volatile Fatty Acids (VFA) and the rate of degradation of organic material, which is measured through the measurement of suspended solids (SS) and the Chemical Oxygen Demand (COD). Previously it was determined that in the range of 6 to 60 hours of hydraulic retention time (HRT), the higher degradation of SS was obtained at an HRT of 48 hours. Liquid medium preparation and characterization

The solution that was fed into the reactors was prepared using the organic fraction of urban solid waste, which is ground and suspended in tap water up to 5 to 7% of total solids. The characteristics of this suspension are presented in Table 1. Results and discusions

Figures 2 and 3 show the variation of the concentration in each reactor of TSS and VSS respectively, Figures 4 and 5 show the percentages of degradation obtained for the same parameters. As can be seen in these figures, there was hydrolysis and acidification in each reactor, measured as the decrease in TSS and VSS, with similar percentages of degradation. In all of them initially the concentration of TSS and VSS increased, probably due to microbial growth; at this stage it is not possible to differentiate which part of the VSS corresponds to the organic residues or to microorganisms. When the microbial growth reached the 1. 2. 3. 4. 5. 6. 7. 8.

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Figure 1 Experimental set-up Table 1 Organic suspension characteristics Parameter

Total COD TSS Total nitrogen N-nitrites N-nitrates N-ammoniac pH 250

All concentrations in mg/L

54,000 50 1,400 <0.001 <0.001 <0.1 6.6 *

Feed Refrigerated chamber Mixer Peristaltic pump Sampler Outlet spillway Gas meter Reactor

TSS (mg/L) 50 45 40 35 Control R.

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Ope r ation T ime (d)

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pH 8

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Figure 2 Concentration of TSS in each reactor VSS (mg/L) 35 30 Control R.

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Figure 3 Concentration of VSS in each reactor Degradat ion of TSS (%) 100 80 Control R.

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Figure 4 Percentage of degradation of TSS in each of the reactors

maximum concentration population and it remained stable, the SS began to decrease until it reached a degradation near to 60% for TSS and near to 80% for VSS. The best results for the degradation of TSS and VSS were obtained in the reactors operating at pH 7 and pH 8: 75% of TSS degradation and 85% of VSS degradation. For the reactors operating at pH 6 and without control of pH, a degradation less than 80% of VSS and about 60% of TSS was achieved. The results regarding the effect of the pH on VFA (acetic, butyric and propionic acid) production are shown in Figures 6 to 9. Figure 6 shows the evolution of the pH in the reactor without pH control. As can be seen, the pH in the reactor ranges between 6.5 and 8.2, this is an appropriate range for allowing the hydrolytic stage of the anaerobic digestion. The most alkaline values were obtained at

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Degradat ion of VSS (%) 100

80 Control R.

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pH 6 pH 7

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Figure 5 Percentage of degradation of VSS in each reactor Concentra tion (g/L) 10

35 33

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4 3 2

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Figure 6 Concentration of accumulated VFA in reactor without pH control Concentration (g/L) 35 33 30 28 25 23 20 18 15 13 10 8 5 3 0 1

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Figure 7 Accumulated concentrations of VFA in reactor at pH 6

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the beginning of the operation, which confirms what was stated before, that at the beginning there was no hydrolysis but there was microbial growth. The concentrations of VFA are similar in all the reactors, and as expected, the acetic acid is found in higher amounts in all the reactors (between 25 and 29 g/L). These results do not allow us to conclude which of the reactors was operating at the most favorable condition. In the reactor operating at pH 8 the lower concentration of butyric acid (18 g/L) was found, in the other reactors the concentration of butyric acid was in the range of 22 and 26 g/L. Although in the reactor without pH control and the reactor at pH7, this acid has a similar concentration to the acetic acid.

Concentration(g/L)

1

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35 33 30 28 25 23 20 18 15 13 10 8 5 3 0

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Figure 8 Accumulated concentrations of VFA in reactor at pH 7

Concentration (g/L) 35 33 30 28 25 23 20 18 15 13 10 8 5 3 0 1

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Figure 9 Accumulated concentrations of VFA in reactor at pH 8

Conclusions

The results obtained allow us to conclude that in the hydrolytic-acidogenic stage of the anaerobic digestion of the organic fraction of urban solid waste, it is not necessary to control the pH, as the reactor remains stable without the need to add alkali or acid. Even though the hydrolytic stage is VFA generating and the acidification of the reactors is expected, the presence of proteins and other compounds gives to the reactor adequate buffer capacity. This is verified in the evolution of the pH in the non controlled reactor, where the value of the pH fluctuated in a range between 6.5 and 8.2. The results obtained for the degradation of SS are relatively similar, achieving better results in the reactors at pH7 and 8, (75% of degradation of TSS and 85% of degradation of VSS), although the obtained values for the reactor without control are slightly lower (78% for VSS degradation). It allows us to conclude that there is no reason, especially from an economical and operational point of view, to control the pH at a given value. The results obtained for the concentration of VFA, with very similar concentrations of the different acids, allow us to confirm what was started before, that the pH is not important for these types of solid residues, and it is not necessary to keep this variable controlled, for operating and economic reasons. Acknowledgement

This work was possible thanks to the support of the Fondecyt Project 1020795. 253

References

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Jimeno, A., Bermúdez, J., Cánovas-Días, M., Manjon, A. and Iborra, J.L. (1990). Effect of feed proceedure and different temperatures on anaerobic trincklin filters: intermediate consumption profiles. Process Biochem., 25(2), 55–60. Rivard, C.J. (1993). Anaerobic bioconversion of municipal solid wastes using a novel high-solid reactor design. Appl. Biochem. & Biotech., 39(40), 71–82. Rivard, C.J., Nagle, N.J., Adney, W.S. and Himmel, M.E. (1993). Anaerobic bioconversion of municipal solid wastes. Appl. Biochem. & Biotech., 39(40), 107–117. Six, W. and de Baere, L. (1992). Dry anaerobic convertion of municipal solid waste by means of the DRANCO process. Wat. Sci. Tech., 25(7), 295–300. Ten Brummeler, E., Horbach, H.C. and Koster, I.W. (1991). Dry anaerobic batch reactor digestion of the organic fraction of municipal solid wastes. J. Chem. Tech. Biotechnol., 50, 191–209.