Detailed study of anaerobic digestion of Spirulina maxima algal biomass

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<ul><li><p>Detailed Study of Anaerobic Digestion of Spirulina maxima Algal Biomass </p><p>Rejean Samson* and Anh LeDuyt Department of Chemical Engineering, Laval University, Ste-Fo y, Quebec, Canada GIK 7P4 </p><p>Accepted for publication August 15, 1985 </p><p>Biomass of the blue-green alga Spirulina maxima was converted to methane using continuous stirred tank digesters with an energy conversion efficiency of 59%. Digesters were operated using once-a-day feeding with a retention time (8) between 5 and 40 days, volatile solid concentrations (S,) between 20 and 100 kg VS/m3, and temperatures between 15 and 52C. The results indicated a maximum methane yield of 0.35 m3 (STP)/kg VS added at 6 = 30 days and S, = 20 kg VS/m3. Under such con- ditions, the energy conversion of the algal biomass to methane was 59%. The maximum methane production rate of 0.80 m3 (STP)/m3 day was obtained with 6 = 20 days and S, = 100 kg VS/m3. The mesophilic condition at 35C produced the maximum methane yield and pro- duction rate. The process was stable and characterized by a high production of volatile acids (up to 23,200 mg/L), alkalinity (up to 20,000 mg/L), and ammonia (up to 7000 mg/L), and the high protein content of the biomass produced a well-buffered environment which reduced in- hibitory effects. At higher loading rates, the inhibition of methanogenic bacteria was observed, but there was no clear-cut evidence that such a phenomenon was due to nonionized volatile acids or gaseous ammonia. The ki- netic analysis using the model proposed by Chen and Hashimoto indicated that the minimum retention time was seven days. The optimum retention time increased gradually from 11 to 16 days with an increase in the initial volatile solid concentration. The kinetic constant K de- creased with the improvement in the digester perform- ance and increased in parallel with the ammonia con- centration in the culture media. </p><p>INTRODUCTION </p><p>Solar energy can be exploited for the production of the chemical energy, methane, through the combined algal-bacterial process. In this process, the algae are mass produced in photobioreactors from light and car- bon in the first step. In the second step, the algal bio- </p><p>* Present address: National Research Council of Canada, Bio- technology Research Institute, 750 Bel-Air, Montreal, Quebec, Can- ada H4C 2K3. Issued as NRCC No. 24665. </p><p>t To whom all correspondence should be addressed. </p><p>mass is then used as a nutrient for feeding an anaerobic digester for the production of methane by anaerobic bacteria. The carbon source for the production of the algal biomass can be derived from the organic carbons in wastewaters (for eukaryotic algae), from the carbon dioxide in the atmosphere or from the combustion of exhaust gases (for both prokaryotic and eukaryotic algae). </p><p>Technical feasibility data on anaerobic digestion of algal biomass have been reported for many species of algae including the macroscopic algae such as the giant brown kelp, Macrocystis pyrifera, ' the red marine alga, Gracilaria ceae,' and the green marine alga, Ulva lac- </p><p>Among the microscopic algae, the following cul- tures have been successfully used for the production of methane: the mixed culture of Scenedesmus spp. and Chlorella ~ p p . , ~ the culture of Scenedesmus sp. alone or together with either Spirulina sp., Euglena sp., Microactinum sp., Melosira sp. or Oscillatoria sp. ,4 and the mixed culture of Hydrodictyon reticula- tum and Cladophora g l ~ m e r a t a . ~ </p><p>Research in our laboratory uses the semimicroscopic blue-green alga Spirulina maxima as the sole substrate for this combined algal-bacterial process."" This spe- cies of algae is very attractive for the process because of its high growth rate, its capability of using atmo- spheric carbon dioxide as a carbon source and its sim- ple harvesting methods. Furthermore, it appeared that the fermentability of S . maxima was sigmficantly higher than other microscopic algae.4 </p><p>This article presents the results on the detailed anal- ysis of the anaerobic digestion of S . maxima algal bio- mass. The influence of various combinations of reten- tion times, volatile solid concentrations, and tempera- tures in the range of psychrophilic to thermophilic have also been investigated. A correlation matrix has been utilized to evaluate the interdependence of the parameters such as biogas production and composi- tion, methane production rate and yield, volatile solids (VS) reduction, energy efficiency, ionized and non- ionized volatile acids, alkalinity, ammonia (NH3 + </p><p>Biotechnology and Bioengineering, Vol. XXVIII, Pp. 1014-1023 (1986) 0 1986 John Wiley 81 Sons, Inc. CCC 0006-3592/86/071014-10$04.00 </p></li><li><p>4 3 </p><p>t t I I0 </p><p>Figure 1. Schematic diagram of the 2-L digesters used for anaerobic digestion of S. maxima algal biomass: (1) rotary platform, (2) con- trolled temperature room, (3) fan, (4) thermostat, ( 5 ) thermometer, (6) 2-L digester, (7) feeding or sampling tube, (8) biogas outlet, (9) biogas sampling port, (10) water trap, and (11) gas measurement system. </p><p>NHz)-nitrogen, pH, and electrode potential. A kinetic analysis of the obtained data has also been used to give a complete picture of the process. </p><p>MATERIALS AND METHODS </p><p>Reactors </p><p>The digestion experiments were carried out in a con- tinuous stirred tank reactor with once-a-day feeding using a series of 20 erlenmeyer flasks of 2 L, containing 1.5 L of culture media. These erlenmeyer flasks were provided with two sampling ports for biogas with- drawal and feeding (Fig. 1). These units were placed in a temperature-controlled, hand-made Plexiglas cab- inet (35C) which was installed on a continuously work- ing rotary platform shaker (140 rpm). When the effects </p><p>of other temperatures were studied, some digesters were placed in a water bath equipped with cooling and heating units. Mixing was then provided intermittently several times a day. Gas produced was collected with water displacement vessels filled with saturated NaCl solution to reduce to a minimum the solubility of the C02. In all cases, the volume of biogas or methane produced was corrected for normal temperature and pressure (STP). </p><p>Substrate </p><p>The semimicroscopic blue-green alga S. maxima was used as a substrate. It was maintained and cultivated in a synthetic medium as reported elsewhere.I2 The algal biomass grown in a 64-L laboratory-built flat tank photobioreactoP was concentrated to a slurry con- taining 30% total solids with the DeLaval model Gy- rotest continuous centrifuge. The algal slurry was kept frozen at - 30C until use. The frozen algal slurry was thawed at room temperature and then diluted to a de- sired volatile solid (SJ or chemical oxygen demand (COD) concentration prior to being fed to the anaerobic digesters. </p><p>Experimental Design </p><p>Table I presents the experimental conditions used to study the effects of retention time (5 &lt; 8 &lt; 40 days), feed concentration (20 &lt; S,, &lt; 100 kg VS/m3), and temperature (15C &lt; T &lt; 52C) on the anaerobic diges- tion of the algal biomass. These experiments were car- ried out in duplicate. Adapted sludge coming from two larger anaerobic digesters was used as inoculum for each series of experiments. For example, in series 1, the 10 digesters, operated with 8 = 40 days, were inoculated anaerobically with sludge adapted to this retention time and with S, = 40 kg VS/m3. Thereafter, different loading rates were achieved by changing the VS concentration between 2 and 10%. This procedure </p><p>Table I. S . maxima algal biomass. </p><p>Experimental design for the study of the anaerobic digestion of </p><p>Loading rate (kg VS/m3 days) </p><p>VS concentration (kg VS/m3) T e </p><p>(C) (days) 20 40 60 80 100 </p><p>Series 1 35 40 0.51 1.01 1.53 2.02 2.55 35 30 0.67 1.35 2.01 2.68 3.55 </p><p>Series 2 35 20 1.01 2.02 3.03 4.04 5.05 35 10 2.02 4.04 6.06 8.08 10.10 </p><p>Series 3 35 5 - 8.08 - 16.16 - Series 4 15 20 </p><p>- 2.02 - - - 25 20 35 20 </p><p>- 2.02 - - - 52 20 </p><p>- - 2.02 - </p><p>2.02 - </p><p>- </p><p>- - - </p><p>SAMSON AND LEDUY: ANAEROBIC DIGESTION OF S. MAXIMA 1015 </p></li><li><p>took at least one month before sampling could begin. In series 4, in which the effect of temperature was studied, experiments began with sludge adapted to 30"C, 8 = 20 days, and S, = 40 kg VS/m3, and were con- tinued for a period of 60 days (38). Thereafter, the temperature was increased gradually (4"C/week) to reach the thermophilic temperature of 52C. At the end of these experiments, new digesters were started at 35C and the temperature was decreased slowly to 25 and 15C prior to measurements being taken. </p><p>Under steady-state, different analyses were per- formed to assess performance. Steady-state conditions were defined primarily by a relatively constant biogas production and composition. However, pH, volatile acids, ammonia, and alkalinity also remained fairly constant during at least two retention times. In prac- tice, steady-state conditions were achieved after 1.58, although, experiments were maintained for a period up to 2.50-38. </p><p>Methods of Analysis </p><p>Routine reactor performance was assessed by de- termination of the rate of gas production, gas com- position, volatile acid content, alkalinity, and ammo- nia-nitrogen content three times per week, and the COD and VS once a week. The volatile acid concen- tration, alkalinity, COD, and VS were determined in accordance with Standard Methods (APHA, 1980) and the ammonia-nitrogen content was determined using an ion selective e le~t rode . '~ . '~ </p><p>Gas composition and individual volatile acid content were measured by gas chromatography. Gas compo- sition was determined on a Fisher model 1200 Gas Partitioner equipped with a TC detector. The standard manufacturer's columns (Fisher Activated Molecular sieve 13X and Columpak T) were kept at 50C and the carrier gas, helium, was maintained at a flow rate of 40 mL/min. The injector was kept at 100C and the detector current was set at 175 mA. A standard gas mixture was used for the calibration of the chromato- graph. The content of the volatile acids was determined by the method of O'Rourke15 using a (Hewlett- Packard) Porapak QS glass column (2 m x 2 mm) on a Hewlett-Packard model 5790 Gas chromatograph us- ing a FI detector with a model 3390A integrator. In- jector and detector temperatures were both set at 225C and the oven temperature started at 175C increasing by 5"C/min to 200"C, where it was held for 20 min. Digester effluent was centrifuged at 2 x lo4 g for 10 min, 0.1 mL of the supernatant was added to 0.1 mL of the internal standard [0.3% (v/v) of methanol], and an aliquot of 3 pL of the mixture was injected in the chromatograph. The chromatograph was calibrated with a standard mixture containing 1 g/L of each acetic, propionic, butyric, isobutyric, valeric, and isovaleric acids (Sigma Chemicals). </p><p>Tests on volatile solids proved to be very inaccurate in determining the destruction of the volatile solids because of the high concentration of volatile acids and alkalinity present in the effluent. The COD tests were also inaccurate because of the high dilution factor (up to 200) necessary. To circumvent these problems, data on VS destruction were calculated from the biogas production rate using Varel's equation"? </p><p>VS destroyed (8) = (mol COz + mole CH4) x (12/0.53) </p><p>in which, the number 12 represents the molecular weight of carbon and 0.53 is the carbon fraction found in S. maxima volatile solids. </p><p>Because of the great number of parameters and vari- ables studied, it was difficult to make an accurate eval- uation of the chemical and microbiological interactions found in the anaerobic digestion process. For this rea- son, a correlation matrix was used between the dif- ferent parameters and variables. l7 The significant level for degree of freedom of (n - 2) = 21 - 2 = 19 is r &gt; 0.433 at 5% and r &gt; 0.549 at The correlations measured were valid only when the relations were lin- ear. On the other hand, nonlinear relations gave poor correlation Nevertheless, this tech- nique is very useful to evaluate the level of interde- pendence of any pair of parameters. </p><p>RESULTS AND DISCUSSION </p><p>Chemical Composition of Algal Biomass </p><p>The most important characteristic of S. maxima algal biomass was its very high protein content (up to 60%) with a lipid concentration below 2.5% (Table 11). This </p><p>Table 11. Characteristics of S. maxima algal biomass. </p><p>Concentration </p><p>Constituent mdg TS mp/La </p><p>Proteins Carbohydrates Lipids Ash Humidity PH Electrode potential Volatile acids Alkalinity Ammonia nitrogen Total nitrogen Total carbon Total solids Volatile solids Total COD Soluble COD </p><p>596.7 178.7 24.1 70.0 82.5 - - 21.5 55.3 8.0 </p><p>112.9 470.2 </p><p>1000 889.5 </p><p>1243 357 </p><p>21,600 6469 </p><p>2534 872.4 </p><p>- </p><p>6.40 25 mV </p><p>780 2000 290 4090 </p><p>17,020 36,200 52,200 44,980 12,900 </p><p>a Concentration is shown in mg/L for an algal slurry containing 30 kg VSlm3. </p><p>1016 BiOTECHNOLOGY A N D BIOENGINEERING, VOL. 28, JULY 1986 </p></li><li><p>explains the low C/N ratio of 4.2. The high concentra- tion of volatile acids and alkalinity indicated that freez- ing broke the algal cells and liberated the cellular con- tent. However, our previous results showed that this phenomenon does not change the performance in terms of m3 CH, producedlkg COD added, when a digester used thawed algae instead of living algae. lo It was also observed that 29% of COD was in a soluble form. The high alkalinity was a positive factor because the NH4HC03 acted as a buffer to stabilize the pH and reduced a possible inhibition by volatile acids and NH,. </p><p>Methane Production </p><p>Results reported here represent the average of data obtained during 2.58-38- No data are reported for 8 = 5 days and S,, = 80 kg VS/m3 because biogas produc- tion ceased after a few days of operation. This occurred when the volatile acid concentration increased abruptly followed by a decrease in pH below 6.8, resulting in a complete inhibition of methane production. </p><p>The methane yields obtained were between 0.037 and 0.353 m3/kg VS added. These usually decreased with the increase in feed VS concentration [Fig. 2(A)], especially for shorter retention times (e.g. 10 days). For longer retention times (20 and 30 days) and low volatile solid concentrations (below 40 kg VS/m3), the decrease in yields was less important. Results also in- dicated that the decrease in methane yield was related to the increase in loading rate (r = -0.87). Further- more, the methane yield decreased in correspondence with the decrease in CH, content of the biogas (r = 0.86). </p><p>A maximum methane yield of 0.35 m3/kg VS added was obtained with reactors fed at 8 = 30 days and S, = 20 kg VS/m3. In comparison with other organic substrates such as farm waste and sewage sludge,20 the methane yield for S. maxima was quite similar (0.2-0.3 m3 CH,/kg VS added). Data reported for other algal biomass, such as Scenedesmus spp. and Chlorella spp. biomass growing in oxidation ponds, converted to a maximurn methane yield of 0.31 m3 CHdkg VS added.3 Methane yield for S . maxima did not increase much when the retention time was above 20 days. However, at shorter retenti...</p></li></ul>


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