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The aim of this study was to investigate the influence of residual glycerine (5 and 10% w/w) from the biodiesel industry, used as a co-substrate, on biogas production from maize silage. The experiments were conducted in a laboratory-scale, single-stage anaerobic digester at 39ºC and hydraulic retention time (HRT) of 60 d. Addition of 5% residual glycerine caused organic load rate (OLR) to increase to 1.82 compared with 1.31 g organic dry matter (ODM) L-1d-1 for maize silage alone. The specific biogas production rate and biogas yield were 1.34 L L-1d-1 and 0.71 L g ODM-1 respectively, i.e. 86% and 30% higher than for maize alone. Increasing the residual glycerine content to 10% increased OLR (2.01 g ODM L-1d-1), but clearly decreased the specific biogas production rate and biogas yield to 0.50 L L-1d-1 and 0.13 L g ODM-1 respectively. This suggested that 10% glycerine content inhibited methanogenic bacteria and organics conversion into biogas. As a result, there was accumulation of propionic and valeric acids throughout the experiment. INTRODUCTION Methane production through anaerobic digestion is currently one of the most intensively developed biofuel technologies. Since 2001 the total production of biogas in European Union (EU) countries has constantly increased at 2.7 million tonnes of oil equivalent per year . The application of plant biomass for biogas production has assumed a growing importance. According to Amon et al. , in the EU the annual potential of anaerobically digested biomass is 1500 million tonnes, and half of this will be supplied by agricultural crops. Of various agricultural crops used in biogas production in Germany, a European leader in the utilization of biomethane technologies , maize is dominant due to its high biogas yield. From technological and economics points of view, in agricultural systems using plant biomass (i.e. grass, maize silage or fibrous plant residues) for biogas production, the application of other materials as co-substrates offer several advantages, i.e. improved nutrient balance, optimized rheological qualities, and cost savings for farmers. For other reasons, the availability of some waste as co-substrates for biogas production in agricultural areas is crucial. Some substrates, including food industry waste (oils, whey, fruit and potato slops) show great potential for biogas production. Other substrates from breweries introduce risks of H2S production, chicken manure contains inhibiting NH3, whereas oils and fats can cause scum layer forming. Among co-substrates, by-products from biodiesel production are relatively poorly recognized. In EU countries, therein Poland, the production of methyl esters as biofuels for the transport sector is showing tremendous growth. Biodiesel production for farmers' needs (as permitted by law from 2006 in Poland), usually uses simple systems without glycerine-refining devices. The glycerine phase contains residual alcohols, catalyst residues, carry-over fats/oils, and some esters. Their using for industrial purposes is limited, but they are not excluded from being processed into biogas. Up to now, residual glycerine has been used as a co-substrate during industrial and agricultural wastewater anaerobic treatment [1, 12, 17]. The use of residual glycerine to enhance biogas production from maize silage and changes in the digestion process performance has not previously been investigated. Glycerol, the main component of residual glycerine, is known as a readily digestible compound; however, its implementation in digesters at too high a concentration has negative effects on microorganisms growth due to increased osmotic pressure in bacterial cells. In present study the effect of different glycerine contents in the feedstock containing maize silage on biogas production and digestion stability has been examined. The influence of residual glycerine on biogas production was assessed by comparing the biogas/CH4 yield obtained experimentally with theoretical biogas/CH4 potential, calculated from the elementary composition of the feedstock. MATERIALS AND METHODS Substrate Maize silage, consisting of leaves, stalks, and cobs, was obtained from the Department of Agrotechnology and Crop Management, University of Warmia and Mazury in Olsztyn (north of Poland). The silage was chopped in a cutting mill (Retsch SM100, Germany) and passed through 1-mesh screen. The chaff was stored in plastic bags at 4°C. Directly before feeding into the digester, the maize was diluted with tap water to approximately 8% of dry matter (DM). Residual glycerine was supplied by a local biodiesel production plant in Lubomin (north of Poland). The characteristics of substrates are given in Table 1. Table 1. Main characteristics of maize silage and residual glycerine (standard deviations are in parentheses) Parameter Dry matter (DM) Organic dry matter (ODM) Chemical oxygen demand (COD) Carbon Hydrogen Oxygen Nitrogen C:N Unit g kg-1 g kg-1 g O2 kg-1 % % % % g g-1 Maize silage 425 (±28) 397 (±18) 1006 (±1.8) 43.13 (±0.191) 7.12 (±0.021) 48.88 (±0.318) 0.89 (±0.106) 48.46 Residual glycerine 816 (±5.2) 740 (±0.9) 2520 (±25) 55.93 (±0.099) 10.15 (±0.057) 33.92 (±0.042) 0.06 (±0.002) 932.17 Residual glycerine was marked by higher DM; however, the percentage of organic dry matter (ODM) was comparable to maize silage with 93 and 91%, respectively. The carbon and hydrogen content of glycerine was higher than that for maize silage, but nitrogen's - much lower. Consequently, the C:N ratio was about 19 times higher for residual glycerine compared to maize silage. Experimental set-up The experiments were conducted in laboratory-scale, single-stage anaerobic digester (CSTR), with the working volume of 6 L. The reactor was constructed of stainless steel and equipped with a propeller mixer with horizontal axis of rotation for continuous stirring of the mixed liquid at 50 rpm. Appropriately mounted valves enabled reactor feeding and biogas and digestate collection. The biogas was collected in Tedlar sample bags. The temperature was maintained at 39 ± 0.2°C by a surrounding water jacket. The reactor was inoculated with anaerobic sludge originating from the municipal wastewater treatment plant in Olsztyn (north of Poland, population 175,000). Once daily each reactor was supplied with 100 mL of the feedstock after 100 mL of mixed liquid had been withdrawn. Therefore, a constant hydraulic retention time of 60 d was obtained. The experiment was conducted continuously for 180 d in three successive runs differing in the residual glycerine content in the feedstock. During 160 d (run 1) maize silage alone was fed into the reactor; during 61120 d (run 2) maize silage mixed with 5% (w/w) residual glycerine; and during 121160 d (run 3) maize silage mixed with 10% (w/w) residual glycerine was used. Analytical methods Dry matter (DM) and organic dry matter (ODM), soluble chemical oxygen demand (COD), ammonia N, and Kjeldahl N were determined according to standard methods . The pH was measured immediately after sampling using a pH meter (Hanna HI 221, US. The total alkalinity was determined by titration to pH 4.3 with normalized 0.1 M HCl using a Schott titroline system. Analyses for COD, ammonia N and alkalinity were performed for filtered supernatant samples previously centrifuged at 8693 × g for 10 min. Volatile fatty acids (VFAs) were analyzed using a gas chromatograph (GC, Varian 3800, Australi equipped with a capillary column (FactorFour VF-1ms, 30 m × 0.25 mm i.d., 1.0-m film; Varian) and flame ionization detector (FID). Helium was used as a carrier gas with a flow rate of 1.0 mL min-1. The samples were analyzed according to Gilroyed et al. . Total VFA concentration was the sum of the concentrations of individual VFAs analyzed. Biogas production was measured daily following standard methods  using apparatus consisting of a cylinder filled with a saturated solution of sodium chloride combined with an equalizing tank equipped with a side tube. The CH4 and carbon dioxide (CO2) contents were measured for samples collected in Tedlar sample bags using a GA 2000+ automatic analyzer (Geotechnic Instruments, UK). The volume of biogas and CH4 was corrected for standard pressure (101 kP and 0°C. Organic dry matter removal efficiency, specific biogas production rate and CH4 content during runs 13 were compared using ANOVA and Tukey's test. Mean values were considered to be significantly different for p < 0.05. RESULTS AND DISCUSSION Characteristics of the feedstock and operational conditions Maize silage was marked by a low pH of 4.3. The addition of 5% (run 2) and 10% (run 3) residual glycerine increased pH to 5.3 and 5.7, respectively, but the mixtures still remained acidic (Table 2). Crop ensiling is based on the conversion of water-soluble carbohydrates into organic acids. Nkosi et al.  reported that during ensiling pH declined from 5.7 to < 4.2 in maize water extract. The increase in pH for runs 2 and 3 probably resulted from the presence of a basic catalyst remaining in the glycerine phase after the oil transesterification. Table 2. Main characteristics of feedstock in runs 13 (standard deviations are in parentheses) Parameter Feedstock liquor: pH Volatile fatty acids (VFAs) Ammonium nitrogen Feedstock: Dry matter (DM) Organic dry matter (ODM) Kjeldahl nitrogen Unit Run 1 Run 2 Run 3 pH 4.3 (±0.16) 1457 (±150.2) mg L-1 mg NH4-N L-1 134.0 (±10.93) g L-1 g L-1 g N kg-1 84.0 (±7.94) 78.4 (±7.37) 2.42 (±0.20) 5.3 (±0.12) 5.7 (±0.15) 1697 (±180.1) 1784 (±155.6) 126.0 (±9.84) 110.6 (±11.56) 119.4 (±3.89) 112.9 (±2.32) 2.05 (±0.25) 149.9 (±8.17) 129.6 (±6.67) 1.71 (±0.14) In all the runs, VFAs were present in the liquid phase. Their concentrations increased in accordance with the glycerine content of the feedstock. Residual glycerine contains methyl esters at 1820% concentration . Esters undergo auto-oxidation at different rates depending upon the number and position of the double bonds and results in formation of a series of by-products, e.g. acids, aldehydes, ketones and lactones . This explains the higher concentration of VFAs in runs 2 and 3 compared to maize silage alone. The addition of residual glycerine to the maize silage clearly increased the organic matter concentration (ODM) in the feedstock; approximately 44% and 65% higher with 5% and 10% glycerine content, respectively, compared to controls. Kjeldahl N concentration was highest in run 1 and lowest in run 3, with an analogous effect for ammonium N. The experiments (runs 13) were carried out at constant HRT = 60 d; the different organic load rate (OLR) increased from 1.31 ± 0.12 to 2.01 ± 0.11 g ODM L-1d-1 due to increased residual glycerine concentration in the feedstock (Table 3). Table 3. The organic load rate (OLR) during anaerobic digestion of maize silage and its mixtures with residual glycerine Parameter Organic load rate: maize silage glycerine b) Unit g ODM L d -1 -1 Run 1 1.31 (±0.12) 1.31 0.0 Run 2 1.82 (±0.04) 1.20 (65.9)b) 0.62 (34.1) Run 3 2.01 ±0.11) 0.80 (39.8) 1.21 (60.2) standard deviation percentage of OLR for single substrate in the feedstock mixture The range of OLR used in present study was within typical values employed in biogas plants. According to Weiland , biogas plants that use energy crops as the main substrate are operated with OLR of 13 g ODM L-1d-1. Effect of residual glycerine supplementation on digestion performance Anaerobic digestion of maize silage and co-digestion of maize silage with residual glycerine were performed for 180 d, starting from fermentation of maize silage alone (run 1) and adding co-substrate after 60 d (run 2) and 120 d (run 3) of the experiment. Organic dry matter removal efficiency The ODM concentrations in the digestate (Xdigestate) and the efficiency of ODM removal (E) in the runs are shown in Fig. 1. X digestate (g ODM L-1) 0 run 1 run 2 run 3 Organic dry matter Organic dry matter remov al ef f iciency Fig. 1. Concentration of organic dry matter in the digestate (in g ODM L-1) and the efficiency of ODM removal (E) during the digestion of maize silage (run 1) and maize silage mixed with 5% (run 2) and 10% w/w (run 3) residual glycerine (error bars represent standard deviations) Using maize silage alone as a feedstock (run 1) gave high efficiency of organic dry matter removal at 74.9%. In run 2, despite increased OLR, due to 5% residual glycerine addition to the silage, there was a significantly higher ODM removal efficiency of 81.1% (Tukey's test at p < 0.05). Thus the 5% residual glycerine content in the feedstock was an appropriate co-substrate with a favorable effect on digestion performance. The enhancement of process efficiency by adding 5% glycerine phase (corresponding to 34.1% ODM) could result from its higher biodegradability compared to maize silage. Hongwei et al.  studied anaerobic biodegradability of 47 kinds of aliphatic compounds, i.e. carboxylic acids, esters, ketones, cycloalkanes, cycloalkenes, alcohols, and included glycerol. The batch experiments were conducted at 35°C for 50 d. Based on integrated assessment indices (IAI), tested compounds were classified into three groups: readily biodegradable (IAI >1.0), partially biodegradable (0.3 < IAI 1.0) and poorly biodegradable (IAI 0.3). Glycerol had IAI = 1.44, and so was classed as readily biodegradable. The high biodegradability of glycerol after removing catalyst (KOH) by acidification with phosphoric acid, followed by vacuum distillation E (%) with methanol and water, was also documented by Siles Lopez et al. . Using granular sludge gave organics removal (expressed as COD) of about 100% ; implementation of non-granular sludge lowered substrate removal efficiency to 75%. In contrast, an increase of glycerine content to 10% (run 3), brought about a strong decrease in the ODM removal efficiency to 30.6%. Hence, the majority of the organic matter was not converted into biogas, but remained in digestate, leading to an increase in post-methanation potentials. Since the OLRs of runs 2 and 3 did not significantly differ, reduction of organic dry matter removal efficiency was probably caused by the osmotic properties of glycerine phase. Glycerine penetrates bacterial cells by facilitated diffusion but cannot bring about an efflux of water from the cell; as a result, osmotic pressure rises and causes weakening of the membrane and cellular lysis depending on the resistance of the cell wall . Zhang and Yang  showed the influence of glycerol concentration on propionic acid yield, and found no significant difference in productivities and yields between batch fermentation at glycerol concentrations of 20 and 40 g L-1. This suggested that glycerol osmotic pressure at such concentrations had a negligible effect on propionic acid fermentation kinetics. However, at glycerol concentration >60 g L-1, the cell growth was remarkably inhibited. In the present study there was no inhibition of the process at 5% (i.e. 50 g L-1) residual glycerine content in the feedstock, which confirmed a high efficiency of organic dry matter removal; however, increasing glycerine content to 10% significantly limited the organic degradation process. Stability of digestion performance Maintenance of an adequate buffering capacity and keeping the pH at a safe and stable level (Fig. 2) required addition of 5 M sodium hydroxide (NaOH) solution throughout the whole process. b) 8000 Alkalinity (mg CaCO3 L-1) 7.6 7.4 7.2 pH 7 6.8 6.6 6.4 6.2 run 1 run 2 run 3 2000 0 run 1 run 2 run 3 Fig. 2. Mean pH ( and alkalinity (b) values after digestion of maize silage (run 1) and maize silage mixed with 5% (run 2) and 10% w/w (run 3) residual glycerine; (error bars represent standard deviations) The pH of the digestate was approximately constant during experiments (Fig. 2), with a mean value of 7.28 ± 0.04 using maize silage alone (run 1) and 7.53 ± 0.05 using maize silage mixed with 5% glycerine phase (run 2). In the biogas digester fed with maize silage containing 10% glycerine phase (run 3), pH averaged 7.06 ± 0.29, and was difficult to control despite adjustment with NaOH (shown by large standard deviations). These values remained within the optimal range for methanogens [7, 23]. The concentration of individual VFAs in the digestate, as well as specific biogas production rate and CH4 concentration, during anaerobic digestion of maize silage and its mixture with co-substrate are shown in Fig. 3. 9000 8000 Volatile fatty acids (mg L -1) 7000 6000 5000 4000 3000 2000 1000 0 0 20 40 60 80 100 120 140 160 180 Reactor day Ac Pro iso-But But iso-Val Val run 1 run 2 run 3 b) Specific biogas production rate (L L -1 d-1) Reactor day c) Methane concentration (%) 0 0 20 40 60 80 100 120 140 160 180 Reactor day Fig. 3. Concentration of individual VFAs in the digestate (; specific biogas production rate (b); and CH4 concentration (c) during mesophilic anaerobic digestion of maize silage and maize silage mixed with 5% and 10% w/w residual glycerine (Ac acetic acid; Pro propionic acid; iso-But isobutyric acid; But butyric acid; iso-Val isovaleric acid; Val valeric acid) After commencing digestion of maize silage alone (run 1), acetic acid appeared in digestate liquor. From 10 d it was detected sporadically. Propionic acid was present at a mean concentration of 1003.0 ± 250.2 mg L-1 throughout run 1. Specific biogas production rate was stable after 29 d of reaction and maintained an average of 0.72 ± 0.07 L L-1d-1 to the end of run 1. CH4 concentration during the stable phase averaged 57.6 ± 1.83%. After addition of 5% (34.1% ODM) glycerine phase as a co-substrate, during the first 18 d of the experiment (6178 d of reaction) propionic and isovaleric acids were detected in digestate liquor. The ratio of isovaleric to the dominant propionic acid ranged from 0.4 at the start to 0.25 at 18 d of run 2. Hill and Holmbert  showed that iso-acids are suitable indicators of the onset of digester instability, as confirmed in the present research. By 19 d (79 d of reaction), propionic acid dominated the digester at mean concentration of 1933.4 ± 183.4 mg L-1, and iso-valeric acid was not detected or appeared sporadically (Fig. 3. This resulted from the adaptation of microorganisms to the glycerine content in the feedstock; after this, specific biogas production rate stabilized. Mean specific biogas production rate during 79120 d of reaction was 1.34 ± 0.13 L L-1d-1 and CH4 concentration reached 61.6 ± 1.83%. The results were significantly higher compared to those of maize silage alone (Tukey's test for p < 0.05). The increase of residual glycerine content to 10% (60.2% ODM) in run 3, had a negligible effect on propionic acid concentration but caused clear increases in concentrations of acetic and valeric acids, during the first 12 d (121132 d of reaction). During this period there were significant decreases in specific biogas production rate from 1.73 to 0.23 L L-1d-1 and CH4 concentration from 58.1 to 39.5%. This may be related to the inhibitory effect of 10% residual glycerine on methanogens, which cannot convert acetate into CH4. After 133 d of reaction, there was an increase in propionic acid concentration; by 150 d, valeric acid was also found, and like propionic acid, its concentration tended to increase until the end of the experiment. Compared to conventional substrates, such as glucose and other carbohydrates, glycerol has a much lower reducing state, which favors the production of more reduced metabolites (e.g. propionic acid) . Conversion of glycerol to pyruvate would generate two moles of NADH per mole of glycerol. When pyruvate is converted to acetate, one additional mole of NADH would be generated, resulting in redox imbalance. However, only 1.44 moles of NADH would be consumed for each mole of pyruvate used in cell biomass production, causing accumulation of some NADH and a decrease in the NAD+ pool. Thus, cell growth on glycerol could be inhibited due to the higher NADH/NAD+ ratio. To maintain the redox balance in the cell, pyruvate must be converted to propionic acid, thus allowing consumption of the excess NADH. Despite the increase of propionic acid content from 2150.6 to 7895 mg L-1 during 140180 d of reaction, specific biogas production rate and CH4 content was stable and reached a mean level of 0.50 ± 0.08 L L-1d-1 and 51.9 ± 3.47%, respectively (Fig. 3b and c). However, the obtained values were significantly lower compared to runs 1 and 2 (Tukey's test for p < 0.05). The results indicate that microorganisms adapted to high glycerine concentration through higher propionic acid production. The adapted biomass had acquired higher tolerance to the propionic acid as shown by stable biogas production by 140 d of reaction. There are differences in the literature data concerning the influence of propionic acid on digestion processes. Concentrations of propionic acid >3000 mg L-1 have been previously shown to inhibit methanogenesis ; however, more recent studies by Pullammanappallil et al.  revealed that propionic acid was an effect rather than cause of inhibition of anaerobic processes as also confirmed by the present research. Hajarnis and Ranade  examined the influence of propionic acid toxicity on methanogens and found that at propionic acid concentration of 5000 mg L-1 and neutral pH, the CH4 production was inhibited to 2238% of control values; and a pH reduction drastically increased the inhibition. Pullammanappallil et al.  showed that propionic acid concentration up to 2750 mg L-1 did not adversely affect CH4 production even at pH <6.5. McCarty and Brousseau  reported that propionate concentration of up to 8000 mg L-1 was tolerated in anaerobic digesters. Similarly, in the present study, due to microorganism adaptation, propionic acid at concentration around 7900 mg L-1 and pH 7.06 ± 0.29 did not inhibit CH4 formation; however, specific biogas production rate and CH4 concentration were low. To sum up, addition of glycerine phase to maize silage influenced both OLR and chemical composition of the feedstock. The experiments showed that specific biogas rate after the addition of 5% residual glycerine increased by approximately 86% in relation to maize silage alone that resulted from OLR increase. A higher residual glycerine addition (10%) slightly increased OLR, but the clear decrease in specific biogas rate was probably due to the inhibiting effect of glycerine phase. Effect of residual glycerine supplementation on biogas potential In the present study, based on elementary composition of the feedstock, theoretical biogas and CH4 potential (TBP and TMP, respectively; Table 4) were estimated and compared with biogas and CH4 yield obtained experimentally. For calculations O'Rourke's equation  was applied. Assuming the elementary composition of the maize silage and residual glycerine (Table 1), equations (1) and (2) were formulated to calculate the theoretical number of moles of CH4 and CO2 produced from one mole of feedstock: Maize silage: C56.51H112O48.07N + 5.3H2O 29.9CH4 + 26.6CO2 + NH3 Residual glycerine: C1045.70H2277.24O474.77N + 239.3H2O 668.7CH4 + 357.0CO2 (1) (2) Theoretical biogas potential for maize silage amounted to 842.1 L kg-1 ODM and 445.3 L CH4 kg-1 ODM. For residual glycerine, TBP and TMP were higher and amounted to 1155.6 and 761.1 L kg-1 ODM, respectively. Supplementation of maize silage with 5% residual glycerine increased the TBP by 10% and TMP by 20% compared to maize silage alone, whereas at 10% glycerine content TBP and TMP were 17.5 and 38% higher, respectively. Measured biogas/CH4 yield (Y) was calculated as the slope of the regression line of accumulated biogas/CH4 per accumulated added organic dry matter for the stable process (Fig. 4ac). The process was considered to be stable by several weeks after feeding had started, when there were no operational disturbances. Experimental data showed process stability by 29, 19 and 20 d of runs 13, respectively, until the end of the experiment. 500 400 biogas volume (L) 300 200 100 0 0 50 100 150 200 250 y = 0.3143x - 8.9794 R2 = 0.9995 300 350 400 y = 0.5469x - 13.199 R2 = 0.9996 b) 500 400 biogas volume (L) 300 200 100 0 0 100 200 300 400 500 600 700 y = 0.4466x - 15.704 R2 = 0.9987 y = 0.726x - 17.026 R2 = 0.9993 c) 500 400 biogas volume (L) 300 200 100 0 0 biogas 100 200 300 400 500 methane organic dry m atter (g) y = 0.2493x + 29.711 R2 = 0.9979 y = 0.1303x + 16.128 R2 = 0.9976 600 700 800 Fig. 4. Accumulated biogas and CH4 production versus added ODM during mesophilic anaerobic digestion of maize silage (run 1) and maize silage mixed with 5% (run 2) and 10% w/w (run 3) residual glycerine Supplementation of maize silage with 5% residual glycerine increased the measured biogas (Ybiogas) and CH4 (Ymethane) yields compared to controls (maize silage alone); however, for 10% glycerine phase, Ybiogas and Ymethane were lower (Table 4). For maize silage alone (run 1) Ybiogas and Ymethane values comprised 69 and 74% of TBP and TMP, respectively (Fig. 5). Similarly, higher Ymethane/TMP ratio in relation to Ybiogas/TBP was obtained for maize silage supplemented with 5% glycerine phase (run 2). According to Weiland , the real CH4 content is generally higher in practice than the Table 4. Feedstock load, and theoretical and observed potential of biogas and CH4 for maize silage (run 1) and its mixture with 5% (run 2) and 10% w/w (run 3) residual glycerine Parameter Feedstock load (Fadded): maize silage glycerine Theoretical biogas potential (TBP): maize silage glycerine Theoretical methane potential (TMP): maize silage glycerine Measured biogas yield (Ybiogas) Measured methane yield (Ymethane) Unit g ODM d-1 % ODM % ODM L g-1ODM Run 1 7.84 100 0.0 0.80 0.80 0.0 0.42 0.42 0.0 0.55 0.31 Run 2 10.90 66.1 33.9 0.88 0.52 0.35 0.51 0.28 0.23 0.73 0.45 Run 3 12.40 40.0 60.0 0.94 0.32 0.62 0.58 0.17 0.41 0.25 0.13 L g-1 ODM L g-1 ODM L g -1 ODM 60 (%) 40 biogas methane 0 run 1 run 2 run 3 Fig. 5. Percentage of measured biogas yield (Ybiogas) and CH4 yield (Ymethane) in relation to theoretical biogas potential (TBP) and CH4 potential (TMP) during mesophilic anaerobic digestion of maize silage (run 1) and maize silage mixed with 5% (run 2) and 10% w/w (run 3) residual glycerine theoretical values, due to part of CO2 being solubilized in the digestate. In contrast, in run 3 the addition of 10% glycerine decreased both ratios, but Ymethane/TMP was lower than Ybiogas/TBP. This could be explained by the inhibitory impact of glycerine phase at 10% concentration leading to accumulation of propionic and valeric acids in the digestate and limited CH4 production. The supplementation of maize silage with 5% glycerine phase was beneficial in regard to biogas yield. Amon et al.  achieved analogous results in 1-L eudiometer batch experiments, in which they investigated the effect of increasing glycerine dosage (3, 6, 8 and 15%) on CH4 yield during anaerobic digestion of a pig manure, maize silage and maize grain mixture (54:31:15 fresh matter basis). They found the highest specific biogas and CH4 yields in feedstock enriched with 6% glycerine (0.68 and 0.44 L g-1 ODM, respectively) a result that corresponds to data obtained in the present study. At higher concentrations the CH4 yield diminished, achieving 0.37 L g-1 ODM at 8% glycerine content. However, for all tested glycerine dosages they found increased specific biogas yield and CH4 content in biogas compared to controls (basic mixture without glycerine). In the present research conducted in CSTR, at 10% glycerine phase in feedstock, there were decreases in specific biogas production rate. The discrepancy between these two sets of data could result from the use of a continuously operated reactor and distinct feedstock chemical compositions. CONCLUSIONS Our results revealed that residual glycerine derived from biodiesel manufacturing had a favorable effect on anaerobic digestion of maize silage when it contributed 34.1% of ODM (5% w/w) in the feedstock. Compared to maize silage alone, there was higher specific biogas production (1.29 ± 0.09 vs. 0.72 ± 0.07 L L-1d-1) and measured biogas yield in relation to the TBP (81 vs. 69%). The increase of residual glycerine to 60.2% ODM (10% w/w) caused an accumulation of propionic and valeric acids in digestate liquor, and inhibitory effects were evident in the significant reduction in efficiency of organics consumption (as ODM) and, in consequence, a decreased specific biogas production rate. ACKNOWLEDGEMENTS This research was financially supported by the Polish Ministry for Science and Higher Education in the frame of the Project No. NN310 209137.
Archives of Environmental Protection – de Gruyter
Published: Dec 23, 2014
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