Effect of digester surface area on biogas yield

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Agricultural Engineering International (E-Journal, CIGR - International Commission of... 64 October, 2017 AgricEngInt: CIGR Journal Open access at http://www.cigrjournal.org Vol. 19, No. 3 Effect of digester surface area on biogas yield Gbolabo Abidemi Ogunwande*, Ayodele Joseph Akinjobi (Department of Agricultural & Environmental Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria) Abstract: Poultry manure, cow dung and swine manure were digested in cube-shaped anaerobic digesters of equal volume but different surface areas (SA) (5.38, 10.0 and 11.6 dm2) in a two-factor experiment. during the experiment. The digesters were agitated once a day The results showed that manure type and SA only had significant (p≤0.05) effect on biogas yield (BY). Poultry manure had the highest yield, followed by swine manure and cow dung (1.00, 0.59 and 0.11 L kg-1 VS fed day-1, respectively). Within the SA investigated, BY increased as digester SA increased (0.27, 0.68 and 0.74 L kg-1 VS fed day-1 for 5.38, 10.0 and 11.6 dm2, respectively). The results of the regression analysis showed that a polynomial model presented an ideal situation and best described the relationship between digester diameter to substrate height (DD/SH) ratio and BY through a dome-shaped curve, indicating DD/SH ratios at which biogas production would commence, peak and stop. Keywords: biogas yield, digester surface area, poultry manure, swine manure, cow dung Citation: Ogunwande, G. A., and A. J. Akinjobi. 2017. Effect of digester surface area on biogas yield. Agricultural Engineering International: CIGR Journal, 19(3): 64–69. 1 Introduction For effective digestion, digester shape must take into consideration the construction practicalities of both Anaerobic digestion is a biochemical technology for mixing and heat loss (Ward et al., 2008). It is a known the treatment of organic wastes and the production of fact that the bigger the digester capacity, the more the biogas, which can be used as a fuel for heating, electricity biogas production, provided decomposition factor levels or vehicle. The process takes place in engineered are kept at optimum. Several studies have focused on the containment vessels, called bioreactors or digesters, design parameters of digesters for optimum biogas designed to exclude air and promote the growth of production (Ortolani et al., 1991; Florentino, 2003; Thy et methane bacteria. Most studies on anaerobic digestion al., 2005; Umaru, 2012; Kaur et al., 2017). Such and optimization of biogas production have focused on parameters ranged from digester surface area, diameter feedstock properties, digester temperature, retention time, and height; positioning of the inlet and outlet pipes; usable loading rate, mixing and co-digestion. The basic and gross volumes of the digester; level of the substrate to requirements of an anaerobic digester design are to allow volume of biogas needed per day; etc. In addition, some for a continuously high and sustainable organic loading authors have used mathematical models to size digesters rate, a short hydraulic retention time (to minimize for effective biogas production. While results from these digester volume) and to maximize biogas yield (Ward et studies have advanced knowledge on effective biogas al., 2008). Igoni et al. (2008) stated that some factors to production, the effect of digesters’ configuration in terms be considered when designing digesters, including type of the surface area (SA) and height on biogas production and nature of waste, rate of waste generation and local and the mathematical relationship between digester environmental conditions, like the ambient temperature. diameter to digester working height (substrate height) (DD/SH) ratio and biogas yield (BY) are not well Received date: 2016-06-15 Accepted date: 2017-07-11 * Corresponding author: Gbolabo Abidemi Ogunwande, PhD, Department of Agricultural & Environmental Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria. Tel. +234 803 400 7128, Email: [email protected], [email protected]. understood. This study therefore aimed to fix the digester volume and vary the surface area and height with a view to determining the effect of digester SA on biogas production and the optimum DD/SH ratio for biogas October, 2017 Effect of digester surface area on biogas yield connect each digester to the water tank and the water Materials and methods 2.1 65 plastic kegs, respectively. Rubber hose was used to production. 2 Vol. 19, No. 3 tank to the water collector. Materials and analytical methods After the moisture content determination, each The anaerobic digestion experiment was conducted manure was diluted with clean tap water to 8% TS, as under ambient conditions in a laboratory at the recommended by Zennaki et al. (1996), agitated Department Environmental vigorously and poured through a 6 mm plastic mesh to Engineering of the Obafemi Awolowo University, Ile-Ife, remove gross solids. The digesters were loaded once Nigeria. The ambient temperatures during digestion were during the experiment to 70% of their capacities, hence, a mesophilic, ranging between 26°C and 34°C. Fresh working volume of 17.5 L, with substrate height and poultry manure (PM), swine manure (SM) and cow dung surface area dimensions of 3.24, 1.75 and 1.50 dm and (CD) were collected from the University Teaching and 5.38, 10.0 and 11.6 dm2, respectively. The experiment Research Farm. Samples were analysed for total solids was set up as a 3 × 3 completely randomized block design (TS) content (oven drying at 105°C for 24 h); volatile with manure type (MT) and digester SA as the variable solids (VS) content (ashing of TS at 550°C for 5 h); total factors, and each treatment triplicated. The biogas nitrogen (TN) (regular-Kjeldahl method; Bremner, 1996); produced was estimated by water displacement method pH (1:10 w/v sample:water extract, using a digital pH (Archimedes’ principle) and measured using a calibrated of Agricultural and meter). The total carbon (TC) content was estimated from the ash content according to the Equation (1) (Mercer and Rose, 1968): cylinder (Ogunwande et al., 2013). The digesters were manually agitated once daily to avoid long period settlement of the substrates and ensure uniform TC (%) = [100 – Ash(%)]/1.8 (1) The initial properties of the manures are presented in Table 1. distribution of micro-organisms and heat within the substrates. Ambient and substrate temperatures and BY were measured daily while substrate pH was measured Table 1 Initial properties of the manures weekly. Properties (dry weight basis) 2.3 MT pH VS, % TC, % TN, % C:N ratio PM 6.88 66.4 36.9 3.41 10.8 CD 7.13 73.3 40.8 1.88 21.7 SW 6.98 61.2 34.0 2.42 14.0 Statistical analysis Data collected were subjected to two-way analysis of variance (ANOVA) to compare variations in substrate temperature and substrate pH, and BY. Where Note: carbon to nitrogen (C:N). significance was indicated at p≤0.05, Duncan’s multiple 2.2 range test was used to separate the means. Experimental set up The experimental set up comprises of digesters, The mathematical relationship between DD/SH ratio water tanks and water collectors. The digesters were and BY was established by regression analysis. The SA adapted using cube-shaped 25 L plastic kegs. The kegs were assumed to be circular and the respective diameters were positioned on each of the three sides to give the were estimated and divided by the corresponding 2 following surface (dm ) and height (dm) dimensions: substrate heights to give the DD/SH ratios. The BY 2.15 × 2.50 and 4.65, 2.15 × 4.65 and 2.5, and 2.50 × obtained from the three SA represented yields from the 4.65 and 2.15, respectively, resulting in 5.38, 10.0 and corresponding DD/SH ratios. Four mathematical models 2 11.6 dm surface areas from the three positions. A drain (logarithmic, power, polynomial and linear) were fitted to plug was fitted at the base of each digester for collection the ratio and yield data to obtain equations which were of samples for pH analysis. Each digester had a digital used to predict biogas yields from given DD/SH ratios. thermometer temperature Both ANOVA and regression analysis were performed measurement. Similarly, the water tanks and water using the Statistical Analysis Systems software (SAS, collectors were adapted using 10 L and 5 L rectangular 2002). probe fitted to it for 66 3 October, 2017 AgricEngInt: CIGR Journal Open access at http://www.cigrjournal.org SM treatments showed a different pattern with S10.0 Results and discussion The livestock wastes were digested for 49 days. The initial properties were within the range obtained in previous studies (Ogunwande et al., 2008; Bernal et al., 2009; Ogunwande et al., 2013), but the C:N ratios of PM and SM were below the range of 20:1-30:1 recommended for effective biodegradation (Rynk et al., 1992). However, having significantly lower temperatures and S5.38 and S11.6 having same trend and close temperature values. No significant (p>0.05) correlation was established between substrate temperature and other parameters measured. Table 3 Duncan’s multiple range tests showing the means separation the low levels did not have adverse effect on BY as the Parameter PM within the range. The results of the statistical analysis Table 2 ANOVA table showing the effects of MT and SA on pH Biogas SM a a 5.38 10.0 11.6 a a 30.8 31.0 30.1 30.8 30.0 31.1a pH 6.32a 6.34a 6.27a 6.28a 6.33a 6.33a Biogas, L kg-1 VS fed day-1 1.00a 0.11c 0.59b 0.27b 0.68a 0.74a Note: Superscripts with the same letter are not statistically different at p ≤ 0.05 significance level. measured parameters Temperature a CD Temperature, °C revealed that MT and SA only had significant (p≤0.05) effect on biogas yield (Table 2). SA, dm2 MT manures still produced more than CD that had C: N ratio Parameter Vol. 19, No. 3 Source Df SS MS F-value Pr>F MT 2 3.754 1.877 1.182 0.330 SA 2 5.599 2.799 1.762 0.200 MT*SA 4 7.285 1.821 1.147 0.367 Error 18 28.594 1.589 MT 2 0.025 0.013 0.266 0.769 SA 2 0.016 0.008 0.169 0.846 MT*SA 4 0.066 0.016 0.347 0.843 Error 18 0.854 0.047 MT 2 3.600 1.800 16.789 <0.0001 SA 2 1.157 0.578 5.394 0.015 MT*SA 4 0.735 0.184 1.714 0.191 Error 18 1.930 0.107 a. PM Note: p≤0.05 indicates significance. Df, degrees of freedom; SS, sum of squares; MS, mean of squares. 3.1 Substrate temperature The average substrate temperature during digestion did not differ (p>0.05) across the MT and SA (Table 3). The daily temperature (DT) of substrates ranged between b. CD 28.7°C and 34.7°C during digestion. The agitation of the digesters once daily would have enhanced uniformity of substrate temperatures. The DT values were averaged weekly and presented in Figure 1. The average temperatures exhibited similar trends in all the treatments by rising from an initial value of 29.4°C to 30.2°C30.9°C (PM), 30.2°C-32.3°C (CD) and 29.0°C-31.8°C (SM) within the first week of the experiment. The PM c. SM treatments attained peak temperatures during weeks 4 and Notes: Error bars show standard errors of means (n = 3). 5 before dropping gradually to 29.9°C-30.6°C by week 7. Figure 1 The CD treatments had the highest temperatures during 3.2 Variation of substrate temperature with digestion time Substrate pH the week 1 and fluctuated slightly (±1.25°C) before The average pH values of the substrates during dropping to final values between 29.5°C and 31.0°C. The digestion were not significantly different (p>0.05) in all October, 2017 Effect of digester surface area on biogas yield the treatments (Table 3). The initial pH of the manures 3.3 Vol. 19, No. 3 67 Biogas yield were within the range (6.8-7.2) considered ideal for The results of the Duncan’s multiple range tests anaerobic digestion (Ward et al., 2008). Starting from (Table 3) showed that PM produced the highest quantity initial values of 6.88, 7.13 and 6.98, the pH dropped of biogas while CD produced the least. This finding was gradually with digestion time in all the treatments to final in conformity with previous anaerobic digestion studies values between 5.69-5.92, 5.83-5.94 and 5.69-5.85 in PM, (Adewumi, 1995; Itodo and Awulu, 1999; Ojolo et al., CD and SM substrates, respectively (Figure 2). The drops 2007) and was attributed to high biodigestibility of PM implied the production of volatile fatty acids as the easily (Odeyemi and Adewumi, 1982). SA of 11.6 dm2 digestible fraction of the substrates was being hydrolyzed produced the highest quantity of biogas while SA of 5.38 (Comino et al., 2009). The rate of pH drop per week dm2 produced the least (Table 3). This implied that BY ranged from 0.11 to 0.15 (R2 from 0.88 to 0.98) in PM increased with digester SA. The effects of SA showed that 2 substrate, 0.16 to 0.19 (R from 0.88 to 0.98) in CD 10.0 and 11.6 dm2 produced the same quantity (p >0.05) substrate and 0.15 to 0.19 (R2 from 0.90 to 0.92) in SM of biogas (Table 3). The non-significance in their yield substrate. The final pH values (5.69-5.94) were below the may be related to the closeness in the SA. SA of 10.0 dm2 range of 6.0-8.5 recommended for organic matter is 86.2% of 11.6 dm2 while that of 5.38 dm2 is 46.4% of compatibility with most plants (Lasaridi et al., 2006). 11.6 dm2. Biogas production started within 24 h in all the However, the digestate could be left to cure before treatments. The daily biogas production of each treatment application to plants. fluctuated repeatedly and peaked at different days during digestion. The differences in peak periods were attributed to the differences in organic matter content and the degree of biodigestibility of the manures (Odeyemi and Adewumi, 1982). PM and SM substrates produced biogas throughout the digestion period but CD substrates had productions stopped on the 42nd, 39th and 45th day in C5.38, C10.0 and C11.6, respectively. The stoppage of a. PM production suggested completion of the digestion process or process inhibition due to volatile fatty acid accumulation (Bouallagui et al., 2001). The weekly production clearly showed that P10.0 and P11.6 followed closely throughout the digestion time (Figure 3a). This pattern was also exhibited by the S10.0 and S11.6 (Figure 3c). The peak productions for the PM substrates were recorded during weeks 1 and 4 for P5.38, and P10.0 and b. CD P11.6, respectively. However, for the CD and SM substrates, the peak productions were during week 1 (Figure 3b and 3c). The cumulative yield showed that SA of 5.38 dm2 produced the least quantity of biogas (Table 4). Although CD substrate produced the least volume of biogas during digestion, by week 2 the substrate had produced 55%-77% of the total biogas while PM and SM substrates c. SM Notes: Error bars show standard errors of means (n = 3). Figure 2 Variation of substrate pH with digestion time produced 33%-55% and 35%-41%, respectively. The early high productions may be attributed to the initial 68 October, 2017 AgricEngInt: CIGR Journal Open access at http://www.cigrjournal.org Vol. 19, No. 3 C: N ratio of the CD which was within the recommended curves (Figure 4) showed that only the polynomial model range of 20:1-30:1 for effective biodegradation. gave a practicable scenario as logarithmic, power and linear models gave exponential increase in BY as the DD/SH ratio increased with substrate depth tending to zero. In a digester, biogas, scum, supernatant, digested slurry and inorganic solids layers are formed during digestion. It would therefore be impossible to keep having increase in biogas production with these layers assuming infinitesimal values. The polynomial model presented an ideal situation with a dome-shaped curve a. PM (Figure 4), predicted that the minimum DD/SH ratio at which biogas production would commence is ≈ 0.3 while at ratio ≈ 2.5, maximum production would be recorded (Figure 4). Also, biogas production would cease when DD/SH ratio is greater than 4.6. Table 5 Regression equations showing the relationship between DD/SH ratio and BY b. CD Model Equation Linear R2 y = 0.3183x + 0.0586 0.970 Logarithmic y = 0.4211 ln(x) − 0.4159 0.995 Polynomial y = −0.3183x 2 + 0.7777x − 0.2043 Power y = 0.3729x 0.9113 1.0 0.988 Note: y: BY, x: DD/SH ratio. c. SM Notes: Error bars show standard errors of means (n = 3). Figure 3 Variation of weekly biogas yield during digestion for different substrates Table 4 Cumulative biogas production (L kg-1 VS fed) SA, dm2 MT p-value 5.38 (0.7) 10.0 (1.8) 11.6 (2.3) PM 20.3a 65.0b 61.6b 0.003 CD 4.16c 2.37b 8.92a 0.001 SW a b b 0.026 15.8 33.4 38.2 Note: Superscripts with the same letter are not statistically different at p ≤0.05. Values within parentheses are DD/SH ratios. Figure 4 Regression curves relating DD/SH ratio and BY. Biogas production commenced at DD/SH ratio ≈ 0.3, peaked at ≈ 2.5 and ceased at ≈ 4.6 4 Conclusions The significant (p≤0.05) correlation established The results from the study indicated that MT and between substrate pH and BY showed that the former may digester SA significantly affected biogas production. PM have affected the latter. The drops in the substrates pH was found to be the best in terms of BY. It was observed below 6.6 (observed between weeks 2 and 3) may have that BY increased as the SA increased within the three SAs greatly reduced the growth rate of methanogens (Mosey tested. Neither the digester SA nor MT significantly and Fernandes, 1989). affected substrate temperature or pH. The results of the The regression equations derived from the DD/SH regression analysis showed that a polynomial model ratio and BY data are presented in Table 5. 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