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Enhancing biomethane production from flush dairy manure with turkey processing wastewater
Applied Energy 87 (2010) 3171–3177
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Enhancing biomethane production from flush dairy manure with turkey processing wastewater J.A. Ogejo *, L. Li Biological Systems Engineering Department, Virginia Tech, Blacksburg, VA 24061, USA
a r t i c l e
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Article history: Received 25 September 2009 Received in revised form 14 April 2010 Accepted 25 April 2010 Available online 21 May 2010 Keywords: Biogas Dairy manure Anaerobic digestion Co-digestion Turkey processing wastewater Poultry wastewater
a b s t r a c t The objective of this study was to assess the quantity and quality of biogas produced by co-digesting flushed dairy manure (FDM) and turkey processing wastewater (TPW). An attached growth digester with working volume of 15 L and a 3 L head space was operated at a 5 d hydraulic retention time using five feed mixes containing 100, 67, 50, 33, and 0% FDM by volume. The biogas yield ranged from 0.072 to 0.8 m3 [g VS1] and the methane content (quality) of the gas ranging from 56% to 70%. Both the quantity and quality of the biogas increased as the proportion of TPW in the feed increased. An energy balance for the digester based on a dairy farm with 150 animals, showed that augmenting FDM with TPW at 1:1 and 1:2 ratios, feeds C and D, respectively, produced biogas with net positive energy to all year round. The gas produced was enough to run a 50 kW generator to produce electricity for about 5.5 and 9 h for the 1:1 and 1:2 feed mixes. However, the economics were not favorable if the benefits of the digester are based only on the value electricity to be produced. Either, other possible revenues such as carbon credit, renewable energy credits, green tags for electricity, putting a value to the environmental benefits of AD should be considered or subsidies from grants or other incentives programs to make the system economically viable. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The search for non-fossil based alternative and secure renewable energy sources is of high priority in many economies around the world. In the US, there are several initiatives to increase the production and use of ‘‘home grown” from renewable sources as strategy to secure an economically and environmentally sustainable energy future. The ‘‘2525” campaign (http://www.25x25. org), is one such effort, with a vision to obtain 25% of the total energy consumed in the United States from agricultural sources by the year 2025, while still producing abundant, safe and affordable food, feed, and fiber, in an environmentally responsible and sound manner. One major agricultural source of biomass is dairy manure, from which energy can be obtained using anaerobic digestion (AD) technology. Anaerobic digestion is a proven technology which is commonly used to treat sludge at municipal wastewater treatment plants (WWTP) and high strength industrial wastes to convert organic matter to biomethane [1]. The AD technology is also being used in animal feeding operations (AFOs) to digest manure to provide: electricity and thermal energy, stable liquid fertilizer and a high quality solid soil amendment, odor reduction, reduction in * Corresponding author. Tel.: +1 540 231 6815; fax: +1 540 231 3199. E-mail address: [email protected] (J.A. Ogejo). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.04.020
ground and surface water contamination, and additional revenue potential from sales of digested manure (liquid and solids) and excess electricity and/or processing off-site organic waste [2]. The use of AD in dairy operations in the US is on the rise, however, the technology is purportedly feasible for dairies milking more than 500 cows due to the high capital investment cost of equipment and the low biomethane yield from manure per unit weight of manure, thus requiring more animals to produce adequate gas [3]. In general, a large fraction of fiber contained in dairy manure is highly recalcitrant to digestion and only about 36% of volatile solids are biodegradable [4,5]. Typical methane (CH4) yields ranging from 0.075 to 0.223 m3 CH4 kg1 VS have been reported for dairy manure with up to 7% total solids (TS) in various digester configurations has been reported [5–7]. Methods to increase biogas produced from dairy manure to make the economics of ADs favorable have included: co-digestion with other types of organic wastes, pre-treating dairy manure (chemical and physical methods), and providing longer retention times [7–12]. Also, multi-digester configuration (phased AD) separating the acid and methane producing phases during AD process have resulted in improved biogas production [13,14]. Co-digestion, the focus of this study, has been reported to enhance the AD process by creating better nutrient balance from the materials mixed to feed the digester and/or provide positive synergism for bacterial growth [15,16]. Specific to dairy manure, co-digestion with food
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waste [12], fruit and vegetable wastes [17], and municipal solid waste [18] have resulted in increased CH4 yield. El-Mashad and Zhang [12] obtained double CH4 yield food waste mixed with dairy manure was increased from 32% to 48%. Double CH4 yields have also been reported when the quantity of fruit and vegetable wastes co-digested with dairy manure was increased from 20% to 50% [17]. Hartmann and Ahring [18] reported over 50% increase in CH4 yields when the quantity of organic fraction of municipal solid waste co-digested with dairy manure was increased. Digestion or co-digestion of high solid waste streams including poultry processing waste by-products such as blood, meat and bones to produce biogas has successfully been demonstrated with the right kind of management and controls to produce increased biogas with methane yields of 0.52–0.55 m3 kg1 VS [19–22]. However, specific research on co-digestion of cow manure with poultry processing wash water generated from cleaning carcasses, equipment, and floors and typically containing some blood, fat from skin, and oils desorbed during scalding to remove feathers and immersion chilling [22–24], has not been reported to our knowledge. The objective of this study was to assess the quantity and quality of biogas produced by co-digesting dairy manure and wastewater from a turkey processing plant. The work reported here is specifically directed at identifying methods to make biogas production feasible on small dairies with an average of 150 milking cows. The Central Shenandoah Valley region, our target area, has several small dairy farms collocated with other organic matter sources, including poultry processing plants. The results obtained from this study provides useful information to consultants who design anaerobic digesters and farmers to assess the feasibility of implementing AD on small dairy farms which may be collocated with other organic material sources. This work is part of a broader study by our research group to develop a biomass inventory for the state of Virginia to assess their potential as alternative energy sources. 2. Materials and methods 2.1. Feed collection 2.1.1. Flushed dairy manure The manure was obtained from the dairy farm at Virginia Tech, Blacksburg, VA. The farm has a free stall barn where the manure accumulated on the floor was flushed four times a day using 150 m3 recycled effluent from the manure treatment system. Details of the farm and manure treatment system are described by Debusk et al. [25]. Briefly, manure treatment system consisted of a rotary press separator with mesh screen openings of 0.79 mm and brushes to move manure across the screen surfaces (Integrity Nutrient Control System, Nutrient Control Systems, Inc., Chambersburg, PA.), a settling tank, and a series of three circular above-ground concrete tanks in which are surface aerated for odor control. The FDM was collected at a location between the solids separator and the settling tank. The manure was collected every Monday, Wednesday, and Friday and brought to the Bioresidue Management and Utilization lab in the Biological Systems Engineering department, Virginia Tech, where the experiments were conducted. 2.1.2. Turkey processing wastewater The turkey processing wastewater (TPW) was obtained from a processing plant in central Shenandoah Valley, Virginia. The material flow through the processing plant is shown in Fig. 1. The TPW was collected from the waste stream at the location designated FC in Fig. 1, before the rotary screen separator (screen). The TPW was a mix of wastewater generated from cleaning at various locations
in the plant and the bird slaughter area. The wastewater was collected once every 2 months in 20 L buckets and stored in a freezer at 20 °C. When needed, a bucket was taken out a day prior to use and thawed at room temperature before feeding to the reactors. 2.2. Reactor design An attached growth reactor (Fig. 2) 0.25 m in diameter, 0.40 m tall with a 15 L working volume, and a 3 L headspace was used in the study. The reactor was made from a schedule 40 PVC pipe capped off with 0.95 cm thick PVC plates. The media in the reactor were polypropylene material 1.6 cm in diameter (Jaeger Products, Houston, Texas). An attached growth reactor was chosen to increase the surface for bacterial growth, resulting in higher bacteria biomass and solids retention time [26]. The reactor was operated at a 5 d HRT [27]. Three pumps (Masterflex, Model 7553-70 and 77200-62, Cole Parmer Instrument Co., Chicago, IL) were used to feed, mix, and discharge the reactor contents. The digester was heated using two heating blankets (Model SSHB-1212-360-120, Omega Engineering, Stamford, CT) wrapped around the tank. The sections of the reactors not covered by the heating blanket were insulated with fiberglass material (Model SP24, Thermwell Products Co., Mahwah, NJ). The digester temperature was maintained at 37 (±1) °C by a temperature controller (Model 11-463-47A, Fisher Scientific, Waltham, MA). The heating blanket temperature was set at 45 °C managed by a controller (Model CN79022, Omega Engineering, Stamford, CT). 2.2.1. Feed and digester start-up and operation Five feed mixtures designated A, B, C, D, and E containing 100%, 67%, 50%, 33%, and 0% FDM by volume, respectively, were used. Experimentation started with feed A (100% DM) followed by other feeds sequentially. The feed was adjusted by reducing FDM and increasing TPW volumes, accordingly. The digester was seeded with 9 L of anaerobic digester sludge from Christiansburg WWTP (Christiansburg, VA) and 6 L feed A at start-up. After seeding, the reactor was fed 0.75 L d1 of feed A for 35 days, increased to 1.5 L d1 on day 36 and 3.0 L d1 on day 41 to obtain the desired 5 d HRT. The reactor was fed and effluent discharged six times a day. Steady gas production was achieved on day 54. Data collection the test using feed A commenced on day 59. Experiments with each feed mix was conducted for at least 45 d. Data collection was started after 5 d (1 HRT) of each period and only the data from the last 40 d of a period were used in the analysis. 2.3. Sample collection and analysis The quantity of biogas produced was measured using a wet-tip gas meter (Rebel wet-tip gas meter company, Nashville, TN). Feed and effluent samples were collected once a week and analyzed for total solids (TS), volatile solids (VS), volatile fatty acids (VFA), and pH. The TS and VS were determined using standard methods for wastewater treatment [28]. The pH was measured using an Orion 5 Star pH/ISE/dissolved oxygen/conductivity, meter (Thermo Fisher Scientific, Fort Collins, CO, USA). The VFA was determined using the method described by Gungor et al. [29]. Every 2 weeks biogas samples were collected in 1 L Tedlar bags and analyzed for composition (CH4 and CO2) using a gas chromatograph (Model GC-14A, Shimadzu Scientific Instruments, Columbia, MD) equipped with thermal conductivity detector; a 1.8 m packed column with Haysept D packing, and nitrogen as a carrier gas. The feed and effluent samples were collected every two weeks and analyzed for COD and nutrients (total Kjeldahl nitrogen (TKN), TAN, potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), and sulfur (S)). The nutrient analyses were done at the Agricultural
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Fig. 1. Material flow through the turkey processing plant showing the feed collection (FC) location.
Fig. 2. The attached growth digester system showing components and material flow (1 – feed pump; 2 – mixing pump; and 3 – effluent pump).
Services Lab at Clemson University, South Carolina, using standard methods for wastewater analysis [28]. 2.4. Energy balance The energy balance was based on a dairy farm with 150 cows collocated with a poultry processing plant. The average liquid temperature is about 7 and 24 °C during the coldest and warmest periods of the year, respectively. A mesophilic digester operated at a temperature of 37 °C providing biogas to a 50 kW engine/generator to produce electricity with waste heat recovery at 35% and 55% efficiencies, respectively, and 10% of the heat is lost (unrecover-
able), was assumed. A simple energy balance was conducted to evaluate whether the total energy content of the biogas produced by the feed balanced the energy required to maintain the temperature of the reactor under mesophilic (37 °C) conditions. The difference in the two energy values (net energy) was used to determine the potential of co-digesting FDM and TPW as an energy source. The system was also assumed to have a regenerative heat exchanger to take the heat from the digester outflow and transfer it to the digester inflow at 70% efficiency [30,31]. A positive net energy is desirable for a feed mix to meet the first step of suitability criteria for biogas production. In general, the net energy (Enet) is estimated as shown in the following equation:
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Enet ¼ EP EC
ð1Þ
respectively. The characteristics of the mixed feeds B, C, and D were between the values of feeds A and E (Table 1).
1
where Enet is the net energy produced per day, kJ d ; Ep is the energy from the methane produced per day, kJ d1, calculated as the product of volume of methane (Vm) produced per day and the lower heating value (LHV = 35.8 kJ L1) of methane (Eq. (2)), and Ec is the energy consumed by the process.
EP ¼ V M LHV
3.2. Biogas yield The biogas yield was calculated and reported as the volume per unit mass of VS fed to the digester per day, as routinely done in the literature [10,12,14,18,32]. The biogas yields ranged from 0.072 to 0.8 m3 kg VS1 (Fig. 3) and was highest in 100% TPW (feed E) and lowest in 100% FDM (feed A). The biogas yield increased as the proportion of TPW in the feed increased. The biogas yield from feed E was approximately 10, 6, 3, and 2 times more compared to feeds A, B, C, and D, respectively. There was no significant difference in biogas yields from feeds A and B, but yields from feeds C, D, and E were significantly different from each other and feeds A and B. The biogas gas yields and the VS destruction decreased as proportion of FDM in the feed increased (Table 1). Feed E had higher VS reduction compared to the other feed mixes although the mass of VS fed was lowest when feed E was used. The higher biogas yields obtained with feeds C, D, and E with at least 50% TPW were consistent with findings by other studies [5,12,33]. Angelidaki and Ellegaard [5] noted that combining manure with waste streams with more easily degradable organic matter yielded higher biogas than manure alone. El-Mashad and Zhang [12] also observed
ð2Þ
The energy consumed (Ec) by the process is the sum of the energy (i) required to raise the feed and maintain the digester temperature (Ef), (ii) lost through the walls, floor and roof of the digester (Ew), and (iii) for mixing, feeding, and effluent pumps (Emisc). The energy, Ef, was calculated using Eq. (3) at 7 and 24 °C (45 and 75 °F) to reflect heating during the cold (December, January, and February) and warm (June, July, August, and September) months, respectively. The Ef was assumed to come from (i) a regenerative heat exchanger (HR), (ii) waste heat from the engine/generator (HW), and (iii) external source (HE).
Ef ¼ Q C P q ðT d T in Þ ðHW þ HR þ HE Þ
ð3Þ
where Q is the volume flow rate (m3 d1), Cp is the specific heat of the feed (4.186 kJ kg1 °C1), q is the density of the feed, Td is the digester operating temperature (37 °C), and Tin is the feed temperature (7 or 24 °C). 2.5. Statistical analysis JMPÒ 7.0.2 statistical software (SAS, Cary, NC) was used for statistical analyses. Analysis of variance (ANOVA) on biogas production, biogas yield, volatile solids destruction, and methane content of biogas were conducted. Multiple means comparisons were conducted using Tukey’s HSD to test statistical differences. Significance differences were reported at a of 0.05. 3. Results 3.1. Feed characteristics The characteristics of the raw (100% FDM and 100% TPW) and mixed feeds are presented in Table 1. In general, the TS, VS, pH, and the VFAs were higher in feed A (100% FDM) compared to feed E (100% TPW). The average TS for feeds A and E were 1.5% and 0.24%, respectively, with VS accounting for 60% and 80% of TS,
Fig. 3. Mean biogas yield per unit mass of volatile solids (VS) fed from different feed mixtures of flushed dairy manure (FDM) and turkey processing wastewater (TPW) (A – 100% FDM, B – 67% FDM, C – 50% FDM, D – 33% FDM, and E – 100% PPW).
Table 1 Mean characteristics of the feed and digester effluent pertinent to anaerobic digestion for the different feed mixtures of flushed dairy manure (FDM) and turkey processing wastewater (TPW) (A – 100% FDM, B – 67% FDM, C – 50% FDM, D – 33% FDM, and E – 100% PPW). Parameter
Digester Feed
Digester Effluent
A
B
C
D
E
A
B
C
D
E
TS (%)
1.5 (±0.17)
Total VFA, mg as HAc Acetate, mg as HAc Propionate, mg as HAc Iso-butyrate, mg as HAc Butyrate, mg as HAc Iso-valerate mg as HAc pH VS loading (g d1)
238 (±28) 209 (±25) 23 (±5) 2.4 (±0.6)
0.49 (±0.04) 0.36 (±0.03) 170 (±40) 131 (±31) 23 (±6) 4.2 (±1.0)
0.25 (±0.02) 0.20 (±0.02) 97 (±13) 57 (±6) 20 (±2) 3.5 (±0.4)
0.65 (±0.33) 92 (±14) 86 (±13) 4.3 (±1.5) 1.3 (±0.3)
0.63 (±0.22) 0.35 (±0.12) 62 (±12) 56 (±11) 3.9 (±1.6) 2.1 (±0.6)
0.45 (±0.15) 0.22 (±0.08) 31 (±7) 27 (±7) 2.7 (±0.5) 1.5 (±0.2)
0.34 (±0.2)
0.9 (±0.13)
0.78 (±0.11) 0.53 (±0.07) 169 (24) 133 (±17) 22 (±6) 5.4 (±0.5)
1.2 (±0.4)
VS (%)
0.86 (±0.07) 0.54 (±0.02) 230 (±32) 184 (±24) 30 (±8) 6.3 (±1.1)
0.18 (±0.13) 48 (±14) 36 (±9) 9.7 (±4.4) 2.5 (±0.4)
0.12 (±0.13) 0.06 (±0.03) 7 (±1.5) 7 (±1.5) nd nd
6.0 (±1.6) 2.7 (±0.3) 8.4 (±0.01) 25.4 (±1.49)
6.5 (±1.6) 3.6 (±0.9) 8.2 (±0.03) 16.3 (±0.18)
4.7 (±0.2) 4.4 (±0.1) 8.1 (±0.06) 15.9 (±1.1)
6.5 (±2.9) 2.4 (±0.2) 7.6 (±0.17) 10.9 (±0.7)
12.2 (±2.1) 5.0 (±0.3) 6.5 (±0.07) 6.2 (±0.43)
nd nd 7.8 (±0.02)
nd nd 7.5 (±0.04)
nd nd 7.4 (±0.07)
nd nd 7.1 (±0.06)
nd nd 7.0 (±0.1)
35 (±4.0) 56 (±0.5)
49 (±9.5) 66 (±0.6)
63 (±8.8) 66 (±0.5)
74 (±11.8) 68 (±5)
81 (±3.2) 70 (±5)
VS destroyed (%) Methane content (%) Total VFA = sum of acetate, propionate, iso-butyrate, butyrate, iso-valerate.
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increased biogas production when manure was augmented with food waste. These increases are due to increase in the amount of degradable material in the co-digestate added to dairy manure [5,12]. The higher reduction of VS using feed E compared to feed A is supported by the fact that TPW has more biodegradable matter compared to FDM. This observation can also be confirmed by calculating the biodegradability (B0) of feed A and E using Eq. (4) [4]:
B0 ¼
g VS destroyed as time for biodegradation goes to infinity g VS added ð4Þ
Eq. (4) predicts the biodegradability of feed A and E as 0.31, 0.81 g VS added g1 VS destroyed for the attached growth digester. The biodegradability of feed A was slightly lower than 0.36 reported by Hill [4] for dairy manure. This result is expected because the manure used in this study was flushed and more dilute than the manure used by Hill [4]. Also, the dairy manure used in this study was found to have volatile matter that is non biodegradable as result of recycling the flush liquid several times through the system [29]. Overall, the biodegradability of FDM was 38% of the biodegradability of TPW. The VS reductions were not different in the following feed comparisons A and B; B and C; and C, D, and E. Another parameter that can be used to explain the quantities of biogas produced is COD of the feed. Only CODs for feeds C, D, and E were reported and were approximately 4000 mg L1 (Table 2). The COD reductions were much higher for feed E, which is consistent with the biomethane production recorded. However caution should be used in interpreting COD information because there may be some inorganic constituents in the feed that may exert COD [1,33]. The quality (methane content) of the biogas ranged from 56% to 70% methane by volume (Table 1). The gas quality was higher with feed E and lowest with feed A suggesting that increasing the TPW in the digester feed increased the energy or fuel value of the biogas. The methane content from feed A was slightly lower than what some the studies [6,7] that used flushed dairy manure only have reported. The differences in methane content in this study with those reported by may be due to the VS content of the FDM used; 60% in this study and 72% in both Powers et al. [6] and Wen et al. [7]. One possible reason is may be because of the quality of flush liquid used in the studies; Powers et al. [6] and Wen et al. [7] used fresh dairy manure (as excreted) to simulate flush manure by dilution with tap water, while in this study, the liquid used for flushing was recycled treated manure containing volatile non biodegradable material as explained earlier. The average pH in the digesters for the different feeds ranged from 7.0 to 7.8. The average pH was lowest for the feed with 100% TPW (feed E) and highest for the 100% FDM (feed A). The pH
in the digester increased as the FDM component of the feed was increased. The accepted pH range for optimal growth of methanogens is between 6.5 and 8.5 [34]. Methane production under conditions outside this range is steeply decreased. The pH values recorded in this study were not out of the recommended range, thus, pH may not have been a limiting factor on the growth of the methanogens. The low VFA concentrations in the digester both in the digester feed and effluent decreased as the quantity of TPW in the feed increased corroborates the non acidic pH values reported. 3.3. Energy balance The energy balance for feed mixes that contained FDM is presented in Table 3. During the warm conditions, the net energy was 27%, 38%, 78%, and 71% of the total energy in the biogas produced for feeds A, B, C, and D, respectively. For cold weather conditions, the net energy were: 140%, 88%, 16%, and 13% of the energy in the biogas produced from feeds A, B, C, and D, respectively. The negative net energy means the digester will need external energy (HE) to maintain the 37 °C operating temperature. HE is zero for all the positive net energies. The amount of electricity that can be produced and the time to operate a 50 kW engine are also presented in Table 3. Based on the criteria set for feasibility, i.e. a need for positive net energy, then, operating the digester with feed mixes C and D, passes the test, with feed D providing enough biogas to generate electricity for 9 h in a day compared to 5.5 h for feed C. 3.4. Other nutrients Other nutrients are presented in Table 2 for feeds C, D, and E. The digester influent and effluent contains macro (N, P, K) and micro (Na, Ca, Mg, S, Zn, Cu, Mn) nutrients which have fertilizer value. With respect the operation of the AD, the TAN concentrations in the digester effluent were higher than the effluent and ranged from 100 to 190 mg L1. These TAN concentrations are far much lower
Table 3 Partial energy balance on the reactor. Feed
A B C D
Energy, kJ d1 Output (biogas produced)
Input (maintain digester temperature) Warm weather
Cold weather
5.21 105 1.03 106 2.81 106 4.56 106
3.53 105 5.81 105 4.88 105 1.12 106
1.22 106 1.88 106 2.23 106 3.76 106
KWH
Time to run engine, (h d1)
51 100 274 443
1 2 5.5 9
Table 2 Other digester feed and effluent characteristics for feeds C, D, and E containing 50% FDM, 33% FDM, and 100% PPW, respectively. Parameter
1
TKN, mg L TAN, mg L1 TP, mg L1 Ortho-P, mg L1 K, mg L1 Ca, mg L1 Mg, mg L1 S, mg L1 Zn, mg L1 Cu, mg L1 Mn, mg L1 Na, mg L1 COD, mg L1
Feed
Effluent
C
D
E
C
D
E
190 (±36) 53 (±29) 18 (±3) 10 (±4) 54 (±4) 26 (±10) 11 (±0.6) 23 (±4) 0.5 (±0.28) 0.1 (±0.03) 0.06 (±0.04) 78 (±4) 4000 (±130)
165 (±7) 20 (±0) 16 (±0) 7 (±1.4) 40 (±4) 25 (±11) 11 (±1.4) 24 (±10) 0.18 (±0.06) 0.1 (±0.03) 0.05 (±0.0) 69 (±17) 3930 (±555)
203 (±25) 100 (±35) 20 (±3) 9 (±1.7) 48 (±3) 44 (±14) 17 (±2.1) 20 (±7) 0.36 (±0.15) 0.1 (±0.01) 0.11 (±0.01) 60 (±5) 3910 (±568)
223 (±77) 187 (±55) 34 (±14) 15(±6) 170 (±159) 75 (±16) 36 (±29) 28 (±17) 0.62 (±0.56) 0.78 (±0.63) 0.44 (±0.11) 97 (±27) 2280 (±274)
160 (±28) 110 (±14) 15 (±0) 11 (±4) 41 (±0) 30 (±1) 11 (±0.7) 10 (±0) 0.28 (±0.01) 0.18 (±0.06) 0.19(±0.02) 69 (±1) 2240 (±482)
153 (±21) 100 (±30) 15 (±6) 9 (±5) 41 (±6) 24 (±5) 14 (±1) 11 (±3) 0.3 (±0.11) 0.16 (±0.09) 0.13 (±0.12) 56 (±9) 640 (±186)
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Table 4 Cost analysis on the digester based on electricity production from a 50 kW engine/ generator. Description
Capital cost Annual capital cost Annual operating cost Equipment maintenance and repair Labor Taxes and insurance Total annual costs Revenue from generated electricity
Feed mix C
D
300,000 21,300
315,000 22,290
1200 4380 1800 28,610 11,000
1500 4380 2250 30,420 17,800
than reported concentrations of >700 mg L1 known to inhibit biomethane in anaerobic digesters [35]. The TAN concentrations in the feed were lower than 100 mg L1 but it is not known why the TAN in feed D were lowest compared to C and E. There were minimal changes in all the other nutrients are reported for information as they may not directly have any impact on biogas production. 3.5. Costs The economics of an anaerobic digester is site specific and usually includes estimation of capital costs, operating expenses, and revenues from different products of anaerobic digestion. The cost analysis in this study are presented in Table 4 and was performed for AD systems fed mixes C and D which had positive net energy in both the cold and warm seasons. The cost analysis was based on interest in generation of electricity. The cost analysis was based on the following assumptions [36–38]: biogas is used to generate electricity using a 50 kW engine/generator with a conservative life of 20 years; low interest (4%) loan available to install the system with a payback period of 20 years; value of electricity produced of $0.11 per KWH; annual labor cost estimated at 1 h per day at the rate of $12/h; taxes and insurance at 3% of equipment cost; equipment repair and maintenance at 2% equipment cost. If the farm bases the value of digester on electricity produced only, then it will be operated at a loss of $17,610 and $12,615 annually using feeds C and D, respectively. The breakeven value of electricity for feeds C and D are $0.285 and $0.185, respectively. For the benefits to be positive, additional sources of income related to the AD should be developed or pursued including carbon credit, renewable energy credits, green tags for electricity, putting a value to the environmental benefits of AD, e.g. odor control, and possible sale of fiber from digested material for animal bedding. All these revenues may not be attainable at locations in Virginia because of policy and regulatory issues establishing them are not in place yet. An alternative to make the economics work would be to participate in grants or governmental incentive programs to help install digesters. For example, if a cost share program for building digesters supported 80% and 55% of the installation costs of systems using feeds C and D, then the AD systems would be viable when only the value of electricity derived from the digesters is considered. This however, is heavy subsidy and not a sustainable model. It should the biogas could be used to produce heat to generate hot water, however, the economics of that option was not considered in this study. 4. Conclusions The quality and quantity of biogas produced by co-digesting flushed dairy manure with turkey processing wastewater increased compared to digesting dairy manure alone. The biogas yield also increased as the proportion of TPW in the feed increased.
The energy balance for a dairy farm with 150 animals, showed that augmenting FDM with TPW at 1:1 and 1:2 ratios, feeds C and D, respectively, produced biogas with net positive energy to all year round. The gas produced was enough to run a 50 kW generator to produce electricity for about 5.5 and 9 h for the 1:1 and 1:2 feed mixes. The economics were not favorable if the benefits of the digester are based only on the electricity to be produced. Other possible revenues such as carbon credit, renewable energy credits, green tags for electricity, putting a value to the environmental benefits of AD should be considered. Also, if government incentives programs and grants are available, subsidizing over 55% of the digester installation costs may make the system economically viable.
Acknowledgements Funding for this work was provided by the Virginia Agricultural Council and Bioprocessing and Biodesign Center, Virginia Tech. References [1] Tchobanoglous G, Burton FL, Stensel HD. Wastewater engineering treatment and reuse. 4th ed. New York: McGraw Hill; 2004. [2] Ross CC, Walsh JL. Handbook of biogas utilization. 2nd ed. US Department of Energy; 1996 [contract TV 93503V]. [3] US EPA. Anaerobic digestion capital cost for dairy farms. The United States Environmental Protection Agency, AgSTAR Program. [accessed 12.09]. [4] Hill DT. Simplified Monod kinetics of methane fermentation of animal wastes. Agr Wastes 1983;5:1–16. [5] Angelidaki I, Ellegaard L. Codigestion of manure, organic wastes in centralized biogas plants. Appl Biochem Biotechnol 2000;109:95–105. [6] Powers WJ, Wilkie AC, Van Horn AC, Nordstedt RA. Effects of hydraulic retention time on performance and effluent odor of conventional and fixedfilm anaerobic digesters fed dairy manure wastewater. Trans ASAE 1997;40:1449–55. [7] Wen Z, Frear C, Chen S. Anaerobic digestion of liquid dairy manure using a sequential continuous-stirred tank reactor system. J Chem Technol Biotechnol 2007;82:758–66. [8] Nagle NJ, Rivard CJ, Adney WS, Himmel ME. Efficiency of hydrolytic enzyme augmentation and thermochemical pre-treatment for increased secondary anaerobic-digestion of treated municipal sewage sludge. Appl Biochem Biotechnol 1992;34–35:737–51. [9] Angelidaki I, Ahring BK. Methods for increasing the biogas potential from the recalcitrant organic matter contained in manure. Water Sci Technol 2000;41:189–94. [10] Hartmann H, Angelidaki I, Ahring BK. Increase of anaerobic degradation of particulate organic matter in full-scale biogas plants by mechanical maceration. Water Sci Technol 2000;41:145–53. [11] Nielsen HB, Mladenovska Z, Westermann P, Ahring BK. Comparison of twostage thermophilic (68 °C/55 °C) anaerobic digestion with one-stage thermophilic (55 °C) digestion of cattle manure. Biotechnol Bioeng 2004;86:291–300. [12] El-Mashad HM, Zhang RH. Co-digestion of food waste, dairy manure for biogas production. Trans ASAE 2007;50:1815–21. [13] Chugh S, Chynoweth DP, Clarke W, Pullammanappallil P, Rudolph V. Degradation of unsorted municipal solid waste by a leach-bed process. Bioresour Technol 1999;69:103–15. [14] Kaparaju P, Ellegaard L, Angelidaki I. Optimisation of biogas production from manure through serial digestion: lab-scale and pilot studies. Bioresour Technol 2009;100:701–9. [15] Mata-Alvarez J, Mace S, Llabres P. Anaerobic digestion of organic solid wastes: an overview of research achievements and perspectives. Bioresour Technol 2000;74:3–6. [16] Mshandete A, Kivaisi A, Rubindamayugi A, Mattiason B. Anaerobic digestion of sisal pulp, fish wastes. Bioresour Technol 2004;95:19–24. [17] Callaghan FJ, Wase DAJ, Thayanithy K, Forster CF. Continuous co-digestion of cattle slurry with fruit, vegetable wastes, chicken manure. Biomass Bioenergy 2002;22:71–7. [18] Hartman H, Ahring BK. Anaerobic digestion of the organic fraction of municipal solid waste: influence of co-digestion with manure. Water Res 2005;39:1543–52. [19] Hejnfelt A, Angelidaki I. Anaerobic digestion of slaughterhouse by-products. Biomass Bioenergy 2009;33:1046–54. [20] Fountoulakis MS, Drakopoulou S, Terzakis S, Georgaki E, Manios T. Potential for methane production from typical mediterranean agro-industrial by-products. Biomass Bioenergy 2008;32:155–61.
J.A. Ogejo, L. Li / Applied Energy 87 (2010) 3171–3177 [21] Salminen E, Rintala J. Anaerobic digestion of organic solid poultry sluagterhouse waste – a review. Bioresour Technol 2002;83:13–26. [22] Desai M, Patel V, Madamwar D. Effect of temperature, retention time on biomethanation of chees whey-poultry waste-cattle dung. Environ Pollut 1994;83:311–5. [23] del Nery V, de Nardi IR, Damianovic MHRZ, Pozzi E, Amorim AKB, Zaiat M. Long-term operation performance of a poultry slaughterhouse wastewater treatment plant. Resour Conserv Recycl 2007;50:102–14. [24] Johns MR. Developments in wastewater treatment in the meat processing industry: a review. Bioresour Technol 1995;54:203–16. [25] Debusk JA, Ogejo JA, Knowlton KF, Love NG. Chemical phosphorus removal for separated flushed dairy manure. Appl Eng Agr 2008;24(2008):499–506. [26] Grady CPL, Daigger GT, Lim HC. Biological wastewater treatment. 2nd ed. New York: CRC press; 1999. [27] NRCS. Anaerobic digester-controlled temperature. Natural resources conservation practice standard code 366. NRCS Iowa, USA; 2005. [28] APHA. Standard methods for the examination of water and wastewater. 19th ed. Washington (DC): American Public Health Association; 1995. [29] Gungor K, Muftugil MB, Ogejo JA, Knowlton KF, Love NG. Prefermentation of liquid dairy manure to support biological nutrient removal. Bioresour Technol 2009;100:2124–9. [30] Zupancic GD, Ros M. Heat, energy requirements in thermophilic anaerobic sludge digestion. Renew Energy 2003;28:2255–67. [31] Angelidaki I, Ellegaard L, Ahring BK. Applications of the anaerobic digestion process. Adv Biotechnol Eng/Biotechnol 2003;82:1–33.
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[32] Kaparaju P, Luostarinen S, Kalmari E, Kalmari J, Rintala J. Co-digestion of energy crops, industrial confectionery by-products with cow manure: batchscale and farm-scale evaluation. Water Sci Technol 2002;45:272–80. [33] Angelidaki I, Sanders W. Assessment of the anaerobic biodegradability of macropollutants. Rev Environ Sci Biotechnol 2004;3:117–29. [34] Speece RE. Anaerobic biotechnology for industrial wastewaters. Nashville (TN, USA): Archae Press; 1996. [35] Nielsen HB, Angelidaki I. Strategies for optimizing recovery of the biogas process following ammonia inhibition. Bioresour Technol 2008;99:7995–8001. [36] Abeles T, Freedman D, Ellswoth D, DeBaere L. Energy and economic assessment of anaerobic digesters for rural waste management. US EPA report #600778174; 2008. p. 171. [37] Lazarus WF. Farm-based anaerobic digesters as an energy and odor control technology: background and policy issues. Agricultural economic report no. 843, United States Department of Agriculture; 2008. p. 31. [accessed 03.10]. [38] Beddoes JC, Bracmort KS, Butns RT, Lazarus WF. An analysis of energy production costs from anaerobic digestion system on US livestock production facilities. Technical note no. 1. Natural Resource Conservation Service, United States Department of Agriculture; 2007. p. 31. [accessed 03.10].
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Enhancing biomethane production from flush dairy manure with turkey processing wastewater J.A. Ogejo *, L. Li Biological Systems Engineering Department, Virginia Tech, Blacksburg, VA 24061, USA
a r t i c l e
i n f o
Article history: Received 25 September 2009 Received in revised form 14 April 2010 Accepted 25 April 2010 Available online 21 May 2010 Keywords: Biogas Dairy manure Anaerobic digestion Co-digestion Turkey processing wastewater Poultry wastewater
a b s t r a c t The objective of this study was to assess the quantity and quality of biogas produced by co-digesting flushed dairy manure (FDM) and turkey processing wastewater (TPW). An attached growth digester with working volume of 15 L and a 3 L head space was operated at a 5 d hydraulic retention time using five feed mixes containing 100, 67, 50, 33, and 0% FDM by volume. The biogas yield ranged from 0.072 to 0.8 m3 [g VS1] and the methane content (quality) of the gas ranging from 56% to 70%. Both the quantity and quality of the biogas increased as the proportion of TPW in the feed increased. An energy balance for the digester based on a dairy farm with 150 animals, showed that augmenting FDM with TPW at 1:1 and 1:2 ratios, feeds C and D, respectively, produced biogas with net positive energy to all year round. The gas produced was enough to run a 50 kW generator to produce electricity for about 5.5 and 9 h for the 1:1 and 1:2 feed mixes. However, the economics were not favorable if the benefits of the digester are based only on the value electricity to be produced. Either, other possible revenues such as carbon credit, renewable energy credits, green tags for electricity, putting a value to the environmental benefits of AD should be considered or subsidies from grants or other incentives programs to make the system economically viable. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The search for non-fossil based alternative and secure renewable energy sources is of high priority in many economies around the world. In the US, there are several initiatives to increase the production and use of ‘‘home grown” from renewable sources as strategy to secure an economically and environmentally sustainable energy future. The ‘‘2525” campaign (http://www.25x25. org), is one such effort, with a vision to obtain 25% of the total energy consumed in the United States from agricultural sources by the year 2025, while still producing abundant, safe and affordable food, feed, and fiber, in an environmentally responsible and sound manner. One major agricultural source of biomass is dairy manure, from which energy can be obtained using anaerobic digestion (AD) technology. Anaerobic digestion is a proven technology which is commonly used to treat sludge at municipal wastewater treatment plants (WWTP) and high strength industrial wastes to convert organic matter to biomethane [1]. The AD technology is also being used in animal feeding operations (AFOs) to digest manure to provide: electricity and thermal energy, stable liquid fertilizer and a high quality solid soil amendment, odor reduction, reduction in * Corresponding author. Tel.: +1 540 231 6815; fax: +1 540 231 3199. E-mail address: [email protected] (J.A. Ogejo). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.04.020
ground and surface water contamination, and additional revenue potential from sales of digested manure (liquid and solids) and excess electricity and/or processing off-site organic waste [2]. The use of AD in dairy operations in the US is on the rise, however, the technology is purportedly feasible for dairies milking more than 500 cows due to the high capital investment cost of equipment and the low biomethane yield from manure per unit weight of manure, thus requiring more animals to produce adequate gas [3]. In general, a large fraction of fiber contained in dairy manure is highly recalcitrant to digestion and only about 36% of volatile solids are biodegradable [4,5]. Typical methane (CH4) yields ranging from 0.075 to 0.223 m3 CH4 kg1 VS have been reported for dairy manure with up to 7% total solids (TS) in various digester configurations has been reported [5–7]. Methods to increase biogas produced from dairy manure to make the economics of ADs favorable have included: co-digestion with other types of organic wastes, pre-treating dairy manure (chemical and physical methods), and providing longer retention times [7–12]. Also, multi-digester configuration (phased AD) separating the acid and methane producing phases during AD process have resulted in improved biogas production [13,14]. Co-digestion, the focus of this study, has been reported to enhance the AD process by creating better nutrient balance from the materials mixed to feed the digester and/or provide positive synergism for bacterial growth [15,16]. Specific to dairy manure, co-digestion with food
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waste [12], fruit and vegetable wastes [17], and municipal solid waste [18] have resulted in increased CH4 yield. El-Mashad and Zhang [12] obtained double CH4 yield food waste mixed with dairy manure was increased from 32% to 48%. Double CH4 yields have also been reported when the quantity of fruit and vegetable wastes co-digested with dairy manure was increased from 20% to 50% [17]. Hartmann and Ahring [18] reported over 50% increase in CH4 yields when the quantity of organic fraction of municipal solid waste co-digested with dairy manure was increased. Digestion or co-digestion of high solid waste streams including poultry processing waste by-products such as blood, meat and bones to produce biogas has successfully been demonstrated with the right kind of management and controls to produce increased biogas with methane yields of 0.52–0.55 m3 kg1 VS [19–22]. However, specific research on co-digestion of cow manure with poultry processing wash water generated from cleaning carcasses, equipment, and floors and typically containing some blood, fat from skin, and oils desorbed during scalding to remove feathers and immersion chilling [22–24], has not been reported to our knowledge. The objective of this study was to assess the quantity and quality of biogas produced by co-digesting dairy manure and wastewater from a turkey processing plant. The work reported here is specifically directed at identifying methods to make biogas production feasible on small dairies with an average of 150 milking cows. The Central Shenandoah Valley region, our target area, has several small dairy farms collocated with other organic matter sources, including poultry processing plants. The results obtained from this study provides useful information to consultants who design anaerobic digesters and farmers to assess the feasibility of implementing AD on small dairy farms which may be collocated with other organic material sources. This work is part of a broader study by our research group to develop a biomass inventory for the state of Virginia to assess their potential as alternative energy sources. 2. Materials and methods 2.1. Feed collection 2.1.1. Flushed dairy manure The manure was obtained from the dairy farm at Virginia Tech, Blacksburg, VA. The farm has a free stall barn where the manure accumulated on the floor was flushed four times a day using 150 m3 recycled effluent from the manure treatment system. Details of the farm and manure treatment system are described by Debusk et al. [25]. Briefly, manure treatment system consisted of a rotary press separator with mesh screen openings of 0.79 mm and brushes to move manure across the screen surfaces (Integrity Nutrient Control System, Nutrient Control Systems, Inc., Chambersburg, PA.), a settling tank, and a series of three circular above-ground concrete tanks in which are surface aerated for odor control. The FDM was collected at a location between the solids separator and the settling tank. The manure was collected every Monday, Wednesday, and Friday and brought to the Bioresidue Management and Utilization lab in the Biological Systems Engineering department, Virginia Tech, where the experiments were conducted. 2.1.2. Turkey processing wastewater The turkey processing wastewater (TPW) was obtained from a processing plant in central Shenandoah Valley, Virginia. The material flow through the processing plant is shown in Fig. 1. The TPW was collected from the waste stream at the location designated FC in Fig. 1, before the rotary screen separator (screen). The TPW was a mix of wastewater generated from cleaning at various locations
in the plant and the bird slaughter area. The wastewater was collected once every 2 months in 20 L buckets and stored in a freezer at 20 °C. When needed, a bucket was taken out a day prior to use and thawed at room temperature before feeding to the reactors. 2.2. Reactor design An attached growth reactor (Fig. 2) 0.25 m in diameter, 0.40 m tall with a 15 L working volume, and a 3 L headspace was used in the study. The reactor was made from a schedule 40 PVC pipe capped off with 0.95 cm thick PVC plates. The media in the reactor were polypropylene material 1.6 cm in diameter (Jaeger Products, Houston, Texas). An attached growth reactor was chosen to increase the surface for bacterial growth, resulting in higher bacteria biomass and solids retention time [26]. The reactor was operated at a 5 d HRT [27]. Three pumps (Masterflex, Model 7553-70 and 77200-62, Cole Parmer Instrument Co., Chicago, IL) were used to feed, mix, and discharge the reactor contents. The digester was heated using two heating blankets (Model SSHB-1212-360-120, Omega Engineering, Stamford, CT) wrapped around the tank. The sections of the reactors not covered by the heating blanket were insulated with fiberglass material (Model SP24, Thermwell Products Co., Mahwah, NJ). The digester temperature was maintained at 37 (±1) °C by a temperature controller (Model 11-463-47A, Fisher Scientific, Waltham, MA). The heating blanket temperature was set at 45 °C managed by a controller (Model CN79022, Omega Engineering, Stamford, CT). 2.2.1. Feed and digester start-up and operation Five feed mixtures designated A, B, C, D, and E containing 100%, 67%, 50%, 33%, and 0% FDM by volume, respectively, were used. Experimentation started with feed A (100% DM) followed by other feeds sequentially. The feed was adjusted by reducing FDM and increasing TPW volumes, accordingly. The digester was seeded with 9 L of anaerobic digester sludge from Christiansburg WWTP (Christiansburg, VA) and 6 L feed A at start-up. After seeding, the reactor was fed 0.75 L d1 of feed A for 35 days, increased to 1.5 L d1 on day 36 and 3.0 L d1 on day 41 to obtain the desired 5 d HRT. The reactor was fed and effluent discharged six times a day. Steady gas production was achieved on day 54. Data collection the test using feed A commenced on day 59. Experiments with each feed mix was conducted for at least 45 d. Data collection was started after 5 d (1 HRT) of each period and only the data from the last 40 d of a period were used in the analysis. 2.3. Sample collection and analysis The quantity of biogas produced was measured using a wet-tip gas meter (Rebel wet-tip gas meter company, Nashville, TN). Feed and effluent samples were collected once a week and analyzed for total solids (TS), volatile solids (VS), volatile fatty acids (VFA), and pH. The TS and VS were determined using standard methods for wastewater treatment [28]. The pH was measured using an Orion 5 Star pH/ISE/dissolved oxygen/conductivity, meter (Thermo Fisher Scientific, Fort Collins, CO, USA). The VFA was determined using the method described by Gungor et al. [29]. Every 2 weeks biogas samples were collected in 1 L Tedlar bags and analyzed for composition (CH4 and CO2) using a gas chromatograph (Model GC-14A, Shimadzu Scientific Instruments, Columbia, MD) equipped with thermal conductivity detector; a 1.8 m packed column with Haysept D packing, and nitrogen as a carrier gas. The feed and effluent samples were collected every two weeks and analyzed for COD and nutrients (total Kjeldahl nitrogen (TKN), TAN, potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), and sulfur (S)). The nutrient analyses were done at the Agricultural
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Fig. 1. Material flow through the turkey processing plant showing the feed collection (FC) location.
Fig. 2. The attached growth digester system showing components and material flow (1 – feed pump; 2 – mixing pump; and 3 – effluent pump).
Services Lab at Clemson University, South Carolina, using standard methods for wastewater analysis [28]. 2.4. Energy balance The energy balance was based on a dairy farm with 150 cows collocated with a poultry processing plant. The average liquid temperature is about 7 and 24 °C during the coldest and warmest periods of the year, respectively. A mesophilic digester operated at a temperature of 37 °C providing biogas to a 50 kW engine/generator to produce electricity with waste heat recovery at 35% and 55% efficiencies, respectively, and 10% of the heat is lost (unrecover-
able), was assumed. A simple energy balance was conducted to evaluate whether the total energy content of the biogas produced by the feed balanced the energy required to maintain the temperature of the reactor under mesophilic (37 °C) conditions. The difference in the two energy values (net energy) was used to determine the potential of co-digesting FDM and TPW as an energy source. The system was also assumed to have a regenerative heat exchanger to take the heat from the digester outflow and transfer it to the digester inflow at 70% efficiency [30,31]. A positive net energy is desirable for a feed mix to meet the first step of suitability criteria for biogas production. In general, the net energy (Enet) is estimated as shown in the following equation:
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Enet ¼ EP EC
ð1Þ
respectively. The characteristics of the mixed feeds B, C, and D were between the values of feeds A and E (Table 1).
1
where Enet is the net energy produced per day, kJ d ; Ep is the energy from the methane produced per day, kJ d1, calculated as the product of volume of methane (Vm) produced per day and the lower heating value (LHV = 35.8 kJ L1) of methane (Eq. (2)), and Ec is the energy consumed by the process.
EP ¼ V M LHV
3.2. Biogas yield The biogas yield was calculated and reported as the volume per unit mass of VS fed to the digester per day, as routinely done in the literature [10,12,14,18,32]. The biogas yields ranged from 0.072 to 0.8 m3 kg VS1 (Fig. 3) and was highest in 100% TPW (feed E) and lowest in 100% FDM (feed A). The biogas yield increased as the proportion of TPW in the feed increased. The biogas yield from feed E was approximately 10, 6, 3, and 2 times more compared to feeds A, B, C, and D, respectively. There was no significant difference in biogas yields from feeds A and B, but yields from feeds C, D, and E were significantly different from each other and feeds A and B. The biogas gas yields and the VS destruction decreased as proportion of FDM in the feed increased (Table 1). Feed E had higher VS reduction compared to the other feed mixes although the mass of VS fed was lowest when feed E was used. The higher biogas yields obtained with feeds C, D, and E with at least 50% TPW were consistent with findings by other studies [5,12,33]. Angelidaki and Ellegaard [5] noted that combining manure with waste streams with more easily degradable organic matter yielded higher biogas than manure alone. El-Mashad and Zhang [12] also observed
ð2Þ
The energy consumed (Ec) by the process is the sum of the energy (i) required to raise the feed and maintain the digester temperature (Ef), (ii) lost through the walls, floor and roof of the digester (Ew), and (iii) for mixing, feeding, and effluent pumps (Emisc). The energy, Ef, was calculated using Eq. (3) at 7 and 24 °C (45 and 75 °F) to reflect heating during the cold (December, January, and February) and warm (June, July, August, and September) months, respectively. The Ef was assumed to come from (i) a regenerative heat exchanger (HR), (ii) waste heat from the engine/generator (HW), and (iii) external source (HE).
Ef ¼ Q C P q ðT d T in Þ ðHW þ HR þ HE Þ
ð3Þ
where Q is the volume flow rate (m3 d1), Cp is the specific heat of the feed (4.186 kJ kg1 °C1), q is the density of the feed, Td is the digester operating temperature (37 °C), and Tin is the feed temperature (7 or 24 °C). 2.5. Statistical analysis JMPÒ 7.0.2 statistical software (SAS, Cary, NC) was used for statistical analyses. Analysis of variance (ANOVA) on biogas production, biogas yield, volatile solids destruction, and methane content of biogas were conducted. Multiple means comparisons were conducted using Tukey’s HSD to test statistical differences. Significance differences were reported at a of 0.05. 3. Results 3.1. Feed characteristics The characteristics of the raw (100% FDM and 100% TPW) and mixed feeds are presented in Table 1. In general, the TS, VS, pH, and the VFAs were higher in feed A (100% FDM) compared to feed E (100% TPW). The average TS for feeds A and E were 1.5% and 0.24%, respectively, with VS accounting for 60% and 80% of TS,
Fig. 3. Mean biogas yield per unit mass of volatile solids (VS) fed from different feed mixtures of flushed dairy manure (FDM) and turkey processing wastewater (TPW) (A – 100% FDM, B – 67% FDM, C – 50% FDM, D – 33% FDM, and E – 100% PPW).
Table 1 Mean characteristics of the feed and digester effluent pertinent to anaerobic digestion for the different feed mixtures of flushed dairy manure (FDM) and turkey processing wastewater (TPW) (A – 100% FDM, B – 67% FDM, C – 50% FDM, D – 33% FDM, and E – 100% PPW). Parameter
Digester Feed
Digester Effluent
A
B
C
D
E
A
B
C
D
E
TS (%)
1.5 (±0.17)
Total VFA, mg as HAc Acetate, mg as HAc Propionate, mg as HAc Iso-butyrate, mg as HAc Butyrate, mg as HAc Iso-valerate mg as HAc pH VS loading (g d1)
238 (±28) 209 (±25) 23 (±5) 2.4 (±0.6)
0.49 (±0.04) 0.36 (±0.03) 170 (±40) 131 (±31) 23 (±6) 4.2 (±1.0)
0.25 (±0.02) 0.20 (±0.02) 97 (±13) 57 (±6) 20 (±2) 3.5 (±0.4)
0.65 (±0.33) 92 (±14) 86 (±13) 4.3 (±1.5) 1.3 (±0.3)
0.63 (±0.22) 0.35 (±0.12) 62 (±12) 56 (±11) 3.9 (±1.6) 2.1 (±0.6)
0.45 (±0.15) 0.22 (±0.08) 31 (±7) 27 (±7) 2.7 (±0.5) 1.5 (±0.2)
0.34 (±0.2)
0.9 (±0.13)
0.78 (±0.11) 0.53 (±0.07) 169 (24) 133 (±17) 22 (±6) 5.4 (±0.5)
1.2 (±0.4)
VS (%)
0.86 (±0.07) 0.54 (±0.02) 230 (±32) 184 (±24) 30 (±8) 6.3 (±1.1)
0.18 (±0.13) 48 (±14) 36 (±9) 9.7 (±4.4) 2.5 (±0.4)
0.12 (±0.13) 0.06 (±0.03) 7 (±1.5) 7 (±1.5) nd nd
6.0 (±1.6) 2.7 (±0.3) 8.4 (±0.01) 25.4 (±1.49)
6.5 (±1.6) 3.6 (±0.9) 8.2 (±0.03) 16.3 (±0.18)
4.7 (±0.2) 4.4 (±0.1) 8.1 (±0.06) 15.9 (±1.1)
6.5 (±2.9) 2.4 (±0.2) 7.6 (±0.17) 10.9 (±0.7)
12.2 (±2.1) 5.0 (±0.3) 6.5 (±0.07) 6.2 (±0.43)
nd nd 7.8 (±0.02)
nd nd 7.5 (±0.04)
nd nd 7.4 (±0.07)
nd nd 7.1 (±0.06)
nd nd 7.0 (±0.1)
35 (±4.0) 56 (±0.5)
49 (±9.5) 66 (±0.6)
63 (±8.8) 66 (±0.5)
74 (±11.8) 68 (±5)
81 (±3.2) 70 (±5)
VS destroyed (%) Methane content (%) Total VFA = sum of acetate, propionate, iso-butyrate, butyrate, iso-valerate.
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increased biogas production when manure was augmented with food waste. These increases are due to increase in the amount of degradable material in the co-digestate added to dairy manure [5,12]. The higher reduction of VS using feed E compared to feed A is supported by the fact that TPW has more biodegradable matter compared to FDM. This observation can also be confirmed by calculating the biodegradability (B0) of feed A and E using Eq. (4) [4]:
B0 ¼
g VS destroyed as time for biodegradation goes to infinity g VS added ð4Þ
Eq. (4) predicts the biodegradability of feed A and E as 0.31, 0.81 g VS added g1 VS destroyed for the attached growth digester. The biodegradability of feed A was slightly lower than 0.36 reported by Hill [4] for dairy manure. This result is expected because the manure used in this study was flushed and more dilute than the manure used by Hill [4]. Also, the dairy manure used in this study was found to have volatile matter that is non biodegradable as result of recycling the flush liquid several times through the system [29]. Overall, the biodegradability of FDM was 38% of the biodegradability of TPW. The VS reductions were not different in the following feed comparisons A and B; B and C; and C, D, and E. Another parameter that can be used to explain the quantities of biogas produced is COD of the feed. Only CODs for feeds C, D, and E were reported and were approximately 4000 mg L1 (Table 2). The COD reductions were much higher for feed E, which is consistent with the biomethane production recorded. However caution should be used in interpreting COD information because there may be some inorganic constituents in the feed that may exert COD [1,33]. The quality (methane content) of the biogas ranged from 56% to 70% methane by volume (Table 1). The gas quality was higher with feed E and lowest with feed A suggesting that increasing the TPW in the digester feed increased the energy or fuel value of the biogas. The methane content from feed A was slightly lower than what some the studies [6,7] that used flushed dairy manure only have reported. The differences in methane content in this study with those reported by may be due to the VS content of the FDM used; 60% in this study and 72% in both Powers et al. [6] and Wen et al. [7]. One possible reason is may be because of the quality of flush liquid used in the studies; Powers et al. [6] and Wen et al. [7] used fresh dairy manure (as excreted) to simulate flush manure by dilution with tap water, while in this study, the liquid used for flushing was recycled treated manure containing volatile non biodegradable material as explained earlier. The average pH in the digesters for the different feeds ranged from 7.0 to 7.8. The average pH was lowest for the feed with 100% TPW (feed E) and highest for the 100% FDM (feed A). The pH
in the digester increased as the FDM component of the feed was increased. The accepted pH range for optimal growth of methanogens is between 6.5 and 8.5 [34]. Methane production under conditions outside this range is steeply decreased. The pH values recorded in this study were not out of the recommended range, thus, pH may not have been a limiting factor on the growth of the methanogens. The low VFA concentrations in the digester both in the digester feed and effluent decreased as the quantity of TPW in the feed increased corroborates the non acidic pH values reported. 3.3. Energy balance The energy balance for feed mixes that contained FDM is presented in Table 3. During the warm conditions, the net energy was 27%, 38%, 78%, and 71% of the total energy in the biogas produced for feeds A, B, C, and D, respectively. For cold weather conditions, the net energy were: 140%, 88%, 16%, and 13% of the energy in the biogas produced from feeds A, B, C, and D, respectively. The negative net energy means the digester will need external energy (HE) to maintain the 37 °C operating temperature. HE is zero for all the positive net energies. The amount of electricity that can be produced and the time to operate a 50 kW engine are also presented in Table 3. Based on the criteria set for feasibility, i.e. a need for positive net energy, then, operating the digester with feed mixes C and D, passes the test, with feed D providing enough biogas to generate electricity for 9 h in a day compared to 5.5 h for feed C. 3.4. Other nutrients Other nutrients are presented in Table 2 for feeds C, D, and E. The digester influent and effluent contains macro (N, P, K) and micro (Na, Ca, Mg, S, Zn, Cu, Mn) nutrients which have fertilizer value. With respect the operation of the AD, the TAN concentrations in the digester effluent were higher than the effluent and ranged from 100 to 190 mg L1. These TAN concentrations are far much lower
Table 3 Partial energy balance on the reactor. Feed
A B C D
Energy, kJ d1 Output (biogas produced)
Input (maintain digester temperature) Warm weather
Cold weather
5.21 105 1.03 106 2.81 106 4.56 106
3.53 105 5.81 105 4.88 105 1.12 106
1.22 106 1.88 106 2.23 106 3.76 106
KWH
Time to run engine, (h d1)
51 100 274 443
1 2 5.5 9
Table 2 Other digester feed and effluent characteristics for feeds C, D, and E containing 50% FDM, 33% FDM, and 100% PPW, respectively. Parameter
1
TKN, mg L TAN, mg L1 TP, mg L1 Ortho-P, mg L1 K, mg L1 Ca, mg L1 Mg, mg L1 S, mg L1 Zn, mg L1 Cu, mg L1 Mn, mg L1 Na, mg L1 COD, mg L1
Feed
Effluent
C
D
E
C
D
E
190 (±36) 53 (±29) 18 (±3) 10 (±4) 54 (±4) 26 (±10) 11 (±0.6) 23 (±4) 0.5 (±0.28) 0.1 (±0.03) 0.06 (±0.04) 78 (±4) 4000 (±130)
165 (±7) 20 (±0) 16 (±0) 7 (±1.4) 40 (±4) 25 (±11) 11 (±1.4) 24 (±10) 0.18 (±0.06) 0.1 (±0.03) 0.05 (±0.0) 69 (±17) 3930 (±555)
203 (±25) 100 (±35) 20 (±3) 9 (±1.7) 48 (±3) 44 (±14) 17 (±2.1) 20 (±7) 0.36 (±0.15) 0.1 (±0.01) 0.11 (±0.01) 60 (±5) 3910 (±568)
223 (±77) 187 (±55) 34 (±14) 15(±6) 170 (±159) 75 (±16) 36 (±29) 28 (±17) 0.62 (±0.56) 0.78 (±0.63) 0.44 (±0.11) 97 (±27) 2280 (±274)
160 (±28) 110 (±14) 15 (±0) 11 (±4) 41 (±0) 30 (±1) 11 (±0.7) 10 (±0) 0.28 (±0.01) 0.18 (±0.06) 0.19(±0.02) 69 (±1) 2240 (±482)
153 (±21) 100 (±30) 15 (±6) 9 (±5) 41 (±6) 24 (±5) 14 (±1) 11 (±3) 0.3 (±0.11) 0.16 (±0.09) 0.13 (±0.12) 56 (±9) 640 (±186)
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Table 4 Cost analysis on the digester based on electricity production from a 50 kW engine/ generator. Description
Capital cost Annual capital cost Annual operating cost Equipment maintenance and repair Labor Taxes and insurance Total annual costs Revenue from generated electricity
Feed mix C
D
300,000 21,300
315,000 22,290
1200 4380 1800 28,610 11,000
1500 4380 2250 30,420 17,800
than reported concentrations of >700 mg L1 known to inhibit biomethane in anaerobic digesters [35]. The TAN concentrations in the feed were lower than 100 mg L1 but it is not known why the TAN in feed D were lowest compared to C and E. There were minimal changes in all the other nutrients are reported for information as they may not directly have any impact on biogas production. 3.5. Costs The economics of an anaerobic digester is site specific and usually includes estimation of capital costs, operating expenses, and revenues from different products of anaerobic digestion. The cost analysis in this study are presented in Table 4 and was performed for AD systems fed mixes C and D which had positive net energy in both the cold and warm seasons. The cost analysis was based on interest in generation of electricity. The cost analysis was based on the following assumptions [36–38]: biogas is used to generate electricity using a 50 kW engine/generator with a conservative life of 20 years; low interest (4%) loan available to install the system with a payback period of 20 years; value of electricity produced of $0.11 per KWH; annual labor cost estimated at 1 h per day at the rate of $12/h; taxes and insurance at 3% of equipment cost; equipment repair and maintenance at 2% equipment cost. If the farm bases the value of digester on electricity produced only, then it will be operated at a loss of $17,610 and $12,615 annually using feeds C and D, respectively. The breakeven value of electricity for feeds C and D are $0.285 and $0.185, respectively. For the benefits to be positive, additional sources of income related to the AD should be developed or pursued including carbon credit, renewable energy credits, green tags for electricity, putting a value to the environmental benefits of AD, e.g. odor control, and possible sale of fiber from digested material for animal bedding. All these revenues may not be attainable at locations in Virginia because of policy and regulatory issues establishing them are not in place yet. An alternative to make the economics work would be to participate in grants or governmental incentive programs to help install digesters. For example, if a cost share program for building digesters supported 80% and 55% of the installation costs of systems using feeds C and D, then the AD systems would be viable when only the value of electricity derived from the digesters is considered. This however, is heavy subsidy and not a sustainable model. It should the biogas could be used to produce heat to generate hot water, however, the economics of that option was not considered in this study. 4. Conclusions The quality and quantity of biogas produced by co-digesting flushed dairy manure with turkey processing wastewater increased compared to digesting dairy manure alone. The biogas yield also increased as the proportion of TPW in the feed increased.
The energy balance for a dairy farm with 150 animals, showed that augmenting FDM with TPW at 1:1 and 1:2 ratios, feeds C and D, respectively, produced biogas with net positive energy to all year round. The gas produced was enough to run a 50 kW generator to produce electricity for about 5.5 and 9 h for the 1:1 and 1:2 feed mixes. The economics were not favorable if the benefits of the digester are based only on the electricity to be produced. Other possible revenues such as carbon credit, renewable energy credits, green tags for electricity, putting a value to the environmental benefits of AD should be considered. Also, if government incentives programs and grants are available, subsidizing over 55% of the digester installation costs may make the system economically viable.
Acknowledgements Funding for this work was provided by the Virginia Agricultural Council and Bioprocessing and Biodesign Center, Virginia Tech. References [1] Tchobanoglous G, Burton FL, Stensel HD. Wastewater engineering treatment and reuse. 4th ed. New York: McGraw Hill; 2004. [2] Ross CC, Walsh JL. Handbook of biogas utilization. 2nd ed. US Department of Energy; 1996 [contract TV 93503V]. [3] US EPA. Anaerobic digestion capital cost for dairy farms. The United States Environmental Protection Agency, AgSTAR Program.
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