Co-digestion of pig slaughterhouse waste with sewage sludge

Waste Management 40 (2015) 119–126

Contents lists available at ScienceDirect

Waste Management journal homepage: www.elsevier.com/locate/wasman

Co-digestion of pig slaughterhouse waste with sewage sludge Sebastian Borowski a,⇑, Przemysław Kubacki b a b

´ ska 171/173, 90-924 Łódz´, Poland Lodz University of Technology, Institute of Fermentation Technology and Microbiology, Wólczan ´ ska 213, 90-924 Łódz´, Poland Lodz University of Technology, Faculty of Process and Environmental Engineering, Wólczan

a r t i c l e

i n f o

Article history: Received 7 January 2015 Accepted 14 March 2015 Available online 31 March 2015 Keywords: Anaerobic digestion Sewage sludge Animal byproducts Slaughterhouse waste

a b s t r a c t Slaughterhouse wastes (SHW) are potentially very attractive substrates for biogas production. However, mono-digestion of these wastes creates great technological problems associated with the inhibitory effects of ammonia and fatty acids on methanogens as well as with the foaming in the digesters. In the following study, the co-digestion of slaughterhouse wastes with sewage sludge (SS) was undertaken. Batch and semi-continuous experiments were performed at 35 °C with municipal sewage sludge and pig SHW composed of meat tissue, intestines, bristles and post-flotation sludge. In batch assays, meat tissue and intestinal wastes gave the highest methane productions of 976 and 826 dm3/kg VS, respectively, whereas the methane yield from the sludge was only 370 dm3/kg VS. The co-digestion of sewage sludge with 50% SHW (weight basis) provided the methane yield exceeding 600 dm3/kg VS, which was more than twice as high as the methane production from sewage sludge alone. However, when the loading rate exceeded 4 kg VS/m3 d, a slight inhibition of methanogenesis was observed, without affecting the digester stability. The experiments showed that the co-digestion of sewage sludge with large amount of slaughterhouse wastes is feasible, and the enhanced methane production does not affect the digester stability. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Production and processing of meat in Poland constitutes the largest branch of the national food economy. According to the Central Statistical Office, in 2013 Poland produced 5206 thousand tons (in live weight) of animals for slaughter, of which 2059 thousand tons were pigs (Witkowski, 2014). It is estimated that approximately 25% of the total animal weight slaughtered is not used for food consumption (Hejnfelt and Angelidaki, 2009). This gives around 500,000 tons of slaughterhouse wastes annually generated in Poland. Most slaughterhouse wastes belong to category 2 of animal byproducts, as classified by the Animal By-Product Regulation of European Union, ABPR 1069/2009/EC (EC regulation, 2009). Materials of this category may be processed in biogas plants only after sterilization ongoing at least 20 min without interruption at a core temperature of more than 133 °C and an absolute steam pressure of not less than 3 bar. Animal by-products are potentially very attractive substrates for biogas production mainly due to the high lipid content. The theoretical biogas yield from fats reaches 1250 dm3/kg TS with around 67–68% methane content, whereas the corresponding ⇑ Corresponding author. Tel.: +48 42 6313484. E-mail addresses: [email protected] (S. Borowski), [email protected] (P. Kubacki). http://dx.doi.org/10.1016/j.wasman.2015.03.021 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

values for carbohydrates are 790–800 dm3/kg TS and 50% CH4 (Weiland, 2010). Edstorm et al. (2003) estimated the total biogas potential from waste generated during slaughter of 1300 MJ/cattle and 140 MJ/pig. However, the high solids and nitrogen contents usually preclude the anaerobic treatment of slaughterhouse wastes in their original undiluted form. The anaerobic treatment of these wastes as mono-substrates often leads to the accumulation of ammonia, volatile fatty acids (VFA) and long chain fatty acids (LCFA), which may inhibit methanogenesis and thereby lower or even cease biogas production. Moreover, a high concentration of lipids in SHW may lead to foam formation and sludge flotation in anaerobic digesters as the lipids are adsorbed onto biomass (Cuetos et al., 2008; Hejnfelt and Angelidaki, 2009; Pitk et al., 2013; Salminen and Rintala, 2002). A solution of this problem may be the co-digestion of slaughterhouse wastes with other waste types. Co-digestion is defined as the simultaneous anaerobic treatment of two or more substrates usually of different characteristics in order to improve biogas and methane production. Other benefits achieved by co-digestion include: an improved balance of macroand micronutrients, dilution of inhibitory or/and toxic substances, increased digestion and stabilization rates (greater VS reduction rates), and often an increased organic loading rate (OLR). Moreover, co-digestion allows to reduce the costs of biogas production as different wastes can jointly be processed in one installation (Mata-Alvarez et al., 2014). Slaughterhouse wastes

120

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126

have been successfully co-digested with other organic wastes including pig manure (Alvarez and Liden, 2008; Edstorm et al., 2003; Hejnfelt and Angelidaki, 2009), fruit and vegetable waste (Alvarez and Liden, 2008), organic fraction of municipal solid waste (Cuetos et al., 2008; Zhang and Banks, 2012), pharmaceutical waste (Braun et al., 2003), rendering waste (Bayr et al., 2012) and food waste (Braun et al., 2003; Edstorm et al., 2003). In Poland, most biogas installations have been designed to treat sewage sludge generating at wastewater treatment plants (WWTP). It has been demonstrated that anaerobic digesters located at WWTPs are often oversized, thus providing a free digestion capacity of 15–30% (Braun et al., 2003; Mata-Alvarez et al., 2014). Therefore, treatment of animal by-products together with sewage sludge may provide additional incomes for these plants due to the increased biogas production and gate fees (Luostarinen et al., 2009; Luste and Luostarinen, 2010). The feasibility of co-digestion of sewage sludge with waste grease (as a substrate of similar characteristics to SHW) was demonstrated in the WWTP in Brzeg (Gazda et al., 2012). However, little information is available in the literature regarding the co-digestion of SHW with sewage sludge (Luste and Luostarinen, 2010; Pitk et al., 2013). Generally the cited authors have focused on cattle and bovine manure or its mixture with pig manure, and the manure-to-sludge ratios in the feed were rather low. Therefore, in the following study, the biogas and methane yields of various types of pig slaughterhouse wastes as well as municipal sewage sludge were determined in batch assays. Then the semi-continuous experiments were conducted to assess the feasibility of SHW co-digestion with municipal sewage sludge in order to improve biogas and methane production and to provide stable digester operation. The emphasis was also made to the effect of the co-digestion on digestate quality and the fate of ammonia and volatile fatty acids during the semi-continuous trials. 2. Materials and methods 2.1. Materials The following pig slaughterhouse wastes were used in this study: meat tissue, intestinal wastes, bristle and post flotation sludge. Other fractions were not investigated due to their low biodegradability and expected operational problems (bones) or other way of their use by the slaughterhouse (blood). The considered SHW belonged to the Category 2 of animal by-products, according to the EC regulation no. 1069/2009. The slaughterhouse wastes were collected at PINI Polonia Company in Kutno, which is one of the largest plant of this kind in Poland with a capacity of up to 1000 pigs slaughtered per hour. After delivering to the laboratory, all SHW groups were ground and then stored at 30 °C prior to use. For co-digestion experiments, the individual waste fractions were mixed in the proportion (wet weigh basis) of 50% intestines, 21% meat tissue, 21% post flotation sludge and 8% bristle, to reflect the real amounts generated in the PINI pig slaughterhouse. Municipal sewage sludge (the mixture of primary and waste activated sludge) was collected at the Municipal Wastewater Treatment Plant in Kutno. The plant annually produces 30,000 tons of dewatered sludge, which is stabilized by liming. However, lime was not added to the sludge prepared for the purpose of this research. The characteristics of slaughterhouse wastes and sewage sludge are depicted in Table 1. Sewage sludge and all investigated SHW were abundant in nitrogen showing the C/N ratio lower than 10. Simultaneously, the sludge was rich in phosphorus because this sludge originated from a WWTP operating with an enhanced biological phosphorus removal system. The contents of lipids in slaughterhouse wastes were generally lower that the values reported in the literature (Bayr et al., 2012; Edstorm et al., 2003; Palatsi et al., 2011; Pitk

et al., 2012). The SHW mixture composed for co-digestion experiments gave the fat content of around 35% TS. 2.2. Batch experiments Batch tests were performed to determine biomethane potential of individual substrates as well as the slaughterhouse waste mixture composed as described in Section 2.1. Anaerobically digested sewage sludge sampled at the municipal WWTP in Łódz´ was used as inoculum. The inoculum had total and volatile solids concentrations of 28.54 gTS/kg and 18.26 g VS/kg, respectively. The batch assays were conducted using glass bottles of 1000 cm3 volume. Each bottle was closely connected to a 1000 cm3 gas collecting tank to measure the daily biogas production by a water displacement method. The bottles were filled with 500 g of inoculum and then the substrates were added to meet the inoculum to substrate ratio of 2 (VS-basis). Finally, distilled water was also added into the bottles to reach the liquid volume of 600 cm3. Before closing, the bottles were flushed with nitrogen gas to ensure anaerobic conditions in the headspace. The reactors were incubated in a thermostat, which maintained a constant temperature of 35 °C. The batch tests were carried in triplicates to achieve reliable results. Additionally, three control (blank) assays were run with inoculum and water only in order to establish the residual methane yield from the inoculum. Net biogas and methane productions were then calculated by subtracting the corresponding values for control and substrate runs. 2.3. Semi-continuous experiments The co-digestion experiments were performed in a semicontinuously fed reactor with a total capacity of 5 dm3 and an active volume of 3 dm3. The reactor was placed in a thermostat to ensure constant mesophilic temperature of 35 ± 1 °C. The headspace of the reactor was coupled with a 4 dm3 gas collecting tank to control the biogas yield by a water displacement method. Semi-continuous operation was achieved by the daily discharge of digestate, followed by a substrate addition using a peristaltic pump. At the start-up, the reactor was filled with inoculum (the same as in case of batch experiments), and then operated with sewage sludge at an SRT of 30 days for a month to provide a suitable acclimation of anaerobic biomass prior to the experiments. In the first two semi-continuous periods (runs R1 and R2) the reactor was exclusively fed with sewage sludge with SRT values of 20 and 15 days, respectively and at equivalent organic loading rates of 2.15 and 3.03 kg VS/m3 d. Then, in runs R3 and R4, a mixture of sewage sludge with 30% SHW (w/w) was treated, and the reactor was operated with SRTs of 20 and 15 days and corresponding OLRs of 2.98 and 3.97 kg VS/m3 d, respectively. Finally, in runs R5 and R6, the content of SHW in the digested mixture was increased to 50% (w/w), and the OLR values were established on 3.10 and 4.10 kg VS/m3 d, respectively. The mixtures prepared for the semi-continuous experiment were diluted with distilled water in order to increase the moisture content and achieve the OLR values not exceeding around 4 kg VS/m3 d. For each experimental run, the reactor was continuously operated for at least 4 consecutive SRTs to approximate steady-state conditions and to provide statistically comparable results. 2.4. Analyses Total and volatile solids (TS,VS), total alkalinity (TAL), chemical oxygen demand (COD) and pH were analyzed according to standard methods (APHA, 2005). The total ammonium nitrogen (TAN) was determined using a HACH-Lange DR2800 spectrophotometer and a modified Nessler method (no. 8038) adopted by HACH. Free ammonia (FAN) concentrations were then calculated using

121

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126 Table 1 Characteristics of slaughterhouse wastes and sewage sludge used for the experiments. Indicator

Unit

Meat tissue

Intestines

Post flotation sludge

Bristles

Sewage sludge

TS VS

g/kg g/kg % TS gO2/kg TS % TS % TS % TS % TS % TS % TS –

383.09 ± 13.43 361.34 ± 17.95 94.30 869.7 ± 64.6 47.46 ± 2.32 70.25 ± 1.34 8.91 ± 0.06 0.51 ± 0.06 5.05 ± 0.21 0.025 ± 0.008 7.89

294.19 ± 10.76 280.57 ± 10.71 95.37 725.7 ± 57.6 23.08 ± 1.25 63.80 ± 0.92 10.25 ± 0.66 0.74 ± 0.03 4.85 ± 0.12 0.015 ± 0.007 6.29

218.52 ± 3.08 185.43 ± 2.66 84.86 1207.4 ± 101.1 25.37 ± 1.20 74.10 ± 0.54 8.44 ± 0.38 0.25 ± 0.01 5.50 ± 0.47 0.025 ± 0.01 8.81

235.97 ± 3.27 223.65 ± 7.11 94.77 1211.2 ± 26.5 8.68 ± 0.86 65.10 ± 0.42 9.35 ± 0.17 1.62 ± 0.13 6.10 ± 1.07 0.045 ± 0.003 6.96

148.615 ± 14.51 123.99 ± 8.41 83.43 11543.3 ± 69.3 n.a. 63.60 ± 1.87 7.38 ± 0.39 2.36 ± 0.18 5.24 ± 0.13 0.712 ± 0.04 8.65

COD Lipids C N P H S C/N ± Standard deviation. n.a. – not analyzed.

the equation as described by Hansen et al. (1998). For determination of phosphates (PO4) and total volatile fatty acids (TVFA) a DR2800 spectrophotometer was also used and HACH-Lange cuvette tests no 8048 and LCK365, respectively. Individual VFAs were measured with high performance liquid chromatography using a Finnigan Surveyor chromatograph (Thermo Scientific, USA) equipped with an Aminex HPX 87H column (Bio-Rad, USA), a refractive index detector. Separation during the HPLC tests was performed using a mobile phase of sulfuric acid (5 mmol/L) at a flow rate of 0.6 cm3/min. Elemental composition (C, N, H, P, S) was determined using a Flash Elemental Analyzer (Thermo Finnigan, Italy), following the manufacturer’s procedures. The concentration of fats was determined using a Standard Soxhlet method (APHA, 2005). The CH4 content in the biogas was measured using a 8610C gas chromatograph (SRI Instrument) coupled with a molecular sieve-packed column (1 m  1/800 ), a silica gel-packed column (1 m  1/800 OD), and a thermal conductivity detector (TCD). The column and detector temperatures were established at 60 °C and 150 °C, respectively. Helium was used as the carrier gas at a flow rate of 10 cm3/min. The analyses of individual samples were performed in at least triplicates. The analysis of variance (single factor ANOVA) was used to compare the results of individual runs. A confidence level of 0.05 was selected for all statistical comparisons.

3. Results and discussion 3.1. Batch experiments In order to establish the maximal biogas and methane yields from individual substrates, batch experiments were performed. Data of these trials are summarized in Table 2. The batch experiments showed generally high methane potential of tested SHW excluding bristles. The highest methane yield of 976 dm3 CH4/kg VS was obtained for meat tissue, while only a slightly lower yield of 826 dm3 CH4/kg VS was achieved for intestinal wastes. These values were higher than the yields reported in the literature (Braun et al., 2003; Hejnfelt and Angelidaki, 2009;

Pitk et al., 2012). This can be attributed to greater fat contents in our wastes compared to the values reported by the authors cited above. Theoretical methane yield from fats is nearly twice as high as the production of methane from proteins and carbohydrates (Weiland, 2010). The production of methane from flotation sludge reached 585 dm3 CH4/kg VS, whereas bristles gave only 53 dm3 CH4/kg VS. Methane production potentials for flotation sludge reported by other authors reach 690 dm3 CH4/kg VS (Braun et al., 2003; Pitk et al., 2012) however these authors investigated wastes containing much higher concentrations of fats and phosphorus (57.9% TS and 1.29% TS, respectively). Low biogas production from bristles could be related to a high content of hardly degradable proteins (mainly keratin), which are strongly resistant to the action of proteolytic enzymes (Suzuki et al., 2006). The batch assays with sewage sludge gave a specific methane yield of 371 dm3/kg VS, which was slightly higher than the typical productions of 200–350 dm3/kg VS reported in the literature (Braun et al., 2003; Luostarinen et al., 2009; Pitk et al., 2013; Borowski and Weatherley, 2013) and this may be related to a relatively high VS content in the sludge originated from the treatment plant in Kutno. Moreover, BMP tests give higher methane yields compared to the productions obtained in semi-continuous or continuous trials, because batch tests are performed with high amounts of inoculum, which dilutes potential toxic and inhibitory substances, and provides the necessary nutrients for microorganisms (Fang et al., 2011; Ortner et al., 2014). It is worth mentioning that the average methane yield achieved in trials with SHW mixture (composed based on the amounts generated in slaughterhouse) was higher than the sum of the amounts produced when the same individual substrates were digested separately (839 over 789 dm3 CH4/kg VS). This can be explained by a more balanced composition of the mixture. In particular, low concentration of phosphorus in post flotation sludge was supplemented by the addition of wastes richer in this nutrient. 3.2. Semi-continuous experiments Operating parameters and performances of the semi-continuous reactor are summarized in Table 3, while the profile of biogas

Table 2 Parameters of the batch digestion tests. Parameter

Unit

Meat tissue

Intestines

Post flotation sludge

Bristles

SHW mixture

Sewage sludge

Mass of substrate Substrate VS Mass of inoculum Inoculum VS SGP SMP

g g g g dm3/kg VS dm3 CH4/kg VS

12 361.34 ± 17.95 500 18.26 ± 0.03 1278.8 ± 16.7 976.2 ± 33.6

16 280.57 ± 10.71 500 18.26 ± 0.03 1066.0 ± 24.7 825.6 ± 22.7

21 185.43 ± 2.66 500 18.26 ± 0.03 727.1 ± 17.8 585.3 ± 14.6

19 223.65 ± 7.11 500 18.26 ± 0.03 115.2 ± 7.7 52.7 ± 3.8

16 290.32 ± 10.83 500 18.26 ± 0.03 1055.1 ± 37.4 839.2 ± 38.3

31 123.99 ± 8.41 500 18.26 ± 0.03 460.9 ± 18.5 370.9 ± 7.0

± Standard deviation.

122

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126

production is plotted in Fig. 1. During the first two experimental periods (R1 and R2), the semi-continuous reactor was fed with sewage sludge as a sole substrate. The specific biogas productions obtained in these trials were 330 and 360 dm3/kg VS for SRT of 20 and 15 days, respectively, whereas methane yield did not differ significantly (p = 0.748) between the trials and averaged 255 dm3 CH4/kg VS (Table 3). These values were considerably lower to the biogas and methane yields obtained in batch experiments but comparable to the findings of other authors (Luostarinen et al., 2009; Pitk et al., 2013; Wan et al., 2011). Simultaneously, the average VS removal rate determined in SS mono-digestion experiments was 50.6% with no significant differences between the trials (p = 0.913). Co-digestion of SS with 30% SHW was initiated on day 141 and the process was continued for 180 days with two SRTs of 20 and 15 days (runs R3 and R4, respectively). A 30% addition of slaughterhouse wastes to the sludge significantly increased the digestion performance compared to the SS mono-digestion. The specific biogas yield significantly (p < 0.01) increased and averaged 611 and 750 dm3/kg VS for trials R3 and R4, respectively, whereas the corresponding methane productions were 473 and 560 dm3 CH4/ kg VS. The VS removal rates during these periods were 61.3% and 57.0% respectively. In the last two experimental runs (R5 and R6), the reactor was fed with the mixture of SS with 50% SHW. The maximal digestion efficiency in terms of biogas production and VS removal was achieved in run R5, when the reactor was operated at SRT of 20 days. The production of biogas and methane significantly increased (p < 0.01) to 893 dm3/kg VS and 609 dm3 CH4/kg VS, whereas the removal of volatile solids achieved 62.2%, which was insignificantly higher (p = 0.517) than the corresponding figure for R3. However, when SRT was reduced to 15 days in run R6 and the process was operated with OLR of 4.10 kg VS/m3 d, the generation of biogas was slightly inhibited, which might have been attributed to the highest levels of ammonia (2000–2900 gN/m3) and volatile fatty acids (1900–5550 g/m3) recorded in this study. The high specific biogas and methane yields achieved in co-digestion experiments (especially in runs R5) can be linked to the considerable amounts of SHW in the digested mixture. As mentioned earlier, SHW are abundant in fats and proteins which in turn have high theoretical methane potentials (Weiland, 2010). As stated in the literature, the anaerobic digestion of slaughterhouse waste as a sole substrate is feasible but large reactor volumes and low organic loading rates need to be implemented to maintain the process stable and to avoid some exploitation problems. Methane yields of around 720 dm3 CH4/kg VS were obtained by Bayr et al. (2012) in a continuously stirred reactor operated with an organic loading rate of 1.0–1.5 kg VS/m3 d and a SRT of 50 days. Salminen and Rintala (2002a) studied semi-continuous monodigestion of poultry SHW. The process appeared feasible with OLR of up to 0.8 kg VS/m3 d and a SRT of 50–100 days with methane yield of up to 550 dm3 CH4/kg VS. Similar methane yields

were achieved in the experiments performed by Cuetos et al. (2008). In spite of applying low OLR of 0.9–1.7 kg VS/m3 d and mechanical stirring system, the authors were not able to prevent the formation of foam in the digester. The literature concerning co-digestion of sewage sludge with slaughterhouse wastes is scarce, and describes the research in which SHW were mainly composed of cattle byproducts. Luste and Luostarinen (2010) treated in mesophilic digesters the SHW and sewage sludge mixture in a ratio of 1:3 (v/v). The highest methane yield of 410 dm3 CH4/kg VS were achieved with 20-days of SRT. Co-digestion of sewage sludge with SHW from cattle and bovine was also investigated by Pitk et al. (2013). The authors obtained the maximal methane production of 645 dm3 CH4/kg VS from the mixture of SS with only 5% SHW, and a further increase in the SHW contribution resulted in the inhibition of methanogenesis attributed to the substantial concentrations of free ammonia (over 500 g/m3) in the digester. 3.3. Digestate quality and digester stability Changes of ammonium nitrogen and phosphates in the course of experiments are illustrated in Fig. 2 whereas the average values of these nutrients in the digestate are given in Table 4. Based on pH and temperature, the free ammonia nitrogen contents were also calculated, according to the formula described by Hansen et al. (1998). From these data, it is clearly visible that the addition of SHW to the sludge led to the increase in the concentration of total ammonium nitrogen in the supernatant. The maximum TAN content of around 2450 gN/m3 was reported in experiment R5, which was significantly more (p < 0.01) that 1560 gN/m3 measured in experiment R1. The highest TAN concentration in the liquid from experiment R5 might be attributed to the greatest digestion rate (measured by VS removal rate – Table 3) in that run. Moreover, the calculated absolute N concentration in sewage sludge fed into the reactor in experiment R1 was 3.77 g/kg of fresh substrate, whereas the corresponding value in the of R5 was 5.76 g/kg, what might have been reflected in the supernatant quality. Ammonium nitrogen at higher concentrations may affect anaerobic digestion, especially its unionized form of ammonia (FAN), because it passes through cell membranes of bacteria and methanogens thus leading to the proton imbalance and the pH change (Chen et al., 2008). The calculated free ammonia concentrations varied between 44.5 gNH3/m3 in run R1 to 116.3 gNH3/m3 in run R5 (Table 4), whereas the experiences with slaughterhouse waste reported in the literature showed stable digestion operations at FAN concentrations of 250–400 gNH3/m3 (Cuetos et al., 2008; Hejnfelt and Angelidaki, 2009; Pitk et al., 2013). Hence, it may be stated that the levels of ammonia in all semi-continuous runs were too low to inhibit the anaerobic digestion and biogas production. Considering phosphates, it was observed that their concentrations in the liquid phase varied in a similar manner to the changes

Table 3 Operating parameters and performances of the semi-continuous experiments. Parameter

Unit

R1

R2

R3

R4

R5

R6

Content of SS Content of SW SRT Feed TS Feed VS OLR VSreduction GPR SGP

% (w/w) % (w/w) d g/kg g/kg kg VS/m3 d % cm3/dm3 d dm3/kg VSfed dm3/kg VSreduced dm3 CH4/kg VS %

100 0 20 51.05 42.95 2.15 50.53 708.8 330.1 653.2 254.4 77.10

100 0 15 53.29 45.41 3.03 50.66 1080.4 356.9 704.5 256.8 72.0

70 30 20 67.02 59.64 2.98 61.31 1822.8 611.3 997.0 472.8 77.4

70 30 15 66.69 59.51 3.97 57.00 2977.5 750.5 1316.8 559.3 74.5

50 50 20 68.07 61.99 3.10 62.16 2767.5 892.8 1436.3 608.6 68.2

50 50 15 67.52 61.45 4.10 61.73 3124.5 762.7 1235.5 513.7 67.4

SMP CH4 content

123

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126

12.0

1200

R3

R2

R5

R4

R6 10.0

1000

SGP OLR 800

8.0

600

6.0

400

4.0

200

2.0

OLR (kgVS/m3 ·d)

Specific biogas producon (SGP, dm3/kgVS)

R1

0.0

0 0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

duraon me (weeks) Fig. 1. Weekly average specific biogas production during co-digestion experiments.

R1

Total ammonium nitrogen (gN/m3)

2700

R3

R2

R5

R4

R6

900 810

TAN PO4

2400

720

2100

630

1800

540

1500

450

1200

360

900

270

600

180

300

90

Ortophosphates (gP/m3)

3000

0

0 0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

duraon me (weeks) Fig. 2. Variations in ammonium nitrogen and phosphate concentrations during the semi-continuous experiments.

Table 4 Characteristics of digestate from semi-continuous experiments. Indicator

Unit

R1

R2

R3

R4

R5

R6

pH TS VS

– g/kg g/kg % TS gN/m3 gNH3/m3 gP/m3 g/m3 g/m3 g/m3 g/m3 g/m3 g/m3 g/m3 g/m3 –

7.42 ± 0.09 29.16 ± 2.86 21.25 ± 2.48 72.81 ± 3.22 1562 ± 249 44.5 ± 12.3 255 ± 47 1879 ± 522 1166 ± 355 282 ± 76 75 ± 29 245 ± 54 76 ± 20 35 ± 13 6664 ± 862 0.28 ± 0.08

7.40 ± 0.05 30.01 ± 2.03 22.40 ± 1.76 74.61 ± 1.07 1752 ± 318 47.5 ± 10.8 269 ± 51 1406 ± 491 797 ± 260 267 ± 101 72 ± 25 169 ± 42 70 ± 28 31 ± 15 7018 ± 939 0.20 ± 0.09

7.50 ± 0.09 30.99 ± 2.94 23.07 ± 2.09 74.62 ± 4.07 1646 ± 268 59.6 ± 14.5 266 ± 59 1823 ± 538 1084 ± 303 431 ± 112 110 ± 30 74 ± 16 106 ± 36 18 ± 5 6761 ± 639 0.27 ± 0.08

7.56 ± 0.06 32.52 ± 2.53 25.59 ± 1.90 78.77 ± 0.62 2413 ± 245 95.8 ± 18.8 371 ± 64 2227 ± 475 1264 ± 227 481 ± 92 160 ± 36 144 ± 17 143 ± 37 35 ± 9 9541 ± 742 0.23 ± 0.06

7.60 ± 0.17 29.81 ± 2.26 23.46 ± 2.07 78.63 ± 1.25 2449 ± 251 116.3 ± 25.2 338 ± 42 2350 ± 243 1110 ± 166 641 ± 83 245 ± 49 104 ± 11 188 ± 51 63 ± 15 9550 ± 679 0.25 ± 0.04

7.52 ± 0.14 29.74 ± 2.69 23.46 ± 2.07 79.02 ± 1.87 2241 ± 264 86.0 ± 28.8 388 ± 52 3774 ± 1376 1460 ± 467 948 ± 398 752 ± 301 223 ± 76 324 ± 146 66 ± 11 9604 ± 592 0.39 ± 0.13

TAN FAN PO4 TVFA Acetic Propionic Iso-butyric n-Butyric Iso-valeric n-Valeric TAL TVFA/TAL ± Standard deviation.

124

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126

of TAN, which is illustrated in Fig. 2. However, in contrast to nitrogen, the concentration of phosphorus in sewage sludge was around 4 times higher than the calculated P-content in SHW mixture (23.60 gP/kg TS versus 5.93 gP/kg TS). Therefore, the monodigestion of sewage sludge should theoretically give much greater phosphate release than the co-digestion process irrespective of the fact of the lowest digestion rates reported in runs R1 and R2. It should be noted, however, that during anaerobic digestion, phosphates easily precipitate in the form of inorganic salts, mainly as struvite (MgNH4PO46H2O) and hydroxyapatite (Ca5OH(PO4)3). Hence, in addition to ammonia and phosphates, also Ca2+ and Mg2+ cations contribute to the precipitation of these salts (Bolzonella et al., 2012; Marti et al., 2008). In the following research, neither potassium nor magnesium were determined, but based on the literature data and our previous findings (Borowski and Weatherley, 2013; Pitk et al., 2012; Wan et al., 2011) it may be stated that concentrations of both cations are much higher in sewage sludge that in slaughterhouse wastes. Therefore, more phosphates might have precipitated during the sewage sludge mono-digestion (runs R1 and R2) thus giving lower concentrations of these ions in the supernatant. Fig. 3 illustrates the evolution of volatile fatty acids and pH in co-digestion experiments. Generally, the VFA and pH levels in the digestate remained stable throughout the whole experimental period with a mild upward trend until the run R6. The average total volatile fatty acid (TVFA) concentrations in sewage sludge mono-digestion experiments (R1 and R2) were 1879 and 1406 g/m3 for SRTs of 20 and 15 days, respectively (Table 4). These concentrations increased to 1823 and 2227 g/m3 in experiments R3 and R4, respectively, when the sludge was co-digested with 30% SHW. A further increase in the SHW content in the feed resulted in a slight growth of volatile fatty acids to average 2350 g/m3 in experiment R5 operated with SRT of 20 days. Simultaneously the TVFA to total alkalinity ratio remained within the range of 0.2 and 0.28 showing lack of inhibition by volatile fatty acids. As stated in the literature, a TVFA/TAL level not exceeding 0.3–0.4 indicates stable digestion operation, whereas significant instability occurs when the TVFA/TAL ratio exceeds 0.7–0.8 (Callaghan et al., 2002; Raposo et al., 2009). In experiment R6, the TVFA concentration averaged 3774 g/m3 with a peak value of 5550 g/m3, whereas the average TVFA/TAL ratio was 0.39 with periodic increases to 0.55, which indicated a slight inhibition of methanogenesis. Total VFA concentrations above 4000 g/m3 are regarded as inhibitory for methanogens (Weiland,

R1

8

2010), however individual fatty acids must also be considered. The concentrations of individual acids were determined three times during each semi-continuous run, and data of these analyses are summarized in Table 4 and Fig. 4. The main acids found in the digestates from experiments R1 and R2 were acetic acid (predominant), propionic and n-butyric acids, which is in agreement with the observations of Bolzonella et al. (2012) who also investigated the anaerobic digestion of municipal sewage sludge. In experiments with the sludge and 30% SHW (R3 and R4), the concentration of propionic acid increased to 431–481 g/m3 whereas the content of n-butyric acid decreased (74–144 g/m3), compared to the corresponding values reported for sewage sludge mono-digestion (R1 and R2). However, the greatest changes in the structure of VFAs were noticed in experiment R6, performed with the mixture of SS and 50% SHW and with the highest loading rate of 4.10 kg VS/m3 d. In particular, the levels of propionic acid and branched fatty acids should be considered. Propionic acid achieved the average content of 948 g/m3 (25% of TVFA) with the peak concentration of 2000 g/ m3, which may have affected the biogas production. The average concentrations of iso-butyric and iso-valeric acids reported in experiment R6 were 752 and 324 g/m3, respectively. It should also be noted that iso-butyric acid constituted 20% of TVFA and its concentration was 10 times higher that the corresponding values determined in experiments R1 and R2. Propionic acid is considered to be the most important intermediate product of anaerobic decomposition of complex organic wastes, and its accumulation almost always leads to anaerobic digestion failure (Amani et al., 2011; Nielsen et al., 2007; Wang et al., 2009). Degradation rate of propionic acid is relatively slow because it involves unique enzymatic reactions and its oxidation is thermodynamically unfavorable in anaerobic digestion (Wang et al., 1999). Therefore, propionic acid is suggested to be the best indicator of stable digestion operation. The reported concentrations of propionic acid that could inhibit methane production vary between 900 and 1500 g/m3 (Ma et al., 2009; Wang et al., 2009). Accumulation of propionic acid can also be linked to the effect of free ammonia, which inhibits propionate breakdown (Giuliano et al., 2013). Considering fatty acids with longer chains, it was reported that VFAs with straight chains degrade faster than their respective isomers with branched chains (Wang et al., 1999). Furthermore, high concentrations of branched VFAs (iso-butyric, iso-valeric) indicate slow growth of acetate-utilizing methanogens (Luste and

R3

R2

R5

R4

R6

6000

7.8 7.6

5000

7.4 7.2

4000

pH

7 6.8

3000

6.6 6.4

2000

6.2 1000

6 5.8 5.6

0 0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

duraon me (weeks) Fig. 3. The pH and VFA concentration profiles in digestates of semi-continuous experiments.

Total volale fay acids (g/m3)

pH TVFA

125

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126

100% 90% 80% 70%

n-Valeric 60%

iso-Valeric

50%

n-Butyric iso-Butyric

40%

propionic acec

30% 20% 10% 0% R1

R2

R3

R4

R5

R6

Fig. 4. Percentages of individual volatile fatty acids in total VFA of digestates.

Luostarinen, 2010). This was also visible in our study. In experiment R6, acetic acid was the dominating species of volatile fatty acids however it constituted only around 38% of TVFA, whereas iso-butyric and iso-valeric acids accounted for 20% and 9% of the total VFA, respectively. Conversely, in experiment R1, acetic acid also dominated making up 62% TVFA, but both iso-butyric and iso-valeric acids consisted of approximately 4% TVFA (Fig. 4). However, despite relatively high contents of propionic acid and branched acids, the biogas production remained stable throughout the experiment R6 operated with the largest contribution of SHW and with the highest OLR. Long chain fatty acids have been reported to be the key factors affecting the anaerobic degradation of slaughterhouse wastes due to the slow growth of LCFA-consuming bacteria and their tendency to adsorb into the microbial cells. The inhibition is assigned to transport limitation of substrates and metabolites through microbial cell membranes due to LCFA adsorption to the cells (Palatsi et al., 2011; Salminen and Rintala, 2002). In the following study, despite a relatively high SHW content in the mixture subjected to digestion, the semi-continuous reactor remained stable throughout the experiments with high biogas and methane production. No foaming problems, as reported in the literature (Cuetos et al., 2008; Hejnfelt and Angelidaki, 2009; Pitk et al., 2013), were also observed. Based on data obtained from the analysis of individual substrates (Table 1), the calculated fat content in SHW was around 35% TS, however that concentration must have been considerable lower in the mixture of SS and SHW. Therefore, the initial fat content was fairly lower than the values reported by Palatsi et al. (2011) and Rodriguez-Abalde et al. (2011) who performed the experiments with slaughterhouse wastes. While the treatment of sterilized SHW containing 33% of fats (in total COD), Rodriguez-Abalde et al. (2011) did not observe the inhibition of biogas production by LCFA. Palatsi et al. (2011) noticed a fast recovery of the anaerobic digester after increasing the COD concentration in the feed up to 15 g/dm3, with 80% fat content. Hence, it might be concluded that in our research, there was no visible inhibition of biogas and methane production by LCFA. 4. Conclusions This study showed that slaughterhouse wastes derived from pig processing have a high potential of biogas and methane production

by far exceeding the biogas yields obtained from sewage sludge. The highest methane yields of 826 and 976 dm3/kg VS were achieved from intestinal wastes and meat tissue, respectively, whereas the mixture of individual SHW in the proportions reflecting the amounts generated by the slaughterhouse gave nearly 840 dm3/kg VS, which confirms the advantageous effect of co-digestion. The study also proved that slaughterhouse waste can be successfully co-digested with sewage sludge. Moreover, a stable digestion operation can be achieved with the mixture containing as much as 50% of slaughterhouse wastes in the total mass of digested substrates, and the applied organic loading rates can be as high as 4 kg VS/m3 d. The co-digestion of sewage sludge with 50% slaughterhouse wastes allows to increase the specific biogas and methane production by over 2-fold, compared to the production obtained from this sludge alone, whereas based on the volumetric biogas production, this increase is even higher. Both sewage sludge and all SHW types were characterized by a relatively high nitrogen content giving the carbon-to-nitrogen ratio not exceeding 10, which was below the optimal range for co-digestion. Despite this, the levels of ammonia measured in the digestates were far below the threshold concentrations regarded as inhibitory for methanogens. A slight inhibition of biogas production was only observed when the digester was operated with the loading rate greater than 4 kg VS/m3 d. Under such conditions, volatile fatty acids, especially propionic, slightly inhibited methane production, however, the digestion process remained stable. Acknowledgement The authors greatly appreciate to Leszek Komarowski from EKOSPOT Company for logistical support of the research. References Alvarez, R., Liden, G., 2008. Semi-continuous co-digestion of solid slaughterhouse waste, manure, and fruit and vegetable waste. Renewable Energy 33, 726–734. Amani, T., Nosrati, M., Mousavi, S.M., Kermanshahi, R.K., 2011. Study of syntrophic anaerobic digestion of volatile fatty acids using enriched cultures at mesophilic conditions. Int. J. Environ. Sci. Technol. 8, 83–96. APHA, 2005. Standard Methods for the Examination of Water and Wastewater. 21st ed. American Public Health Association, Washington, DC.

126

S. Borowski, P. Kubacki / Waste Management 40 (2015) 119–126

Bayr, S., Rantanen, M., Kaparaju, P., Rintala, J., 2012. Mesophilic and thermophilic anaerobic co-digestion of rendering plant and slaughterhouse wastes. Bioresour. Technol. 104, 28–36. Bolzonella, D., Cavinato, C., Fatone, F., Pavan, P., Cecchi, F., 2012. High rate mesophilic, thermophilic, and temperature phased anaerobic digestion of waste activated sludge: a pilot scale study. Waste Manage. 32, 1196–1201. Borowski, S., Weatherley, L., 2013. Co-digestion of solid poultry manure with municipal sewage sludge. Bioresour. Technol. 142, 345–352. Braun, R., Brachtl, E., Grasmug, M., 2003. Codigestion of proteinaceous industrial waste. Appl. Biochem. Biotechnol. 109, 139–153. Callaghan, F.J., Wase, D.A.J., Thayanithy, K., Forster, C.F., 2002. Continuous codigestion of cattle slurry with fruit and vegetable wastes and chicken manure. Biomass Bioenergy 27, 71–77. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Cuetos, M.J., Gomez, X., Otero, M., Moran, A., 2008. Anaerobic digestion of solid slaughterhouse waste (SHW) at laboratory scale: influence of co-digestion with the organic fraction of municipal solid waste (OFMSW). Biochem. Eng. J. 40, 99– 106. Edstorm, M., Nordberg, A., Thyselius, L., 2003. Anaerobic treatment of animal byproducts from slaughterhouses at laboratory and pilot scale. Appl. Biochem. Biotechnol. 109, 127–138. Fang, C., Boe, K., Angelidaki, I., 2011. Anaerobic co-digestion of by-products from sugar production with cow manure. Water Res. 45, 3473–3480. Gazda, M., Rak, A., Sudak, M., 2012. Research on cofermentation of sewage sludge with waste fats for the wastewater treatment plant in Brzeg. Infrastruct. Ecol. Rural Areas 03/3, 79–90, . Giuliano, A., Bolzonella, D., Pava, n.P., Cavinato, C., Cecchi, F., 2013. Co-digestion of livestock effluents, energy crops and agro-waste: feeding and process optimization in mesophilic and thermophilic conditions. Bioresour. Technol. 128, 612–618. Hansen, K.H., Angelidaki, I., Ahring, B.K., 1998. Anaerobic digestion of swine manure: inhibition by ammonia. Water Res. 32, 5–12. Hejnfelt, A., Angelidaki, I., 2009. Anaerobic digestion of slaughterhouse by-products. Biomass Bioenergy 33, 1046–1054. Luostarinen, S., Luste, S., Sillanpaa, M., 2009. Increased biogas production at wastewater treatment plants through co-digestion of sewage sludge with grease trap sludge from a meat processing plant. Bioresour. Technol. 100, 79– 85. Luste, S., Luostarinen, S., 2010. Anaerobic co-digestion of meat-processing byproducts and sewage sludge – effect of hygienization and organic loading rate. Bioresour. Technol. 101, 2657–2664. Ma, J., Carballa, M., Van De Caveye, P., Verstraete, W., 2009. Enhanced propionic acid degradation (EPAD) system: proof of principle and feasibility. Water Res. 43, 3239–3248. Marti, N., Bouzas, A., Seco, A., Ferrer, J., 2008. Struvite precipitation assessment in anaerobic digestion process. Chem. Eng. J. 141, 67–74. Mata-Alvarez, J., Dosta, J., Romero-Guiza, M.S., Fonoll, X., Peces, M., Astals, S., 2014. A critical review on anaerobic co-digestion achievements between 2010 and 2013. Renew. Sustain. Energy Rev. 36, 412–427.

Nielsen, H.B., Uellendahl, H., Ahring, B.K., 2007. Regulation and optimization of the biogas process: propionate as a key parameter. Biomass Bioenergy 31, 820–830. Ortner, M., Leitzinger, K., Skupien, S., Bochmann, G., Fuchs, W., 2014. Efficient anaerobic mono-digestion of N-rich slaughterhouse waste: influence of ammonia, temperature and trace elements. Bioresour. Technol. 174, 222–232. Palatsi, J., Vinas, M., Guivernau, M., Fernandez, X., Flotats, X., 2011. Anaerobic digestion of slaughterhouse waste: main process limitations and microbial community interactions. Bioresour. Technol. 102, 2219–2227. Pitk, P., Kaparaju, P., Palatsi, J., Affes, R., Vilu, R., 2013. Co-digestion of sewage sludge and sterilized solid slaughterhouse waste: methane production efficiency and process limitations. Bioresour. Technol. 134, 227–232. Pitk, P., Kaparaju, P., Vilu, R., 2012. Methane potential of sterilized solid slaughterhouse wastes. Bioresour. Technol. 116, 42–46. Raposo, F., Borja, R., Martin, M.A., Martin, A., De la Rubia, M.A., Rincon, B., 2009. Influence of inoculum-substrate ratio on the anaerobic digestion of sunflower oil cake in batch mode: process stability and kinetic evaluation. Chem. Eng. J. 149, 70–77. Regulation (EC) No. 1069/2009 of the European Parliament and of the Council of 2009 October 2009 laying down health rules as regards animal by-products and derived products not intended for human consumption. Official Journal of European Union L273 14/11/2009. Rodriguez-Abalde, A., Fernandez, B., Silvestre, G., Flotats, X., 2011. Effects of thermal pre-treatment on solid slaughterhouse waste methane potential. Waste Manage. 31, 1488–1493. Salminen, E., Rintala, J., 2002. Anaerobic digestion of organic solid poultry slaughterhouse waste – a review. Bioresour. Technol. 83, 13–26. Salminen, E., Rintala, J., 2002a. Semi-continuous anaerobic digestion of solid poultry slaughterhouse waste: effect of hydraulic retention time and loading. Water Res. 36, 3175–3182. Suzuki, Y., Tsujimoto, Y., Matsui, H., Watanabe, K., 2006. Decomposition of extremely hard-to degrade animal proteins by thermophilic bacteria. J. Biosci. Bioenergy 102, 77–81. Wan, C., Zhou, Q., Fu, G., Li, Y., 2011. Semi-continuous anaerobic co-digestion of thickened waste activated sludge and fat, oil and grease. Waste Manage. 31, 1752–1758. Wang, Q., Kuninobu, M., Ogawa, H.I., Kato, Y., 1999. Degradation of volatile fatty acids in highly efficient anaerobic digestion. Biomass Bioenergy 16, 407–416. Wang, Y., Zhang, Y., Wang, J., Meng, L., 2009. Effect of volatile fatty acid concentrations on methane yield and methanogenic bacteria. Biomass Bioenergy 33, 848–853. Weiland, P., 2010. Biogas production: current state and perspectives. Appl. Microbiol. Biotechnol. 85, 849–860. Witkowski, J., 2014. Concise statistical yearbook of Poland. Publication available on website: http://www.stat.gov.pl. Zhang, Y., Banks, C.J., 2012. Co-digestion of the mechanically recovered organic fraction of municipal solid waste with slaughterhouse wastes. Biochem. Eng. J. 68, 129–137.