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Semi-continuous anaerobic co-digestion of sugar beet byproduct and pig manure: Effect of the organic loading rate (OLR) on process performance
Bioresource Technology 194 (2015) 283–290
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Semi-continuous anaerobic co-digestion of sugar beet byproduct and pig manure: Effect of the organic loading rate (OLR) on process performance Kaoutar Aboudi ⇑, Carlos José Álvarez-Gallego, Luis Isidoro Romero-García Department of Chemical Engineering and Food Technology, Faculty of Sciences, Agrifood Campus of International Excellence (CeiA3), University of Cádiz, 11510 Puerto Real, Cadiz, Spain
h i g h l i g h t s The anaerobic co-digestion of sugar beet byproducts with pig manure is feasible. The highest methane productivity was obtained at hydraulic retention time of 6 days. The organic loading rate maximizing the methane production was 11.2 gVS/Lreactor d. The hydraulic retention time of 5 days leads to volatile fatty acids accumulation. System recovery is possible by increasing hydraulic retention time from 5 to 6 days.
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Article history: Received 24 May 2015 Received in revised form 10 July 2015 Accepted 11 July 2015 Available online 17 July 2015 Keywords: Co-digestion Organic loading rate Hydraulic retention time Semi-continuous tank reactor Methane production
a b s t r a c t Anaerobic co-digestion of dried pellet of exhausted sugar beet cossettes (ESBC-DP) with pig manure (PM) was investigated in a semi-continuous stirred tank reactor (SSTR) under mesophilic conditions. Seven hydraulic retention times (HRT) from 20 to 5 days were tested with the aim to evaluate the methane productivities and volatile solids (VS) removal. The corresponding organic loading rates (OLR) ranged from 4.2 to 12.8 gVS/Lreactor d. The findings revealed that highest system efficiency was achieved at an OLR of 11.2 gVS/Lreactor d (6 days-HRT) with a methane production rate (MPR) and volatile solids (VS) reduction of 2.91 LCH4/Lreactor d and 57.5%, respectively. The HRT of 5 days was found critical for the studied process, which leads to volatile fatty acids (VFA) accumulation and sharp drop in pH. However, the increase of HRT permits the recovery of system. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Sugar beet byproducts (SBB) generated during industrial sugar extraction from sugar beet plant (Beta vulgaris) are mainly composed of pulp and molasses. It may cause serious environmental problems if not properly treated before discharge. The exhausted sugar beet cossettes (ESBC) is produced in several sugar beet processing factories, and it consists of a mixture of exhausted slices of sugar beet pulp (cossettes) with molasses. This by-product is also dried and pelletized and it is sold as complement for animal feed. However, sugar-manufacturing plants require electrical and thermal energy at different stages of the process, which makes it energy intensive process. Moreover, the use of SBB as animal feed is not a promising strategy while its use as renewable energy resource could add economic and environmental benefits. ⇑ Corresponding author. Tel.: +34 956016474; fax: +34 956016411. E-mail address: [email protected] (K. Aboudi). http://dx.doi.org/10.1016/j.biortech.2015.07.031 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Previous studies showed that SBB are a suitable material for biological treatment by means of anaerobic digestion (AD) giving the advantages of agro-food wastes to produce clean energy such as methane from biogas (Aboudi et al., 2015; Alkaya and Demirer, 2011; Fang et al., 2011; Hutnan et al., 2000; Lehtomäki et al., 2007; Montañés et al., 2015; Ohuchi et al., 2014; Suhartini et al., 2014). However, bioconversion of agro-food byproducts is still having limitations due to the presence of hardly bio-degradable lignocellulosic content and nitrogen deficiency despite high carbohydrate content (Anwar et al., 2014; Sawatdeenarunat et al., 2015). Co-digestion of agro-food byproducts with nitrogen rich livestock manures can resolve this problem by balancing the nutrient content in the anaerobic digester, providing the required buffering capacity and adding a variety of microorganisms coming from animal digestive tract who are capable to degrade vegetal fibers (Hindrichsen et al., 2006; Mata-Alvarez et al., 2014; Rodriguez-Verde et al., 2014; Yan et al., 2014).
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Abbreviations Acronyms
Parameters (units)
Analytical methods C/N carbon/nitrogen ratio chemical oxygen demand; t: total; s: soluble (gO2/kg) t/sCOD DOC dissolved organic carbon HPr/HAc propionic/acetic ratio (gC/kg) N-NH4+ ammonia nitrogen (gN/kg) TKN total Kjeldahl nitrogen (gN/kg(TS)) TS total solids (g/kg) TVFA total volatile fatty acidity (gHAc/kg) VFA volatile fatty acid (g/L) VS volatile solids ((%TS)) Feedstock ESBC-DP PM
SBB SBP CM
sugar beet byproducts sugar beet pulp cow manure
Opeartive conditions AcoD anaerobic co-digestion AD anaerobic digestion CSTR continuous stirred tank reactor HRT hydraulic retention time (days) MPR methane production rate (LCH4/Lr d) OLR organic loading rate (gVS/Lr d) OM organic matter SMP specific methane production (mLCH4/gVSfed) SSTR semi-continuous stirred tank reactor
dried pellets of exhausted sugar beet cossettes pig manure
AD reactors can be performed in mainly three operating modes according to the feeding strategy including batch, semi-continuous and continuous operation. A batch system is run by feeding the reactor only once at the start of the process. The reactor is then sealed and monitored for the duration of the process until the degradation of all biodegradable material mainly giving information such as the maximum biogas productivity. In a semi-continuous system, the reactor is periodically loaded with the organic matter (OM) according to the established conditions. In continuous system, the reactor is constantly fed with the OM. Continuous (CSTR) and semi-continuous (SSTR) systems led to assess the stability (constant biogas production) and performance of the digester in the studied conditions and permit to define the capacity threshold of the digester for OM treatment (Chowdhury and Fulford, 1992). Some studies have investigated the anaerobic digestion of SBB and, very few studies were focused on sugar beet pulp (SBP). Fang et al. (2011) studied the semi-continuous thermophilic anaerobic co-digestion (AcoD) of SBP (wet pulp) with cow manure (CM) operating at HRT of 20 days and OLR of 6.75 gVS/Lreactor d. The authors found that co-digestion of SBP and CM with 50% SBP proportions achieved a specific methane production (SMP) of 280 mL CH4/gVSfed. However, a relatively high volatile fatty acids (VFA) concentration was observed in the reactor due to the high proportion of SBP in the mixture. Lehtomäki et al. (2007) investigated anaerobic co-digestion (AcoD) of different crop residues with CM in a continuous stirred tank reactor (CSTR). The authors obtained a SMP of 229 mLCH4/gVSfed in the co-digestion of 30% of sugar beet tops with CM, which was significantly higher than that obtained from CM mono-digestion. Nevertheless, the increase of OLR to 4 gVS/Lreactor d disturbed the AcoD process by decreasing the methane generation. Moraes et al. (2015) working with a CSTR for the treatment of sugar beet vinasse with 3% CM have reported that CM addition provided stability to AD process and enhanced the methane yield compared with mono-digestion of vinasse. So far, no studies were conducted on anaerobic co-digestion of exhausted sugar beet cossettes as dried pellets (ESBC-DP) with pig manure (PM) using semi-continuous stirred tank reactors. The main objective of this study was to determine the maximum OLR and the minimum HRT allowed by a semi-continuous type anaerobic reactor co-digesting the ESBC-DP with PM under mesophilic conditions.
2. Methods 2.1. Feedstock Dried pellets of exhausted sugar beet cossettes (ESBC-DP) were used in this study, which were composed by 85% of pulp and 15% of residual molasses. ESBC-DP were collected from sugar beet processing factory located at Jerez de la Frontera (Cádiz, Spain). Pig manure (PM) was collected from a semi-intensive farm facility at El Puerto de Santa Maria (Cádiz, Spain). The used PM corresponds to the entire animal excrement because the farm does not have any system for urine and feces separation at source. The used dry pellets had around 20–70 mm of length and 6 mm diameter and its total solids (TS) content ranged 80–90%. The pellets were stored at 4 °C to avoid its degradation. PM was periodically collected and stored under the same conditions as ESBC-DP. The mixture of both substrates was prepared within C/N ratio of 18.5, which was the best mixture ratio proposed in previous studies using similar wastes (Aboudi et al., 2015). Kivaisi and Mtila (1997) also proposed a C/N range of 16–18 for lignocellulosic-type wastes. The TS content was fixed to 8%, which is in agreement with those suggested in previous studies by Hutnan et al. (2000, 2001). The rheological behavior of ESBC-DP (very fibrous material) makes infeasible to work at higher TS content, because it leads to inadequate mixing conditions in the reactor and inducing the mass transfer limitations (Dai et al., 2014; Stoyanova et al., 2014). 2.2. Reactor configuration and start-up The photo and the detailed scheme of the SSTR used is this study, are shown in Fig. 1. The reactor was entirely made up of stainless steel. Agitation was performed at 12 rpm by a motor installed at the top of the reactor (Heidolph-RZR-2102) with a stirring blade. The temperature was maintained at 37 ± 0.25 °C by propelling water from a circulating bath (Ultraterm200-Selecta) through the reactor jacket. The top of the reactor has several ports: inlet port for feeding, output port for biogas collection, and an overpressure valve. The startup period of the SSTR was as follow: a 10 L working volume reactor was fed with the effluent coming from another SSTR treating ESBC-DP (as the only substrate) under mesophilic
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Srring motor
Srring blade
40L Tedlar biogas bag
Feed
Thermostac bath circulang
Effluent
Fig. 1. Photograph and detailed scheme of the SSTR used in the mesophilic co-digestion of ESBC-DP and PM.
Table 1 Physico-chemical characteristics of the substrates, inoculum and the feedstock mixture.
*
Components
Units
ESBC-DP
PM
Inoculum
Feedstock
pH TS VS DOC* sCOD* tCOD* TVFA* Alkalinity* N-NH4+* TKN Ratio C/N
– g/kg (%TS) gC/kg gO2/kg gO2/kg gHAc/kg gCaCO3/kg gN /kg gN/kg(TS) –
5.72 ± 0.31 887.30 ± 15.34 91.65 ± 2.34 48.82 ± 1.26 52.00 ± 4.45 134.28 ± 3.52 2.53 ± 0.46 2.85 ± 0.27 0.28 ± 1.32 12.18 ± 2.51 37.00 ± 0.22
6.44 ± 0.15 221. 61 ± 20.82 71.76 ± 11.84 6.44 ± 12.19 15.71 ± 3.84 44.78 ± 1.92 7.58 ± 1.77 50.2 ± 0.2 3.2 ± 0.7 35.74 ± 1.24 13.12 ± 0.27
7.54 ± 0.85 34.97 ± 10.59 21.75 ± 3.28 4.68 ± 0.72 8.97 ± 2.13 11.64 ± 2.52 0.08 ± 0.27 30.4 ± 2.5 1.1 ± 0.2 6.25 ± 1.28 19.42 ± 2.36
6.75 ± 0.07 86.88 ± 20.21 85.90 ± 5.92 5.85 ± 1.01 19.62 ± 4.34 73.41 ± 3.86 1.45 ± 0.42 73.25 ± 5.33 0.21 ± 0.05 26.62 ± 1.02 18.52 ± 0.26
g/kg (wet basis).
conditions at 20 days HRT. Once the reactor was filled, a feeding strategy with a mixture of ESBC-DP and PM was started under operation at 20 days-HRT (OLR of 4.2 gVS/Lreactor d) with different OLRs conditions. Each HRT tested in this study was maintained at least for a period of time equivalent to three HRT in order to allow the pseudo-steady state could be achieved. Table 1 shows the physico-chemical characteristics of the feedstock used in the seven runs of different OLRs (corresponding to HRTs range from 20 to 5 days). 2.3. Analytical methods The TS, volatile solids (VS), total and soluble chemical oxygen demand (tCOD, sCOD), dissolved organic carbon (DOC), pH, alkalinity and ammonium were analyzed according to the Standard Methods (APHA, 2005). For the parameters determined in soluble form (sCOD, DOC and VFA), the samples were previously lixiviated with deionized water for 2 h and filtered by 0.47 mm glass fiber filter (Álvarez-Gallego, 2005). Total COD was measured directly from the diluted effluent without any filtrations. The dissolved organic carbon (DOC) analysis was carried out in an Analytic-Jena Multi N/C 3100 carbon analyzer with chemiluminescence detector
(CLD) according to combustion-infrared method (5310B) of Standard Methods (APHA, 2005). The oxidizing gas was oxygen 5.0 (Abelló-Linde) at a pressure of 4–6 bars. Samples from the previous lixiviation and filtration were filtered again by a Teflon filter of 0.22 lm for VFA analysis, and were analyzed with a gas chromatograph (Shimadzu GC-2010) equipped with a flame ionization detector (FID) and capillary column filled with Nukol™ from Supelco (diameter of 0.25 lm and 30 m length). Biogas generated during the assays was collected in a 50L Tedlar bag (SKC) and the volume of gas was measured daily using a high precision drum-type gas meter (Ritter TG5). The gas composition (CH4, CO2, and H2) was determined by using a gas chromatograph (Shimadzu GC-2014) with a stainless steel column packed with Carbosieve™ SII from Supelco (diameter of 3.2 mm and 3.0 m length) and thermal conductivity detector (TCD) (Aboudi et al., 2015).
3. Results and discussion In this section, different parameters associated with the process stability and the production of methane gas, were discussed by
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comparing the experimental results obtained for the seven OLRs studied. Table 2 shows the different stages (runs) performed, indicating the HRT corresponding to each OLR tested and the operation time for each run. 3.1. Effect of OLR on the evolutions of pH and VFA Fig. 2-a shows the pH profile in the semi-continuous co-digestion reactor. At the beginning of the startup period (run 0), pH values were below 7 as a consequence that hydrolysis and acidification of the organic matter were not coupled with the utilization of VFAs by methanogenic microorganisms. This is an adaptation stage and hence, the pH correction by adding an alkali (NaOH, 8 M) was necessary to avoid the startup failure by acidification. Subsequently, the pH increased gradually from 6.7 (on day 6) to 7.4–7.8 from day 16 onwards. The pH correction strategy was used when the reactor effluent had a pH below 7, especially when changing the feedstock and/or increasing the OLR to ensure optimum conditions for the microbial growth (Brooks et al., 2008; Cysneiros et al., 2012). Once the system was stabilized, the pH levels were self-maintained above 7 during the whole assay for all OLRs tested. This pH stability and regulation by the system itself indicated that the digester was working within optimum conditions for methane generation and was not under inhibitory conditions despite of increasing OLR. However, a sharp decrease in the pH value was observed upon increasing the OLR to 12.8 gVS/Lreactor d (run VII) as a result of the acidification of the medium despite of daily pH corrections. The digester operation was severely affected with no biogas production and high levels of VFAs accumulation (acetic, propionic and butyric acids, mainly). It shows that the growth rate of acidogenic bacteria was higher than that of the highly sensitive methanogenic archaea (Demirel et al., 2008). Hence, an imbalance between the metabolic activities of different microorganisms involved in the process was occurred, and the reactor was overloaded under operation at 12.8 gVS/Lreactor d (run VII). In this situation, the best strategy was to stop feeding the reactor, and to give necessary time for the growth of acetoclastic microorganisms, which effectively led to the decrease in VFAs concentration and the system finally recovered after about 11 days. At that time, the feeding was started again at the previous OLR of run VI (11.7 gVS/Lreactor d). The system was effectively recovered without any inhibition effect and achieved the similar methane generation as before. Fig. 2-a represents the trends of TVFA (i.e. the weighted sum of the individual VFAs concentrations considering the molecular weight of each VFA, expressed as acetic acid) during the assay. TVFA evolution was in agreement with the behavior of pH variation as discussed before. Thus, higher values of TVFA were observed during the startup period, which was related with the adaptation stage of the microorganisms to the substrate mixtures. The inoculum used for the start-up of the process was adapted for the treatment of ESBC-DP individually and, hence, an adaptation phase was required as a consequence of the addition of PM as co-substrate. The average TVFA concentration of 3.2 ± 0.8 g/L was Table 2 Characteristics of the different runs tested for the semi-continuous co-digestion of ESBC-DP and PM. Runs
HRT (days) OLR (gVS/Lreactor d) Operation time (days)
0
I
II
III
IV
V
VI
VII
VI0
20 4.2 18
20 4.2 76
18 4.7 54
15 5.9 99
12 7.4 62
8 8.5 37
6 11.2 24
5 12.8 15
6 11.7 67
observed in the start-up period. Nevertheless, after about 30 days of the assay, TVFA concentration decreased gradually to below 1 g/L which were maintained during all the period of run I (Fig. 2-b). Moreover, very high values of TVFA (above 6 g/L) were observed at OLR of 12.8 gVS/Lreactor d (run VII) (Fig. 2-b). The high VFAs accumulation was the main reason for pH drop during this period and, hence, 5 days-HRT was the critical HRT leading to inactivation and washout of methanogenic microorganisms. Therefore, an operational imbalance was occurred in AD phases at 5 days-HRT. The increase in VFAs concentration was observed because their production rate by acidogenic microorganisms was not coupled with their degradation rate by the activity of acetogenic and methanogenic microorganisms (Ahring et al., 1995). Nevertheless, the reactor recovery was possible by stopping semi-continuous regime of feeding and, consecutively, using again the previous OLR of 11.2 gVS/Lreactor d (run VI0 ). The operation at this stage was maintained and stabilized for a long period of 60 days. Fig. 2-a and c shows that the main VFAs generated at Run VII were acetic, propionic and butyric acids with maximum values of 3.6 ± 0.2 and 1.4 ± 0.3 g/L, for acetic and propionic acids, respectively. Butyric acid generation was also observed with a maximum average value around 0.7 ± 0.1 g/L. However, the HPr/HAc ratio in this study does not exceed 1.4 (Table 3), which is the maximum admissible ratio reported in the literature (Hill et al., 1987). The HPr/HAc ratio is an important parameter to check process stability because the increase in propionic acid concentration is related normally to the inhibition of the process. Indeed, it has been pointed out by several authors (Hill et al., 1987; Li et al., 2014; Pullammanappallil et al., 2001) that acetic acid increases is a much more easily recoverable problem in the AD process than the increase in propionic acid concentrations. At run VII, methanogens were inhibited by organic overloading of the reactor and the high acetic acid concentration was attained in the medium. The acidification of AD system is also related with the buffering capacity inside the digester, which refers to the capacity of the medium to neutralize the possible VFAs accumulation in the reactor and hence to maintain the required pH for stable AD operation. Accordingly, the ratio between acidity and alkalinity (TVFA/alkalinity) is largely considered as an earlier indicator of the system stability (Chen et al., 2008; Hill et al., 1987; Kleyböcker et al., 2012). TVFA/alkalinity ratio of 0.4 is considered as the threshold for the system stability and upper values indicates the unstable operating conditions in the digester (Ahring et al., 1995). In the present study, the buffering capacity of the system was good enough to maintain the stable conditions in the digester excepting the startup period and operation at the OLR of 12.8 gVS/Lreactor d (run VII) (Table 3). At this stage, the TVFA/alkalinity ratio reached to 1.3, which later on decreased upon stopping the feeding to the reactor, and resulting in the decrease of TVFA concentration. It should be noted that the main buffering capacity in the SSTR was likely provided by PM, which is obvious from the characteristic of the substrate (refer to Table 1). Aboudi et al. (2015) have studied the batch anaerobic co-digestion of ESBC-DP with PM by testing different mixture ratios and comparing results with the mono-digestion of each substrate. They observed the high VFAs production for the individual digestion of ESBC-DP. However, for the co-digestion of ESBC and PM mixtures, the VFAs were stabilized by the buffering capacity provided to the medium by the livestock manure. 3.2. Effect of OLR on the organic matter evolution and degradation (DOC and VS removal) Fig. 3 shows the evolution of dissolved organic carbon (DOC) throughout the seven OLRs tested. The findings revealed that despite that OLR was increased initially (runs 0–IV), the residual
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(a) Stop feeding 0
Runs
I
II
III
V
IV
V
VII
VI’
(b) 0
Runs
I
(c) Stop feeding
Runs
VII
VI
VI’
Fig. 2. (a) pH and TVFA profile during the co-digestion of ESBC-DP and PM in the SSTR. (b) Detailed evolution of pH, TVFA and individual VFAs during the start-up period. (c) Detailed evolution of pH, TVFA and individual VFAs during the failure period.
DOC in the effluents was decreased. Which indicates that the system was well adapted to the substrates mixture. Subsequently, the slight increase in DOC was observed in runs V and VI. However, the transition to the run VII leads to very high
values of DOC in the effluent, which indicates the destabilization of the system and inactivation of microorganisms. Therefore, an imbalance between the rates of hydrolysis, acidogenesis and methanogenesis was occurred at 5 days-HRT and OLR of
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Table 3 Summarize of the main parameters related to the process stability. Runs *
TVFA/alkalinity ratio HPr/Hac ratio* *
0
I
II
III
IV
V
VI
VII
VI0
0.48 ± 0.36 0.31 ± 0.15
0.028 ± 0.08 0.69 ± 0.96
0.002 ± 0.01 0.47 ± 0.18
0.003 ± 0.01 0.74 ± 0.68
0.012 ± 0.01 0.45 ± 0.84
0.015 ± 0.08 0.25 ± 0.12
0.054 ± 0.04 0.23 ± 0.12
0.898 ± 0.52 0.54 ± 0.34
0.01 ± 0.10 0.55 ± 0.37
Average of stable data of the steady state for each phase is shown in the table.
Stop feeding Runs
0
I
II
III
IV
V
VI
VII
VI’
Fig. 3. Cumulative biogas production and DOC profile during the co-digestion of ESBC-DP and PM in the SSTR.
12.8 gVS/Lreactor d, and the solubilised organic matter was not converted into CH4, which ultimately leading to the VFAs accumulation in the digester (Fig. 2). Ahring et al. (1995) reported that the interdependence of the microorganisms in an AD system is the key factor for biogas production and system stability. At run VII of the present study, the VFAs generation was higher than the consumption, which leads to the distortion of the process (Pind et al., 2003). The DOC is an important parameter directly related to the organic matter content in the system but it also gives information about biological activity inside the digester. Thus, during the startup period (run 0), the DOC was decreasing gradually and constantly implying that even during the adaptation stage, the organic carbon was used by the microorganisms provided by the inoculum despite the change in the feedstock composition (including 68% of PM and 32% of ESBC-DP). Therefore, the release and consumption of DOC were rapidly balanced in the process, which shows the quality of the inoculum used. Upon comparing the test runs from I to V, it can be observed that the highest DOC degradation was obtained at HRT of 12 days (run IV) with residual DOC values in the medium. Moreover, the DOC level in the medium was increased during runs V (HRT: 8 days) and VI (HRT: 6 days) in comparison with run IV. If compared with the previous runs, the higher DOC concentrations in the medium was an earlier indicator that during this stage the system was close to critical conditions, which was effectively confirmed upon changing to run VII (HRT: 5 days). On the basis of findings related with DOC evolution, it can be concluded that highest treatment capacity can be obtained by working at 12 days-HRT (OLR of 7.4 gVS/Lreactor d). Fig. 4-a shows the VS removal during the co-digestion of ESBC-DP and PM. The higher VS removal were obtained at runs
III, IV and V with a maximum removal of 68% at 12 days-HRT (run IV), which confirm that highest treatment capacity was achieved at this run. For SSTR operation in phase VI (HRT: 6 days), the VS removal was around 57%. The lowest VS removal was observed at run VII (HRT: 5 days) because of the system instability by organic overloading. 3.3. Effect of OLR on biogas and methane generation Fig. 3 shows the accumulated biogas volume (Lbiogas/Lreactor) produced during the assay and Fig. 4 shows the different parameters associated with biogas generation: Fig. 4a shows the average values of biogas quality (% CH4) and the methane production rate (MPR) for the different runs performed, while Fig. 4b represents the daily volumes of methane produced and the specific methane production (SMP) for each run. Figs. 3 and 4 show that the biogas production was relatively low during the startup period (Run 0). In this period, only 6.1 LCH4/d were produced (Fig. 4-b). However, the progressive increase in OLR for the subsequent stages (runs I–VI) leads to the continuous increase in methane productions, which were 12.4 LCH4/d (run I), 15.4 LCH4/d (run II), 19.3 LCH4/d (run III), 19.4 LCH4/d (run VI), 27.7 LCH4/d (run V) and 30.3 LCH4/d (run VI). Nevertheless, the increase of OLR to 12.8 gVS/Lreactor d (runVII) caused a sharp decreasing in methane production due to the overloading of reactor at this stage. Fig. 4a shows that the CH4 production rates (MPR) were increased with the increase in OLR, excepting for the critical OLR of 12.8 gVS/Lreactor d (run VII), where the MPR decreased to 0.8 L/Lreactor d, which reflecting the system failure at this OLR due to system overloading. Moreover, the highest methane content
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(a)
SMP
(b) 400
CH4 producon
40
30
300
25 20
200
15
100
10
CH4 Volume (L/day)
SMP (mLCH4/gVSfed)
35
5 0
0 0
I
II
III
IV
V
VI
VII
VI’
Runs
Fig. 4. (a) Volatile solids removal, biogas quality (%CH4) and methane production rate (MPR) during the co-digestion of ESBC-DP and PM in the SSTR. (b) Daily methane productions and specific methane productions (SMP).
was obtained at run I (HRT: 20 days) with an average value of 60.5%. In addition, for all the runs performed, the CH4 content was above 50%, while for the critical conditions (run VII) the CH4 content dropped to around 48.5%. The specific methane production (SMP) increases in run 0 to run IV and decreases in run IV to run VII. This trend is similar to the evolution of organic matter degradation as discussed in previous section. Therefore, the maximum SMP of 362.2 mLCH4/gVSfed, was obtained in run IV (12 days-HRT and OLR of 7.4 gVS/Lreactor d). Aboudi et al. (2015) studying the anaerobic mesophilic co-digestion of ESBC-DP and PM in batch reactors with the same feed (68% PM and 32% ESBC) and reported that SMP was 494 mL CH4/gVSfed. Therefore, the value obtained under the best operating conditions in semi-continuous process (HRT: 12 days) represents a 73% of the value obtained for batch system. For other runs, the SMP
values ranged from 54% to 66% with respect to the values achieved in batch assay. Finally, for run VII (HRT: 5 days), the SMP was lower (only 12% of batch data) as a consequence of system distortion by organic overloading. Data obtained in the batch co-digestion process must be considered as the maximum yield of methane, since the digestion continued until a negligible biogas production was observed. Therefore, the SMP obtainable operating in the semicontinuous regime is necessarily lower than that.
4. Conclusion Anaerobic co-digestion of ESBC-DP with PM was conducted successfully in mesophilic SSTR operating at HRT ranged from 20 to 6 days. The highest VS removal of 68% was achieved at a HRT of
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12 days (7.4 gVS/Lreactor d). However, the higher methane production was observed at a HRT of 6 days (4.2 gVS/Lreactor d). The OLR of 12.8 gVS/Lreactor d, (HRT: 5 days) was found critical for the semi-continuous co-digestion assay, mainly due to high VFA accumulation and pH drop. The digester recovery was possible by stopping feeding the reactor and restarting with previous conditions of 6 days HRT. Acknowledgements This research was supported by the Spanish Ministry of Economy and Finance (project CTM2013-43938-R) and the MICINN (project UNCA08-1E-035) and co-funded by the European Regional Development Funds (ERDF). The authors acknowledge to the University of Cadiz (Spain) for the Scholarship UCA-2010-063PU/EPIF-FPI-A/BC and the Agrifood Campus of International Excellence (Ceia3). References Aboudi, K., Álvarez-Gallego, C.J., Romero-García, L.I., 2015. Improvement of exhausted sugar beet cossettes anaerobic digestion process by Co-digestion with pig manure. Energy Fuels 29 (2), 754–762. Ahring, B.K., Sandberg, M., Angelidaki, I., 1995. Volatile fatty acids as indicators of process imbalance in anaerobic digestors. Appl. Microbiol. Biotechnol. 43 (3), 559–565. Alkaya, E., Demirer, G.N., 2011. Anaerobic mesophilic co-digestion of sugar-beet processing wastewater and beet-pulp in batch reactors. Renewable Energy 36 (3), 971–975. Álvarez-Gallego, C. 2005. Testing different procedures for the start up of a dry anaerobic co-digestion process of OFMSW and sewage sludge at thermophilic range. in: Ph.D. thesis, University of Cádiz. Spain. Anwar, Z., Gulfraz, M., Irshad, M., 2014. Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review. Journal of Radiation Research and Applied Sciences 7 (2), 163–173. APHA, 2005. Standards Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association Washington DC, USA. Brooks, L., Parravicini, V., Svardal, K., Kroiss, H., Prendl, L., 2008. Biogas from sugar beet press pulp as substitute of fossil fuel in sugar beet factories. Water Sci. Technol. 58, 1497–1504. Cysneiros, D., Banks, C.J., Heaven, S., Karatzas, K.A.G., 2012. The effect of pH control and ’hydraulic flush’ on hydrolysis and Volatile Fatty Acids (VFA) production and profile in anaerobic leach bed reactors digesting a high solids content substrate. Bioresour. Technol. 123, 263–271. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: A review. Bioresour. Technol. 99 (10), 4044–4064. Chowdhury, R.B.S., Fulford, D.J., 1992. Batch and semi-continuous anaerobic digestion systems. Renewable Energy 2 (4–5), 391–400. Dai, X., Gai, X., Dong, B., 2014. Rheology evolution of sludge through high-solid anaerobic digestion. Bioresour. Technol. 174, 6–10. Demirel, B., Neumann, L., Scherer, P., 2008. Microbial community dynamics of a continuous mesophilic anaerobic biogas digester fed with sugar beet silage. Eng. Life Sci. 8 (4), 390–398. Fang, C., Boe, K., Angelidaki, I., 2011. Anaerobic co-digestion of by-products from sugar production with cow manure. Water Res. 45 (11), 3473–3480.
Hill, D.T., Cobb, S.A., Bolte, J.P., 1987. Using volatile fatty acid relationships to predict anaerobic digester failure. Transactions of the American Society of Agricultural Engineers 30 (2), 496–501. Hindrichsen, I.K., Wettstein, H.R., Machmüller, A., Knudsen, K.E.B., Madsen, J., Kreuzer, M., 2006. Digestive and metabolic utilisation of dairy cows supplemented with concentrates characterised by different carbohydrates. Anim. Feed Sci. Technol. 126 (1–2), 43–61. Hutnan, M., Drtil, M., Derco, J., Mrafkova, L., Hornak, M., Mico, S., 2001. Two-Step Pilot-Scale Anaerobic Treatment of Sugar Beet Pulp. Polish Journal of Environmental Studies 10 (4), 237–243. Hutnan, M., Drtil, M., Mrafkova, L., 2000. Anaerobic biodegradation of sugar beet pulp. Biodegradation 11 (4), 203–211. Kivaisi, A.K., Mtila, M., 1997. Production of biogas from water hyacinth (Eichhornia crassipes) (Mart) (Solms) in a two-stage bioreactor. World J. Microbiol. Biotechnol. 14 (1), 125–131. Kleyböcker, A., Liebrich, M., Verstraete, W., Kraume, M., Würdemann, H., 2012. Early warning indicators for process failure due to organic overloading by rapeseed oil in one-stage continuously stirred tank reactor, sewage sludge and waste digesters. Bioresour. Technol. 123, 534–541. Lehtomäki, A., Huttunen, S., Rintala, J.A., 2007. Laboratory investigations on codigestion of energy crops and crop residues with cow manure for methane production: Effect of crop to manure ratio. Resour. Conserv. Recycl. 51 (3), 591– 609. Li, L., He, Q., Wei, Y., He, Q., Peng, X., 2014. Early warning indicators for monitoring the process failure of anaerobic digestion system of food waste. Bioresour. Technol. 171, 491–494. Mata-Alvarez, J., Dosta, J., Romero-Güiza, 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. Montañés, R., Solera, R., Pérez, M., 2015. Anaerobic co-digestion of sewage sludge and sugar beet pulp lixiviation in batch reactors: Effect of temperature. Bioresour. Technol. 180, 177–184. Moraes, B.S., Triolo, J.M., Lecona, V.P., Zaiat, M., Sommer, S.G., 2015. Biogas production within the bioethanol production chain: use of co-substrates for anaerobic digestion of sugar beet vinasse. Bioresour. Technol. 190, 227–234. Ohuchi, Y., Ying, C., Lateef, S.A., Ihara, I., Iwasaki, M., Inoue, R., Umetsu, K., 2014. Anaerobic co-digestion of sugar beet tops silage and dairy cow manure under thermophilic condition. J. Mater. Cycles Waste Manage. Pind, P.F., Angelidaki, I., Ahring, B.K., 2003. Dynamics of the anaerobic process: Effects of volatile fatty acids. Biotechnol. Bioeng. 82 (7), 791–801. Pullammanappallil, P.C., Chynoweth, D.P., Lyberatos, G., Svoronos, S.A., 2001. Stable performance of anaerobic digestion in the presence of a high concentration of propionic acid. Bioresour. Technol. 78 (2), 165–169. Rodriguez-Verde, I., Regueiro, L., Carballa, M., Hospido, A., Lema, J.M., 2014. Assessing anaerobic co-digestion of pig manure with agroindustrial wastes: The link between environmental impacts and operational parameters. Sci. Total Environ. 497, 475–483. Sawatdeenarunat, C., Surendra, K.C., Takara, D., Oechsner, H., Khanal, S.K., 2015. Anaerobic digestion of lignocellulosic biomass: Challenges and opportunities. Bioresour. Technol. 178, 178–186. Stoyanova, E., Forsthuber, B., Pohn, S., Schwarz, C., Fuchs, W., Bochmann, G., 2014. Reducing the risk of foaming and decreasing viscosity by two-stage anaerobic digestion of sugar beet pressed pulp. Biodegradation 25 (2), 277–289. Suhartini, S., Heaven, S., Banks, C.J., 2014. Comparison of mesophilic and thermophilic anaerobic digestion of sugar beet pulp: Performance, dewaterability and foam control. Bioresour. Technol. 152, 202–211. Yan, B.H., Selvam, A., Wong, J.W.C., 2014. Application of rumen microbes to enhance food waste hydrolysis in acidogenic leach-bed reactors. Bioresour. Technol. 168, 64–71.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Semi-continuous anaerobic co-digestion of sugar beet byproduct and pig manure: Effect of the organic loading rate (OLR) on process performance Kaoutar Aboudi ⇑, Carlos José Álvarez-Gallego, Luis Isidoro Romero-García Department of Chemical Engineering and Food Technology, Faculty of Sciences, Agrifood Campus of International Excellence (CeiA3), University of Cádiz, 11510 Puerto Real, Cadiz, Spain
h i g h l i g h t s The anaerobic co-digestion of sugar beet byproducts with pig manure is feasible. The highest methane productivity was obtained at hydraulic retention time of 6 days. The organic loading rate maximizing the methane production was 11.2 gVS/Lreactor d. The hydraulic retention time of 5 days leads to volatile fatty acids accumulation. System recovery is possible by increasing hydraulic retention time from 5 to 6 days.
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Article history: Received 24 May 2015 Received in revised form 10 July 2015 Accepted 11 July 2015 Available online 17 July 2015 Keywords: Co-digestion Organic loading rate Hydraulic retention time Semi-continuous tank reactor Methane production
a b s t r a c t Anaerobic co-digestion of dried pellet of exhausted sugar beet cossettes (ESBC-DP) with pig manure (PM) was investigated in a semi-continuous stirred tank reactor (SSTR) under mesophilic conditions. Seven hydraulic retention times (HRT) from 20 to 5 days were tested with the aim to evaluate the methane productivities and volatile solids (VS) removal. The corresponding organic loading rates (OLR) ranged from 4.2 to 12.8 gVS/Lreactor d. The findings revealed that highest system efficiency was achieved at an OLR of 11.2 gVS/Lreactor d (6 days-HRT) with a methane production rate (MPR) and volatile solids (VS) reduction of 2.91 LCH4/Lreactor d and 57.5%, respectively. The HRT of 5 days was found critical for the studied process, which leads to volatile fatty acids (VFA) accumulation and sharp drop in pH. However, the increase of HRT permits the recovery of system. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Sugar beet byproducts (SBB) generated during industrial sugar extraction from sugar beet plant (Beta vulgaris) are mainly composed of pulp and molasses. It may cause serious environmental problems if not properly treated before discharge. The exhausted sugar beet cossettes (ESBC) is produced in several sugar beet processing factories, and it consists of a mixture of exhausted slices of sugar beet pulp (cossettes) with molasses. This by-product is also dried and pelletized and it is sold as complement for animal feed. However, sugar-manufacturing plants require electrical and thermal energy at different stages of the process, which makes it energy intensive process. Moreover, the use of SBB as animal feed is not a promising strategy while its use as renewable energy resource could add economic and environmental benefits. ⇑ Corresponding author. Tel.: +34 956016474; fax: +34 956016411. E-mail address: [email protected] (K. Aboudi). http://dx.doi.org/10.1016/j.biortech.2015.07.031 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Previous studies showed that SBB are a suitable material for biological treatment by means of anaerobic digestion (AD) giving the advantages of agro-food wastes to produce clean energy such as methane from biogas (Aboudi et al., 2015; Alkaya and Demirer, 2011; Fang et al., 2011; Hutnan et al., 2000; Lehtomäki et al., 2007; Montañés et al., 2015; Ohuchi et al., 2014; Suhartini et al., 2014). However, bioconversion of agro-food byproducts is still having limitations due to the presence of hardly bio-degradable lignocellulosic content and nitrogen deficiency despite high carbohydrate content (Anwar et al., 2014; Sawatdeenarunat et al., 2015). Co-digestion of agro-food byproducts with nitrogen rich livestock manures can resolve this problem by balancing the nutrient content in the anaerobic digester, providing the required buffering capacity and adding a variety of microorganisms coming from animal digestive tract who are capable to degrade vegetal fibers (Hindrichsen et al., 2006; Mata-Alvarez et al., 2014; Rodriguez-Verde et al., 2014; Yan et al., 2014).
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Abbreviations Acronyms
Parameters (units)
Analytical methods C/N carbon/nitrogen ratio chemical oxygen demand; t: total; s: soluble (gO2/kg) t/sCOD DOC dissolved organic carbon HPr/HAc propionic/acetic ratio (gC/kg) N-NH4+ ammonia nitrogen (gN/kg) TKN total Kjeldahl nitrogen (gN/kg(TS)) TS total solids (g/kg) TVFA total volatile fatty acidity (gHAc/kg) VFA volatile fatty acid (g/L) VS volatile solids ((%TS)) Feedstock ESBC-DP PM
SBB SBP CM
sugar beet byproducts sugar beet pulp cow manure
Opeartive conditions AcoD anaerobic co-digestion AD anaerobic digestion CSTR continuous stirred tank reactor HRT hydraulic retention time (days) MPR methane production rate (LCH4/Lr d) OLR organic loading rate (gVS/Lr d) OM organic matter SMP specific methane production (mLCH4/gVSfed) SSTR semi-continuous stirred tank reactor
dried pellets of exhausted sugar beet cossettes pig manure
AD reactors can be performed in mainly three operating modes according to the feeding strategy including batch, semi-continuous and continuous operation. A batch system is run by feeding the reactor only once at the start of the process. The reactor is then sealed and monitored for the duration of the process until the degradation of all biodegradable material mainly giving information such as the maximum biogas productivity. In a semi-continuous system, the reactor is periodically loaded with the organic matter (OM) according to the established conditions. In continuous system, the reactor is constantly fed with the OM. Continuous (CSTR) and semi-continuous (SSTR) systems led to assess the stability (constant biogas production) and performance of the digester in the studied conditions and permit to define the capacity threshold of the digester for OM treatment (Chowdhury and Fulford, 1992). Some studies have investigated the anaerobic digestion of SBB and, very few studies were focused on sugar beet pulp (SBP). Fang et al. (2011) studied the semi-continuous thermophilic anaerobic co-digestion (AcoD) of SBP (wet pulp) with cow manure (CM) operating at HRT of 20 days and OLR of 6.75 gVS/Lreactor d. The authors found that co-digestion of SBP and CM with 50% SBP proportions achieved a specific methane production (SMP) of 280 mL CH4/gVSfed. However, a relatively high volatile fatty acids (VFA) concentration was observed in the reactor due to the high proportion of SBP in the mixture. Lehtomäki et al. (2007) investigated anaerobic co-digestion (AcoD) of different crop residues with CM in a continuous stirred tank reactor (CSTR). The authors obtained a SMP of 229 mLCH4/gVSfed in the co-digestion of 30% of sugar beet tops with CM, which was significantly higher than that obtained from CM mono-digestion. Nevertheless, the increase of OLR to 4 gVS/Lreactor d disturbed the AcoD process by decreasing the methane generation. Moraes et al. (2015) working with a CSTR for the treatment of sugar beet vinasse with 3% CM have reported that CM addition provided stability to AD process and enhanced the methane yield compared with mono-digestion of vinasse. So far, no studies were conducted on anaerobic co-digestion of exhausted sugar beet cossettes as dried pellets (ESBC-DP) with pig manure (PM) using semi-continuous stirred tank reactors. The main objective of this study was to determine the maximum OLR and the minimum HRT allowed by a semi-continuous type anaerobic reactor co-digesting the ESBC-DP with PM under mesophilic conditions.
2. Methods 2.1. Feedstock Dried pellets of exhausted sugar beet cossettes (ESBC-DP) were used in this study, which were composed by 85% of pulp and 15% of residual molasses. ESBC-DP were collected from sugar beet processing factory located at Jerez de la Frontera (Cádiz, Spain). Pig manure (PM) was collected from a semi-intensive farm facility at El Puerto de Santa Maria (Cádiz, Spain). The used PM corresponds to the entire animal excrement because the farm does not have any system for urine and feces separation at source. The used dry pellets had around 20–70 mm of length and 6 mm diameter and its total solids (TS) content ranged 80–90%. The pellets were stored at 4 °C to avoid its degradation. PM was periodically collected and stored under the same conditions as ESBC-DP. The mixture of both substrates was prepared within C/N ratio of 18.5, which was the best mixture ratio proposed in previous studies using similar wastes (Aboudi et al., 2015). Kivaisi and Mtila (1997) also proposed a C/N range of 16–18 for lignocellulosic-type wastes. The TS content was fixed to 8%, which is in agreement with those suggested in previous studies by Hutnan et al. (2000, 2001). The rheological behavior of ESBC-DP (very fibrous material) makes infeasible to work at higher TS content, because it leads to inadequate mixing conditions in the reactor and inducing the mass transfer limitations (Dai et al., 2014; Stoyanova et al., 2014). 2.2. Reactor configuration and start-up The photo and the detailed scheme of the SSTR used is this study, are shown in Fig. 1. The reactor was entirely made up of stainless steel. Agitation was performed at 12 rpm by a motor installed at the top of the reactor (Heidolph-RZR-2102) with a stirring blade. The temperature was maintained at 37 ± 0.25 °C by propelling water from a circulating bath (Ultraterm200-Selecta) through the reactor jacket. The top of the reactor has several ports: inlet port for feeding, output port for biogas collection, and an overpressure valve. The startup period of the SSTR was as follow: a 10 L working volume reactor was fed with the effluent coming from another SSTR treating ESBC-DP (as the only substrate) under mesophilic
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Srring motor
Srring blade
40L Tedlar biogas bag
Feed
Thermostac bath circulang
Effluent
Fig. 1. Photograph and detailed scheme of the SSTR used in the mesophilic co-digestion of ESBC-DP and PM.
Table 1 Physico-chemical characteristics of the substrates, inoculum and the feedstock mixture.
*
Components
Units
ESBC-DP
PM
Inoculum
Feedstock
pH TS VS DOC* sCOD* tCOD* TVFA* Alkalinity* N-NH4+* TKN Ratio C/N
– g/kg (%TS) gC/kg gO2/kg gO2/kg gHAc/kg gCaCO3/kg gN /kg gN/kg(TS) –
5.72 ± 0.31 887.30 ± 15.34 91.65 ± 2.34 48.82 ± 1.26 52.00 ± 4.45 134.28 ± 3.52 2.53 ± 0.46 2.85 ± 0.27 0.28 ± 1.32 12.18 ± 2.51 37.00 ± 0.22
6.44 ± 0.15 221. 61 ± 20.82 71.76 ± 11.84 6.44 ± 12.19 15.71 ± 3.84 44.78 ± 1.92 7.58 ± 1.77 50.2 ± 0.2 3.2 ± 0.7 35.74 ± 1.24 13.12 ± 0.27
7.54 ± 0.85 34.97 ± 10.59 21.75 ± 3.28 4.68 ± 0.72 8.97 ± 2.13 11.64 ± 2.52 0.08 ± 0.27 30.4 ± 2.5 1.1 ± 0.2 6.25 ± 1.28 19.42 ± 2.36
6.75 ± 0.07 86.88 ± 20.21 85.90 ± 5.92 5.85 ± 1.01 19.62 ± 4.34 73.41 ± 3.86 1.45 ± 0.42 73.25 ± 5.33 0.21 ± 0.05 26.62 ± 1.02 18.52 ± 0.26
g/kg (wet basis).
conditions at 20 days HRT. Once the reactor was filled, a feeding strategy with a mixture of ESBC-DP and PM was started under operation at 20 days-HRT (OLR of 4.2 gVS/Lreactor d) with different OLRs conditions. Each HRT tested in this study was maintained at least for a period of time equivalent to three HRT in order to allow the pseudo-steady state could be achieved. Table 1 shows the physico-chemical characteristics of the feedstock used in the seven runs of different OLRs (corresponding to HRTs range from 20 to 5 days). 2.3. Analytical methods The TS, volatile solids (VS), total and soluble chemical oxygen demand (tCOD, sCOD), dissolved organic carbon (DOC), pH, alkalinity and ammonium were analyzed according to the Standard Methods (APHA, 2005). For the parameters determined in soluble form (sCOD, DOC and VFA), the samples were previously lixiviated with deionized water for 2 h and filtered by 0.47 mm glass fiber filter (Álvarez-Gallego, 2005). Total COD was measured directly from the diluted effluent without any filtrations. The dissolved organic carbon (DOC) analysis was carried out in an Analytic-Jena Multi N/C 3100 carbon analyzer with chemiluminescence detector
(CLD) according to combustion-infrared method (5310B) of Standard Methods (APHA, 2005). The oxidizing gas was oxygen 5.0 (Abelló-Linde) at a pressure of 4–6 bars. Samples from the previous lixiviation and filtration were filtered again by a Teflon filter of 0.22 lm for VFA analysis, and were analyzed with a gas chromatograph (Shimadzu GC-2010) equipped with a flame ionization detector (FID) and capillary column filled with Nukol™ from Supelco (diameter of 0.25 lm and 30 m length). Biogas generated during the assays was collected in a 50L Tedlar bag (SKC) and the volume of gas was measured daily using a high precision drum-type gas meter (Ritter TG5). The gas composition (CH4, CO2, and H2) was determined by using a gas chromatograph (Shimadzu GC-2014) with a stainless steel column packed with Carbosieve™ SII from Supelco (diameter of 3.2 mm and 3.0 m length) and thermal conductivity detector (TCD) (Aboudi et al., 2015).
3. Results and discussion In this section, different parameters associated with the process stability and the production of methane gas, were discussed by
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comparing the experimental results obtained for the seven OLRs studied. Table 2 shows the different stages (runs) performed, indicating the HRT corresponding to each OLR tested and the operation time for each run. 3.1. Effect of OLR on the evolutions of pH and VFA Fig. 2-a shows the pH profile in the semi-continuous co-digestion reactor. At the beginning of the startup period (run 0), pH values were below 7 as a consequence that hydrolysis and acidification of the organic matter were not coupled with the utilization of VFAs by methanogenic microorganisms. This is an adaptation stage and hence, the pH correction by adding an alkali (NaOH, 8 M) was necessary to avoid the startup failure by acidification. Subsequently, the pH increased gradually from 6.7 (on day 6) to 7.4–7.8 from day 16 onwards. The pH correction strategy was used when the reactor effluent had a pH below 7, especially when changing the feedstock and/or increasing the OLR to ensure optimum conditions for the microbial growth (Brooks et al., 2008; Cysneiros et al., 2012). Once the system was stabilized, the pH levels were self-maintained above 7 during the whole assay for all OLRs tested. This pH stability and regulation by the system itself indicated that the digester was working within optimum conditions for methane generation and was not under inhibitory conditions despite of increasing OLR. However, a sharp decrease in the pH value was observed upon increasing the OLR to 12.8 gVS/Lreactor d (run VII) as a result of the acidification of the medium despite of daily pH corrections. The digester operation was severely affected with no biogas production and high levels of VFAs accumulation (acetic, propionic and butyric acids, mainly). It shows that the growth rate of acidogenic bacteria was higher than that of the highly sensitive methanogenic archaea (Demirel et al., 2008). Hence, an imbalance between the metabolic activities of different microorganisms involved in the process was occurred, and the reactor was overloaded under operation at 12.8 gVS/Lreactor d (run VII). In this situation, the best strategy was to stop feeding the reactor, and to give necessary time for the growth of acetoclastic microorganisms, which effectively led to the decrease in VFAs concentration and the system finally recovered after about 11 days. At that time, the feeding was started again at the previous OLR of run VI (11.7 gVS/Lreactor d). The system was effectively recovered without any inhibition effect and achieved the similar methane generation as before. Fig. 2-a represents the trends of TVFA (i.e. the weighted sum of the individual VFAs concentrations considering the molecular weight of each VFA, expressed as acetic acid) during the assay. TVFA evolution was in agreement with the behavior of pH variation as discussed before. Thus, higher values of TVFA were observed during the startup period, which was related with the adaptation stage of the microorganisms to the substrate mixtures. The inoculum used for the start-up of the process was adapted for the treatment of ESBC-DP individually and, hence, an adaptation phase was required as a consequence of the addition of PM as co-substrate. The average TVFA concentration of 3.2 ± 0.8 g/L was Table 2 Characteristics of the different runs tested for the semi-continuous co-digestion of ESBC-DP and PM. Runs
HRT (days) OLR (gVS/Lreactor d) Operation time (days)
0
I
II
III
IV
V
VI
VII
VI0
20 4.2 18
20 4.2 76
18 4.7 54
15 5.9 99
12 7.4 62
8 8.5 37
6 11.2 24
5 12.8 15
6 11.7 67
observed in the start-up period. Nevertheless, after about 30 days of the assay, TVFA concentration decreased gradually to below 1 g/L which were maintained during all the period of run I (Fig. 2-b). Moreover, very high values of TVFA (above 6 g/L) were observed at OLR of 12.8 gVS/Lreactor d (run VII) (Fig. 2-b). The high VFAs accumulation was the main reason for pH drop during this period and, hence, 5 days-HRT was the critical HRT leading to inactivation and washout of methanogenic microorganisms. Therefore, an operational imbalance was occurred in AD phases at 5 days-HRT. The increase in VFAs concentration was observed because their production rate by acidogenic microorganisms was not coupled with their degradation rate by the activity of acetogenic and methanogenic microorganisms (Ahring et al., 1995). Nevertheless, the reactor recovery was possible by stopping semi-continuous regime of feeding and, consecutively, using again the previous OLR of 11.2 gVS/Lreactor d (run VI0 ). The operation at this stage was maintained and stabilized for a long period of 60 days. Fig. 2-a and c shows that the main VFAs generated at Run VII were acetic, propionic and butyric acids with maximum values of 3.6 ± 0.2 and 1.4 ± 0.3 g/L, for acetic and propionic acids, respectively. Butyric acid generation was also observed with a maximum average value around 0.7 ± 0.1 g/L. However, the HPr/HAc ratio in this study does not exceed 1.4 (Table 3), which is the maximum admissible ratio reported in the literature (Hill et al., 1987). The HPr/HAc ratio is an important parameter to check process stability because the increase in propionic acid concentration is related normally to the inhibition of the process. Indeed, it has been pointed out by several authors (Hill et al., 1987; Li et al., 2014; Pullammanappallil et al., 2001) that acetic acid increases is a much more easily recoverable problem in the AD process than the increase in propionic acid concentrations. At run VII, methanogens were inhibited by organic overloading of the reactor and the high acetic acid concentration was attained in the medium. The acidification of AD system is also related with the buffering capacity inside the digester, which refers to the capacity of the medium to neutralize the possible VFAs accumulation in the reactor and hence to maintain the required pH for stable AD operation. Accordingly, the ratio between acidity and alkalinity (TVFA/alkalinity) is largely considered as an earlier indicator of the system stability (Chen et al., 2008; Hill et al., 1987; Kleyböcker et al., 2012). TVFA/alkalinity ratio of 0.4 is considered as the threshold for the system stability and upper values indicates the unstable operating conditions in the digester (Ahring et al., 1995). In the present study, the buffering capacity of the system was good enough to maintain the stable conditions in the digester excepting the startup period and operation at the OLR of 12.8 gVS/Lreactor d (run VII) (Table 3). At this stage, the TVFA/alkalinity ratio reached to 1.3, which later on decreased upon stopping the feeding to the reactor, and resulting in the decrease of TVFA concentration. It should be noted that the main buffering capacity in the SSTR was likely provided by PM, which is obvious from the characteristic of the substrate (refer to Table 1). Aboudi et al. (2015) have studied the batch anaerobic co-digestion of ESBC-DP with PM by testing different mixture ratios and comparing results with the mono-digestion of each substrate. They observed the high VFAs production for the individual digestion of ESBC-DP. However, for the co-digestion of ESBC and PM mixtures, the VFAs were stabilized by the buffering capacity provided to the medium by the livestock manure. 3.2. Effect of OLR on the organic matter evolution and degradation (DOC and VS removal) Fig. 3 shows the evolution of dissolved organic carbon (DOC) throughout the seven OLRs tested. The findings revealed that despite that OLR was increased initially (runs 0–IV), the residual
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(a) Stop feeding 0
Runs
I
II
III
V
IV
V
VII
VI’
(b) 0
Runs
I
(c) Stop feeding
Runs
VII
VI
VI’
Fig. 2. (a) pH and TVFA profile during the co-digestion of ESBC-DP and PM in the SSTR. (b) Detailed evolution of pH, TVFA and individual VFAs during the start-up period. (c) Detailed evolution of pH, TVFA and individual VFAs during the failure period.
DOC in the effluents was decreased. Which indicates that the system was well adapted to the substrates mixture. Subsequently, the slight increase in DOC was observed in runs V and VI. However, the transition to the run VII leads to very high
values of DOC in the effluent, which indicates the destabilization of the system and inactivation of microorganisms. Therefore, an imbalance between the rates of hydrolysis, acidogenesis and methanogenesis was occurred at 5 days-HRT and OLR of
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Table 3 Summarize of the main parameters related to the process stability. Runs *
TVFA/alkalinity ratio HPr/Hac ratio* *
0
I
II
III
IV
V
VI
VII
VI0
0.48 ± 0.36 0.31 ± 0.15
0.028 ± 0.08 0.69 ± 0.96
0.002 ± 0.01 0.47 ± 0.18
0.003 ± 0.01 0.74 ± 0.68
0.012 ± 0.01 0.45 ± 0.84
0.015 ± 0.08 0.25 ± 0.12
0.054 ± 0.04 0.23 ± 0.12
0.898 ± 0.52 0.54 ± 0.34
0.01 ± 0.10 0.55 ± 0.37
Average of stable data of the steady state for each phase is shown in the table.
Stop feeding Runs
0
I
II
III
IV
V
VI
VII
VI’
Fig. 3. Cumulative biogas production and DOC profile during the co-digestion of ESBC-DP and PM in the SSTR.
12.8 gVS/Lreactor d, and the solubilised organic matter was not converted into CH4, which ultimately leading to the VFAs accumulation in the digester (Fig. 2). Ahring et al. (1995) reported that the interdependence of the microorganisms in an AD system is the key factor for biogas production and system stability. At run VII of the present study, the VFAs generation was higher than the consumption, which leads to the distortion of the process (Pind et al., 2003). The DOC is an important parameter directly related to the organic matter content in the system but it also gives information about biological activity inside the digester. Thus, during the startup period (run 0), the DOC was decreasing gradually and constantly implying that even during the adaptation stage, the organic carbon was used by the microorganisms provided by the inoculum despite the change in the feedstock composition (including 68% of PM and 32% of ESBC-DP). Therefore, the release and consumption of DOC were rapidly balanced in the process, which shows the quality of the inoculum used. Upon comparing the test runs from I to V, it can be observed that the highest DOC degradation was obtained at HRT of 12 days (run IV) with residual DOC values in the medium. Moreover, the DOC level in the medium was increased during runs V (HRT: 8 days) and VI (HRT: 6 days) in comparison with run IV. If compared with the previous runs, the higher DOC concentrations in the medium was an earlier indicator that during this stage the system was close to critical conditions, which was effectively confirmed upon changing to run VII (HRT: 5 days). On the basis of findings related with DOC evolution, it can be concluded that highest treatment capacity can be obtained by working at 12 days-HRT (OLR of 7.4 gVS/Lreactor d). Fig. 4-a shows the VS removal during the co-digestion of ESBC-DP and PM. The higher VS removal were obtained at runs
III, IV and V with a maximum removal of 68% at 12 days-HRT (run IV), which confirm that highest treatment capacity was achieved at this run. For SSTR operation in phase VI (HRT: 6 days), the VS removal was around 57%. The lowest VS removal was observed at run VII (HRT: 5 days) because of the system instability by organic overloading. 3.3. Effect of OLR on biogas and methane generation Fig. 3 shows the accumulated biogas volume (Lbiogas/Lreactor) produced during the assay and Fig. 4 shows the different parameters associated with biogas generation: Fig. 4a shows the average values of biogas quality (% CH4) and the methane production rate (MPR) for the different runs performed, while Fig. 4b represents the daily volumes of methane produced and the specific methane production (SMP) for each run. Figs. 3 and 4 show that the biogas production was relatively low during the startup period (Run 0). In this period, only 6.1 LCH4/d were produced (Fig. 4-b). However, the progressive increase in OLR for the subsequent stages (runs I–VI) leads to the continuous increase in methane productions, which were 12.4 LCH4/d (run I), 15.4 LCH4/d (run II), 19.3 LCH4/d (run III), 19.4 LCH4/d (run VI), 27.7 LCH4/d (run V) and 30.3 LCH4/d (run VI). Nevertheless, the increase of OLR to 12.8 gVS/Lreactor d (runVII) caused a sharp decreasing in methane production due to the overloading of reactor at this stage. Fig. 4a shows that the CH4 production rates (MPR) were increased with the increase in OLR, excepting for the critical OLR of 12.8 gVS/Lreactor d (run VII), where the MPR decreased to 0.8 L/Lreactor d, which reflecting the system failure at this OLR due to system overloading. Moreover, the highest methane content
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CH4 producon
40
30
300
25 20
200
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Fig. 4. (a) Volatile solids removal, biogas quality (%CH4) and methane production rate (MPR) during the co-digestion of ESBC-DP and PM in the SSTR. (b) Daily methane productions and specific methane productions (SMP).
was obtained at run I (HRT: 20 days) with an average value of 60.5%. In addition, for all the runs performed, the CH4 content was above 50%, while for the critical conditions (run VII) the CH4 content dropped to around 48.5%. The specific methane production (SMP) increases in run 0 to run IV and decreases in run IV to run VII. This trend is similar to the evolution of organic matter degradation as discussed in previous section. Therefore, the maximum SMP of 362.2 mLCH4/gVSfed, was obtained in run IV (12 days-HRT and OLR of 7.4 gVS/Lreactor d). Aboudi et al. (2015) studying the anaerobic mesophilic co-digestion of ESBC-DP and PM in batch reactors with the same feed (68% PM and 32% ESBC) and reported that SMP was 494 mL CH4/gVSfed. Therefore, the value obtained under the best operating conditions in semi-continuous process (HRT: 12 days) represents a 73% of the value obtained for batch system. For other runs, the SMP
values ranged from 54% to 66% with respect to the values achieved in batch assay. Finally, for run VII (HRT: 5 days), the SMP was lower (only 12% of batch data) as a consequence of system distortion by organic overloading. Data obtained in the batch co-digestion process must be considered as the maximum yield of methane, since the digestion continued until a negligible biogas production was observed. Therefore, the SMP obtainable operating in the semicontinuous regime is necessarily lower than that.
4. Conclusion Anaerobic co-digestion of ESBC-DP with PM was conducted successfully in mesophilic SSTR operating at HRT ranged from 20 to 6 days. The highest VS removal of 68% was achieved at a HRT of
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12 days (7.4 gVS/Lreactor d). However, the higher methane production was observed at a HRT of 6 days (4.2 gVS/Lreactor d). The OLR of 12.8 gVS/Lreactor d, (HRT: 5 days) was found critical for the semi-continuous co-digestion assay, mainly due to high VFA accumulation and pH drop. The digester recovery was possible by stopping feeding the reactor and restarting with previous conditions of 6 days HRT. Acknowledgements This research was supported by the Spanish Ministry of Economy and Finance (project CTM2013-43938-R) and the MICINN (project UNCA08-1E-035) and co-funded by the European Regional Development Funds (ERDF). The authors acknowledge to the University of Cadiz (Spain) for the Scholarship UCA-2010-063PU/EPIF-FPI-A/BC and the Agrifood Campus of International Excellence (Ceia3). References Aboudi, K., Álvarez-Gallego, C.J., Romero-García, L.I., 2015. Improvement of exhausted sugar beet cossettes anaerobic digestion process by Co-digestion with pig manure. Energy Fuels 29 (2), 754–762. Ahring, B.K., Sandberg, M., Angelidaki, I., 1995. Volatile fatty acids as indicators of process imbalance in anaerobic digestors. Appl. Microbiol. Biotechnol. 43 (3), 559–565. Alkaya, E., Demirer, G.N., 2011. Anaerobic mesophilic co-digestion of sugar-beet processing wastewater and beet-pulp in batch reactors. Renewable Energy 36 (3), 971–975. Álvarez-Gallego, C. 2005. Testing different procedures for the start up of a dry anaerobic co-digestion process of OFMSW and sewage sludge at thermophilic range. in: Ph.D. thesis, University of Cádiz. Spain. Anwar, Z., Gulfraz, M., Irshad, M., 2014. Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review. Journal of Radiation Research and Applied Sciences 7 (2), 163–173. APHA, 2005. Standards Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association Washington DC, USA. Brooks, L., Parravicini, V., Svardal, K., Kroiss, H., Prendl, L., 2008. Biogas from sugar beet press pulp as substitute of fossil fuel in sugar beet factories. Water Sci. Technol. 58, 1497–1504. Cysneiros, D., Banks, C.J., Heaven, S., Karatzas, K.A.G., 2012. The effect of pH control and ’hydraulic flush’ on hydrolysis and Volatile Fatty Acids (VFA) production and profile in anaerobic leach bed reactors digesting a high solids content substrate. Bioresour. Technol. 123, 263–271. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: A review. Bioresour. Technol. 99 (10), 4044–4064. Chowdhury, R.B.S., Fulford, D.J., 1992. Batch and semi-continuous anaerobic digestion systems. Renewable Energy 2 (4–5), 391–400. Dai, X., Gai, X., Dong, B., 2014. Rheology evolution of sludge through high-solid anaerobic digestion. Bioresour. Technol. 174, 6–10. Demirel, B., Neumann, L., Scherer, P., 2008. Microbial community dynamics of a continuous mesophilic anaerobic biogas digester fed with sugar beet silage. Eng. Life Sci. 8 (4), 390–398. Fang, C., Boe, K., Angelidaki, I., 2011. Anaerobic co-digestion of by-products from sugar production with cow manure. Water Res. 45 (11), 3473–3480.
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