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Mesophilic co-digestion of palm oil mill effluent and empty fruit bunches a
a
a
Sang-Hyoun Kim , Seon-Mi Choi , Hyun-Jun Ju & Jin-Young Jung
b
a
Department of Environmental Engineering , Daegu University , Jillyang, Gyeongsan, Gyeongbuk 712–714, South Korea b
Department of Environmental Engineering , Yeungnam University , Gyeongsan, Gyeongbuk 712–749, South Korea Published online: 08 Oct 2013.
To cite this article: Sang-Hyoun Kim , Seon-Mi Choi , Hyun-Jun Ju & Jin-Young Jung (2013) Mesophilic co-digestion of palm oil mill effluent and empty fruit bunches, Environmental Technology, 34:13-14, 2163-2170, DOI: 10.1080/09593330.2013.826253 To link to this article: http://dx.doi.org/10.1080/09593330.2013.826253
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2163–2170, http://dx.doi.org/10.1080/09593330.2013.826253
Mesophilic co-digestion of palm oil mill effluent and empty fruit bunches Sang-Hyoun Kima , Seon-Mi Choia , Hyun-Jun Jua and Jin-Young Jungb∗ a Department
of Environmental Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk 712–714, South Korea; of Environmental Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 712–749, South Korea
b Department
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(Received 28 March 2013; accepted 3 July 2013 ) The palm oil mill industry generates palm oil mill effluent (POME) and empty fruit bunches (EFB) as by-products. This study reports the mesophilic co-digestion of POME with EFB. The biochemical methane potential (BMP) of POME and EFB was 0.397 L CH4 /g volatile solids (VS) and 0.264 L CH4 /g VS, respectively. In a series of batch tests at various EFB to POME ratios, the maximum methane production rate was achieved at an EFB:POME ratio of 0.25–0.31:1. Performance data from lab-scale digesters confirmed the positive synergism by the addition of EFB to POME, which was attributed to the balanced chemical composition, for example the chemical oxygen demand (COD) to total Kjeldahl nitrogen (TKN) ratio. The EFB addition enhanced the acceptable organic loading rate, methane production, COD removal, and microbial activity. The mesophilic co-digestion of POME and EFB promises to be a viable recycling method to alleviate pollution problems and recover renewable energy in the palm oil mill industry. Keywords: palm oil mill effluent; empty fruit bunch; co-digestion; mesophilic; synergism
1. Introduction The palm oil production chain generates large quantities of biomass by-products of little value such as palm oil mill effluent (POME) and empty fruit bunches (EFB).[1] The conversion of these by-products to energy would offer a potential for designing a certified and sustainable, food oil and biodiesel production system based on the palm oil industry. POME is generated from the sterilization of fresh fruit bunches (FFB), clarification of palm oil, and effluent from hydrocyclone operation.[2] It has a total solids content of 5–7%, of which a little over half is dissolved solids, with the other half being a mixture of various forms of organic and inorganic suspended solids.[3] About 0.675 m3 of POME is generated for every tonne of FFB [4] and 59.3 million m3 of POME is generated yearly in Malaysia, equivalent to 3.62 million tons of chemical oxygen demand (COD).[5] In developing countries, an anaerobic lagoon has been employed by most palm oil mills as their primary treatment of POME.[1] However, the low treatment efficiency of the conventional treatment results in high atmospheric methane gas emissions.[6] Yacob et al. [6] showed that an average of 116 kg of carbon dioxide greenhouse gas is emitted for every tonne of POME discharged to an anaerobic lagoon. Therefore, closed anaerobic digester systems including continuous stirred tank reactor, upflow anaerobic sludge blanket reactor, anaerobic filter, and fluidized bed reactor have been widely examined. POME is
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
discharged at temperatures around 80–90◦ C which makes both mesophilic and thermophilic digestion feasible.[2] However, most of the existing digesters are conducted in the mesophlic temperature owing to better process stability.[1] EFB is the other problematic by-product of palm oil. EFB is barely used and suffers from problematic disposal due to the banning of open field/pile burning.[7] A high moisture content of 60–70% hinders the use of EFB as fuel.[8] About 0.23 ton of EFB is generated for every tonne of FFB.[4] Co-digestion is a technology that is increasingly being applied for the simultaneous treatment of several solid and liquid organic wastes.[9] This strategy offers economic advantages such as increased organic loading rate (OLR) of digester and biogas yield, minimizing equipment for different waste streams, and easier handling of mixed waste.[10] It can also increase the efficiency of the digestion process, by improving the methane yields due to the positive synergistic effects of the mixed waste with complementary characteristics such as nutrients balancing and toxicity dilution.[11] EFB has high content of organic matter and has the potential to be used for biogas production.[12] In spite of its lignocellulosic structure, the biochemical methane potential (BMP) of EFB was reported to be 0.15–0.20 L CH4 /g volatile solids (VS) in thermophilic condition (55◦ C).[13,14] O-thong et al. [15] reported batch-type co-digestion of EFB and POME in thermophilic condition. They reported that the addition of POME enhanced biodegradability of EFB at mixing ratios
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of 0.6:1–3.4:1 on a COD basis. However,the information of co-digestion of POME with EFB at mesophilic temperature range is not available yet. This study, therefore, investigates the feasibility for the co-digestion of POME and EFB at mesophilic temperature range. The main study goals are to (i) examine the anaerobic digestibility of POME and EFB, (ii) study the effect of the EFB to POME ratio on biogas production, and (iii) research the continuous operation performance of the mesophilic co-digestion. 2. Materials and methods 2.1. Feedstock and inoculum POME and EFB were obtained from a palm oil mill in Malaysia. EFB was ground by a hammer mill and sieved to a final powder size below 1 mm. The characteristics of POME and EFB were analysed and summarized in Table 1. Anaerobic digester sludge from a full-scale digester in Gyeongsan wastewater treatment plant was used as methanogenic inoculum. The sludge digester was operated at 35◦ C, at a hydraulic retention time (HRT) of 25 d, and an OLR of 1.8 g VS/L.d by feeding a mixture of primary and secondary sludge. The pH, alkalinity, and volatile suspended solids (VSS) concentration of the digested sludge were 7.6, 2.9 g CaCO3 /L, and 16.8 g/L, respectively. 2.2. Batch test for anaerobic degradability The BMPs of POME and EFB were examined using a 250-mL serum vial with a working volume of 150 mL.
Glucose was used as a reference substrate. To the serum bottle, was added 30 mL of inoculum, to give an initial biomass concentration of 3.3 g VSS/L, and then POME, EFB, or glucose, equivalent to 3.0 g VS/L as the initial substrate concentration. All the cases were duplicated. Blank bottles without a substrate were also prepared. The following nutrients were supplied to each bottle: 6.0 g/L of NaHCO3 , 0.5 g/L of KH2 PO4 , 0.53 g/L of NH4 Cl, 0.1 g/L MgCl2 · 6H2 O, 75 mg/L CaCl2 · 2H2 O, 20 mg/L FeCl2 · 4H2 O, 0.5 mg/L of MnCl2 · 4H2 O, 0.25 mg/L of H3 BO3 , 0.25 mg/L of ZnCl2 , 0.15 mg/L of CuCl2 , 0.05 mg/L of NaMoO4 · 2H2 O, 2.5 mg/L of CoCl2 · 6H2 O, 0.25 mg/L of NiCl2 · 6H2 O, and 0.25 mg/L of Na2 SeO4 . The bottle was then filled to the working volume with distilled water and the pH was adjusted to 7.0–7.7 using 1 M HCl or KOH. Subsequently, the bottle was purged with N2 gas for 3 min, sealed with butyl-rubber septum and aluminium cap, and agitated at 150 rpm and 35◦ C. The septum was periodically pricked with a needle attached to a glass syringe to measure the biogas production and to maintain the headspace pressure below 2 atm. Figure 1(a) depicts the schematic diagram of batch experiments. The effect of EFB addition on POME digestion was investigated using a series of batch tests at various EFB to POME ratios on a COD basis (0, 0.12, 0.25, 0.31, 0.62, and 1.0). The initial POME concentration was maintained
(a) Periodic measurement
GC analysis for gas composition
of gas volume by glass syringe Butyl-rubber septum
Table 1.
Aluminum cap
Characteristics of POME and EFB.
Item Total solids (TS) VS Total suspended solids (TSS) VSS COD Soluble chemical oxygen demand (SCOD) TKN Total phosphorus (TP) VS/TS (g/g) VSS/TSS (g/g) VSS/VS (g/g) COD/VS (g/g) SCOD/COD (g/g) COD/TKN (g/g) pH Fe (mg/g COD) Ca (mg/g COD) Mg (mg/g COD) Mn (mg/g COD) Zn (mg/g COD)
POME
EFB
46.2 ± 5.9 g/L 37.0 ± 4.6 g/L 15.2 ± 1.8 g/L
350 ± 16 g/kg 929 ± 43 g/kg TS 1000 ± 0 g/kg TS
14.4 ± 1.8 g/L 49.2 ± 6.2 g/L 29.5 ± 3.3 g/L
929 ± 43 g/kg TS 1114 ± 53 g/kg TS 0 g/kg TS
1.00 ± 0.12 g/L 103 ± 14 mg/L
7.13 ± 0.41 g/kg TS 0.23 ± 0.01 g/kg TS
0.80 ± 0.14 0.95 ± 0.16 0.39 ± 0.07 1.33 ± 0.24 0.60 ± 0.10 49 4.3 ± 0.2 6.92 4.19 11.0 0.140 0.0688
0.93 ± 0.04 0.93 ± 0.04 1.00 ± 0 1.20 ± 0.08 0.00 156 ± 11 – 1.81 3.94 1.75 0.0347 0.0374
• •
Reciprocating shaking at 150 rpm Temperature = 35ºC
Serum bottle with working volume 150 mL
(b) POME + EFB
Effluent
Fill (2 min)
React (23 hr 56 min)
Draw (2 min)
Figure 1. Schematic diagrams of: (a) batch experiments and (b) the three phases of the intermittent–continuous stirred tank reactor.
Environmental Technology Table 2. Experimental conditions for the batch tests at various EFB to POME ratios. EFB to POME ratio (COD basis)
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POME (g COD/L) EFB (g COD/L) Total Feed (g COD/L) COD/TKN (g/g)
0
0.12
0.25
0.31
0.62
1.0
4.8 0.0 4.8 49
4.8 0.6 5.2 53
4.8 1.2 6.0 57
4.8 1.5 6.3 59
4.8 3.0 7.8 67
4.8 4.8 9.6 75
at 4.8 g COD/L, while the amount of added EFB was varied to give the desired EFB to POME ratios, as shown in Table 2. In this case, no inorganic compound was added except 6.0 g/L of NaHCO3 . The other experimental procedures were the same as those for the batch test using POME, EFB, or glucose as the sole substrate.
2.3. Lab-scale digester operation Two identical intermittent-continuous stirred tank reactors with a working volume of 10 L (318 mm in liquid depth and 200 mm in inner diameter) were fabricated as lab-scale digesters and operated in a room at constant 35 ± 1◦ C temperature. Each reactor was connected to a gas collector with water displacement to measure the biogas production. A reactor (R–A) was inoculated with 9.75 L of the digester sludge and operated in a Fill–React–Draw operation on a 24-h cycle. Once a day, 0.25 L of POME supplemented with 1.0 g of NaHCO3 was fed as the sole substrate (HRT 40 d). The addition of NaHCO3 raised the feedstock pH to around 6.0. In this study, NaHCO3 was selected due to its high solubility and lack of the need for CO2 neutralization. In a full-scale operation, it could be replaced by less costly chemicals, such as lime or sodium carbonate.[16] Fill and Draw were completed within 2 min each. After 52 days, half of the biomass (5 L) was transferred to the other reactor (R–B) anaerobically. Then, R–A was fed with 0.40 L of POME plus 1.6 g of NaHCO3 once a day without the Draw phase. For R–B, 0.40 L of POME, 10.3 g of EFB, and 1.6 g of NaHCO3 were added every day without the Draw phase. When the working volume of the reactors reached 10 L, the operational sequence was reverted to the regular one composed of Fill, React, and Draw phases. Figure 1(b) shows the schematic diagram of Table 3.
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the three phases of the intermittent–continuous stirred tank reactors. For R–A, HRT was decreased to 25, 20, and 16 d stepwise. For R–B, HRT was decreased to 25, 20, 16, and 14 d with EFB to POME ratios of 0.25 and 0.31 on a COD basis. The reactor operating conditions are summarized in Table 3. In each condition, the pseudo-steady-state performance with stable CH4 production and effluent COD concentration (±10%) were maintained for at least 3 consecutive days before performance data were measured. 2.4.
Specific methanogenic activity
The specific methanogenic activity (SMA) of the lab-scale digesters was analysed using serum vials (working volume 150 mL).[17] The methanogenic microflora were taken at the end of each operating mode and washed with the basal medium used in the BMP test three times. The SMA of the microflora (0.6–0.8 g VSS) was measured for a specific substrate (2 g COD/L). In this study, acetate and propionate were used as substrates. The other experimental procedures were same as that of the BMP test. 2.5.
Analysis and assay
The measured biogas production was corrected to standard temperature (0◦ C) and pressure (760 mmHg). The CH4 content in the biogas was determined by gas chromatograph (SRI 310, SRI Instrument) with a 0.9 m × 3.2 mm stainless steel column with porapak Q (80/100 mesh). Organic acids were measured using high-performance liquid chromatography (Spectrasystem P2000, Spectraphysics) with an ultraviolet detector and a 100 × 7.8 mm fast acid column (Aminex). Solids, COD, total Kjeldahl nitrogen (TKN), and total phosphorus were quantified according to standard methods.[13] The micronutrient contents were analysed using Inductively Coupled Plasma/Atomic emission spectrometer (ICP/AES) (Polyscan 60E, Thermo Jarell Co.). The methane production curve in the batch test was fitted to a modified Gompertz equation[11] RM M = P × exp −exp × (λ − t) × e + 1 , (1) P where M = is the cumulative methane production (mL CH4 ), P the ultimate methane production (mL CH4 ),
Operating conditions for the lab-scale digesters condition.
HRT (d) POME (g COD/L) EFB (g COD/L) EFB to POME Total OLR (g COD/L.d) POME OLR (g COD/L.d)
I-A
II-A
II-B-1
II-B-2
III-A
III-B-2
IV-A
IV-B-2
V-B-1
V-B-2
40 53.3 0 0 1.33 1.33
25 40.9 0 0 1.64 1.64
25 40.9 10.2 0.25 2.04 1.64
25 40.9 12.6 0.31 2.14 1.64
20 48.0 0 0 2.40 2.40
20 48.0 14.9 0.31 3.14 2.40
16 54.4 0 0 3.40 3.40
16 54.4 16.9 0.31 4.46 3.40
14 54.4 13.6 0.25 4.86 3.89
14 54.4 16.9 0.31 5.09 3.89
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BMP (L CH4 /g VS) BMP (L CH4 /g COD) Relative BMP compared with glucose CH4 production rate (mL CH4 /gVSS.d) Relative CH4 productivity compared with glucose Lag-phase time (d) Relative lag-phase time compared with glucose R2 for the Gompertz equation
RM the methane production rate (L CH4 /d), λ the lag-phase time (d), t the time (d), and e the exponential 1.
POME
EFB
Glucose
0.397 ± 0.009 0.301 ± 0.007 0.99 ± 0.02 55.1 ± 2.1 1.31 ± 0.05 2.2 ± 0.4 2.4 ± 0.5 0.9978 ± 0.0003
0.264 ± 0.007 0.221 ± 0.005 0.73 ± 0.02 20.1 ± 0.9 0.48 ± 0.02 6.0 ± 1.1 6.7 ± 1.4 0.9933 ± 0.0017
0.323 ± 0.002 0.305 ± 0.002 – 42.2 ± 0.5 – 0.9 ± 0.1 – 0.9988 ± 0.0010
3.1. Anaerobic degradability of POME and EFB The cumulative CH4 production curves obtained from the batch test using POME, EFB, or glucose as the sole substrate were described well by the modified Gompertz equation (Equation (1)) with correlation coefficients, R2 , larger than 0.997, as presented in Table 4. The methane yield (BMP)
and the specific methane production rate were calculated from P and the added substrate, and from RM and the biomass concentration, respectively. Table 4 shows that the BMP of POME expressed, based on VS and COD, were nearly equal to glucose with 86% of COD in POME being converted to methane, although 40% of COD was insoluble. The specific methane production rate was 1.31 times higher than that of glucose for MBP on the basis of VS. The lag-phase time was also less than 3 days. Therefore, POME is a relevant feedstock for mesophilic digestion.
Figure 2. Effect of EFB to POME ratio on (a) the specific methane production rate and (b) the methane yield in the batch test.
Figure 3. Daily variation in methane production rate from (a) the digester fed with POME only and (b) the digester fed with POME and EFB.
3.
Results and discussion
Environmental Technology EFB showed a longer lag-phase time and a lower methane yield and production rate compared with those of POME due to its lignocellulosic structure.[4] However, the BMP of EFB, 0.264 L CH4 /g VS and 0.221 L CH4 /g COD, was in the range of the BMP for conventional feedstock in anaerobic digestion (0.2 to 0.4 L CH4 /g VS).[18,19] Furthermore, it is higher than reported values of EFB in thermophilic digestion.[14,15] From the data obtained, it can be established that EFB also could be an effective feedstock for mesophilic digestion.
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3.2.
Effect of EFB addition on the anaerobic digestion POME As the cumulative CH4 production curves were well fitted with Equation (1) (R2 > 0.997), the methane yield and
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methane production rate obtained from a series of batch tests at various amounts of EFB addition on POME were calculated from P and the added substrate and from RM and the biomass concentration, respectively. EFB addition to POME increased the specific rate and the ultimate amount of methane production. The specific methane production rate of co-digestion at an EFB:POME ratio of 0.25–0.31 was 50.8 mL CH4 /g VSS.d, which was 1.2 times higher than that of POME (Figure 2(a)). The increase in specific methane production rate by codigestion has usually been attributed to mitigate toxicant levels, improve nutrients balance, or enhance microbial activity.[15,20–24] As POME had the high methane yield and production rate similar to those of glucose, the toxicant level of POME was considered negligible in this batch test. Among the nutrients balance, the carbon to nitrogen (C/N),
Figure 4. Performance comparison of the digesters on (a) CH4 production rate, (b) CH4 yield based on TCOD, (c) CH4 yield based on POME COD, (d) TCOD removal efficienty, (e) effluent COD concentration, and (f) Effluent pH.
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in other words COD to TKN, ratio is the most common interpretation of the synergism in co-digestion. A low COD to TKN ratio could cause a lack of substrate, whereas an excess COD to TKN could result in a nitrogen deficiency in microbial growth.[9,21,23–25] In this study, as the EFB to POME ratio was increased from 0 to 0.25–0.31, the COD to TKN ratio was increased from 49 to 57–59. The raised COD to TKN ratio was probably beneficial to co-digestion of the wastes from the palm oil production chain. Other effects including enhanced cellulase activity by the addition of the cellulosic by-products may have also caused the increased SMA.[15,19,23] However, the balance of micronutrients including Fe, Ca, Mg, Mn, and Zn would not result in the synergism, as POME had higher contents of them than EFB. As the BMP of EFB was lower than that of POME, the methane yield based on total COD (POME + EFB) decreased linearly according to the amount of EFB, as depicted in Figure 1(a). However, the EFB to POME ratio up to 1 did not hinder the anaerobic conversion of POME to methane, but did increase the total methane production amount. The methane yield based on POME increased linearly without any elongated lag-phase time, as shown in Figure 2(b), which indicated that EFB was also utilized as the substrate. 3.3.
Performance of POME and EFB co-digestion
Figure 3 shows the dynamics of the methane production rates for the lab-scale digester fed with POME and EFB and the control digester fed with POME only. In the control digester, the methane yield and COD removal efficiency were maintained above 0.26 L/g COD and 80%, respectively, up to POME loading rates of 2.40 g COD/L.d (HRT 20 d), but declined suddenly at 3.40 g COD/L.d (HRT 16 d). This performance deterioration occurred simultaneously with an effluent pH drop to 6.2 and an increase in effluent-soluble COD concentration, which implied a damaged balance between the several microbial groups for anaerobic digestion.[17] The addition of EFB increased the absolute amount of methane production at every POME loading rate. Furthermore, the co-digestion improved the reactor stability at the high organic loading rates. The performance enhancement by co-digestion was greater at an EFB to POME ratio of 0.31 than at a ratio of 0.25. The synergistic effect of co-digestion was attributed to the balanced chemical composition, for example the COD to TKN ratio [17,19,23]. As the POME loading rate was increased to 3.40 g POME COD/L.d (HRT 16 d; IV-B-2) and 3.89 g POME COD/L.d (HRT 14 d; V-B-1 and V-B-2), the methane yield and TCOD removal efficiency in the co-digestion were also decreased. However, the co-digestion still showed better performance than the single digestion of POME, and the effluent pH values were maintained over 7.0. Figure 4 summarizes the average performance data of the pseudo-steady state at each operational condition.
Figure 5. Organic acid production at (a) EFB:POME = 0:1 (POME only), (b) EFB:POME = 0.25:1, and (c) EFB:POME = 0.31:1.
The effluent organic acids concentration is depicted in Figure 5. In the control digester, organic acids, the intermediates of anaerobic degradation were accumulated at 3.40 g COD/L.d. Acetate, propionate, and n-butyrate were over 95% of the total organic acids detected. Among these three, the propionate concentration was the highest, implying that the conversion of propionate to acetate was one of the rate-limiting steps [11,17]. Organic acids accumulation, with propionate as the major component, was also found in the co-digestion at 3.40 and 3.89 g COD/L.d, but the amounts were lower than those for the control. Table 5 compares the SMA of the biomass fed to the single substrate (POME) and to the co-substrate (POME and EFB) at every POME loading rate. The test was conducted using precursor substrates of methane, acetate, and propionate, to identify the effects of co-digestion and
Environmental Technology Table 5.
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SMA of methanogenic biomass. SMA (mL CH4 /g VSS.d) Co-digestion of POME and EFB
POME loading (g POME/CODd)
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1.33 1.64 2.40 3.40
Single digestion of POME
Acetate
Propionate
Glucose
Acetate
Propionate
Glucose
81.2 30.9 26.6 –
43.5 36.5 6.7 –
45.1 37.2 11.3 –
86.0 48.1 47.8 34.6
42.4 21.0 18.0 34.6
42.6 22.2 18.8 11.3
the organic loading rate on the microbial activities of the anaerobic microflora. In most cases, the SMA on acetate was higher than that on propionate, which was concurrent with the propionate accumulation in the digester operation. The SMA decreased with increasing organic loading rate, but co-digestion mitigated an abrupt decline. For the biomass fed with POME at 3.40 g POME COD/L.d, the SMA on propionate declined below 7 mL CH4 /g VSS.d. A pH drop resulting from organic acid accumulation is the most probable limitation that seriously reduces the SMA of the biomass.[11] By contrast, the SMA on propionate for the biomass fed with the co-substrate up to 3.89 g POME COD/L.d was maintained over 17 mL CH4 /g VSS.d. Co-digestion improved both the microbial activity of the biomass and the performance. 4. Conclusions The authors investigated the feasibility for co-digestion of POME with EFB at mesophilic temperature range. The BMPs of the waste by-products were within the general range of anaerobic digestion. At various tested EFB to POME ratios, the methane production rate was maximized at EFB:POME = 0.25–0.31:1. The synergistic effect of co-digestion was attributed to the balanced chemical composition, for example the COD to TKN ratio. The EFB addition also improved the acceptable organic loading rate, methane production, COD removal, and microbial activity of a lab-scale digester. The mesophilic co-digestion of POME and EFB is a viable recycling method to alleviate pollution and recover energy in the palm oil mill industry. Funding This research was supported by Yeungnam University research grant in 2013.
References [1] Poh PE, Chong MF. Development of anaerobic digestion methods for palm oil mill effluent (POME) treatment. Bioresour Technol. 2009;100:1–9. [2] Borja R, Banks CJ. Anaerobic digestion of palm oil mill effluent using an up-flow anaerobic sludge blanket reactor. Biomass Bioenergy. 1994;6:381–389.
[3] Bhatia S, Othman Z, Ahmad AL. Pretreatment of palm oil mill effluent (POME) using Moringa oleifera seeds as natural coagulant. J Hazard Mater. 2007;145:120–126. [4] Baharuddin AS, Hock LS, Yusof MZM, Rahman NAA, Shah UM, Hassan MA, Wakisaka M, Sakai K, Shirai Y. Effects of palm oil mill effluent (POME) anaerobic sludge from 500 m3 of closed anaerobic methane digested tank on pressed-shredded empty fruit bunch (EFB) composting process. Afr J Biotechnol. 2010;9(16):2427–2436. [5] Loh SK, Choo YM. Achievements and challenges in adoption of biofuels and bio-energy. 3rd International Symposium on Biofuels and Bio-Energy; New Delhi; 2012. p. 19–20. [6] Yacob S, Hassan MA, Shirai Y, Wakisaka M, Subash S. Baseline study of methane emission from open digesting tanks of palm oil mill effluent treatment. Chemosphere. 2005;59:1575–1581. [7] Husain Z, Zainal ZA, Abdullah MZ. Analysis of biomassresidue-based cogeneration system in palm oil mills. Biomass Bioenergy. 2003;24(2):117–124. [8] Paepatung N, Nopharatana A, Songkasiri W. Bio-methane potential of biological solid materials and agricultural wastes. Asian J Energy Environ. 2009;10(1):19–27. [9] Bouallagui H, Lahdheb H, Ben Romdan E, Rachdi B, Hamdi M. Improvement of fruit and vegetable waste anaerobic digestion performance and stability with co-substrates addition. J Environ Manage. 2009;90:1844–1849. [10] Garcia-pena EI, Parameswaran P, Kang DW, Canul-Chan M, Krajmalnik-Brown R. Anaerobic digestion and co-digestion processes of vegetable and fruit residues: process and microbial ecology. Bioresour Technol. 2011;102:9447–9455. [11] Chen GY, Zheng Z, Yang SG, Fang CX, Zou XX, Zhang JB. Improving conversion of Spartina alterniflora into biogas by co-digestion with cow feces. Fuel Process Technol. 2010;91:1416–1421. [12] Hosseini SE, Wahid MA. Feasibility study of biogas production and utilization as a source of renewable energy in Malaysia. Renew Sust Energy Rev. 2013;19:454–462. [13] Purwandari FA, Sanjaya AP, Millati R, Cahyanto MN, Horvath IS, Niklasson C, Taherzadeh MJ. Pretreatment of oil palm empty fruit bunch (OPEFB) by N-methylmorpholine-N-oxide (NMMO) for biogas production: structural changes and digestion improvement. Bioresour Technol. 2013;128:461–466. [14] Nieves DC, Karimi K, Horvath IS. Improvement of biogas production from oil palm empty fruit bunches (OPEFB). Ind Crop Prod. 2011;34:1097–1101. [15] O-Thong S, Boe K, Angelidaki I. Thermophilic anaerobic co-digestion of oil palm empty fruit bunches with palm oil mill effluent for efficient biogas production. Appl Energ. 2012;93:648–654. [16] Speece RE. Anaerobic biotechnology for industrial wastewaters. Nashville, TN: Archae Press; 1996.
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[17] Kim HW, Nam JY, Shin HS. A comparison study on the high-rate co-digestion of sewage sludge and food waste using a temperature-phased anaerobic sequencing batch reactor system. Bioresour Technol. 2011;102: 7272–7279. [18] APHA, AWWA, and WEF. Standard methods for the examination of water and wastewater. 20th ed. Washington, DC: American Public Health Association; 2005. [19] Yen HW, Brune DE. Anaerobic co-digestion of algal sludge and waste paper to produce methane. Bioresour Technol. 2007;98:130–134. [20] El-Mashad HM, Zhang R. Biogas production from co-digestion of dairy manure and food waste. Bioresour Technol. 2010;101:4021–4028. [21] Labatut RA, Angenent LT, Scott NR. Biochemical methane potential and biodegradability of complex organic substrates. Bioresour Technol. 2011;102:2255–2264.
[22] Xie S, Lawlor PG, Frost JP, Hu Z, Zhan X. Effect of pig manure to grass silage ratio on methane production in batch anaerobic co-digestion of concentrated pig manure and grass silage. Bioresour Technol. 2011;102:5728–5733. [23] Zhong W, Zhang Z, Luo Y, Qiao W, Xiao M, Zhang M. Biogas productivity by co-digesting Taihu blue algae with corn straw as an external carbon source. Bioresour Technol. 2012;114:281–286. [24] Kayhanian M, Hardy S. The impact of four design parameters on the performance of a high-solids anaerobic digestion of municipal solid waste for fuel gas production. Environ Technol. 1994;15:557–567. [25] Najafpour GD, Zinatizadeh AAL, Mohamed AR, Hasnain IM, Nasrollahzadeh H. High-rate anaerobic digestion of palm oil mill effluent in an upflow anaerobic sludge-fixed film bioreactor. Process Biochem. 2006;41: 370–379.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Mesophilic co-digestion of palm oil mill effluent and empty fruit bunches a
a
a
Sang-Hyoun Kim , Seon-Mi Choi , Hyun-Jun Ju & Jin-Young Jung
b
a
Department of Environmental Engineering , Daegu University , Jillyang, Gyeongsan, Gyeongbuk 712–714, South Korea b
Department of Environmental Engineering , Yeungnam University , Gyeongsan, Gyeongbuk 712–749, South Korea Published online: 08 Oct 2013.
To cite this article: Sang-Hyoun Kim , Seon-Mi Choi , Hyun-Jun Ju & Jin-Young Jung (2013) Mesophilic co-digestion of palm oil mill effluent and empty fruit bunches, Environmental Technology, 34:13-14, 2163-2170, DOI: 10.1080/09593330.2013.826253 To link to this article: http://dx.doi.org/10.1080/09593330.2013.826253
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 2163–2170, http://dx.doi.org/10.1080/09593330.2013.826253
Mesophilic co-digestion of palm oil mill effluent and empty fruit bunches Sang-Hyoun Kima , Seon-Mi Choia , Hyun-Jun Jua and Jin-Young Jungb∗ a Department
of Environmental Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk 712–714, South Korea; of Environmental Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 712–749, South Korea
b Department
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(Received 28 March 2013; accepted 3 July 2013 ) The palm oil mill industry generates palm oil mill effluent (POME) and empty fruit bunches (EFB) as by-products. This study reports the mesophilic co-digestion of POME with EFB. The biochemical methane potential (BMP) of POME and EFB was 0.397 L CH4 /g volatile solids (VS) and 0.264 L CH4 /g VS, respectively. In a series of batch tests at various EFB to POME ratios, the maximum methane production rate was achieved at an EFB:POME ratio of 0.25–0.31:1. Performance data from lab-scale digesters confirmed the positive synergism by the addition of EFB to POME, which was attributed to the balanced chemical composition, for example the chemical oxygen demand (COD) to total Kjeldahl nitrogen (TKN) ratio. The EFB addition enhanced the acceptable organic loading rate, methane production, COD removal, and microbial activity. The mesophilic co-digestion of POME and EFB promises to be a viable recycling method to alleviate pollution problems and recover renewable energy in the palm oil mill industry. Keywords: palm oil mill effluent; empty fruit bunch; co-digestion; mesophilic; synergism
1. Introduction The palm oil production chain generates large quantities of biomass by-products of little value such as palm oil mill effluent (POME) and empty fruit bunches (EFB).[1] The conversion of these by-products to energy would offer a potential for designing a certified and sustainable, food oil and biodiesel production system based on the palm oil industry. POME is generated from the sterilization of fresh fruit bunches (FFB), clarification of palm oil, and effluent from hydrocyclone operation.[2] It has a total solids content of 5–7%, of which a little over half is dissolved solids, with the other half being a mixture of various forms of organic and inorganic suspended solids.[3] About 0.675 m3 of POME is generated for every tonne of FFB [4] and 59.3 million m3 of POME is generated yearly in Malaysia, equivalent to 3.62 million tons of chemical oxygen demand (COD).[5] In developing countries, an anaerobic lagoon has been employed by most palm oil mills as their primary treatment of POME.[1] However, the low treatment efficiency of the conventional treatment results in high atmospheric methane gas emissions.[6] Yacob et al. [6] showed that an average of 116 kg of carbon dioxide greenhouse gas is emitted for every tonne of POME discharged to an anaerobic lagoon. Therefore, closed anaerobic digester systems including continuous stirred tank reactor, upflow anaerobic sludge blanket reactor, anaerobic filter, and fluidized bed reactor have been widely examined. POME is
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
discharged at temperatures around 80–90◦ C which makes both mesophilic and thermophilic digestion feasible.[2] However, most of the existing digesters are conducted in the mesophlic temperature owing to better process stability.[1] EFB is the other problematic by-product of palm oil. EFB is barely used and suffers from problematic disposal due to the banning of open field/pile burning.[7] A high moisture content of 60–70% hinders the use of EFB as fuel.[8] About 0.23 ton of EFB is generated for every tonne of FFB.[4] Co-digestion is a technology that is increasingly being applied for the simultaneous treatment of several solid and liquid organic wastes.[9] This strategy offers economic advantages such as increased organic loading rate (OLR) of digester and biogas yield, minimizing equipment for different waste streams, and easier handling of mixed waste.[10] It can also increase the efficiency of the digestion process, by improving the methane yields due to the positive synergistic effects of the mixed waste with complementary characteristics such as nutrients balancing and toxicity dilution.[11] EFB has high content of organic matter and has the potential to be used for biogas production.[12] In spite of its lignocellulosic structure, the biochemical methane potential (BMP) of EFB was reported to be 0.15–0.20 L CH4 /g volatile solids (VS) in thermophilic condition (55◦ C).[13,14] O-thong et al. [15] reported batch-type co-digestion of EFB and POME in thermophilic condition. They reported that the addition of POME enhanced biodegradability of EFB at mixing ratios
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of 0.6:1–3.4:1 on a COD basis. However,the information of co-digestion of POME with EFB at mesophilic temperature range is not available yet. This study, therefore, investigates the feasibility for the co-digestion of POME and EFB at mesophilic temperature range. The main study goals are to (i) examine the anaerobic digestibility of POME and EFB, (ii) study the effect of the EFB to POME ratio on biogas production, and (iii) research the continuous operation performance of the mesophilic co-digestion. 2. Materials and methods 2.1. Feedstock and inoculum POME and EFB were obtained from a palm oil mill in Malaysia. EFB was ground by a hammer mill and sieved to a final powder size below 1 mm. The characteristics of POME and EFB were analysed and summarized in Table 1. Anaerobic digester sludge from a full-scale digester in Gyeongsan wastewater treatment plant was used as methanogenic inoculum. The sludge digester was operated at 35◦ C, at a hydraulic retention time (HRT) of 25 d, and an OLR of 1.8 g VS/L.d by feeding a mixture of primary and secondary sludge. The pH, alkalinity, and volatile suspended solids (VSS) concentration of the digested sludge were 7.6, 2.9 g CaCO3 /L, and 16.8 g/L, respectively. 2.2. Batch test for anaerobic degradability The BMPs of POME and EFB were examined using a 250-mL serum vial with a working volume of 150 mL.
Glucose was used as a reference substrate. To the serum bottle, was added 30 mL of inoculum, to give an initial biomass concentration of 3.3 g VSS/L, and then POME, EFB, or glucose, equivalent to 3.0 g VS/L as the initial substrate concentration. All the cases were duplicated. Blank bottles without a substrate were also prepared. The following nutrients were supplied to each bottle: 6.0 g/L of NaHCO3 , 0.5 g/L of KH2 PO4 , 0.53 g/L of NH4 Cl, 0.1 g/L MgCl2 · 6H2 O, 75 mg/L CaCl2 · 2H2 O, 20 mg/L FeCl2 · 4H2 O, 0.5 mg/L of MnCl2 · 4H2 O, 0.25 mg/L of H3 BO3 , 0.25 mg/L of ZnCl2 , 0.15 mg/L of CuCl2 , 0.05 mg/L of NaMoO4 · 2H2 O, 2.5 mg/L of CoCl2 · 6H2 O, 0.25 mg/L of NiCl2 · 6H2 O, and 0.25 mg/L of Na2 SeO4 . The bottle was then filled to the working volume with distilled water and the pH was adjusted to 7.0–7.7 using 1 M HCl or KOH. Subsequently, the bottle was purged with N2 gas for 3 min, sealed with butyl-rubber septum and aluminium cap, and agitated at 150 rpm and 35◦ C. The septum was periodically pricked with a needle attached to a glass syringe to measure the biogas production and to maintain the headspace pressure below 2 atm. Figure 1(a) depicts the schematic diagram of batch experiments. The effect of EFB addition on POME digestion was investigated using a series of batch tests at various EFB to POME ratios on a COD basis (0, 0.12, 0.25, 0.31, 0.62, and 1.0). The initial POME concentration was maintained
(a) Periodic measurement
GC analysis for gas composition
of gas volume by glass syringe Butyl-rubber septum
Table 1.
Aluminum cap
Characteristics of POME and EFB.
Item Total solids (TS) VS Total suspended solids (TSS) VSS COD Soluble chemical oxygen demand (SCOD) TKN Total phosphorus (TP) VS/TS (g/g) VSS/TSS (g/g) VSS/VS (g/g) COD/VS (g/g) SCOD/COD (g/g) COD/TKN (g/g) pH Fe (mg/g COD) Ca (mg/g COD) Mg (mg/g COD) Mn (mg/g COD) Zn (mg/g COD)
POME
EFB
46.2 ± 5.9 g/L 37.0 ± 4.6 g/L 15.2 ± 1.8 g/L
350 ± 16 g/kg 929 ± 43 g/kg TS 1000 ± 0 g/kg TS
14.4 ± 1.8 g/L 49.2 ± 6.2 g/L 29.5 ± 3.3 g/L
929 ± 43 g/kg TS 1114 ± 53 g/kg TS 0 g/kg TS
1.00 ± 0.12 g/L 103 ± 14 mg/L
7.13 ± 0.41 g/kg TS 0.23 ± 0.01 g/kg TS
0.80 ± 0.14 0.95 ± 0.16 0.39 ± 0.07 1.33 ± 0.24 0.60 ± 0.10 49 4.3 ± 0.2 6.92 4.19 11.0 0.140 0.0688
0.93 ± 0.04 0.93 ± 0.04 1.00 ± 0 1.20 ± 0.08 0.00 156 ± 11 – 1.81 3.94 1.75 0.0347 0.0374
• •
Reciprocating shaking at 150 rpm Temperature = 35ºC
Serum bottle with working volume 150 mL
(b) POME + EFB
Effluent
Fill (2 min)
React (23 hr 56 min)
Draw (2 min)
Figure 1. Schematic diagrams of: (a) batch experiments and (b) the three phases of the intermittent–continuous stirred tank reactor.
Environmental Technology Table 2. Experimental conditions for the batch tests at various EFB to POME ratios. EFB to POME ratio (COD basis)
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POME (g COD/L) EFB (g COD/L) Total Feed (g COD/L) COD/TKN (g/g)
0
0.12
0.25
0.31
0.62
1.0
4.8 0.0 4.8 49
4.8 0.6 5.2 53
4.8 1.2 6.0 57
4.8 1.5 6.3 59
4.8 3.0 7.8 67
4.8 4.8 9.6 75
at 4.8 g COD/L, while the amount of added EFB was varied to give the desired EFB to POME ratios, as shown in Table 2. In this case, no inorganic compound was added except 6.0 g/L of NaHCO3 . The other experimental procedures were the same as those for the batch test using POME, EFB, or glucose as the sole substrate.
2.3. Lab-scale digester operation Two identical intermittent-continuous stirred tank reactors with a working volume of 10 L (318 mm in liquid depth and 200 mm in inner diameter) were fabricated as lab-scale digesters and operated in a room at constant 35 ± 1◦ C temperature. Each reactor was connected to a gas collector with water displacement to measure the biogas production. A reactor (R–A) was inoculated with 9.75 L of the digester sludge and operated in a Fill–React–Draw operation on a 24-h cycle. Once a day, 0.25 L of POME supplemented with 1.0 g of NaHCO3 was fed as the sole substrate (HRT 40 d). The addition of NaHCO3 raised the feedstock pH to around 6.0. In this study, NaHCO3 was selected due to its high solubility and lack of the need for CO2 neutralization. In a full-scale operation, it could be replaced by less costly chemicals, such as lime or sodium carbonate.[16] Fill and Draw were completed within 2 min each. After 52 days, half of the biomass (5 L) was transferred to the other reactor (R–B) anaerobically. Then, R–A was fed with 0.40 L of POME plus 1.6 g of NaHCO3 once a day without the Draw phase. For R–B, 0.40 L of POME, 10.3 g of EFB, and 1.6 g of NaHCO3 were added every day without the Draw phase. When the working volume of the reactors reached 10 L, the operational sequence was reverted to the regular one composed of Fill, React, and Draw phases. Figure 1(b) shows the schematic diagram of Table 3.
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the three phases of the intermittent–continuous stirred tank reactors. For R–A, HRT was decreased to 25, 20, and 16 d stepwise. For R–B, HRT was decreased to 25, 20, 16, and 14 d with EFB to POME ratios of 0.25 and 0.31 on a COD basis. The reactor operating conditions are summarized in Table 3. In each condition, the pseudo-steady-state performance with stable CH4 production and effluent COD concentration (±10%) were maintained for at least 3 consecutive days before performance data were measured. 2.4.
Specific methanogenic activity
The specific methanogenic activity (SMA) of the lab-scale digesters was analysed using serum vials (working volume 150 mL).[17] The methanogenic microflora were taken at the end of each operating mode and washed with the basal medium used in the BMP test three times. The SMA of the microflora (0.6–0.8 g VSS) was measured for a specific substrate (2 g COD/L). In this study, acetate and propionate were used as substrates. The other experimental procedures were same as that of the BMP test. 2.5.
Analysis and assay
The measured biogas production was corrected to standard temperature (0◦ C) and pressure (760 mmHg). The CH4 content in the biogas was determined by gas chromatograph (SRI 310, SRI Instrument) with a 0.9 m × 3.2 mm stainless steel column with porapak Q (80/100 mesh). Organic acids were measured using high-performance liquid chromatography (Spectrasystem P2000, Spectraphysics) with an ultraviolet detector and a 100 × 7.8 mm fast acid column (Aminex). Solids, COD, total Kjeldahl nitrogen (TKN), and total phosphorus were quantified according to standard methods.[13] The micronutrient contents were analysed using Inductively Coupled Plasma/Atomic emission spectrometer (ICP/AES) (Polyscan 60E, Thermo Jarell Co.). The methane production curve in the batch test was fitted to a modified Gompertz equation[11] RM M = P × exp −exp × (λ − t) × e + 1 , (1) P where M = is the cumulative methane production (mL CH4 ), P the ultimate methane production (mL CH4 ),
Operating conditions for the lab-scale digesters condition.
HRT (d) POME (g COD/L) EFB (g COD/L) EFB to POME Total OLR (g COD/L.d) POME OLR (g COD/L.d)
I-A
II-A
II-B-1
II-B-2
III-A
III-B-2
IV-A
IV-B-2
V-B-1
V-B-2
40 53.3 0 0 1.33 1.33
25 40.9 0 0 1.64 1.64
25 40.9 10.2 0.25 2.04 1.64
25 40.9 12.6 0.31 2.14 1.64
20 48.0 0 0 2.40 2.40
20 48.0 14.9 0.31 3.14 2.40
16 54.4 0 0 3.40 3.40
16 54.4 16.9 0.31 4.46 3.40
14 54.4 13.6 0.25 4.86 3.89
14 54.4 16.9 0.31 5.09 3.89
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BMP (L CH4 /g VS) BMP (L CH4 /g COD) Relative BMP compared with glucose CH4 production rate (mL CH4 /gVSS.d) Relative CH4 productivity compared with glucose Lag-phase time (d) Relative lag-phase time compared with glucose R2 for the Gompertz equation
RM the methane production rate (L CH4 /d), λ the lag-phase time (d), t the time (d), and e the exponential 1.
POME
EFB
Glucose
0.397 ± 0.009 0.301 ± 0.007 0.99 ± 0.02 55.1 ± 2.1 1.31 ± 0.05 2.2 ± 0.4 2.4 ± 0.5 0.9978 ± 0.0003
0.264 ± 0.007 0.221 ± 0.005 0.73 ± 0.02 20.1 ± 0.9 0.48 ± 0.02 6.0 ± 1.1 6.7 ± 1.4 0.9933 ± 0.0017
0.323 ± 0.002 0.305 ± 0.002 – 42.2 ± 0.5 – 0.9 ± 0.1 – 0.9988 ± 0.0010
3.1. Anaerobic degradability of POME and EFB The cumulative CH4 production curves obtained from the batch test using POME, EFB, or glucose as the sole substrate were described well by the modified Gompertz equation (Equation (1)) with correlation coefficients, R2 , larger than 0.997, as presented in Table 4. The methane yield (BMP)
and the specific methane production rate were calculated from P and the added substrate, and from RM and the biomass concentration, respectively. Table 4 shows that the BMP of POME expressed, based on VS and COD, were nearly equal to glucose with 86% of COD in POME being converted to methane, although 40% of COD was insoluble. The specific methane production rate was 1.31 times higher than that of glucose for MBP on the basis of VS. The lag-phase time was also less than 3 days. Therefore, POME is a relevant feedstock for mesophilic digestion.
Figure 2. Effect of EFB to POME ratio on (a) the specific methane production rate and (b) the methane yield in the batch test.
Figure 3. Daily variation in methane production rate from (a) the digester fed with POME only and (b) the digester fed with POME and EFB.
3.
Results and discussion
Environmental Technology EFB showed a longer lag-phase time and a lower methane yield and production rate compared with those of POME due to its lignocellulosic structure.[4] However, the BMP of EFB, 0.264 L CH4 /g VS and 0.221 L CH4 /g COD, was in the range of the BMP for conventional feedstock in anaerobic digestion (0.2 to 0.4 L CH4 /g VS).[18,19] Furthermore, it is higher than reported values of EFB in thermophilic digestion.[14,15] From the data obtained, it can be established that EFB also could be an effective feedstock for mesophilic digestion.
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3.2.
Effect of EFB addition on the anaerobic digestion POME As the cumulative CH4 production curves were well fitted with Equation (1) (R2 > 0.997), the methane yield and
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methane production rate obtained from a series of batch tests at various amounts of EFB addition on POME were calculated from P and the added substrate and from RM and the biomass concentration, respectively. EFB addition to POME increased the specific rate and the ultimate amount of methane production. The specific methane production rate of co-digestion at an EFB:POME ratio of 0.25–0.31 was 50.8 mL CH4 /g VSS.d, which was 1.2 times higher than that of POME (Figure 2(a)). The increase in specific methane production rate by codigestion has usually been attributed to mitigate toxicant levels, improve nutrients balance, or enhance microbial activity.[15,20–24] As POME had the high methane yield and production rate similar to those of glucose, the toxicant level of POME was considered negligible in this batch test. Among the nutrients balance, the carbon to nitrogen (C/N),
Figure 4. Performance comparison of the digesters on (a) CH4 production rate, (b) CH4 yield based on TCOD, (c) CH4 yield based on POME COD, (d) TCOD removal efficienty, (e) effluent COD concentration, and (f) Effluent pH.
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in other words COD to TKN, ratio is the most common interpretation of the synergism in co-digestion. A low COD to TKN ratio could cause a lack of substrate, whereas an excess COD to TKN could result in a nitrogen deficiency in microbial growth.[9,21,23–25] In this study, as the EFB to POME ratio was increased from 0 to 0.25–0.31, the COD to TKN ratio was increased from 49 to 57–59. The raised COD to TKN ratio was probably beneficial to co-digestion of the wastes from the palm oil production chain. Other effects including enhanced cellulase activity by the addition of the cellulosic by-products may have also caused the increased SMA.[15,19,23] However, the balance of micronutrients including Fe, Ca, Mg, Mn, and Zn would not result in the synergism, as POME had higher contents of them than EFB. As the BMP of EFB was lower than that of POME, the methane yield based on total COD (POME + EFB) decreased linearly according to the amount of EFB, as depicted in Figure 1(a). However, the EFB to POME ratio up to 1 did not hinder the anaerobic conversion of POME to methane, but did increase the total methane production amount. The methane yield based on POME increased linearly without any elongated lag-phase time, as shown in Figure 2(b), which indicated that EFB was also utilized as the substrate. 3.3.
Performance of POME and EFB co-digestion
Figure 3 shows the dynamics of the methane production rates for the lab-scale digester fed with POME and EFB and the control digester fed with POME only. In the control digester, the methane yield and COD removal efficiency were maintained above 0.26 L/g COD and 80%, respectively, up to POME loading rates of 2.40 g COD/L.d (HRT 20 d), but declined suddenly at 3.40 g COD/L.d (HRT 16 d). This performance deterioration occurred simultaneously with an effluent pH drop to 6.2 and an increase in effluent-soluble COD concentration, which implied a damaged balance between the several microbial groups for anaerobic digestion.[17] The addition of EFB increased the absolute amount of methane production at every POME loading rate. Furthermore, the co-digestion improved the reactor stability at the high organic loading rates. The performance enhancement by co-digestion was greater at an EFB to POME ratio of 0.31 than at a ratio of 0.25. The synergistic effect of co-digestion was attributed to the balanced chemical composition, for example the COD to TKN ratio [17,19,23]. As the POME loading rate was increased to 3.40 g POME COD/L.d (HRT 16 d; IV-B-2) and 3.89 g POME COD/L.d (HRT 14 d; V-B-1 and V-B-2), the methane yield and TCOD removal efficiency in the co-digestion were also decreased. However, the co-digestion still showed better performance than the single digestion of POME, and the effluent pH values were maintained over 7.0. Figure 4 summarizes the average performance data of the pseudo-steady state at each operational condition.
Figure 5. Organic acid production at (a) EFB:POME = 0:1 (POME only), (b) EFB:POME = 0.25:1, and (c) EFB:POME = 0.31:1.
The effluent organic acids concentration is depicted in Figure 5. In the control digester, organic acids, the intermediates of anaerobic degradation were accumulated at 3.40 g COD/L.d. Acetate, propionate, and n-butyrate were over 95% of the total organic acids detected. Among these three, the propionate concentration was the highest, implying that the conversion of propionate to acetate was one of the rate-limiting steps [11,17]. Organic acids accumulation, with propionate as the major component, was also found in the co-digestion at 3.40 and 3.89 g COD/L.d, but the amounts were lower than those for the control. Table 5 compares the SMA of the biomass fed to the single substrate (POME) and to the co-substrate (POME and EFB) at every POME loading rate. The test was conducted using precursor substrates of methane, acetate, and propionate, to identify the effects of co-digestion and
Environmental Technology Table 5.
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SMA of methanogenic biomass. SMA (mL CH4 /g VSS.d) Co-digestion of POME and EFB
POME loading (g POME/CODd)
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1.33 1.64 2.40 3.40
Single digestion of POME
Acetate
Propionate
Glucose
Acetate
Propionate
Glucose
81.2 30.9 26.6 –
43.5 36.5 6.7 –
45.1 37.2 11.3 –
86.0 48.1 47.8 34.6
42.4 21.0 18.0 34.6
42.6 22.2 18.8 11.3
the organic loading rate on the microbial activities of the anaerobic microflora. In most cases, the SMA on acetate was higher than that on propionate, which was concurrent with the propionate accumulation in the digester operation. The SMA decreased with increasing organic loading rate, but co-digestion mitigated an abrupt decline. For the biomass fed with POME at 3.40 g POME COD/L.d, the SMA on propionate declined below 7 mL CH4 /g VSS.d. A pH drop resulting from organic acid accumulation is the most probable limitation that seriously reduces the SMA of the biomass.[11] By contrast, the SMA on propionate for the biomass fed with the co-substrate up to 3.89 g POME COD/L.d was maintained over 17 mL CH4 /g VSS.d. Co-digestion improved both the microbial activity of the biomass and the performance. 4. Conclusions The authors investigated the feasibility for co-digestion of POME with EFB at mesophilic temperature range. The BMPs of the waste by-products were within the general range of anaerobic digestion. At various tested EFB to POME ratios, the methane production rate was maximized at EFB:POME = 0.25–0.31:1. The synergistic effect of co-digestion was attributed to the balanced chemical composition, for example the COD to TKN ratio. The EFB addition also improved the acceptable organic loading rate, methane production, COD removal, and microbial activity of a lab-scale digester. The mesophilic co-digestion of POME and EFB is a viable recycling method to alleviate pollution and recover energy in the palm oil mill industry. Funding This research was supported by Yeungnam University research grant in 2013.
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