Acidogenesis of Palm Oil Mill Effluent to Produce Biogas: Effect of Hydraulic Retention Time and pH

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  Procedia - Social and Behavioral Sciences 195 ( 2015 ) 2466 – 2474 World Conference on Technology, Innovation and Entrepreneurship

  Acidogenesis of Palm Oil Mill Effluent to Produce Biogas: Effect of Hydraulic Retention Time and pH a a a a _a

Bambang Trisakti *, Veronica Manalu , Irvan Taslim , Muhammad Turmuzi

  Chemical Engineering Department, University of Sumatera UtaraJalan Almamater Komplek USU, Medan 20155, Indonesia Abstract

Acidogenesis of palm oil mill effluent to produce biogas was conducted base on anaerobic digestion process. This process was

consisted of four different stages using various types of microorganisms which came from various groups of bacteria. This

research studied the effect of hydraulic retention time (HRT) and pH to the change of the organic compounds concentration and

solids content of POME to the growth of microorganisms during acidogenesis stage. Initially, the suitable loading up was

determined by varying the HRT 6.7; 5.0; and 4.0 days in the continuous stirred tank reactor (CSTR) with mixing rate 50 rpm,

pH 6.0 ± 0.2, and room temperature. Next, suitable pH was determined by varying the pH at 5.0; 5.5; and 6.0 with mixing rate

_o 100-110 rpm and temperature 55

  C. Analysis of TS, VS, TSS, VSS, COD, and VFA were conducted in order to study the growth

of microorganisms and their abilities in converting organic compound to produce VFA. The highest growth of microorganisms

was achieved at HRT 4.0 day with microorganism concentration was 20.62 mg VSS/L and COD reduction was 15.7%. The

highest production of total VFA achieved was 5622.72 mg/L at pH 6.0, with the concentration of acetic acid, propionic acid and

butyric acid were 2257.34; 975.49; and 2389.90 mg/L, respectively. While degradation VS and COD were 11 and 23%, respectively.

  © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

  Peer-review under responsibility of Istanbul University.

  Peer-review under responsibility of Istanbul Univeristy.

  Keywords: acidogenesis; anaerobic digestion; biogas; POME * Corresponding author. Tel.: +62-812-6362-2066; fax: +62-61-821-3250.

  E-mail address: b_trisakti@yahoo.com

  1. Introduction

  Indonesia is currently the largest producer of palm oil in the world with a total production reached 33 million tons (USDA FAS, 2015). Palm oil is produced from approximately 8.54 million hectares of oil palm plantations (Wright & Rahmanulloh, 2015), which mostly located on the island of Sumatra and Borneo (Jaenicke et al., 2008). The process to extract oil requires significantly large amounts of water for steam sterilization of fresh palm fruit bunches (FFB) and clarification of extracted oil. As a result, palm oil mill (POM) produces large amounts of wastewater or commonly called palm oil mill effluent (POME). For every ton of FFB processed will produce approximately 0.6 to

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  0.87 m POME or 2.4 to 3.7 tons of POME per one ton of palm oil produced (Alam, Jamal, & Nadzir, 2008; O- Thong, Boe, & Angelidaki, 2012). POM also generates large amount of solids wastes such as empty fruit bunch (EFB) (23%), shell (5%), and mesocarp fiber (12%) for every ton of FFB processed in the mills (O-Thong, Boe, & Angelidaki, 2012).

  Most of the existing POME processing in POMs is anaerobic-aerobic lagoon system (Wu, Mohammad, Jahim, & Anuar, 2010). This system aims to reduce waste pollution parameters before being discharged into water bodies or for land application (Lam & Lee, 2011). POME as feedstock utilization of biogas (CH , CO , H S, etc.) through the

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  process of anaerobic digestion has been widely reported by the researchers, even has been applied on a commercial scale in several POMs in Malaysia and Indonesia (Hosseini & Wahid, 2013). POME conversion into biogas would provide more benefits because this process in addition will reduce waste pollution parameters such as organic solids, microbial pathogens, and toxicity. Furthermore, this technique will also produce biogas which can be used as an alternative energy (Stamatelatou, Antonopoulou, & Lyberatos, 2011). Although it has been applied on a commercial scale, but the hydraulic retention time (HRT) is still high enough so that the investment needed to build this process is high (Yingyuad, 2007).

  Anaerobic digestion is a biochemical process without the presence of oxygen which complex organic compounds are broken down by different types of anaerobic bacteria (Seadi et al., 2008). This process involves a four-step process that engages four different types of bacterial groups (hydrolysis, acidogenic, acetogenic, and methanogenic). Each of these bacteria have a different physiology and nutritional requirements (Olvera & Lopez, 2012). If all four groups of bacteria are working under the same conditions, there will be an imbalance between the formations of acid and methane that biogas production time will be longer (Jung et al., 2000). In an effort to accelerate the rate of biogas production, many researchers separate the anaerobic digestion process into two stages (Jung et al., 2000). In the first stage, the organic compounds contained in POME are converted into compounds of volatile fatty acids (VFA) by hydrolysis, acidogenesis, and acetogenesis (commonly referred as acidogenesis stage) and subsequently the VFA is converted into biogas through methanogenesis stage (Wong et al., 2013). Therefore, to improve the effectiveness of the two-stage anaerobic digestion process, the optimum operating conditions such as pH, temperature, and agitation rate of each of the stages should be known (Veeken et al., 2000; Polprasert, 2007). This paper will report the effect of variations of HRT and pH on the acidogenesis stage.

  2. Literature Review

  Indonesian palm oil production in the coming years is expected to continue to rise, as shown on Figure 1 that the growth of the average production during the last 10 years is increasing. The high production of palm oil led to the amount of liquid waste or POME produced is also high. It is estimated that in 2014, the number of POME generated is about 79.2 - 122.1 million tons for every one ton of palm oil (Alam, Jamal, & Nadzir, 2008; O-Thong, Boe, & Angelidaki 2012).

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  Fresh POME is a liquid brownish viscous mud with temperature around 80-90

  C, acidic (pH 3.8-4.5), and the concentration of organic particles is high enough so the COD and BOD are also high (Zinatizadeh et al., 2006). POME contains large amounts of carbohydrates, protein, and fat with composition 29.55, 12.75, and 10.21% respectively. In addition, there are also some macro and micro mineral compounds such as potassium (K), sodium

  (Na), calcium (Ca), iron (Fe), zinc (Zn), chromium (Cr), and others (Salihu & Alam, 2012). Therefore, POME can be utilized as substrates and nutrients for microorganisms in methanogenic anaerobic digestion for biogas production (Seadi et al., 2008).

  

Fig. 1. Indonesia's palm oil production growth in 2004-2014 (Source: Wright & Rahmanulloh, 2015)

  Methanogenic anaerobic digestion is anaerobic bioconversion process that actually has been practiced for a long time and has also been applied on a commercial scale. This process is complicated because it involves a mixed population of microorganisms and produces various gases and fluids, so it requires accurate process control and product purification (Li & Yu, 2011). As a comparison, a two-stage process of anaerobic digestion, which separates the stage acidogenesis and methanogenesis stages, is allowing for more flexible operations. Furthermore the amount and purity of the product is higher (Blonskaja, Menert, & Vilu, 2003;. Demirer & Chen, 2005).

  Some researchers report that the methanogenesis stage takes place at a neutral pH range, with optimum ranges between 7-8, while microorganisms acidogenesis generally are optimum at lower pH (Seadi et al., 2008). Environmental factors that influence the acidogenesis stages are substrate, inoculum, pH, temperature, and HRT (He et al., 2012). Low pH and short HRT are conditions favored by acid-forming bacteria to grow and may inhibit the growth of microorganisms forming methane (Solera, Romero, & Sales, 2002). High alkalinity is often necessary to maintain the pH in connection with high CO content in the biogas (Zhang, 2014).

2 Control of HRT is needed to help multiply the microorganisms of hydrolysis and acidogenesis (Fang and Yu,

  2000) and will increase the production of VFA (Maharaj & Elefsiniotis, 2001). Retention time should be long enough to ensure the number of dead microorganisms in the digester is not higher than the amount of reproduced microorganisms (Seadi et al., 2008)

3. Methodology

3.1. Research Goal

  This paper reports the effect of loading up (variations HRT) and pH variations of acidogenesis stage or first stage of a two stage anaerobic digestion of conversion POME into biogas.

3.2. Materials, Experimental set-up, and Data Collection

  x Materials Fresh POME was collected from a fat pit of Adolina POM, Sumatera Utara, Indonesia, and stored in 300 L clean high-density polyethylene (HDPE) container. It was transported to the laboratory, mixed well, put in 1-liter plastic bottles, and stored at 4 °C until further use. Fresh POME characteristics are shown in Table 1. Acidogenic bacteria as starter were collected from acidification pond of WWTP of POM at Torgamba, Sumatera Utara, Indonesia. Sodium bicarbonate (NaHCO

  3 ) was used as pH adjusting.

  Table 1. Characteristics of fresh POME Parameters Units Results Methods pH - 3.70 – 4.70 Chemical Oxygen Demand (COD) mg/L 48,300 APHA 5220B Total Solid (TS) mg/L 13,420 – 37,020 APHA 2540B Volatile Solid (VS) mg/L 10,520 – 31,220 APHA 2540E Total Suspended Solid (TSS) mg/L 2,080 – 27,040 APHA 2540D Volatile Suspended Solid (VSS) mg/L 1,920 – 25,800 APHA 2540E Oil and Grease mg/L

  6.25 APHA 5520B Protein %

  0.53 Kjeldahl Carbohydrate % trace Lane Eynon

  x Experimental set-up The laboratory scale of continuous stirred tank reactor (CSTR) as fermenter is shown in Figure 2. The CSTR is

  EYELA model MBF 300ME with working volumes is 2 L. The fermenter comprises an integrated on-line temperature and pH data recording system. No pH adjustment was applied to the fermenter. However, pH of the raw POME was controlled with sodium bicarbonate (NaHCO

  3 ), then after that, fed into the fermenter.

  

Figure 2. Experimental set-up x Data Collection Data collected during the experiment were pH, alkalinity, TS, VS, TSS, VSS, COD, and VFA which according to the American Public Health and Association (APHA) standard methods for water and wastewater (APHA, 1999) as presented in Table 2.

  Table 2. Analysis methods of data collection (APHA, 1999) Parameters Units Methods pH - Alkalinity mg/L APHA 2320B Total Solid (TS) mg/L APHA 2540B Volatile Solid (VS) mg/L APHA 2540E Total Suspended Solid (TSS) mg/L APHA 2540D Volatile Suspended Solid (VSS) mg/L APHA 2540E Chemical Oxygen Demand (COD) mg/L APHA 5220B Volatile Fatty Acids (VFA) mg/L APHA 5560B

3.3. Results and Discussion

  x Effect of hydraulics retention time (HRT) Microbial adaptation and growth of the starter were carried out by reducing the HRT or organic loading up. During loading up, HRT was reduced slowly and evaluated at HRT 6.7; 5.0; and 4.0 days. The fermenter was controlled at room temperature, mixing rate 50 rpm, and the pH was kept constant at 6 (± 0.2). The experiment was conducted in intermittent operation mode, where fresh feed and effluent where flown in every 4 hours or 6 times per 24 hours. pH was maintained by adding sodium bicarbonate (NaHCO ) in the fresh feed. pH, alkalinity, TS, and VS

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  of the feed and effluent were analyzed every day, while the TSS, VSS, COD and VFA were analyzed after the stable data was achieved. The results of experiments on organic loading up are presented in a graph showing the effect of HRT reduction on pH and alkalinity, also the microbial growth, and COD reduction.

  The effect of HRT reduction on pH and alkalinity is presented in Figure 3. The pH value slightly fluctuates but remains constant at 6 days because the addition of sodium bicarbonate in the feed. As a result, large fluctuation in the value of alkalinity occurred, although it is still in the range where the acidogenesis process takes place i.e. in the range of 542 - 3580 mg/L (Senthil Kumar 2010; Schmit & Ellis, 2001)

  Alkalinity range Effect of HRT reduction on microbial growth is described by the change of VSS concentration in the fermenter. Metabolism and growth of microbial are also highly dependent on the pH and alkalinity (SenthilKumar 2010; Schmit & Ellis, 2001). Figure 4 shows VSS profile simultaneously with the pH and alkalinity during the process.

  The pattern of microbial growth is significantly influenced by changes in pH and alkalinity. The highest concentration of microbes achieved at HRT 4 days was 20.62 mg VSS/L, although at this is the shortest HRT.

  

Figure 4. Effect of HRT reduction on microbial growth

  Effect of reduction HRT on microbial growth can also be explained by the amount of COD decrease in inlet and outlet of the fermenter as shown in Figure 4. Although the decrease of COD indicates the occurrence of microbial growth but the expected decrease COD is not too large, because the desired product of acidogenesis process is VFA.

  Figure 5. Effect of reduction of HRT on COD reduction

  x Effect of pH Effect of pH on acidogenesis process was studied by conducting a series of experiments by varying the pH at 5.0;

  5.5; and 6.0, mixing rate at 100 rpm, temperature of 55 °C, and HRT 4 days. The experiment was conducted in intermittent operation mode, where fresh feed and effluent where flown in every 4 hours or 6 times per 24 hours. The pH was maintained by adding sodium bicarbonate (NaHCO ) in the feed. The pH, alkalinity, TS, and VS from

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  the feed and effluent were analyzed every day, while the TSS, VSS, COD and VFA were analyzed after the stable data was achieved. The results of experiments are presented in a graph, showing the effect of pH on the profile of

  Effect of pH on alkalinity and microbial growth are presented in Figure 6. At the beginning of pH 5.0 the alkalinity is decreased, because the pH of fresh POME is 4.7 and thus the addition of sodium bicarbonate in feed is only small. After day 10, alkalinity started to rise and eventually fluctuated at 3000 mg/L. Similarly, the growth of microbes, although initially decreased but from day 10 to be stable in the range of 12000-16000 mg/L.

  a b

  

Figure 6. Effect of pH on (a) alkalinity and (b) microbial growth

  Effect of pH on VS and COD reduction is presented in Figure 7. Both graphs are actually used to look for microbial activity. VS and COD reduction is best achieved at a pH of 5.5 for the reduction is not too large.

  a

b

  

Figure 7. Effect of pH on (a) VS reduction and (b) COD reduction

  Effect of pH on VFA production is presented in Figure 8 (a) while its effect on the VFA alkalinity ratio is presented in Figure 8 (b). VFA production is expressed with the production of acetic acid, propionic acid, and butyric acid. Although the VFA production was highest at pH 5.0, but high production of propionic acid, if the amount is greater than 1500 mg/L, its presence will disrupt next stage (methanogenesis) (Senthilkumar et al., 2010). Thus, the highest VFA production are at pH 6 but considering the VS and COD reduction (Figure 7), then the VFA production is best at pH 5.5. The VFA alkalinity ratio (Figure 8b) is to describe the stability of process acidogenesis where it is assumed stable if the VFA alkalinity ratio is greater than one (Li et al., 2012). a

b

  

Figure 8. Effect of pH on (a) VFA production and (b) VFA Alkalinity ratio

4. Conclusion

  On the loading up process, the highest growth of microorganisms was achieved at HRT 4.0 day with microorganism concentration was 20.62 mg VSS/L and COD reduction was 15.7%. Meanwhile, on the pH variation, the highest production of total VFA achieved was 5622.72 mg/L at pH 6.0, with the concentration of acetic acid, propionic acid and butyric acid were 2257.34; 975.49; and 2389.90 mg/L, respectively. While degradation VS and COD were 11 and 23%, respectively.

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