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Treatment of Palm Oil Mill Effluent by Closed Endless Stream Anaerobic Digesters

Kazumasa KAMACHI*, Akihiro IKEDA**
Toshihiro SUZUKI*, Yuji SENDA*
* Swing Engineering Corporation
** Former Swing Corporation

A full-scale plant with two parallel 12 000 m3 closed endless stream anaerobic digesters was installed at a palm oil mill in Perak, Malaysia. The digesters treat palm oil mill effluent (POME). The designed flow of palm oil mill effluent (POME) is 1 270 m3/d, and a chemical oxygen demand (CODCr) volumetric loading rate is 4.0 kg/(m3·d) under mesophilic conditions (35 ℃). Two fluidity investigations were conducted before the start of operation. Based on the results, the sludge fluidity was deemed sufficient for the digester. During the first reactor start-up, the CODCr volumetric loading rate of 3.9 kg/(m3·d) was achieved on the 28th day. After startup of both reactors and introduced all POME, with the CODCr volumetric loading rate of 2.9 kg/(m3·d), The CODCr removal rate was 87.9 to 91.5 % (average 89.2 %), and the CH4 concentration in the biogas was 60.1 to 60.3 % (average 60.3 %). The methane gas volume relative to removed CODCr was 0.306 to 0.453 m3/kg-CODCr removed (average 0.357 m3/kg-CODCr removed, NTP), which is almost the same as the theoretical value (0.35 m3/kg-CODCr removed, NTP). Thus almost all removed CODCr was supposed to be recovered as methane.

Keywords: Palm oil mill effluent (POME), Anaerobic digester, Start-up, Mesophilic, Endless stream, Full-scale plant

1. Introduction

Palm oil, which is made from oil palm fruit (Figure 1), is one of the most important vegetable oils worldwide. Global palm oil production was 69 million tons in 2017, and it has been increasing annually (Figure 2) 1). Palm oil produced in Malaysia and Indonesia accounts for 86% of global palm oil production, because their climates favor the growth of the oil-palm tree (Figure 3) 1). The extraction and purification processes during palm oil production generate wastewater known as palm oil mill effluent (POME), which has high organic content, mainly consisting of lipids and fatty acids2),3). POME is most commonly treated in anaerobic lagoons, which are open pond systems requiring long treatment periods4). This treatment process releases a large amount of methane gas into the atmosphere. Methane is a greenhouse gas and contributes to global warming. In Malaysia, the amount of methane gas released from anaerobic lagoons used for treating POME in 2009 was equivalent to 6.4% of its total greenhouse gas emissions in 20065).

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Fig. 1 Oil palm and fruit
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Fig. 2 Palm oil production
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Fig. 3 Palm oil production by countries (2017)

 Recently, methane gas recovery systems such as closed anaerobic digesters for treating POME have attracted attention. The electricity-generating potential of the methane produced from the anaerobic treatment of POME in Malaysia is predicted to be 602 MW by 2020, and then as a result, emission of 3.20 Mt of CO2 equivalent can be potentially avoided by 20206).
 Other methane recovery systems, such as rubber-covered anaerobic lagoons and closed reactors with stirrers, are available4). In previous studies, a CODCr volumetric loading rate for anaerobic treatment using lagoons is limited to 1.4 kg/(m3・d) typically, which suggested that they could handle only low loads7). On the other hand, closed anaerobic digesters8) and anaerobic MBRs require only small installation space, but entail high costs in construction and operation. Therefore, more effective POME treatment system with low costs is required.
 We have developed a new closed endless stream anaerobic digester with excellent processing performance. We report the sludge fluidity by stirrers and the start-up performance of the digester in this paper.

2. Treatment system and Methods

2.1 Treatment system

A closed endless stream anaerobic digester is elliptical, and a maintenance bridge spans its minor axis. The four submergible mixers installed under the maintenance bridge stir the water in the reactor (Figures 4 and 5). A settling pond is located downstream of the reactor, and sludge is returned from the settling pond to the reactor. The reactor is covered with a High-Density PolyEthylene (HDPE) sheet. The recovered biogas is desulfurized using a biological desulfurizer and sent to electric generators. A part of the electricity generated is sold to a local power company and the rest is consumed by the palm oil mill. Table 1 shows the specifications of the system. The full-scale plant has two reactors with the volume of 12 000 m3. The design POME flow is 1 270 m3/d, with a hydraulic retention time (HRT) of 18.9 days, and a volumetric organic loading rate (OLR) of 4.0 kg/(m3・d) under mesophilic conditions.

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Fig. 4 Process diagram for an endless stream anaerobic digester
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Fig. 5 System appearance
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2.2 Method of analysis

The POME characteristics were identified by US EPA test methods9). During the start-up period, we measured pH (PM-10, DKK-TOA Corporation, Japan), CODCr (DR890, HACH), TS (FD-720, Kett Electric Laboratory, Japan), and biogas composition (CH4, CO2, H2S) (COMBIMASS* Portable Gasanalyzer GA-m, BINDER, Germany). Flow velocity was measured by a 3-D velocity sensor (ACM3-RS, JFE Advantech).

2.3 Method of investigation

Two tests were conducted to evaluate the fluidity in the reactors. In the first test, the flow velocity near the maintenance bridge was measured at 21 different points (6 horizontal positions and 2 to 5 vertical positions) in the reactor filled with water to check whether the low flow velocity causes sludge deposition near the bottom of the reactor (Figure 6). In the second test, in order to confirm the performance of four submergible mixers, we checked changes of sludge concentration with operational time using seed sludge, at 1 m under the water surface above the mixers (Figure 4, A-D).

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Fig. 6 Flow velocity measurement points (on one side of the maintenance bridge)

2.4 Activity evaluation of seed sludge

Prior to the tests, the methanogenic activity of seed sludge was evaluated . 800 mL of seed sludge and 60 mL of POME were fed into a plastic bottle. The bottle was immersed in a water tank with temperatures of around 35 ℃ with a gas-collection bag at the top of the bottle. The generated gas volume was measured at predetermined time intervals.

2.5 Start-up method

Prior to the start-up of the plant, sludge from the existing anaerobic lagoon was seeded into each reactor. POME from the acidification pond (HRT of 5 days) was used as influent.

3.Results and Analysis

3.1 POME characteristics and seed sludge activity

POME, dark brown waste water (Figure 7), was fed into the reactor after cooling because it was discharged at a high temperature. Table 2 shows the results of water quality analysis conducted before the start-up.

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Fig. 7 POME appearance
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 The POME has the pH of 4.3 (acid), TS of 69 700 mg/L, VS of 61 600 mg/L, VS/TS of 88 %, MLSS of 23 900 mg/L, MLVSS of 12 900 mg/L, MLVSS/MLSS of 54%, CODCr of 91 200 mg/L, Kj-N of 423 mg/L, and T-P of 312 mg/L. The ratio of CODCr, Kj-N, and T-P was almost 1 000:5:1, which is suitable for methane fermentation. On the other hand, the MLVSS/MLSS indicated that POME contains about 10 000 mg/L of inorganic substances.
 Figure 8 shows the results of a test of methanogenic activity of seed sludge. Gas generation was observed immediately after the start of the test. Assuming 60% of the gas to be methane, the methanogenic activity was 0.18 g-CODCr/(g-MLVSS・d). Therefore, it was estimated that the seed sludge has sufficient methanogenic activity.

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Fig. 8 Methanogenic capacity of seed sludge

3.2 Investigation of fluidity

3.2.1 Test 1

The results of Test 1, and the average flow velocities near the bottom and the average overall flow velocities, are shown in Figures 9 and 10, respectively. The flow velocities near the bottom of the reactor (0.1 m above the bottom) were 0.15 m/s or more (an average of 0.28 m/s). The overall velocities were from 0.15 to 0.58 m/s (an average of 0.36 m/s). Japan’s oxidation ditch process requires at least a flow velocity of 0.1 m/s near the bottom and an average velocity of around 0.25 m/s10). Based on this investigation, we conclude that there was little risk of sludge deposition.

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Fig. 9 Flow velocity at bottom of reactor
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Fig. 10 Flow velocity at bottom of reactor and overall

3.2.2 Test 2

Figure 11 shows changes in TS after the mixers were started. It was confirmed that TS became constant at 2% after 2 hours since the mixers were started, and the sludge fluidity was sufficient for the digester.

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Fig. 11 Changes in TS after the mixers were started

3.3 Start-up

Table 3 shows the POME characteristics of the first reactor during the start-up period. Figure 12 shows the treatment results.

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Fig. 12 Operation results (at start-up)

 During the start-up period, the average pH was 4.4, the average CODCr was 94 700 mg/L, and the average TS was 60 000 mg/L.
 A CODCr volumetric loading rate of 3.9 kg/(m3・d) was achieved on the 28th day. The average rate between the 28th and 39th days was 3.4 kg/(m3・d). The average CODCr removal rate was 71.6% for the reactor effluent, and 81.1% for the settling pond effluent. We found that the average CODCr removal rate was improved by 10% due to the solid-liquid separation in the settling pond.

3.4 Process performance when all POME was fed

Table 4 shows the process performance when all POME was fed after the completion of the start-up of the second reactor. The average CODCr volumetric loading rate was 2.9 kg/(m3・d), the average CODCr removal rate was 89.2%, the average biogas generation volume was 37 200 m3/d, and the average methane concentration of the biogas was 60.3%. The average methane generation volume was 22 500 m3/d, which is equivalent to 0.357 m3/kg-CODCr. This value was almost the same as the theoretical value (0.350 m3/kg-CODCr, NTP) for converting CODCr to methane. Thus almost all the removed CODCr was recovered as methane.

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 As an example of a real full-scale plant with anaerobic digesters, its specifications were reported as follows: three 2 500 m3 reactors, raw water CODCr = 45 000-70 000 mg/L, HRT = 18 days, CODCr volumetric loading rate = 2.6-3.5 kg/(m3・d), and CODCr removal rate = 80-85%8). In this proposed process, the process performance was equal to or exceeded that of the anaerobic digester in above example.

4. Conclusion

We have developed a closed endless stream anaerobic digester and demonstrated its performance in a real plant for POME treatment. The following is a summary of the results.
 We confirmed that our developed closed endless stream anaerobic digesters were working well.
 In 28 days, 3.9 kg/(m3・d) of CODCr volumetric loading rate and 81.1% of CODCr removal rate at the settling pond have been achieved, resulting in a short start-up time.
 When all POME was fed, the average CODCr volumetric loading rate was 2.9 kg/(m3・d), the average CODCr removal rate was 89.2 %, the average methane concentration of the biogas was 60.3 %, and the average methane volume converted to the removed CODCr was 0.357 m3/kg-CODCr, NTP, all of results indicate good process performance.

*COMBIMASS is a trademark of BINDER GmbH.

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