Upflow anaerobic sludge blanket-hollow centered packed bed (UASB-HCPB) reactor for thermophilic palm oil mill effluent (POME) treatment

b i o m a s s a n d b i o e n e r g y 6 7 ( 2 0 1 4 ) 2 3 1 e2 4 2

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Upflow anaerobic sludge blanket-hollow centered packed bed (UASB-HCPB) reactor for thermophilic palm oil mill effluent (POME) treatment P.E. Poh a,b, M.F. Chong a,* a

Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor D.E., Malaysia b Chemical Engineering Discipline, School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor D.E., Malaysia

article info

abstract

Article history:

Upflow anaerobic sludge blanket-hollow centered packed bed (UASB-HCPB) reactor was

Received 30 July 2012

developed with the aim to minimize operational problems in the anaerobic treatment of

Received in revised form

Palm Oil Mill Effluent (POME) under thermophilic conditions. The performance of UASB-

6 May 2014

HCPB reactor on POME treatment was investigated at 55
C. Subsequent to start-up, the

Accepted 9 May 2014

performance of the UASB-HCPB reactor was evaluated in terms of i) effect of hydraulic

Available online

retention time (HRT); ii) effect of organic loading rate (OLR); and iii) effect of mixed liquor volatile suspended solid (MLVSS) concentration on thermophilic POME treatment. Start-up

Keywords:

up of the UASB-HCPB reactor was completed in 36 days, removing 88% COD and 90% BOD

Anaerobic digestion

respectively at an OLR of 28.12 g L1 d1, producing biogas with 52% of methane. Results

Bioreactors

from the performance study of the UASB-HCPB reactor on thermophilic POME treatment

Wastewater treatment

indicated that HRT of 2 days, OLR of 27.65 g L1 d1 and MLVSS concentration of 14.7 g L1

Bioremediation

was required to remove 90% of COD and BOD, 80% of suspended solid and at the same time

Palm oil mill effluent

produce 60% of methane. © 2014 Elsevier Ltd. All rights reserved.

Biogas

1.

Introduction

In year 2011, 18.91 Mt of crude palm oil (CPO) was produced in Malaysia [1] and in year 2008 alone, 44 Mt of palm oil mill effluent (POME) was generated from CPO production [2]. POME has to be treated due to its high polluting strength which causes detrimental effects with direct discharge to the watercourses. Anaerobic digestion using high-rate reactors was proven to effective for POME treatment. Borja and Banks [3] showed that

at least 96% of chemical oxygen demand (COD) can be removed from POME using upflow anaerobic sludge blanket (UASB) reactor; while at least 90% COD removal from POME was achieved with the upflow anaerobic sludge-fixed film (UASFF) reactor [4]. Furthermore, anaerobic filters and fluidized bed could reduce 94% and 78% of COD in POME respectively [5,6]. However, high suspended solids and oil and grease contents in POME posed operational problems to most of these systems (i.e.: clogging, foaming and scum formation), which led to the compromise of certain operating conditions (i.e.: OLR, HRT) to avoid reactor upset. Hence, a novel design of

* Corresponding author. Tel.: þ60 3 8324 8347; fax: þ60 3 8924 8017. E-mail addresses: [email protected] (P.E. Poh), [email protected] (M.F. Chong). http://dx.doi.org/10.1016/j.biombioe.2014.05.007 0961-9534/© 2014 Elsevier Ltd. All rights reserved.

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upflow anaerobic sludge blanket-hollow centered packed bed (UASB-HCPB) reactor was proposed in this study to address these operational problems, so that it can be a more viable option for POME treatment. The concept on the UASB-HCPB reactor design will be elaborated in the subsequent section. POME treatment under thermophilic conditions (45e60
C) was considered in this study due to high discharge temperatures of POME (80e90
C) [4]. This will eliminate the necessity for cooling facilities prior treatment. In addition, higher rate of reaction under thermophilic condition enables shorter retention times and produces better treatment efficiency as compared to mesophilic condition (30e42
C). Cail and Barford [7] reported that POME treatment under thermophilic condition had treatment rates that are more than four times faster than operation under mesophilic temperature range. An investigation conducted by Ibrahim et al. [8] on POME treatment using an anaerobic contact digester at 55
C showed that 90% of BOD in POME can be removed from the system. Therefore, the UASB-HCPB reactor was designed based on the parameters for operation under thermophilic condition. OLR and HRT are the two important parameters that should be studied due to the fact that operation at an extreme OLR will cause decline in COD removal efficiency, methane yield and biogas production [9]; while short HRT will reduce the contact time between substrate and microbes. Nevertheless, short HRT will also enhance granulation, leading to better treatment efficiencies [10]. Yuan et al. [11] stated that VFA yields increased with decreasing mixed liquor volatile suspended solid (MLVSS) concentration. An increase in VFA concentration is undesirable as VFA/alkalinity greater than 0.4 will cause instability during anaerobic digestion [9]. Therefore, MLVSS is also an important parameter to be investigated to

achieve satisfying treatment efficiency and stable operating conditions. In this research, the start-up performance of the UASBHCPB reactor on POME treatment under thermophilic condition was evaluated. Subsequently, investigations were also made on the performance of UASB-HCPB reactor under the effect of OLR, HRT and MLVSS. Finally, an optimized set of operating parameters for thermophilic POME treatment was obtained from the performance study.

2.

UASB-HCPB reactor

Fig. 1 displays the UASB-HCPB reactor design. The UASB-HCPB reactor had an internal diameter of 12 cm and height of 50 cm. The total and working volume of the reactor was 5.65 L and 5 L respectively. 80% of the working volume at the lower section of the reactor has the configuration of an UASB reactor. The UASB section facilitates the formation of granules with better settling property. In addition, the dense granular sludge will provide higher retention of biomass in the system [12]. This in turn will allow higher loading potential for the system, allowing treatment of wastewater with high suspended solids loading. The middle section of the reactor (HCPB section) consists of a packed bed with a cylindrical channel of 6 cm in the middle. It is randomly packed with plastic pall rings with length of 1 cm, internal diameter of 0.34 cm and external diameter of 0.51 cm, occupying 20% of the working reactor volume. The packed bed section in the UASB-HCPB reactor functions to immobilize biomass which does not form granules to reduce sludge washout and also to reduce the

Fig. 1 e Design of UASB-HCPB reactor.

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volume required for sludge settling after treatment. The immobilized biomass which forms biofilm on the packing of HCPB should improve the treatment efficiency as it provides greater biomass surface area for biological digestion. Packed bed systems are more prone to clogging problems when operated at high suspended solids loading. In order to avoid clogging at high suspended solids loading, a cylindrical channel in the middle of the packed bed is introduced for the first time in this design. Biomass still can be immobilized in the packed bed section and the hollow channel in the middle of the packed bed will allow smooth flow of effluent and biogas out from the reactor, reducing trapped gas bubbles within the packing. The reduction on the packed bed area will reduce the cost required for packing media and also maintenance of the packed bed section as the HCPB is expected to be less susceptible to clogging. The top section of the UASB-HCPB reactor consists of a gasliquid-solid (GLS) separator with a shape of an inverted funnel which works to channel the biogas into the gas collection system. Based on Fig. 1, POME will be fed from the bottom of the reactor to the sludge bed where most of the microbes in the granular form would be in contact with the substrate. A recirculation pump is attached to the bottom of the reactor to allow adequate contact of granules with the substrate by providing efficient mixing in the UASB section. The upflow velocity and mixing due to recirculation at the bottom of the reactor will allow microbes to be suspended in the blanket section and enhances the contact between substrate and microbes. In the HCPB section, microbes that are suspended will be attached on the pall rings to form an attached growth system. Biogas produced from the UASB and the packed bed

Table 1 e Designed, actual operating conditions and expected performance for UASB-HCPB reactor. Design parameters

Temperature (
C) OLR (g L1 day1) pH HRT (d) Average MLSS in sludge bed (mg L1) Biomass yield coefficient, Ya Maximum specific growth rate, mm (day1)a Endogenous decay coefficient, kd (day1)a Half velocity constant, Ks (mg L1)a Reactor working volume (L)

Designed value

55 25 6.8e7.2 1e5 57,000

Actual operating value during start-up 55 0.5e28.22 6.8e8.0 1.5 14,000

0.89 0.49

e e

0.17

e

1500 5

e e

Expected performance from UASB-HCPB reactor based on designed value Parameters

Units

Value

COD removal efficiency BOD removal efficiency Methane yield Methane production

% % L g1 L d1

>90 >90 0.3821 55

a

Values were obtained from Yeoh et al. [15].

section will rise through the hollow channel in the HCPB section leading to the GLS separator connected to the biogas collection system. Heavier solids that are carried to the top of the reactor will slide back into the reactor through the baffle as shown in Fig. 1. This will avoid washout of huge amount of solids from the reactor. Effluent will exit from the reactor to the settling tank through the overflow weir. Table 1 shows a list of design parameters and the expected performance for the UASB-HCPB reactor. The design OLR for UASB-HCPB reactor was fixed at 25 g L1 d1 based on literature references of various thermophilic anaerobic wastewater treatment systems and POME characteristics. Most high-rate reactors were able to operate at an OLR of more than 10 g L1 d1 (except for coffee waste) and the maximum achievable OLR was found to be 150 g L1 d1 [13]. However, taking into consideration that POME has high solid and oil and grease content in the wastewater [14], a conservative OLR of 25 g L1 d1 was selected as the design OLR of UASB-HCPB reactor to avoid problems of overloading.

3.

Materials and methods

3.1.

Palm oil mill effluent

POME fed into the UASB-HCPB reactor was collected weekly from the outlet of oil trap from Golconda Palm Oil Mill (3.138049, 101.386999), Selangor, Malaysia. Samples were stored at 4
C before use. The characteristics of POME were listed in Table 2. POME was diluted with tap water for the initial stage of start-up to achieve desired operating OLR. The pH of the feed was adjusted to 6.8 with 1 mol dm3 sodium hydroxide solution and was further adjusted to 7.0 using sodium bicarbonate to provide an alkalinity value of >1.5 g L1. The pH of the feed was not adjusted with sodium bicarbonate after the alkalinity of the UASB-HCPB reactor can be maintained above 1.5 g L1.

3.2.

Seed sludge

The inoculum that was used to seed the UASB-HCPB reactor was a thermophilic mixed culture which was cultivated using POME as a substrate in the previous study [16]. The volatile suspended solid (VSS) concentration of the seed sludge was 11.62 g L1. The seed sludge was black in colour and in a mixture of fine particles and granules.

Table 2 e Characteristics of POME. Parameters

Units Average Standard deviation

Temperature pH Biochemical oxygen demand (BOD) Chemical oxygen demand (COD) Suspended solid (SS) Oil and grease (O&G) Total nitrogen (TN) Volatile fatty acid (VFA) (as acetic acid)




C e mgL1 mgL1 mgL1 mgL1 mgL1 mgL1

47 4.7 17,000 32,580 15,000 6100 852 3540

±4.52 ±0.19 ±2500 ±9500 ±1350 ±1094 ±107 ±510

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3.3.

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Start-up of UASB-HCPB

Fig. 2 shows the process flow diagram of the UASB-HCPB reactor for POME treatment. Under normal circumstances, a reactor can be started-up by acclimatizing the seed sludge with glucose to produce readily measurable intermediate metabolites for anaerobic digestion [17]. Since the seed sludge used was cultivated using POME as the substrate under thermophilic condition, the seed sludge had the capability to degrade POME under thermophilic condition in the UASBHCPB reactor. Therefore, 500 ml of seed sludge which accounts for 10% of the reactor's working volume was inoculated in the reactor and directly fed with raw POME. Furthermore, temperature shift from mesophilic to thermophilic condition was unnecessary since the seed sludge was cultivated under thermophilic conditions. As such the reactor was consistently maintained at 55
C throughout the entire process with a heating water bath which circulates hot water in the jacket of the UASB-HCPB reactor. The rest of the reactor was filled with diluted raw POME. Contents in the UASB-HCPB reactor was purged with nitrogen gas to remove oxygen from the system prior to start-up. In previous investigations on anaerobic digestion of wastewater, researchers started-up a reactor by either: i) increasing OLR by reducing HRT while operating at the same influent feed concentration [20e22] or ii) increasing OLR by increasing influent feed concentration while operating at the same HRT throughout the start-up [4,23,24]. The latter

strategy was employed for the start-up of the UASB-HCPB reactor in this study as start-up by shortening the HRT may cause serious problem of sludge washout due to high upflow sheer force [25], leading to unstable reactor operation. Table 3 summarizes the strategy employed during the start-up of the UASB-HCPB reactor. The UASB-HCPB reactor was initially operated in batch where the diluted raw POME was fed once a day for two consecutive days for development of microbial population in the system. This was equivalent to HRT of 1 day and OLR of 0.5 g L1 d1. On the third day of operation (Run II), diluted POME was continuously fed into the UASB-HCPB reactor from the feed tank as shown in Fig. 2 at a HRT of 1.5 days. The HRT of 1.5 days was maintained throughout the start-up period while the feed COD concentration into the system was increased after each run by reducing the dilution factor of POME until undiluted POME was fed into the system. During start-up, the initial average MLSS concentration in the reactor was found to be 3.18 g L1. However, based on the study conducted by Poh and Chong [16], MLSS concentration of 14.0 g L1 was required to achieve 90% COD removal efficiency. Hence, the MLSS concentration was allowed to increase and then maintained at 14.0 g L1 throughout the start-up. The recirculation pump was fixed at a speed of 3.36 L h1 throughout the operation of the reactor. pH of the reactor was constantly monitored to ensure that the pH did not fall below 6.5. Sodium bicarbonate was added into the system to adjust the pH of the feed once pH of the reactor falls below 6.5.

Fig. 2 e Schematic diagram of the UASB-HCPB reactor (numbers 1e7 represents the 7 sampling ports along UASB-HCPB reactor).

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Table 3 e Start-up strategy for UASB-HCPB reactor. Fixed parameters

Units

Value

Temperature pH MLSS HRT




55 >6.5 14,000 1.5

Start-up strategy experimental run Batch feed Run I Continuous feed Run II Run III Run IV Run V Run VI Run VII

C e mg L1 d

Day

Feed COD concentration (g L1)

Dilution factor

1e2

0.89

30

4.5 L

3e6 7e12 13e14 15e20 21e28 29e36

2.35 10.40 27.30 18.40 30.80 42.60

10 2.5 1 2 1 1

3.31 3.31 3.31 3.31 3.31 3.31

BOD, COD, SS, MLSS, MLVSS, VFA, pH and alkalinity of the effluent was analysed daily in accordance to AHPA Standard Methods [18]. MLSS and MLVSS at the 7 sampling ports along the reactor (Fig. 2) were also analysed. The composition of biogas produced was tested daily with the biogas meter (GFM 416, Gas Data, UK) by sampling from the gas sampling port (Fig. 2). Start-up of the reactor will be considered complete when COD removal efficiency and methane gas concentration remains constant at more than 80% and 55% respectively, with less than 5% variation for 3 consecutive days at its designed OLR of 25 g L1 d1.

3.4. Performance study of thermophilic POME treatment using UASB-HCPB reactor The performance of POME treatment in the UASB-HCPB reactor was studied in terms of HRT, OLR and MLVSS concentration. To evaluate the performance and optimize the thermophilic POME treatment process using UASB-HCPB reactor, HRT of the reactor was varied while maintaining feed and MLVSS concentration at a constant value. Meanwhile, the effect of OLR on the treatment efficiency was evaluated by varying the feed COD concentration whilst operating at a constant HRT and MLVSS concentration. In order to study the effect of MLVSS concentration on POME treatment efficiency, MLVSS concentration was varied by adding or removing the inoculated thermophilic mixed culture from sampling port 1 (Fig. 2). The HRT of the UASBHCPB reactor was fixed at 5 days to study the effect of MLVSS concentration in the reactor on POME treatment efficiency. This strategy was employed to improve the methane purity in the biogas. Parameters that were monitored throughout the experimental runs were COD, BOD, TSS, alkalinity, methane concentration and VFA. Once effluent COD and methane concentration in the biogas produced remain constant with less than 5% variation for 4 days in a particular run, it can be considered that steady-state has been achieved and parameters were varied for subsequent runs.

Feed volume/ flowrate

L/day L/day L/day L/day L/day L/day

Corresponding OLR (g L1 d1)

MLSS concentration (mg L1)

0.50

3.18

1.00 6.89 18.05 12.19 20.40 28.20

3.20 4.00 7.80 3.70 13.50 14.00

3.5. Environmental scanning electron microscopy (ESEM) ESEM (FEI Quanta 400F, USA) was used to examine the physical appearance of the thermophilic sludge. ESEM is an alternative option to conventional SEM for biological samples as it is able to view samples which produce significant amount of vapour and coating of samples is unnecessary [19]. As thermophilic sludge has high moisture content and not conductive, ESEM was selected to examine the physical appearance. Thermophilic sludge was sampled from sampling ports of the UASB-HCPB reactor into petri dishes. To improve image quality and avoid evaporation, the petri dishes containing the thermophilic seed sludge samples were air dried for at least 8 h to reduce the moisture in the samples.

4.

Results and discussion

4.1.

UASB-HCPB reactor start-up

Fig. 3 shows the changes in OLR, corresponding COD, BOD, SS removal efficiency and VFA level in the UASB-HCPB reactor during the start-up period. After batch feeding (Run I), the reactor was then fed continuously at a higher OLR of 1 g L1 d1 by feeding with an influent COD of 2.35 g L1 (Run II). In Run II, the COD removal efficiency remained constant at approximately 62% for 3 consecutive days. In Run III, the influent COD was further increased to 10.4 g L1, resulting in an OLR of 6.89 g L1 d1. The COD removal efficiency of Run III was increased from 62% (Run II) to 90%, as shown in Fig. 3(a). This result indicated that at higher influent feed concentration where substrate was in abundance, microbes did not have to compete for substrate. This encouraged the growth of microbial population where the MLSS concentration increased from 4.0 g L1 at the beginning of Run III to 7.8 g L1 at the start of Run IV, leading to a better treatment efficiency of the system. The VFA concentration in the UASB-HCPB reactor during this period remained low, fluctuating

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Methane Gas Concentration (%)

80

I

75

II

III

IV

V

VI

VII

70 65 60 55 50 45 40 35 30

0

5

10

15

20 Time (d)

25

30

35

Fig. 4 e Methane composition in biogas during start-up (I e Run 1; II e Run 2; III e Run 3; IV e Run 4; V e Run 5, VI e Run 6; VII e Run 7).

Fig. 3 e Performance of UASB-HCPB reactor along start-up period (a) OLR applied to the UASB-HCPB reactor and the COD removal efficiency; (b) BOD removal efficiency; (c) SS removal efficiency; (d) VFA level in the UASB-HCPB reactor during start-up (I e Run 1; II e Run 2; III e Run 3; IV e Run 4; V e Run 5, VI e Run 6; VII e Run 7).

between 0.12 and 0.27 g L1 (Fig. 3(d)) while the methane gas composition in the biogas (Fig. 4) in Run III peaked at 76% on day 9. The relatively low VFA concentration indicated that VFA produced by the acidogens were effectively consumed by the methanogens to produce methane. This was reflected in the increase in methane production, indicating that the microbial consortium have adapted to the system. The population of the acidogens and methanogens were in balance as the population of methanogens in the system was sufficient to consume the VFA produced by the acidogens [26]. Based on the encouraging results obtained in Run III, undiluted POME with an influent COD of 27.3 g L1 (Run IV) was fed into the reactor (OLR ¼ 18.05 g L1 d1). The COD removal efficiency of the system immediately declined to 6.6% while the VFA concentration in the reactor increased from 0.25 g L1 to 1.39 g L1 in day 13 as shown in Fig. 3(a) and (d). Nevertheless, the reactor resumed to its performance on day 14, removing 82.6% of COD with a VFA concentration 0.846 g L1, but with scum formation observed. This was due to the fact that the UASB-HCPB experienced a shock loading due to increase in feed concentration. The immediate response by the

reactor under stress due to fluctuation in feed concentration would be accumulation of VFA, reduction in methane composition [27] which was reflected in Figs. 3(d) and 4. Scum formation contributed to loss of methanogens in the UASBHCPB reactor, causing VFA accumulation as the methanogens were unable to cope with the rate of organic matter conversion to VFA. There was no massive decline in the methane composition as shown in Fig. 4. The methane concentration was maintained above 65% during Run IV. However, a decreasing trend in the methane concentration in the biogas produced was observed. This suggests that the methanogens in the system were affected by the shock loading and counter measures were required to avoid continuous scum formation. To eliminate scum formation and avoid reactor upset, the influent COD concentration in Run V was reduced to 18.4 g L1 (OLR of 12.19 g L1 d1). The COD removal efficiency for day 15 (Run V) was 80% before declining to 5.8% on day 16 as shown in Fig. 3(a) and was also accompanied by a sharp decline in SS removal (Fig. 3(c)). The VFA concentration increased from 0.60 g L1 to 0.90 g L1 (Fig. 3(d)) during the decline in COD and SS removal on day 16 (Run V) and methane in the biogas produced also declined from 67.3% to 61.3% as shown in Fig. 4. Scum was also visible at the top of the settling tank. These observations indicated that there was washout of solids from the reactor. The solids washout from the reactor could be related to the fact that the reactor was trying to cope with the instantaneous change in the feed concentration, as reported ^´ˆ o et al. [28]. Sufficient ‘buffer’ capacity in the reactor by Leita will be required to absorb the instantaneous change applied to the system in terms of feed concentration and OLR to avoid drastic reduction in the reactor performance. The solids washout from the system, the low COD and SS removal clearly showed that rapid change in feed concentration of the system was not desirable to allow the system to gain sufficient ‘buffering’ capacity to respond to any shock changes applied to the system. As such, the feed COD was maintained at 18.4 g L1 (OLR 12.19 g L1 d1) to allow the system to stabilize in terms of COD and SS removal and eliminate the problem of

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may be conducted to investigate the reason of the difference in the trend in BOD and COD removal efficiency during anaerobic digestion. Referring to Fig. 3(d), the VFA level in the effluent was considerably higher than other systems operated under mesophilic system. This result was in concurrence with the results obtained from studies conducted to compare the VFA level of high-rate anaerobic reactors operated under mesophilic and thermophilic condition. Mesophilic anaerobic systems generally have VFA concentrations less than 0.50 g L1 while thermophilic anaerobic systems have VFA concentrations greater than 1.0 g L1 [32,33]. The higher VFA concentration in the reactor could be attributed to low substrate affinity of thermophilic microorganisms i.e. Methanosarcina sp. and Methanobacterium sp. [12]. Nevertheless, the VFA concentration was lower than the concentration of sour digesters (2.0e4.0 g L1) which causes instability to the system. This implies that the reactor was stable as the VFA level was considerably lower than the range of VFA concentration of sour digesters. Furthermore, the alkalinity in the reactor (Fig. 5) increased gradually during the start-up period and pH adjustments of the UASB-HCPB reactor was not required to maintain the reactor at optimum pH range (6.8e7.2). This also shows that the reactor had sufficient buffering capacity to counter pH change due to the increase in OLR. The VFA concentration in effluent from the UASB-HCPB reactor was generally lower compared to the VFA concentration in effluent from the upflow anaerobic filter for POME treatment under thermophilic condition during start-up process studied by Mustapha et al. [30]. Mustapha et al. [30] reported that during the early stage of start-up, the reactor faced accumulation of VFA (>3.0g L1), drop of alkalinity (<1.0g L1) and deterioration of reactor efficiency, in which the COD removal efficiency declined from 94% to 60%. It was due to the inoculated mesophilic seed sludge which was adapting to the temperature shift from mesophilic to thermophilic condition. This shows that the use of thermophilic seed sludge in this study reduced the problem of VFA accumulation during early stage of start-up as the microbial consortia in the seed sludge was acclimatized to the thermophilic condition. Moreover, the lower level of VFA in the effluent of the UASB-HCPB reactor was attributed to the features in the reactor. The HCPB section

3

I

II

III

IV

V

VI

VII

2.5 Alkalinity (g L-1)

solids washout. The corresponding COD and SS removal efficiencies at the end of Run V stabilized at 81% and 78% respectively, indicating that the reactor gained its performance after the change in loading rate. Subsequently, problem of solids washout was completely eliminated. Thus, it was unnecessary to lower the influent feed concentration as the reactor had the capacity to cope with the load in Run V with improved performance as compared to Run IV. The influent COD into the reactor was increased to 30.8 g L1 which corresponds to an OLR of 20.40 g L1 d1 in Run VI. The reactor responded well with no deterioration in COD, BOD and SS removal efficiency, as illustrated in Fig. 3(a)e(c) respectively. However, there was a sharp decline in the concentration of methane in the biogas produced (Fig. 4). The decline in methane concentration corresponded to the increasing trend of VFA concentration of the system as shown in Fig. 3(d). This could be due to the application of high OLR, causing inhibitory effects to the methanogens which lead to the inability to effectively utilize VFA that was being produced. When OLR in the reactor increases, hydrolytic, acidogenic and acetogenic bacteria will convert POME to simpler compounds at a faster rate, contributing to higher VFA concentration. However, due to slow rate of growth of methanogens, excess VFA and carbon dioxide produced by acidognes and acetogens cannot be consumed effectively by the methanogens, causing the accumulation of VFA in the reactor. This affects the composition of methane in the biogas to decrease; the carbon dioxide and VFA concentration to increase. This phenomenon was in concurrence to the studies  nchez et al. [9] and Borja et al. [29] which conducted by Sa indicated that methane concentration in biogas decreases with the increase in OLR. Subsequently, the OLR was increased to 28.20 g L1 d1 by feeding POME with COD concentration of 42.6 g L1 in Run VII after the COD and methane concentration in Run VI remained at 86% and 54% respectively. A steady COD removal efficiency of 88%, BOD removal efficiency of 93% and SS removal efficiency of 88% was attained throughout the Run VII as shown in Fig. 3(a)e(c), respectively. The reactor showed stable operating performance as the VFA concentration of the effluent reduced from 1.2 g L1 to less than 0.60 g L1 with no major fluctuations detected from other monitored parameters (pH and methane concentration). The start-up was considered completed when the reactor reached a steady-state treatment performance on day 36 as the UASB-HCPB reactor was operated above the design OLR of 25 g L1 d1. The BOD removal efficiency throughout the start-up period as shown in Fig. 3(b) fluctuated in between the range of 79.1%e 97.7% from Run III to Run VII. This does not correspond to the trend of COD removal efficiency as shown in Fig. 3(a) during start-up which had a drastic decline under shock loading and instantaneous change in feed concentration. Although most studies [30,31] showed that trend of BOD and COD removal efficiencies were similar, the BOD removal efficiency obtained during the start-up in this study was similar to that observed in thermophilic treatment of bulk drug pharmaceutical industrial wastewaters [17]. As reported by Sreekanth et al. [17], no drastic decline in BOD removal efficiency was detected when COD removal efficiency of the hybrid UASB reactor showed drastic decline due to shock loading. Further studies

2 1.5 1 0.5 0

0

5

10

15

20 Time (d)

25

30

35

Fig. 5 e Effluent alkalinity during start-up (I e Run 1; II e Run 2; III e Run 3; IV e Run 4; V e Run 5, VI e Run 6; VII e Run 7).

7.32 9.51 28.93 10.16 19.26 19.40 33.22 10.53 36.76 7.57 7.59 7.78 7.78 51.9 54.7 59.8 66.9 67.5 58.4 53.8 49.4 57.4 65.0 65.6 64.8 64.1 83.8 83.8 84.0 82.6 86.8 86.9 84.9 77.9 86.7 93.5 98.3 98.2 96.1 95.7 95.1 95.0 92.0 93.7 93.1 93.7 95.0 91.2 98.2 97.3 96.0 96.8 81.9 90.5 91.9 88.8 92.6 92.6 93.5 67.9 91.8 95.7 97.5 97.0 97.1 8.12 7.75 7.45 8.19 8.25 7.62 7.40 7.27 7.72 8.22 7.75 8.15 8.11 14.40 14.40 14.40 14.50 14.50 14.50 14.70 14.70 14.70 13.60 14.98 16.70 18.56 17.9 17.9 17.9 23.5 23.5 23.5 33.0 33.0 33.0 17.0 19.0 21.0 23.0 1.25 1.67 2.50 1.25 1.67 2.50 1.25 1.67 2.50 1.00 1.00 1.00 1.00 4 3 2 4 3 2 4 3 2 5 5 5 5 4.28 5.71 8.55 6.88 9.19 13.75 13.83 18.47 27.65 6.66 6.66 6.66 6.66 17.1 17.1 17.1 27.5 27.5 27.5 55.3 55.3 55.3 33.3 33.3 33.3 33.3 1 2 3 4 5 6 7 8 9 10 11 12 13

Biogas production rate (L day1) OLR (g L1 d1) Influent COD (g L1) Run

Parameters of steady-state runs

HRT (d)

Feed flow rate (L d1)

Average MLSS (g L1)

Average MLVSS (g L1)

Effluent pH

Overall COD removal (%)

Overall BOD removal (%)

Overall SS removal (%)

Overall CH4 (%)

Value

The results of performance study were listed in Table 4. Runs 1e9 were aimed to study the effects of HRT and OLR on POME treatment efficiency. HRT in Runs 1e3, 4e6 and 7e9 were varied between 2 and 4 days while the influent COD and MLVSS were maintained at a certain concentration as shown in Table 4 in order to investigate the effects of HRT. The influent COD concentration was increased after every 3 runs to investigate the effect of OLR. Effect of MLVSS was studied from Run 10e13 by increasing the MLVSS concentration and maintaining the HRT and OLR at 5 days and 6.66 g L1 d1. Based on runs 1e9 of Table 4, increase in OLR up to 9.19 g L1 d1 (Run 5) showed a fluctuating behaviour but with increasing trend in overall COD and SS removal, overall methane concentration and biogas production rate. However, the overall COD and SS removal remained almost stable when the OLR was further increased to 27.65 g L1 d1 (Run 9). On the other hand, the overall methane concentration dropped to less than 60% at the end of Run 9. The overall BOD removal efficiency of the system did not show significant changes with increasing OLR with values fluctuating between 91.2 and 95.7% throughout Run 1e9. The trend of COD removal and methane gas production corresponded to the study conducted by Zhou et al. [25] on the effect of loading rate on granulation process which showed an increase in COD removal when OLR was increased before succumbing to poorer performance at higher OLRs. The decrease in reactor performance at higher OLRs was mainly due to the imbalance of the acidogen and methanogen populations. Zhou et al. [25] found that although granules formed more quickly under higher OLRs, SEM observations and microbial activities indicated that the loss of

55
C >6.5

Performance study of the UASB-HCPB reactor

Temperature pH

4.2.

Fixed parameter

enabled the attachment of biomass on the packing whilst allowing more contact between the biomass and substrate to increase the rate of conversion of VFA to methane. Fig. 4 shows the composition of methane in the biogas throughout the start-up period. The biogas contained 42.5e76.1% of methane with the remaining being carbon dioxide. Hydrogen sulphide was also present in the biogas but the level remained below 0.80 g L1 throughout the start-up period. The level of hydrogen sulphide is normally maintained at a level of 0.70e1.50 g L1 or less when used in internal combustion engines for combined heat and power (CHP) production [34]. In Denmark, biogas is upgraded through removal of carbon dioxide in order to blend into natural gas. For biogas which has not undergone upgrading, 8% of biogas (with 65% methane) could be blended into the natural gas supplied in a natural gas network [34]. Hence, the biogas produced from this study was still acceptable for use as boiler fuel without further purification to remove the hydrogen sulphide. Overall, the MLSS concentration in the UASB-HCPB reactor gradually increased with increase in influent feed concentration (as shown in Table 3) except during Run IV where the formation of scum caused washout of solids. This indicated that there was a significant development of biomass in the system throughout the start-up period and the reactor retained sufficient amount of biomass to achieve high COD, BOD and SS removal during the start-up.

Parameter

b i o m a s s a n d b i o e n e r g y 6 7 ( 2 0 1 4 ) 2 3 1 e2 4 2

Table 4 e Results of the performance study of thermophilic POME treatment by UASB-HCPB reactor.

238

b i o m a s s a n d b i o e n e r g y 6 7 ( 2 0 1 4 ) 2 3 1 e2 4 2

balance between acidogens and methanogens in the reactor caused poor performance of anaerobic digestion system. Hence, it is necessary to operate at a suitable OLR which will not compromise on the effluent quality and biogas yield. However, this study did not consider the effect of further increase in OLR from 27.65 g L1 d1 on COD removal efficiency. This is because the OLR of 27.65 g L1 d1 was already higher than the maximum design OLR of 25 g L1 d1 and the overall methane concentration already showed a decreasing trend since Run 6. The overall COD removal efficiency also remained almost stable for 4 consecutive runs (Run 5e7 and 9) except for Run 8. A massive decline in reactor performance during Run 8 was encountered with scum formation in the GLS section of the UASB-HCPB reactor. The scum also overflowed into the settling tank, causing a decline in overall COD removal efficiency to 67.9%. Furthermore, the biogas production rate was greatly affected due to scum formation. The scum did not subside even though the pH of the system remained in the region of 7.27, which was optimum for methanogenesis. Scum formation was observed in Run 8 due high oil and grease (O&G) concentration (11.2 g L1) in the system at high OLR. Halalsheh et al. [35] found that longer solid retention time (SRT) will reduce scum forming potential of a system as lipid removal efficiency is better at longer SRT. Since the O&G loading is high in Run 8, the O&G tends to adsorb onto the sludge particles to float, forming scum in the system [35]. Hence, in order to prevent the reactor from further upset, the feed into the reactor was halted to increase the SRT of the reactor until the scum subsided before operating the system with the parameters in Run 9 of Table 4. In Run 9, the performance of the reactor on POME treatment achieved the overall COD, BOD and SS removal efficiencies of 91.8%, 91.2% and 86.7%, respectively. The methane concentration in the biogas produced in Run 9 was 57.4%. The results indicated that the reactor resumed normal operation following the scum formation in Run 8. Based on the results in Table 4, at a constant influent COD and MLVSS concentration, it can be concluded that the effluent quality of the UASB-HCPB reactor improved with a decrease in HRT due to increasing OLR, although the observed trend is fluctuating. Nevertheless, the methane concentration in the biogas produced increased with the decreasing HRT at low OLR (up to Run 5) but reduced with the decreasing HRT at higher corresponding OLR (Run 6e9). This shows that the effect of HRT on the treatment efficiency is closely related to the corresponding OLR. The better performance at shorter HRT with low corresponding OLR for UASB-HCPB reactor was due to granulation of microbes in the UASB section of the reactor. A study conducted by Alphenaar et al. [10] on the effects of liquid upflow velocity and HRT on the treatment of high sulphate content wastewater in UASB reactor indicated that granulation of microbes was favoured at short HRT. Thus, higher amount of biomass was retained in the UASB section of the UASB-HCPB reactor and this improved the overall reactor performance. On the other hand, Table 3 shows that the pH of the effluent was lower when operated at shorter HRTs with constant influent COD and MLVSS concentration. This implied that although granulation might be enhanced at shorter HRT in the UASB-HCPB reactor producing effluent of improved

239

quality, the population of methanogens was not sufficient to quickly convert the VFA into methane under the situation of increasing OLR due to decreasing HRT at constant MLVSS concentration. This result shows that HRT is a very important variable that should be considered to obtain high methane concentration in the biogas without compromising the effluent quality. In Run 10e13 (Table 4), the MLVSS concentration in the UASB-HCPB reactor was varied at constant influent COD concentration and HRT to study the effect of MLVSS concentration on the reactor performance. Based on results in Table 4, operating parameters in run 11 (MLVSS concentration of 14.98 g L1) produced effluent with the highest quality, with overall COD, BOD, SS removal efficiency of 97.5%, 97.3%, 98.3%, respectively and overall methane concentration of 65.6%. Operation with higher MLVSS did not show significant improvement on the effluent quality or the overall methane concentration and yield. In fact, the methane concentration and yield declined when the MLVSS concentration was greater than 14.98 g L1. According to Contois' kinetic model, cell mass concentration (microbial population) is inversely proportional to the specific growth rate (m) in a system. Hence, excessive MLVSS concentration will lead to lower methane yield. Nevertheless, the specific growth rate is also affected by substrate concentration. Therefore, MLVSS is also an important operating parameter besides OLR and HRT to ensure acceptable methane yield. Based on the performance results presented in Table 4, the responses of COD, BOD, SS removal, methane concentration and methane yield are dependent on the three individual operating parameters e OLR, HRT and MLVSS. COD and BOD removal efficiencies of greater than 90% are desired to ensure that the discharge limit can be achieved in the subsequent stage of treatment. Meanwhile, biogas production with methane concentration greater than 60% is desired to ensure that no subsequent treatment is required before using the biogas for energy generation. Best performance of the UASBHCPB reactor was achieved under OLR of 6.66 g L1 d1, HRT of 5 days and MLVSS concentration of 14.98 g L1 (Run 11, Table 4). However, the operating conditions to achieve selected target value for responses (Table 4) were OLR of 27.65 g L1 d1, HRT of 2 days and MLVSS concentration of 14.7 g L1 (Run 9, Table 4).

4.3. ESEM imaging of granules from sludge bed and attached growth in HCPB section Fig. 6 shows the ESEM images of the granules from the sludge bed of the UASB section while Fig. 7 shows the ESEM images of the attached growth in the HCPB section. Fig. 6 indicated that majority of microbes present in the UASB section were coccus-shaped Methanosarcina thermophila which grew in aggregates (Fig. 6(a)) or attached to biomass (Fig. 6(b)). The population of microbes at this stage of operation was distinctly different from the population of the thermophilic mixed culture which was developed in the batch fermenter [16], as shown in Fig. 8. This was presumably due to the shift in microbial population growth to favour Methanosarcina sp. with n change in operating conditions. Studies conducted by Leve et al. [36], Noike et al. [37] and Sasaki et al. [38] found

240

b i o m a s s a n d b i o e n e r g y 6 7 ( 2 0 1 4 ) 2 3 1 e2 4 2

Fig. 6 e ESEM image of granules from the sludge bed in the UASB section. (a) aggregates of coccus which form the granule, (b) attachment of microbes on solid substrate and (c) mixture of rod and coccus-shaped bacteria in the sludge bed.

Fig. 7 e ESEM images of attached growth from the HCPB section of the UASB-HCPB reactor. (a) rod-shaped microbes and (b) rod-shaped microbe (dimensions: L £ D e 0.75 mm £ 4.52 mm).

b i o m a s s a n d b i o e n e r g y 6 7 ( 2 0 1 4 ) 2 3 1 e2 4 2

241

for methane production helps to maintain the partial pressure of hydrogen in the system. This will maintain the stability of an anaerobic digestion system as high hydrogen partial pressure will cause the inhibition of converting long chain fatty acids into acetate or propionate [42]. Thus, the presence of Methanosarcina in addition to Methanosaeta and Methanobacterium sp. in the microbial population of UASB-HCPB for thermophilic POME treatment is beneficial to the operation of the reactor due to higher maximum substrate utilization rate which enables operation at shorter HRT and higher OLR.

5.

Fig. 8 e ESEM image of thermophilic seed sludge.

difference in microbial population when there were changes in operating conditions and VFA level. Amount of Methanosarcina sp. was found to increase as HRT [38] and SRT were shortened [37]. In the operation of the UASB-HCPB reactor for thermophilic POME treatment, the HRT was much shorter (2e4 days) as compared to the operating HRT during the cultivation of thermophilic mixed culture (>6 days) [16]. When HRT is shorter, the corresponding OLR of the UASB-HCPB reactor also increases. Under high VFA concentration, the growth of Methanosarcina sp. will be greater than Methanosaeta sp. [39]. Thus, Methanosarcina sp. forms the majority in the microbial population of the thermophilic mixed culture in the UASB section. In the HCPB section, the microbes attached on the pall rings mostly consisted of rod-shaped microbes (Fig. 7). Measurements made using the imaging software of SEM indicated that most of the rod-shaped microbes had a width between 0.5 and 1.2 mm and length greater than 2 mm. Based on Bergey's Manual of Determinative Bacteriology [40], the dimensions of these long rods indicated the presence of Methanobacterium sp. and Methanosaeta sp. in the microbial population. This is supported by Fig. 7(a) which shows the ESEM image of rods which are connected (as shown by the circle), indicating the presence of Methanosaeta sp. The Methanosaeta sp. is characterized as large sheathed rods which sometimes may appear continuous under phase-contrast microscopy [40]. This is in contrast with the population obtained in the UASB section which had Methanosarcina as the majority. The higher maximum substrate utilization rate by Methanosarcina as compared to Methanosaeta [37] will be more advantageous for thermophilic POME treatment which requires operation at higher OLR so that the UASB-HCPB reactor can be operated at shorter HRTs by controlling the SRT of the reactor [37]. Furthermore, the ability of Methanosarcina to use alternative substrate (hydrogen and formate) besides VFA is the reason for its dominance in a steady system with high level of VFA [41]. The ability of Methanosarcina to consume hydrogen

Conclusion

A novel high-rate anaerobic reactor, UASB-HCPB reactor was successfully investigated for the treatment of POME under thermophilic condition. The UASB-HCPB reactor had a successful start-up after 36 days of operation, achieving COD and BOD removal of 88% and 90% respectively and 52% of methane in the biogas at an OLR of 28.12 g L1 d1. Best performance of the reactor was achieved under OLR of 6.66 g L1 d1, HRT of 5 days and MLVSS concentration of 14.98 g L1. OLR of 27.65 g L1 d1, HRT of 2 days and MLVSS of 14.7 g L1 were found to be the operating conditions to remove more than 90% of BOD and COD from POME and produce biogas with approximately 60% of methane.

Acknowledgements We thank the University of Nottingham Malaysia Campus and MOSTI e-Science Fund (03-02-12-SF0030) for the financial support as well as Golconda Palm Oil Mill for allowing us to collect samples for this study.

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