Next Article in Journal
Compliance with the Verification of Environmental Technologies for Agricultural Production Protocol in Ammonia and Particulate Matter Monitoring in Livestock Farming: Development and Validation of the Adherence VERA Index
Previous Article in Journal
Prenatal Metal Exposure and Neurodevelopmental Changes in Children up to 18 Months of Age: PIPA Cohort Project, Rio de Janeiro
Previous Article in Special Issue
Plasma-Assisted Valorization of Liquid Digestate from the Ravda Wastewater Treatment Plant: Microbiological and Chemical Aspects
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Environments
Volume 13
Issue 1
10.3390/environments13010022
environments-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Palm Oil Fuel Ash-Enhanced Biofilm Reactor: Performance and Microbial Dynamics in POME Treatment

by
Pei Ling Soo
1,
Lai Peng Wong
1,*,
Mohammed J. K. Bashir
1,2,*,
Xinxin Guo
1 and
Yuansong Wei
1,3
1
Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Kampar 31900, Perak, Malaysia
2
School of Engineering and Technology, Central Queensland University, 120 Spencer St., Melbourne, VIC 3000, Australia
3
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Rd., Haidian District, Beijing 100084, China
*
Authors to whom correspondence should be addressed.
Environments 2026, 13(1), 22; https://doi.org/10.3390/environments13010022 (registering DOI)
Submission received: 11 November 2025 / Revised: 16 December 2025 / Accepted: 21 December 2025 / Published: 1 January 2026
(This article belongs to the Special Issue Advances in Wastewater Bio-Management: Microbial Community Relationships, Monitoring and Assessment)
Browse Figures
Versions Notes

Abstract

The rapid growth of the palm oil industry produces large amounts of palm oil mill effluent (POME), which contains high organic content and is challenging to treat using conventional ponding systems. These traditional systems often fail to meet discharge standards for biochemical oxygen demand (BOD) and chemical oxygen demand (COD). This study tested anaerobic biofilm reactors enhanced with biochips and chemically treated palm oil fuel ash (TPOFA) to improve POME degradation and biogas production. Two 3 L reactors were operated at the same feed-to-microorganism (F/M) ratio: a control (C) and a combination of both (P + B). Biochips helped microbes attach and form biofilms, while TPOFA acted as an adsorbent, creating better conditions for anaerobic breakdown. The P + B reactor outperformed others, achieving over 95% COD removal, high microbial biomass (MLVSS: 24,500 mg/L), and the highest biogas yield at 917 mL per day. Microbial analysis showed dominant groups, including phyla groups of Halobacterota, Bacteroidota, and Firmicutes. Class Methanosarcina in archaeal phylum of Halobaterota was key in converting acetate to methane. Bacteroidota primarily aided organic matter breakdown and nutrient removal, while Firmicutes supported hydrolysis and electron transfer. Less abundant Desulfobacterota also helped by interacting with methanogenic archaea. Overall, combining biochips with TPOFA in anaerobic biofilm reactors offers an effective, sustainable method for treating POME and recovering renewable energy through biogas.

Graphical Abstract

1. Introduction

The rapid growth of the palm oil industry produces large amounts of palm oil mill effluent (POME), which contains high organic content and is challenging to treat using conventional ponding systems [1,2]. It is a vital component in a vast array of products, from food to cosmetics and industrial applications. It follows that the global demand for palm oil has surged over the past few decades, making it an important economic article of trade in palm oil-producing countries like Indonesia, Malaysia, Thailand, and Brazil. Vast development of the palm oil industry yields numerous advantages for the country’s economy; however, it also gives rise to significant environmental challenges, such as water pollution by POME and greenhouse gas emissions. Failure to handle it properly may result in a devasting effect on the ecosystem, as POME has higher oxygen depletion (100 times more than sewage wastewater) [3,4]. Existing treatment technologies such as Fluidized Bed Biofilm Reactors (FBBR), Up-flow Anaerobic Sludge Blanket (UASB), and Up-flow Anaerobic Filter (UAF) are insufficient to achieve the desired chemical oxygen demand (COD) removal efficiency of over 95% [5,6,7,8,9]. Conventional ponding systems for POME treatment remain widely utilized in countries like Indonesia and Malaysia, primarily due to the ample availability of extensive land and steady warm climate that enable POME treatment [10,11]. This method is commonly employed because of its low initial investment, simple technical demands, and manageable operation expenses. Despite that, this approach requires extended retention time and substantial large land allocation to accommodate the treatment facility [12,13,14]. These traditional systems often fail to meet discharge standards for BOD and COD.
Palm oil fuel ash (POFA) recently has been receiving attention for its effective utilization in wastewater treatment for pollutants removal. POFA is a potential adsorbent in conventional surface modification methods (i.e., chemical activation, thermal treatment and metal impregnation), enabling more effective pollutants removal [15]. Research by Manikam et al. [16] proved the ability of POFA as an adsorbent to reduce COD from sewage wastewater, with up to 100% removal. The studies above have shown a promising result of applying POFA into wastewater treatment. The presence of high organic content in POME has been always a major threat to water quality, requiring immediate action. Current technologies and methods applied for POME treatment are inadequate to address the increasing volume of wastewater, and the potential of using POFA in POME treatment remains undiscovered. Thus, there is a need for innovative solutions to reduce the environmental impacts of POME and POFA on water bodies by utilizing these waste by-products.
On the other hand, biocarrier are widely used in various wastewater treatment plant. In this study, biochips were chosen as the biocarrier to act as the hub for biofilm attachment and suitable for high-strength wastewater treatment like POME due to its high specific surface area and protection against shock loads [17,18]. Mazioti et al. [19] mentioned that biofilm attached to the media could not be removed easily due to its morphology making it resistant to washout and clogging. Application of biochips has demonstrated substantial enhancement in treating various wastewaters, such as municipal and sewage wastewater. Biochip applications in wastewater have been proven to enhance BOD removal up to 88% in sewage wastewater [20]. In addition, Allegue et al. [21] suggested the use of biochips in the anoxic environment as it favors the development of anaerobic conditions. A study from Dan et al. [22] showed that the best COD removal was 79% in the biochip tank and has an organic loading rate (OLR) of 1.74 kgCOD/m3.d as compared to no biocarrier. This groundbreaking research of integrating POFA and biochips into an anaerobic biofilm reactor will enhance the performance in terms of organic degradation and biogas production.
This study aims to test the hypothesis that anaerobic biofilm reactors is enhanced with biochips and chemically treated palm oil fuel ash (TPOFA) to improve POME degradation and biogas production. Microbial diversity in the integrated anaerobic biofilm reactors is studied and discussed.

2. Materials and Methods

2.1. Sample Characteristics

The study used POME, POFA, and palm oil sludge (POS), obtained from local palm oil mill located in Perak, Malaysia. Effluent from the treated POME was collected and tested daily.

2.2. Activated POFA as Adsorbents

In this study, activated POFA is labeled TPOFA and it is activated with chemicals to increase the surface porosity of the ash to enhance bacterial growth [23]. Collected POFA was chemically treated with potassium hydroxide (KOH) with a ratio of POFA (1:1 in weight), and the TPOFA was dried in an oven at 105 °C for two hours and then ground to obtain a size smaller than 500 µm after drying, which was further kept in a desiccator to avoid contact with moisture [24]. Both POFA and TPOFA underwent comprehensive analysis consisting of pH, Scanning Electron Microscope (SEM) imaging, and Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy. SEM imaging (JEOL Ltd., Tokyo, Japan; model JSM-6701F) was performed to comprehend the surface morphology of TPOFA and POFA particles under magnification of 1000×. FTIR ((PerkinElmer Inc., Waltham, MA, USA; model RX1) analysis was conducted on both POFA samples at a scanning resolution of 4000 cm−1 to 400 cm−1 to discern the surface functional groups’ difference before and after KOH pre-treatment.

2.3. Biochip Specifications

Biochip is utilized as microbial biocarrier in the bioreactor during anaerobic treatment, with specifications as shown in Figure 1 and Table 1.

2.4. POS the Seed Sludge

POS collected from the oil mill was used as seed sludge to the anaerobic reactors in this experiment. The POS was left to sediment overnight to obtain a thick layer of sludge rich in microorganisms. The cultivation begins with untreated POME feeding to the sludge at an F/M ratio of 1.0 and stirred slowly and manually, both clockwise and anticlockwise, for homogeneous mixing for about 3 weeks to reach a MLVSS value above 10,000 mg/L, as reported by various studies for a healthy sludge condition for start-up [25,26].

2.5. Experimental Set-Up

The set-up of a bioreactor is shown in Figure 2. In a real setting, two different anaerobic bioreactors are fabricated and set up in this study, which consists of one reactor each for C (Control) and P + B (POFA and Biochip incorporation).
The bioreactors were placed at room temperature (26 ± 2 °C) and operated at an F/M ratio of 1.0 [27], with daily feed of neutralized (pH 6.5–7.5), untreated POME by sodium bicarbonate for two bioreactors, with additional feed for the P + B bioreactor with an added 2 g/L dosage of TPOFA [24,28], as well as a biochip constituting 8.3% of the working volume with reference to research by [29]. The working volume of both reactors are maintained at 3 L (total 5 L bottle with 2 L headspace, which is 40%), with influent flow rate of 0.15 L/day operated at a hydraulic retention time (HRT) of 18 days. The reactor underwent nitrogen purging for approximately 3–5 min daily after feeding of neutralized POME to create anaerobic conditions in the reactor [30]. The HRT can be calculated as HRT = V/Q; V = working volume (3 L), Q is the amount of fresh POME added to the reactor per day. An OLR of 1025 mg L−1 d−1 was maintained in both reactors. Treated samples known as effluent of all bioreactors were collected and tested daily for characterization analysis. Weekly sludge samples from each bioreactor were collected for MLVSS and pH analysis. The COD, MLVSS, and pH were performed in triplicate (n = 3) and an average was calculated to be tabulated. The biogas produced was monitored by the water displacement method to determine the amount of biogas daily, and analyzing the produced biogas using GC-Natural Gas Analysis (NGA) was implemented to find out the composition of the biogas samples.

2.6. Microbial Analysis

Total DNA samples were extracted from all two reactors’ sludge using the UltraClean® Soil DNA Isolation Kit (Vivantis Technologies Sdn. Bhd., Selangor, Malaysia) The total DNA samples underwent DNA amplification (16S rRNA) through PCR. High-throughput sequencing was conducted on the NovaSeq platform (Illumina Inc., San Diego, CA, USA) by Novogene Co., Ltd. (Beijing, China). Taxonomic annotation database of the project is Silva 138.1.

3. Results and Discussion

3.1. POME Characteristics

POME stands as a complex by-product of the palm oil extraction process, and its effluent is characterized by its high BOD, COD and oil and grease (O&G) content. This heightened organic content renders POME a potent contributor to environmental pollution if not properly managed. Additionally, the effluent typifies a low pH and high solid content which is a challenge to aquatic ecosystems. The nuanced characteristics of untreated POME are outlined in Table 2.

3.2. POFA Characteristics

Subsequent tests were performed to provide better mechanism insights, especially the effectiveness of TPOFA as a potential adsorbent for pollutants in POME treatment. The pH of POFA is 5.5–7.0 and TPOFA is 7.2–7.4, as tested during the experiment.
Both POFA and TPOFA were characterized using FTIR to identify the chemical functional groups, allowing the identification of different types of chemical bonds in both materials at the molecular level [33]. It is evident from Figure 3 that both ashes exhibited similar characteristic spectra between 400 cm−1 and 4000 cm−1. The FTIR results show similar patterns for POFA and TPOFA because the alkaline treatment primarily enhances the surface reactivity and hydroxyl content without significantly altering the underlying silica–alumina framework of POFA. The treatment with KOH increases the availability of hydroxyl groups, which can participate in adsorption reactions without changing the fundamental chemical bonds detected by FTIR [24,28]. Similar results on the morphology change after KOH treatment in a study by Abdeldjouad et al. [34]. The distinct peak at ~1070 cm−1 corresponded to asymmetric Si-O-Si and Si-O-K stretching vibration. The dominant functional groups identified in POFA and TPOFA, such as Si-O-Si, O-H, and H-O-H, are the key functional groups responsible for treating POME, involving adsorption mechanisms that help remove organic matter and other pollutants.
SEM analysis is important in identifying the morphologies of POFA and TPOFA. In Figure 4a, there were mixtures of spherical shapes of POFA as they were missed out in the grinding process due to their existing small size. Oyehan and Salami [15,35] mentioned that ungrounded POFA can range from as petite as 10 to 20 µm, and grounded POFA has a smaller particle size of 7–10 µm. Hence, it has a similar morphology as that carried in TPOFA samples, but with a larger pore size after alkali activation, as shown in Figure 4b. It is likely because alkalis like KOH and sodium hydroxide (NaOH) dissolve Si effectively, leading to large pores and greater surface area [34], showing potential higher adsorption capability. Similar results on the morphology change after KOH treatment in a study by [34]. It is likely because alkalis like KOH and sodium hydroxide (NaOH) dissolve Si effectively, leading to large pores and greater surface area. According to [36], TPOFA has almost double the surface area of raw POFA, showing potential higher adsorption capability [37].

3.3. COD Removal Efficiency

Steady state of a bioreactor is achieved when the daily fluctuation in COD decrement is continuously <10% [38]. COD removal efficiency in P + B gradually increased from 82% to 95% for the first six weeks and then fluctuated between 89% and 96% for the subsequent 14 weeks. In Figure 5, P + B has achieved its steady state from Week 20 onwards with COD removal >95%. The control has also reached its steady state at Week 24 (fluctuates between 87 and 93%), much later than P + B. It was observed that the COD removal spiked from 38% to 84% within one week, which is reflected by its high MLVSS concentration (14,300 mg/L). But the removal later gradually dropped to the lowest 46% and slowly rose, maintaining at between 81% and 90%, for about 16 weeks before reaching a steady state. The average effluent COD concentration during the steady state of P + B and control were 2582 ± 382 mg/L and 5237 ± 604 mg/L, respectively.
A notable drop in COD removal efficiency occurred between Weeks 8 and 9 across all reactors, particularly in C. This decline could be attributed to a shift in microbial community dynamics, possible accumulation of intermediate fermentation products, or temporary inhibition due to substrate overload. High organic loads can inhibit efficient processing because its own set of inhibitory challenges; i.e., lipid-rich wastes like fats, oils, and greases tend to contain long-chain fatty acids that may affect key microbial community [39]. Previous studies have highlighted that sudden fluctuations in COD removal efficiency often occur during the microbial adaptation phase, particularly when biofilm-based carriers are introduced into anaerobic systems. The incorporation of biochips and TPOFA may have influenced initial microbial attachment and acclimatization, leading to temporary instability before the system regained efficiency [40].
P + B showed the best performance in terms of COD removal, up to 96%, reaching a steady state within 20 weeks. COD removal by P + B was 7.6% greater than control. The performance of anaerobic process was enhanced by the combination of biocarrier and activated carbon. In this study, TPOFA acts as a high-surface area adsorbent in the adsorption of POME treatment. Molecules of the organic content in POME are attracted and retained on the surface of the adsorbent (TPOFA) [24].
At the same time, the biochip is hydrophobic, which contributes to higher adhesion of bacteria on the surface on the numerous surface macropores [29]. Along with the POME feed-in, the organic compounds are attracted to POFA and degraded by the microbes attached to the biochip. Both materials complimentarily support each other, where TPOFA offers biomass attachment and biochip offers microbial attachment during the treatment system [21,36].

3.4. Mixed Liquor Volatile Suspended Solids (MLVSS) Growth Profile

The growth of MLVSS analysis in this study represents the activity and concentration of microbial biomass to monitor the stability and efficiency of anaerobic digestion of organic matter and biogas production. Based on Figure 6, the MLVSS concentration dropped at week 26 and stabilized in ranges of 22,500–24,500 mg/L for control and 21,500–23,000 mg/L for P + B. These values align with optimal MLVSS ranges reported in the POME anaerobic treatment literature, where 22,000–25,000 mg/L supported maximum COD (95–96%) removal efficiencies in sequencing batch reactors (SBRs) [41]. The adaptation period of the sludge took around three weeks for P + B and control. At the beginning of cultivation period, sharp drop of MLVSS concentration was observed in all bioreactors. It happens when large amount of cell death are unable to adapt to new surroundings in anaerobic bioreactors as well as to the addition of TPOFA and biochips. After adapting, the MLVSS concentration slowly increased due to bacterial growth with the continuous POME feed. MLVSS growth of control in this study outperformed the other reactors (Average: 24,952 mg/L) observed during the experiment, as observed from the dense sludge and low sludge washout despite the continuous feed of POME. It might be attributed to the bacteria entrapment on the support structure within the biocarrier and TPOFA, leading to the formation of bacteria biofilm. The lower MLVSS concentration observed in the P + B reactor (POFA + biochip) compared to the control likely happened from bottom-sampling methodology (Figure 2), which adequately captures suspended biomass in the control but underestimates total solids in P + B, where most activated sludge adheres to biocarriers, leaving only periodically detached (washed-out) portion at the base. This reflects fundamental biofilm reactor dynamics, characterized by temporally separated phases of accumulation (growth on carriers), detachment (sloughing), and regrowth, contrasting with the simultaneous biomass processes in suspended-growth systems and resulting in oscillating microscale patterns despite macroscale steady-state performance. Homogenizing samples or incorporating carrier-attached biomass would yield more representative MLVSS values, aligning measurements with actual reactor biomass distribution [42,43]. Overall, MLVSS growth of P + B was lower than control; however, the COD removal efficiency was stable and achieved a steady state earlier than control. It is possible with the presence of attached growth system in the P + B reactor. Incorporation of biochip and TPOFA maximized microbial attachment on the media surface, creating a high-density population that efficiently degrades the organic matter.

3.5. Biogas Yield and Methane Yield

Table 3 summarizes the percentage composition of the main biogas components measured in both bioreactors. The biogas production changes from Week 1, Week 15, and Week 30 are shown in Table 4. Surprisingly, the C (control) reactor yields higher biogas production during the first 15 weeks of anaerobic treatment, perhaps attributed to the superior adaptation of indigenous microorganisms, which kickstart the high rate of organic matter degradation over time [44]. However, as the process goes on, biogas production slows down in the control (Refer to Figure 7), a similar trend as stated by Hossain [45], with rapid biogas generation at the initial stage of digestion and slow deterioration at the final stage of digestion. This happened at the later stage of the experiment, as the sludge was slowly exhausted because of the lack of inoculum addition in the reactor system. Also, Reactor C does not contain any additional carriers to host the microorganism to avoid sludge washout. The control reactor was able to produce 53.51% of methane and remove up to 93% of COD content.
In a study by Tabassum et al. [3], using an anaerobic granular sludge bed enables removal of up to 93% COD and generates 57% methane from POME sludge [3]. In addition, the removal efficiency of COD and BOD with up-flow anaerobic sludge blanket (UASB) can reach up to 90% [46] shows the similar results of COD removal efficiency of anaerobic reactors. Subsequently, biogas production by P + B is relatively lower than C at the beginning of the treatment. In this study, P + B generated an overall average of 893 mL CH4/g COD methane with 62.2% of methane purity. In general, methane content is in the range of 60–65% [47], in which the experimental result matches the expected methane purity. Other than the adaptation period of microorganisms, the synergistic effect of biochips and TPOFA further enhances biogas production. The microorganism-rich biochips can utilize the substrate more efficiently, while TPOFA plays a role in maintaining optimal growth conditions by reducing the inhibitory factors (long-chain fatty acids, H2S, and ammonia) [48].
Table 5 presents a comparison of the performance of the anaerobic reactors investigated in this study with those reported in the literature. The biogas production of both reactors in this study may not be as impressive as expected; however, the methane content of biogas produced, especially from P + B (methane purity: 62.2%, methane yield: 893 mL CH4/g COD), demonstrates a strong qualitative improvement. This result is likely comparable to Bayonita et al. [49], who reported 65.1% methane purity from 5.5 L biogas/day of biogas yield from anaerobic reactor supplemented with support media and trace nickel (Ni). Although their system operated at a much larger scale (40 L) and under optimized continuous-mode conditions, the methane purity achieved in our lab-scale reactor aligns closely with their performance benchmark. This suggests that the integration of TPOFA and biochips in reactor P + B is capable of promoting a methane-rich biogas stream. The initial methane composition in the control reactor may be higher due to the direct anaerobic digestion process without adsorption or retention effects from biochips or TPOFA. The average biogas generated obtained in this study by P + B from week 15 to week 30 is 490–917 mL biogas/day, and reactor C produces a range of 780–1697 mL biogas/day (shown in Table 4), which both falls within the 0.23–0.4 L CH4/gCOD [50,51]. However, previous studies by Abera [52] indicated that biofilm-based anaerobic systems often outperform suspended systems in methane yield over extended operation periods. P + B has the higher cumulative methane yield once microbial colonization is fully established and optimized, as shown in Figure 7 [46].

3.6. Taxonomic Classification of the Microbial Communities of Reactors’ Sludge

Anaerobic digestion is a multi-step process that involves various microorganisms. This includes fermentative bacteria (acidogens), hydrogen-producing and acetate-forming bacteria (acetogens), and archaea that convert acetate or hydrogen to methane (methanogens). An imbalance at any stage can cause the entire system to collapse [54]. Figure 7 depicts the bacterial composition of the two sludge samples collected from reactor C and reactor P + B at phylum and class levels. Each sample has its different abundant bacterial phyla. Phyla-level bacteria groups Bacteroidota, Cloacimonadota, and Firmicutes are the most dominant in samples C and P + B, respectively.
In this study, phylum Halobacterota exhibits higher relative abundance in C (≈20%) compared to P + B (≈13%). For the archaea group like Halobacterota is the dominant phylum. Halobacterota are methane-producing archaea that transform substances like H2, CO2, methyl compounds (including formate, methanol, and methylamine), and VFAs into methane [55]. The high average methane yield production of reactor C correlates to the high Halobacterota population as methanogen in the sludge class. Despite this higher relative abundance of Halobacterota in reactor C, the methane fraction in the biogas is higher in reactor P + B (62.2% CH4) than in reactor C (53.51% CH4), while reactor C shows higher CO2 (23.34% vs. 21.93%) and N2 (19.39% vs. 12.83%). This suggests that methane percentage is controlled not only by the proportion of methanogens but also by substrate availability, syntrophic partners, and overall reactor performance. Relative abundance indicates the proportion of a particular class of phylum, whereas the final CH4 percentage in the gas phase is additionally influenced by conversion efficiency and gas dilution effects (e.g., N2), meaning that a community with slightly lower Halobacterota abundance (reactor P + B) can still achieve a higher methane concentration when conditions favor more complete methanogenesis [56,57].
Among them, Desulfobacterota has the most relative abundance, accounting ≈6% of bacteria in reactor P + B. Desulfobacterota plays a role in organic matter degradation and holds an abundance of electrochemically active class, which enables syntrophic interactions with methanogenic archaea [58]. Desulfobacterota is mainly composed of sulfate-reducing bacteria group, which are competitive with methanogens while simultaneously inhibiting methane production, as the growth rate increases with higher affinity for the substrates [59,60]. Li [61] reported that Desulfobacterota also involves methane consumption and sulfur cycling. This information may infer that Desulfobacterota may suppress the growth of methanogens. Biogas production of reactor P + B was much less than the reactor control since the beginning, and it slowly becomesbecom higher than the control by day 30, which may be due to higher abundance of Desulfobacterota. However, the high population of Chloroflexi plays a role in acetic acid formation [62]. Acetic acid can be an accelerant for methanogens in methane production [63].
Ibáñez-López [54] also showed the presence of phylum Halobacterota (Percentage abundance 17%) in the anaerobic sludge of the UASB reactor of winery wastewater. Similar results were also reported by Gao et al. [64], where Firmicutes, Bacteroidita, Chloroflexi, Halobacterota, and Euryarchaeota were found in AD of waste-activated sludge, with methane production of 167.4 ± 4.2 to 246.0 ± 6.2 mL/g. The genera Methanosarcinia in the phylum Halobacterota is the major methanogen in the samples by Yang [65], as shown in Figure 8 and Figure 9.
Methanosarcinia, which is capable of electron transfer between acetogens and methanogens, is able to increase the methane production rate [66]. Methanosarcinia is one of the methanogens that is capable of converting acetate to methane from three pathways—hydrgenotrophic, acetoclastic, and methylotrophic methogeneses. According to Tirapanampai [67], Methanosarcinia is expected to be abundant with elevated acetate levels, indicating acid build-up.
Bacteroidota and Firmicutes were the next anaerobic sludge’s most abundant phylum level. These bacteria are widely present in conventional wastewater treatment plants, playing a crucial part in the removal of organic matter and nutrients [68,69]. Bacteroidota was the most dominant bacteria phylum in sample P + B, followed by Firmicutes and Halobacterota. The best results were obtained in reactor P + B, in which the COD removal efficiency >95%, with high microbial biomass (MLVSS: 24,500 mg/L) and the highest biogas yield at 917 mL per day. Under these conditions, the anaerobic sludge was dominated by Bacteroidota, an anaerobic bacteria with the ability to convert organic substances to volatile fatty acids (VFAs) [70]. The same phylum and Methanosarcina were also found in mesophilic reactors by Patil [71] in treating swine wastewater. Acetate is a major source of methane production, but the capability to catabolize this substrate is limited to the class of Methanosaeta and Methanosarcina (also found in this study) [72]. It was also mentioned that Methanosarcinia has the capability to adapt to reduced substrate availability, possessing three methanotrophic pathways: acetic, hydrogen, methylotrophic [73].
Firmicutes, a common hydrolytic bacterium in the anaerobic system, has become the most dominant bacteria for control and has potential electron transfer capability in AD [58,70]. The higher abundance of Firmicutes in AD might be linked to their role in consuming volatile fatty acids (VFAs) or their adaptation to environments with high VFA concentrations and low pH [54]. These phyla also involve in the rate-limiting step of AD and are capable of VFA degradation and acetic acid production [74,75].

4. Conclusions

The microbial community analysis revealed that the anaerobic digestion (AD) process across different reactors was strongly influenced by the dominance and interaction of specific bacterial and archaeal groups, directly affecting organic degradation and methane production efficiency. Among the tested reactors, reactor P + B demonstrated the best overall performance with >95% COD removal efficiency, attributed to the synergistic role of Bacteroidota and Methanosarcinia. Bacteroidota, the dominant phylum in P + B, is known for its ability to degrade complex organic substances into volatile fatty acids (VFAs), which serve as essential intermediates for methane generation. The coexistence of Methanosarcinia—a versatile methanogen capable of acetoclastic, hydrogenotrophic, and methylotrophic pathways—further enhanced methane formation through efficient acetate utilization. In reactor P + B, phylum Bacteroidota exhibited syntrophic interaction with the methanogenic class Methanosarcinia in enhancing organic substrate degradation and elevated biogas yield.
Comparatively, reactors C exhibited higher relative abundance of phylum Halobacterota (≈20 and 55%, respectively), which correlated with moderate methane yields due to their role in converting CO2, H2, and methyl compounds into methane. However, in reactor P + B, Halobacterota abundance was lower (≈13%), suggesting that methane generation here was more dependent on acetate-driven pathways facilitated by class Methanosarcinia. The presence of phylum Desulfobacterota (~6%) in reactor P + B indicated active sulfate reduction and sulfur cycling, which may have initially suppressed methanogenesis, but its electrochemically active interactions later supported syntrophic stability and gradual methane increase. Additionally, supporting bacterial phyla groups such as Firmicutes, Cloacimonadota, and Syntrophomonas contributed to the conversion of VFAs, propionate, and butyrate into acetate, promoting efficient substrate turnover. Overall, the microbial dynamics in reactor P + B reflected a well-balanced community, where synergistic relationships between hydrolytic, fermentative, acetogenic, and methanogenic populations optimized organic degradation and methane conversion efficiency, leading to its superior reactor performance. Overall, integrating TPOFA and biochips significantly enhanced both biogas production and organic degradation in POME treatment. This demonstrates the innovative use of waste-derived TPOFA as a value-added material to improve wastewater treatment performance, highlighting both the environmental and social benefits of resource recovery and sustainable waste management.

5. Limitations and Future Perspectives

Anaerobic bioreactors are fully sealed to prevent any gas leakage or permit of air. Hence, the only passages are the valves; however, due to the small orifice, biochip samples are unable to be extracted for analysis. Extensive molecular studies are performed to understand the bacterial dominancy. The bioreactors are expected to be maintained even after the experiment. This research focuses on the effect of the addition of POFA and biochips in the performance of organic degradation of raw POME, biogas production, and microbial growth. Both bioreactors operated with similar operational conditions, and optimization studies were not performed in this study. The operation may not be the best condition for the particular manipulated variable. Future studies should incorporate surface area and pore structure analyses of POFA and TPOFA before and after POME treatment to quantify material changes during reactor operation. Including these parameters would strengthen optimization efforts by linking shifts in surface characteristics to treatment performance, extending beyond the current focus on biochip and TPOFA integration. Although this study focused on extending the application of POFA/TPOFA, as previously identified, as a promising material for wastewater treatment for POME degradation through its integration with biochips, future work could explore the co-fermentation of POFA-supported systems with additional organic substrates, such as palm olive pomace or other agro-industrial residues. Such co-utilization strategies may provide synergistic benefits, potentially enhancing microbial activity, improving organic degradation, and further increasing biogas productivity. Future studies could investigate the catalytic effects of metal ions, such as K and Ca, present in substrates like POFA instead of palm olive pomace in POME treatment to evaluate whether low concentrations of these metals could enhance microbial activity, improve organic degradation, and increase biogas production in POME treatment systems. In addition, future work could incorporate ANOVA, RSM, or other statistical modeling approaches to evaluate factor significance and optimize operational conditions, providing a more comprehensive understanding of how key parameters influence POME treatment performance.

Author Contributions

Conceptualization, L.P.W., M.J.K.B., and P.L.S.; Methodology, L.P.W., M.J.K.B., and P.L.S.; Validation, L.P.W., M.J.K.B., and P.L.S.; Formal Analysis, P.L.S.; Investigation, P.L.S.; Resources, L.P.W. and M.J.K.B.; Data Curation, L.P.W., M.J.K.B., and P.L.S.; Writing—Original Draft Preparation, P.L.S.; Writing—Review and Editing, L.P.W., M.J.K.B., X.G., and Y.W.; Visualization, P.L.S.; Supervision, L.P.W. and M.J.K.B.; Project Administration, L.P.W. and M.J.K.B.; Funding Acquisition, M.J.K.B. and L.P.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Universiti Tunku Abdul Rahman, Malaysia, Project Number: IPSR/RMC/UTARRF/2021-C1/M02.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ADAnerobic digestion
BODBiochemical oxygen demand
CO2Carbon dioxide
CODChemical oxygen demand
F/MFeed-to-microorganism
H2Hydrogen
N2Nitrogen
NiNickel
KOHPotassium hydroxide
POFAPalm oil fuel ash
POMEPalm oil mill effluent
TPOFATreated palm oil fuel ash
TSTotal solids
TSSTotal suspended solids
TVSTotal volatile solids
UASBUp-flow anaerobic sludge bioreactor
VFAVolatile fatty acids

References

  1. Bashir, M.J.K.; Hong, L.J.; Amr, S.S.A.; Wong, L.P.; Sim, Y.L. Post-treatment of palm oil mill effluent using electro-coagulation-peroxidation (ECP) technique. J. Cleaner Prod. 2019, 208, 716–727. [Google Scholar] [CrossRef]
  2. Kusnadi, K.; Wahid, M.A.; Yasin, M.F.M.; Setiawan, A.; Anwar Fudholi, A. Environmental and socioeconomic impacts of palm oil mill effluent (POME) management in Indonesian oil palm plantations: Challenges and opportunities. IOP Conf. Ser. Earth Environ. Sci. 2025, 1534, 012002. [Google Scholar] [CrossRef]
  3. Tabassum, S.; Zhang, Y.; Zhang, Z. An integrated method for palm oil mill effluent (POME) treatment for achieving zero liquid discharge—A pilot study. J. Clean. Prod. 2015, 95, 148–155. [Google Scholar] [CrossRef]
  4. Yahaya, S.M.; Lau, S. Palm oil mill effluent (POME) from Malaysia palm oil mills: Waste or resource. Int. J. Sci. Environ. Technol. 2013, 2, 1138–1155. [Google Scholar]
  5. Bodkhe, S. Development of an improved anaerobic filter for municipal wastewater treatment. Bioresour. Technol. 2008, 99, 222–226. [Google Scholar] [CrossRef] [PubMed]
  6. Habeeb, S.A.; Latiff, A.B.A.A.; Daud, Z.; Ahmad, Z. A biodegradation and treatment of palm oil mill effluent (POME) using a hybrid up-flow anaerobic sludge bed (HUASB) reactor. Int. J. Energy Environ. 2011, 2, 653–660. [Google Scholar]
  7. Nelson, M.J.; Nakhla, G.; Zhu, J. Fluidized-Bed Bioreactor Applications for Biological Wastewater Treatment: A Review of Research and Developments. Engineering 2017, 3, 330–342. [Google Scholar] [CrossRef]
  8. Al-Amshawee, S.K.; Yunus, M.Y.; Azoddein, A.A. A Novel Microbial Biofilm Carrier for Wastewater Remediation. IOP Conf. Ser. Mater. Sci. Eng. 2020, 736, 072006. [Google Scholar] [CrossRef]
  9. Khadaroo, S.N.B.A.; Grassia, P.; Gouwanda, D.; He, J.; Poh, P.E. Enhancing the biogas production and the treated effluent quality via an alternative Palm Oil Mill Effluent (POME) treatment process: Integration of thermal pretreatment and dewatering. Biomass Bioenergy 2021, 151, 106167. [Google Scholar] [CrossRef]
  10. Lok, X.; Chan, Y.J.; Foo, D.C.Y. Simulation and optimisation of full-scale palm oil mill effluent (POME) treatment plant with biogas production. J. Water Process Eng. 2020, 38, 101558. [Google Scholar] [CrossRef]
  11. Kacaribu, A.A.; Aisyah, Y.; Febriani, F.; Darwin, D. Development of wastewater treatment methods for palm oil mill effluent (POME): A comprehensive review. Resour. Chem. Mater. 2025, 4, 100130. [Google Scholar] [CrossRef]
  12. Wu, T.Y.; Mohammad, A.W.; Jahim, J.M.; Anuar, N. Pollution control technologies for the treatment of palm oil mill effluent (POME) through end-of-pipe processes. J. Environ. Manag. 2010, 91, 1467–1490. [Google Scholar] [CrossRef] [PubMed]
  13. Zahrim, A.Y.; Nasimah, A.; Hilal, N. Pollutants analysis during conventional palm oil mill effluent (POME) ponding system and decolourisation of anaerobically treated POME via calcium lactate-polyacrylamide. J. Water Process Eng. 2014, 4, 159–165. [Google Scholar] [CrossRef]
  14. Ng, K.H. Adoption of TiO2-photocatalysis for palm oil mill effluent (POME) treatment: Strengths, weaknesses, opportunities, threats (SWOT) and its practicality against traditional treatment in Malaysia. Chemosphere 2021, 270, 129378. [Google Scholar] [CrossRef]
  15. Oyehan, T.A.; Salami, B.A.; Badmus, S.O.; Saleh, T.A. Technological trends in activation and modification of palm oil fuel ash for advanced water and wastewater treatment—A review. Sustain. Chem. Pharm. 2022, 29, 100754. [Google Scholar] [CrossRef]
  16. Manikam, M.K.; Abdul Halim, A.; Hanafiah, M.M.; Krishnamoorthy, R.R. Removal of ammonia nitrogen, nitrate, phosphorus and COD from sewage wastewater using palm oil boiler ash composite adsorbent. Desalination Water Treat. 2019, 149, 23–30. [Google Scholar] [CrossRef]
  17. Mohammad, S.; Baidurah, S.; Kobayashi, T.; Ismail, N.; Leh, C.P. Palm Oil Mill Effluent Treatment Processes—A Review. Processes 2021, 9, 739. [Google Scholar] [CrossRef]
  18. Soo, P.L.; Bashir, M.J.K.; Wong, L.P. Recent advancements in the treatment of palm oil mill effluent (POME) using anaerobic biofilm reactors: Challenges and future perspectives. J. Environ. Manag. 2022, 320, 115750. [Google Scholar] [CrossRef]
  19. Mazioti, A.A.; Koutsokeras, L.E.; Constantinides, G.; Vyrides, I. Untapped Potential of Moving Bed Biofilm Reactors with Different Biocarrier Types for Bilge Water Treatment: A Laboratory-Scale Study. Water 2021, 13, 1810. [Google Scholar] [CrossRef]
  20. Bobade, G.; Joshi, R.A. Progression of various MBBR media in Wastewater Treatment Process. Int. Res. J. Eng. Technol. 2023, 10, 252–257. [Google Scholar]
  21. Allegue, T.; Carballo-Costa, M.N.; Fernandez-Gonzalez, N.; Garrido, J.M. Simultaneous nitrogen and dissolved methane removal from an upflow anaerobic sludge blanket reactor effluent using an integrated fixed-film activated sludge system. J. Environ. Manag. 2020, 263, 110395. [Google Scholar] [CrossRef] [PubMed]
  22. Dan, N.H.; Le Luu, T. High organic removal of landfill leachate using a continuous flow sequencing batch biofilm reactor (CF-SBBR) with different biocarriers. Sci. Total Environ. 2021, 787, 147680. [Google Scholar] [CrossRef] [PubMed]
  23. Chan, W.L.; Bashir, M.J.K.; Wong, L.Y.; Lim, J.W.; Sethupathi, S.; Ng, C.A. Ancillary palm oil fuel ash (POFA) in sequencing batch reactor for enhancing recalcitrant pollutants removal from domestic wastewater. Chemosphere 2021, 265, 129050. [Google Scholar] [CrossRef] [PubMed]
  24. Abuabdou, S.M.A.; Teng, O.W.; Bashir, M.J.K.; Aun, N.C.; Sethupathi, S. Adsorptive treatment of stabilized landfill leachate using activated palm oil fuel ash (POFA). In Proceedings of the International Symposium on Green and Sustainable Technology; Ipoh, Malaysia, 23–26 April 2019, AIP Publishing: Melville, NY, USA, 2019; p. 020002. [Google Scholar] [CrossRef]
  25. Yee, T.; Rathnayake, T.; Visvanathan, C. Performance Evaluation of a Thermophilic Anaerobic Membrane Bioreactor for Palm Oil Wastewater Treatment. Membranes 2019, 9, 55. [Google Scholar] [CrossRef]
  26. Ng, C.A.; Wong, L.Y.; Chai, H.Y.; Bashir, M.J.K.; Ho, C.-D.; Nisar, H.; Lo, P.K. Investigation on the Performance of Hybrid Anaerobic Membrane Bioreactors for Fouling Control and Biogas Production in Palm Oil Mill Effluent Treatment. Water Sci. Technol. 2017, 76, 1389–1398. [Google Scholar] [CrossRef]
  27. Muenmee, S.; Theepharaksapan, S.; Boonnorat, J. Effect of Food to Microbe (F/M) Ratio on Anaerobic Digestion of Refinery Waste Sludge under Mesophilic Conditions: Biogas Potential and Phytotoxicity. Curr. Appl. Sci. Technol. 2021, 22, 1–11. [Google Scholar] [CrossRef]
  28. Abuabdou, S.M.; Wei Teng, O.; Bashir, M.J.K.; Choon Aun, N.; Sethupathi, S.; Pratt, L.M. Treatment of tropical stabilised landfill leachate using palm oil fuel ash: Isothermal and kinetic studies. Desalination Water Treat. 2019, 144, 201–210. [Google Scholar] [CrossRef]
  29. Bassin, J.P.; Dias, I.N.; Cao, S.M.S.; Senra, E.; Laranjeira, Y.; Dezotti, M. Effect of increasing organic loading rates on the performance of moving-bed biofilm reactors filled with different support media: Assessing the activity of suspended and attached biomass fractions. Process Saf. Environ. Prot. 2016, 100, 131–141. [Google Scholar] [CrossRef]
  30. Chang, P.Y.; Chan, Y.J.; Arumugasamy, S.K.; Wan, Y.K.; Lim, J.W. Optimisation of anaerobic digestion of palm oil mill effluent with biochar addition: Synergistic application of Artificial neural network and response Surface Methodology. Fuel 2025, 398, 135514. [Google Scholar] [CrossRef]
  31. Environmental Quality Act. Environmental Quality (Prescribed Premises) (Crude Palm Oil) (Amendment) Regulations 1982; Environmental Quality Act: Kuala Lumpur, Malaysia, 1982. [Google Scholar]
  32. Environmental Quality Act. Environmental Quality (Industrial Effluent) Regulations. Malaysia Industrial Effluent Discharge Standards. In Environmental Quality Act 1974; Environmental Quality Act: Kuala Lumpur, Malaysia, 2009. [Google Scholar]
  33. Li, Q.; Xu, H.; Li, F.; Li, P.; Shen, L.; Zhai, J. Synthesis of geopolymer composites from blends of CFBC fly and bottom ashes. Fuel 2012, 97, 366–372. [Google Scholar] [CrossRef]
  34. Abdeldjouad, L.; Dheyab, W.; Gamil, Y.; Asadi, A.; Shukla, S.K. Thermal curing effects on alkali-activated treated soils with palm oil fuel ash. Case Stud. Constr. Mater. 2023, 19, e02455. [Google Scholar] [CrossRef]
  35. Salami, B.A.; Megat Johari, M.A.; Ahmad, Z.A.; Maslehuddin, M. Impact of added water and superplasticizer on early compressive strength of selected mixtures of palm oil fuel ash-based engineered geopolymer composites. Constr. Build. Mater. 2016, 109, 198–206. [Google Scholar] [CrossRef]
  36. Zulkifli, M.; Abu Hasan, H.; Sheikh Abdullah, S.R.; Muhamad, M.H. A review of ammonia removal using a biofilm-based reactor and its challenges. J. Environ. Manag. 2022, 315, 115162. [Google Scholar] [CrossRef] [PubMed]
  37. Abuabdou, S.M.A.; Bashir, M.J.K.; Aun, N.C.; Sethupathi, S.; Yong, W.L. Development of a novel polyvinylidene fluoride membrane integrated with palm oil fuel ash for stabilized landfill leachate treatment. J. Clean Prod. 2021, 311, 127677. [Google Scholar] [CrossRef]
  38. Heng, G.C.; Isa, M.H.; Lock, S.S.M.; Ng, C.A. Process Optimization of Waste Activated Sludge in Anaerobic Digestion and Biogas Production by Electrochemical Pre-Treatment Using Ruthenium Oxide Coated Titanium Electrodes. Sustainability 2021, 13, 4874. [Google Scholar] [CrossRef]
  39. El Sayed, A.; Ismail, A.; Rabii, A.; Hamze, A.; Hamza, R.A.; Elbeshbishy, E. Biochemical Methane Production Potential of Different Industrial Wastes: The Impact of the Food-to-Microorganism (F/M) Ratio. Processes 2025, 13, 802. [Google Scholar] [CrossRef]
  40. Dehghani, M.; Rezaei, M.; Shamsedini, N.; Maleknia, H.; Javaheri, M.R. The Effects of Temperature on the Performance of Anaerobic Sequencing Batch Reactor in the Treatment of Synthetic Dairy Wastewater. Jundishapur J. Health Sci. 2014, 6, 23455. [Google Scholar] [CrossRef]
  41. Chan, Y.J.; Chong, M.F.; Law, C.L. Biological treatment of anaerobically digested palm oil mill effluent (POME) using a Lab-Scale Sequencing Batch Reactor (SBR). J. Environ. Manag. 2010, 91, 1738–1746. [Google Scholar] [CrossRef]
  42. Lewandowski, Z.; Beyenal, H.; Myers, J.; Stookey, D. The effect of detachment on biofilm structure and activity: The oscillating pattern of biofilm accumulation. Water Sci. Technol. 2007, 55, 429–436. [Google Scholar] [CrossRef]
  43. Arabgol, R.; Vanrolleghem, P.A.; Piculell, M.; Delatolla, R. The impact of biofilm thickness-restraint and carrier type on attached growth system performance, solids characteristics and settleability. Environ. Sci. 2020, 6, 2843–2855. [Google Scholar] [CrossRef]
  44. Soleimaninanadegani, M.; Manshad, S. Enhancement of Biodegradation of Palm Oil Mill Effluents by Local Isolated Microorganisms. Int. Sch. Res. Not. 2014, 2014, 727049. [Google Scholar] [CrossRef] [PubMed]
  45. Hossain, M.S.; Karim, T.U.; Onik, M.H.; Kumar, D.; Rahman, M.A.; Yousuf, A.; Uddin, M.R. Impact of temperature, inoculum flow pattern, inoculum type, and their ratio on dry anaerobic digestion for biogas production. Sci. Rep. 2022, 12, 6162. [Google Scholar] [CrossRef] [PubMed]
  46. Lokman, N.A.; Ithnin, A.M.; Yahya, W.J.; Yuzir, M.A. A brief review on biochemical oxygen demand (BOD) treatment methods for palm oil mill effluents (POME). Environ. Technol. Innov. 2021, 21, 101258. [Google Scholar] [CrossRef]
  47. Isa, M.H.; Bashir, M.J.K.; Wong, L.P. Anaerobic treatment of ultrasound pretreated palm oil mill effluent (POME): Microbial diversity and enhancement of biogas production. Environ. Sci. Pollut. Res. 2022, 29, 44779–44793. [Google Scholar] [CrossRef]
  48. Loganath, R.; Senophiyah-Mary, J. Critical review on the necessity of bioelectricity generation from slaughterhouse industry waste and wastewater using different anaerobic digestion reactors. Renew. Sustain. Energy Rev. 2020, 134, 110360. [Google Scholar] [CrossRef]
  49. Bayonita, S.; Purnomo, C.W.; Cahyono, R.B. Performance of anaerobic reactor with media support and Ni addition for palm oil mill effluent treatment. In Proceedings of the 11th Regional Conference on Chemical Engineering, Yogyakarta, Indonesia, 7–8 November 2018; AIP Publishing: Melville, NY, USA, 2019; p. 020024. [Google Scholar]
  50. Sarwani, M.K.I.; Fawzi, M.; Osman, S.A.; Nasrin, A.B. Bio-methane from palm oil mill effluent (POME): Transportation fuel potential in Malaysia. J. Adv. Res. Fluid Mech. Therm. Sci. 2019, 63, 1–11. [Google Scholar]
  51. Chen, J.W.; Chan, Y.J.; Arumugasamy, S.K.; Yazdi, S.K. Process modelling and optimisation of methane yield from palm oil mill effluent using response surface methodology and artificial neural network. J. Water Process Eng. 2023, 52, 103493. [Google Scholar] [CrossRef]
  52. Abera, G.B.; Trømborg, E.; Solli, L.; Walter, J.M.; Wahid, R.; Govasmark, E.; Horn, S.J.; Aryal, N.; Feng, L. Biofilm application for anaerobic digestion: A systematic review and an industrial scale case. Biotechnol. Biofuels Bioprod. 2024, 17, 145. [Google Scholar] [CrossRef]
  53. Wadchasit, P.; Suksong, W.; O-Thong, S.; Nuithitikul, K. Development of a novel reactor for simultaneous production of biogas from oil-palm empty fruit bunches (EFB) and palm oil mill effluents (POME). J. Environ. Chem. Eng. 2021, 9, 105209. [Google Scholar] [CrossRef]
  54. Ibáñez-López, M.E.; Frison, N.; Bolzonella, D.; García-Morales, J.L. Enhancing Anaerobic Digestion with an UASB Reactor of the Winery Wastewater for Producing Volatile Fatty Acid Effluent Enriched in Caproic Acid. Fermentation 2023, 9, 958. [Google Scholar] [CrossRef]
  55. Zhang, C.; Yang, R.; Sun, M.; Zhang, S.; He, M.; Tsang, D.C.W.; Luo, G. Wood waste biochar promoted anaerobic digestion of food waste: Focusing on the characteristics of biochar and microbial community analysis. Biochar 2022, 4, 62. [Google Scholar] [CrossRef]
  56. Walag, A.M.P.; Canencia, M.O.P. Physico-chemical parameters and macrobenthic invertebrates of the intertidal zone of Gusa, Cagayan de Oro City, Philippines. Adv. Environ. Sci. Int. J. Bioflux Soc. 2016, 8, 71–82. [Google Scholar]
  57. Wu, B.; Yang, Q.; Yao, F.; Chen, S.; He, L.; Hou, K.; Pi, Z.; Yin, H.; Fu, J.; Wang, D.; et al. Evaluating the effect of biochar on mesophilic anaerobic digestion of waste activated sludge and microbial diversity. Bioresour. Technol. 2019, 294, 122235. [Google Scholar] [CrossRef] [PubMed]
  58. Wang, X.; Zhang, M.; Zhou, Z.; Qu, T.; Ran, J.; Zhang, J.; Li, X.; Zhang, L.; Zhang, A. Effect of extracellular polymeric substances removal and re-addition on anaerobic digestion of waste activated sludge. J. Water Process Eng. 2024, 57, 104702. [Google Scholar] [CrossRef]
  59. Zan, F.; Tang, W.; Jiang, F.; Chen, G. Diversion of food waste into the sulfate-laden sewer: Interaction and electron flow of sulfidogenesis and methanogenesis. Water Res. 2021, 202, 117437. [Google Scholar] [CrossRef]
  60. Yang, S.; Xue, W.; Liu, P.; Lu, X.; Wu, X.; Sun, L.; Zan, F. Revealing the methanogenic pathways for anaerobic digestion of key components in food waste: Performance, microbial community, and implications. Bioresour. Technol. 2022, 347, 126340. [Google Scholar] [CrossRef]
  61. Li, J.; Chen, Q.; Wang, X.; Tan, Y.; Li, L.; Zhang, B.; Guo, B.; Zhao, C. Methane Anaerobic Oxidation Potential and Microbial Community Response to Sulfate Input in Coastal Wetlands of the Yellow River Delta. Sustainability 2023, 15, 7053. [Google Scholar] [CrossRef]
  62. Lv, L.; Feng, C.; Li, W.; Ren, Z.; Wang, P.; Liu, X.; Gao, W.; Sun, L.; Zhang, G. Accelerated performance recovery of anaerobic granular sludge after temperature shock: Rapid construction of protective barriers (EPS) to optimize microbial community composition base on quorum sensing. J. Clean Prod. 2023, 392, 136243. [Google Scholar] [CrossRef]
  63. Haosagul, S.; Vikromvarasiri, N.; Sawasdee, V.; Pisutpaisal, N. Impact of acetic acid in methane production from glycerol/acetic acid co-fermentation. Int. J. Hydrogen Energy 2019, 44, 29568–29574. [Google Scholar] [CrossRef]
  64. Gao, J.; Li, Z.; Chen, H. Untangling the effect of solids content on thermal-alkali pre-treatment and anaerobic digestion of sludge. Sci. Total Environ. 2023, 855, 158720. [Google Scholar] [CrossRef]
  65. Yang, Y.; Yu, K.; Xia, Y.; Lau, F.T.K.; Tang, D.T.W.; Fung, W.C.; Fang, H.H.P.; Zhang, T. Metagenomic analysis of sludge from full-scale anaerobic digesters operated in municipal wastewater treatment plants. Appl. Microbiol. Biotechnol. 2014, 98, 5709–5718. [Google Scholar] [CrossRef] [PubMed]
  66. Shi, Z.; Campanaro, S.; Usman, M.; Treu, L.; Basile, A.; Angelidaki, I.; Zhang, S.; Luo, G. Genome-Centric Metatranscriptomics Analysis Reveals the Role of Hydrochar in Anaerobic Digestion of Waste Activated Sludge. Environ. Sci. Technol. 2021, 55, 8351–8361. [Google Scholar] [CrossRef] [PubMed]
  67. Tirapanampai, C.; Toewiwat, N.; Weeranoppanant, N.; Chaiyen, P.; Wongnate, T. Processing of palm oil mill effluent (POME) into food waste digesting microbes: An investigation of acclimatization strategies. Sustain. Energy Technol. Assess. 2022, 52, 102287. [Google Scholar] [CrossRef]
  68. Chuda, A.; Otlewska, A.; Ziemiński, K. Insights into the microbial community structure in the biodegradation process of high-strength ammonia digestate liquid fraction in conventional activated sludge system. Bioresources 2023, 18, 3540–3559. [Google Scholar] [CrossRef]
  69. Ríos-Castro, R.; Cabo, A.; Teira, E.; Cameselle, C.; Gouveia, S.; Payo, P.; Novoa, B.; Figueras, A. High-throughput sequencing as a tool for monitoring prokaryote communities in a wastewater treatment plant. Sci. Total Environ. 2023, 861, 160531. [Google Scholar] [CrossRef]
  70. Du, X.; Zhang, Y.; Ma, Y.; Feng, S.; Zhang, Y.; Kou, H.; Sun, Y. The synergistic effect of chemical oxidation and microbial activity on improving volatile fatty acids (VFAs) production during the animal wastewater anaerobic digestion process treated with persulfate/biochar. Sci. Total Environ. 2023, 857, 159276. [Google Scholar] [CrossRef]
  71. Patil, S.S.; Kumar, M.S.; Ball, A.S. Microbial community dynamics in anaerobic bioreactors and algal tanks treating piggery wastewater. Appl. Microbiol. Biotechnol. 2010, 87, 353–363. [Google Scholar] [CrossRef]
  72. Sworkin, M. The Prokaryotes; Falkow, S., Rosenberg, E., Schleifer, K.H., Stackebrandt, E., Eds.; Springer: New York, NY, USA, 2006; Volume 3, pp. 165–207. [Google Scholar]
  73. Yang, Y.; Wang, M.; Yan, S.; Yong, X.; Zhang, X.; Awasthi, M.K.; Xi, Y.; Zhou, J. Effects of hydrochar and biogas slurry reflux on methane production by mixed anaerobic digestion of cow manure and corn straw. Chemosphere 2023, 310, 136876. [Google Scholar] [CrossRef]
  74. Singh, S.P.; Prerna, P. Review of recent advances in anaerobic packed-bed biogas reactors. Renew. Sustain. Energy Rev. 2009, 13, 1569–1575. [Google Scholar] [CrossRef]
  75. Sun, M.T.; Fan, X.L.; Zhao, X.X.; Fu, S.F.; He, S.; Manasa, M.R.K.; Guo, R.B. Effects of organic loading rate on biogas production from macroalgae: Performance and microbial community structure. Bioresour. Technol. 2017, 235, 292–300. [Google Scholar] [CrossRef]
Environments 13 00022 g001
Figure 1. Image of biochip used in this study.
Figure 1. Image of biochip used in this study.
Environments 13 00022 g001
Environments 13 00022 g002
Figure 2. Schematic diagram of the anaerobic treatment system of POME.
Figure 2. Schematic diagram of the anaerobic treatment system of POME.
Environments 13 00022 g002
Environments 13 00022 g003
Figure 3. FTIR spectra of TPOFA and POFA materials.
Figure 3. FTIR spectra of TPOFA and POFA materials.
Environments 13 00022 g003
Environments 13 00022 g004
Figure 4. SEM images of (a) POFA and (b) TPOFA at 1000× magnification.
Figure 4. SEM images of (a) POFA and (b) TPOFA at 1000× magnification.
Environments 13 00022 g004
Environments 13 00022 g005
Figure 5. COD removal efficiency of both reactors.
Figure 5. COD removal efficiency of both reactors.
Environments 13 00022 g005
Environments 13 00022 g006
Figure 6. MLVSS growth profile for both reactors.
Figure 6. MLVSS growth profile for both reactors.
Environments 13 00022 g006
Environments 13 00022 g007
Figure 7. Biogas yield of both reactors.
Figure 7. Biogas yield of both reactors.
Environments 13 00022 g007
Environments 13 00022 g008
Figure 8. Bar chart analysis depicting the relative abundance and distribution of the sample assigned to phylum taxonomic rank.
Figure 8. Bar chart analysis depicting the relative abundance and distribution of the sample assigned to phylum taxonomic rank.
Environments 13 00022 g008
Environments 13 00022 g009
Figure 9. Bar chart analysis depicting the relative abundance and distribution of the sample assigned to class taxonomic rank.
Figure 9. Bar chart analysis depicting the relative abundance and distribution of the sample assigned to class taxonomic rank.
Environments 13 00022 g009
Table 1. Specifications of biochip used in this study [20].
Table 1. Specifications of biochip used in this study [20].
SpecificationDescription
ShapeRound
Diameter30 mm
Thickness≈1.1 mm
MaterialPolyethylene (PE)
Specific surface areaUp to 5500 m2/m3
Pore systemVary due to raw material
BrandMulti Umwelttechnologie AG (Aue, Saxony, Germany)
Table 2. Characteristics of collected untreated POME as compared to the environmental standard discharge limit.
Table 2. Characteristics of collected untreated POME as compared to the environmental standard discharge limit.
ParametersValueUnitStandard Discharge Limit [31]
AppearanceBrown--
Temperature43.0 ± 3.2-°C45
pH4.4 ± 0.2-5.0–9.0
BOD35,770 ± 8108mg/L100
COD54,150 ± 24,416mg/L200 *
O&G3048 ± 2618mg/L50
TS32,694 ± 5930mg/L-
TVS14,944 ± 6924mg/L400
* Obtained from Standard B of the Environmental Quality Act (Industrial Effluent) Regulations 2009 [32].
Table 3. Percentage composition of different components in biogas samples of both bioreactors.
Table 3. Percentage composition of different components in biogas samples of both bioreactors.
CompositionPercentage (%)
CP + B
CO223.3421.93
O23.763.04
N219.3912.83
CH453.5162.20
Table 4. Average biogas generated by each anaerobic bioreactor in Week 1, 15, and 30, respectively.
Table 4. Average biogas generated by each anaerobic bioreactor in Week 1, 15, and 30, respectively.
WeekAverage Biogas Production (mL Biogas/Day)
CP + B
116670
151697490
30780917
Table 5. Comparative performance of anaerobic reactors in this study and in the literature.
Table 5. Comparative performance of anaerobic reactors in this study and in the literature.
SourceHRT
(Days)		COD Removal
Efficiency (%)		Methane Yield (mLCH4/g COD)Methane
Content (%)
This study18C: 81
P + B: 96		C: 776
P + B: 893		C; 53
P + B: 62
Bayonita et al. [49]2088-65
Wadchasit et al. [53]15–2581–8929364–71
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Soo, P.L.; Wong, L.P.; Bashir, M.J.K.; Guo, X.; Wei, Y. Palm Oil Fuel Ash-Enhanced Biofilm Reactor: Performance and Microbial Dynamics in POME Treatment. Environments 2026, 13, 22. https://doi.org/10.3390/environments13010022

AMA Style

Soo PL, Wong LP, Bashir MJK, Guo X, Wei Y. Palm Oil Fuel Ash-Enhanced Biofilm Reactor: Performance and Microbial Dynamics in POME Treatment. Environments. 2026; 13(1):22. https://doi.org/10.3390/environments13010022

Chicago/Turabian Style

Soo, Pei Ling, Lai Peng Wong, Mohammed J. K. Bashir, Xinxin Guo, and Yuansong Wei. 2026. "Palm Oil Fuel Ash-Enhanced Biofilm Reactor: Performance and Microbial Dynamics in POME Treatment" Environments 13, no. 1: 22. https://doi.org/10.3390/environments13010022

APA Style

Soo, P. L., Wong, L. P., Bashir, M. J. K., Guo, X., & Wei, Y. (2026). Palm Oil Fuel Ash-Enhanced Biofilm Reactor: Performance and Microbial Dynamics in POME Treatment. Environments, 13(1), 22. https://doi.org/10.3390/environments13010022

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Article metric data becomes available approximately 24 hours after publication online.
Back to TopTop