The palm oil industry supplies more than 35.0% of global vegetable oil production^[1]. Malaysia is the world’s second-largest palm oil producer after Indonesia, with approximately 5.61 million hectares of oil palm plantations and production of 19.34 million tonnes of palm oil in 2024^[2]. Palm oil processing generates solid and liquid by-products that can cause substantial environmental impacts when they are not managed appropriately^[3]. Palm oil mill effluent (POME) has high biochemical oxygen demand and chemical oxygen demand, but its organic content also provides a feedstock for renewable energy and value-added chemical production. Advanced anaerobic digestion converts POME into biogas and can provide economic and environmental benefits^[4]. Solid residues, including empty fruit bunches and palm kernel shells, can also be converted into biochar and activated carbon. Palm oil mill biomass comprises empty fruit bunches, palm mesocarp fibers, and palm kernel shells, whereas POME is the principal liquid residue. Processing one tonne of fresh fruit bunches generates approximately 22.0%-25.0% empty fruit bunches, 13.0%-16.0% mesocarp fibers, and 5.0%-7.0% palm kernel shells^[3]. Empty fruit bunches have attracted attention as feedstocks for renewable energy and other applications. Mesocarp fibers and palm kernel shells have high calorific values and are commonly used in mill boilers, contributing to mill energy self-sufficiency. POME remains a major liquid-waste management challenge. Palm oil milling generates POME at approximately 0.7-0.9 m³ per tonne of fresh fruit bunches processed^[5]. This review examines advances in technologies that convert palm oil processing residues into renewable energy or valuable products and examines the economic and technical constraints on scale-up.

POME is a brown, viscous wastewater generated mainly during fruit sterilization and crude palm oil clarification^[4]. Its organic and inorganic constituents include carotene, lignin, phenolic compounds, tannins, carbohydrates, organic acids, and minerals, which require appropriate treatment. Methane released during uncontrolled anaerobic degradation contributes to greenhouse-gas emissions^[6]. Untreated oily POME also contains high biochemical oxygen demand and chemical oxygen demand and can acidify and eutrophicate receiving waters^[3]. POME treatment methods include biological, chemical, physical, and physicochemical processes^[4], such as coagulation and flocculation^[7], anaerobic and aerobic digestion^[8], integrated sequential treatment^[9], photocatalysis^[10], and biofilm reactors^[11]. Anaerobic digestion can simultaneously reduce the organic load and recover biogas. POME can also serve as a substrate for biological production of value-added chemicals because it contains phenolic compounds, organic acids, residual oils, and other biodegradable organics. Bacterial systems can degrade phenolic compounds^[12], whereas microbial fermentation converts POME into succinic acids^[13,14].

The current practices, limitations, and prospects for POME treatment and resource recovery are discussed. A Scopus search identified publications up to September 2026 on resource recovery from POME using the terms “POME” AND “removal” AND “recovery” in titles, abstracts, and keywords. The search retrieved 68 documents, comprising 56 research articles, 4 review papers, and 8 conference papers. The records were screened for POME treatment, pollutant removal and resource recovery. The review uses POME characteristics and regulatory requirements to evaluate ponding systems, anaerobic digestion, and bioreactors and to compare their treatment performance and limitations. It also examines scalability, waste-management costs, and nutrient-recovery potential. Adsorption, advanced oxidation processes, and integrated treatment systems are assessed as emerging approaches. This combined technological, environmental, regulatory, and economic analysis provides a structured assessment of POME management and recovery options.

Figure 1 summarizes the stages used to process fresh fruit bunches into crude palm oil. Fresh fruit bunches arrive at the loading bay and are sterilized with steam to loosen the fruits. Sterilizer condensate contains organic material leached from the fruits and is a major source of POME. Threshers then separate the fruits from the bunches, and digestion breaks down the fruit before oil extraction. The digested fruit is fed to a press, which produces crude palm oil. Clarification subsequently separates the oil from water and solids. Wastewater from the clarifier and decanter forms part of the POME stream. Aerobic and anaerobic ponds are commonly used to biodegrade organic matter and reduce the pollutant load before discharge^[6].

Steam generated in boilers fired with fibers and shells is used for fruit sterilization and oil extraction. Solid sludge from the milling stages is collected for disposal or further processing as a potential fertilizer or soil conditioner. POME accounts for approximately 60.0% of the waste generated during fresh-fruit-bunch processing^[15]. Effective management is necessary because POME has high biochemical oxygen demand and chemical oxygen demand and can release methane when untreated. Biogas-recovery systems convert this liability into an energy resource^[6]. Palm oil milling therefore combines mechanical, thermal, and biological operations, and integrated POME management is necessary for environmental compliance and sustainable production. POME was previously regarded primarily as an environmental problem but is increasingly recognized as a renewable-resource feedstock because of its high organic content. Anaerobic digestion can reduce its biochemical oxygen demand and chemical oxygen demand while producing methane and biohydrogen^[16]. Improved anaerobic treatment systems can recover methane and support renewable-energy production. Treating POME as a resource rather than solely as a pollutant supports more sustainable palm oil production.

Raw POME is a dense, brown, viscous wastewater with an unpleasant odor and a high concentration of colloidal solids, making it a pollutant that can threaten water resources^[17,18]. Water constitutes most of the stream, while soil particles, suspended solids, and residual oil account for less than 5.0%^[19,20]. The physicochemical characteristics of raw POME demonstrate that it is a highly concentrated and heterogeneous wastewater requiring a combination of treatment mechanisms rather than a single-stage process, as shown in Table 1.

Raw POME is a highly concentrated and heterogeneous wastewater by its high total dissolved solids, total solids, suspended solids, turbidity, and color intensity. Its COD concentration of 50,000-78,290 mg/L indicates an exceptionally high organic loading that could rapidly deplete dissolved oxygen if the effluent were discharged without adequate treatment^[8,22]. The acidic pH of 4-5 is also below the Malaysian permissible discharge range of 5.0-9.0, reflecting the presence of residual free fatty acids and other acidic products generated during palm-oil processing^[22,24]. Although the ammoniacal-nitrogen concentration of 87 mg/L is below the stipulated limit of 150 mg/L, the total nitrogen concentration of 1,200 ± 200 mg/L exceeds the corresponding limit of 200 mg/L by approximately 5-7 times^[25]. This substantial difference indicates that nitrogen is predominantly present as organic nitrogen or other non-ammoniacal forms, which may subsequently be mineralized to ammonium during biological degradation. Moreover, the suspended-solids concentration exceeds the discharge limit of 400 mg/L by approximately 45-118 times, demonstrating that effective solids separation is essential before final discharge^[22,24]. The absence of specific statutory limits for COD, total solids, total dissolved solids, turbidity, and color under the cited crude-palm-oil regulations should not be interpreted as evidence that these parameters are environmentally insignificant, because they directly influence oxygen depletion, light penetration, sludge accumulation, and the aesthetic quality of receiving waters. Considering the high solids and organic content, preliminary screening, oil recovery, sedimentation, decanting, or dissolved-air flotation should be employed first to remove coarse solids, residual oil, and settleable material. High-rate anaerobic digestion is subsequently appropriate for reducing the biodegradable organic load while recovering methane-rich biogas; however, anaerobic treatment alone is unlikely to achieve the required suspended-solids, nitrogen, color, and residual chemical oxygen demand (COD) levels. The anaerobic effluent should therefore undergo aerobic treatment to oxidize residual biodegradable compounds and facilitate ammonium nitrification. An anoxic denitrification stage, or alternatively partial nitritation-anammox for nitrogen-rich effluent with limited biodegradable carbon, may be required to achieve the total-nitrogen discharge limit. Coagulation-flocculation, electrocoagulation, or membrane separation can provide further removal of colloidal particles and residual suspended solids, whereas adsorption, advanced oxidation, or photocatalytic processes may be applied as tertiary polishing treatments for persistent color and refractory dissolved organics. Accordingly, an integrated treatment configuration comprising physical pretreatment, anaerobic digestion, aerobic-anoxic nitrogen removal, solids separation, and tertiary polishing is more technically appropriate than a single treatment process for managing the complex pollutant profile of raw POME.

POME is suitable for conversion into methane and hydrogen through advanced anaerobic digestion. Optimized operating conditions can increase gas yields and provide resource and economic benefits relative to conventional wastewater treatment, although the benefits depend on scale, energy prices, reactor configuration, and downstream biogas utilization. Organic fractions in POME can also serve as feedstock for biopolymers, organic acids, and biofuels, but the techno-economic performance of these recovery pathways remains insufficiently evaluated^[4]. A transition from pollutant removal to resource recovery can reduce environmental impacts and improve economic performance when recovered energy and products offset treatment costs. Untreated POME depletes dissolved oxygen in receiving waters and alters aquatic microbial communities^[18]. Anaerobic degradation releases odorous compounds that affect nearby communities^[28]. Its high suspended-solids concentration, organic content, and acidic pH can also cause severe pollution^[29]. POME exposure has reduced Daphnia magna reproduction^[30] and caused sublethal effects in zebrafish embryos^[31]. Direct discharge is therefore prohibited. Extreme rainfall can cause pond overflows and increase the risk of uncontrolled release.

Discharge limits define the physicochemical quality required for treated POME^[32]. Chemical oxygen demand and biochemical oxygen demand remain the principal compliance challenges. Moderate influent chemical oxygen demand can support methane production in biogas-recovery systems^[33], but final effluent must still meet regulatory limits. Residual color arises from suspended and dissolved compounds^[34], and other anthropogenic inputs can complicate attribution of river pollution. Supplementary real-time monitoring would help mill operators verify compliance and identify abnormal discharges. Industry-wide implementation requires shared commitment from regulators and mill operators.

The transition from waste management to resource recovery offers a pathway towards more sustainable palm oil production. Treatment denotes removal or reduction of a constituent, whereas recovery requires harvesting, regeneration, or reuse of the captured resource. Anaerobic digestion recovers methane and hydrogen as usable energy products^[35]. The summarized POME studies on advanced oxidation processes predominantly report degradation, mineralization, chemical oxygen demand reduction, or color removal and are therefore classified as pollutant-removal processes. Adsorption and biological nutrient uptake enable recovery only when the nutrient-bearing adsorbent or biomass is subsequently separated and reused or when nutrients are desorbed. More than 85.0% of palm oil mills use ponding systems for POME treatment^[26]. A typical system comprises a de-oiling tank, acidification ponds, anaerobic ponds, and facultative or aerobic ponds. The aerated upper layer supports microorganisms that oxidize residual organic matter and reduce ammonia, odor, biochemical oxygen demand, and chemical oxygen demand. Mill capacity determines the number of ponds required^[36]. Reported final chemical oxygen demand and biochemical oxygen demand concentrations are 100-1,725 mg/L and 100-610 mg/L, respectively^[37]. Ponding systems are widely used as they require little energy, maintenance, process control, or mechanical mixing^[15]. However, sludge accumulation reduces the effective pond volume and can increase the hydraulic retention time required for treatment^[38]. Microbial degradation and natural aeration in successive ponds reduce pollutant concentrations before discharge. Ponding systems have low capital costs and simple operation, but they require large land areas and long hydraulic retention times. Other limitations include odor, mosquito breeding, variable effluent quality, and greenhouse-gas emissions when biogas from anaerobic ponds is not captured. These constraints can prevent consistent compliance with stringent discharge standards.

Conventional POME treatment can provide inadequate control of residual pollution^[15]. New treatment methods increasingly combine pollutant removal with resource recovery^[11]. Anaerobic digestion produces biogas but depends on balanced microbial activity and controlled temperature, pH, loading, and retention time. Changes in POME composition can disrupt the microbial community and reduce gas yield and process stability. More resilient microbial consortia and improved process control are therefore required. Membrane bioreactors, adsorption, and biological nutrient-recovery systems provide complementary treatment functions but face distinct operational and economic constraints.

Membrane bioreactors produce high-quality effluent by integrating biological degradation with membrane-based solid-liquid separation. However, membrane fouling reduces permeate flux and increases the frequency of backwashing, chemical cleaning, and membrane replacement. Fouling results from cake-layer formation, pore blockage, microbial attachment, extracellular polymeric substances, soluble microbial products, membrane properties, and operating conditions^[93]. Fouling-resistant membrane materials, optimized cleaning protocols, and energy-efficient fouling-control strategies are therefore required to reduce operating costs and extend membrane service life^[93].

Adsorption provides an operationally simple method for removing contaminants from POME, but its classification as resource recovery requires the subsequent release and utilization of the captured material. Spent-adsorbent management remains a major limitation because regeneration efficiency, contaminant desorption, adsorbent deterioration, secondary waste generation, and final disposal affect overall process sustainability. Future studies should quantify desorption efficiency, nutrient concentration in the recovered solution, adsorbent regeneration efficiency, and performance over repeated adsorption-desorption cycles^[94]. Hybrid biological and advanced-oxidation systems can improve the treatment of complex wastewater containing biodegradable and recalcitrant contaminants, although their energy demand and process compatibility require evaluation^[95].

Nutrient removal using microalgae and aquatic plants is constrained by light availability, inhibitory wastewater constituents, biomass harvesting, culture stability, environmental variability, and integration with existing treatment systems^[96]. High biomass concentrations, suspended solids, and inadequate mixing restrict light penetration and reduce photosynthetic productivity, whereas closed photobioreactors require greater capital investment and operational control. Excess ammonia and other wastewater contaminants can inhibit microalgal growth, while variations in nutrient composition, temperature, irradiance, and pH affect biomass productivity and treatment performance. Microalgal harvesting remains a major scale-up constraint because centrifugation and membrane separation have high energy and operating requirements. Floating aquatic macrophytes require regular harvesting to maintain plant density, preserve treatment capacity, and remove accumulated nutrients from the treatment system^[97]. Outdoor systems are also affected by temperature, irradiance, nutrient availability, culture composition, and biological contamination. Commercial application therefore requires low-energy biomass harvesting, stable outdoor cultivation, safe biomass processing, and defined markets for recovered biomass products^[97].

Future research should prioritize commercial-scale POME recovery pathways that generate and capture usable products, particularly biogas, biomass, and biologically produced chemicals. Adsorption studies should quantify desorption, regeneration, nutrient concentration, recovery yield, and reuse of nutrient-loaded adsorbents. Studies using microalgae and aquatic plants should quantify nutrient partitioning into harvested biomass, harvesting efficiency, biomass quality, and end-use performance. Advanced oxidation research should optimize pollutant degradation, energy demand, reagent consumption, toxicity reduction, and biodegradability. Techno-economic and life-cycle assessments are needed to compare the treatment and recovery functions of integrated POME biorefineries.

Combining anaerobic and aerobic treatment can improve POME treatment performance^[98]. Anaerobic treatment reduces complex organic matter and produces biogas, after which aerobic treatment oxidizes residual biodegradable compounds. A pilot-scale integrated anaerobic-aerobic bioreactor started within 24 days and operated at organic loading rates up to 30.0 g-COD/(L·day)^[98]. Process simulation and optimization have also examined biogas production and economic and environmental performance^[99]. Further development should optimize solids retention time, recirculation, and temperature and validate performance at commercial scale. Adding biochar to bioreactors can support POME treatment [Figure 3]. In anaerobic systems, biochar provides a porous surface for microbial attachment and can adsorb inhibitory organic compounds and nutrients^[100]. These functions can improve process stability and biogas production. In aerobic treatment, biochar can remove dissolved contaminants^[101]. However, the overall environmental benefit depends on biochar production, dosage, regeneration or disposal, and the energy and chemical requirements of the treatment system^[102].

AOPs can polish treated POME by degrading persistent organic compounds, reducing color, and improving biodegradability^[70,103]. Ozonation, Fenton oxidation, and ultraviolet/H₂O₂ treatment generate reactive species that break down organic pollutants. Their implementation requires control of reagent dosage, pH, reaction time, and energy use. Future POME studies should report mineralization, residual toxicity, biodegradability, oxidant and catalyst consumption, energy demand, and secondary residues in addition to chemical oxygen demand and color removal. Adsorption research should progress from removal efficiency to demonstrated recovery by quantifying desorption conditions, nutrient concentration in the regenerant, recovery yield, adsorbent regeneration over repeated cycles, and spent-adsorbent management. Direct agricultural use of nutrient-loaded biochar or other adsorbents requires evidence of nutrient release, crop response, contaminant safety, and soil compatibility. Biological nutrient-recovery studies should report biomass productivity, harvesting efficiency, nitrogen and phosphorus content of the harvested biomass, and the fraction of influent nutrients recovered. Proposed uses as biofertilizer, feed, or biofuel also require product-composition, safety, performance, and downstream-processing data. Emerging biological nutrient-recovery technologies focus on reactor design, strain selection, integration with membrane separation, and lower-energy biomass harvesting. Photobioreactors can use controlled lighting and sensors to maintain growth conditions, while hybrid membrane systems retain biomass and improve nutrient removal. Floating treatment wetlands and constructed wetlands use plants and rhizosphere microorganisms to assimilate nutrients. These approaches require POME-specific validation of nutrient recovery, biomass quality, energy demand, operating stability, and commercial-scale performance.

Figure 4 presents an integrated POME treatment concept in which each unit operation performs a distinct function. Anaerobic digestion reduces the organic load and produces recoverable biogas. Aerobic treatment, advanced oxidation, coagulation and flocculation, and membrane filtration primarily reduce residual pollutants and polish the effluent. Adsorption transfers dissolved contaminants to a solid phase and contributes to resource recovery only when the loaded adsorbent is regenerated with product recovery or is beneficially reused. These pollutant-removal processes support the biorefinery by enabling water reuse and protecting operations that recover energy, biomass, nutrients, or chemicals.

Hybrid treatment trains can increase nutrient capture from POME. Photobioreactor effluent can feed constructed wetlands, where plants and associated microorganisms remove remaining nutrients. Chemical crystallization can then recover phosphorus as struvite. However, the viability of these integrated systems depends on demonstrated mass balances, product quality, energy demand, capital and operating costs, and performance under field conditions. These data are required before modular treatment systems can be considered reliable resource-recovery platforms. Reorienting POME management from pollutant mitigation towards integrated treatment and resource recovery can provide environmental and resource benefits. Direct recovery is most clearly demonstrated for methane, hydrogen, and biologically produced organic acids. Advanced oxidation processes primarily provide pollutant degradation and polishing, whereas adsorption and microalgal or plant-based treatment capture nutrients that require subsequent recovery. Future POME biorefinery studies should report product yield, recovery efficiency, regeneration or harvesting performance, product quality, techno-economic performance, and life-cycle impacts alongside conventional treatment metrics.

Integrated POME management can provide environmental and economic benefits when treatment systems recover usable energy, nutrients, biomass, or chemicals. Anaerobic digesters directly recover methane and hydrogen, while membrane bioreactors, adsorption, biological uptake, and advanced oxidation provide complementary separation and polishing functions. Claims of nutrient recovery require downstream evidence of desorption, regeneration, biomass harvesting, product quality, or beneficial reuse. Commercial implementation depends on technology maturity, scale, product markets, energy prices, and treatment costs. Future work should therefore combine process optimization with pilot- or industrial-scale validation, techno-economic analysis, and life-cycle assessment. Hybrid and multistage treatment systems should be designed around clearly defined treatment and recovery functions rather than removal efficiency alone.