Enhancing sustainability: microalgae cultivation for biogas enrichment and phycoremediation of palm oil mill effluent - a comprehensive review

ABSTRACT

Phycoremediation, employing microalgae, effectively treats Palm Oil Mill Effluent (POME) by removing pollutants, curbing eutrophication, and yielding valuable substances. Microalgae utilize photosynthesis and their high surface area to convert carbon dioxide into biomass, reducing pollutant concentrations and enhancing water body oxygenation. This dual mechanism acts as a natural biofilter, addressing environmental concerns while bolstering the economic potential of the palm oil industry. The integrated system of biogas production and phycoremediation covers crucial aspects such as cultivating microalgae on POME, producing valuable compounds, and biogas upgrading. This approach harmonizes POME treatment, resource recovery, and renewable energy production, showcasing environmental and economic advancements.

KEYWORDS:

• Palm oil mill effluent
• phycoremediation
• biogas upgrading
• microalgae
• value-added product, integrated system

1. Introduction

Effective treatment of palm oil mill effluent (POME) remains a critical challenge for palm oil manufacturers. In the commercial wet milling process of palm oil, it is projected that for every ton of fresh fruit, approximately 2.5 tons of effluent are generated, resulting in a staggering annual production of over 200 million tons of POME within the oil palm industry in worldwide [1]. Characterized by high concentrations of organic compounds, nitrogenous compounds, proteins, and minerals, POME poses significant environmental concerns and necessitates rigorous treatment processes before safe disposal [2–4]. Compliance with POME treatment standards is not merely a regulatory requirement but a pivotal operational necessity for sustainable palm oil production.

Various methods are used to manage POME in the palm oil industry. Some facilities use dark fermentation to convert POME into biogas, while others compost treated POME as a nutrient source for palm oil trees. However, AD remains the preferred choice for most manufacturers due to its cost-effectiveness in reducing biological and chemical pollutants like chemical oxygen demand (COD), and biological oxygen demand (BOD). Moreover, the biogas produced through this process can be used to generate electricity for the palm oil mill [5]. In certain regions like Southeast Asia, smaller-scale facilities employ a conventional approach by using ponds to treat POME [6]. This method involves covering the ponds to capture the biogas produced. Its appeal lies in its simplicity and lower cost compared to alternative methods such as biodigesters or membrane filtration systems.

The utilization of microalgae in treating POME has emerged as a promising approach for sustainable remediation [7,8]. This method not only addresses sustainability concerns but also potentially reduces the cost associated with the biomass produced by microalgae cultivated in POME [9]. Phycoremediation, employing microorganisms like algae, plays a pivotal role in removing pollutants from wastewater, specifically in the case of palm oil industry effluents [10]. Algae exhibit the capability to eliminate contaminants such as oil, grease, and nutrients that contribute to eutrophication in water bodies. These microorganisms utilize various nutrients found in waste streams, like nitrogen and phosphorus, fostering their growth and the production of valuable secondary metabolites [11–13]. Moreover, algae can contribute to biofuel production, presenting an added advantage in the treatment of POME. This eco-friendly and cost-effective process holds promise, and this study delves into the potential of microalgae in not only purifying polluted POME but also yielding valuable by-products.

On a global scale, around half of the total renewable energy consumption in 2017 originated from modern bioenergy. Projections suggest that this prominent contribution will persist shortly and is anticipated to sustain its position as the primary renewable energy source (RES) until 2023 [14]. It is estimated to constitute approximately 30% of the growth in renewable energy sources over the upcoming five years. As previously mentioned, the emission rates of methane (CH₄) and carbon dioxide (CO₂) in the anaerobic pond during the treatment of POME in Indonesia were notably elevated, measuring 261.93 grams per square meter per day for CH₄ and 595.99 grams per square meter per day for CO₂. This equates to an overall emission of 48.572 metric tons of CO₂ equivalent per day or 14,571.5 metric tons of CO₂ equivalent per year [15]. Notably, the emission of CO₂ exceeded that of CH₄ by more than two times, both in terms of spatial distribution and temporal occurrence. Upgrading biogas to pure methane can eliminate this carbon dioxide component. In East Africa, reports indicate that replacing fossil fuels with biodigesters can potentially save between 0.6 and 5.7 million tons of carbon dioxide emissions annually per country [16]. The reduction in carbon dioxide emissions could be significantly enhanced if 40% of the carbon dioxide in biogas is removed through the process of upgrading biogas before its utilization. Additionally, capturing carbon dioxide before the release of flue gas from used biogas is another viable method to diminish greenhouse gas emissions [17].

Besides contributing to global warming through carbon dioxide emissions, the presence of contaminants in biogas poses a challenge to its widespread application by diminishing its energy density. Typically, biogas consists of varying compositions, generally containing 50–75% methane, 25–65% carbon dioxide, and 1–5% hydrogen sulfide [18]. It also contains trace amounts of water, nitrogen, oxygen, siloxanes, ammonium, and hydrogen [19]. To be efficiently used as engine fuel, biogas necessitates a high energy density with a methane content exceeding 96% mol [20]. Additionally, the inclusion of hydrogen sulfide in biogas can induce corrosion, resulting in the deterioration of metallic components in appliances and engines [18].

Enhancing the value of biogas entails more than its mere generation; it involves a purification and upgrading process that significantly improves its utility. Purification procedures are aimed at removing an array of contaminants present in biogas, including hydrogen sulfide, water vapor, oxygen, nitrogen, ammonia, and siloxanes. On the other hand, the upgrading process specifically targets the removal of carbon dioxide from biogas, effectively boosting the concentration of biomethane and elevating its energy density to optimal levels. Following this upgrading, the biogas becomes viable for utilization as a potent fuel source across various applications. Its newfound suitability extends to powering boilers, stoves, engines, and gas turbines for electricity generation, as well as serving as a fuel for vehicles and fuel cells [21]. This comprehensive enhancement process significantly amplifies the versatility and applicability of biogas in diverse energy-related sectors.

Numerous studies have explored modern technologies for both purification and upgrading of biogas [22–24]. These technologies encompass diverse methods including membrane separation, cryogenic separation, pressure swing adsorption, water scrubbing, physical and chemical scrubbing, chemical absorption, hydrate formation, and biological conversion processes. These commercially available upgrading technologies have demonstrated effectiveness in capturing carbon dioxide and trace components from biogas. However, a significant drawback lies in their substantial investment capital and operating costs [25]. An alternative approach involves utilizing low-cost natural adsorbents for biogas cleaning. Despite their potential cost-effectiveness, there exists limited information regarding the use of natural materials in biogas purification. This study aims to critically review and document the utilization of POME as a medium growth of microalgae as well as to upgrade biogas from POME by using microalgae. The aim of this review paper is to propose concept to integrate the production of biomass from microalgae and upgraded biogas from POME. To our knowledge this is the first time the complete integrated system together with the microalgae species and their valuable products are reviewed.

2. Pome

The majority of liquid waste from agro-industries in Indonesia has the potential to pollute the environment. However, the micronutrient content in these wastes can be utilized for the cultivation of microalgae [9]. POME has become a serious concern for the national palm oil industry due to its high COD, BOD, and Total Solid values (Table 2). Treating POME with a digestion process can significantly reduce COD/BOD concentrations, but a drawback is that this process also decreases nutrient content, including nitrogen, phosphorus, and other minerals [26]. The use of membranes provides advantages in recovering essential components from POME. However, membrane technology is currently not economically feasible for application in POME treatment.

Presently, treatment methods involving biological systems, such as anaerobic and aerobic oxidation ponds, are employed for plantations and agriculture (fertilizer production) [27]. However, these treatment systems have yet to yield waste byproducts that meet quality standards. Therefore, there is a need for methods to utilize the residual waste from POME. By employing microalgae, it is anticipated that there will be advantages in harnessing the remaining nutrients, particularly nitrogen and phosphorus.

As previously discussed, the potential release of methane (CH₄) from various components of palm oil waste, including empty fruit bunches (EFB), fibers, and POME, presents a substantial threat to greenhouse gas (GHG) emissions when left untreated (Table 1) [28]. EFB, a byproduct of the palm oil extraction process, and fibrous residues can contribute to GHG emissions, particularly methane, through their decomposition in open fields or landfills. Similarly, POME is a major concern for GHG emissions, with the anaerobic decomposition of organic matter in POME producing significant amounts of methane. These emissions pose a potent threat due to methane’s higher warming potential compared to carbon dioxide. To address these environmental concerns, effective waste management and treatment strategies, such as anaerobic digestion (AD) and the use of microalgae, are crucial. By adopting integrated and sustainable approaches, the palm oil industry can mitigate methane emissions from its waste components, reducing its environmental impact and potentially harnessing energy from these waste streams, contributing to environmental conservation.

Table 1. Waste from Crude Palm Oil (CPO) processing [28].

Kind of waste	Average production (t/year)	Waste conversion (%)	Total waste (t/year)	Potential emission (t CH4/year)	Emission (t CO2/year)
Palm oil production	9.816.393
Empty fruit bunch (t/year)		23	2.257.770	27.093.24	568.958
Shell (t/year)		12	785.311	9.423	197.898
Fibers (t/year)		8	1.177.967	14.135	296.847
POME (m3/t)		66	6.478.819	1.285.721.706	27.000.155

Evaluating the quality of POME holds significant importance in formulating effective strategies for future microalgae cultivation. This assessment encompasses a range of parameters, including chemical, physical, and biological attributes as detailed in Table 2. Consequently, the elimination of such particles becomes imperative, requiring the application of various filtration and sedimentation processes [29].

Table 2. Physico-chemical constituents of POME.

Physico-chemical characteristic		Amino acids		Fatty acids
Parameters	Concentrations	Components	Concentration (mg/g protein)	Components	Concentrations (mg/g lipid)
COD (g/L)	15.0–100.0	Alanine	77.0	Arachidic acid	75.6
BOD (g/L)	10.25–43.75	Arginine	41.5	Arachidonic acid	11.2
Total solid (g/L)	11.50–79.0	Aspartic acid	96.6	Behenic acid	26.2
Total suspended solid (g/L)	5.0–54.0	Cystine	33.7	Capric acid	42.9
Total volatile solid (g/L)	9.0–72.0	Glutamic acid	1.08	Eicosatrienoic acid	14.9
Total nitrogen (g/L)	0.18–1.4	Glycine	94.3	Hepadecanoic acid	13.9
Oil and grease (g/L)	0.13–18.0	Histidine	14.3	10-Heptadecanoic acid	11.2
Lignin (g/L)	4.7	Isoleucine	45.3	Lauric acid	92.2
Phenolics (g/L)	5.8	Leucine	68.6	Linoleic acid	47.2
Pectin (g/L)	3.4	Lysine	56.6	Linolenic acid	47.2
Carotene (g/L)	0.008	Methionine	68.8	Myristric acid	126.6
Calcium, Ca (g/L)	0.27–0.40	Phenylalanine	32.0	Oleic acid	85.4
Cobalt, Co (g/L)	0.04–0.06	Proline	45.7	Palmitic acid	144.5
Iron, Fe (g/L)	0.07–0.16	Serine	68.6	Stearic acid	114.1
Magnesium, Mg (g/L)	0.25–0.34	Threonine	25.8
Manganese, Mn (g/L)	0021–0.004	Tyrosine	32.6
Phosphorus, P (g/L)	0.094–0.13	Tryptophan	12.6
Potassium, K (g/L)	1.28–1.92	Valine	35.6
Zinc, Zn (g/L)	0.0012–0.0018

[30,31].

3. Phycoremediation

Phycoremediation is an environmentally friendly and sustainable approach that leverages the unique properties of microalgae, bacteria, and fungi to remediate and enhance environmental conditions. This innovative technique has gained recognition for its efficiency in treating various pollutants in diverse ecosystems, ranging from industrial wastewater to agricultural runoff [32].

The mechanism behind phycoremediation involves the collective efforts of microalgae, bacteria, and fungi in absorbing and transforming pollutants, leading to the purification of contaminated environments [33]. Microalgae serve as natural biofilters, utilizing photosynthesis and metabolic processes to sequester contaminants like heavy metals, nutrients, and organic compounds [34,35]. Meanwhile, bacteria and fungi contribute through processes such as biodegradation and bioaccumulation [33]. Each type of microorganism plays a unique role, with bacteria often involved in breaking down complex organic pollutants, and fungi being efficient in absorbing heavy metals [36].

However, the use of microalgae in phycoremediation offers several advantages over bacteria and fungi. Microalgae, through their rapid growth and efficient nutrient uptake, generate biomass during the remediation process [34]. This biomass can serve as a valuable resource with multiple applications. Unlike bacteria and fungi, microalgae are photosynthetic organisms, utilizing sunlight for energy, which makes them highly efficient in converting pollutants into biomass through photosynthesis. This biomass can be utilized for bioenergy production, such as the generation of biogas or biofuels, offering a sustainable and economically viable solution [9].

Moreover, microalgae have a higher surface area, allowing for more effective absorption of contaminants [34]. They can be recovered from water bodies, facilitating the removal of pollutants from the environment. The versatility of microalgae in treating a wide range of pollutants and their potential for biomass utilization make them a preferred choice in phycoremediation, offering a holistic and environmentally friendly solution to pollution-related challenges. Ongoing research aims to further optimize and expand the applications of microalgae in phycoremediation to address diverse environmental concerns [37].

4. Microalgae as agent of phycoremediation

Microalgae, encompassing diverse species such as Chlorella, Spirulina, and Scenedesmus, stand out as potent agents in phycoremediation, offering a natural and efficient means to mitigate environmental pollution [38]. This unique class of photosynthetic microorganisms plays a pivotal role in absorbing and transforming various pollutants, contributing to the purification and restoration of contaminated ecosystems.

4.1. Mechanism of phycoremediation

Microalgae employ several key mechanisms that make them highly effective in the process of phycoremediation. Through photosynthetic sequestration, microalgae harness sunlight to convert carbon dioxide and pollutants into biomass, not only reducing pollutant concentrations but also yielding valuable microalgal biomass [35]. Their exceptional nutrient uptake capacity is showcased as they efficiently remove excess nitrogen and phosphorus from water bodies, addressing concerns associated with eutrophication, a common consequence of nutrient runoff [9]. Moreover, certain microalgal species exhibit the unique ability to accumulate heavy metals within their cellular structures, providing a natural and efficient approach to heavy metal remediation in aquatic environments [39]. Additionally, microalgae contribute to phycoremediation through the release of extracellular substances, which aid in the binding and immobilization of contaminants (i.e. microplastic), further enhancing the overall efficiency of the remediation process [40].

4.2. Advantages of microalgae in phycoremediation

Microalgae offer several distinct advantages in the context of phycoremediation, providing efficient and environmentally friendly solutions to various forms of pollution. An inherent advantage of microalgae-based phycoremediation is the generation of biomass during the remediation process [33]. This biomass, containing valuable components like proteins, lipids, and carbohydrates, holds substantial potential for applications in bioenergy production, including the generation of biogas and biofuels [9].

Microalgae demonstrate exceptional versatility by efficiently addressing a broad spectrum of pollutants. From nutrients such as nitrogen and phosphorus to heavy metals and organic compounds, microalgae exhibit adaptability to different environmental conditions, making them suitable for diverse polluted settings as demonstrated previously [39]. Nur et al. [35], reported that the color and COD of POME could be degraded by using the combination of Spirulina and photodegradation during the cultivation. Microalgae are characterized by their rapid growth rates, enabling swift and effective uptake of pollutants. This characteristic is particularly advantageous for addressing short-term environmental challenges, providing a dynamic and responsive solution to pollution-related issues [41].

5. Cultivation of microalgae on POME

Microalgae cultivation can be divided into 3 type: microalgae monoculture, polyculture, and co-culture. The three types are distinguished as follows: 1) microalgae monoculture with one type of microalgae and fewer functional genes, so they are not stable and not resistant to species invasion; 2) microalgae polycultures with higher diversity have more functional genes, so they are more stable and resistant to species invasion, and 3) co-cultivation microalgae co-exist harmoniously with other heterotrophic microorganisms like fungi, bacteria, and yeast as well as other microalgae. Co-culture microalgae cultivation with other microorganisms for lipid production in POME under outdoor condition in Indonesia had been reported by Nur et. al [42]. In addition, polyculture microalgae biomass production in large-scale open raceway pond using excess energy and effluent from palm oil mills was also reported in Indonesia [43].

For successful microalgae cultivation, it is essential to provide a growth medium that contains all the necessary nutrients to support robust biomass production. Numerous studies [11,13,44–46] have highlighted the ability of microalgae to thrive in a nutrient-rich environment, emphasizing the significance of essential nutrients in sustaining their growth and biomass production. This underscores the importance of ensuring an adequate nutrient supply in the cultivation medium to facilitate the optimal growth of microalgae.

The growth of algae necessitates essential nutrients such as carbon, nitrogen, phosphorus, and micronutrients, which are abundantly present in POME, as highlighted in the study by Nur and Buma [9]. However, the quality of POME can be further enhanced to optimize its utility in fostering the growth of specific algal forms for pollution control and the production of valuable biomass. Studies conducted by Ahmad et al. [47], Low et al. [48], Nur et al. [35], and Nur and Buma [9] have underscored the potential for enhancing POME quality through the reduction of parameters such as COD, BOD, lipids, proteins, and tannins. By minimizing these components, the availability and accessibility of essential nutrients in POME can be increased. This enhancement in nutrient availability can be particularly advantageous for cultivating specific strains of algae tailored for pollution remediation purposes and for the generation of valuable biomass.

By reducing the levels of COD, BOD, lipids, proteins, and tannins in POME, researchers aim to create an environment more conducive to fostering the growth of targeted algal species. This optimized environment can facilitate the efficient utilization of POME as a nutrient-rich medium to support the growth of specific algae strains, thereby serving dual purposes in pollution control and the production of valuable biomass. COD is the main dominating parameter in POME that should be optimized for the wastewater as a culture medium for microalgae either a single strain or consortia (cultivating with other microorganisms) [49].

The optimal level of COD holds paramount importance in maximizing the generation of microalgal biomass, as highlighted in the research by Nur et al. [50]. An excessive COD concentration can potentially hinder the growth of microalgae, while a lower level might not provide sufficient organic carbon substrate for robust biomass production. Therefore, achieving a moderate COD level is crucial to support the ideal conditions for maximum microalgal growth.

Dilution of the original wastewater can be a viable strategy to attain this moderate COD level, thereby creating a more favorable environment for microalgal cultivation. This dilution process not only helps in reaching the desired COD concentration but also aids in reducing turbidity and coloration present in the wastewater. These reductions contribute to optimizing light penetration, which is essential for facilitating the ideal conditions necessary for microalgal growth, as noted in the research by Tan et al. [7,8].

Furthermore, maintaining an optimal COD level through dilution sustains microalgal growth in open pond cultivation systems. Additionally, dilution aids in mitigating the extremes in pH levels that may exist in the original wastewater. Such extreme pH ranges could otherwise impede the growth of microalgae. Hence, the dilution of wastewater not only regulates the COD levels but also assists in creating a more conducive environment for microalgal growth by mitigating factors such as turbidity, coloration, and extreme pH conditions. POME vary greatly in terms of physical properties and chemical composition (Table 2), and many are not suitable for direct use for microalgae cultivation. As a result, proper pre-treatment is required based on POME characteristics and microalgal cultivation requirements. Many different types of pre-treatment processes, such as thermal, chemical, mechanical, and biological processes, are used to treat wastewater before cultivation [27,51].

The pre-treatment of POME plays a crucial role in enhancing light penetration and creating favorable conditions for microalgae cultivation [52]. Various techniques such as coagulation and adsorption have been employed for this purpose. In the coagulation process, rice straw starch was utilized, as described in the research conducted by Mahmod et al. [53]. This method involves the application of rice straw starch as a coagulant to improve the clarity of POME. By facilitating the aggregation of particles, coagulation helps in reducing turbidity and enhances light penetration, thereby optimizing the conditions necessary for microalgae growth.

On the other hand, the adsorption process involves the use of activated carbon derived from other biomass sources like palm kernel shells [54]. Activated carbon from biomass serves as an effective adsorbent, aiding in the removal of impurities and colorants from POME. This process contributes to improving the clarity of the effluent, thereby further enhancing light penetration crucial for microalgal cultivation. The pre-treatment process of POME, depicted in Figure 1, illustrates the steps involved in coagulation and adsorption. These processes play a pivotal role in optimizing the quality of POME, facilitating better light penetration, and providing an improved environment for microalgae cultivation, ultimately contributing to enhanced biomass production.

Figure 1. Pre-treatment process of POME as growth medium of microalgae.

Acid-heat treatment stands out as an effective method for pre-treating POME, as highlighted in the research by Cheah et al. [51]. The dark coloration of POME primarily stems from the presence of lignin derived from plant components. Within this dark liquid, various components like lignin, cellulose, hemicellulose, hexoses, and pentoses are present. During the pre-treatment process, the highly soluble POME lignin is effectively removed in acidic solutions. This removal of lignin not only introduces more sugars into the medium, promoting microalgae growth but also eliminates the dark-colored compounds, thereby allowing increased light penetration – an essential factor for optimal microalgae growth.

Another viable pre-treatment option involves a combination of ozone and biological treatments for POME, as mentioned by Jürgensen [55]. The use of ozone gas, particularly before anaerobic-aerobic treatment, has shown promising results in improving the removal efficiency of COD and BOD in POME. Ozone gas is acknowledged for its efficacy as a sterilizer in various bioprocessing industries, contributing to enhanced treatment efficiency when employed in combination with biological treatments.

Both acid-heat treatment and the use of ozone in combination with biological treatments offer effective strategies for pre-treating POME. Acid-heat treatment facilitates the removal of lignin and dark-colored compounds while introducing more sugars into the medium, promoting microalgae growth. Meanwhile, the application of ozone gas aids in improving the removal efficiency of organic pollutants, thereby enhancing the overall treatment of POME, especially in combination with biological treatment processes.

6. Microalgae as a potential phycoremediation agent for POME

Microalgae are regarded as versatile microorganisms with substantial importance in various biotechnological applications, as highlighted in numerous studies [11,46,56–58]. Their significance spans across diverse sectors including food, health, pharmaceuticals, and specialty products. One of the notable attributes of microalgae is their ability to utilize solar energy, providing eco-friendly solutions for carbon dioxide sequestration and nutrient utilization in biomass production, as noted in the works of Sarkar et al. [59] and Yaakob et al. [13].

These microorganisms possess the unique capability to harness solar energy efficiently, thereby aiding in the eco-friendly conversion of carbon dioxide and nutrients into valuable biomass. Microalgae exhibit adaptability to a wide range of environments, thriving in both closed photo-bioreactors and open ponds, as evidenced by studies conducted by Rahul et al. [60] and Ranga Rao and Ravishankar [61]. Their ability to grow in diverse environments, coupled with their efficient utilization of solar energy and capability for eco-friendly biomass production, positions microalgae as promising candidates for various biotechnological applications across multiple industries, showcasing their potential to contribute to sustainable solutions and innovative products. Microalgae-based wastewater treatment is a sustainable method that can reduce pollution [62]. Additionally, the biomass produced by the microalgae that grow in POME has the potential to serve as a significant source of biofuel and other valuable products, making the use of microalgae to treat POME very cost-effective [9]. Microalgae have the added benefit of being able to be grown on non-agricultural land. Table 3 lists the constituents of various species of microalgae growing on POME.

Table 3. Phycoremediation of POME by microalgae.

Microalgae	Mode of cultivation	Media cultivation	Removal efficiency	References
Co-culture C. vulgaris and bacteria Azospirillum brasilense	Batch cultivation with aeration	100% POME (initial COD 750 mg/L)	Removal of ammonium (84%) phosphorus (87%), and COD (52%)	Halim et al. [49]
Co-cultivation Scenedesmus sp. and bacteria	Batch cultivation with aeration	Anaerobic unsterilized POME (COD 1800 mg/L)	TP removal (70%), TN removal (80%), COD removal (40%)	Udaiyappan et al. [63]
Spirulina (Arthrospira) platensis	Batch cultivation at saturated light intensity, no aeration	Anaerobic and autoclaved POME (initial COD 1425 mg/L)	Phenolic compound removal (94%), color removal (70%), and total COD removal (35%)	Nur et al. [35]
Scenedesmus sp.	Batch cultivation with aeration	Anaerobic POME (initial COD 2012 mg/L)	Photoperiod and loading rate influence pollutant removal (COD 67%, Ammonia N 100%, PO4^3− (61%))	Udaiyappan et al. [64]
Chlorella pyrenoidosa, Coelastrellia sp., Chlorella sorokiniana	Batch cultivation with sparged 1% CO2	10% POME from facultative pond (initial COD 1935 mg/L)	TN (65%) and TP (56%) removal efficiency	Ding et al. [65]
Chlorococcum oleofaciens	Batch cultivation with dilution of POME	Pre-treated anaerobic -aerobic POME (initial COD 700 mg/L)	TP and TN removal (>90%), COD removal (65%)	[7,8]
Chlorococcum oleofaciens	Immobilized cell cultures batch cultivation	Pretreated anaerobic -aerobic POME (initial COD 700 mg/L)	TP (90%), total nitrogen (93%), COD (50.72%) removal efficiency	[66]
Nannochloropsis sp.	Immobilized cells and diluted POME	Treated POME (initial COD 3250 mg/L)	COD removal (71%)	[67]

Comparative analysis of mode microalgae cultivation and value-added products.

Numerous studies have extensively explored the utilization of various microalgae species for the removal of nitrogen and phosphorus from POME. Past research has delved into the potential of co-cultivating microalgae with bacteria to effectively reduce pollutants present in POME. These co-cultivation efforts, conducted in aerated batch cultivation systems, demonstrated successful reductions in COD, nitrogen, and phosphorus content [49,63]. The strategy of co-cultivating microalgae with bacterial consortia is known to facilitate pollutant removal by capitalizing on the bacterial degradation of complex organic content into simpler forms. Simultaneously, microalgae utilize these simpler organic compounds from POME and supply oxygen, promoting aerobic bacterial activities. Moreover, microalgae, through the process of photo-degradation, exhibit the capability to reduce the dark brown coloration of POME. Studies have highlighted the effectiveness of microalgae, such as S. platensis, in degrading phenolic compounds (94%), reducing color (70%), and lowering COD (35%) under high saturated light intensity [35].

Other species like Chlorococcum oleofaciens, Scenedesmus sp., and locally isolated microalgae strains like Chlorella sorokiniana UKM2, Coelastrella sp UKM4, and Chlorella pyrenoidosa UKM7 have demonstrated efficiency in degrading pollutants in POME, as reported in studies [7,8,64,65]. Additionally, immobilized cells of microalgae such as Nannochloropsis sp. and Chlorococcum oleofaciens have shown promise in degrading complex organic components like proteins and lipids into carboxylic acids, ammonium, and phosphate [66,67]. The advantage of using immobilized cells lies in their easy separation from the effluent post-treatment.

The environmentally and economically beneficial nature of using microalgae in treating POME is evident, as it aids palm oil manufacturers in managing wastewater while offering the added advantage of absorbing carbon dioxide and releasing oxygen into the system. However, the selection of the most effective microalgae species is critical to achieving cost-effective bioremediation, as emphasized in studies [68–70]. The choice of microalgae species is pivotal in ensuring the efficacy and efficiency of the bioremediation process for POME.

7. Production of valuable compounds from microalgae growing on POME

Cultivating microalgae in POME goes beyond nutrient reduction, offering a versatile approach illuminated by recent studies [9,71,72]. This cultivation strategy allows microorganisms to thrive by utilizing the effluent’s nutrients, resulting in the accumulation of biomass rich in proteins, lipids, carbohydrates, and other vital components. The biomass of microalgae cultivated in wastewater presents limitations in its utilization; it cannot be directly used for food, pharmaceuticals, or cosmetics due to certain restrictions. However, there are numerous applications where this biomass can be effectively employed. It serves as a valuable resource for various purposes such as biofuel production, biocement, biofertilizers, bio-stimulants, animal feed, and fermentation products. Generally, this biomass can be utilized directly or undergo chemical or bioprocessing to extract its benefits (Figure 2). The utilization of POME for microalgae cultivation presents an environmentally friendly and economically feasible approach, converting waste into a resourceful medium for cultivation. This strategy not only aids in mitigating POME’s environmental impact but also holds promise for revenue generation through valuable product extraction, emphasizing the potential of sustainable waste management, resource recovery, and high-value commodity production.

Figure 2. Utilization of microalgae growing on POME.

Cultivating microalgae in POME presents an opportunity for extracting an array of value-added products, a point emphasized by De Carvalho et al. [73] and Nur and Buma [9] (Table 4). These products encompass a wide spectrum, ranging from biofuels to bioactive compounds, with versatile applications spanning multiple industries, including food, feed, pharmaceuticals, cosmetics, and fuel. This approach leverages the potential of microalgae biomass derived from POME cultivation as a source for multifaceted, high-value products, contributing to diverse industrial sectors and underscoring the versatility and economic potential of microalgae-based solutions.

Table 4. Comparative analysis of mode microalgae cultivation and value-added products.

Microalgae	Media cultivation	Mode of cultivation	Valuable products	References
Isochrysis sp.	POME modified medium	Batch outdoor condition	PUFAs	Vairappan and Yen [74]
Spirulina platensis	Digested POME	Two step semi continuous cultivation batch cultivation	C-Phycocyanin	Nur et al. [75,76]
Botryococcus braunii	Digested POME	Batch cultivation	Hydrocarbons	Nur et al. [50]
Phaeodactylum tricornutum	Digested POME	Batch cultivation	Fucoxanthin	Nur et al. [75,76]
Phaeodactylum tricornutum	Digested POME	Batch cultivation with medium and environmental modification	EPS	Nur et al. [75,76]
Haematococcus pluvialis	Diluted raw POME	Batch cultivation	Astaxanthin	[77]
Chlorella zofingiensis	Diluted raw POME	Batch cultivation	Astaxanthin
Arthrospira platensis	Digested POME	Batch cultivation with UV-C irradiation	PHB, C-phycocyanin	[78]
Botryococcus braunii	Digested POME	Batch cultivation, mixotrophic condition	PHB	[42]
Chlorella vulgaris	Digested POME	Batch cultivation, diluted with BG11 medium	PUFAs	[79]
Thalassiosira pseudonana	Digested POME	Batch cultivation, diluted with f/2 medium	Lipids (dominated with palmitoleic acid and myristic acid)	[80]
Chlorella sorokiniana CY-1	Diluted POME	Batch cultivation, mixotrophic condition injected with 2.5% CO2	Lipids (dominated with saturated fatty acid)	[81]
Chaetoceros affinis	Diluted digested POME	Batch cultivation	Lipids (dominated with oleic acid)	[80]
Chlamydomonas biconvexa	Digested POME	Batch cultivation	Carbohydrate (31%), lipid (11%, dominated with palmitic acid)	[1]
Mixed algae	Digested POME	Batch co-cultivation outdoor condition	PUFAs	[82]
C. calcitrans and A. platensis	Digested POME	Batch co-cultivation, outdoor condition	Fucoxanthin and C-phycocyanin	[83]
Diatom and green algae	Digested POME plus external organic source	Batch co-cultivation	PUFAs and fucoxanthin	[35]
Aurantiochytrium limacinum	Digested POME plus external organic source	Heterotrophic batch cultivation	DHA	[84]

Microalgae possess the remarkable capability to produce high-value biofuels and bioactive compounds that are in high demand across multiple industries. This biomass acts as a rich source for the extraction of biofuel resources and bioactive compounds, serving as a sustainable solution for meeting the demands of various sectors. The extraction of these valuable products from microalgae biomass cultivated in POME not only adds economic value but also aligns with environmentally conscious practices by utilizing waste streams for beneficial purposes. Production of polyunsaturated fatty acids (PUFAs) was increased 26% when Isochrysis sp was cultured in f/2 media enriched with POME in outdoor conditions [74].

The cultivation of S. platensis in 100% POME using a two-step semi-continuous approach has shown promising results in enhancing C-phycocyanin productivity. This method has demonstrated nearly similar production rates compared to Zarrouk’s medium, yielding approximately 4.08 mg/L/day of C-phycocyanin. Additionally, various microalgae species such as Botryococcus braunii, Chlorella zofingiensis, Haematococcus pluvialis, and Chlamydomonas sp have exhibited robust growth in digested POME medium. Cultivation techniques involving batches with serial dilution or enrichment with specially formulated commercial medium have been employed to harness these microalgae to produce valuable compounds (Table 4).

Studies conducted by Cheah et al. [81], Fernando et al. [77], Kumaran et al. [79], Nur et al. [50,82], and Pascoal et al. [1] have highlighted the successful cultivation of these microalgae species in POME, resulting in the production of various valuable compounds. These include lipids rich in PUFAs, lipids for bioenergy purposes, carbohydrates, biopolymers such as polyhydroxybutyrate (PHB), and pigments like astaxanthin and C-phycocyanin.

Moreover, diatoms such as Phaeodactylum tricornutum, Thalassiosira pseudonana, and Chaetoceros affinis have also exhibited successful cultivation in POME. These diatoms have displayed the capability to produce high-value compounds such as sulfated polysaccharides, fucoxanthin, and lipids, as reported in studies by Nur et al. [75,76] and Palanisamy et al. [80,85].

The utilization of mixed cultures of microalgae has proven advantageous in producing high-lipid content, particularly rich in Polyunsaturated Fatty Acids (PUFAs), under outdoor cultivation conditions, as reported in research by Nur et al. [42]. This approach of cultivating a mixed culture of cyanobacteria and diatoms in digested POME has resulted in the production of C-phycocyanin and fucoxanthin, as highlighted in the study by Nur et al. [83]. Co-cultivating diatoms with green algae in POME medium under mixotrophic conditions has shown promise in promoting the content of fucoxanthin and PUFAs, as demonstrated in the research conducted by Nur [86].

nuThe utilization of microalgae biomass as ruminant feed for animals due to its high protein content presents an opportunity to address the economic strain by 67.11% in effluent treatment, as reported by Chia et al. [2]. Integrating this approach with phytoremediation offers a dual benefit by concurrently addressing effluent treatment and producing valuable biomass. This integration strategy offsets the energy inputs of palm oil industries, offering a sustainable solution that leverages microalgae’s high-protein content for animal feed while aiding in effluent treatment. Moreover, digested POME has been found suitable as a growth medium for heterotrophic algae, facilitating the production of Docosahexaenoic Acid (DHA), a potential alternative to fish fatty oil. Research by Tada et al. [84] has demonstrated that POME-enriched medium supports the robust growth of DHA-producing microalgae, such as the Aurantiochytrium limacinum 4W-1b strain, exhibiting higher growth rates compared to conventional microalgae. This growth medium not only sustains satisfactory algae growth but also facilitates the production of DHA at elevated concentrations.

8. Production of biogas from POME

Biomass containing essential components like carbohydrates, proteins, fats, cellulose, and hemicellulose holds potential as a feedstock for generating biogas. POME abundant in carbohydrates, proteins, and lipids, stands out as an ideal biomass source for biogas production. Moreover, POME has the versatility to be transformed into biodiesel, biobutanol, biohydrogen, and polymers. It also serves as a promising resource for algae-based biorefineries and finds utility in compost production due to its distinctive properties as discussed earlier.

Around 25,000 kilograms of POME is generated during the production of one ton of crude palm oil, yielding approximately 70 cubic meters of biogas [87]. Hence, one ton of POME can potentially produce 28.13 cubic meters of biogas. Considering the calorific value of biogas as 21.5 megajoules per cubic meter, the biogas quantity was estimated at 6 kilowatt-hours per cubic meter [88,89]. However, without proper storage and treatment, biogas production from POME can disrupt the biogeochemical cycle and release substantial amounts of CH4 into the atmosphere. Previous researchers suggest that methanogenic reactions from POME components could yield over 0.8 liters per gram of biogas with a methane concentration exceeding 50% [90].

During the AD process of POME, a series of sequential reactions occur, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis [91,92]. Hydrolysis involves the breakdown of particulate organic compounds or complex molecules like carbohydrates, proteins, and lipids into simpler molecules such as sugars and amino acids. This breakdown is facilitated by extracellular enzymes produced by hydrolytic microorganisms. In acidogenesis, acidogenic bacteria further decompose the hydrolyzed molecules, leading to the production of organic acids and intermediates. These compounds subsequently form acetic acid, carbon dioxide, and hydrogen, along with inorganic compounds like ammonia and hydrogen sulfide. Finally, in methanogenesis, methanogenic bacteria transform the produced acids into methane gas [3].

The POME treatment systems integrated with biogas capture facilities have garnered considerable attention as a complement to the financially unproductive conventional POME treatment methods alone. Table 5 provides a summary of high-rate bioreactor technologies utilized for POME treatment and biogas production. Notably, the most studied POME bioreactor technologies over the last 15 years include UASB, UASFF, CSTR, MAS, and IAAB. In-depth discussions regarding the advantages and drawbacks of these technologies have been presented previously (A [2]; [27, 100, 101] These reviews highlight that due to more advanced control and process systems, several improved high-rate AD bioreactors have been developed or modified from conventional high-rate AD systems. These modifications aim to enhance the efficiency of POME treatment, increase biogas yield, and improve capturing efficiencies within shorter Hydraulic retention times (HRT) and smaller footprints for broader large-scale applications. However, it is important to note that these systems exhibit significant drawbacks, notably high Capital Expenditure (CAPEX), Operational Expenditure (OPEX), and the necessity for skilled workers to operate these relatively advanced processes.

Table 5. Biogas production from POME using different digestion tanks. VS means volatile solids.

Reactor type	HRT (days)	COD removal (%)	Methane yield	References
Two stage UASFF	1 (1^st stage), 1 (2^nd stage)	83	770 ml/g COD	[93]
Two stage CSTR-MECs	2 (1^st stage), 8 (2^nd phase)	91	290 ml/g COD	[94]
Two stage UASB-CSTR	0.38 (1^st stage), 12 (2^nd stage)	85	155.87 ml/g COD	[95]
Two stage ASBR-UASB	2 (1^st stage), 15 (2^nd stage)	84	240.65 ml/g VS	[96]
IAAB	1–4	>90	131 ml/g COD	[97]
UMAS	7.5	96.6–98.4	240–590 ml/g COD	[98]
CSTR	3–3.3	>85	240–343 mg/g COD	[99]

UASFF was considered the most promising technology due to its superior treatment efficiency, high removal of COD, and elevated methane (CH₄) yield at minimal HRT compared to other technologies [101,102]. The latest researcher informed that UASFF with a two-stage bioreactor could remove 83% COD from POME and produce 94% CH4 using 100% raw POME and HRT 4 h [93]. However, other technologies such as CSTR, modified anaerobic sludge (MAS), and integrated anaerobic-aerobic bioreactor (IAAB) provide simple installation and process [103,104].

Additionally, existing biogas bioreactors like UASB, CSTR, and UASFF, underwent further enhancements. Despite its advantages, membrane technology encountered hindrances due to fouling and high operational costs, impeding its widespread commercial application. Some of these technologies successfully underwent scaling up and were tested at pilot plants. The outcomes of pilot plant investigations of IAAB [105], AnEG [4], and CSTR [106] aligned well with findings from bench-scale studies. Among these technologies, only continuous stirred tank reactor (CSTR) and up-flow anaerobic sludge blanket (UASB) are commonly employed for commercial operations. CSTR is recognized for its straightforward design, operational simplicity, and improved mixing rates [5]. Like UASB, its notable advantages include the production of high-quality effluent and CH4 yield [101].

9. Biogas upgrading by using microalgae

Methods for upgrading biogas conventionally involve physicochemical approaches to remove CO₂ and other contaminations such as sulphuric acid [107]. Chemical methods include CO₂ absorption using solvents or mineral carbonation, while physical separation methods encompass membrane separation, pressure swing adsorption, and cryogenic separation, among others [108]. Despite commercialization, these technologies endure significant drawbacks like energy penalties (3–6% of biogas energy content) and high costs (up to 30% of upgraded biogas total cost) [25,107]. Given the urgency of meeting the Paris Agreement’s goals to avert climate change risks European Academies Science Advisory Council (EASAC), negative emission technologies are crucial. Bioenergy with carbon capture and reuse emerges as a solution to lower the CO₂ footprint of biogas systems [109]. Combining biological biogas upgrading with microalgae cultivation to create extra revenue and valuable products seems like a practical and efficient solution. This approach can help capture CO₂ better, balance energy needs, and increase energy production using algal biofuels. Some recent findings related to biogas upgrading by using microalgae can be seen in Table 6.

Table 6. Biogas upgrading from AD method using microalgae.

Microalgae species	Biogas source	Upgraded CH4/biogas	CO2 removal efficiency	Optimal condition	References
Chlorella vulgaris, Ganoderma lucidum, and endophytic bacteria	Biogas pig farm. Purified biogas: 64.21% CH4, 32.78% CO2	80–82%	53–63%	Cultivation: 25 ± 1°C, period of 10 days, intensity of 200 μmol·m − 2·s − 1, and 12 h light/12 h dark+ strigolactone analogs	[110]
Consortia of microalgae and bacteria	WWTP: 63.7% CH4, 33.7% CO2, 0.45% O2 and 1.59% N2	91%	97%	Supplementation of NaHCO3 and Na2CO3 in the cultivation at HRAP outdoor condition.	[111]
Chlorella vulgaris + activated sludge	Anaerobic digester plant. Desulfurization: 62.87% CH4, 33.62% CO2	88–89%	>60%	0.74 g/L initial inoculum of microalgae and 3.74 g/L TSS activated sludge + strigolactone analogs	[112]
Chlorella vulgaris, Stigeoclonium tenue, Nitzschia closterium and Navicula amphora,	AD microalgae biomass: (13.7%), H2S (0.1%) and CH4 (86.2%).	94.1%–98.8%	>98%	Biogas flow rate of 100 L/d, L/G ratio 0.5 and the supplementation of 2.0 L d^−1 of carbonate solution to the absorption column	[113]
Scenedesmus obliquus FACHB 416 with fungus Ganoderma lucidum 5.765	farm biogas plant. Desulfurised: 67% CH4, 29% CO2, 2.95% H2O, 0.21% O2, <0.005% H2S	>90% CH4	79.92%	initial inoculum 120 mg/L, 25°C, 200 μmol/m^2/s, mixed LED light red:blue as 5:5, 14 h:10 h light/dark cycle	[114]
Chlorella vulgaris and nitrifying-denitrifying activated sludge	Pretreated anaerobic digester: 64% CH4, 31% CO2, 3.15% H2O, 0.54% O2, <0.005% H2S	92.58% CH4	66.93–88.27%	initial inoculum 113 mg/L, 25°C, 200 μmol/m^2/s, mixed LED light red:blue as 5:5, 16 h:8 h light/dark cycle	[115]
Chlorella sorokiniana	Raw biogas from in-house diluted wine treating anaerobic reactor with 65% CH4, 32% CO2	89–93%	>90% CH4	M-8a medium, ambient temperature, light intensity of μmol/m^2/s, light dark cycle of 12 h:12 h, continuous culture with dilution rate 0.1 day^−1	[116]
Chlorella vulgaris FAHCB31 and Ganoderma lucidum	synthetic domestic sewage with desulfurisation: 67.6% CH4, 28.4% CO2, 3.54% H2O, 0.47% O2, <0.005% H2S	93.25% CH4	65–84%	influent C/N ratio of 5/1, 25°C, 200 μmol/m^2/s, 160 rpm, 7 days	[117]
Scenedesmus sp.	Raw biogas from swine WWTP with 70.7% CH4, 26.1% CO2, 0.23% O2, 1550ppmv H2S	50.4% CH4	N.A.	initial biomass 30% v/v, 22 ± 2°C, red light at 630 nm, 148.5 μmol/m^2/s, light/dark cycle 12 h:12 h	[118]
Chlorella vulgaris FAHCB 31	Desulfurized biogas from piggery wastewater with 61.38% CH4, 32.57% CO2, 5.52% H2O, 0.54% O2, <0.005% H2S	83.46% CH4	N.A.	initial biomass of 0.068 g/L, 25°C, light intensity 150 μmol/m^2/s, light dark cycle as 14 h:10 h, mixing by shaking the bag thrice a day	[119]
Scenedesmus obliquus FACHB 31	AD treated piggery wastewater. Desulfurisation: 58.67% CH4, 37.54% CO2, 3% H2O, 0.79% O2, <50 ppm H2S	88.25% CH4	78.81%	initial biomass of 153 mg/L, 25°C, light intensity 200 μmol/m^2/s, light dark cycle as 12 h:12 h, mixing by shaking the bag thrice a day	[120]
Chlorella sp.	Desulfurised biogas: Desulfurized raw biogas with 64.21% CH4, 31.38% CO2, 3.79% H2O, 0.68% O2, <50 ppm H2S	93.68% CH4	57.7%	initial biomass of 877.68 mg dry weight, 25°C, light intensity 800 μmol/m^2/s, light dark cycle as 12 h:12 h, mixed LED light with red:blue at 5:5, mixing by shaking the bag thrice a day	[121]
Chlorella vulgaris	Raw biogas from POME: 77% CH4, 22% CO2	98% CH4	>98%	4800 lux, photobioreactor 0.9 m^3	Handayani et al. [122],

Maintaining low oxygen levels is crucial to prevent a potentially explosive environment in commercial biogas upgrading systems. These systems need to operate continuously, keeping CH₄ concentrations above 97% to meet the standard [123]. However, challenges persist, including the generation of oxygen during photosynthesis, which elevates O₂ levels in the upgraded biomethane, posing a major hurdle in this biological biogas upgrading process. Additionally, issues such as low CO₂ transfer, inadequate control of process parameters (like gas and liquid flow rates, and pH), diurnal fluctuations in operations due to photo-autotrophy, and intermittent functioning due to seasonal temperature changes affecting microalgae growth, present significant challenges [109].

Most microalgae selected for biogas upgrading can withstand typical methane (CH₄) levels in biogas, although the reasons for biological methane consumption by microalgae remain unclear [118,124]. Nannochloropsis gaditana CCMP 567 (wild type) demonstrated no impact on biomass concentrations and growth rates when cultivated in methane concentrations of 0%, 50%, and 100% [125]. Three microalgal strains—C. protothecoides TISTR 8243, Chlorella sp. TISTR 8263, and marine Chlorella sp.—showed adaptability in the presence of 50% CH₄ and 50% CO₂ resembling biogas composition, with the marine Chlorella sp. exhibiting robust growth and CO₂ removal [126]. Additionally, a mutant microalga such as Chlorella sp. MM-2 and Chlorella sp. MB-9 developed by random mutagenesis displayed resilience to 80% CH₄ and demonstrated biomass productivity [127]

Hydrogen sulfide (H₂S), another crucial component in biogas, poses challenges for microalgal growth due to its effect on pH levels [128]. However, certain strains like Scenedesmus sp. and Spirulina platensis have exhibited tolerance to H₂S up to 3000–5000 ppm, enhancing CO₂ removal [118,129]. Despite the absence of isolated mutant strains resistant to high H₂S levels, most microalgae typically tolerate H₂S concentrations of 50–100 ppm, commonly found in desulfurized biogas (Table 6).

A promising outdoor and scalable condition for biogas upgrading was proposed previously [111]. A 100-liter anaerobic digester connected with a 180-liter high-rate algal pond (HRAP) was optimized across four stages: (I) during winter with a greenhouse; (II) without a greenhouse; (III) using NaHCO₃ supplementation; (IV) using Na₂CO₃ supplementation. The produced biogas consisted of 63.7 ± 2.9% CH₄, 33.7 ± 1.9% CO₂, 0.5 ± 0.3% O₂, and 1.6 ± 1.1% N₂. The digester showed an average methane production of 324.7 ± 75.8 mL CH₄ g VSin − 1 and removed 48 ± 20% of total COD. The CH₄ content in the biomethane decreased to 87.6 ± 2.0% and 85.1 ± 1.3% in stages I and II due to inorganic carbon loss in the HRAP. NaHCO₃ and Na₂CO₃ supplementation increased CH₄ content to 90.4 ± 1.5% and 91.2 ± 0.7% in stages III and IV. CO₂ removal rates were steady at 90% and 88% in stages I and II, and increased to 95.7% and 97.6% in stages III and IV. Daily harvesting maintained constant biomass productivity at 22 g m − 2 d − 1, removing N and P supplied via central. This setup ensured efficient biogas production and nutrient removal.

10. Integration of phycoremediation and biogas upgrading from POME by microalgae

Many potential microalgae have yet to be used for water pollution treatment, and the key is to find suitable and efficient microalgae production. The use of POME as a growth medium for microalgae at an industrial scale is still challenging despite the high nutrient levels. First off, the presence of a lot of organic compounds like tannins, lignin, and phenolic compounds may hurt growth [47,75,76,130].

High suspended solids concentrations may cause a dark coloration that could prevent light from penetrating, which is essential for the development of photosynthetic organisms [69,75,76]. Before using the wastewater for microalgae, the growth medium, pH, and salinity must be corrected [75,76]. Additionally, in some circumstances, the presence of heavy metals in POME may prevent the use of the bioactive compounds from the algae for pharmaceutical, cosmeceutical, and human consumption. However, the conditions mentioned above could be avoided by using the previously described pre-treatment process to reduce COD, color, and heavy metals in the POME [9]. To increase salinity, the cultivation could be relocated to seashore areas. Furthermore, the wastewater could be combined with hypersaline wastewater produced by industrial activities such as chemical manufacturing, making marine microalgae cultivation more feasible.

The proposed concept involves a holistic approach combining phycoremediation of POME with biogas upgrading through microalgae utilization (Figure 3). Initially, the fresh POME undergoes digestion in a tank, aiming to reduce its COD and BOD, concurrently producing biogas. The effluent resulting from this digestion process, enriched with nutrients due to lowered COD, is then directed into a high-rate algal pond. In this pond, microalgae thrive using the effluent as a growth medium. As the microalgae reach the end of their exponential growth phase, they are moved to an absorption column.

Figure 3. Integration process phycoremediation of POME and biogas upgrading using microalgae. Picture caption high rate of algae pond is the phycoremediation process of POME by microalgae.

In the absorption column, the microalgae are brought into contact with biogas derived from the digested POME. Before entering the absorption column, the biogas undergoes a cleansing process in a desulphurization tank to reduce sulfur levels, enabling the microalgae to effectively absorb carbon dioxide (CO₂) in the absorption column. The outcome of this process is an upgraded form of biogas with increased methane content. Meanwhile, the microalgae that exit the absorption column are harvested for various purposes, including animal feed [131], biofertilizers and biostimulant [132], biopolymers [133], biocement [134] and other valuable compounds [9] (Figure 2). The effluent resulting from the harvested microalgae holds potential applications as a bio-stimulant and for soil remediation within the palm oil plantation area [135]. This integrated approach not only offers a sustainable method for POME treatment but also maximizes resource utilization by extracting value-added products from microalgae while producing upgraded biogas for energy purposes.

11. Future prospect

The combined approach of using microalgae for cleaning POME and upgrading the biogas product presents an innovative solution with a significant beneficial potential. However, several challenges need addressing to fully realize its benefits. Key technical hurdles involve optimizing conditions for microalgae growth, ensuring effective absorption of CO₂, and maintaining consistent biogas quality. Overcoming these challenges will require further research and development efforts tailored to the diverse conditions and compositions found in different palm oil mills.

Looking ahead, the future of this integrated approach relies on improving efficiency and scalability. This includes developing advanced technologies for enhancing microalgae growth and upgrading biogas, as well as creating cost-effective systems for large-scale implementation. Additionally, finding efficient ways to utilize the harvested microalgae and identifying viable markets for the derived products will be crucial for its economic feasibility. Collaboration between researchers, industry players, and policymakers will be vital for optimizing and implementing these integrated systems in palm oil mills, paving the way for sustainable practices and a more environmentally friendly palm oil industry. Future recommendations are regulation and funding for the integrated system in the CPO factory using more efficient harvester and more diverse algae products.

Abbreviations

AD	=	Anaerobic digestion
BOD	=	biological oxygen demand
CAPEX	=	Capital Expenditure
CO2	=	carbon-dioxide
COD	=	chemical oxygen demand
CSTR	=	continuous stirred tank reactor
DHA	=	Docosahexaenoic acid
EPS	=	extracellular polysaccharides
HRAP	=	high-rate algal pond
HRT	=	hydraulic retention time
IAAB	=	integrated anaerobic-aerobic bioreactor
LCA	=	life cycle assessment
MAS	=	membrane anaerobic system
OPEX	=	Operational Expenditure
PHB	=	polyhydroxy butyrate
POME	=	palm oil mill effluents
PUFAs	=	polyunsaturated fatty acids
RES	=	renewable energy source
TN	=	total nitrogen
TP	=	total phosphorus
UASB	=	up-flow anaerobic sludge blanket
UASFF	=	up-flow anaerobic sludge fixed-film
UMAS	=	ultrasonic membrane anaerobic system
UV-C	=	ultra-violet-C
WWTP	=	wastewater treatment plant

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is supported by New Energy and Renewable Energy Funding from the National Research and Innovation Agency (BRIN).