An insight into potential phosphate bioremediation and renewable energy from agricultural waste via integrated wastewater treatment systems in Indonesia

Indonesia is renowned as an agricultural powerhouse, ranking first globally in oil palm production. This prominence in agriculture leads to the consistent generation of agro-industrial waste, notably Palm Oil Mill Effluent (POME). Effectively addressing these waste concerns is important due to their adverse impacts on aquatic ecosystems and the nation’s health and economy. Anthropogenic wastewater with excessive phosphorus content can trigger eutrophication and toxic algal blooms, posing environmental risks and potentially precipitating a future clean water crisis. Thus, a comprehensive approach is necessary to restore the environment and biogeochemical cycles. Treatment efforts involving bioremediation agents aim to recycle organic and inorganic pollutants in the environment. Photosynthetic organisms like plants and microalgae serve as effective bioremediation agents, capable of absorbing excess phosphorus. They can utilize phosphate as an energy source to boost biomass. Integrating these bioremediation agents with bioengineering technology optimizes the treatment efficacy while simultaneously producing valuable biomass for products and bioenergy. This review article explores photosynthetic organisms’ multifunctional role as phosphorus bioremediation agents for wastewater treatment, minimizing environmental pollutant impacts, and providing biomass for fertilizers, polymers, bioplastics, and renewable energy. Furthermore, this study unveils opportunities for future technological advancements in this field.


Introduction
The utilisation of phosphorus (P)-usually in the form of phosphates-to support human activities for growing food has caused various alarming results.Not only does the inefficient system of extraction cause a significant decrease in the natural resources of P, but the process also leads to waste generation that harms the environment.Though Indonesia is a P producer, the country is still one of the top two P importers after India.Most of P and other fertiliser component materials imported into Indonesia are from Russia, Jordan, Canada and Myanmar (Fang et al 2021).Most of the P production in Indonesia has been dependent on natural phosphate rock (Hilman et al 2007, Venkiteshwaran et al 2018).Another alternative source are crucial to fulfil the demand for P. Phosphorus is critical to support the country's agricultural needs and economy (Bunce et al 2018, Slocombe et al 2020).Optimizing agricultural practices, such as precision farming, can improve the efficiency of phosphate use.This includes targeted application of fertilizers based on the specific needs of crops, reducing excess runoff into water bodies.
Most naturally mined phosphate rock is converted into soluble fertilizers and animal feeds, mainly for farming and aquaculture in form of phosphate supplement.However, these P inputs are not used efficiently, leading to excess P becoming part of the farming system.The use of phosphorus (P) is often inefficient due to several reasons.One significant factor is that traditional fertilization practices may result in excessive application of phosphorus-based fertilizers, leading to runoff and leaching of unused phosphorus into water bodies.This excess phosphorus can contribute to eutrophication, disrupting aquatic ecosystems.Additionally, phosphorus applied to soils can become immobilized or fixed by soil minerals, making it less available for plant uptake (Bowler et al 2010, Zakaria et al 2022).Furthermore, phosphorus can form insoluble compounds with other elements in the soil, reducing its effectiveness as a nutrient for plants.Improper management of phosphoruscontaining wastes, such as animal manure and sewage sludge, can also contribute to inefficiencies in phosphorus use (Slocombe et al 2020, Zulfahmi et al 2021).
In Indonesia, phosphate levels were recorded at 10-16 mg L −1 in grey water and 0.4-1.3mg L −1 in mixed wastewater.The overall phosphorus concentration in the country was determined to be 24 mg l −1 in grey water and 3-12 mg l −1 in mixed wastewater.As a result, grey water and black water accounted for 11% and 79%, respectively, of the total phosphorus in domestic wastewater (Widyarani et al 2022).The cumulative effect of these factors contributes to the excess accumulation of phosphorus in agricultural systems, posing environmental risks and sustainability challenges.Efforts to improve the efficiency of phosphorus use in agriculture often involve better management practices, precision agriculture techniques, and the development of more targeted and environmentally friendly fertilizer formulations (Liew et al 2015, Kerstens et al 2016).This excess P can be found in soil, recycled through livestock manure and crop residues, and is vulnerable to loss during rainfall and runoff, eventually entering nearby water bodies (Shukla et al 2017).Some is transported off the farm and becomes part of food products distributed to urban areas.In addition, inorganic P (Pi) is added during food processing and for non-food uses like detergents and fertilizers.Urban areas concentrate P, which is then disposed of as solid waste in landfills, discharged through wastewater, and carried into water bodies through runoff.
Phosphorus (P) in urban sewage primarily comes from human activities, and its main sources include detergents and cleaning products such as household detergents and soaps.Besides that, industrial discharges may release wastewater containing phosphorus through their manufacturing processes (Mohammad et al 2021, Mahmod et al 2023).This can include sectors such as food processing, chemical manufacturing, and other industries where phosphorus-containing compounds are used.In urban areas, stormwater runoff can transport phosphorus from various sources to sewage systems.This runoff may pick up phosphorus from fertilizers, pet waste, and other outdoor sources.However, studies have estimated that 50% to 80% of phosphoru applied to agricultural fields may not be utilized by crops and instead ends up in the environment.The fertilizers may not be taken up by plants and may be lost through runoff, leaching, or immobilization in the soil (Suwarno et al 2014, Slocombe et al 2020).This linear movement of P from mines to oceans is much larger than the closed P cycles in natural ecosystems.Less than 20% of P is typically reclaim, resulting in significant wastage and leakage into water bodies throughout the food production and consumption chain (Slocombe et al 2020, Zakaria et al 2022).According to Schaum (2018) this trend is expected to worsen due to climate change and the increasing global demand for food.Therefore, it is important to protect the phosphate rock reserves and to find solutions for overabundance of phosphate in wastewater treatment systems.
Phosphorus is a vital element necessary for the development of vital cellular components in both aquatic plants and animals.These components include phospholipid cell membranes, DNA, RNA, and adenosine triphosphate (ATP), which plays a critical role in intracellular energy transfer (Xing et al 2021).Apatite, a mineral containing calcium phosphate, constitutes a significant part of bones and teeth, while the form of P commonly found in the diet, phytate, plays a regulatory role in the availability of other dietary minerals such as calcium, magnesium, iron, and zinc (Duodu andApea-Bah 2017, Wagner 2023).Inadequate levels of P can lead to various adverse effects in both plants and animals.These include stunted growth in plants, weakened root systems, blue-green discoloration in leaves, brittle bones in animals, and reduced growth rates in fauna (Ajmera et al 2019).The role of P in the natural world is essential and cannot be substituted with any other element (Bowler et al 2010).
Phosphorus is currently present in excessive quantities in many freshwater ecosystems worldwide, whether in developed or developing regions.This surplus is primarily the result of widespread use of P-rich fertilizers and the direct discharge of untreated or partially treated human waste into water bodies through sewerage systems (Bennett et al 2001, Zakaria et al 2022).In undisturbed natural environments, P is introduced into freshwater systems through inherent natural mechanisms, including the weathering and erosion of bedrock and catchment soil, as well as the input of organic materials like leaf litter and animal excrement from terrestrial surroundings.Typically, P is the nutrient element that is most scarce and conventionally regarded as the primary factor limiting primary production in pristine aquatic ecosystems, in contrast to other essential macro-nutrients like carbon and nitrogen (Correll 1999, Slocombe et al 2020, Zakaria et al 2022) (figure 1).
When P levels are elevated beyond what is natural, a cascade of undesirable consequences can occur.These include the overgrowth of algae and plants, shifts in microbial communities towards the dominance of cyanobacteria and the production of associated cyanotoxins, increased water turbidity, a decline in macrophytes as they are overshadowed by algae, and a decrease in biodiversity (Śliwińska-Wilczewska et al 2018, Eland et al 2019).Furthermore, when algal blooms eventually subside, they can lead to additional ecological issues, such as the decrease of dissolved oxygen levels that caused by the decomposition of organic matter resulting from the cell death of algae, which affecting the aquatic population (Kusumawati and Mangkoedihardjo 2021).The costs of water treatment are elevated to remove algal toxins and decomposition by-products.Additionally, there is a significant decrease in the attractiveness and property values of affected areas due to the presence of unsightly algal blooms, which give rise to 'murky' waters that are perceived as a health risk (Schaum 2018).
To overcome P waste problems, photosynthetic organism such as algae and plants play a crucial role in bioremediation by effectively removing pollutants from wastewater, contributing to the purification of water in treatment plants.In phytoremediation method, specific types of aquatic plants and algae can absorb and store pollutants from water, essentially serving as natural filtration systems.Aquatic plants like duckweed (Lemna spp.) and water hyacinth (Eichhornia crassipes) are widely recognized for their efficacy in wastewater treatment, given their rapid nutrient absorption rates, particularly for nitrogen (N) and phosphorus (P).(Su et al 2019, Rezania et al 2021).Algae, including microalgae and macroalgae, can absorb pollutants such as heavy metals, organic compounds, and nutrients.Wastewater treatment plants often integrate these biological agents into treatment systems, creating constructed wetlands or algae ponds as tertiary treatment stages (Zulfahmi et al 2021).The plants and algae facilitate nutrient removal, reduce suspended solids, and enhance overall water quality through their metabolic activities.This environmentally friendly approach shows the potential of utilizing the inherent abilities of algae and plants to enhance sustainable and effective wastewater treatment procedures (Slocombe et al 2020).
In this present-day setting of anthropogenically-enriched aquatic ecosystems, P is therefore considered as an agent of ecological harm and a threat to human health and well-being, rather than a vital nutrient essential for ecosystem functioning.This review seeks to investigate and suggest methods for reclaiming P from wastewater systems, with a specific focus on the context of Indonesia.The goal is to transform dispersed bioresources into valuable products that can effectively contribute to closing the P cycle and establish a more sustainable management approach for the removal of excessive phosphate from these systems.

Methods
In this review, we explore into the potential of harnessing photosynthetic organisms, including plants and microalgae especially cyanobacteria to address Indonesia's phosphorus problem from agricultural activities such as palm oil cultivation, which generates palm oil mill effluent (POME).Our aim is to provide a comprehensive analysis of utilizing these biological agents for effective phosphate remediation.The study focuses on the integration of photosynthetic organisms within wastewater treatment systems as a solution.We conducted an extensive literature review by gathering information from various reputable sources, including Web of Science, Scopus, ScienceDirect, Google Scholar, ResearchGate, and the University of Otago interlibrary loan program.Subsequently, we meticulously extracted key findings from research articles, books, and book chapters, which we then synthesized in both tabular and textual formats within this review paper.We used these keywords, phosphorus use, phosphorus waste, phosphorus recycling, bioremediation, phytoremediation, microalgae for phosphate uptake, cyanobacteria, wastewater management, waste circular economy.This literature review is based on 157 relevant articles related to the discussion.The selection of these articles is associated with the scope of the discussion as well as quantitative data that can be utilized for analysis and depicting the current conditions.The data underwent rigorous analysis, enabling us to draw meaningful conclusions and formulate recommendations for future research endeavours.

Results and discussion
3.1.Water pollution in Indonesia through agriculture activities 3.1.1.Palm oil industries and phosphorus waste Agricultural activities, including from palm oil industries involving the discharge of high-nutrient compounds are a primary source of water pollution (Mohammad et al 2021).One significant consequence of this pollution is eutrophication, which occurs when significant quantities of nutrients, usually N and P compounds, are discharged into water bodies.Excessive nutrient input leads to heightened primary productivity in aquatic ecosystems (Schaum 2018).Eutrophication can result from both natural processes, such as upwelling, and human activities including the palm oil industry (Ribet et al 2021).During the production time palm oil mill effluent enters streams and rivers, introducing high nutrient loads that can cause bioaccumulation and the formation of toxic algal blooms.These events, in turn, have significant consequences for aquatic ecosystems and impact the quality of drinking water for nearby communities.Moreover, pollutants from these water bodies flow downstream into estuarine and coastal marine ecosystems, further deteriorating these areas.
Indonesia, as a developing country, has reported phosphate data as follows: phosphate concentrations in greywater typically range from 10 to 16 mg L −1 , while in mixed wastewater, they fall between 0.4 to 1.3 mg L −1 .In terms of total P concentration, levels of 24 mg L −1 in greywater and 3 to 12 mg L −1 in mixed wastewater have been noted.Greywater contributes around 11% to total P in domestic wastewater, whereas blackwater accounts for approximately 79%.These concerns carry notable consequences, as they result in elevated nitrogen and phosphate concentrations in aquatic settings, fostering conditions where aquatic plants and algae proliferate, thereby disrupting the balance of the aquatic ecosystem.This overgrowth obstructs sunlight, depletes oxygen levels, and leads to a deterioration in water quality (Kogawa et al 2017).
One potential solution to reduce runoff and mitigate these environmental issues involves the treatment of waste produced by palm oil mills and converting this waste into biogas or other valuable products.This approach could make the mill self-sustainable or even allow it to generate surplus energy for additional economic benefit (Sutherland andRalph 2020, Cheah et al 2023).Palm oil production generates a substantial amount of waste, both on the plantation and in the mill.In the harvest season, there are two primary waste categories: oil palm fronds (OPF) and oil palm trunks (OPT).Furthermore, in the mills where oil is extracted, various waste byproducts are generated, such as pressed fruit fiber (PFF), palm kernel shell (PKS), mesocarp fiber (MF), empty fruit bunches (EFB), oil palm shell (OPS), and palm oil mill effluent (POME) as reported by Dungani et al (2018).Palm oil mill effluent (POME) is a byproduct generated during the palm oil milling process (figure 2).While POME is considered a waste, it has potential applications and can be utilized for various purposes.POME is rich in organic matter, making it a suitable feedstock for anaerobic digestion.Anaerobic digestion of POME in biogas plants can produce biogas, which is primarily composed of methane.This biogas can be used as a renewable energy source for electricity generation or as a fuel for various industrial processes.The solid residue obtained after anaerobic digestion of POME, known as digestate, can be utilized as an organic fertilizer.It contains nutrients that are beneficial for soil, promoting plant growth and enhancing soil fertility (Mohammad et al 2021).
POME contains high concentrations of organic matter such as nitrogen, and phosphorus (table 1).With proper treatment processes, it can be used in biological treatment systems for nutrient removal, contributing to the purification of wastewater.Treated POME can be used for irrigation in oil palm plantations (Azmi and Yunos 2014).Properly treated POME contains fewer pollutants, and when applied in controlled quantities, it can serve as a water and nutrient source for the cultivation of crops.Meanwhile, the solid fraction obtained after the treatment of POME can be composted to produce organic compost.This compost can be used as a soil conditioner and fertilizer in agriculture.POME has been explored as a potential source to produce biosurfactants.Biosurfactants have applications in various industries, including agriculture, cosmetics, and the petroleum industry (Cheah et al 2023).
Most palm oil waste is composed of biodegradable lignocellulosic materials that can be converted into biocomposite (Kamyab et al 2018).Lignocellulosic materials, found in components such as EFB and palm press fiber, are rich in cellulose, hemicellulose, and lignin.These materials are considered biodegradable because microorganisms can break down their complex structures over time (Ren et al 2009).However, the biodegradability and decomposition rate can be influenced by various factors, including treatment methods, environmental conditions, and the specific composition of the waste.In biocomposite form it has potential as a sustainable raw material that can be recycled back into other useful products.POME, which is made up of water, production waste, solid particle sediment, and fiber, is also biodegradable.However, the release of excessive amounts of raw POME is likely a risk to the environment.Kamyab et al (2018) mentioned that the untreated POME contains extremely high levels of substantial quantities of solid sediment with significant macronutrients and micronutrients.The characteristics of POME also encompass parameters such as high macronutrients and micronutrients.More detailed examples can be seen in table 1.These contents could be a threat to the environment by changing the physiochemical properties of soil.Increased pH by exposure of POME also could affect the nutrient availability in the soil.Untreated POME also influences the capacity of soil to hold water and alters the composition of organic compounds (Nmaduka et al 2018).Phytotoxicity of POME caused by the high concentrations of polyphenol also interferes with plant and microorganism metabolism (Nwoko et al 2010,   Anaerobic conditions and high temperatures caused by newly released POME may also reduce the population of microorganisms and alter enzyme activities, however, exposure to old POME may in fact increase the microorganism population as a result of elevated levels of organic compounds (Orji et al 2006, Ibe et al 2014).Organic nutrients in the POME also disturb the aquatic environment by causing algal blooms, leading to a such as high chemical oxygen demand (COD) and biochemical oxygen demand (BOD) becoming a threat to aquatic life (Kamyab et al 2018).To mitigate the negative impact of POME release, various treatments are employed including the common anaerobic or facultative pond series.This method relies on bacteria to break down organic matter in POME into less harmful compounds.However, the method is sensitive to factors like temperature and pH and requires long treatment times as well as large land areas, and these systems produce unpleasant odorous biogas (Ahmad et al 2003, Azmi andYunos 2014).Marleni et al (2020) have highlighted the widespread use of untreated wastewater in developing countries.Globally, approximately 20 million hectares of land are irrigated using untreated wastewater, posing health risks to farmers, vendors, consumers, and contributing significantly to the global burden of diseases associated with excreta-related illnesses.In cases where wastewater treatment plants adopt a resource recovery approach, the primary focus often centers on waste sludge streams resulting from biological treatment processes.These sludge streams, having lower volumes compared to the main wastewater flow and higher concentrations, are conducive to resource recovery with minimal adjustments to the wastewater infrastructure (UNESCO 2020).As for the alternative treatments for POME, it can involve various biotic and abiotic methods.Biotic treatments encompass phytoremediation, closed anaerobic digesters, and activated sludge systems (Hadiyanto et al 2013, Wun 2017).In contrast, abiotic methods, such as evaporation and membrane technology, are employed by the palm oil industry to manage POME waste (Ahmad et al 2003, Azmi and Yunos 2014, Mahmod et al 2023).As per Minister of Environmental and Forestry's Decree No. 28/2003, utilizing wastewater from the palm oil industry in oil palm plantations necessitates compliance with environmental regulations, which entail conducting an Environmental Impact Analysis (AMDAL) study, implementing Environmental Management Efforts (UKL), and conducting Environmental Monitoring Efforts (UPL).Validation procedures may be carried out to evaluate potential pollution, aligning with the wastewater quality standards outlined in the Ministry of Environment andForestry's Regulation No. 5/2014. (Leela andNur 2019).
Organic contaminants in agricultural wastewater can include plant residues, pesticide residues, and organic nutrients.However, the specific organic contaminants and their concentrations can vary depending on the agricultural practices, crops grown, and the use of agricultural inputs.Effective management and treatment of agricultural wastewater, in this case POME, are important to minimize the environmental impact and protect water quality in surrounding (Arun et al 2020).The release of excessive P into surface water bodies from residential, agricultural, and industrial origins leads to substantial water pollution and plays a crucial role in eutrophication (Du et al 2017).Addressing eutrophication potential is a vital component of wastewater management, particularly in developing nations.This study suggests wastewater reuse as a strategy to mitigate eutrophication potential by redirecting high-nutrient loads away from direct discharge into the land and rivers.The excessive release of P into surface water bodies from residential, agricultural, and industrial sources results in significant water pollution and plays a pivotal role in eutrophication (Du et al 2017).Addressing the potential for eutrophication is a crucial aspect of wastewater management, especially in developing countries.Du et al (2017) suggests the reuse of wastewater as a strategy for mitigating the potential for eutrophication by redirecting high-nutrient loads away from direct discharge into the land and rivers.For this reason, we need various alternative solutions to overcome problems related to waste generated from agricultural activities.

Phosphate uptake in microalgae
Photosynthetic microorganisms such as microalgae offer economical and eco-friendly solutions for P remediation.Their self-sustaining nature involves phosphate uptake and storage, and they serve as primary producers in diverse environments from freshwater to harsh conditions like deserts where they form biofilmlike structures.While these microorganisms are highly adaptable, studies have shown that they can dominate in blooms, sometimes releasing toxins into the environment.They efficiently store polyphosphate (PolyP) in granules, known as PolyP bodies, regardless of fluctuating nutrient conditions, making them valuable in addressing P-related environmental challenges (Gomez-Garcia et al 2013, Śliwińska-Wilczewska et al 2018, Sanz-Luque et al 2020).
As noted above, P is essential for various cellular functions, including cell structure, energy production, and genetic information storage and transfer (Blank et al 2012, Wagner 2023).Certain microalgae espesially cyanobacteria are known to employ a competitive adaptation strategy, storing P during periods of high availability to support growth during periods of lower P concentration (Carey et al 2012, Yue et al 2013, Jentzsch et al 2023).In the realm of cyanobacterial research, Synechocystis sp.PCC 6803 cells have been identified as having two phosphate-specific transport (Pst) systems, Pst1 and Pst2, while lacking a phosphate-inorganic transport (Pit) system (Burut-Archanai et al 2011, Lis et al 2019).These transport systems play a significant role in phosphate uptake in cyanobacteria, including the synthesis of PolyP.Many studies have explored the potential of microalgae and cyanobacteria for bioremediation, with additional details provided in table 2.
Polyphosphate is a universally present polymer in all living organisms, serving as a reservoir of chemical energy (Slocombe et al 2020).In algal cells it is stored as granules in acidocalcisomes and in cyanobacterial as granules in the cytosol, influencing various cellular processes.The PolyP acts as a reservoir for inorganic phosphate, aids in metabolism, maintains cation balance, modifies protein functions, and helps cells adapt to stress (Burut-Archanai et al 2013).It also plays roles in symbiotic and parasitic interactions.Reduced PolyP levels in higher organisms can impact cancer, apoptosis, and other critical processes (Sanz-Luque et al 2020).This review delves into the metabolic pathways, storage mechanisms, and roles of PolyP in photosynthetic microorganisms, specifically examining studies conducted on microalgae.
Polyphosphates serve a variety of functions in bacteria and lower eukaryotes, primarily acting as storage molecules for phosphate and energy ( Kornberg et al 1999, Gray and Jakob 2015, Kus et al 2022).Additionally, owing to their negative charge, PolyPs interact with cations such as K + , Ca 2+ , Mg 2+ as highlighted by van Groenestijn et al (1988) and Akbari et al (2021).This interaction plays a crucial role in metal homeostasis and enhances tolerance to heavy metals (Kornberg et al 1999, Andreeva et al 2014, Kulakovskaya 2018).Additionally, PolyPs and the enzymes involved in their synthesis play crucial roles in regulating various vital processes in bacteria.These functions encompass stress response management, virulence control, facilitation of motility, assistance in biofilm formation, and regulation of sporulation (Kornberg et al 1999, Shi et  Powell et al (2009) conducted research on microalgae and cyanobacteria in waste stabilization ponds (WSP) and found that these organisms have the ability to accumulate PolyP.The existence of PolyP can be dynamic in cells.Two terms related to the availability of PolyP inside cells are 'luxury uptake' and 'overplus response' (Powell et al 2009, Da Ros andMansfield 2020).Luxury uptake occurs when there is an abundance of P in the environment.In this situation, cells take up more P than needed for growth, leading to the accumulation of PolyP.Polyphosphate reserves stored within cells serve as a valuable resource to sustain metabolic functions and facilitate growth during periods of P scarcity.Conversely, when cells that have experienced P deficiency are provided with a fresh supply of P, they exhibit an excessive response by promptly absorbing P to synthesize PolyP, thereby aiding in their recuperation from P deficiency (Li and Dittrich 2019).
The luxury uptake of P can be influenced by a range of factors, including temperature, light intensity, CO 2 levels, and phosphate concentration (Da Ros and Mansfield 2020).Research studies investigating luxury uptake have involved the chemical extraction of acid-soluble and acid-insoluble fractions of PolyP within microalgae.Light intensity was found to affect both the accumulation and utilization of acid-soluble PolyP, while temperature primarily influenced the accumulation of acid-insoluble PolyP.Acid-soluble PolyP contributes to metabolic processes and short-term P storage, whereas acid-insoluble PolyP is regarded as a form of long-term P storage (Powell et al 2009, Slocombe et al 2020).In autotrophic metabolism, algae primarily rely on photosynthesis to produce energy from light, carbon dioxide, and water.On the other hand, microalgae with mixotrophic metabolism, involves a combination of photosynthesis and the uptake of organic compounds to meet energy and nutrient requirements (Slocombe et al 2020).
To enhance the efficient absorption of P by microalgae, a concept called a 'luxury uptake pond' has been suggested.In this specialized pond, microalgae would be exposed to conditions with high phosphate concentrations and intense light for a short duration, promoting optimal accumulation of PolyP.Following this accumulation phase, the P-rich microalgae could be harvested and removed from the system (Powell et al 2009).Notably, PolyP also has many potential applications with various usages for medical material, biofertilizer, water softening, the food industry, heavy metal remediation, and building construction.
Recent studies have increasingly recognized the potential of PolyP as a valuable resource that can be reclaimed from wastewater treatment processes (Chu et al 2022).PolyP recovery from wastewater has garnered attention due to its dual benefits-environmental sustainability and the potential for resource extraction.Advances in recovery technologies, such as enhanced biological phosphorus removal (EBPR) and chemical precipitation methods, have shown promise in efficiently extracting PolyP from wastewater streams (Klein et al 2020).In terms of feasibility, PolyP's versatility makes it applicable in various industries.Its role in agriculture as a phosphorus fertilizer has been well-established, and recent research suggests that recovered PolyP from wastewater can be processed and reused in agricultural settings, reducing dependence on traditional phosphate fertilizers.
The ability of PolyP to sequester metals and improve water quality makes it a valuable asset in addressing water treatment challenges.The implementation scale of PolyP recovery is subject to factors such as technological advancements, economic viability, and regulatory frameworks.Pilot projects and small-scale applications have demonstrated the efficacy of PolyP recovery, but widespread adoption may require further research, development, and policy support.Therefore, the recovery of Polyphosphate from wastewater is an evolving area of research with promising implications for circular bioeconomy principles.Its feasibility in diverse applications, ranging from agriculture to water treatment, showcases its potential as a valuable resource.As technology continues to advance and awareness grows, the implementation scale of PolyP recovery is likely to expand, contributing to sustainable practices and reduced reliance on traditional phosphorus sources.

Phosphate uptake by plants
The pursuit of innovative approaches to remove P is a lengthy history through research and development.Conventional approaches for phosphorus removal encompassed biological methods like activated sludge, chemical processes such as iron precipitation, alum ion exchange, and lime utilization, as well as hybrid combinations of chemical and biological techniques.Since years of 2000, it is found that chemical precipitation was just as effective at removing P from water as other organic compounds.A decade later, it was revealed that alum residues could achieve remarkable P removal rates, ranging from 94% to 99%.Over time, these approaches have gained widespread use, with various refinements and considerations for cost-effectiveness.In parallel with these methods, the utilization of plants as photosynthetic organism for P removal has also been investigated (Rezania et al 2021).
Phytoremediation is a method that directly utilizes plants to diminish or eradicate diverse forms of pollutants from the environment (Ojoawo et al 2015, Darajeh et al 2017).This approach has been applied in treating various wastewater types to mitigate pollutants in aquatic ecosystems.In this approach, plants are employed to extract, translocate, and transport pollutants from soil, water, and sediments (Mani and Kumar 2014, Lu et al 2018, Rezania et al 2021).Its advantages, such as distinct operation, affordability, low energy consumption, and dependence on natural processes, render it a viable remediation technique.Phytoremediation is an environmentally friendly and cost-effective method that uses various plant species to remove, stabilize, or degrade pollutants from soil, water, or sediments, including excess phosphate.Plants can remediate excessive phosphate in the environment through uptake and accumulation, adsorption and precipitation, interaction with rhizosphere microorganisms, phytoaccumulation and harvesting, phytoextraction, and enhanced phytoremediation (Schaum 2018).
Plants take up phosphate from the surrounding environment through their root systems.This process is similar to how plants naturally absorb essential nutrients from the soil.Once absorbed, the phosphate is stored in the plant's tissues.Species like water hyacinth and water lettuce are known for their high phosphate uptake capabilities (Slocombe et al 2020).Plants also release substances from their roots or leaves that can chemically react with phosphate ions in the soil or water (Da Ros and Mansfield 2020).This reaction can result in the formation of insoluble compounds or complexes, effectively immobilizing phosphate and reducing its bioavailability.This process helps prevent excessive phosphate from entering water bodies and causing eutrophication.Besides that, plants can influence the composition and activity of microorganisms in their root zone, known as the rhizosphere, and plant microbe interactions incorporating phosphate uptake participate in the formation of mycorrhiza (Schützendübel and Polle 2002).Some of these microorganisms can assist in phosphate remediation by converting soluble phosphate into less mobile forms or by promoting the precipitation of phosphate compounds.In some cases, plants are grown specifically for the purpose of accumulating high levels of phosphate in their tissues (Ahemad 2015, Su et al 2019).
Once these plants have upatake a significant amount of phosphate, they can be harvested and removed from the environment (table 3).This approach is particularly useful for treating contaminated water bodies.In cases of soil contamination, some hyperaccumulator plants can be used to extract excess phosphate from the soil (Ahemad 2015).These plants accumulate phosphate in their above-ground biomass, which can then be harvested and disposed of properly.Many researchers are working on enhancing the phytoremediation potential of plants by using genetic engineering or by selecting plant varieties that have a higher affinity for phosphate or other pollutants (Slocombe et al 2020).Soft-tissue plants have emerged as promising candidates for phosphate removal, offering the advantage of maintaining treatment effectiveness, aesthetic appeal, composting options, and the efficiency of nitrogen and phosphate removal (Martinez et al 2014, Slocombe et al 2020).The rate of dry weight accumulation has been noted to be substantial, attributed to the rapid accumulation of nutrients in plant tissue, reaching approximately 0.2% (Haritash et al 2017).Analysis of different plant's organs have shown that shoots and roots possess the highest storage capacity.In sediment, calcium and magnesium can temporarily bind with phosphate, immobilizing it under alkaline conditions, while iron and aluminium primarily remove phosphate at neutral pH levels (Su et al 2019).pH levels are critical in regulating phosphate availability in aquatic environments.Lowering acidic conditions may promote the solubilization of phosphate, increasing the susceptibility of a water body to eutrophication.Therefore, it is essential to monitor both the overall load and fractionation of phosphate in a water body to accurately assess its trophic status and evaluate the effectiveness of plants in phosphate removal (Haritash et al 2017).
Enhancing habitat quality and boosting contaminant removal efficiency can be achieved by employing a variety of plant species.It's important to highlight that ideal plant species for phytoremediation should demonstrate high rates of bioaccumulation, rapid propagation, and significant above-ground biomass (Rezania et al 2021).In recent years, there has been a growing interest in the removal of P by various aquatic plants.While several review papers have explored topics like nutrient uptake by aquatic plants (Mustafa and Hayder 2021), global P management (Chowdhury et al 2017), and phytoremediation of contaminants in air, water, and soil, there has been a notable absence of specific reviews on P removal by different aquatic plants.The choose of plant species influenced by the method used.For floating treatment, plants that can float on the water surface are required, such as Typha glauca and Lemna minor (Chapman and Boucher 2020), also Eichhornia crassipes and Pistia Stratioes (Sundaralingam and Gnanavelrajah 2014).There are also plants that live in marshy and swampy areas, such as Rumex verticillatus and Rumex orbiculatus (Chapman and Boucher 2020).
Characteristics of plants that contribute to phytoremediation encompass tolerance to potentially toxic metals (PTMs), swift growth rate, ability to accumulate PTMs in their aboveground biomass, robust and extensive root systems, as well as high bioconcentration and translocation factors (Mehmood et al 2022).Phytoremediation involves employing plants to eliminate a broad spectrum of pollutants, such as hydrocarbons, alkanes, phenols, polychlorinated solvents, pesticides, chloroacetamides, explosives, trace elements, toxic heavy metals, metalloids, and landfill leachates.Plant uptake effectively lowers P concentrations in soil solutions, thereby reducing the movement of dissolved P into surface waters.
Quan et al (2016) conducted research on using artificial wetlands to treat urban runoff and remove P.They studied rainfall runoff in Xi'an City, simulating pollutants like ammonia nitrogen and total P.The wetlands, with plants like reed and canna, effectively removed total P (42.23%-60.89%).Wetlands with plants showed better P removal (18.66% contribution) than those without.Phosphorus removal relied on plant and substrate absorption (over 95% of the effect).Longer hydraulic retention time improved P removal but decreased after 7 days due to substrate effects.Canna and reed's net photosynthesis and Total Phosphate (TP) removal rates correlated positively (0.941 and 0.915), as plant photosynthesis played a role in energy and substance synthesis for pollutant removal.In the same vein, Wang et al (2021b) investigated how the introduction of calcium ions to the submerged plant Elodea nuttalli influenced the removal of P from aquatic environments and the subsequent P accumulation within plants in a simulated water-sediment setting.Elodea nuttalli demonstrated efficient absorption of all P forms present in the water throughout its growth phase.Additionally, it was observed that Elodea nuttallii could decrease the levels of various P forms within the sediment.
Meanwhile, Zulfahmi et al (2021) focuses on the phytoremediation of palm oil mill effluent (POME) using water spinach (Ipomoea aquatica Forsk).The chosen plant, water spinach, is investigated for its effectiveness in remedying the environmental impact of POME.The results indicated that water spinach was able to reduce COD, nitrate, phosphate and color as 86.3%, 21.5%, 90.9% and 95.3%, respectively. Sa'at et al (2022) investigates the polishing treatment of POME using phytoremediation with Scirpus grossus.From the result, Scirpus grossus plant was capable to enhance the pollutant removal efficiencies with 91% of SS, 88% of COD, 96% of ammonia, 99% total nitrogen and 56% of total phosphorus.These collective findings highlight the diverse range of plant species that can contribute to efficient nutrient and pollutant removal, providing valuable insights for the development of sustainable and ecologically friendly remediation practices.Table 4 illustrates the advantages and disadvantages of using microorganisms and plants for phosphate remediation.

Recycling of the nutrient from wastewater into biomass and other valuable products
Agricultural wastewater is a nutrient-rich and sustainable source to produce valuable products, for instance, biodegradable bioplastics.Environmental problems have arisen with non-degradable plastic while biodegradable bioplastic usage is still insignificant to the plastics overall in the world (Rahman and Bhoi 2021).There is a trend of usage of bioplastics from polylactic acid (PLA) but it was found to be a non-degradable plastic in marine environments (Andrady 2015).Limitations with PLA applications has led researchers to investigate another class of bioplastics poly-hydroxy-alkanoates (PHAs) that come with better degradation properties.Another variant of the PHA class can be obtained from microorganisms, this is known as poly-hydroxybutyrate (PHB).Figure 3 illustrates the diverse applications of PHB.
The primary method for producing PHB involves fermentation using heterotrophic bacteria, such as Cupriavidus necator or Escherichia coli (Chen 2009, Koch et al 2020).However, this approach requires additional resources.An alternative strategy for PHB production is to utilize photosynthetic microorganisms like cyanobacteria (Akiyama et al 2011, Balaji et al 2013, Rueda et al 2023).Koch et al (2020) discovered that engineered cyanobacteria, specifically Synechocystis sp.PCC 6803, can be optimized to produce PHB under nutrient-limiting conditions, such as nitrogen starvation.When cyanobacteria experience starvation, they enter a state known as chlorosis (Allen andSmith 1969, Neusmann et al 2021).During chlorosis, cyanobacteria break down their photosynthetic machinery and accumulate significant amounts of glycogen as a carbon and energy storage source (Jackson et al 2015, Klotz et al 2016).In the later stages of chlorosis, the cells begin to degrade glycogen and convert it into PHB (Koch et al 2019).
Polyhydroxybutyrate (PHB) is a biodegradable biopolymer that can be produced by certain bacteria as a form of energy storage (Koch et al 2020).The usage of polyhydroxybutyrate has garnered interest in various applications, including bioenergy and biofertilizer production.For biogas production, PHB can be used as a substrate for anaerobic digestion, a process in which microorganisms break down organic materials to produce biogas, mainly methane.During anaerobic digestion, bacteria can metabolize PHB, releasing methane that can be captured and used as a renewable energy source (Koch et al 2019).This process contributes to the sustainable generation of bioenergy from organic waste.Beside that, PHB can also be used as a feedstock for the production of biofuels.Through microbial fermentation processes, PHB can be converted into bio-based fuels such as bioethanol or biodiesel (Slocombe et al 2020).These biofuels can serve as alternatives to conventional fossil fuels, contributing to the reduction of greenhouse gas emissions and promoting a more sustainable energy landscape.
PHB also can be functioned as a soil amendment or a component of biofertilizers.When PHB-containing microorganisms are introduced to the soil, they can enhance soil fertility by promoting nutrient availability and improving soil structure.Some bacteria that produce PHB may also exhibit plant growth-promoting properties.These bacteria can establish symbiotic relationships with plants, promoting root development, enhancing nutrient uptake, and conferring resistance to certain stresses.The biodegradation of PHB releases nutrients Table 4. Advantages and disadvantages of using plants and microalgae as phosphate bioremediation agents.

Organisms
Pros Cons

Plants
• Environmentally Friendly: Phytoremediation is an eco-friendly approach that does not involve the use of chemicals or mechanical processes, reducing the environmental impact of phosphate removal.
• Slow Process: Phytoremediation can be a slow process, especially in large-scale applications, as it relies on the plant physiological condition.
• Cost-Effective: Phytoremediation is often costeffective compared to traditional remediation methods.It requires minimal maintenance once the plants are established.
• Species Selection: Determine the appropriate plant species for a specific remediation method can be challenging, and not all plants are equally effective at phosphate removal.• Sustainable: Many plant species used in phytoremediation are native or adaptable to the local environment, making them a sustainable choice for phosphate removal.
• Limited Depth: Plants are primarily effective at removing phosphates from the upper soil layers or near the water's surface, making them less suitable for deep phosphate contamination.• Aesthetic Benefits: In some cases, phytoremediation can enhance the aesthetic value of the site, as it involves the growth of vegetation.• Biomass Utilization: Harvested plant biomass can be used for various purposes, such as bioenergy production, composting, or as a soil amendment, providing additional benefits.

Microalgae/ Cyanobacteria
• Rapid Growth: Microalgae have high growth rates, allowing for the quick removal of phosphates from water bodies.
• Microalgal Management: Maintaining microalgal cultures can be complex and may require regular monitoring to prevent overgrowth and crashes.• High Phosphate Uptake: Microalgae are efficient at absorbing and accumulating phosphorus from the water column.
• Nutrient Competition: Microalgae can be outcompeted by other microorganisms, leading to limited effectiveness in highly nutrient-rich environments.
• Bioenergy Potential: The harvested microalgal biomass can be used for biofuel production or as a feedstock for other valuable products.
• Harvesting Challenges: The harvesting and separation of microalgae from water can be energy-intensive and costly.• Modularity: Microalgal bioremediation systems can be implemented in various settings, including smallscale and large-scale applications.
• Potential Harmful Algal Blooms: If not managed properly, microalgal bioremediation can lead to harmful algal blooms (HABs), which can have adverse ecological impacts.• Species Selection: Choosing the right microalgal species for a specific remediation project can be crucial, as different species have varying abilities to thrive and remove phosphates.
gradually, serving as a slow-release fertilizer.As a result, PHB-containing biofertilizers have the potential to improve crop productivity and sustainability.This can contribute to better nutrient management in agriculture.Meanwhile, P removal using plants, relies on the natural capacity of plants to absorb, and accumulate P from the surrounding water.Plants uptake P and other nutrients from the water, helping to reduce nutrient concentrations.This is especially important in eutrophic water bodies where excess P can lead to harmful algal blooms and water quality degradation.Plants can be integrated into constructed wetlands or wastewater treatment ponds to enhance the removal of P from municipal and industrial wastewater.Phosphate uptake by plants and the subsequent use of plant biomass for biogas production is an interesting and sustainable approach that can contribute to both nutrient removal and renewable energy generation (Bunce et al 2018, Mehmood et al 2022).
Plants can play a significant role in the purification of eutrophic water.Su et al (2019) conducted a study to assess the effectiveness of nine aquatic plant species in removing total nitrogen (TN) and total phosphorus (TP) from wastewater.The findings indicated that aquatic plant species, whether alone or in combinations, made significant contributions to water purification within a timeframe of 36 to 46 days.Importantly, the biomass of these plants increased under different pollutant conditions, showcasing their robust ecological adaptability to nitrogen and phosphorus nutrient stress.Their study also indicated that plants had a more significant impact on TP removal compared to TN removal, with TP removal rates ranging from 21% to 91%.The initial six days witnessed higher TN and TP removal rates for all nine plant species in eutrophic water.This phenomenon was attributed to the initial damage to the newly transplanted plants' roots, which subsequently stimulated new root growth.Consequently, plants with high TP uptake capabilities hold significant promise for phytoremediation efforts.
In terms of nutrient accumulation, Ipomoea aquatica and Eleocharis plantagineiformis were found to have the most effective purification effect for TP and TN, respectively.Water bodies with high pollutant concentrations would benefit from a combination of Salvinia natans, E. plantagineiformis, and Hydrocotyle vulgaris for treatment.The influence of P availability on growth is a result of its impact on physiological processes, with a significant emphasis on photosynthetic biochemistry.Phosphorus serves as a crucial plant nutrient found in various plant components, such as membranes, nucleic acids, and energy molecules.It plays a vital role in multiple physiological processes, encompassing light reactions and the Calvin-Benson cycle within photosynthesis (Trial et al 2021).Efficient uptake of nitrogen (N) and P could serve as a complementary factor in achieving nutrient reduction goals, in conjunction with substantial biomass production and the recovery of valuable elements such as bio-fertilizer, thereby reinforcing efforts to promote a circular economy (Cheah et al 2023).
3.3.Model systems of phosphorus removal 3.3.1.Enhanced biological phosphorus removal Most of the current practices of P removal in wastewater are using chemical treatment through precipitation with aluminium salts and iron or via the biological process.Hirota et al (2010) suggested P recovery be applied in municipal sewage treatment facilities throughout Japan, where they managed to release PolyP from sewage with a simple heating procedure.Biological P removal involves enhanced biological P removal (EBPR) with PolyP accumulating organisms (PAOs) under anaerobic and aerobic conditions plus organic carbon (Mbamba et al 2019, Wang et al 2021a).Alternatively, photosynthetic organisms like microalgae have been explored for P removal.They assimilate nitrogen and P without organic carbon for organism with mixotrophic metabolism, and microalgae biomass can be used as fertilizer (Slocombe et al 2020).EBPR relies on sludge microorganisms accumulating PolyP.In anaerobic phases, these organisms take up organic compounds and store them as biopolymers like PHAs and glycogen.PolyP provides energy, releasing phosphate.PHAs supply energy and carbon for phosphate uptake during aerobic stages, and glycogen aids organic uptake in anaerobic conditions (Lemos et al 1998, Mino et al 1998, Janpum et al 2022).
3.3.2.High-rate algal ponds (HRAP) HRAP, originally designed in the late 1950s to foster microalgae growth, is a shallow, raceway-style pond used for wastewater treatment and resource recovery in the form of algal biomass (Oswald and Golueke 1960, Craggs et al 2015, Sutherland and Ralph 2020).Despite their global use in managing agricultural and industrial waste, conventional pond systems are still prevalent in many wastewater treatment plants.These ponds rely on microalgal photosynthesis and growth to absorb nutrients and supply oxygen for aerobic bacterial decomposition of organics.However, they often yield inconsistent effluent quality and limited nutrient and pathogen removal due to low microalgal activity (Park et al 2013, Park et al 2018).To address the growing need for reduced nutrient discharge, HRAPs have gained renewed attention for their ability to provide more effective and consistent wastewater treatment, producing higher-quality effluent (Sutherland et al 2014).The microalgae in HRAP could be replaced with other photosynthetic microorganisms including cyanobacteria.For example, using the cyanobacterium Arthrospira platensis, Holanda et al (2021) reported the removal of P from effluent in shrimp biofloc systems could reach up to 90%.Similar observations have also been made using Synechocystis sp.PCC 6803 (Burut-Archanai et al 2013, Asih et al 2022).Moreover, HRAPs offer resource recovery through microalgal biomass, which can be used as fertilizer, protein-rich feed, or as the source of biomass rich in carbohydrates or lipids by adjusting these conditions of environmental condition in cultivating system and the species used.for biofuel production.While HRAPs alone are not economically viable for biofuel production, their integration with wastewater treatment makes this approach financially feasible (Rawat et al 2011, Cheah et al 2023).

Photobioreactors (PBRs)
A Photobioreactor (PBR) is described as a container, whether open, closed, or semi-closed, constructed from transparent and waterproof materials.Its purpose is to create an optimal growth environment for photosynthetic microorganisms, as outlined by Ting et al (2017).In contrast to natural systems relying on sunlight and weather conditions, the PBR system efficiently utilizes space by extending vertically in threedimensional space with greater depth.The treatment process in natural systems is often slow and variable due to dependence on climatic factors.However, the PBR system, equipped with artificial illumination and temperature control, ensures a more consistent and expedited treatment process, offering enhanced control over parameters.Open systems are prone to contamination, evaporation losses, and limited control over species selection, while the PBR system provides comprehensive control over species, losses, and contamination, making it well-suited for achieving desired outcomes in a shorter timeframe (Ashok et al 2019).Additionally, artificial environments supporting biomass growth, such as light, temperature, nutrients, and mixing, can be meticulously controlled in PBRs.Notably, the growth of algae in PBRs is characterized by being rapid, stable, and predictable, as highlighted by Ngo et al (2019).
Closed Photobioreactors (PBRs) commonly employed today encompass various designs such as flat panel, vertical tube (including bubble column and airlift configurations), horizontal tube, stirred tank, and their modified forms.The classification of PBRs is based on their configurations, including flat plate, column, tubular, and soft-frame, as indicated by Ngo et al (2019).Addressing Low-Fluence Light (LFL) issues often requires the combination of two or more methods.In this study, a novel approach is being explored, involving the synergistic use of photobiodegradation and adsorption processes facilitated by microalgae and powder-activated carbon (PAC), respectively, for LFL treatment.The combined treatment system combines the functions of photosynthetic microalgae, Chlorella vulgaris, and adsorption, offering a promising symbiotic blend for the simultaneous elimination of nutrients (ammonium and phosphate) and organic pollutants.The Adsorption-Photobioreactor (APBR) showcased enhanced microalgae growth rates in comparison to the traditional PBR treatment.Maximum removal levels for COD, TN, TP, and UV254 in the PBR treatment were 15.2%, 65.4%, 55.8%, and 7.8%, respectively.Conversely, the application of the APBR treatment yielded significantly higher removal percentages, with values of 47.9%, 87.7%, 75.2%, and 34.3%, respectively.Additionally, the addition of PAC helped maintain the pH within the natural range in the photobioreactor (Ganjian et al 2017).
While numerous PBRs are designed for the pure cultivation of microalgae using culture medium, specialized PBRs tailored for wastewater treatment remain scarce (Ting et al 2017).Wastewater composition is complex, leading to potentially different photosynthetic effects in microalgae compared to those grown in a controlled cultivation medium.Consequently, there is a need to redesign and enhance PBRs for wastewater treatment, building upon existing PBR designs tailored for microalgae pure cultivation.

Recirculating aquaculture system
Recirculating aquaculture systems (RAS) have garnered favor in contemporary aquaculture due to their capacity for increased production yield, enhanced water quality, and their commitment to environmentally sustainable practices.In conventional intensive aquaculture systems, wastewater is known to accumulate high nutrient concentrations, notably P, often reaching levels as elevated as 20 mg P L −1 (Burut-Archanai et al 2013).Consequently, effective P removal from aquaculture wastewater is of paramount importance and necessitates rigorous monitoring.While most RASs currently maintain around a 5% daily water exchange rate, this does not categorize them as true closed systems.To augment recirculation efficiency and curtail water exchange to less than 1%, advanced treatment methods such as denitrification and phosphate removal become imperative in order to mitigate the buildup of nitrate and phosphate.These advanced treatments are especially critical for marine RASs situated in proximity to seawater sources, or for specialized systems like pathogen-free, biosecure broodstock culture systems.Nitrogen removal in RASs involves a blend of nitrification and denitrification processes (Gelfand et al 2003, Wiesmann 2005, Neori et al 2007, Li et al 2023).Burut-Archanai et al (2013) conducted research involving engineered cyanobacteria in RASs, achieving significant reductions in P levels from 8 mg P L −1 to below the P detection limit of 0.01 mg P L −1 .It is worth noting that while P toxicity is lower compared to ammonia or nitrite-nitrogen (Epifanio and Srna 1975), the indirect consequences of eutrophication can still pose threats to aquatic ecosystems (Abeysinghe et al 1996).

Phosphorus removal pathways in water stabilization ponds
The system in water stabilization ponds (WSP) transforms dissolved P into solid forms via biological assimilation, adsorption, and precipitation.Effective phosphorus removal in water stabilization ponds often involves a combination of multiple pathways, including physical, chemical, and biological processes (Mahapatra et al 2022).The synergistic interaction of these pathways enhances the overall efficiency of reducing phosphorus concentrations in wastewater.To remove P from wastewater, it's essential to harvest and extract these solid forms (Marleni et al 2020).Nonetheless, a significant number of WSPs lack specialized harvesting mechanisms and instead rely on natural sedimentation within the ponds to separate and remove P solids.This practice leads to the buildup of P compounds within the anaerobic sludge layer at the pond's bottom, which is seldom extracted.Consequently, the interplay between the sludge and the liquid above it plays a pivotal role in the removal of P in WSPs (Eland et al 2019).
Powel et al (2011) observed in full-scale WSP systems, the sludge layers release P from 3 to 12.4 μg P per g total soluble solids (TSS) per day with overall P removal efficiency in WSP systems being generally low and variable, ranging from 15% to 50%.While the exact cause of this variability remains uncertain in the literature, it is suggested that algal P uptake is the dominant P removal mechanism in WSPs (Larsdotter et al 2007, Powell et al 2011, Sells et al 2018).The following section provides a more detailed examination of cyanobacteria and plant for P uptake.

Suggestions for phosphorus remediation via integrated wastewater system
We have delineated an integrated system design in which agricultural wastewater treatment employs anaerobic fermentation, notably through a biodigester, synergistically coupled with an open HRAP infused and a maturation pond with aquatic plant as photosynthetic bioremediation agents (figure 4).Indonesia features a consistently warm and moist climate, characterized by minimal temperature fluctuations year-round.Coastal zones generally have temperatures ranging from 77 °F (25 °C) to 95 °F (35 °C), while inland areas can experience even higher temperatures, which would be ideal for the thriving of photosynthetic microorganisms in HRAPs.Besides that, the utilization of plants to uptake phosphate from the environment also has two advantageous aspects.Plants that possess phosphate hyperaccumulator capabilities can utilize the existing phosphate to enhance their biomass.Consequently, the biomass of these plants can then be utilized and degraded for use in a biodigester that can generate biogas.The residual material left after anaerobic digestion, known as digestate, is rich in nutrients, including P. This digestate can be further processed and used as a nutrient-rich fertilizer.The anaerobic digestion process within the biodigester yields a trifecta of outputs: digestate, biogas, and a liquid effluent, the latter of which retains utility for subsequent P remediation within the HRAP, yielding valuable products such as PolyP and PHB.Within the HRAP, we envision the deliberate inoculation of select photosynthetic microorganism and aquatic plant candidates, all of which can be subject to bioengineering methodologies.
It is incumbent upon us to implement a stringent biocontainment strategy, assuring the safety and precision of these bioengineered microorganisms, thereby ensuring the consistent production of the desired valuable products.Moreover, the integrated wastewater system has the potential to harness CO 2 emissions from industrial sources, satiating the photosynthetic requisites of the system while concurrently mitigating CO 2 emissions across Indonesia.This innovative approach is poised to extend its accessibility to small farming communities residing in remote Indonesian regions, owing to its practicality and cost-effectiveness.The intricacies of the P remediation scheme are elucidated in the subsequent illustration (figure 4).The current focus is on implementing a more sustainable approach within the circular bioeconomy framework, with a specific transition targeted at the palm oil industry (Cheah et al 2023).This concept prioritizes resource efficiency and waste reduction, aiming to achieve cost-effective production, alleviate adverse environmental effects, and preserve valuable resources.
Palm Oil Mill Effluent (POME) stands out as the primary waste product generated by palm oil mills.At present, the predominant method for treating POME involves anaerobic digestion in a sequence of open anaerobic ponds, followed by the application of the treated POME to the land.Given the substantial volume of POME and its elevated organic content, there is a significant risk of environmental pollution if proper treatment measures are not in place.In Indonesia, the treatment of Palm Oil Mill Effluent (POME) has predominantly utilized anaerobic digesters.However, a combination with the utilization of bioremediation through microalgae and plants has not been explored.Apart from that, Indonesia itself also has a diversity of indigenous species of microalgae and plants that have the potential to be used as bioremediation agents.The method we consider most feasible and challenging to implement in the Indonesian context is the use of a modified High-Rate Algal Pond (HRAP) combined with the utilization of aquatic plants in the maturation pond.The biomass from these bioremediation agents can then be harnessed as a source of biomass and biogas, which can be further degraded in a biodigester.Therefore, the effective utilization of POME has the potential to yield valuable materials or energy, playing a crucial role in promoting the sustainability of both palm oil plantations and palm oil mills.

Conclusion
The utilization of photosynthetic bioremediation techniques presents an innovative solution for addressing Indonesia's pressing agricultural waste challenges.As the foremost global producer of palm oil, Indonesia contends with a substantial waste burden, primarily in the form of POME, which can be ingeniously repurposed within this framework.Bioremediation, a transformative process, relies upon the strategic deployment of photosynthetic organisms, including microalgae, cyanobacteria, and higher plants.This process not only ameliorates waste but also yields valuable byproducts such as PolyP and PHB.This intricate remediation endeavor is a delicate interplay of various factors, where temperature, light intensity, CO 2 levels, and phosphate concentrations exert profound influence.From this crucible of reclamation emerges PolyP and PHB materials, poised to serve diverse purposes, including fertilizers, biodegradable plastics, construction materials, medicinal compounds, and water purification agents.
Furthermore, the narrative extends to the realm of resilient plants endowed with the ability to sequester phosphate, efficiently utilizing it as a nutrient source to augment their biomass.Building upon this foundation, we propose a conceptual design that integrates a waste treatment system, harnessing the power of a biodigester in tandem with a HRAP and aquatic flora.Through this intricate orchestration, the alchemy of agricultural waste transformation into utilitarian assets is realized, simultaneously affording respite to environmental challenges.Indeed, the potential for harnessing microalgae and plants as stalwarts of bioremediation within agricultural waste treatment resonates with the call for environmental sustainability.In this symphony of bioresources, we discern the harmonious notes of alternative biomass, renewable energy, and the virtuous circle of P reincorporation into the natural milieu in the form of PolyP, culminating in the creation of a pristine and salubrious environment.

Figure 2 .
Figure 2. The palm oil production process and the resulting effluent.
al 2004, Recalde et al 2021, Neville et al 2022).In certain bacterial species, PolyPs are vital for survival during the stationary phase and act as a safeguard against cytoplasmic alkalization (Kornberg et al 1999, Müller et al 2019).Additionally, PolyPs serve as chaperones (Gray et al 2014) and play a role in membrane ion channels in conjunction with PHB and calcium ions, as discussed (Das et al 2002, Elustondo et al 2016).

Figure 4 .
Figure 4. Schematic diagram of Integrated wastewater system for phosphorus remediation.

Table 1 .
Raw POME and characteristics and regulatory discharge limit.
* All values in mg L −1 except for temperature (°C) and pH and taken from (Department of Environment Ministry of Natural Resources, Environment and Climate Change 1982, Alhaji et al 2016, Zainal et al 2017).
(Darajeh et al 2017, Ubani et al 2017)been observed that POME can hinder the germination of seeds and the initial growth stages of plants(Darajeh et al 2017, Ubani et al 2017).

Table 3 .
Research on plants as phosphate bioremediation agents in several systems.