Operational Design Considerations for Phosphorus Adsorption Media (PAM)

Abstract

Phosphorus Adsorption Media (PAM) is an emerging technology used to remove phosphorus from water and has the advantage of minimal operation and maintenance support when compared to biological and chemical treatments. Although the capacity of PAM has been researched, the understanding of important design parameters for PAM is lacking. Therefore, this study focused on determining critical design parameters for PAM, such as hydraulic loading, Empty Bed Contact Time (EBCT), and its impact on the media’s capacity. In addition, the regeneration potential of PAM and the mathematical model for predicting the exhaustion of PAM are discussed to provide a practice tool for designing PAM. The results indicate that hydraulic loadings do not show a strong effect on PAM performance, as there are no significant differences between hydraulic loadings of 0.05, 0.12, and 0.22 mL/min/cm². This study also showed that the higher EBCT (190 min) has higher removal rates than the lower EBCT (60 and 90 min). This indicated that EBCT is a critical design parameter for PAM. Laboratory studies demonstrating the regeneration of exhausted media by washing with a caustic solution have been conducted, and a qualitative study showed that exhausted media can be used in hydroponics. Batch testing showed that over 99% of the sorbed phosphorus was eliminated after six cycles of the regeneration process.

1. Introduction

Phosphorus fertilizer is key to modern intensified agricultural production and the global annual consumption of phosphorus fertilizer increased from 43.7 million tonnes in 2015 to 48 million tons in 2022 [1]. Phosphorus fertilizer is largely produced by mining non-renewable phosphate rocks [2]. The majority of phosphorus rock reserves are found in Morocco, China, Algeria, Syria, Jordan, South Africa, the United States, Russia, Peru, and Saudi Arabia [3]. At the current rate, existing phosphate rock reserves are predicted to be exhausted within 40–140 years [4]. Phosphorus utilization efficiencies, a ratio of the mass of harvested P in crop production to the mass of total P inputs in the system, in most countries are below 20% [4,5,6]. Phosphorus exists both in scarcity and excess. Excess phosphorus can cause significant environmental and health problems. An estimated 50% of United States rivers and streams have excessive levels of phosphorus [7]. Sources include the agricultural application of manure and synthetic fertilizers (72% combined), wastewater treatment plant effluent (5%), and onsite wastewater treatment systems (4–25%) [8]. Therefore, developing and implementing phosphorus efficiency and recovery technologies, the focus of this manuscript, is urgent.

Phosphorus is not volatile. Therefore, its removal from wastewater is unlike that of nitrogen, which can be microbiologically converted to nitrogen gas that escapes passively into the atmosphere. Well-established phosphorus treatment options at municipal wastewater reclamation facilities include biological and chemical precipitation systems. Biological phosphorus removal entails microbial uptake in an anaerobic/aerobic sequencing system, with phosphorus accumulation in the biomass and removal with the excess biosolids. In a chemical system, phosphorus is precipitated with metal salts, such as iron or aluminum, or combined with magnesium and ammonium to produce struvite. As with biological systems, the precipitated phosphorus complex must be physically removed from the wastewater.

An onsite treatment system is one in which wastewater is treated at or near the site where it is produced. The simplest type is a septic tank followed by a drain field. The septic tank allows for the settling of heavier particles and the flotation of fats, oils, and grease. A minimum amount of biodegradation of soluble material may also occur. Septic tank effluent is distributed in a drain field and treated as it percolates through unsaturated soil beneath the trench. State regulation dictates the minimum depth of this unsaturated soil. Biological, physical, and chemical mechanisms remove pollutants from the water. The mechanism of phosphorus removal from septic tank effluent is primarily by sorption to soil. When the sorption capacity of the soil is exceeded, phosphorus is no longer retained. The removal of phosphorus from wastewater before soil application by established methods at wastewater treatment plants is challenging for onsite wastewater applications because of the need for chemical addition and/or routine operation and maintenance. An emerging technology for soluble phosphorus management is the use of sorption media. The primary mechanism driving the adsorption process is the chemisorption interaction between the metals and phosphate [9]. Adsorption kinetics are complex, requiring diffusion from the bulk liquid into the moisture layer around the media, to the surface of the adsorption media, and then actual adsorption onto the media. The rate limiting step determines the overall time for adsorption to occur [10]. Phosphorus removal through adsorption can be influenced by the chemical complexity of the aqueous environment, particularly the presence of competing anions, including NO₃⁻ and HCO₃⁻ [11]. This technology is particularly advantageous for applications that have minimal operation and maintenance support.

Table 1. Summary of PAM.

Category	General Characteristic	Media Name	Manufacturer	Capacity (mg/g)	HRT (min)	References	Comments
Manufactured Nanomaterial	Nano-Coated Ceramic	PO4Sponge	MetaMateria	50	60–90	[12,13,14]	Limited production capacity
	Nano-Coated Resin	BioPhree	Aquacare (Netherlands)	2.5	5	[15,16]	Highly researched and many variations but trials with actual wastewater show substantial interference with capacity. Research to improve selectivity is ongoing
		FerrIXA33E/Lewatit	Purolite (USA)/LanXess (Norway)	3	N/A	[17]
		LayneRT	REPCO)	2.5	17.6	[10,18,19,20]
		PhosX	Hagen (USA and Europe)	0.01	8.5	[15]
Manufactured Natural Material	Activated Aluminum	ActiGuard AAFS50	Axens (Alcan)	13	17.5	[21]	Established and well-studied
		Compalox	Huber/Martin-werk (UK and other)	10.4	N/A	[22,23]
	Torrefied Biomass	Biochar		30	N/A	[24]	Emerging
	Iron/Manganese Oxy-Hydroxide	AquAsZero	Laufaksis Chemical	10.4	44	[22]	No longer appears to be available
	Metal Oxide	FerroSorp AW	HeGo Biotech (Germany)	3.1	N/A	[10,19]
	Expanded Clay	Filtralite Nature P	Filtralite	0.3	N/A	[25]
	Ferric Hydroxide	GFH	GEH Wasserchemi Osnabruck (Germany)	4.2	N/A	[19,23]	Established and well-studied
	Rock Opoka Thermal Treated	Polonite Nordic	Bioptech	7	N/A	[26]	Not recommended by Coffey Group

N/A: not available.

provides a summary of phosphorus sorption media (PAM) that has been studied previously. In Table 1, PAM is divided into manufactured nanomaterials, manufactured natural materials, and sized natural and waste materials. These are listed in order of the highest to lowest cost per gram of PAM, although this may not be the order regarding the cost per unit of phosphorus removed from wastewater. PAMs in the two manufactured categories are generally considered regenerable once dissolved phosphorus breakthrough occurs in the flowing wastewater. In each category, PAMs are listed that are or have been commercially available.

Although the capacity of many PAMs has been studied, the understanding of critical parameters for designing PAM is lacking. The critical parameters include the geometry of reactor systems and PAM contact time. In addition, guidance on the evaluation of PAM adsorption capacity and a mathematical model for prediction are needed to design the PAM system effectively and economically. This paper focuses on the following:

(1)

Determining important design parameters for PAM. Included are the impact of hydraulic loading, empty bed contact time (EBCT), and the impact of EBCT on the media’s capacity;

(2)

Exploring options to regenerate the media;

(3)

Utilizing a mathematical model to predict the exhaustion of PAM;

(4)

Providing a PAM design approach.

For this research, PO4Sponge was selected as a representative manufactured nano-material that has been proven effective for both synthetic and actual wastewater. However, the protocol and need to determine the above-listed factors in designing a PAM system apply to any type of media. By focusing on both performance and practical implementation, this research advances the field of phosphorus recovery and offers actionable insights for sustainable nutrient management in decentralized systems.

2. Materials and Methods

2.1. Experimental Design

Laboratory reactors were used to evaluate the design parameters and adsorption capacity for PAM through multiple phases. The phases include evaluation of the impacts of hydraulic loading and EBCT on PAM’s phosphorus removal, the estimation of PAM adsorption capacity, the assessment of potential PAM regeneration/recovery through its use in hydroponics, and prediction of the exhaustion time and adsorption process for a given adsorbate concentration through a mathematical model.

2.1.1. Phosphorus Adsorption Media

The representative media used in this research was PO4Sponge, manufactured by MetaMateria Technologies, LLC. (Columbus, OH, USA). However, there are several other commercially available products in this category, including those listed in Table 1, and the approaches described in this manuscript are applicable to all. PO4Sponge is engineered to have interconnected, hierarchical porosity, with pores ranging from 500 µm to nanometer size. An iron coating manufactured on the nano-scale was affixed to the iron foam. The general approach for producing the media started with the formation of an inorganic geopolymeric material from reactive liquids that polymerize and gel at ambient temperatures to form a solid aluminosilicate ceramic that bonds together iron powder and other materials. During polymerization, gases and surfactants were used to create the interconnected hierarchical three-dimensional porosity. After curing and drying, this porous iron foam typically had a surface area of 15 m²/g. This porous base was chemically modified to grow iron oxide nano-crystals. The surface area of the final media depended upon the composition and the processing conditions, but generally ranged from 60 to 80 m²/g, with a density of ~0.65 g/mL. In addition to the high porosity and high surface area, this is a liquid process that can be used to prepare a variety of shapes, as illustrated in Figure 1. In this study, the media used was in the form of a granule, prepared by crushing larger pieces (Figure 2). Figure 3 shows scanning electron microscope images that illustrate the microstructure of the media.

2.1.2. Reactor Design

The reactors were constructed from schedule 40 PVC pipe, approximately 30 cm (12 in.) in length and 3.81 cm (1.5 in.) in diameter (Figure 4). Before use, the reactors were cleaned with a phosphate-free detergent and rinsed with deionized (DI) water. Approximately 30 mL of clean pea stone was first added to the column to prevent media from exiting through the bottom hose barb. Before adding the media, it was rinsed repeatedly with DI water until the rinse water became colorless, and was then air-dried for several days. Between 50 to 150 mL of air-dried media was used in each column, depending on the experiment. This resulted in approximately 7.6 to 12.7 cm (3 to 5 inches) of column headspace. The media was weighed before adding. A portion of the media was set aside to be tested for phosphorus and moisture content and examined using a scanning electron microscope. The columns were mounted on a specially constructed stand, and flexible 1.6 mm (0.06 in) inner diameter tubing was used to connect the influent to the bottom hose barb. Wastewater was pumped from the bottom of the reservoir to the top using a Masterflex positive displacement pump (Vernon Hills, IL, USA). Wastewater was collected weekly from an onsite wastewater treatment system fed by 25 houses in Dimondale, Michigan. The system has multiple treatment steps consisting of a septic tank, an advanced textile treatment system, and sand filtration.

Hydraulic loading is a measure of the flux of fluid flowing across a cross-section of the media bed and has units of volume per time per surface area. Three single reactors in parallel were operated. These columns had various hydraulic loading rates, but the flow rates and, consequently, empty bed contact times (EBCTs) were identical. An EBCT of 60 min was selected as this seemed like a realistic time for actual installations. Differences were achieved by using columns of varying diameters. The influent to the three reactors came from a common container (

Table 2. Design parameters for the effectiveness of the hydraulic loading experiment.

Columns	Diameter of Column (cm)	Flow (mL/min)	EBCT (min)	Vol. of Media (mL)	Hydraulic Loading (mL/min/cm2)
1	3.8	2.5	60	150	0.22
2	5.1	2.5	60	150	0.12
3	7.6	2.5	60	150	0.05

).

Initially, only the influent and effluent total phosphorus levels for each column were measured. As the study progressed, spikes of effluent total phosphorus were observed, as well as visible particulate matter in the effluent. This was hypothesized to be caused by the sloughing of biological material growing in the reactors. Consequently, soluble phosphorus was measured thereafter. To verify the attachment of biofilm and the presence of phosphorus crystals, samples of the media were taken from the top of the reactors and scanning electron microscopy (SEM) images and electron dispersive spectroscopy (EDS) analyses were conducted.

2.2. Effectiveness of Empty Bed Contact Time on Phosphorus Removal Efficiency

To test different EBCTs, the volume of media in each test reactor was different, but the flow and hydraulic loading remained constant (

Table 3. Design parameters for the effectiveness of the empty bed contact time experiment.

Columns	Diameter of Column (cm)	Flow (mL/min)	EBCT (min)	Vol. of Media (mL)	Hydraulic Loading (mL/min/cm2)
1	3.81	0.8	190	150	0.07
2	3.81	0.8	90	70	0.07
3	3.81	0.8	60	50	0.07

). The soluble phosphorus was measured every week. Flow rates were checked weekly and adjusted if needed.

2.3. Analytical Methods and Quality Assurance/Quality Control (QA/QC)

To measure soluble phosphorus, the samples were first filtered through a 0.45-micron filter. Analyses were performed using the HACH 8190 reagent kits, which is U.S. EPA accepted, with a HACH model DR5000 spectrophotometer. The detection range for phosphorus is 0.06–3.50 mg PO₄/L. Quality was assessed for all parameters throughout this study, with a minimum of 1 field duplicate, 1 lab duplicate, 1 standard, and 1 blank for every 10 to 20 samples.

2.4. Fixed-Bed Adsorption Modeling

A mathematical fixed-bed adsorption model was used to estimate breakthrough and the effect of the variables on adsorption. A kinetic model to predict the exhaustion time and adsorption process for a given adsorbate concentration was used [27]. The Yoon and Nelson model is expressed in Equation (1).

$${\displaystyle \frac{C_e}{C_o}}={\displaystyle \frac{\left(C_o\ast \exp \left[K_{YN}\left(t-\tau \right)\right]\right)}{1+\exp \left[K_{YN}\left(t-\tau \right)\right]\ast C_o}}$$

(1)

where C_e represents effluent concentration (mg/L), C_o represents influent concentration (mg/L), K_YN is the rate constant (min⁻¹), t is the breakthrough time (day), $\tau$ is the time corresponding to 50% contaminant breakthrough (days). The Yoon and Nelson model equation, shown as Equation (2), can be used to determine K_YN and $\tau$.

$$ln\left({\displaystyle \frac{C_e}{C_o-C_e}}\right)=K_{YN}t-\tau K_{YN}$$

(2)

The model performance was evaluated using the Mean Bias Error (MBE) and the Index of Agreement (IA), which are presented in Equations (3) and (4), respectively.

$$MBE={\displaystyle \frac{1}{N}}{\sum }_{i=1}^N\left(P_i-M_i\right)$$

(3)

$$IA=1-{\displaystyle \frac{\sum _{i=1}^N{\left(M_i-P_i\right)}^2}{\sum _{i=1}^N{\left(\left|P_i-\overline{M}\right|+\left|M_i-\overline{M}\right|\right)}^2}}$$

(4)

where N is the sample size, M is the measured (reactor study) value, P is the predicted (Yoon and Nelson model) value, and $\overline{M}$ is the average measured value.

3. Result and Discussion

3.1. Effectiveness of Hydraulic Loading on Phosphorus Removal Efficiency

The reactors were operated at an EBCT of 60 min, but with different hydraulic loadings. Figure 5 shows the influent and effluent soluble phosphorus concentrations. No significant trends were found among hydraulic loadings of 0.05, 0.12, and 0.22 mL/min/cm². Consequently, hydraulic loading does not appear to have a strong effect on media performance at these hydraulic loading values.

3.2. SEM to Examine the Mechanisms and Impact of Biomass Loading

Much variability was observed in the effluent total phosphorus concentrations (Figure 5), which was attributed to the periodic sloughing of biofilm that naturally grew from the carbon and nutrients remaining in the treated wastewater. Although not planned, this growth is not surprising and is commonly observed in drinking water and wastewater treatment applications. Small quantities of biofilm in an analytical sample can greatly increase the phosphorus concentrations, as well as be a source of phosphorous removal from the system. The growth of biofilm in the reactors, which was an unplanned occurrence, is illustrated in the SEM photo (Figure 6). This observation emphasizes the importance of using highly treated wastewater before removing phosphorus with engineered sorption media.

Surface complexation of phosphorus with the media was verified by conducting an EDS analysis of crystals found on the media’s surface. A substantial increase in phosphorus was noted in the used media (right image) compared to the fresh media (left image) (Figure 7).

3.3. Effectiveness of Empty Bed Contact Time on Phosphorus Removal Efficiency

Each of the three columns operated at a 0.07 mL/min/cm² hydraulic retention time, based on the performance shown in Figure 5 and capabilities of the equipment used in the experiments. However, they operated at different EBCTs: 60 min, 90 min, and 190 min. The results are shown in Figure 8. Days from the start of phosphorus loading to the breakthrough of 1.0 mg/L for column EBCTs of 60 min, 90 min, and 190 min were 123 days, 166 days, and 390 days, respectively. The phosphorus removed (in mg phosphorus removed/g of dry media) for 60, 90, and 120 min columns upon reaching the 1 mg/L breakthrough were 26.57, 24.58, and 27.88 mg/g, respectively. Statistical analysis indicated a significant difference between pre- and post-breakthrough conditions (p-valve < 0.001). This shows consistency in media removal rates with different EBCTs. In a previous study, researchers tested steel furnace slag (SFS) and nano-engineered media (NEM) in column experiments with EBCTs ranging from 30 to 120 min. They also observed that phosphorus concentrations dropped from 0.5 mg/L to below 0.05 mg/L, with higher EBCTs yielding better performance [28].

3.4. Impact of EBCT on Media Capacity

To determine the impact of EBCT on the media’s capacity, experimentation continued beyond breakthrough at 1 mg/L (Figure 8). Once the media reached exhaustion, the operation was discontinued; however, the column with an EBCT of 190 min never reached exhaustion after 643 days. Another means of examining this relationship is through the use of bed volume. Bed volume is defined as the volume of wastewater passing through the media per unit time, per volume that the media occupies. Figure 9 shows that the longer the EBCT, the higher the media capacity, as measured in terms of bed volume. Furthermore, the data confirm the findings that increased EBCT results in improved removal efficiency.

Preliminary data under different operating conditions indicate that this and other varieties of PAM can still be effective at substantially lower EBCTs than 60 min. However, the impact on capacity at these lower EBCTs was not analyzed, and further research is required.

3.5. Design Approach

Mass transfer of a solute to a sorbing solid occurs in four steps [29]:

• Diffusion from bulk to liquid film surrounding the solid;
• Diffusion through the liquid film;
• Diffusion into the pores of the sorbing solid;
• Sorption of solute to the solid.

The step that takes the longest is rate limiting, controlling the overall rate of the process. A lower hydraulic loading (larger media surface area for a given flow rate) increases the contact time of wastewater with any given particle of media, thus providing more time for steps two, three, and four to occur. Over the ranges of hydraulic loading studied, decreasing hydraulic loading did not appear to affect the effluent phosphorus concentration. In these ranges, it appears that the rate limiting step is diffusion from the bulk liquid to liquid film surrounding the solid. Therefore, for this reactor configuration, EBCT appears to be more important than hydraulic loading as a design factor for efficient phosphorus removal and a previous study found similar results [30]. This indicates that the configuration of the reactor is less important than the overall volume of the reactor, allowing reactor shapes to be customized for specific applications, provided that reactor short circuiting does not occur. Achieving an adequate EBCT may require a recirculation and/or equalization/pumping system. Such a system can take advantage of the inherent, periodic high and low flows of water typical of onsite wastewater and reduce peak flows by spreading the volume out with time.

The results show the substantial amount of capacity that remained and the importance of operating multiple columns in series (Figure 8 and Figure 9). In a three-column arrangement, the first column is considered roughing and removes most of the phosphorus. The second sequential column is used for polishing the water, removing the phosphorus concentration to below the breakthrough level. The third is fresh media that is on standby. Once breakthrough occurs, the polishing column is switched to the roughing position, the roughing column is regenerated and put on standby, and the standby column is used in the polishing position. However, this arrangement complicates operations and for many applications, such as for small onsite systems, using the media to exhaustion may not justify the added complexity.

3.6. Regeneration of the Media

Exhausted media must be environmentally and economically managed for this approach to be financially viable. Laboratory studies demonstrated that the media can be chemically regenerated by washing with a caustic solution. According to the manufacturer of the PAM used during this testing, batch testing indicated that three cycles of the regeneration process removed 75–85% of the sorbed phosphorus and six cycles removed over 99%. The regenerated media retains a high capacity for capturing phosphorous even after six cycles, although further research is needed. The resulting concentrated phosphorus can then be used for beneficial purposes, which is an important consideration as phosphorus is a finite resource. However, economic analyses of different PAMs, both with and without regeneration, are critical, as described in a case study [26].

Additionally, media regeneration recovers phosphorus, although the quantities recovered are relatively small compared to those required for fertilizers. An alternative to chemical regeneration is the direct use of the media as a slow-release phosphorus source. A preliminary hydroponics study demonstrated this feasibility, as illustrated in Figure 10. In this research, lettuce was grown with an optimized hydroponic nutrient solution without phosphorus, with phosphorus provided only by the exhausted media. Plant development was delayed by about 2 weeks when the media was used as the sole phosphorus source, but the visual health of the plant, including color, size, root mass, and extension, appeared to be healthy.

3.7. Fixed-Bed Adsorption Modeling

Figure 11 shows a comparison between the experimental data and the Yoon and Nelson model predictions for EBCTs of 60 and 90 min. The performance of the model prediction was evaluated using the MBE and IA. The MBE value for the EBCT of 60 min indicates that the model overestimated exhaustion by 0.002 (mg/mg). A strong correlation was found between the model’s predicted values and the experimental values at an EBCT of 60 min (IA = 0.999). The MBE value for the EBCT of 90 min indicates that the model overestimated exhaustion by 0.1 (mg/mg). Similar to the IA statistical comparison at an EBCT of 60 min, there was a strong correlation between the model’s predicted values and the experimental values at an EBCT of 90 min (IA = 0.996). Overall, the Yoon and Nelson model predicts the trend of PAM exhaustion well for both EBCT of 60 and 90 min. Although the model overestimated the exhaustion of PAM by 0.002 to 0.1, the model remains a useful tool to predict the exhaustion time and adsorption process at a given adsorbate concentration, which ultimately helps the practical design of the PAM. A previous study has also used the model to describe micropollutant adoption in a fixed-bed column and reported that the model effectively captured strongly asymmetric breakthrough data, but slightly overestimated exhaustion time. This supports the results observed at an EBCT of 90 min [31].

4. Conclusions

The uniqueness of this study lies in its comprehensive and practical approach to evaluating phosphorus adsorption media (PAM) for decentralized wastewater treatment. Unlike many previous studies that focus solely on media capacity, this work emphasizes critical operational design parameters, particularly the Empty Bed Contact Time (EBCT) and hydraulic loading rate, under long-term conditions. The experiment ran for up to 643 days, with a breakthrough point at 390 days for 1.0 mg/L of phosphorus, demonstrating the media’s durability and real-world applicability.

Many PAMs have been developed recently, and practical design of these PAMs is important for effective implementation. Each PAM has its own characteristics; thus, the evaluation of the design parameters should be performed for each individually. This study shows that EBCT was the critical design parameter that impacted the performance of adsorption capability, whereas hydraulic loading was not significant. Research by others indicates that shorter EBCTs are as effective as those tested in this research. Further research is needed under the conditions used for this research to determine the effect of EBCT on the effluent concentration and on PAM’s capacity. The influent concentration is also dependent on the performance of phosphorus removal using PAM. Further study on the impact of influent concentration on the importance of hydraulic loading and EBCT should be studied.

The removal of phosphorus using sorption media is particularly advantageous for onsite wastewater treatment systems due to its low-maintenance nature. For small-flow applications, the media can be packaged into modular containers and conveniently exchanged every 1 to 2 years. The high hydraulic conductivity of the media allows for gravity-fed distribution, simplifying system design. However, biofilm growth can lead to clogging and phosphorus and total phosphorus breakthrough as the biofilm sloughs into the liquid, although soluble phosphorus adsorption continues. Consequently, highly treated effluent from an advanced treatment system is recommended. This study further reinforces the importance of operational design, showing that EBCT is a critical parameter influencing phosphorus removal efficiency, whereas hydraulic loading within the tested range had minimal impact. Regeneration of the media using a caustic solution was highly effective, with over 99% phosphorus recovery after six cycles, supporting long-term reuse and cost savings. Designers of PAM-based systems should prioritize EBCTs of at least 90–120 min, plan for periodic regeneration, and consider simple flow configurations. Highly treated effluent from an advanced treatment system is required to prevent biofilm growth on the media, which can cause plugging and biofilm sloughing, resulting in total phosphorus spikes. These findings offer practical guidance for implementing sustainable, low-maintenance phosphorus removal systems in decentralized wastewater treatment settings.