Analysis of Modifications to an Outdoor Field-Scale Rotating Algal Biofilm Reactor with a Focus on Biomass Productivity and Power Usage

Abstract

Filtrate from dewatering anaerobically digested biosolids is a side-stream of wastewater treatment that contains high concentrations of nitrogen and phosphorus compounds that can serve as nutrients for cultivating microalgae biomass as biofilms for bioproduct production at Water Resource Recovery Facilities (WRRFs). One system used to cultivate attached microalgae biofilms is the rotating algal biofilm reactor (RABR). A pilot RABR with 72 m² growth surface area, 11.5 m² footprint area, and a liquid volume of 11,500 L was operated in an outdoor environment at the largest WRRF in Utah, U.S.A, the Central Valley Water Reclamation Facility (CVWRF). The configuration of the RABR was altered from the previous configuration with regard to temperature and duty cycle with the goal to maximize biomass productivity. Results included an increase in dry biomass productivity on a footprint basis from 8.8 g/m²/day to 26.8 g/m²/day (205%) while power requirements changed from 28.3 W to 91 W. The increase in biomass productivity has direct benefits for bioproducts including bioplastic, biofertilizer, and the extraction of lipids for conversion to biofuels.

1. Introduction

There exists a need for empirical demonstrations of scalable biomass production and associated power consumption. The U.S. Water Environment Federation has summarized the relatively few pilot and larger scale algae biofilm reactors and emphasized the need for pilot scale testing before implementing field-scale algae biofilm reactors [1]. Pilot scale testing of an attached algae system (Revolving Algae Biofilm) has been demonstrated for yearlong performance in outdoor conditions and for in situ biomass harvest [2,3,4,5]. Hillman [6] and Goldsberry [7] identified a fertilizer bioproduct of a pilot scale RABR resulting from the effect of photosynthesis increasing the pH of the biofilm that was correlated with the precipitation of struvite within the biofilm matrix [6,7,8,9]. Sandia National Laboratories developed an integrative algae flow-way system that produces a net-CO₂ negative biofuel production from waste nutrients resulting from a treating waterway. This system remediates (or reclaims) major nutrients, metals, and organic and inorganic carbon from surface waters [10]. These designs illustrate different patterns of contacting biofilm, liquid, and gas phases to produce algae biomass. A summary and review of several microalgae biofilm technologies have been conducted by Moreno Osorio et al. [11]. However, a literature and industry survey of pilot-scale and full-scale algae biofilm reactors demonstrated a lack of information on increasing biomass productivity related to both bioproducts production and power requirements [12,13].

The U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy (EERE), Bioenergy Technology Office (BETO), awarded a multi-year project to the Sustainable Waste to Bioproducts Engineering Center (SWBEC) at Utah State University in partnership with Central Valley Water Reclamation Facility (CVWRF), the largest water resources recovery facility (WRRF) in the State of Utah, WesTech Engineering (Salt Lake City, UT), and Algix Bloom Sustainable Materials, Meridian, MS [14]. The goal for the project was to expand the domestic resource potential of the bioeconomy through creation of a low-cost supply of algae biomass for bioproducts that utilized biofilm microalgae biomass [14]. The importance of the project relates to evaluating the potential to create algae biomass through the scale-up of a specific outdoor biofilm microalgae system identified as a rotating algae biofilm reactor (RABR) for the potential for full scale application to the 1300 municipal WRRFs utilizing anaerobic digestion (AD) in the U.S. The overall goal of the project was the integration of municipal wastewater AD streams in a sustainable operation with upcycling of the nutrients nitrogen and phosphorus into nutrient-rich value bioproducts and producing a technoeconomic analysis (TEA) for compostable bioplastic production from RABR-generated biomass [14]. Because of the large amount of nutrients in a WRRF, including phosphorus and nitrogen, which are present in the AD stream, the focus of the project was on utilizing that stream as a medium for algae cultivation.

Anaerobic digestion is a solids treatment process where microorganisms decompose organic material in the absence of oxygen. This digestion process produces methane gas, stable biosolids, and liquid filtrate after separation of the solids from the water. The methane gas can be utilized as a source of power for several wastewater treatment operations. The biosolids can be dewatered and processed into compost. The CVWRF utilizes the methane and stable biosolids as sustainable bioproducts. The liquid removed during the dewatering process is colored and contains some turbidity and is generally returned to the influent of the WRRF and recycled through the treatment system. Filtrate from dewatering anaerobically digested biosolids, which will be referred to as filtrate, is a high strength nutrient feed that is the cultivation medium for the microalgae biofilm in the pilot RABR [7,13,15,16].

Facilities such as WRRFs also contain bioresources such as nutrients, water, and sunlight exposure, and already include infrastructure that could be utilized to produce biomass that can be processed into value-added bioproducts [1,6,7,8,9,17]. Converting wastewater into bioproducts such as bioplastics, biofuels, and biofertilizers is an example of upcycling, where waste is transformed into a quality and valuable product [8]. Microalgae cultivation at WRRFs can utilize the bioresources available at WRRFs with some alteration to existing infrastructure to produce bioproducts having significant economic value [7,8,9]. Side-stream nutrient removal is a treatment train of stages of removal processes to treat filtrate [8]. A RABR could be incorporated at the end of the treatment train to produce microalgae biomass as a feedstock for high value bioproducts [7,8].

The RABR is an outdoor technology for treating municipal wastewater through the cultivation of mixed culture microalgae as a biofilm that is rotated to alternately expose the biofilm to the wastewater and to the atmosphere. The RABR utilizes the nutrients phosphorus and nitrogen in the water, carbon dioxide supplied by microbial processes in the water and from the atmosphere, and sunlight to cultivate nutrient-rich microalgae biomass that can be controlled for moisture content and utilized in downstream processing for conversion into bioproducts, including compostable bioplastics [8]. Advantages of the RABR system are that it: (1) combines algae cultivation and separation in one reactor, eliminating the need for another physical unit for clarification, (2) minimizes power requirements through harvesting by simple scraping and through eliminating the need for oxygen addition, (3) has good gas exchange with the atmosphere (no O₂ toxicity), (4) can function in turbid and/or colored water, and (5) avoids the limitation of shallow depth for light penetration that occurs in suspended open systems. Disadvantages of the RABR system are that: (1) performance is affected by seasonal changes in temperature that affect the AD water temperature as well as the air temperature of the biofilm environment, and (2) there is a lack of photosynthetically active radiation at night and on cloudy overcast days [1,12,13].

One difference between this project and other published work is the evaluation of power requirements. Power is used to rotate the biofilm and support additional systems such as pumping media. For example, rotating the biofilm in a pond requires about 0.65 W m⁻², while another 0.22 W m⁻² is needed for operations like pumping water and running blowers [18]. The duty cycle or percentage of time the RABR shaft is rotating the biofilm platform also has a direct effect on the power required for rotation.

For this U.S. DOE project, the baseline productivity used for the bioplastic TEA [8] based on treating whole algae rather than fractionated biomass was determined using the surface area productivity achieved by Jeppesen [13]. This productivity was applied to treating 2.3 million liters per day of filtrate, which was 231 metric tons per year dry weight (8.8 g/m²/day) seasonal average. This determination established a minimum plastic selling price (MPSP) of $4570 per metric ton and a ratio of income to expenses of 106%. Intermediate and final biomass productivity targets were identified as 391 and 577 metric tons per year that would achieve MPSP values of $3720 and $3520 per metric ton with ratios of income to expenses of 115% and 120%, respectively [8]. Other conversion processes investigated in the TEA included lipids for biodiesel and hydrothermal liquefaction (HTL) to produce bio-oil that had income to expenses ratios of 100% or less, which agreed with HTL projections by others [8,15,19]. This investigation only looked at RABR biomass for use in HTL to produce biocrude and did not look at other bioproduct pathways using HTL [20].

In the study reported here, the pilot RABR operation utilized by Jeppesen [13] was modified using sustained control of increased temperature and duty cycle with the goal to increase the biofilm biomass productivity above the baseline value toward achieving targets identified in the TEA for intermediate and final biomass productivities of 16 and 22 g/m²/day, respectively [8,13]. The principal bioproduct of interest was compostable bioplastic that was produced using the RABR biofilm algae biomass by Algix Bloom Sustainable Materials that served as the basis for the TEA [8,14].

Achieving one or more of these targets would increase the ratio of income to expenses over the baseline value and decrease the MPSP based on the TEA produced by Watkins [8] while comparing power requirements.

2. Materials and Methods

2.1. Modifications

The field-scale RABR was constructed as described by Jeppesen, including inoculation with trickling filter microalgae from the CVWRF in South Salt Lake City, Utah, USA [13]. The construction was altered from the design described by Jeppesen using the modifications explained below [13].

The first modification was that the influent was changed to be the same influent used for the CVWRF anammox process: filtrate with solids removed by settling and heated to 25 °C. The influent used by Jeppesen was at ambient temperature. This change was made to potentially increase microalgae growth rates. The filtrate used in this test was sourced from the CVWRF. The RABR was located on site at the CVWRF and was placed in parallel to the side-stream nitrogen removal process. The number of shelves were doubled, with 4 sections of shelves, each with 3 bays of shelves and 3 shelves per shelving unit. This updated design totaled 36 shelves with substratum on the top and bottom of each shelf for a surface area of 72 m², or double the amount used by Jeppesen [13]. The doubling of surface area and the tank volume remaining the same at 11,500 L were modifications made to increase productivity on a footprint basis. This configuration is seen in Figure 1, Figure 2 and Figure 3. Harvesting groups were not divided per bay or section, only per shelf, based off the recommendations made by Jeppesen [13]. Lack of statistical significance between bays and sections led to reducing the number of sampling groups from 18 to 3. Only 3 biofilm categories were collected: one each for the top, middle, and bottom shelves across all bays and sections. This change was made to simplify harvesting, storage, and quantification of microalgae biomass, thus reducing the time and labor required.

The RABR was spun clockwise with reference to the side with the VFD to save power based on raw data from Jeppesen [13]. The VFD was set to 15 Hz and set to rotate the RABR at 0.896 RPM to replicate the conditions used by Jeppesen [13]. A different power meter was used for more accurate measurements; the power meter used was the Powersight™ PS3550 Power Analyzer (Pleasant Hill, CA, USA). This power meter was purchased new and calibrated at the factory. Both atmospheric and fluid temperature were recorded hourly throughout the year. During autumn and winter, the RABR was insulated and covered in clear polycarbonate shielding to retain heat. The flow rate was set to a 48 h or 11 h hydraulic retention time (HRT) depending on the quarter. The flow rate of the 48 h HRT was 4 L/min and the flow rate of the 11 h HRT was 18 L/min, which was the highest flow rate achievable with the RABR setup. Finally, the laboratory at the CVWRF was used to analyze total phosphorus (TP) and total Kjeldahl nitrogen (TKN) concentration.

2.2. Sampling

The RABR was sampled weekly, with the following data collected: daily light integral (DLI) over the last seven days, photosynthetic photon flux density (PPFD) from the center of each shelf, including the underside of the bottommost shelf, and a 50 mL sample of the influent, effluent, and a mixed biofilm sample that was 1/3 of biofilm from each shelf. In addition, the sample temperature and pH, atmospheric temperature at the site at the time of sampling, the pH of the biofilm on each shelf of a random section in bay 2, and the energy consumption over each week from the power meter were also collected.

2.3. Harvesting

The harvesting side used was the side to the left of the VFD and the side facing the camera in Figure 2. The RABR was halted to stop rotation so that one section was parallel to the ground, facing the harvesting side. The microalgae were then mechanically removed using garden hoes with the blades shortened by 63.5 mm (2.5 in) and plastic putty knives, then scraped into a container such as a storage tote. The harvesting started at the bottom of the bottommost shelf and proceeded upwards, harvesting the top and bottom of a shelf into a single container. The container was then exchanged for a different container assigned to a different shelf and the middle shelf was then harvested. The same procedure was repeated for the topmost shelf. Harvesting was performed by two or three participants, each responsible for a given bay. Once one participant finished harvesting a shelf on the first bay, the container was passed to the participant at the next bay.

Once all shelves in a section were harvested, the sections were rotated 180° and the harvesting repeated, harvesting into the same container for each shelf. The sections were then rotated 270° and harvested for the third time, and finally the sections were rotated 180° and harvested one final time. This rotation system ensured each section was equally damp at the time of harvesting to minimize unequal removal due to uneven drying.

2.4. Drying

The three containers of microalgae were transported to the SWBEC laboratory where they were poured into five-gallon buckets (of ~18.9 L volume) and were frozen at −20 °C overnight. Once frozen, the buckets were removed and thawed until each bucket formed two distinct layers. The top layer of excess filtrate was decanted off. This freeze, thaw, and decant process was determined to be necessary because the mass decanted off approached 50% of the total mass in some samples, increasing the time the samples would need to dry in a drying oven. The remaining contents of each bucket were poured into oven-safe containers such as aluminum casserole pans and dried in a drying oven at 90 °C until dry. The mass of the pan was recorded prior to use and the mass of biofilm was recorded both before and after the drying process.

For a single harvest, a test was performed if sun drying before harvesting would eliminate the need for decanting in the drying process. Samples were taken after 30 min and 1 h of sun drying and then tested for solids percentage. Results indicated that allowing sun drying for 1 h may eliminate the need to perform the decantation process.

2.5. Incineration

One gram of oven-dried microalgae from each shelf was ground in a mortar and pestle. This process was repeated three times. Each gram of dry microalgae was placed in a small tin with the mass of each tin measured before and after the mass was added. The tins were then placed in a muffle furnace and incinerated at 550 °C for two hours. The tins were cooled in a desiccator and were measured, calculating the ash-free dry weight through mass difference with the dry weight.

2.6. Insulation

During the fall season, the RABR was insulated to allow for high internal tank temperatures while atmospheric temperatures decreased. The top of the tank was insulated by installing 1.5875 mm (1/16 inch) thick polycarbonate plastic to the RABR frame throughout the perimeter of the RABR. The bottom was insulated by installing 2 × 8 nominal size boards (with actual dimensions of 38 mm by 184 mm) along the length of the bottom of the tank, with each piece cut to fit between the supports to prevent air flow under the tank. The sides of the tank were insulated with 88.9 mm (3.5 inch) thick R-13 fiberglass insulation that was covered with a reflective 12.7 mm (0.5 inch) thick R-2 polystyrene foam board. The entire perimeter of the tank was then wrapped with radiant barrier bubble insulation. The insulation is depicted in Figure 4, and a thermal image of the insulation is shown in Figure 5.

2.7. Biomass Productivity Calculation

Biomass productivity was calculated based on mass of dry weight and ash-free dry weight calculated over area and time using the surface area of 72 m² and footprint area of 11.5 m² while considering that the harvests occurred weekly. Biomass productivity was not calculated based on nutrient removal. This is because the filtrate contained excess nitrogen and phosphorus, therefore nutrients were not limited in biomass production.

3. Results and Discussion

3.1. Performance Evaluation of the Pilot RABR

The results of the test are shown in

Table 1. Average and standard deviation of results of weekly sampling of the pilot RABR, divided into quarters of three months and the performance of the final month.

	Quarter 1: Apr–Jun 2024	Quarter 2: Jul–Sep 2024	Quarter 3: Oct–Dec 2024	Quarter 4: Jan–Mar 2025	Final Month: Apr 2025
Footprint Productivity (g/m2/day)	7.8 ± 3.7 4.3 ± 2.1 (ash-free)	11.5 ± 4.1 7.2 ± 2.6 (ash-free)	10.8 ± 7.3 7.2 ± 2.6 (ash-free)	22.3 ± 6.64 13.2 ± 3.9 (ash-free)	29.2 ± 2.3 16.3 ± 1.3 (ash-free)
Substratum Productivity (g/m2/day)	1.3 ± 0.6 0.7 ± 0.3 (ash-free)	1.9 ± 0.7 1.2 ± 0.4 (ash-free)	1.8 ± 1.2 1.2 ± 0.8 (ash-free)	4.3 ± 1.4 2.5 ± 0.8 (ash-free)	5.8 ± 0.4 3.2 ± 0.3 (ash-free)
Instantaneous Power Draw (W)	No Data c	No Data c	49.9 ± 1.4	90.8 ± 2.4	92.6 ± 1.3
Average Set Flow Rate (L/min)	3.6 ± 0.8 a	4.3 ± 0.6 a	4.3 ± 0.8 a	17.7 b	17.7 b
Optimal Power Consumption for N (kWh/kg N removed)	No Data c	No Data c	13.64 ± 33.75	19.11 ± 13.51	14.89 ± 20.28
Optimal Power Consumption for P (kWh/kg P removed)	No Data c	No Data c	130.92 ± 129.74	63.82 ± 63.66	58.52 ± 16.98
Biomass Power Consumption (kWh/kg dry mass)	No Data c	No Data c	13.45 ± 6.77 20.65 ± 10.40 (ash-free) d	9.73 ± 5.02 16.51 ± 8.51 (ash-free)	6.65 ± 0.51 11.96 ± 0.92 (ash-free)
Effluent pH	8.12 ± 0.18	7.88 ± 0.38	8.16 ± 0.24	8.59 ± 0.13	8.51 ± 0.14
Biofilm pH	7.75 ± 0.28	7.53 ± 0.38	7.95 ± 0.55	8.54 ± 0.15	8.27 ± 0.09
Organic Percentage	55.66% ± 3.90%	62.67% ± 7.02%	65.15% ± 4.35%	58.92% ± 3.26%	55.66% ± 5.08%
Percent Ash	44.34% ± 3.90%	37.33% ± 7.02%	34.85% ± 4.35%	41.08% ± 3.26%	44.34% ± 5.08%
Biomass Light Footprint Productivity (g/mol photons)	2.7 ± 1.6 1.5 ± 0.89 (ash-free)	3.5 ± 1.4 2.1 ± 0.88 (ash-free)	8.0 ± 6.3 d 5.2 ± 4.2 (ash-free)	No Data c	13.0 ± 6.0 7.2 ± 3.4 (ash-free)
Average Photoperiod (h/day)	15.32	14.12	11.1	No Data c	13.7
Duty Cycle	25%	25% (50% in Sep)	50%	100%	100%

^a Two-day HRT; ^b 11 h HRT (fastest flow rate possible); ^c power data were not collected for the first two quarters and the DLI probe was broken for quarter 4; ^d quarter 3 had approximately 15 L (4 gallons) of post-decant wet mass shipped to PNNL each workweek during November and December, as well as the second week of October. This missing mass is not accounted for in the data.

and Figure 6. The columns in Table 1 are divided into quarters of a year. Parameters were changed between some quarters, and each quarter represents a season with related environmental factors. Additionally, the month of April 2025 is shown in its own column. Data were only collected from 1 April 2025 until 17 April 2025, when the RABR ceased rotation due to mechanical issues.

3.2. Changes in Parameter

Notable changes in parameters include:

• Increase in duty cycle in September 2024 from 25% to 50% to increase biomass productivity.
• Installation of insulation consisting of fiberglass, polystyrene, and a radiant barrier at the end of October 2024 to reduce heat loss in the above-ground steel tank, thus increasing tank temperature.
• Improvements to the insulation by installing 1.5875 mm (1/16 inch) polycarbonate sheets to cover all sides of the RABR frame in February 2025, as seen in Figure 4. This was performed to reduce impact of winter weather.
• Increase in duty cycle in February 2025 from 50% to 100% to minimize moisture stress on the algae.
• Decrease in HRT from 48 h to 11 h to increase liquid temperature, which resulted from increasing the flow rate to the maximum flow rate possible with the RABR setup of 18 L/min on 28 February 2025. This was performed with the goal of increasing biomass productivity.
• Removal of six shelves from the RABR to fix mechanical issues experienced in January 2025. The reduced surface area is accounted for in substratum productivity calculations. This was performed to prevent the early shutdown of the RABR system.

3.3. Re-Evaluation

Results that indicated a failure to decrease nutrient concentration in the effluent compared to Jeppesen during quarters two and three led to a reevaluation of objectives [13]. After research into economic analyses of RABRs with an emphasis on bioproduct profitability as well as consulting with industry contacts, the decision was made to change focus from removing nitrogen and phosphorus and instead focus on maximizing biomass productivity. One example of a consultation was a conversation during a meeting on the RABR life cycle analysis (LCA) with the general manager of the CVWRF. They mentioned that for side-stream nutrient removal, conventional systems used in WRRFs such as anammox (Anita Mox™ as an example of a specific system) for nitrogen or MagPrex™ for phosphorus lower nutrient concentrations by a larger percentage with greater consistency than the RABR. The value that RABRs bring is that microalgae biomass contains reactive nitrogen and can potentially be processed into value-added bioproducts that can be sold for profit [8,9]. The general manager of the CVWRF suggested that a practical application of an industrial scale RABR could occur in series after an anammox system. In this configuration, the RABR would produce microalgae biomass that could generate revenue for the operating treatment facility if processed into profitable bioproducts [8,9]. According to the CVWRF, the nutrient concentration levels in the Anita Mox™ effluent were estimated to be approximately 100 mg/L of nitrogen and 15 mg/L of phosphorus which are sufficient concentrations to allow for microalgae biofilm cultivation [1,13,21].

The concept of an RABR in series after anammox or similar side-stream treatments harnesses the strengths of the RABR while minimizing the weaknesses that can be interpreted from the results. If the primary objective of an RABR shifts from nutrient remediation to biomass production, the inconsistency of the remediation shown by high standard deviation values is less relevant, and the apparent decrease in removal percentage from increasing the flow rate is an acceptable compromise. The adjustments that occurred in quarter four to increase biomass productivity necessitated working within the physical constraints of the RABR construction with the goal of raising the temperature of the filtrate in the tank. This change required increasing the duty cycle to 100% and increasing the flow rate to the maximum value permitted by the RABR setup. The decision to increase the duty cycle to 100% alongside reducing the HRT was to minimize moisture stress on the algae. The intent was to maximize heat in the steel tank through a lower HRT. Reducing the HRT from 48 h to 11 h would give less time for heat to leave the liquid, thus maintaining a higher temperature in the liquid. Implementing the faster flow rate led to the dry biomass productivity on a footprint basis increasing to 26.8 g/m²/day from 8.8 g/m²/day, as reported by Jeppesen, (an increase of 205%) while decreasing nutrient removal [13]. This value of 26.8 g/m²/day includes all data from after the increase in flow rate: from 28 February 2025, until the end of the project. Quarter four included most of the winter months in the northern hemisphere, so the increased flow rate may have increased productivity to a comparatively larger degree than other potential seasons. It is possible the gap between summer 2024 data and potential summer 2025 data would be smaller than the 173% dry biomass productivity increase on a footprint basis between quarter three and quarter four.

3.4. Discussion of Results

Many of the data gathered were to allow for a comparison to Jeppesen and other comparable RABR experiments, but some data were gathered to provide new information not seen in other studies or to carry out future work as suggested by Jeppesen [13]. One example is collecting data to quantify ash-free photosynthetic efficiency, or ash-free dry weight productivity per moles of photosynthetically active photons. These data are found in Table 1. Few other publicly available studies were found to contain data in these units, but data in these units are of interest to the United States Department of Energy [13,14,17,22].

Jeppesen reported biomass productivity values of 8.8 g/m²/day (4.5 g/m²/day ash-free) on a footprint area basis and 2.8 g/m²/day (1.4 g/m²/day ash-free) on a substratum or surface area basis [13]. As seen in Table 1, quarters two and three were more productive on a footprint basis, but less productive on a substratum basis. Quarters four and five were more productive than Jeppesen 2024 on both bases [13]. The combined results of all harvests after the increased flow rate result in a dry biomass productivity value of 26.8 g/m²/day on a footprint basis. This number is larger than the 22 g/m²/day used as the final estimate in the technoeconomic analysis performed as a part of the U.S. DOE project, and that estimate projected an income to expense ratio of 120% if used to produce bioplastic [8,14].

The high flow-rate data reports an average instantaneous power draw of 91.0 ± 2.1 W due to the 100% duty cycle. The duty cycle was raised to 100% at the same time as the flow rate increase. Therefore, it is unclear whether the observed increase in biomass productivity was primarily driven by flow rate or duty cycle. Based on results reported by Jeppesen, it is plausible that both factors contributed [13]. However, the increase from 25% duty cycle to 50% duty cycle during September 2024 did not appear to affect biomass productivity, so it is possible that the increase in flow rate that caused an increase in the temperature of the tank liquid was the main cause of the increased biomass growth. Maximum flow rate was never tested alongside a duty cycle of less than 100%, but it is possible that nearly the same amount of biomass productivity could be achieved with a 50% duty cycle, which had an average instantaneous power draw of only 49.9 ± 1.4 W. Duty cycles lower than 50% may also be viable if the liquid temperature was maintained at 25 °C.

Comparison of results with other pilot studies puts the RABR results in context. A pilot scale algae turf scrubber by Witarsa et al. [21] with a 122 m² footprint area produced an average of 33.6 g/m²/day of dry weight. A pilot scale revolving algal biofilm by Gross and Wen [2] with an 8.5 m² footprint area produced an average of 21.5 g/m²/day of dry weight. This study with an 11.5 m² footprint area produced an average of 26.8 g/m²/day of dry weight under the 11 h HRT configuration.

3.5. Temperature

Since an objective of this study was to observe the RABR over one calendar year, filtrate temperature was manually recorded during sampling using an analog hand thermometer. Attempts were also made to set up continuous temperature recording with a sampling interval of 1 h, but the initial system had difficulty with wintry weather, and a replacement was not found until March 2025. While there are no continuous temperature data for every month, the average values are similar to the manually recorded values that had only four data points a month, meaning the data in the manual temperature column in Table S1 are representative of the actual temperature. The recording occurred at approximately 8:00 am until February 2025, when sampling was changed to 5:00 pm due to the scheduling of sampling events. The average atmospheric temperature column in Table S1 was included for reference and as a contrast to the effluent temperature, especially after the flow rate increased at the end of February 2025. The atmospheric temperature data were collected from historical data from a weather station at the Salt Lake City Airport.

The manual effluent temperature data supports the observation that the increase in temperature is linked to biomass productivity. For example, the average effluent temperature was 9.1 °C in December 2024 before the increase in flow rate and 24.6 °C in March 2025 after the increased flow rate. The average biomass productivity for December was 13.7 g/m²/day on a footprint basis and was 25.7 g/m²/day on a footprint basis for March.

An example comparing the manual and continuous temperature measurement process for the tank effluent is shown in Figure 7 and Figure 8. Figure 7 shows weekly effluent temperature across quarter three. Figure 8 shows the continuous temperature data collected using an Arduino™-based temperature probe between the first and second data point in Figure 7. Average monthly temperature results were approximately the same between both methods as shown in Table S1 even though the manual method had four data points per month and the continuous method used approximately 700 data points per month.

Figure 7 and Figure 9 show the contrast between the liquid temperature before and after both the tank insulation and the increased temperature due to the increased flow rate. The liquid temperature data in Figure 9 were collected using a new HOBO™ Pendant MX2201 Water Temperature Data Logger (Bourne, MA, USA). The filtrate influent temperature is 25 °C and the data in Figure 9 indicates little to no temperature loss in the liquid, in contrast to Figure 7. This contrast shows the combined impact of the insulation and the high flow rate on the filtrate temperature.

3.6. Section Summary

If the objective of the RABR was to produce biomass using nutrients in wastewater, the RABR design may provide an asset to any future WRRFs that use anaerobic digestion: biomass for profitable bioproducts. Two bioproducts that appear potentially profitable for RABR microalgae biomass based on prior studies are bioplastic and struvite-based fire retardants [8,9]. Both solutions upcycle the nutrients in wastewater into products that are projected to return more than their production cost. This could provide a revenue stream for facilities that incorporate future RABR designs.

4. Conclusions and Future Work

Modifications were made to the RABR design tested by Jeppesen, some of which were suggested in the future work section of that thesis [13]. The work reported here presents new information for the RABR system, increased algae biomass productivity above the baseline productivity, and further investigations into power requirements. The objectives of this study were to compare results with Jeppesen to identify if the modifications to the design led to improved results, observe the field-scale RABRs in outdoor conditions over one calendar year, and in quarter four the objective to maximize biomass productivity was added. This shift in objectives during the study itself weakens the dataset and thus the conclusions that can be drawn from the data. The following statements are intended to be preliminary observations and would require future validation. The modifications made in quarter four led to an increase in biomass productivity on a footprint basis from 8.8 g/m²/day (4.5 g/m²/day ash-free) reported by Jeppesen to 26.8 ± 2.7 g/m²/day (15.5 ± 1.3 g/m²/day ash-free) in this study [13]. The productivity on a substratum basis increased from 2.8 g/m²/day (1.4 g/m²/day ash-free) to 5.3 ± 0.5 g/m²/day (3.07 ± 0.2 g/m²/day ash-free) in this study [13]. This increase was possibly due to modifications that raised the temperature of the filtrate in the tank, such as preheating the filtrate, tank insulation, and increased flow rate. Increased biomass productivity at the field-scale allows for the possibility of future RABR development and scale-up with the intent of using RABR technology to produce biomass for processing into profitable bioproducts such as bioplastic [8,9]. The 26.8 g/m²/day dry mass productivity value exceeded the 22 g/m²/day final estimate used in the U.S. DOE TEA that projected a 120% income to expense ratio [8,14].

There is still potential for future research, including testing the 11 h HRT design at duty cycles lower than 100% to decrease power consumption and measure the impact on biomass productivity or further year-round testing with the 11 h HRT design. The high flow rate configuration was only operated from 28 February 2025 to 10 April 2025, leaving room for investigating productivity during warmer weather conditions. Other future avenues include research into UV radiation exposure and its effects on biomass productivity, feasibility of separating struvite from wet or dry microalgae biomass, and shear force testing since the direction of rotation may affect the associated costs of operating a RABR. Future research may also consider using a more durable RABR parts design such as corrosion-resistant fasteners and perforated metallic sheets between the substrata instead of polycarbonate. Methods to minimize wear on the substratum could also be considered, as well as continued testing on sun drying before harvesting to reach more definitive conclusions on the concept. Finally, there may be value in looking at automatic harvesting designs; a potential design is a self-harvesting Archimedes screw design. A screw-type algal biofilm reactor or SABR may merit further investigation.