Towards phosphorus circularity: biofilm EBPR with subsequent struvite production at the Hias WWTP: insights from full-scale and laboratory testing

Conceptually viewing the Earth as a closed system or as Spaceship Earth (Haas et al. 2020) is useful for understanding the concept circular economy. Maximizing waste prevention through recycling improves resource use in a circular economy (Velenturf & Purnell 2021). No single measure exists for keeping the food system within environmental limits (Springmann et al. 2018), but recovery of phosphorus (P) from wastewater is one approach that is helpful. The extent of P circularity achieved from wastewater treatment depends on the extent of actual reuse of the said P. The P content in the biosolids from wastewater treatment does not equate to P circularity if the P is bound by iron or aluminium and therefore not taken up by plants (Mehta et al. 2015). Thus, efficient P recovery is an important step towards sustainability (Cordell et al. 2009). One way of recovering P from wastewater is in the form of struvite and struvite is an excellent fertilizer (Hertzberger et al. 2020).

The term ‘end of waste’ is useful and meaningful as criteria can be set to determine when a recycled waste is safe to be reused (European Commission 2015). Regarding P reuse as struvite, the criteria set for precipitated phosphate salts in Regulation (EU) 2019/1009 protects the public from bacterial contamination and polycyclic aromatic hydrocarbons (PAHs) and ensures a high-quality product.

Activated sludge systems (ASS) have been widely used for nutrient removal in municipal wastewater (Fanta et al. 2024). One key advantage of enhanced biological phosphorus removal (EBPR) is the reduced use of precipitation chemicals for P removal (Oleszkiewicz & Barnard 2006; Tomei et al. 2020; Zahed et al. 2022). This reduction is beneficial from cost, greenhouse gas, and circularity perspectives, as chemical precipitation has high costs, yields inert sludge, and reduces plant availability of P (Mehta et al. 2015; Shaddel et al. 2019). EBPR sludge provides a P-rich stream that is promising for P recovery as struvite (Mehta et al. 2015; Shaddel et al. 2019). Any wastewater treatment plant (WWTP) with EBPR and an anaerobic digester can produce struvite because of the reject streams containing PO₄, NH₄, and Mg, which can crystallize into struvite.

The Hias WWTP was built in 1974 and originally consisted of primary sedimentation, followed by biological treatment with activated sludge and chemical precipitation of P with aluminium (Al) chloride before final sedimentation. It was attempted to transition the Hias WWTP towards circularity such that it evolved into a Water Resource Recovery Facility (WRRF) and it was believed that the primary hindrance to the sustainable recycling of P was precipitation with metal coagulants, which significantly reduced the availability of P for uptake by plants (Krogstad et al. 2005; Mehta et al. 2015). EBPR was deemed a suitable choice and the Hias WWTP has been operating a continuous biofilm EBPR system (Saltnes et al. 2017), known as the Hias Process, since 2022, achieving excellent operational stability and nutrient removal while significantly reducing CO₂ emissions by minimizing chemical precipitation and improving plant availability of P.

A struvite reactor was commissioned in late 2023 and has been running since. Our goal is to optimize P recovery as struvite while minimizing the use of metal coagulants. The long-term ambition is to recover 40–60% of influent P as struvite.

We will describe the performance of our processes and show what we regard as the main bottlenecks, then explain the actions we will take to move closer and closer towards phosphorus circularity in a sustainable manner. We have applied innovative technologies and are now demonstrating their effectiveness.

Table 1 gives an overview of the key figures of the BNR step at Hias. With two main trains and one prototype, the process is dimensioned for an organic load measured as soluble chemical oxygen demand (SCOD) of 5 g SCOD/m²day and a hydraulic capacity of 250 litres a second. When either of these thresholds is surpassed, a portion of the wastewater is directed to stormwater handling where chemical precipitation is employed. The Hias Process was originally designed with three main trains, but due to budget limitations only two were installed. Consequently, the Hias Process is bypassed on a regular basis following rain events and chemical precipitation is employed to stay within effluent limits of P.

The P-recovery system includes a fluidized bed struvite reactor (Ostara Pearl 2K) and an anaerobic tank for PO₄ release. Two distinct reject streams, A and B, feed the reactor. Feed A comprises the reject from the final dewatering process after digestion, while Feed B contains the reject from dewatered EBPR sludge from disc filters (Figure 1). This sludge is subjected to anaerobic conditions to release PO₄ (Schauer 2013).

Thus, at the Hias WWTP, the incoming P has three ways out, namely, into the lake as effluent, in big bags as struvite, and in the biosolids.

Dissolved phosphorus (PO₄-P) and SCOD were analysed by grab samples taken at specific locations in the plant. All grab samples were analysed in the Hias in-house laboratory on a NOVA Spectroquant 60 spectrophotometer after being filtered through a fibreglass filter of 1.2 μm.

The following analyses were conducted by external laboratory SGS analytics: dry solids (DS), magnesium (Mg), aluminium (Al), iron (Fe), total phosphorus (Tot-P) PAH16, ammonium (NH₄-N), total organic carbon (TOC), Ascaris sp., Salmonella, E. coli, and Clostridium perfringens.

Laboratory P-release experiments at different temperatures were performed as parallel batch tests on EBPR sludge collected after disc filters. In each test, two 1-litre batches were stirred for 24 h. One of the batches was kept at a given temperature and the other batch was maintained at room temperature, as shown in Table 2. The parallel test was repeated six times with different temperatures.

A modification of the sequential extraction procedure of Ajiboye et al. (2004) was used in this study.

The Ostara Pearl 2K struvite reactor was handed over in December 2023 after commissioning was completed. The produced struvite is of high quality containing only very small traces of compounds such as organic carbon, PAH, and unwanted bacteria, as the analysis presented in Table 3 displays.

Although the quality of produced struvite has been excellent, the amount produced and harvested can be improved. The crystallization and harvesting of struvite have been irregular. In May, the daily average harvest of struvite was 96 kilos (Table 4). Reaching our long-term goals for P recovery would require harvesting a magnitude of 400–500 kilos a day.

At the Hias WWTP, the main limiting factor for P recovery as struvite is the amount of PO₄ entering the struvite reactor from the final dewatering reject (Feed A) and stripped EBPR sludge reject (Feed B). Currently, Feed B contributes about 75% of the load. Increasing the PO₄-P load and concentration coming into the reactor is our main objective as this will not automatically translate into higher effluent concentration but instead into higher production and precipitation of struvite.

Most of the P that enters the WWTP becomes concentrated in the EBPR sludge, underscoring the importance of releasing PO₄-P from this sludge for maximizing effective phosphorus recovery. A normal release rate in the P-stripper is currently around 27% of total P, released as PO₄-P.

When the EBPR treatment system is bypassed, the amount of P available for PAOs to take up is reduced and less P enters the P-stripper, leading to lower PO₄-P concentrations in the stripper and subsequently in the struvite reactor, as depicted in Figure 8.

It seems the concentration of PO₄-P in the reject from the anaerobic digester (Feed A) is significantly limited by the presence of metal compounds.

This rise is in concert with the change from activated sludge process to biofilm EBPR (the Hias Process) and was an anticipated consequence of the reduced use of aluminium for precipitation of P (Krogstad et al. 2005; Mehta et al. 2015). During the commissioning of the struvite reactor in 2023, approximately 5 tonnes of struvite were harvested. Production has improved in 2024 and about 13 tonnes of struvite have been harvested by the end of May. Further reduction of aluminium in the biosolids should by itself increase the amount of plant available P, but this will be offset when P is redirected around the digester as struvite harvesting expands.

The fractionation study conducted in mid-April 2024 followed a period of substantial bypass and extensive use of aluminium-based precipitation.

The Hias Process coupled with a P-stripper and struvite reactor has proven to be a powerful step towards P circularity. This comes with added benefits of lower cost and CO₂ footprint due to less use of chemical precipitation. The main P circularity effects are more plant available P in the biosolids, due to low use of chemical precipitation and P recycled as struvite from the struvite reactor. However, our long-term goal of 40–60% P recovery as struvite from incoming wastewater is not yet fulfilled. To move closer to this ambitious goal, we have identified the following actions, presented in order of anticipated impact.

• 1. Expand the capacity of the Hias Process with one additional train (45% capacity increase). This will reduce bypass and use of chemical precipitation. Thus, allowing significant more P to enter the P-recovery system through the EBPR sludge. Furthermore, less aluminium will enter the digester, raising PO₄-P concentration in Feed A.

• 2. Raise the temperature in the P-stripper by heat exchange with exhaust from biogas engine. This will increase release ratio of PO₄-P from the total P entering the stripper and struvite precipitation will rise due to both higher load and higher concentration of PO₄-P in Feed B.

• 3. Improve magnesium dosing control strategy in struvite reactor using online monitoring of effluent PO₄-P. This will increase crystallization of the struvite.

• 4. Keep EBPR sludge from disc filter at highest possible DS by frequent maintenance. This will ensure high retention time, which aids PO₄-P release and high concentration of PO₄-P in the P-stripper, benefitting precipitation rate in the reactor.

• 5. Avoid precipitation chemicals entering the anaerobic digester from external sources. This will allow more P to leave the digester as PO₄-P and more struvite can be produced due to higher PO₄-P load into the reactor.

• 6. Keeping EBPR sludge isolated in a separate anaerobic digester, prohibiting any contact with precipitation metals. This would allow for high rate of PO₄-P release from the EBPR sludge.

Parts of this work have received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement No 869283.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors Gjermund Sørensen and Sondre Eikås are inventors of the Hias Process with royalty benefits attached to its commercialization.