###### Abstract  
This work focused on the potential of simultaneously removing ammonium nitrogen from industrial wastewater using immobilized microalgae and powdered zeolite. Experiments were performed with different species and doses of microalgae embedded in spherical hydrogels in semicontinuous conditions. Ammonium nitrogen uptake by microalgae promoted the slow release of previously adsorbed ammonium nitrogen from zeolite that was then also absorbed by microalgae. Results showed that immobilized microalgae can reach a removal efficiency of up to 60% (_C. vulgaris_) and 42% (_S. armatus_). A higher removal efficiency was obtained for zeolites and immobilized _C. vulgaris_ or _S. armatus_ up to 86% and 79%, respectively. Moreover, a higher maximum sorption capacity for _C. vulgaris_ (13.8 mg/g) was achieved than for _S. armatus_ (5.5 mg/g). The recycling of spent hydrogel, zeolite, and wastewater is possible. Such an approach represents a circular bioeconomy loop. ammonium nitrogen removal; simultaneous sorption; immobilized microalgae; hydrogel beads; alginate; circular bioeconomy +
Footnote †: journal: _Sciences_  
contaminated environment. During the growth of microalgae, CO\({}_{2}\) is absorbed from the air or from industrial sources [5]. Microalgae could be immobilized by many techniques and agents that can form hydrogel beads. According to the literature, hydrogel beads could be utilized to immobilize different microorganisms used for wastewater treatment. [PERSON] et al., 2022 immobilized _Bacillus subtilis_ on polyethylene glycol (PEG)-modified polyvinyl alcohol (PVA)/sodium alginate (SA) hydrogel microspheres to treat starch wastewater [6]. An amplification efficiency of the wastewater treatment during the anammox process during utilization of immobilized microorganisms in polyvinyl alcohol/sodium alginate was achieved [3]. Green adsorbents such as hydrogel-bead biomaterials possess functional groups that can act as binding sites for the uptake or removal of many ions from aqueous solutions [7]. For example, alginate gel possesses carboxyl groups, and polyvinyl alcohol has hydroxyl group as binding sites. In addition, the micropores in hydrogel can help the exchange of the substrates between solid and liquid phases [3]. Immobilized algae are easier to remove after wastewater treatment [8]. Moreover, the utilization of immobilized microalgae can help to improve the tolerance of embedded microalgae to the strong environment in industrial wastewater. Thirdly, alginate gel is characterized by its non-toxicity, high porosity, transparency, and mild physical environment for the cells [9]. Zeolites are highly porous natural minerals that can remove various ions from contaminated environments [10; 11]. Zeolite is a low-cost sorbent for, e.g., water and wastewater treatment [12; 13; 14]. Nowadays, the emphasis is on employing environmental methods that fulfil zero-carbon circular bioeconomy strategies. Microalgae can help to fulfil such aims because during their growth, they absorb carbon dioxide [15]. Microalgae can grow and assimilate ammonium nitrogen during photosynthesis or biosorption. Clinoptiolite can remove ammonium nitrogen by means of adsorption and ion-exchange processes. Natural sorbents such as microalgae and zeolite are environmentally friendly agents in industrial wastewater treatment because they can help to reduce the utilization of chemicals. This also brings benefits to the economy and society. This is directly aligned with the principles of the circular bioeconomy. Several research articles have been published on algal or zeolite wastewater treatment. However, there are no reports on simultaneous sorption and biosorption of ammonium nitrogen from industrial wastewater in alignment with a circular bioeconomy loop.