Development and performance evaluation of a circulating fixed-bed photobioreactor (CFBP) with clinoptilolite fillers for enhanced microalgae cultivation in urine: mechanistic insights

Urine, a major source of nitrogen and phosphorus in domestic wastewater, challenges microalgae-based treatments due to high ammonia-nitrogen concentrations and salinity; therefore, additional processes are required when using conventional photobioreactors. This research developed a novel circulating fixed-bed photobioreactor (CFBP) with clinoptilolite and a three-stage operation mode to solve the above deficiencies. Modes including clinoptilolite cyclic processing (mode 1), microalgae suspension cultivation (mode 2), and microalgae-clinoptilolite cyclic cultivation (mode 3) occurred during the operation of the CFBP system. The results demonstrated that clinoptilolite treatment reduced the concentration of ammonia nitrogen (from 1793.85 mg/L to 129.03 mg/L) and salinity (from 1.22 % to 0.62 %) in synthetic urine of mode 1. Furthermore, some beneficial ions (e.g., Ca²⁺, Mg²⁺, and Fe³⁺) were introduced through ion exchange, which favored the microalgae growth in mode 2 and mode 3. Clinoptilolite loaded with urine components can be used as a slow-releaser for microalgae cultivation in mode 3. The biomass concentration of microalgae reached 1.09 g/L and 1.03 g/L at the end of two modes (mode 2 and mode 3), and the intracellular lipids and pigments were accumulated, which can be used as raw materials for further processing. The data analysis of the 3D excitation−emission matrix (EEM) revealed the humic acid-like substances accumulation induced by microalgae during operation. The novel CFBP system and three-stage mode provide an innovative solution for source-separated urine treatment. The system enables simultaneous pollutant removal and microalgae biomass recovery, advancing sustainable wastewater management and resource recycling.