Correlation of changes in rheological properties with the growth kinetics of Haematococcus pluvialis in municipal wastewater

The objective of this research was to evaluate the biodegradation of municipal wastewater through the cultivation of Haematococcus pluvialis in a 4-L airlift photobioreactor over a period of 14 days, establishing the relationship between growth kinetics and changes in the rheological properties of the medium as a potential process monitoring strategy. To identify the ideal inoculum percentage, a unifactorial experimental design was implemented, where tests were conducted at different levels to determine statistically significant differences between them based on the biodegradation of organic matter. Three inoculum levels (5, 10, and 15%) were analyzed in 400 mL photobioreactors where cell growth, total COD, soluble COD, and pH were monitored. Once in the airlift photobioreactor, changes in shear stress and apparent viscosity were monitored in a range of 0.1–200 rpm, along with cell density growth and pollutant degradation (total COD, soluble COD, total nitrogen, and total phosphorus) every 2 days for 14 days. Laboratory-scale experiments showed soluble COD removals exceeding 95%, with the 15% inoculum level statistically showing the highest organic load removal. In the airlift reactor scale-up, a soluble COD removal percentage of 78% was achieved, and concentrations and concentrations values below the maximum permissible limits established by Mexican regulations (NOM-001-SEMARNAT2021) were obtained for phosphorus and nitrogen content. The rheological monitoring showed an affinity to the Herschel–Bulkley model (R² > 0.99), with a decreasing trend observed in the consistency index (k), which decreased from 8.517 × 10⁻⁵ to 1.453 × 10⁻⁵, and an increasing trend in the flow index(n), which rose from 1.583 to 1.905. This improvement is attributed to the reduction of soluble COD to values as low as 180 mg/L and to the increase in microalgal biomass, which reached up to 1.3 × 10⁷ cells/mL.