Investigation of Green Microalgae Cultivation on Two Different Waste Streams: Growth Kinetics, Nutrient Removal, and Pilot-scale Cultivation
Abstract
The increasing environmental challenges associated with oil sands tailings ponds, particularly the persistence of naphthenic acids (NAs), necessitate innovative bioremediation strategies. Concurrently, the rapid expansion of insect farming has led to the underutilization of cricket frass, a nutrient-rich byproduct with potential applications in microalgal cultivation. This study explores the dual prospects of two microalgae species, Chlorella vulgaris (C. vulgaris) and Parachlorella kessleri (P. kessleri), for NA biodegradation and the utilization of chitin and cricket frass as alternative nitrogen sources in microalgal biotechnology.
In simulated oil sands tailings pond conditions, both microalgal species tolerated salinity up to 4.5 g/L NaCl. C. vulgaris exhibited stable growth across all NA types at 40–130 mg/L, while P. kessleri showed variable responses. COD analysis revealed that C. vulgaris partially degraded T-4 NAs (16% reduction), whereas P. kessleri demonstrated a 70.3% biomass productivity increase in its presence. However, neither species significantly degraded F-1 or Cyclo NAs, indicating the necessity of microbial consortia for enhanced remediation.
Additionally, C. vulgaris growth kinetics were investigated using chitin and cricket frass as sustainable nitrogen sources. Microalgae cultivated in chitin-containing media exhibited a 47% increase in biomass yield compared to nitrate-based media, with 6.67 g/L chitin improving productivity by 33%. Total nitrogen analysis confirmed chitin biodegradation, with soluble nitrogen initially rising before uptake by C. vulgaris. Cricket frass also effectively supported microalgal growth, with concentration of 4 g/L frass optimizing nitrogen removal, while higher concentrations induced light limitation. C. vulgaris successfully formed biofilms on chitin-incorporated polyvinyl alcohol (PVA) gel, penetrating its structure and utilizing biodegraded chitin, demonstrating its potential as a controlled-release nitrogen source. Lipid accumulation was 1.55-fold higher in chitin-fed cultures compared to BBM, whereas frass-grown microalgae exhibited a 14% lipid reduction due to a more balanced nutrient profile.
The scalability of cricket frass as a nutrient source was further assessed in bench-scale (10-L) and pilot-scale (660-L) photobioreactors. Optimal growth occurred at 2 g/L frass, achieving 57.4 billion cells/g frass, while higher concentrations reduced yield due to light limitation. Growth kinetic modelling using a two-phase Monod equation confirmed nitrogen as the limiting substrate, with R² = 0.93 for cell density and R² = 0.80 for soluble nitrogen. Pilot-scale fed-batch cultivation with incremental feeding from day 2 enhanced nitrogen (94.7%) and phosphorus (84.1%) removal, with COD removal reaching 93.9%. A combination of microfiltration and sedimentation strategy in a membrane tank efficiently concentrated biomass by 15.5-fold, though filtrate reuse necessitated sterilization due to coliform presence.
These findings establish C. vulgaris as a viable candidate for cultivation in wastewater containing NA and confirm the feasibility of chitin and cricket frass as sustainable nitrogen sources for large-scale microalgal cultivation. The study also demonstrates the potential of chitin-incorporated PVA gel for controlled nutrient release and biofilm formation, contributing to sustainable wastewater treatment and resource recovery strategies.