Structural equation modeling unveils the dual impact of suspended solids on microalgal growth and nitrogen removal

Cultivating algae in swine wastewater (SWW) leverages the nutrients in the wastewater. However, the role of suspended solids (SS) in algae growth and nitrogen removal remains unclear. Here, we report a 14-day batch trial that employs correlation and structural equation model (SEM) analyses to assess the effects of SS removal treatments. The untreated SWW group achieved the highest ammonium nitrogen (NH₄⁺-N) and total nitrogen (TN) removal efficiencies at 79.7 % and 80.9 %, respectively, whereas the ultrafiltration-treated group produced the maximum algal biomass (404 mg/L). On day 2, NH₄⁺-N and TN removal correlated positively and significantly with wastewater transmittance (p < 0.05), highlighting the inhibition of SS-induced shading. On day 8, these removals were associated with TN, phosphate phosphorus (PO₄^3--P), and total phosphorus (TP) concentrations, suggesting the onset of nutrient release from SS. On day 14, nitrogen assimilation and uptake had nearly ceased, with chl-a and chl-b synthesis correlating with PO₄^3--P (R = 0.93-0.94) and TP concentrations (R = 0.77), respectively (p < 0.01). Furthermore, SEM revealed that, on day 2, the shading effect of SS indirectly inhibited NH₄⁺-N removal by reducing photosynthetic pigment synthesis (standardized coefficient: 0.88). Conversely, on day 8, NH₄⁺-N removal was directly driven by phosphorus availability from SS (standardized coefficient: 0.95). These findings demonstrated that SS plays a dual role in algae growth and nitrogen removal from SWW; this role closely tied to the reciprocal regulation of NH₄⁺-N assimilation and photosynthetic pigment synthesis. Overall, this study offers critical theoretical and technical support for optimizing algal cultivation in SWW.