Phagotrophic microalgae production from waste activated sludge under non-sterile conditions

In this study waste activated sludge (WAS) was sonicated to release bacteria-sized volatile solids (VS) from flocs, after initial pH adjustment to 10 for higher energy efficiency. The released VS supported growth of phagotrophic alga Ochromonas danica. Initial-rate growth experiments confirmed the Monod-type kinetics but the specific cell growth rate, μ, correlated with the prey-to-predator ratio, i.e., the ratio of (fed VS concentration)-to-(initial O. danica concentration), significantly better than with the VS alone, as the typical Monod dependency on soluble substrates. The best-fit kinetics had the following parameters: μ_max = 0.198 h^-1 and K_M = 1.056 (g-VS/g-algae). Post-sonication reflocculation could render particles too large to ingest by O. danica; therefore, pH and VS effects on reflocculation were investigated. Batch cultivations were then conducted in fermentors at pH 5, under nonsterile conditions. Algae number reached 8.86 × 10¹⁰ L^-1 after 20 h, corresponding to ∼2.3 g/L dry-weight and volumetric algae productivity of 2.8 g/L-day. VS reduction was 38%, giving an O. danica VS yield of 44.5%. The ultrasonication-algae process can be used to produce algae while achieving at least partial WAS treatment.