Microalgae-mediated biogas slurry resource utilization: heavy metal risk control and aquaculture benefit enhancement

Biogas slurry with high concentration of heavy metal stemming from intensive breeding of livestock and poultry has been becoming a huge threat to environment in China. The aim of this study was to evaluate the safety risks by quantitatively analyzing the transfer of heavy metals along the ecological chain of biogas slurry - microalgae - GIFT tilapia, and assess the efficacy of resource-oriented utilization of biogas slurry in aquaculture. Biogas slurry was pretreated with FeSO₄. Chlorella was cultivated in 30% pretreated slurry using a 500 L column photobioreactor. The harvested microalgae were incorporated into a commercial fish feed at a 25% dry weight substitution rate and fed to GIFT tilapia (GIFT Oreochromis niloticus) for 60 days. The growth performance and muscle nutritional composition of fish in two experimental groups were compared. ICP - MS was used to detect the contents of heavy metals in various treatment stages of biogas slurry, algal bodies, feed, and the muscle, liver, skin, and gills of the fish. The results shew that slurry turbidity was reduced by 80.8% and the elimination of heavy metals (As, Pb, Hg, Cd, Cr) reached to 47.5%-81.4% with treatment of 800 mg/L FeSO₄ (pH 9). Chlorella cultivation removed 33.5% of COD, 18.2% of ammonia nitrogen, 39.2% of total nitrogen, and 68.9% of total phosphorus from the slurry, while increasing algal biomass by 1.5-fold. Compared to the control group, weight gain rate of fishes feed food containing 25% microalgae significantly increased by 45.4% , while specific growth rate increased by 13.8% (P < 0.001), the concentration of As and Cr in muscles were decreased by 18.1% and 18.8%, respectively. Meanwhile, the activity of serum aspartate aminotransferase (AST) was significantly lower than that of control (P = 0.006), and serum total protein was significantly higher (P = 0.042) . There was no significant change of nutritional profile of the muscle tissue in two group. The data demonstrate that reduction of heavy metal make, microalgae growing in biogas slurry fit to aquaculture and result in resource recovery.