Experiment of the pilot-scale microalgal cultivation system with pig manure biogas slurry

Abstract: A large amount of pig slurry has been generated on the farm each year, particularly with the ever-increasing pig production in recent years. It is a high demand to fully utilize pig manure biogas slurry and microalgae cultivation at a large scale. In this study, a series of experiments were carried out on the multi-stage amplification cultivation with Chlorella E2E using pig manure biogas slurry on a pilot scale, according to the coupled mode of "indoor monoculture + outdoor monoculture + outdoor culture using biogas slurry". The raceway pond reactor was set as the largest area of 12.31 m3 in the pilot-scale experiments of "column reactor + plate reactor + runway pool reactor". An investigation was implemented to analyze the growth of Chlorella E2E performance in the pure and biogas slurry cultivation system. A systematic evaluation was also made to clarify the pollutants removal and the diversity of microorganisms in the microalgal cultivation system using pig manure biogas slurry under natural light conditions. Results showed that the Chlorella E2E was gradually adapted to the reactor environment after the multiple batches of pure culture and acclimatization. Specifically, the Chlorella E2E quickly reached the optimum growth state in the second column reactor in indoor monoculture, where the average maximum biomass reached 227.72 mg/L on the 70th day. The fitting relationship was favorable between the inoculum amount, cultivation time and bioaccumulation in each stage of the indoor reactor. A characteristic equation was obtained to predict the biomass yield or cultivation time, particularly for the management of the monoculture mode of Chlorella E2E. The growth capacity of Chlorella E2E in an outdoor monoculture plate reactor was significantly higher than that in an indoor column reactor, where 266.48 mg/L Chlorella E2E biomass was produced on the 14th day. In addition, the highest daily biomass productivity of Chlorella E2E and mean specific growth rate (μ) were achieved in 11.10 mg/L/d and 0.062 /d, respectively, in the flate bioreactors under natural light conditions. The reason was that the Chlorella E2E received sufficient light to effectively utilize the light energy under outdoor natural light conditions (the local average light intensity was 211.76 μmol/(m2·s)). Correspondingly, the raceway pond was more suitable to culture Chlorella E2E for the pig manure biogas slurry than the flat plate reactor in outdoor culture using biogas slurry. The maximum biomass accumulation reached up to 245.98 mg/L with the highest Chemical Oxygen Demand (COD), NH4+-N, and Total Phosphorus (TP) removal of 40.05%, 95.06%, and 97.52%, respectively. The short light path of the plate reactor was much more conducive to the light reception in the pure culture stage, compared with the raceway pond. Previously, the short light path failed to fully culture microalgae using biogas slurry, due to the low light transmittance of biogas slurry. By contrast, the mechanical mixing enhanced the microalgal production to relieve the dark color of the biogas slurry in the raceway pond. There were diverse microbes in the raceway ponds, particularly with the higher performance. A bacteria-algae symbiotic system was easily formed to maintain the stability of the cultivation system. A better performance was achieved to remove more pollutants from the biogas slurry. This finding can also provide a technical reference for the practical application of microalgae cultivation using biogas slurry.