Effects of different treatments on the low-temperature cultivation of microalgae combining biogas power generation exhaust with biogas slurry

Large-scale biogas has been widely used in recent years. A secondary source of pollution can be found in the disposal of the biogas slurry rich in nitrogen and phosphorus and the exhaust gas with carbon dioxide (CO₂) from biogas power generation. Alternatively, microalgae can be cultivated using the exhaust gas and biogas slurry. Nitrogen and phosphorus can be effectively removed from the biogas slurry. The CO₂ can be concurrently captured in the exhaust gas. The purification can be simultaneously realized on the biogas slurry and exhaust from the power generation. However, the microalgae culture is required for heat preservation and warming measures, thereby increasing the energy consumption and operating costs of microalgae production in northern regions. Therefore, this study aims to explore the growth and pollutant removal of cultivated microalgae using low-temperature exhaust from biogas power generation and biogas slurry. The study area was taken as the Heilongjiang Province in Northern China. Chlorella sp. and Kirchneriella obesa were used as the experimental microalgal species. A graded cooling was used for the microalgae domestication. The procedure was started at 25 ℃ and decreased by 5 degrees each time to the minimum of 10 ℃. The temperature of the microalgae culture was also selected, according to the actual temperatures in winter greenhouses in daylight. A systematic investigation was made on the microalgae growth and nutrient removal from the biogas slurry. Some parameters were optimized, such as the biogas slurry concentration, aeration, CO₂ concentration, and aeration flow rate. The tolerance of microalgae was improved on the biogas slurry and CO₂ exhaust under low-temperature environments. The results showed that it was practically feasible for the microalgae culture in the biogas slurry at low temperatures. Both Chlorella sp. and Kirchneriella obesa grew well under the temperature of 10 ℃. Chlorella sp. was more tolerant to the biogas slurry than Kirchneriella obesa. There was an increase and then a decrease tendency with the increasing biogas concentration. The optimal performance was achieved in the biogas slurry concentration of 10%. The aeration effect of nano aerated disc stone was significantly better than those of the aeration stone, aeration column, and glass straight tube. The glass straight tube was the worst. The removal of ammonia nitrogen (TAN) increased with the increasing aeration flow rate. There was the first increase and then a decrease in the Chlorella sp. biomass and protein concentration, the total nitrogen (TN), TAN, total phosphorus (TP), and chemical oxygen demand (COD), with the increasing CO₂ concentration and aeration flow rate. Chlorella sp. shared a continuous increase in the percentage contents of chlorophyll a and b, with the increasing CO₂ concentration. While the percentage of carotenoids shared a constant decrease. Chlorella sp. grew better under 10% biogas slurry concentration, aeration with a nano gas disc stone, 8% CO₂ concentration, and 1.2 L/min aeration flow rate. The dry weight of Chlorella sp. biomass production and protein concentration reached 2.43 and 1.46 g/L, respectively, at the end of 20 d for optimal cultivation. Meanwhile, the removal rates of TN, TAN, TP, and COD reached 83.73%, 93.00%, 86.02% and 93.83%, respectively. The finding can provide a strong reference for the low-temperature and low-cost cultivation of microalgae in biogas slurry combined with biogas exhaust from power generation. Some implications can also be given to promote the rapid development and application of microalgae and biogas industries in cold regions.