Research progress on the mechanisms and influencing factors for the microalgae-bacteria symbiosis system for treating biogas slurry from livestock and poultry industry

A large number of ancillary products are inevitably produced during the anaerobic digestion of livestock and poultry manure. Among them, biogas slurry contains a high number of available nutrients, like nitrogen, phosphorus and potassium, as well as amino acids, humic acids, and indoleacetic acids. Correspondingly, biogas slurry is often used in agricultural practices in the form of organic liquid fertilizer. However, there are also many pollutants in the biogas slurry, such as high concentrations of ammonia nitrogen, total phosphorus, heavy metals and some refractory organic substances including antibiotics. Therefore, the discharge of biogas slurry without appropriate treatment is extremely likely to cause ecological and environmental risks, such as water eutrophication, soil acidification, and pollutant accumulation. Nevertheless, the reclamation and reuse of biogas slurry can be expected as the potential of promoting a circular economy and sustainable development worldwide from the viewpoint of resource reutilization. Fortunately, the microalgae-bacteria symbiosis system can serve as an emerging technology to treat the biogas slurry in a sustainable manner in recent years. Much attention has been gained in the biogas slurry treatment, due to the lower cost, higher efficiency, and much more environmental friendliness than before. Particularly, the microalgae-bacteria symbiosis system can be used to efficiently remove nitrogen, phosphorus, heavy metals, and antibiotics, in order to provide the sound purification of biogas slurry. The contributions can also be found in the recovery of nutrients from biogas slurry and the utilization of CO₂ for photosynthesis, leading to the microalgae biomass of biofuel value. Therefore, a systematic and latest review was proposed on this symbiosis system to consider the substantial development potential and great application prospects. The symbiotic mechanisms between microalgae and bacteria were first summarized, and then to provide a systematic review from the perspectives of the types of microalgae-bacteria symbiosis system, pollutant removal mechanisms and influencing factors. The symbiotic relationships between microalgae and bacteria included nutrient exchange, signal transduction, and gene transfer, thus supporting the operation of the microalgae-bacteria symbiosis system. Three types were divided, i.e., free type, attached type, and microalgae-bacteria photobioreactor, according to the existential state of microalgae and bacteria in the system and development sequence. No matter what type the system was, it was necessary to fully coordinate the pre-treatment of biogas slurry with the docking of the reactor in the process of system establishment, in order to maximize the purification performance and resource utilization potential. Besides, an overview was provided on the system’s removal of nutrients (i.e., nitrogen and phosphorus), refractory organic compounds, and heavy metals from biogas slurry. The microalgae-bacteria symbiosis system showed a lower energy demand and cost than before, indicating the effective performance of resource recovery. In addition, a brief discussion was conducted on the effects of microalgae-bacterial type/ratio and environmental factors (i.e., nutrients, aeration rate, pH, temperature, light conditions, heavy metals, and antibiotics). The appropriate environmental factors and type/ratio of microalgae-bacteria played a crucial role in the better growth and metabolism of microalgae, the removal of pollutants, and the stability of the system. Finally, the main challenges of this system were proposed for the promising research directions. A relatively complex mechanism was found in the microalgae-bacteria symbiosis under the biogas slurry environment. By far, it was still unclear on the information about signal transduction and gene transfer between microalgae and bacteria. Besides, the promising directions were the coordination and docking between pretreatment and reactors. More importantly, the full chain of resource recycling can be realized from the notorious biogas slurry to the valuable microalgae biomass-based refined products. The study can also provide the scientific basis and theoretical guidance on the future large-scale application of microalgae-bacteria symbiosis system for biogas slurry treatment.