Research progress of biogas slurry treatment and resource utilization

Abstract: A large amount of biogas slurry has gradually become an important part of sewage sources with the rapid development of large and medium-sized biogas projects in China in recent years. Biogas slurry, as a residue of anaerobic fermentation, generally cannot meet the national discharge standard of aquaculture wastes, due to the high concentration of organic matter and nutrients. Effective disposal of biogas slurry can greatly contribute to the surrounding environment, while saving agricultural resources. There are most useful matters in the biogas slurry for plants, including nitrogen, phosphorus, potassium, microelement, amino acids, humic acid, indoleacetic acid, vitamin B, and some auxin. Correspondingly, biogas slurry can widely be expected to serve as the best organic fertilizer in pollution-free agricultural products for better crop growth and soil properties without diseases. Additionally, it can also be used as feed additive for animal growth, where there are essential vitamins and hormones. However, biogas slurry also has many pollutants, such as high concentration of chemical oxygen demand (COD), biochemical oxygen demand (BOD), and ammonia nitrogen (NH4+-N). In addition, it even has harmful substances, such as heavy metals (for instance, mercury, chromium, cadmium, arsenic, and lead), which are easy to settle, antibiotics, and pathogenic microorganisms. Therefore, the disposal and resource utilization of biogas slurry are highly demanding to alleviate the impact of current industrial development on the environment. Traditional wastewater treatment can achieve the purification of biogas slurries, such as activated sludge process, membrane concentration, and chemical flocculation. But how to improve the efficiency of treatment needs to clarify in further researches, particularly for the waste of resources and high-cost investment in current direct processing. Among them, the membrane concentration of biogas slurry can intercept most pollutants, while considering high-value utilization of nutrients recovery. However, it is difficult to achieve a lower effluent concentration of pollutants using whether single-stage or multistage combined treatment, where there are material erosion, pollution, limited service, and the high cost of membrane concentration. Biochemical methods have an obvious removal effect for pollutants in biogas slurry, with the high removal rate, and the effluent over the discharge standard. MBR and SBR processes can achieve a higher removal rate of ammonia nitrogen, compared with the UASB and A/O processes. The removal efficiencies of suspended solid (SS) and COD are significantly improved using chemical crystallization and flocculation as biological pretreatment, while the subsequent treatment pressure and the hydraulic retention time (HRT) are effectively reduced. Biogas slurry reflux can increase the total amount of microorganisms in the fermentation system, the gas production, and methane yield for better performance of anaerobic fermentation with water-saving consumption under the premise of appropriate reflux ratio. However, it is difficult to determine the reflux of biogas slurry, due to the participation of different substrates in the fermentation, the control conditions of fermentation reaction, and the storage conditions of biogas slurry. At the same time, there are also a series of problems, such as ammonia nitrogen inhibition, volatile organic acids, and cellulose accumulation, leading to the reduction of gas production and methane content, even the termination. Therefore, the reflux ratio should be set strictly in practical application, according to the properties of fermentation substrate and biogas slurry to prevent the occurrence of inhibition of gas production. Microalgae culture using biogas slurry is more significant to realize resource reuse, compared with the traditional physical and biochemical way. However, it still remains unsolved to achieve the conversion of microalgae into biofuels, the cost-saving, and high technical level. Biogas slurry has the potential as an organic quick-acting fertilizer for the high standard of farmland irrigation, due to the 17 kinds of bioactive substances in addition to macroelements and microelements. An active role of biogas slurry can be found to improve the crop yield, soaking seeds, insect resistance and bacteriostasis, soil fertility, and livestock breeding. It can thus become an efficient substitute for chemical fertilizers in agricultural development, particularly for the most economical and effective way to the resource utilization of biogas slurry. In view of the considerable amount of biogas slurry production, limited land consumption and low degree of resource utilization in China, it is very urgent to formulate a reasonable and efficient combination of biogas slurry disposal and resource utilization for the most healthy and sustainable agriculture.