Influence of intermittent minimal mixing intensity on high-solids anaerobic digestion energy efficiency of dairy manure in a pilot-scale stirred tank digester

• Published in: International journal of energy and environment 2022-03, Vol.13 (2), p.53-64
• Main Authors: Denka Kariyama, Ibrahim, Li, Weixiang, Yu, Shaoqi, Chen, Long, Qi, Rui, Zhang, Hao, Li, Xiaxia, Deng, Xin, Lin, Jian, Wu, Binxin
• Format: Article
• Language: English
• Subjects: Alternative energy sources; Anaerobic conditions; Anaerobic digestion; Animal wastes; Biogas; Carbon dioxide; Dairy wastes; Digesters; Energy efficiency; Greenhouse gases; Hydraulic retention time; Influence; Load distribution; Loading rate; Manures; Metabolism; Methane; Microorganisms; Organic loading; Refuse as fuel; Renewable resources; Retention; Retention time; Volatile solids
• ISSN: 2076-2895; 2076-2909

The influence of intermittent minimal mixing intensity on high-solids anaerobic digestion energy efficiency of dairy manure was investigated in a pilot-scale anaerobic stirred tank digester operated under mesophilic temperature conditions. A non-mixed experiment was also investigated. Three mixing intensities were investigated; 50, 100 and 150 rpm mixed only once during feeding for a constant mixing time of 5 minutes. The volatile solids concentration ranged from 105.74 to 135 kg m3, with an organic loading rate varying from 3.5 to 4.5 kg VS m3 d"1 for a 30-day hydraulic retention time. The results of the methane yield and specific methane production rate show that the 100 rpm performed better than the 50 rpm which also performed betterthanthe 150 rpm mixing intensity. This research confirms that there exists a mixing intensity threshold for every anaerobic digestion setup and above which increasing the mixing intensity is a waste of energy and does not increase methane production but rather may reduce it. The results of the net energy production in kilowatt hours confirms that the 100 rpm is the economical speed, followed by 50 and 150 rpm. A high mixing intensity is not beneficial to increase methane production but rather waste energy used for mixing and should be avoided. Mixing intensity within the ranges of 50 to 100 rpm is therefore ideal for optimum methane production. Using the net energy production is the best criteria in determining the mixing mode, mixing intensity, mixing time and mixing interval for every anaerobic digestion operating plan.