Biogas fermentation of swine slurry based on the separation of concentrated liquid and low content liquid

This paper aims to demonstrate the benefits of using gravitational sedimentation to separate raw swine slurry into two substances: one thick and highly-concentrated, the other a low-concentration liquid. The volume of the concentrated liquid, accounting for 18.3% of the raw slurry, contained about 61.0% of the chemical oxygen demand or biochemical oxygen demand and could produce about 60% of the total biogas production potential. The digestion temperature could be raised by approximately 22 °C by heating the concentrated liquid with the excess heat of biogas generation but only by about 6.43 °C by heating the raw slurry. The size of digester with the new process could be reduced by two-thirds that of the digester with the conventional process. After separation, the digestate of the concentrated liquid, with nearly three times total nutrient content as that of raw slurry, can readily be used as fertilizer and the low content liquid can easily meet discharge standards after aerobic treatment.