LAUSR

The effects of the solid bed immersion in the anaerobic digestion of straw-cattle manure (SCM) were studied using two batch reactors of 2 m of solid height and 440 L of total volume. The reactors were operated in parallel with total (R1_100%) and partial solid height immersion (R2_74%) and no liquid recirculation. Recovered methane yield was 31.1% higher in R1 than in R2, 33.7 and 23 Nm3 CH4 t−1SCM respectively. The volatile solids (VS) and fiber degradation was studied in layers distributed each 0.5 m of the initial solid bed height profile; VS removal was measured at 16% at 2 m height in contrast to 39.9% at the reactor bottom. VS removal was related with hemicellulose and cellulose biodegradation, maximal hemicellulose and cellulose degradation in the studied layers were 68.2 and 49.5% respectively. Physical and rheological changes of the solid phase were measured between the SCM and the digestate. Macropores volume was reduced from 30.4% to values between 0.82 and 5.57%, this decrease was related to the water content and the fiber degradation state in each layer. Similarly, yield stress values obtained with the slump test depends on total solids content and fiber degradation state, yield stress values ranged from 1.41 and 2.23 kPa for raw and digested SCM. Moreover, values of cohesion and the friction angle were between 1.5 and 2.8 kPa and 15.6 and 47.7° respectively. Physical and rheological properties of digested SCM depends on the solid degradation state and the water availability through the material’s height profile.