Experimental Assessment of the Effects of Ultrasound on the Dewaterability and Agronomic Properties of High-Solid Food Waste Digestate

Dewatering of digestate is an important post-anaerobic digestion treatment of full-scale plants useful to reduce storage volumes, transportation costs and facilitate subsequent agricultural management or other recovery and disposal processes. Dewatering process is particularly challenging for digestate from high-solid anaerobic digestion due to its peculiar characteristics. In this study, high-solid food waste digestate was subjected to ultrasound treatment at a laboratory scale to evaluate for the first time the effects both on dewaterability and agronomic properties. Digestate was ultrasonicated (100 W, 30 kHz) at different time (1, 2.5, 5, 10, 15 and 25 min) and temperature (25 and 60 °C). Capillary suction time and time to filter tests showed better digestate dewaterability with increasing sonication time for both temperatures. Centrifugal dewatering coefficient reached the maximum value of 1.19 at 15 min of sonication and 25 °C. Viscosity, total chemical oxygen demand, extracellular polymeric substances, total solid and volatile solid contents generally decreased with increasing sonication time, while soluble chemical oxygen demand to total chemical oxygen demand ratio increased. The amount of macronutrients (N, P, K) and microelements (Mg, Ca, Fe, Zn) increased in the solid phase recovered after centrifugation of the sonicated digestate, which gained a higher agronomic value. The results of this study demonstrate that sonication notably improves the digestate properties and, thus, can be considered as a useful post-anaerobic digestion treatment for possible large-scale applications.