Application of experimental design techniques in the optimization of the ultrasonic pretreatment time and enhancement of methane production in anaerobic co-digestion

In this study, wheat straw (WS) and dairy manure (DM) were ultrasonically pretreated to optimize the time for ultrasonic pretreatment (UP), identify the most appropriate optimization method, clarify whether UP is a high efficiency method for application, and explain why UP improves total methane production (TMP). The pretreatment conditions were designed using orthogonal experiment design (OED) and central composite design (CCD) and were optimized using the direct measurement method and response surface method, the relationships among the initial digestion characteristics and TMP were subsequently analyzed, the net energy, and energy benefit were calculated. The OEM results showed that mixed DM pretreated for 30minutes (min) and WS pretreated for 20min (DM30WS20) produced the maximum TMP, net energy, and energy benefit of 186mL/g TS (total solid), 6.04kJ/g TS, and 2.88kJ/g TS, respectively. For CCD, the maximum TMP, net energy, and energy benefit were 146.61mL/g TS, 4.91kJ/g TS, and 1.75kJ/g TS, respectively, and were obtained with DM9WS25. The maximum TMP, net energy, and energy benefit of OED were greater than those of CCD, indicating that OED is more suitable for UP in anaerobic digestion (AD). When pretreated with UP, the surface of DM showed an increasingly uniform distribution and WS became increasingly rough and displayed fractures of different degrees. Cellulose activity contributed the maximum (0.3856) direct decision influence to TMP, and the comprehensive decision influence at a pH of 0.328 was the highest in TMP. This study concluded that the direct measurementmethod in OED was best suited for the design and optimization of pretreatment conditions by UP in AD. UP improved TMP, net energy, and energy benefit by changing the initial environment of AD. Therefore, UP is a high-energy benefit method and worth popularizing for AD.