In this study, the effects and mechanism of pretreating digestates with cetyltrimethylammonium bromide (CTAB) and coal fly ash (CFA) were investigated. The water content (WC) and specific resistance of filtration (SRF) of the digestates were measured to evaluate the digestate dewaterability. The protein (PN) content, polysaccharide (PS) content, zeta potential and particle size of the digestates were measured to probe the mechanism of pretreatment. The results indicated that the digestate dewatering performance was effectively improved by joint conditioning with CTAB and CFA. For straw digestate (St-digestate), pig manure digestate (PM-digestate), cow dung digestate (CD-digestate) and kitchen waste digestate (KW-digestate), the optimum dosage of CTAB was 50%, 30%, 20% and 50% dry solid (DS) of digestate, respectively, and the optimum dosage of CFA was 100% DS of digestate. Co-conditioning with CTAB and CFA resulted in clear advantages compared to CTAB or CFA conditioning alone for decreasing the WC of digestates. The proposed mechanism is the synergistic effect of CTAB and CFA. CTAB releases extracellular polymeric substances (EPS) and neutralizes charge, and CFA acts as a skeleton builder in the dewatering process to enhance dewaterability. Finally, a green degree method for the environmental assessment of chemical processes was adopted to evaluate digestate dewatering. (C) 2016 Elsevier B.V. All rights reserved.