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[Display omitted] • Synthesis of S-doped biochar (WPB-S) and N-doped biochar (WPB-T) from watermelon peel. • Investigation of WPB-S and WPB-T as an improver for biogas production from C. myrica. • The supreme biogas production (623 mL/g VS) was achieved using 50 mg/L of WPB-S. • Smaller total pore volume, S-functional groups and higher conductivity of WPB-S may play a key role in enhancing DIET. • A new direct interspecies electron transfer mechanism of WPB-S and WPB-T has been proposed. Although it was suggested that adding biochar to an anaerobic reactor may aid in digestion, its function has not been formally established. It has been suggested that anaerobic digestion (AD) is dominated and participated in by direct interspecies electron transfer (DIET). This study assessed the efficiency of S-modified biochar (WPB-S) and N-modified biochar (WPB-T) using Triethylenetetramine (TETA) derived from watermelon peels biomass as an additive for the production of biogas from brown algae cystoceira myrica (C. myrica) either co-digested or individually with WPB-S or WPB-T. BET, FTIR, TGA, XRD, XPS, SEM, and EDX were used to characterize the prepared biochars. FTIR analysis confirmed the establishment of S and N groups on the surface of biochar. The modified Gompertz kinetic model fits the investigational data satisfactorily, with R 2 ranging between 0.907 and 0.990. Adding WPB-S at doses of 50 mg/L to C. myrica considerably enlarged the biogas yield (623 mL/g VS) compared to all other treatments. Finally, according to RSM study, the maximum biogas production (585.53 mL/g VS) was achieved for C. myrica and WPB-S treatment after 48 days of contact time using 50 mg/L WPB-S dose. The mechanism of DIET has been studied in detail, confirming that WPB-S can support electron transfer and conversion of CO to CH 4. DIET mechanism could be explained in new ways depending on the conductivity, XPS, FTIR and BET findings which confirm that S-functional groups, S% in the surface of WPB-S and smaller total pore volume are the main reason for higher biogas production. [ABSTRACT FROM AUTHOR]