Although the lab-scale SIBPD exhibited its possibility and potentiality for reducing nitrate, the nitrate reduction ability of SIBPD seemed rather lower than that of previous sulfur-based bioreactor (Moon et al., 2008, Sahinkaya et al., 2014 and Kong et al., 2014), the reasons might be speculated as follows: (1) elemental sulfur (S0) might have a relatively more stable and efficient Sirolimus denitrification performance than pyrite serving as electron donor. Batch tests demonstrated that SAD obtained 68.16 mg NO3−-N L−1 d−1 reduction rate while PAD was 44.69 mg NO3−-N L−1 d−1 (data not shown); (2) even though pyrite was ground into small particles, the grain size was still larger than sulfur powder. Therefore, the denitrification reaction was mainly taken place on the surface of the pyrite particles, upon which Fe(OH)3 generated from PAD would accumulate, gradually lowering the nitrate reduction rate; (3) the relatively lower nitrifying capacity in AES might have a negative effect on PAD, causing high residual NH4+-N concentration, and led to a further nitrification in ANS; (4) high influent COD and residual COD in Effluent A inhibited autotrophic denitrification. As organic carbon amount increased, the cultural environment might no longer fit for PAD. Consequently, heterotrophic process was likely to be involved with PAD and becoming dominant in ANS over time. Despite that this mixotrophic process could maintain a well nitrate reducing for a long period, insufficient carbon source would slow down heterotrophic process and the deterioration of autotrophic bacteria (such as T. denitrificans) would also result in nitrate reduction rate decreasing.