the water phase was mainly released from the polyphosphate in
phosphate-accumulating organisms (PAOs). The TP content
released from PFS-T was 26.7 ± 0.3 mg/gVSS, and the TP release
efficiency was 26.4%, which was similar to the TP release efficiency
(29%) reported previously under similar thermal pretreatment conditions
(Zhang and Kuba, 2014). However, the TP content released
from PGS-T and the TP release efficiency (39.3 ± 0.7 mg/gVSS and
43.7%, respectively) were much higher than those from PFS-T. In
the EBPR processes, EPS could act as a reservoir for P. At the end
of the aeration phase, approximately 5–10% of TP in the flocculate
sludge accumulated in EPS (Li et al., 2010; Zhang et al., 2013),
while in the EBPR granular sludge, the TP accumulated in EPS could
be 30% (Wang et al., 2014). These studies demonstrate that PGS has
much more TP reserved in EPS than PFS. Therefore, much more P
could be released from PGS than from PFS via low-temperature
thermal pretreatment, leading to the higher content of TP released
from PGS-T. In addition, PO4 3-P release was also observed in PGS-T
and PFS-T. This is because polyphosphate could be partially
degraded to PO4 3-P at 70–90 C