## Results  
### Culture's Dynamic Performance  
The objective of this study was to evaluate the potential of selecting a PHA-producing MMC capable of using proteins as a nutrients source, eliminating the need for nutrients supplementation. The fCW had a C/N ratio of 100/5 (Table 2). If all the nitrogen content in the proteins of the fCW fed to the selection reactor was used for growth by the PHA-producing organisms, and assuming the general biomass formula (C\({}_{5}\)H\({}_{7}\)NO\({}_{2}\)) and the OLR and SRT applied in this study (40 Cmmol\(\cdot\)L\({}^{-1}\).d\({}^{-1}\) and 4 d, respectively), it would sustain a biomass concentration of ca. 0.9 gVSS\(\cdot\)L\({}^{-1}\) in the SBR. The experiments started with nitrogen supplementation at the levels reported in [PERSON] et al. [15] (Period I) and was gradually decreased until being totally removed, from operational Period I to V. The dynamic system's response to the imposed changes was depicted in the biomass concentration and the F/f ratio trends along time (Figure 1). In Period I, cellular growth was chiefly supported by the ammonia-nitrogen, since the observed biomass concentration (average 1.8 gVSS\(\cdot\)L\({}^{-1}\)) was close to that estimated if all ammonia in the feed, both supplemented and present in the fCW, was used for growth (2.0 gVSS\(\cdot\)L\({}^{-1}\)). This reveals that even though the proteins were consumed (in average 27.7 mg per cycle, \(\Delta\)Prot, Table 3), protein-nitrogen was negligibly used by the cells, when a fair amount of ammonia was available. Under the reference operation conditions of Period I, the selected culture was efficient in storing PHA, accumulating ca. Figure 1: Biomass concentration (\(X\), \(\times\)) and least/famine ratio (_F/f_, \(\blacktriangle\)), over time, in the selection SBR, operated under the five operational periods (I-IV), and representation of the calculated X, if the total nitrogen (_solid line_) and all the ammonia–nitrogen (_dashed line_), present in the feed, were used for cellular growth. 2 wt.%, during the cycles, with a storage yield of 0.5 Cmol-PHA.Cmol-HOrgs\({}^{-1}\) and a specific storage rate of 0.18 Cmol-PHA.Cmol-X\({}^{-1}\).h\({}^{-1}\) (Table 3). When the nitrogen supplementation was diminished to half, from Period I to II, a reduction on biomass concentration was initially observed. Consequently, due to the lower concentration of cells in the reactor, the least-phase increased, which was reflected in the increase of the F/f ratio (Figure 1). Nevertheless, the system promptly recovered, and after 9 days, during which the culture was probably acclimatising to the new conditions, both the biomass concentration and F/f ratio steadied. The biomass concentration stabilised in a similar value to that obtained in Period I (1.7 \(\pm\) 0.1 and 1.8 \(\pm\) 0.1 gVSS\(\cdot\)L\({}^{-1}\), Period II and I, respectively), which could not be supported solely by the supplied ammonia consumption (estimated value of 1.1 gVSS\(\cdot\)L\({}^{-1}\)), indicating that proteins were partially used as a nitrogen source during Period II (in average, ca. 32.5 mg consumed per cycle, Table 3). The F/f also stabilised, approximately, in the value attained in Period I (0.20 \(\pm\) 0.1 and 0.22 \(\pm\) 0.1 in Period I and II, respectively, corresponding to least periods of ca. 2 h). The F/f ratios in Periods I and II were within the range of values (F/f of ca. 0.2) which are generally recognised to induce the selection of organisms with a strong storage-response [32]. The efficient selection of a PHA-storing culture in Periods I and II was confirmed by the culture's performance parameters, both in the selection reactor and in the accumulation tests (Table 3). In the selection reactor, the enriched-culture produced a relatively high amount of PHA, during the cycles (\(\Delta\)PHA = 22 wt.%), indicating that, even though high PHA contents were achieved, during the least phase, the microbial culture exhausted all the stored reserves, during the famine phase.