# Urban Biorefinery Demonstration: Production of Polyhydroxyalkanoates from a Municipal Solid Waste
## 3 Results  
### Enrichment of scl-PHA-Accumulating Microorganisms  
The enrichment process was carried out in a 20 m\({}^{3}\) fermenter over 214 days to develop a microbial culture capable of accumulating scl-PHA.  
Due to the slow growth of MMC observed during the first cycles, and to favor the growth of accumulating bacteria, allythiourea (ATU) was added (2 g/L) to the daily feeding from cycle 28 during three TRH, i.e., up to cycle 43, to reach a faster stabilization of culture broth rich in accumulating bacteria. ATU was employed to remove the nitrifying bacteria that can subsist in the famine phase due to its capacity to obtain energy from ammonium oxidation without a carbon source [46]. The addition of ATU to inhibit nitrifying bacteria in PHA-producing fermenters is a common practice to avoid the competition of these bacteria with PHA-accumulating bacteria for ammonium and oxygen [58; 62]. Figure 2 shows the evolution of TSS, VSS, pH, and COD over all the enrichment cycles performed.  
An example of a F/F cycle at steady-state conditions and COD consumption during the cycle can be observed in Figure 3.  
Figure 3: DO profile and COD consumption of the enrichment fermenter in 20 m\({}^{3}\) during cycle 137.  
Figure 2: Evolution of COD, TSS, VSS, and pH in the enrichment fermenter feeding VFA-rich digestate during 214 days.  
Feast and famine phases were monitored through the variation in DO concentration over the cycle. When an external carbon source was available (VFAs), DO decreased due to substrate consumption and the shift between feast and famine was characterized by a sudden increase in DO as a result of carbon source depletion [47]. The VFA profile during the whole cycle was not monitored but it was indirectly measured through the soluble COD (CODs) as samples were taken at different times during an enrichment cycle. The CODs reached values lower than 100 mg/L from approximately the third hour of the cycle until its end. The CODs monitorization started right once the VFA daily feeding of the enrichment fermenter was completed. Hence, the initial COD measure was 850 mg/L instead of 1000 mg/L.  
Thus, when the OLR was set at 1 g COD/Ld (cycle 51), the COD consumption rate was monitored at different cycles during operation (Table 3).  
The scl-PHA average percentage in the enriched biomass (before accumulation stage), assessed over 214 days of operation, was 0.39 \(\pm\) 0.12 wt.%.  
Figure 4 shows the temperature profile of the MMC during the whole enrichment operation.  
Figure 4: Temperature evolution in fermenter during the enrichment operation.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Cycle**} & \multirow{2}{*}{**TSS (g/L)**} & \multicolumn{3}{c}{**COD Consumption Rate in the**} \\  & & **VSS (g/L)** & \multicolumn{2}{c}{**Enrichment fermenter**} & \multirow{2}{*}{**g COD/g SSV-h**} \\  & & & **(mg COD/L.Min)** & \\ \hline
29–34 & 2.89 \(\pm\) 0.24 & 1.95 \(\pm\) 0.27 & 10.11 & 0.31 \\
35–62 & 3.05 \(\pm\) 0.52 & 2.21 \(\pm\) 0.45 & 10.41 & 0.28 \\
63–81 & 3.30 \(\pm\) 0.66 & 2.28 \(\pm\) 0.48 & 11.98 & 0.32 \\
82–132 & 2.99 \(\pm\) 0.48 & 2.19 \(\pm\) 0.66 & 13.91 & 0.38 \\
133–214 & 2.91 \(\pm\) 0.74 & 1.92 \(\pm\) 0.55 & 19.56 & 0.61 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Evolution of the COD consumption rate monitored at state conditions in the enrichment fermenter at the beginning of different cycles.