Recursos y gobernanza

The common strategy used for the PHA accumulation step is the nutrient-limiting conditions during the entire step. Under this condition, the uptake of carbon is mainly driven for PHA storage until it reaches a saturation level inside the cell. As for culture selection, the evaluation of the maximum PHA storage capacity was performed mostly with pure substrates. Only recently real complex wastes were used to evaluate the storage capacity of the cultures selected.

The influence of the initial substrate concentration on the PHA storage has been mostly investigated to optimize the PHA accumulation step. In order to be able to achieve a saturation PHA content it is necessary to apply a high substrate to biomass ration. However, it has been shown that high substrate concentration can be inhibitory and limit the kinetics of substrate uptake and PHA storage (Serafim et al. 2004). Pulse feeding strategy has been widely used to prevent inhibition by the substrate. Alternatively, a continuous feeding strategy has also been

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reported (Johnson et al. 2009; Albuquerque et al. 2011) which can only be applied when either a very high feed solution concentration can be used (so as not to affect the reaction volume) or if a membrane separation process can be attached to the bioreactor thereby continuously recirculating the cells back to the reactor.

In general, the results obtain with waste-based substrates are lower than those reports for mixed cultures using synthetic feedstocks. The highest values obtain with MMC and synthetic substrates were reported by Johnson et al. 2009 (89% PHB cell dry weigh with acetate) and Jiang et al. 2011b (92% PHB cell dry weigh with lactate). The majority of works that used waste-based substrates and MMC to produce PHA reported values between 20-48% PHA (cell dry weight). As an exception, Albuquerque et al. 2010b reported a PHA content of 75% (cell dry weight) using fermented molasses to fed MMC in a pulse feeding strategy. Later, using the same substrate but with a continuous feeding strategy, Albuquerque et al. 2011 reported a PHA of 77% (cell dry weight). Recently Jiang et al. 2012 have reported a PHA content of 77% of cell dry weight using MMC and paper mill wastewater as substrate. So far, these two works have reported the highest PHA content using MMC and waste-based substrates. The main different among them are the strategy used to fed the MMC and the feedstock used, resulting in different enriched microbial culture and different type of PHA produced. In the case of Jiang et al. only the homopolymer PHB was produced, on the case of Albuquerque et al. a copolymer (PHB-co-HV) was formed providing a higher broad range of application.

The gap existing between synthetic substrates and waste-based substrates can be justified, on one hand, by the fact real substrate typically contain organic matter other than VFA, even after acidogenic fermentation. This fraction is composed by different types of chemical species with different degrees of biodegradability which may include alcohols, unfermented sugars or compounds not susceptible to fermentation. This non-VFA fraction of the total organic matter present in this type of waste based feedstocks may be consumed by PHA-accumulating organisms but not serve as PHA precursors or eventually may be used for PHA storage but at different rates, or it might also be consumed by non storing organisms, which can have a negative impact on the maximum accumulation capacity of the selected culture. Furthermore, the presence of inhibitory compounds may also negatively affect the process kinetics

The composition of the substrate alone cannot account for the full gap between PHA production using MMC selected with synthetic feedstocks and those selected using fermented waste-based substrates. The impact of the type of feedstock on the enrichment reactor may condition the degree of enrichment obtained from these feedstocks, subsequently limiting batch accumulation performance in the final production step. Considering this, Dionisi et al. 2005 used a synthetic medium (ace+prop+lact) to select a culture with high PHA storage capacity and subsequently

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fed with a complex feedstock (fermented olive oil mill effluent (OME)) only in the final accumulation stage achieving a PHA content of 54% (cell dry weight) which is higher than the most cultures selected using fermented feedstocks.

As mention before, VFAs are considered as the main precursors to produce PHAs from MMC. However, a few works have reported the direct used of non-VFA organic matter for PHA storage. Gurieff et al. 2007 obtain a PHA content of 20% (cell dry weight) with primary sludge and 39% with fruit cannery wastewater using a mixed culture enriched with primary sludge. Liu et al. 2008 reported a PHA content of 20% (cell dry weight) using tomato cannery wastewater. Moralejo-Gárate et al. 2011 has able to reach a PHA content of 80% (cell dry weight) using synthetic glycerol as substrate.

Mixed microbial cultures fed with synthetic feedstocks have reported very high specific productivity values (up to 1.97 g PHB/g X.h, Jiang et al. 2011b). This value is about 5 times the highest value reported for pure culture fermentations (0.38 g PHB/g X.h, Lee et al. 1999). These results are one of the major advantages in using MMC to produce PHA production. Since less biomass is necessary to obtain the same amount of biopolymer smaller bioreactors can be used reducing all the adjacent operation costs. However, one of the main drawbacks with using MMC is the low volumetric productivities compared to the ones obtained with pure cultures due to the lower biomass concentrations usually reached in these processes. The maximum cell concentration reported for ADF operated systems was 6.1 g/l (Dionisi et al. 2006), which is much lower than the obtained by pure cultures, usually above 100 g/l (Lee et al., 1999).