Polyhydroxyalkanoates production in aerobic sequential processes by mixed microbial cultures

The current emphasis on sustainability, eco-efficiency and green chemistry has led to intensive search for renewable and environmentally friendly resources. Thus, sustainable development is recognized to be essential for the growth of the economy and industrial productivity. Since global petroleum reserves are finite, there is a need for additional new sources of durable materials. Renewable materials from microorganisms can provide a source of sustainable alternative to petroleum derived chemicals including polymers. Polyhydroxyalkanoates (PHA) are one of the relatively newer families of biodegradable polymers that have great potential in the future due to their variability in properties, they are polyesters of hydroxyalkanoic acids that have received extensive attention due to their inherent biocompatibility and biodegradability. The most widely applied strategy for PHA production is to cultivate one wild-type or genetically recombinant bacterial strain with a renewable but refined substrate. At the same time, some of the bacteria naturally occurring in open mixed microbial cultures (activated sludge) treating wastewaters are able to store PHA. In the wastewater treatment plant, carbon storage in the form of PHA can be enhanced under dynamic conditions respect to microbial substrate. In this way, while the wastewater treatment is applied favoring the PHA storing bacteria, a biopolymer can be harvested as a value-added resource from wastewater treatment system. The research activities of the thesis has been divided in 3 fundamental steps: a) under new dynamic feeding condition (feast-famine) in Sequencing Batch Reactor (SBR), the process performance was investigated in the early days after start up, performing the PHA-accumulation stage in batch, under pulse feeding condition. The new process concept was evaluated in terms of specific storage rate and yield, PHA biomass content and PHA productivity; b) the more traditional steady-state approach has been applied at fixed operating parameters (OLR, HRT, pH, T) with the exception of the frequency of the alternation of the feast and famine phases. Microbial community analysis, based on denaturing gradient gel electrophoresis (DGGE) was carried out in order to evaluate if the cycle length plays a main role on the microbial speciation within the SBR; c) based on the results obtained at the point (b), a continuous system (SBR and accumulation) was developed. The PHA-saturated biomass was daily collected and treated with quenching reagent for PHA extraction; then the stored polymer was purified and characterized via lyophilization and chloroform purification; d) the last part of the experimental program consists in the optimization of the accumulation stage in terms of nutrient content of wastewaters used as substrate, in particular the nitrogen and phosphorous concentration; the COD/N and COD/P ratios covered a wide range of values, changing from starvation to excess condition in the growth medium. The achieved results open new perspectives and solutions for further investments and technology applications, in particular its integration within public services like the traditional wastewater treatment plants and anaerobic digestion plants.