Quick start up of biological sequencing batch reactor (SBR) for Polyhydroxyalkanoates production from renewable resources

In recent years, a lot of research has been dedicated on the production of biodegradable polymers (e.g. polyhydroxyalkanoates, PHAs) from renewable resources. Presently, industrial processes for PHA production are primarily based on the use of pure cultures of specific microorganisms, batch cultivation conditions and ad hoc formulated substrates (usually based on the unbalance between carbon and nitrogen). In order to reduce the PHA production costs and further increase their environmental sustainability, alternative processes have been proposed based on the use of fermented organic waste as a substrate and of microbial mixed cultures previously selected under periodic feeding. Typically, the proposed process includes three stages: the first stage, acidogenic fermentation of organic waste or wastewater (e.g. food farming industry), produces a concentrated mixture of organic acids needed to feed second and third stages. The second stage is performed under periodic feeding at a medium organic load in order to select and produce a biomass with a high storage response. After each feeding cycle, a portion of the selected biomass is extracted, to be used in the third stage at a higher load for the synthesis of the polymer. A key point of the overall process is the appropriate selection of biomass in the second stage, through its cultivation under alternating excess and lack of substrate conditions, usually done in a sequencing batch reactor (SBR). The SBR is usually inoculated by an activated sludge from wastewater treatment plants, which ability to produce PHAs is progressively increased during SBR operation through several mechanisms (physiological adaptation, selection and enrichment); hence, the process start up and the time needed for reaching the steady-state are also key factors for obtaining a good process performances. For these reasons, in the present study, we investigated the process performances in the early days after SBR start up (from two to ten days from the SBR inoculation). By taking the biomass from the SBR during start up and performing the third accumulation stage in batch experiments, it was possible to compare the ability to store PHAs of the biomass after short acclimation with the one of a fully acclimated biomass, obtained after quite longer operation in the same operating conditions of SBR reactor (OLR 8.5gCOD L-1 d-1, pH = 7.5, cycle time 2 h). As expected, during start up the SBR performances was found highly dependent on the used inoculum (four different runs were performed) and highly unstable in terms of yield and rate of PHA formation. The tests carried out after two-three days after start up (just over 30 operating cycles) did not give satisfying results, probably because of too short selective pressure imposed to the biomass. However, already after four-five days operation, the SBR showed good performance and the selected biomass was able to store PHAs at comparable (though slightly lower) rate and yield with those obtained in batch tests carried out with fully acclimated biomass. It was also shown that the time needed for substrate depletion in the SBR cycles was the key parameter to be monitored for ascertain when the biomass had been selected enough and was ready for the next accumulation stage: under chosen operating conditions, a substrate depletion time equal or less than 15% of the cycle length was enough to obtain a good storage performance. These results could open a new perspective for a different arrangement of a semi-continuous process, where the SBR is started up with a new activated sludge inoculums and the selection is operated for the shortest time possible; then the whole amount of selected biomass is used in the accumulation stage.