Reduction of the Microbial Load of Digestate by the Cultivation of Galdieria sulphuraria Under Acidic Conditions

Abstract

Purpose Organic waste streams with a high microbial load have been used as nutrient sources in the cultivation of heterotrophic microalgae. However, it remains unclear whether an alga-based organic waste utilization can also result in a reduction of the microbial load, and thus in a combined waste utilization and hygenization process. Methods The heterotrophic cultivation of the microalga Galdieria sulphuraria at a pH of 2 and 45 °C as hygenization method for digestate, which serves as nitrogen source in algae cultivation, was investigated. Attention has been paid to Salmonella sp. and spores, coming from the organic waste stream digestate and its reduction during hydrolysis and cultivation of G. sulphuraria in the resulting hydrolysate. Results Digestate contained 0.5 × 106 counts per g material, predominantly formed by aerobic, mesophilic organisms. Salmonella sp., yeast and molds, enterobacteriaceae as well as enterococci were diminished within 24 h of hydrolysis or cultivation. During hydrolysis of digestate and cultivation of G. sulphuraria the counts of aerobic, mesophilic organisms could be subsequently reduced by a log reduction factor of 3. The remaining microorganisms were almost exclusively spore forming ones which were reduced by a log reduction factor of 2 during cultivation under acidic conditions. Conclusion It is suggested here to include the cultivation of G. sulphuraria in future waste management as hygenization process. Graphical Abstract

Algae Cultivation as Measure for the Sanitation of Organic Waste—A Case Study Based on the Alga Galdieria sulphuraria Grown on Food Waste Hydrolysate in a Continuous Flow Culture

“…However, the growth of the yeast cells could be kept under control and G. sulphuraria was the dominant species. Nevertheless, the results shown here (where yeasts were dominant) and earlier [20] (where spores were dominant) indicate the importance of a hygenization process that works independently of the applied substrates. The acidic growth conditions did not diminish all the organisms and allowed for control of them.…”

“…In a previous study, it was shown that, during the hydrolysis of digestate and the cultivation of G. sulphuraria, the counts of aerobic, mesophilic organisms could be subsequently reduced by a log reduction factor of 3. The remaining microorganisms were almost exclusively spore-forming ones, which were reduced by a log reduction factor of 2 during cultivation under acidic conditions [20]. In this study using non-sterile food waste hydrolysate, only a few spore-forming microorganisms and no pathogens were found.…”

“…Temperature had a considerable effect on the reduction in S. infantis in a broiler litter during composition, and, in order to achieve a log reduction of 7, 1.5-1.5 days were needed at 60 • C, but 13.7-27.2 days were needed at 30 • C [33]. The elimination of pathogens such as Salmonella enterica was also achieved when hydrolysed waste streams were treated under acidic conditions for G. sulphuraria cultivations [20]. However, even though pathogens could be eliminated, there was still life in the form of remaining spore formers, which can grow-up again under favourable conditions.…”

“…G. sulphuraria is a promising candidate for the bioprocessing of wastes, as it can grow in harsh environments and utilize a wide range of carbon sources [17]. Furthermore, harsh conditions have been shown to suppress or even reduce the microbial load, including pathogens, and thus contribute not only to the sanitation of organic matter, but also to its utilization [18][19][20][21].…”