Novel photobioreactor for moving bed biofilm cultivation of terrestrial cyanobacteria

Terrestrial cyanobacteria grow quite poorly as suspension culture. This is one of the reasons why they have not yet been considered as producers of interesting metabolites such as antibacterial substances. Previous work in our group have shown that surface-associated growth can significantly increase productivity [1]. Moving bed bioreactor technology, which is already established in wastewater treatment, offers a possibility to carry out such growth on a larger scale. In these reactors, the bacteria grow on the surface of solid structured carrier particles in areas protected from mechanical abrasion (protected surface). These particles are usually about 1-5 cm in size and are made of high-density polyethylene (HDPE). Moving bed processes for microalgae have only been described for fabric as a solid substrate [2] whereby only 30% of the biomass was actually immobilized on the carrier particles. For this reason, different HDPE carrier particles and different cyanobacteria were investigated. Three different cyanobacteria could be successfully cultivated on two different particles in a 1.5-liter photobioreactor in a moving bed. As an up-scale step, a larger reactor was developed, which provided a larger cultivation surface in combination with a sufficient illumination.

Photobioreactor

The design of the reactor is similar to Zhuang et al. [2]. Based on an 80x35x40 cm tank, the reactor has a working volume of 65 liters. At a particle filling degree of 27 %, the reactor has a protected cultivation surface area of 11.26 m² within the particles. This corresponds to 173 m² per m³ reactor volume. Their circulation is generated by a gassing unit on the ground. An inclined plate is installed beside the gassing unit, to avoid a flow dead zone at the bottom of the reactor. The reactor is illuminated by LEDs located outside the reactor. The growth is monitored offline by the determination of the dry biomass (bdm) and the measurement of the biofilm thickness by optical coherence tomography (OCT).

Results

Cultivations with the cyanobacterium Trichocoleus sociatus were carried out. The inoculum was added to the reactor as suspended biomass with a concentration of 0.035 g_bdm/L. After two weeks, the complete biomass was immobilized as a thin biofilm on the carrier particles. Between day 18 and day 45, an increase in the median biofilm thickness from 36 µm to 65 µm could be measured with an increase of the dry biomass from 0.44 to 1.56 g/L. This volume-specific yield is similar to cultivations in the 1.5-liter photobioreactors with carrier particles.

 

Funding

The project is financially supported by the DFG (Project number UL 170/16-1) and the Ministry of Education of Rhineland-Palatinate (bm.rlp) (iProcess intelligent process development – from modelling to product)

 

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