LAUSR

The cyanobacterium Nostoc sp. BB 92.3. had shown antibacterial activity. A cultivation as biofilm, a self‐forming matrix of cells and extracellular polymeric substances, increased the antibacterial effect. A new photobioreactor system was developed that allows a surface‐associated cultivation of Nostoc sp. as biofilm. High‐density polyethylene carriers operated as a moving bed were selected as surface for biomass immobilization. This system, well established in heterotrophic wastewater treatment, was for the first time used for phototrophic biofilms. The aim was a cultivation on a large scale without inhibiting growth while maximizing immobilization. Cultivation in a small photobioreactor (1.5 L) with different volumetric filling degrees of carriers (13.4%–53.8%) in a batch process achieved immobilization rates of 70%–85% and growth was similar to a no‐carrier‐control. In a larger photobioreactor (65 L) essentially all of the biomass was immobilized on the carriers and the space‐time yield of biomass (0.018 gcell dry weight L−1 day−1) was competitive compared to phototrophic biofilm cultivations from literature. The use of carriers increased the gas exchange in the reactor by a factor of 2.5–3 but doubled the mixing time. Enriched gassing with carbon dioxide resulted in a short‐term increase in growth rate, but unexpectedly it also adversely changed the growth morphology.