Chemostat selection of a bacterial community able to degrade s-triazinic compounds: Continuous simazine biodegradation in a multi-stage packed bed biofilm reactor

Using a successive transfer method on mineral salt medium containing simazine, a microbial community enriched with microorganisms able to grow on simazine was obtained. Afterwards, using a continuous enrichment culture procedure, a bacterial community able to degrade simazine from an herbicide formulation was isolated from a chemostat. The continuous selector, fed with a mineral salt medium containing simazine and adjuvants present in the commercial herbicide formulation, was maintained in operation for 42 days. Following the lapse of this time, the cell count increased from 5 × 10⁵ to 3 × 10⁸ CFU mL^-1, and the simazine removal efficiency reached 96%. The chemostat's bacterial diversity was periodically evaluated by extracting the culture's bacterial DNA, amplifying their 16S rDNA fragments and analyzing them by thermal gradient gel electrophoresis. Finally, a stable bacterial consortium able to degrade simazine was selected. By PCR amplification, sequencing of bacterial 16S rDNA amplicons, and comparison with known sequences of 16S rDNA from the NCBI GenBank, eight bacterial strains were identified. The genera, Ochrobactrum, Mycobacterium, Cellulomonas, Arthrobacter, Microbacterium, Rhizobium and Pseudomonas have been reported as common degraders of triazinic herbicides. On the contrary, we were unable to find reports about the ability of the genus Pseudonocardia to degrade triazinic compounds. The selected bacterial community was attached to a porous support in a concurrently aerated four-stage packed-bed reactor fed with the herbicide. Highest overall simazine removal efficiencies η _SZ were obtained at overall dilution rates D below 0.284 h^-1. However, the multistage packed bed reactor could be operated at dilution rates as high as D = 3.58 h^-1 with overall simazine removal volumetric rates R _v,SZ = 19.6 mg L^-1 h^-1, and overall simazine removal specific rates R _X,SZ = 13.48 mg (mg cell protein)^-1 h ^-1. Finally, the consortium's ability to degrade 2-chloro-4,6- diamino-1,3,5-triazine (CAAT), cyanuric acid and the herbicide atrazine, pure or mixed with simazine, was evaluated in fed batch processes.