Three laboratory-scale activated sludge treatment trains were operated to investigate the effect on biodiversity in plug flow (PFR) main-stream sewage treatment from input of biomass from side-stream reactors treating anaerobic digester supernatant. One train had a completely mixed (CSTR) side-stream reactor, one a PFR side-stream reactor, and the third train was a control that did not receive input from a side-stream reactor. Restriction endonucleases were used to digest polymerase chain reaction-amplified ammonia monooxygenase subunit A (amoA) genes in monthly samples from each reactor. Restriction fragment banding patterns from polyacrylimide gel electrophoresis indicated that the structure of the ammonia oxidizing bacteria (AOB) populations in all five reactors stabilized by the fourth month of operation and then did not vary subsequently. Furthermore, a dendrogram generated using the Jaccard distance showed that the AOB in each side-stream reactor was most similar to the main-stream reactor in the same train indicating that the AOB population in the side-stream reactor exerts a strong influence on the population in the main-stream reactor. Sequencing results indicated that Nitrosomonas europea, an r-strategist, was the dominant AOB in the PFR side-stream reactor, while Nitrosomonas europea and Nitrosomonas marina, a marine bacterium, were strongly represented in the CSTR side-stream reactor.

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