Combined phosphate and nitrogen removal in a sequencing batch reactor using the aerobic denitrifier, Microvirgula aerodenitrificans

A phosphate removal sludge was bioaugmented with the aerobic denitrifier, Microvirgula aerodenitrificans in order to reduce the nitrate produced during the aerobic nitrifying-phosphate uptake phase. Fluorescent in situ hybridization (FISH) was used to follow the fate of the added strain. In order to maintain the pure strain in the complex ecosystem, diverse physiological and kinetic based strategies of bioaugmentation were tested under the sequencing batch reactor (SBR) type culture. The nature of the M. aerodenitrificans inoculum (adapted to nitrate-aerobic conditions or to anoxic one) had no influence on the SBR performances and did not enhance aerobic denitrifying performances. The optimum quantity of the added strain (10% of the total biomass) seemed to have much more positive influence on the long term maintenance of the pure strain than on the SBR performances. A small but daily supply of M. aerodenitrificans gave exactly the same result than a massive and 1-day supply, i.e. no enhancement of performances and no amelioration of the length of maintenance. A continuous supply of carbon during the first hour of the aerobic phase combined to a 10% supply of M. aerodenitrificans gave the best compromise in terms of phosphate removal, nitrification and aerobic denitrification performances. It was accompanied too by a decreased number of the ammonia and nitrite-oxidizing bacteria and a modification of the nitrite-oxidizing floc structure. FISH on M. aerodenitrificans revealed that (i) before bioaugmentation, the strain was already present in the phosphate removal sludge and (ii) the added bacteria almost disappeared from the reactor after 16 HRT. In a last experiment, M. aerodenitrificans embedded in alginate beads allowed enhancement of both aerobic denitrifying performances and length of strain maintenance.