Possible role of denitrification on aerobic granular sludge formation in sequencing batch reactor

The aim of this work was to evaluate and quantify the influence of denitrification of nitrate on the aerobic granular sludge development. Two parallel sequencing batch reactors (SBRs) were fed with synthetic wastewater, the first reactor (R1) receiving nitrate at a fixed concentration whereas the second reactor (R2) did not receive any external source of nitrate. Both systems were working at the same operational conditions with a low specific air velocity (0.07cms(-1)), a relatively low and acceptable oxygen concentration (1.8+/-0.8mgL(-1)) and without significant biomass selection pressure (minimal velocity 0.4mh(-1)). The results showed that the sludge settling properties of R1 were comparable to granular sludge (settling velocity approximately 10.6mh(-1), SVI(5) approximately 13-30mLg(-1), SVI(30) approximately 11-28mLg(-1) and SVI(5)/SVI(30) ratio approximately 1.05-1.1). In the opposition, biomass of the reactor R2 reveals more traditional properties. Nitrogen measurements revealed that a significant denitrification (up to 80% of fed nitrate) occurred in R1 in the core of aggregates, as aerobic condition was maintained in the bulk. After the mean particle size was reduced from 250 to 125microm by an increase of mechanical stirring rate, it was shown that denitrification decreased from 80% to 20%. These parallel experiments definitively proved that the presence of nitrate in SBRs can assist the densification of the biological aggregates in aerobic condition. The presence of nitrate (commonly provided by nitrification) should be considered as a possible factor which helps to maintain the granulation process.