Hydraulic selection pressure-induced nitrifying granulation in sequencing batch reactors

The effect of hydraulic selection pressure on the development of nitrifying granules was investigated in four column-type sequencing batch reactors (SBR). The nature of SBR is cycle operation, thus SBR cycle time can serve as a main hydraulic selection pressure imposed on the microbial community in the system. No nitrifying granulation was observed in the SBR operated at the longest cycle time of 24 h, due to a very weak hydraulic selection pressure, while the washout of nitrifying sludge was found in the SBR run at the shortest cycle time of 3 h, and led to a failure of nitrifying granulation. Excellent nitrifying granules with a mean diameter of 0.25 mm and specific gravity of 1.014 were developed in a SBR operated at cycle times of 6 h and 12 h, respectively. The results further showed that a short cycle time would stimulate microbial activity, production of cell polysaccharides and also improve the cell hydrophobicity. These hydraulic selection pressure-induced microbial changes favour the formation of nitrifying granules. This work, probably for the first time, shows that nitrifying granules can be developed at a proper hydraulic selection pressure in terms of SBR cycle time. Nitrifying granulation is a novel biotechnology which has a great potential for wastewater nitrification.