Role of exopolymeric protein on the settleability of nitrifying sludges

The relationship of exopolymeric substances and the sludge volume indexes (SVI) of two nitrifying sludge were studied over a period of time (30 d) in two different types of reactors: (1) a stirred tank aerated nitrifying reactor (SANR) and (2) an air-lift nitrifying reactor (ALNR). Concentrations of lipids, carbohydrates and proteins of the EPS were determined in both reactors. The variation in lipids and carbohydrates was low (<10%), while the variation in the exopolymeric protein was higher. The SVI increased in accordance with the increase in the concentration of the exopolymeric protein. The number of bands of the exopolymeric protein was modified as a consequence of the change in the concentration of the exopolymeric protein. The molecular weights of the exopolymeric protein ranged from 31 to 97 in the stirred and 13 to 45 kDa in the air-lift reactor. The variations in the SVI were mainly due to the changes in the molecular weight of the exopolymeric proteins resulting in the modification of the sludge settleability characteristics.