Production of S. meliloti using wastewater sludge as a raw material: effect of nutrient addition and pH control.

The utilization of wastewater sludge as a raw material for the production of legume inoculant is considered as a new viable alternative for recycling. The effect of addition of nutrient sources (yeast extract and glycerol) and the pH control on S. meliloti was investigated in a shake flask and at controlled pH in a 15 l fermentor. Different concentrations of yeast extract (0.5, 1, 2 and 4 g l(-1)) and glycerol (2.5, 5, 7.5 and 10 g l(-1)) were added to the secondary sludge. The cell yield as well as the generation time were affected by the addition of these nutrient sources. The maximum cell yield (8.85x10(9) cfu ml(-1)) was achieved in 32 hours of incubation with the addition of 4 g l(-1) of yeast extract. This value was 3.2 times higher than from the non-supplemented sludge. Moreover, at this yeast extract concentration, the cell concentration in the stationary phase did not decrease. The addition of glycerol to sludge samples containing 4 g l(-1) of yeast extract further improved the rhizobial growth but not significantly compared with the control. The highest yield (16.5x10(9) cfu ml(-1)) was obtained with 7.5 g l(-1) of glycerol and 4 g l(-1) of yeast extract. In fermentor experiments, pH did not seem to be a limiting factor and the increase of pH up to 8.85 in uncontrolled fermentor seems to have no effect on rhizobial growth and cell yield.