Wastewater sludges have been proposed as an effective media for the product
ion of rhizobia. The effect of total suspended solid (TSS) concentrations a
nd pretreatments of sludge on the growth of Sinorhizobium meliloti were inv
estigated. Acid (pH 2.0-6.0 obtained with H2SO4) and alkaline (50-200 mequi
v.wt./L of NaOH) treatments were applied to enhance the biodegradability of
primary (0.325%-3.2% TSS obtained by dilution of original sample) and seco
ndary (0.2%-0.4% TSS obtained by concentration of original sample) sludges.
In primary sludge without pretreatment, the highest cell count (11.10 x 10
9 cfu/mL) was obtained with 1.3% TSS. However, a maximum cell count of 13.0
0 x 109 cfu/mL was reached using an acid treatment of pH 2.0 and a 0.325% T
SS concentration. Moreover, the alkaline treatment with 100 mequiv.wt./L of
NaOH and 0.65% TSS increased the cell yield to 21.00 x 109 cfu/mL. For sec
ondary sludge without pretreatment, no enhancement of growth was observed w
hile increasing TSS concentration. This may be due to the increase of inhib
itory substances, such as heavy metals, and of the Ca and Mg concentrations
. As in primary sludge, some acid and alkaline treatments of secondary slud
ge tend to improve the cell count of S. meliloti. However, the highest valu
e of 9.80 x 109 cfu/mL obtained with 0.4% TSS at pH 2.0 was lower than that
obtained with primary sludge. It was also observed that S. meliloti grown
in treated sludges maintained its capacity to nodulate alfalfa.