**7. Optimization of the time of sulphate reduction**

The sulphate reduction was further optimized in terms of incubation time in the same bioreactor under identical conditions. The desired level of reduction (for environmental discharge) was reached within three and half hours (**Figure 7**). The data were fitted using Origin8pro (**Figure 8**), which resulted in a polynomial equation of order 5 with *r* <sup>2</sup> = 0.97. The following equation perfectly expresses this desulfurization system:

$$Y = 0.941 + 1.60.51 \text{x} - 182.51 \text{x}^2 + 83.8 \text{x}^3 - 18.14 \text{x}^4 + 1.39 \text{x}^5 \tag{1}$$

**Figure 7.** Time course of sulphate reduction by the packed bed bioreactor.

**Figure 8.** Fifth-order polynomial equation of time optimization of sulphate reduction using Origin8pro.

Here *Y* denotes the percentage of sulphate reduction, and *x* signifies the time required for reduction in hours.
