**3. Optimization of inoculum percentage and immobilization matrix**

Inoculum optimization was done based on the extent of sulphate reduction following immo‐ bilization of the consortium onto a matrix as per the method of Nasipuri et al. [1]. The inoculum percentage was varied from 2% to 50% (2%, 5%, 10%, 20%, 30%, 40% and 50%) to maximize the sulphate reduction by the system. Optimum sulphate reduction of 70% was obtained with 10% primary inoculum. It implied that with 10% bacterial inoculum maximum metabolic rate was reached, which eventually resulted in a significant sulphate reduction under optimum condition. But further increase in inoculum percentage resulted in no further increase in efficiency in terms of sulphate reduction.

Two types of matrices (polypropylene and steel) with uniform surface areas were tested for the purpose as per the method of Nasipuri et al. [2]. The stainless steel and polypropylene raschig rings showed an overall sulphate reduction of 72.05% and 69.59%, respectively. They were equally efficient as immobilization matrix in terms of sulphate reduction under the same set of conditions. The comparable range of efficiency between stainless steel and polypropy‐ lene raschig ring in terms of sulphate reduction might be due to equal lower pressure drop at effective surface areas and same gas velocity. Our data were also supported by the study of Kolev et al. [23]. In this regard, the scanning electron microscopic images were further used to visualize the dense biofilm formation on both types of matrices (**Figure 2**).
