I think the brown colored layer formed is a result of high iron and manganese content of bottled water which originated from underground water. Mechanical removal for the layer formed is the solution for the problem. High content of iron and manganese is usually observed in ground water and it should be treated to remove both of them before bottling. I just advice you to change to other bottled water brand which contains less amount of iron and manganese

Thanks for ur advice.

It Might be Higher content of Iron, Nickel  or  Some Microbiological Contamination (Yeast and  Mould)