Excess of which of the following in drinking water causes mottling of teeth?

Which one of the following represents natural pollutants?

$BOD$ stands for:

$A$ lake with an inflow of domestic sewage rich in organic waste may result in:

$A$ : $ESP$ can remove particulate matter by more than $99\;\%$.
$R$ : Anode attracts the particles and provides a positive charge to them,allowing them to fall.

Some toxic substances present in industrial waste water can undergo $.........$ in the aquatic food chain.