Which one of the following plants is effective in reducing water pollution and is also useful for producing biogas?

Answer: (b) Water hyacinth

Water hyacinth absorbs nutrients and heavy metals from water, and its soft biomass is good for biogas.

The idea

Water hyacinth grows extremely fast because it soaks up nutrients, and with them heavy metals, from the water. That makes it a natural cleaner of dirty water, if it is harvested. Its soft, watery mass also rots easily without air, giving biogas.

Floats on polluted water → Roots absorb nitrates, phosphates and metals → Plants are removed with the pollutants in them → Used to make biogas, compost, paper and handicrafts

Why the others are wrong

(a) Eucalyptus is a tree that uses a lot of water — it doesn’t clean ponds.

(c) Salvinia is also a floating weed, but water hyacinth is the standard answer for both uses.

(d) Lotus is not a pollution-control or biogas plant.

The trap

Water hyacinth is an invasive weed that chokes lakes, so it seems like a problem only. Harvested, it is useful.

Also know

Left alone it is a disaster: it can double in about two weeks, covers the whole surface, blocks light and uses up oxygen, killing fish. It came from South America as an ornamental plant and was once called the terror of Bengal.