It's plastic, I've no idea of what specific type of plastic, but I can't believe it would be hazardous.
On Spring Fever, in addition to the 14 gallon forecabin tank, we also have a 13 gallon plastic tank in the front of each cockpit locker too and treat all of them: -
Pump them as dry as possible at the end of each season, to ensure a tank/pipe leak can't fill the boat with water.
In the spring mix 4 Puritabs (a drinking water treatment for fellwalkers/backpackers) into two litres of hot water, pour this into each tank and then slosh it around a bit and let it stand for an hour and then pump it through, letting it stand in the pump/pipes for a further hour, before flushing through with some fresh/untreated water.
About once every three years, I use a strong bleach solution to wipe out the inside of each tank with a clean sponge and replace all of the pipes.
We're living aboard and drinking the tank water for about 6 or 7 months each year and have never suffered from any water contamination problems/ailments yet; though if we have any non-sailing guests aboard, we'll buy in bottled water for the duration of their visit 'just in case'
Bob Carlisle