I believe the water authority could offer to purchase the bond (and then write it off once they own it). It would be up to the current owner to decide if they are willing to sell.
The price of the bond itself is at least 100x the value it would have to pay for eternity
100x the annual yield would be assuming a 1% interest rate, so I'm guessing it's worth quite a bit less than that. Long-term government bonds give 2+% returns and fixed term annuities/MYGAs are giving ~4% returns, as far as I know. So I would expect a value more like 30–50 times its eternal annual payments. Heck, inflation is around 2%, so eventually this will be worthless.
I think they were referring to the historical value of the bond itself makes it worth far more than its strict monetary value over time. So even if its 'worth' 30-50 times more when you do the math, a collector maybe be willing to pay 500 time more due to its rarity.