Sediment, also sometimes called scale, comprises minerals that were dissolved in the water, but precipitated out when the water was heated, settling into the bottom of the tank. It also can include bits of metal that have sloughed off from an aluminum anode.

Sediment can be as fine as sand or come in good-sized chunks. It's usually not the most critical element in the life and death of water heaters, but it's the one people know most about, since there is usually an admonition to drain the tank once in awhile in installation instructions.

That said, it can burn out the lower element in electric water heaters and clog recirculation pumps, as well as causing a variety of lesser ills in all tanks. It also is the source of noise when a tank is firing, which can be quite disconcerting to those who don't know what it is. Noise merely means there is sediment and the amount of noise is not proportional to the amount of sediment.