On 3 Dec, 20:47, robgraham wrote:

If it is an old system in the style of two tanks and an outflow to a
local stream, then experience shows that pumping out annually, or ten
yearly even, isn't necessary. Actually ten years is possibly a bit of
a limit, but seriously for that sort of tank every 5 years is more
than adequate.

I had my tank - of that sort - desludged last year. It certainly
hadn't been done since 1963.

Ian