On 19-Nov-17 7:36 PM, MM wrote:
On Sun, 19 Nov 2017 16:14:02 GMT, Iggy
m wrote:
....
The old has to be removed as
well as possibly a mountain of contamination, then new fill goes in and then
and only then does the new plant go in.

Yes, well, I kind of guessed that already!...

Depending upon the site available and the drain layout, it might be
possible to install the new unit first, divert the drains into it, then
worry about the old one. That would minimise disruption and you might
find that the old unit doesn't need to be removed, although it would
probably need to be filled in for safety.

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Colin Bignell