Has the equipment to empty toilets at Machynlleth depot been installed yet? If not, I would hope that no 158s have gone into the works yet and it is only 150s getting PRM mods right now. Otherwise, I'm assuming the newly-modified 158s will have to have their toilets locked out of use until Machynlleth depot is suitably upgraded.
I don't understand why 158s need to go for works now; surely the major refurbishment which saw the new ocean blue livery applied to the 158s handled PRM mods with only CET (toilet tanks) left to do which while a nice-to-have is not tied to the 2020 deadline so could wait until the necessary depot upgrade is done.
The 158s are not quite PRM ready yet. Whilst they are most of the way after the refurb from the start of the decade, there are still some small works that need doing. As they will need tanks fitting to make them compliant with the new rules for that kind of thing, it was decided to do both at the same time, and whilst the much more challenging work on the 150s also takes place.
Yes they do, but they don't need Machynlleth depot to empty their toilet tanks; correct me if I'm wrong (I AM GUESSING HERE) but presumably Chester, Holyhead, Crewe and Canton can all empty toilet tanks and I expect most of the 150s will visit those depots fairly frequently. With the 158s, I think most stable overnight at Machynlleth so couldn't have their tanks emptied for serveral days.
I wonder if Llandudno (Junction?) and Carmarthen, both of which stable 175s I believe, have the ability to empty toilet tanks.
Whilst it is true that most 158s stable at Machynlleth, they are also kept at various other parts of the network - some with facilities for emptying tanks and some without. Diagrams could possibly be altered in the short term to keep them away from those depots not currently fitted as much as possible.
AFAIK, tanks can currently be emptied at a Holyhead, Chester, Crewe (not currently used by ATW services) and Canton. Carmarthen is a definite no at present - I've no idea if the new franchise plans on changing that, but 175s have been stabling there for years despite not being able to be tanked.