I recall seeing spells to desalinate liquids with ReAq(te). However separating mixtures of liquids would seem to me be controlling the liquid in a gentle way (ReAq: base effect of 1), possibly with an added magnitude for complexity, and if there are dissolved physical components maybe a magnitude with the Te requisite.
The low-tech methods for getting salt and other impurities from water seem to require a state change which seems more than
Creo seems like it could be useful for cleaning other impurities but I figure making salt water “better” would just mean clean salt water.
-PeTe to destroy dirt and salt in the water, base 3 is what is used for the lab cleaning spell in Covenants
-ReAq to change the state of the water without heat leaving salt and vapor which could be collected in some alchemical or other implement, base 3
-ReAq(Te) or ReTe(Aq) depending on base) to separate without boiling ending up with a bucket of water and a bucket of salt, the Terram base guidelines seem worded better here, either Level 2: Control or move dirt in a slightly unnatural fashion or Level 3: Control or move dirt in a very unnatural fashion or, possibly, teleport base of 4 using a Part target to only teleport the water or the impurities
-MuTe(Aq) or, maybe, ReTe(Aq) to facilitate the salt to form larger crystals which can be filtered
I’m sure there are others and it is unfortunate that there is not a canon CrAq “clean this water” guideline that could be used to clean non-salt impurities
Keep in mind that regular clean drinkable water will suffice. I am not looking to create chemically pure H2O (or Ultra-pure water, millipore water, MilliQ or whatever term suits you better). Fish can live i regular drinkable freshwater just fine.
I thought there was a spell to make a pond, puddle, whatever worse (undrinkable) but I must be confusing it with the one for a room of stale air. But if that guideline does exist one could consider the opposite and equal base as a CrAq guideline… the problem I think of doing this with Creo is I think it would need to be a ritual.
Desalinating water is PeAq by the core book. It is the Level 15 base effect "Destroy one property of a liquid". One of its listed examples is "destroy sea water's saltiness" and there are enchanted items that use this effect in later books. Page is 123 if someone wants to take a quick look.
As weird as it seems, Perdo is actually the appropriate Art for just removing so element from water. If you can classify that contamination as one property of a liquid then Perdo will let you destroy it. And it actually fits in with Perdo partial destruction as seen across all the varies Forms such as the ability to specifically destroy some (Target Part) or all (Target Individual) the weight of an object.
I think you are in the right when it comes to desalination.
I am less sold on the idea of turning contaminated fresh water into drinkable water being perdo. Since perdo makes things worse and I think it is hard to argue that drinkable water is a worse version of contaminated water in the hermetic/aristotelian paradigm.
I would never consider perdo as the universal "make bad water drinkable" Art. Its effects are concentrated on destroying a single property. Water just being "bad to drink" is not something I would consider a property.
Even the effect that removes the saltiness from sea water does not make it clean, fresh or tasty. Water that would be unpleasant to drink without the salt is still unpleasant. Water that could kill you if drunk could still kill you after the salt is removed.
I think the MuAq Base 4 "Change a natural liquid into two or more different natural liquids" might actually be the best. Though since they will naturally mix after you need to add a ReAq Base 2 or 3 requisite in to actually separate move them apart.
Something like this
Separate the Fine Draft from the Profane [Mu(Re)Aq Level 15]
R: Touch, D: Momentary, T: Part, Req: Rego
To cast this spell, a Magi must hold a container in one hand and touch a volume of water with the other. A portion of the water is changed into two types; fresh clean drinkable water and doubly contaminated water. The drinkable portion floats through the air into the container while the contaminated portion sinks into the larger body of water before mixing with it.
(Base 4, +1 Touch, +1 Part, +1 Rego requisite)
You could actually do it without the Part Target and save a Magnitude, but with it you can both limit the spell to just making enough clean water to fill the container and allow you to cast it on a body of water far bigger than a base individual. That feels more thematically correct and useful to me since most of the time you would be casting the spell on a large body of water to fill a container.
Though in my specific case I actually intend to craft a magic item which fills a container with salt water and then purifies the content of the container - or in the case of this spell transfers the purified water to second container while discarding the contaminated water.
So in my specific case T:Ind makes more sense since I am, by design, working with a bounded body of water which I intend to target in its entirety.
Yes when having a specified quantity of water that will not exceed a base individual then switching to T: Ind makes sense.
You also might be able to reduce the Rego requisite to a +0 if you set it up as a splitter device. Since the affected water is split into two parts you could have them created aligned each with an output that would separate them as they drop under gravity and thus only require minimal Rego effect to keep them from starting to mix for that fraction of a second before they each exited on their own side.
Possibly consider changing the Duration to Concentration, add item maintains concentration, 2@ day uses, and environmental trigger. That would result in a magical filter in which when water is poured through would separate into a "clean" and "contaminated" section with a total level of 19. It would have the advantage of just requiring water to be poured into it without worrying about activation. You could save a few levels just taking it T/M/I with U@day uses though that would be slower to use and a slightly larger and more physically complex design.