Watts PremierWP560060 RO Water Filtration PH Balancing Ceramic Filter with 1/4 inch QC Fitting, 11 1/2 Inch, Multi

Q: which one is the input and output?

It says on the filter "input on side", "output center"

Q: How should this filter be mounted: vertically or horizontally?

I mounted it vertically. This did it this way, because all filters from my RO are vertical.

Q: Do I need to flow some water through it after installing like carbon filters require?

Important thing here is to install the filter after the membrane just before it enters The split so the water can slowly go through the Alkaline filter absorbing the minerals instead of most people installing it just after the taste before the faucet making the flow to strong and fast giving you too much minerals all at once and nothing after awhile..

Q: The Watts RO-Pure has 1 line to/fro the tank. Water flows thru the post filter as it comes back from tank to faucet. Where does ph filter go? thx

Linked below is the PDF from Watts explaining the installation, this document was also included with the filter when I ordered it. Although the filter is supposedly directional, this official document specifically notes that water can flow in both directions, first going from the membrane to the tank, then going from the tank to the faucet. The filter is officially supposed to go on the line to the tank, but there are many varying opinions of the pros and cons of this setup.https://cdn.shopify.com/s/files/1/0826/7845/files/299657_PH-Filter-Installation-Diagram.pdf?3715988257525716971