That is obviously the return plumbing. You need to include a union between the ball valve and the 90 for servicing.
Why is there a 45 on the top? Normally peeps use a 90.
Any chance you can change the 90 before the reactors to a 45? It will reduce the back pressure and give you better flow. Same question with the 90 after the reactors. Can you possibly change it to a 45 also? I like using two 45's in place of one 90 if possible. If not, it is what it is. It'll work, you'll just need to upsize your pump to handle the additional back pressure.
UPDATE based on turning everything upside down (bottom to top and top to bottom :lol_large ):
You need to include a union, then a ball valve above it, between the bottom 45 (that will connect to the pump) and the first 90 for servicing.
Why do you have the ball valve at the very top? It's unnecessary. Each fitting adds another 1' to head pressure. A ball valve is considered a fitting just like a 90, union, T, etc.
As far as a pump is concerned you can use any of the following: