Actually, I'd recommend you OPEN the filter, and wash it out. Backwashing never fully cleans a DE filter.

I just looked up the manual for that Hayward filter. It's a big filter case, and they are somewhat tricky to re-assemble. You have to be careful to get the O-ring clean and aligned, the O-ring groove clean, and the case in place, without bumping the O-ring. Then you have to install the band and torque it into place.

It works better if you use a machine hammer to tap the ring as you tighten:
1. Snug the bolt up.
2. Starting in the position 180 degrees from the bolt, use a machine hammer to tap the ring lightly, moving from the opposite position evenly to the bolt. Do this on both sides.
3. Then tighten the bolt some more, and 'tap' again.
4. Once you are nearly fully torqued, tap the ring again, and then fully torque the bolt.

If you don't have a torque wrench, here's a well-rated, but inexpensive one. Please note that 150 inch-pounds is specified, but that that is equal to 12.5 foot pounds. (Which actually seems light to me -- but I have enough experience to go by 'feel', and adjust that way.)

Anyhow:
Hayward Pro Grid Manual
Neiko 3/8-Inch 10-80 Foot-Pounds Automatic Torque Wrench