Up to you and your town. But from my own pool and experienc a backwash hose into a slop sink would overwhelm the plumbing rapidly. Think of a solid 1.5" column of water coming out RAPIDLY. I run mine into a storm drain. I recently put in a French Drain system for a soggy back yard and put in a Y-connection to run a 4" drain over to my pool. I run 1.5 inch TigerFlex into that drain for my backwash.

Carl