Plastic bits of <5mm should pass through a common centrifugal pump of relatively modest size without clogging or other difficulty. I'd mock up the system with a ~1HP sprinkler pump or pool pump, run it for a while, then separate the pump, and buy a bigger one if the impeller has retained any plastic.
Even better, don't take the word of a random idiot on the Internet; talk to a pump manufacturer, supply them some samples of your plastic bits, and ask them to recommend a closed impeller large enough to pass your stuff, or an open impeller, a paper stock impeller, or even a trash pump impeller. Pumping bits of plastic in water is not new technology.
Mike Halloran
Pembroke Pines, FL, USA