First, welcome to the forum.
The 300# model may be an S-244T same as mine. It is normal to get sand out the waste line when backwashing as there is nothing in the filter to prevent this. In time the amount you get dumped will decrease. The procedure should be to backwash until the sight glass is clear, then rinse for only 15 seconds or so. Rinse mode is the same as Filter except the output is directed to waste. Any stuff stirred up during backwash is dumped to waste instead of back to the pool. The things on the bottom are called laterals and are simply pipes with slits that let water flow but prevent most of the sand from dumping. The sand should be labelled Filter Sand or something like that.
Hope this helps.
Al
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