Both of the achievements can be obtained in Chapter 5:Black Mesa East. Go through the chapter until the point when you get the Gravity Gun.

Look towards the left at the red, washing maching thing:


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Then grab some boxes and place them so you can climb up to the the ledge:


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Then you can grab the H.E.V Suit Charger Faceplate from the corner:


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Before continuing on to the next area, go the metal bins on the right:


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Grab the Toilet from the left bin, then go to the next area:


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Play with DOG until pods start shooting from the sky towards you, then launch the toilet at one of the headcrabs:

You are now the proud collector of the achievements "Blast from the Past" and "Flushed". You are now two achievements closer to total Half-Life 2 dominance.