How to Sieve a fly pupa from the sawdust

[bioptic]

Hello.
I will start with tell you what I do so maybe is will be more easy for you to try to help

Ok so what I do is breed a fly maggots with a bran.(i give them the barn as a food).
After they are fat Enough I pot them in a sawdust till they become a pupa .

So I got a problem with Sieve (i hope this is the correct word) the fly pupa from the sawdust and the bran. When I do it with a normal Sieve is come very dirty.
And what I need is only a clean pupa .

So there is any other good way to do it?
maybe to feed them with some different food then it will be less dirty and still make them fat?

Really really thanks and sorry about the english