Question: Can a DIY spectrometer be used to measure water turbidity?

By definition, turbidity measures the amount of light scattered at a 90 angle. So there would have to be a change in the optical layout from the basic spectrometer. You also need to check the specific method referenced. Some instruments are getting quite complex ( ratio recording , etc.). If a method specifies something like that, it can change your optical layout dramatically. Having said that, most turbidimeters are very simple and would be easy to duplicate.

Hm, good point -- actually going back to an old post there's some great stuff here:

https://publiclab.org/notes/donblair/08-25-2015/turbidity-001

So perhaps turbidity since it measures reflectivity, is not just an analogue for opacity, and Beer's Law may not apply for all contaminants?

So a design using the Lego Spectrometer, for example, might need a right-angle light source -- but that's not so bad, since already with fluorescence spectrometry we'd be doing this. Interesting!

Absolutely correct. The fluorescence spectrometer is the same layout as for the basic turbidimeter.

The standards are a bit of a pain, but quite doable. The ingredients for the standards didn't use to be a problem to get. Maybe that had changed.

The current epa method for turbidity, with standard prep, is at:

Https://epa.gov/sites/production/files//2015-08/documents/method_180-1_1993.pdf

As you might guess, it is method 180-1.

The photometer listed recently might be a good start on a turbidity meter, but it does need some work on the optical bench. The change needed in the optical bench you documented above. The light source and detector would need to be at 90 degree angles. This is the photometer that used a laser led with the lense removed as the light source. White light would be better, but...