[struggling_kemist]

Hi, I am a high school student doing experiments in chemistry. Here are two questions I have:
(I tried to search for the answer myself but to no avail)

1) what are the common substances/ pollutants in water that aluminium sulphate act on as a coagulant?

2) What are the ways to measure the state of coagulation? I read about two methods: feeding current to metal wire submerged in solution (the example was milk so I wasn't sure if it works in this case) and using a rain gauge to look at the height of precipitation. which one is more reliable?

Wud appreciate any help. Thanks!