Using Alum as a flocculant

 When a pool is really green and has so much algae that you can't even see the bottom, it is either time to drain and refill the entire pool or floc the pool and vacuum to waste. A flocculant drops everything to the bottom of the pool so that it can be vacuumed to waste. 

One of the best flocculants is Alum (i.e. Aluminum Sulfate). Alum is Al₂(SO₄)₃

Here is the reaction when added to water: 

Al₂(SO₄)₃ + 6H₂O ® 2Al³⁺ 3SO₄^2- + 6H⁺ + 6OH^- ® 2Al(OH)₃ + 3H₂SO₄

The resulting product is Al(OH)₃  which is an insoluble floc that will drop everything to the bottom of the pool where it can be vacuumed to waste. This including a good portion of cyanuric acid (stabilizer), carbonate alkalinity, and calcium hardness. (Adjust as needed after vacuuming and refilling.) 

Besides the floc, the remaining by-product is 3H₂SO₄ (Sulfuric acid) meaning that adding Alum will lower the pH. You can deal with this in one of two ways. You can raise the pH to a higher than normal level prior to adding Alum so that the pH doesn’t drop too low after adding it, however, the Alum will work better if you add the Alum when the pH is normal (near 7.2) and then raise back as needed after it drops. 

Temperature is also important as the water needs to be 70 degrees Fahrenheit or higher for a good chemical reaction to take place. 

Dosage isn't precise: dissolve and broadcast 2-6 lbs alum per 10k gallons.

Where to find it. Alum can be purchased from pool stores, but you will find better prices on larger quantities at garden centers as alum is also used in flower gardens.