I used to work at a WWTP that used aluminum sulfate for phosphorus removal, and we had very little problems with the alum causing corrosion. We did have to add soda ash to supplement alkalinity because we were a nitrification plant and used up the alkalinity in the aeration basin.
Do you have a digester? Do you produce biogas?
You can use aluminum sulfate, iron sulfate, or aluminum chloride. Price is less attractive though, and anything containing sulfate generates more H2S. It’s particularly annoying when biogas is produced. And with aluminum oxide, there is a risk of it becoming soluble again if you are spreading the sludge.
There is no perfect solution.
Otherwise, you could try installing a phosphate sensor and regulate injection of FeCl3 depending on the concentration of P. You might have fewer problems if you limit the doses.
We have worked with many utilities fighting this issue. Especially, in NW Arkansas where there is lawsuit between Oklahoma and Arkansas. They have some on limits as low as 0.1 ppm and 0.5 is normal. I think jar testing would help find the best product to remove the P Total. Aluminum Chloride is very effective as well as Aluminum Sulfate. I have successfully used Sodium Aluminate as well which also adds alkalinity while the others reduce it. I think its effectiveness first with the lowest removal balanced with cost. Also, don’t ignore sludge reduction as solids can be determined and calculated as lbs of chemical sludge produced. The iron products do tend to produce a lot of sludge.
Phosphorous removal is currently achieved largely by chemical precipitation, however an alternative is the biological phosphate removal.
1) If you want to continue with the chemical precipitation approach you can use Calcium (in the form of lime Ca(OH)2) as the pH value of the wastewater increases beyond ~10, excess calcium ions will then react with the phosphate and precipitate out. The lime dose required can be approximated at 1.5 times alkalinity as CaCO3.
2) Aluminium and iron can also be used instead of ferric chloride. The dosage rate required is a function of the phosphorous removal required. In practice, an 80-90% removal rate is achieved at coagulant dosage rates between 50 and 200mg/L. The advantages of biological removal of phosphorous is reduced chemical cost, usage and less sludge production. The phosphorous in the influent wastewater is incorporated into cells biomass, which is subsequently removed.
PAO (bacteria used) are encouraged to grow and consume phosphorous. Over the past 20 years several biological suspended growth process configurations have been used to accomplish biological phosphorous removal.