We currently are using PAC, polyaluminum chloride, to settle out our phosphorus.  We started using it this past winter and were getting excellent results.  Recently we've had to add more PAC daily to help keep the phosphorus levels down but our phosphorus levels keep wanting to increase.  We aren't in violation but before it gets there we'd like to get a better handle on this.  I'm wondering if it's working similar to polymer and the charges aren't agreeing between phosphates and PAC.  Or if you have other suggestions that may help us.  Thanks in advance!

Two points to consider beside pH

change in wastewater to be treated
phosphate concentration and composition, total suspended solids

change in the PAC
concentration and quality may have changed.

Kind regards
T. Sobisch

yes. The biology is always a huge factor that chemical salesmen often overlook.(almost never a good idea to rely on them for operations help- the solution is always add more chemicals!) Rapidly settling sludge leaves pin floc behind, maybe the PAC isn't catching all of it.

What operational changes have been made?

Look at the orthophoshate concentration before the PAC addition as stated, has that gone up? Do you use biological phoshorus removal? perhaps an upset? Also check the DO and ORP in the clarifier- a septic clarifier can re-release nutrients.

Ok, I ran a filtered Effluent P test and an unfiltered.  The results I have (this is only for ortho-P) are filtered was .697, unfiltered was .726 and I also did a ortho-P in the clarifier which came out to be .720.  Our phosphorus limit is 1.0mg/L.  Our settling is at already at 120 after 5 minutes of the settle-o-meter test.  We don't have an ORP meter, our pH meter does show mV but I'm not sure that would be an accurate reading to use.  I think that mV pertains to the slope of the pH meter?  We just ordered a portable DO meter and currently take samples only from the top part of the oxidation ditch, clarifier and effluent post aeration tank. (To me our readings of DO aren't as accurate that way, the ditch is 18 foot deep and the clarifiers are about 12 foot deep.  We see a lot of large floc in the clarifier and we add the PAC into the flume between the ditch and the clarifier since it gets decent mixing action there.  Also I was wondering if the level of ortho-P in the tap water would contribute?  That reading is .495.

The P is soluble (in the form of orthophoshate). Increasing the chemical feed will bring it down. You might have to keep a close eye on your alkalinity as increasing it may lower the alkalinity and the pH.

I'm curious why you have to use PAC? it's awfully expensive and alum or ferric chloride will work.

Sounds like your sludge is settling very rapidly, are you increasing the wasting rate yet.