primary sludge thickening performance

[schap]

Primary sludge thickening performance at gravity thickeners at a WWTP decreased significantly in 1998. Prior to 1998 typical thickened primary sludge concentrations were around 5-6 %D.S. Since 1998, thickened sludge concentrations have decreased to around 3-4% D.S. Several tests conducted suggest that the decrease thickening performance is primarily due to changes in the primary sludge thickening characteristics (as opposed to thickener operation).
Prior to 1998, the plant added "pickle liquor" (ferrous) to precipitate phosphorus. In 1998, they switched to ferric chloride. Do you think this change could potentially affect primary sludge thickening characteristics?

 

[mbph]

Sure, every technique depends on the wastestream and conditions. Probably best to run pilot tests before major operational changes.

 

[ischgl99]

I would expect the ferric to settle your sludge better then the ferrous. The ferrous has to be oxidized to ferric, and then form ferric hydroxide, for the coagulating to occur. Is your pH lower now then before? What other changes occured at the plant at that time? Did the flow to the plant increase? Did a new company come to town that changed the sludge chemistry?
Higgins and Novak did a study several years ago that showed when the ratio of Sodium and Potassium to Magnesium and Calcium is above 1:1, sludge settling problems are increased. Can your lab do an analysis of this?
You can try a bench scale test of ferrous and ferric and see how they compare, that may give you an indication of whether the chemical is to blame or if the problem lies elsewhere.

Mike Bensema

http://www.dutchmenservices.com

 

[BobPE]

I would have to second mbph on his opinion....

BobPE