ARTICLE
Metal hydroxide intermediates: Why they matter for phosphorus removal
Aluminum- and iron-based coagulants are commonly used to precipitate phosphorus from wastewater. As these coagulants react with wastewater constituents, a significant amount becomes floc composed of metal hydroxide intermediates, such as Al(OH)3 that removes phosphate by adsorption.
The adsorption process works well to achieve effluent P concentrations around 1 mg/L, but becomes less effective when more stringent limits are required. To reach low P targets operators typically increase the molar dose rate of aluminum and iron coagulants, but this results in higher chemical costs, greater sludge production and adverse impacts on system pH.
The advantages of ClariPhos Rare Earth Coagulant
ClariPhos is different. It’s made with cerium and lanthanum, two rare earth elements that react primarily in a one-step process to bond with phosphorus and precipitate it from wastewater. Bishop Water’s analytical and operational experience with ClariPhos shows that only a small amount of the coagulant reacts to form intermediates such as Ce(OH)3.
The rare earth elements (REE) in ClariPhos (left) have a high affinity for phosphate and when combined form rhabdophane, a dense and insoluble mineral. By comparison, Al- and Fe-based coagulants form intermediates that remove phosphate by adsorption, a weak bond that is reversible and non-specific to phosphate. Adsorption is less efficient when there is little phosphate present.
The table below shows results from a study that evaluated coagulant performance to reduce phosphorus from 0.8 mg/L to 0.06 mg/L. The results revealed that less than seven percent of the ClariPhos chemical sludge was in a metal hydroxide form, whereas almost 70 percent of the alum sludge was as a metal hydroxide. Similar results were seen for iron sludge.
Estimation of sludge generated from coagulant addition. (MW – molecular weight, M – metal, RE – rare earth) From: Rare Earth technology for low-level P removal and enhanced sludge properties, Haneline, M.; Cornish, P. and Gallmann, J.
That means an abundant amount of rare earth elements remain available in the wastewater process to form ionic bonds with phosphate and precipitate as a naturally occurring, insoluble mineral known as rhabodphane.
The crystalized forms of cerium and lanthanum phosphate (CePO4, LaPO4) readily form and are much more stable than phosphated metal hydroxide intermediates, such as Al(OH)2H2PO4. Cerium and lanthanum phosphate are also heavier, denser and settle faster, which improves clarifier performance.
By comparison, metal hydroxide intermediates are more susceptible to changes in pH and interference from other ions, which may reduce adsorption and even cause phosphorus to be released back into the wastewater.
ClariPhos ionic bonds beat hydroxide-intermediate adsorption
The graph below shows another example of the benefits of fewer metal hydroxide intermediates for chemical phosphorus removal. It compares the molar dose rates of ClariPhos against several aluminum- and iron-based coagulants to achieve a range of phosphorus targets. As the phosphorus targets drop, the dose rates for the aluminum- and iron-based coagulants increase significantly.
By switching to ClariPhos, WWTPs can improve phosphorus removal, reduce chemical consumption, produce less sludge, and lower operating costs.
To achieve a phosphorus concentration of 0.1 mg/L in the treated effluent, ClariPhos maintains a 1:1 molar dose rate, whereas Al- and Fe-based coagulants required molar dose rates ranging from 3:1 to 8:1.
Learn more about the advantages of ClariPhos Rare Earth Coagulant.
Contact us to discuss ClariPhos jar testing and dose estimations for your wastewater plant.
