Removal of phosphorus and organic matter removal by alum during wastewater treatment

Ferron reagent and FTIR spectroscopy were used for the identification and characterization of the aluminum species formed during dephosphorization of simulated wastewater with and without organic matter. Evidence from FTIR spectroscopy showed the formation of aluminum hydroxyphosphate, hydroxy-Al–tannate and aluminum complexes containing both phosphorus and tannic acid. The surface reactivity of the solid products is proportional to the rate of reaction with ferron. The measured reactivities indicate that aluminum solid species with different surface properties were formed depending on solution components and method of precipitation. Tannic acid was found to inhibit phosphorus removal and the extent of inhibition increased with increasing concentration. When prehydrolyzed aluminum is exposed to tannic acid, the organic matter forms a coating on the surface of the inorganic solid during the aging process. Coprecipitation of aluminum, phosphate and tannic acid, which is synonymous with the addition of alum before or in the aerator, produces some soluble complex and some hydroxy-Al–phosphate–tannate complex, in the form of solid with very small particle size. This system gives high residual aluminum. The results suggest that it is advantageous to add at least a portion of the alum at the exit of the aerator. This enhances phosphorus removal by coprecipitation under conditions where the concentration of organic matter is relatively low and enhances removal of organic matter by postprecipitation onto the recycled sludge in the aerator.