Adsorption isotherms of Polyquaternium-10 polymers by activated sludge solids

Polymeric quaternary ammonium salts (polyquaternium-10 polymers, PQ-10) are extensively used as components of hair care products, lotions, and makeup and, therefore, are present in domestic sewage. Adsorption of these polymers by activated sludge was assessed to evaluate their removal by a commonly used wastewater treatment process. Activated sludge solids (ASS) were dosed with PQ-10 polymers with relatively high molecular mass (JR-125 and JR-30 M) and relatively low molecular mass (LR-400 and LR-30 M) and equilibrated for 2 h at 20 ± 2 °C. After equilibration, the vessels with the mixtures were centrifuged and the supernatants were analyzed with a phenol method. The relationships between dosed amounts and equilibrium concentrations in the aqueous phase (dose-response curves) for each tested polymer indicated that at relatively low dosing solution concentrations were negligible ("total adsorption" region of the dose-response curve). Light absorbance of the samples prepared for determination of polymer concentration using a phenol method within this region was close to the absorbance of the blank (0 μg (a.i.)/mL) or below absorbance of the blank, resulting in calculated concentrations apparently below zero. At some level of dosing, a subsequent increase in polymer dosage resulted in a quantifiable increase in solution concentration ("solution concentration rising" region). The relationships between concentrations of the polymers in the aqueous phase and concentrations of adsorbed polymers (adsorption isotherms) were non-linear. The ability of activated sludge solids to bind tested polymers may be characterized by apparent "threshold values" of the loading for each polymer. Below the "threshold value," solution concentration of the polymer is close to zero and above this value it becomes measurable. "Threshold loading" for all polymers was from 50 to 100 mg (a.i.)/g, far above concentrations of these compounds expected to be present in the sewage.