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A simple batch settling test has been developed to examine the process effectiveness of polyelectrolytes. Serious degradation and Joss of effectiveness were noted under high shear, in the presence of ferrous ion and with long term storage. Polyelectrolyte application in phosphorus removal is expected to occur in conjunction with "coagulant-precipitants" such as alum, ferric chloride and lime. Thus, before assessing the effect of polyelectrolytes, the residual settled phosphorus concentration and the settling rate resulting from the addition of these three coagulant precipitants were studied by themselves in model phosphate solutions as a function of phosphate type, pH and coagulant-precipitant dosage. The results indicated that substantial near colloidal, non settle able phosphorus precipitates can be formed with all three chemicals. The magnitude of this fraction was determined by pH. Condensed phosphates lowered the optimum precipitation pH in the aluminum and iron systems. The addition of polyelectrolytes slightly increased the removal of residual phosphorus and dramatically increased settling rates in the model solution systems. With polyelectrolytes satisfactory performance is anticipated over a much wider pH range. The results, in domestic wastewaters, with alum as the coagulant- precipitant, appeared similar to those of the model solutions. The agreement was best between maximum permissible overflow rates (although alum only results were predicted conservatively in the laboratory) and worst in terms of predicting effluent concentrations. The plant operated very well with alum and polyelectrolyte addition prior to primary sedimentation over the one month period of the study, with secondary effluent phosphorus concentrations averaging approximately 0.4 mg/l as P. A comparison of a large number of commercial polyelectrolyte indicated that the best polyelectrolyte for phosphorus removal is a 10-20% hydrolyzed polyacrylamide with a molecular weight above 8 x 106. Studies on polyelectrolyte residuals, after sedimentation in model solution studies, indicated that a small concentration of polyelectrolyte can be expected to remain in solution and that this concentration is decreased as the ratio of polyelectrolyte mass to floc mass is reduced. Afin d'examiner l'efficacité des polyélectrolytes, un essai de dcantation en discontinu a été mis au point áa travers une méthode d'interprétation permettant l'extrapolation des donnes du traitement en discontinu aux clarificateurs áa marche continue. On a observé une dégradation et une perte d'efficacité graves dans des conditions de cisaillement accentues, en pršence d'ions ferreux et lors d'un long entreposage. L'application de polyé1ectro1ytes pour l'élimination du phosphore se fait en mãeme temps que celle de "coagulants-prcipitant' comme l'alun, le chlorure ferrique et la chaux. Ces trois substances peuvent produire une quantité importante de prcipitš phosphorš áa l'état presque colloèidal et non dcantables, mesure áa l'aide du pH. Les phosphates condensš abaissent le pH optimal de la prcipitation en pršence de chlorure ferrique et d'alun. Les phosphates condensš ont tendance áa diminuer la dcantabilité des flocs, en pršence d'alun et de chlorure ferrique, et une dose maximale permise de coagu1antprcipitant pourrait ãetre prédite áa partir de l'alcalinité initiale. L'addition de po1yé1ectrolytes accroit légèrement l'efficacité d'élimination du phosphore ršiduel et augmente fortement les vitesses de dcantation dans les solutions modèles. La comparaison d'un grand nombre de polyélectrolytes commerciaux indique que le meilleur est un polyacrylamide de 10 áa 20% hydrolysé et de poids molculaire supérieur áa 8 x 106. Des études des ršidus de polyélectrolytes dans les solutions modèles après dcantation indiquent qu'une petite quantité demeure en solution mais que sa concentration est proportionnelle au rapport de la masse de polyélectrolytes áa la masse du floc. |
