Luxury uptake of phosphate by activated sludge.

Abstract

Phosphate's role in the eutrophication roc ss has been well documented, whether it is the limiting nutrient or not. Substantial efforts have been made re cently by the federal government and municipalities to limit the amount of phosphate ion that is released in effluents from waste water treatment plants. Under ordinary operating conditions the two most commonly used heterotrophic biological treatment processes, trickling filter instal lations and activated sludge aeration facili ties, have been reported to be capable of removing an average of only 20 and 40 percent, respectively, of the soluble phos phate concentrations normally found in municipal wastewaters.1 Clearly, addi tional treatment of phosphate ion is needed to reduce its effluent concentration in modern phosphate-rich waters. Under normal operating conditions, the biomass of the heterotrophic biological treatment plants removes organic carbon and phos phorus from the wastewater in a 106:1 ratio,2 and the sludges from such processes contain approximately 2 to 3 percent of their dry weight as phosphate.3 Current efforts to modify biological treatment systems, which have been in vestigated in the laboratory and to a limited extent in the field, involve the addition of chemicals containing cations that form insoluble phosphate precipitates, which are then removed from the waste water by conventional separation tech niques. Chemicals such as lime and ferric and aluminum salts are being used for this purpose. The chemicals conven tionally are added in a separate third stage process, but in some cases they have been successfully added directly to the secondary aeration tanks. A number of activated sludge waste water treatment plants, however, have reported phosphate removals of appreciably greater than 40 percent.4-7 These removals far exceed any typical 106:1 stoichiometric carbon : phosphorus ratio. At times, these treatment plants reportedly achieve a 90 to 95 percent phosphate removal, even in phosphate-rich wastewaters. Such excessive phosphate removal by an activated sludge process has been termed luxurious uptake of and much controversy has arisen over the mechanism by which this removal takes place. In 1965, Levin and Shapiro8 reported that this uptake was a biological one that was dependent on oxygen transfer into the cell and relatively independent of carbon con centration. In 1970, Menar and Jenkins9 reported that, in their opinion, such luxuri ous removals were a result of a chemical precipitation phenomenon, and, although the extent of removal was a direct function of the aeration rate, the removal was oxygen-transfer independent. In their scheme, the aeration process allegedly served to strip the metabolically-produced carbon dioxide (C02) from the sludge suspension. The resultant increase in pH induced the precipitation of some form of calcium phosphate, which then was re moved with the biomass during sludge separation. Two such divergent hypoth eses are contradictory and mutually ex clusive because both mechanisms cannot