The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal - Part 5: Experimental periods using a ferrous-ferric chloride blend

A blend of ferrous chloride and ferric chloride (FeCl2-FeCl3) was simultane ously dosed into an activated sludge system at pilot scale in order to test the effect on biological P removal. Additional removal due to chemical pre cipitation was measured as the difference in system P removal between paral lel test and control systems. Both systems strongly exhibited biological ex cess P removal (BEPR). The extent of P release in me anaerobic reactors of the two systems was compared by mass balance, as one indicator of the relat ive "magnitude" of BEPR. Phosphorus fractionation of the mixed liquor also served as an indicator of the biological and chemical mechanisms. Evidence was found that the BEPR mechanism is partially inhibited by simultaneous Fe Cl2-FeCl3 addition, even in the absence of effluent phosphate limitation. H owever, the degree of inhibition was relatively low, ranging from 3 to 25% (approximately) for Fe doses in the range ca. 10 to 20 mg/l as Fe, with an average system P removal of 14 to 18 mgP/l in the control. FeCl2-FeCl3 dosi ng in this range was sufficient to produce additional P removal of the orde r of 1 to 8 mgP/l over periods of one to seven sludge ages per experimental period, depending on the experimental conditions. Sustained operation of t he BEPR mechanism in the presence of FeCl2-FeCl3 was possible over a contin uous period of seven sludge ages, under conditions in which effluent phosph ate was at least partially limiting. Under such conditions, the chemical an d biological mechanisms appear to be "disadvantaged" to approximately the s ame extent, as evidenced by the apparent stoichiometry of Fe:P for the chem ical precipitation and magnitude of the poly P containing fractions measure d for the biological mechanism. This suggested mat the biological mechanism is able to compete effectively with the chemical mechanism under condition s of low reactor phosphate concentrations (similar to1 mgP/l orthoP) for su stained periods. However. the presence of simultaneous chemical precipitant significantly reduces the extent to which the biological P removal potenti al is utilised under P-limiting conditions. This could explain the difficul ty sometimes reported in the control of full-scale activated sludge systems with simultaneous precipitant addition where a very low affluent P concent ration (<1 mgP/l) has to be achieved.