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

Abstract

A blend of ferrous chloride and ferric chloride (FeCl 2 -FeCl 3 ) was simultaneously dosed into an activated sludge system at pilot scale in order to test the effect on biological P removal. Additional removal due to chemical precipitation was measured as the difference in system P removal between parallel test and control systems. Both systems strongly exhibited biological excess P removal (BEPR). The extent of P release in the anaerobic reactors of the two systems was compared by mass balance, as one indicator of the relative "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 FeCl 2 -FeCl 3 addition, even in the absence of effluent phosphate limitation. However, 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. FeCl 2 -FeCl 3 dosing in this range was sufficient to produce additional P removal of the order 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 the BEPR mechanism in the presence of FeCl 2 -FeCl 3 was possible over a continuous period of seven sludge ages, under conditions in which effluent phosphate was at least partially limiting. Under such conditions, the chemical and biological mechanisms appear to be "disadvantaged" to approximately the same extent, as evidenced by the apparent stoichiometry of Fe:P for the chemical precipitation and magnitude of the poly P containing fractions measured for the biological mechanism. This suggested that the biological mechanism is able to compete effectively with the chemical mechanism under conditions of low reactor phosphate concentrations (~1 mgP/l orthoP) for sustained periods. However, the presence of simultaneous chemical precipitant significantly reduces the extent to which the biological P removal potential is utilised under P-limiting conditions. This could explain the difficulty sometimes reported in the control of full-scale activated sludge systems with simultaneous precipitant addition where a very low effluent P concentration (<1 mgP/l) has to be achieved.

The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal: Part 7: Application of the IAWQ model

“…This is an acceptable assumption given that most biological systems operate optimally at a pH close to neutral. In Parts 3, 4 & 5 (De Haas et al, 2000c to d;2001a), it was reported that the reactor pH in the pilot plant systems used here fluctuated in the range 6.9 to 7.8, with median values close to pH 7.4. On this basis, the inherent assumption in the model with respect to pH appeared to be acceptable.…”

“…However, the settling problems that emerged in the test unit (alum dosed) and later also in the control unit during the first alum dosing period with 250 mg/l acetate COD (see Part 3, De Haas et al, 2000c) resulted in a revision of the experimental approach. A lower acetate supplement in the influent (usually 150 mg/l as COD) was accepted (De Haas et al, 2000c to d;2001a), with the settled sewage contributing the balance of the influent COD. This resulted in the influent composition being more variable than would ideally have been preferred.…”

“…For a limited number of experimental periods (Periods 3.6.1 and 3.6.2a,b with ferric chloride dosing -see De Haas et al, 2000d), RBCOD was measured using the physico-chemical method of Mamais et al (1993). For these periods, the measured RBCOD values were used for characterisation of the influent.…”

“…Accordingly, an investigation was carried out in which the ASM No. 2 was applied to the experimental results reported in Parts 3, 4 and 5 (De Haas et al, 2000c;2001a). This paper describes the outcome of that investigation with the aim of evaluating the predictive power of ASM No.2 for BNR-activated sludge systems incorporating simultaneous addition of aluminium or iron salts for supplementary phosphorus removal.…”

“…The first five parts of this series of papers (De Haas et al, 2000a to d;2001a) outlined the background and experimental investigation conducted into simultaneous chemical precipitation of phosphate in a biological nutrient removal (BNR) modified activated sludge systems, at pilot scale. In Part 6 (De Haas et al, 2001b), existing models for the chemical precipitation phenomenon were reviewed.…”