Measurement Of Ordinary Heterotroph Organism Anoxic Yield In Anoxic-Aerobic Activated Sludge Systems

BACKGROUND

Integral to the biological nutrient removal (BNR) activated sludge system is the biologically

mediated process of denitrification. Not only does this process reduce the N load to the

environment, but it also has the advantages of alkalinity recovery and reducing the oxygen

demand. Further, the denitrification protects the biological excess P removal (BEPR), if

included, from the deleterious effect of recycling nitrate to the anaerobic reactor (Wentzel et

al., 1990). Accordingly, quantifying the potential denitrification is fundamental to the

design and operation of the BNR activated sludge system. Biological denitrification has

been explicitly incorporated into the steady-state design (e.g., WRC, 1984; Wentzel et al.,

1990; Maurer and Gujer, 1994) and kinetic simulation (Van Haandel et al., 1981; Henze et

al., 1987, 1995; Dold et al., 1980, 1991; Wentzel et al., 1992; B.arker and Dold, 1997;

Mumleitner et al., 1997; Gujer et al., 1999) models developed as aids to the design and

operation of BNR activated sludge systems. In both sets of models, critical as input to

quantify the denitrification is the value for the ordinary heterotrophic organism (OHO)

anoxic cell yield coefficient (Y H,No).

In terms of the models, the heterotrophic cell yield coefficient, Y H, is the fraction of

substrate electrons that are used to synthesize new cell mass, while the remainder, 1 - Y H,

are passed to the terminal electron acceptor to generate energy. Hence, the value of Y H,

designated here Y H,No for anoxic conditions and Y H,AE for aerobic, determines both the

mass of electron acceptor utilized and the new cell biomass produced. In activated sludge

systems treating municipal wastewaters, the effect of YH ,N O on sludge production typic.a lly

is small, since the mass of sludge produced under anoxic conditions is small compared to

that produced under aerobic conditions, due to the relatively low influent TKN/COD ratios

(Barker and Dold, 1997). In contrast, the effect of Y H,NO on the amount of denitrification

achievable (and hence, on system design and operation) is quite significant.

In the IWA Task Group models for activated sludge systems, ASMl (Henze et al., 1987),

ASM2 (Henze et al., 1995) and ASM2d (Henze et al., 1999), and similar (e.g. Dold et al.,

1991; Wentzel et al., 1992), the heterotrophic yield coefficient is assumed to have the same

value under anoxic as under aerobic conditions. However, when nitrate serves as terminal

electron acceptor, ideally only 2 ATP are formed per pair of electrons (e-) transferred to

nitrate as compared to 3 ATP when the transfer is to oxygen (Payne 1981, WRC 1984,

Kuba et al. 1993, Casey et al. 1999, Wentzel et al. 2001). This difference reduces the

energy captured by the organism when nitrate serves as electron acceptor (versus oxygen) in

biological oxidation of organic substrate. Correspondingly, therefore, the cell yield under

anoxic conditions (Y H,No) should be reduced relative to its aerobic value (Y H,A£).

Based on thermodynamic and bioenergetic principles, it is shown (Appendix A) that the

theoretical anoxic cell yield coefficient (Y H,No) is about 83% of its aerobic value (Y H,AE), or

0.35 mgVSS/mgCOD (0.5 mgCOD/mgCOD) compared to the .aerobic yield of 0.42

mgVSS/mgCOD (0.6 mgCOD/mgCOD). Similarly, Orhon et al. (1996) theoretically

quantified the anoxic to aerobic yield ratio for four organic substrates based on bioenergetic

principles set out by McCarty (1971, 1972, 1975). For municipal wastewater, protein,

lactate and carbohydrate substrates, they obtained the following anoxic:aerobic yield ratios,