Biological process modeling is advanced by explicitly describing heterotroph and nitrifier biomass, incorporating formation of soluble microbial products (SMP) from both the bacterial groups, and allowing degradation of SMP by the heterotrophs. Biomass decay now has two parts, endogenous respiration and formation of biomass-associated products (BAP). The model is applied to investigate interactions between heterotrophs and nitrifiers. Main attention is directed to evaluating the role that SMP produced by nitrifiers plays as a supply of organic substrate to heterotrophs and to predicting the COD concentration in the effluent. The model quantitatively describes the observed accumulation of SMP in the effluent at long SRT and at high influent substrate concentration. The significance of SMP from nitrifiers to support growth of heterotrophs is clearly elucidated through the model experiments under various operational conditions. The results indicated that a high NH4+-N/COD ratio in the influent would decrease original substrate COD due to increased heterotrophs whose growth is supported by SMP from nitrifiers, but total COD increases. The minimum substrate concentration, Smin, is reduced for heterotrophs by the additional growth from SMP.

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