Understanding some of the differences between the COD- and BOD-based models offered in STOAT

Within STOAT, there are different models for the activated sludge process. The ASAL models are based on BOD and the IAWQ models are based on COD. When these different models are used, differences in the sludge yields can arise. The reasons for these differences are discussed and the yield values which were obtained for a hypothetical plant were compared with values reported in the literature. With the ASAL range of models, sludge production predictions may be influenced by the mixing regime in the aeration tank. Also, the split between viable and non-viable biomass is poorly defined in the ASAL models.