Primary sedimentation modelling with characterized setting velocity groups

Within a plantwide water and resource recovery facility context, an important requirement for a primary sedimentation unit model is the correct fractionation of the settleable portion (primary sludge - PS) of the raw wastewater total suspended solids (TSS) according to the (i) unbiodegradable particulate organic (UPO), (ii) biodegradable particulate organic (BPO), and (iii) inorganic settleable solid (ISS) components. This paper focuses on improving a current TSS- based primary settling tank (PST) model to account for correct proportions of these three components, with characterized settling velocity groups. The steps taken towards development of the primary sedimentation unit model involved the development of a discrete particle settling model in Microsoft Excel and the utilisation of well characterised municipal wastewater data from previous studies in the discrete particle settling model, to reproduce PS and settled wastewater outputs in settling fractions of UPO, BPO and ISS, via steady state and dynamic calculations and under strict material mass balances. Finally, the insights obtained from discrete particle settling model calculations were implemented in the development of a dynamic University of Cape Town primary sedimentation unit (UCTPSU) model. This dynamic model was rigorously verified to be internally consistent with regards to material mass balances and utilised to simulate plantwide scenarios, under steady state conditions, whereby the impact of incorrect characterisation of TSS components (UPO, BPO and ISS) fractions was evaluated. From these evaluations, it was noted that the incorrect disaggregation of the TSS components of primary sludge can lead to incorrect predictions with regard to parameters such as the settled wastewater composition and the activated sludge system capacity. Thus, the investigation revealed the need to measure key wastewater parameters such as particle settling velocities and the UPO fraction, towards realistically modelling the primary sedimentation unit operations.