Wastewater Characterisation for Design and Modelling of Primary Settlers at Municipal Wastewater Treatment Plants

Abstract: The primary settling tank (PST) is often one of the first treatment steps at a wastewater treatment plant (WWTP) and is the first process in the plant to remove significant amounts of suspended solids. Its role in wastewater treatment however, has oftentimes been neglected and little effort has been made to optimize and model the process. The PSTs have historically been designed based on hydraulic surface loading rates and expected suspended solids removal based on old rules of thumb, without considering the physical properties of the incoming wastewater such as settling properties of the sus- pended solids and composition of the wastewater in terms of solubles/particulates and biodegradable/inert material. In this project, the incoming suspended solids, particulate chemical oxygen demand (COD) as well as the inert and biodegradable parts of the particulate COD at two different WWTPs have been characterized based on their settling velocities. A model of the PST using groups with different settling velocities has also been set up to validate the results and it has been used to compare with current design guidelines for the PST. The characterization has been done with a settleometer consisting of five different columns and a calculation procedure for considering the misplacement of particles in the settleometer has been developed. The results indicate that the settling velocity distribution of the particles vary between different WWTPs and that a model based on the particle settling velocity distribution concept yields good results, but some modification of the assigned distribution had to be made. A broader distribution of the settling velocity groups is needed to model the PST outside its normal overflow range.