Wastewater characterisation for design and modelling of primary settlers at municipal wastewater treatment plants


The primary settling tank (PST) at municipal wastewater treatment plants (WWTP) is most often the first treatment step after screens, and fat and sand traps. Its main function is to reduce the organic load to the subsequent biological treatment step. Less organics into the aerobic treatment leads to less oxygen demand, and aeration is the most energy-intense activity at the WWTP. Furthermore, the biogas production potential of the wastewater sludge increases when primary settling is included in the treatment train. Chemically enhanced primary treatment (CEPT) by addition of coagulants and/or flocculants, may also be pursued for maximising removal of organics. However, traditional biological nutrient removal (BNR) processes are in need of organics, i.e. too efficient PST could generate inadequate biological nitrogen and phosphorus removal, and then external, often fossil-based, carbon sources are added.

PSTs are designed on hydraulic loading and expected removal of suspended solids (SS) and organics based on old rule-of-thumbs, without considering the physical properties of the current wastewater such as settling properties of SS and fractionation of organics into solubles/particulates and biodegradables/inerts. PSTs are often modelled with simple models since these were considered sufficiently robust fulfilling rule-of-thumbs.

In the WWTP of the future, when carbon recovery is even more important, the BNR processes biofilm or membrane based and when digital twins are implemented, the function of the PST will get more attention and the understanding of this initial treatment step needs to be enlarged.


The main objectives of this project are (i) to study methods for characterising settling properties of municipal wastewater, and (ii) to utilise experimental data for more complex PST modelling to be able to compare traditional design with a design based on physical and chemical characterisation of wastewater.


Characterisation of settling properties is conducted at Källby WWTP (Lund) Öresund WWTP (Helsingborg) and used for simulation with a computer model.


Autumn 2022


Chebbo, G. & Gromaire, M.-C. (2009). VICAS – an operating protocol to measure the distributions of suspended solid settling velocities within urban drainage samples. Journal of Environmental Engineering 135(4), 768-775.