Pipeline transportation of sanitised sewage sludge
Creators: name: Mohammed, Nurudeen
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)

description: Wastewater undergoes a series of biological and physical treatments in wastewater treatment plants (WWTPs) to separate water from biosolids and to remove the hazardous matter. The processed water is usually released into rivers, whereas sludge with reduced environmental footprints is landfilled. In recent time, population growth, industrialisation and stringent environmental regulations, however, have resulted in substantial volumes of wastewater generation exceeding new infrastructure development, implying huge volumes of sludge generation. Consequently, WWTPs are under pressure to improve performance by handling highly concentrated sludge. Ideally, sludge is transported in pipelines across various units and into landfill. However, sewage sludge pumping is a process with high energy demand which grows with solid concentrates because of strong non-Newtonian and complex behaviours of highly concentrated sludge. Hence, for an optimised system design, it has become imperative to study the pipe flow of highly concentrated sludge, which depends on the rheology of the material.

Rheological characterisation provides important design and optimisation information utilised by engineers for pump system design and operation of treatment processes. Concentrated sludge shows very complex rheological properties which change with sludge source, treatment and composition. These properties inform sludge behaviour and largely determine flow properties in pipelines. Accurate rheological measurement is key for practical design application as it influences proportionately the energy consumption for pumping that has hitherto accounted for a high share of treatment costs due to oversized pumps.
 
In the pipe flow of concentrated suspension, the pressure drop estimation of laminar and turbulent flow and the prediction of laminar/turbulent transition are extremely important. These predictions entirely determine the efficiency of pumping requirements and translate into the efficiency of the overall pipeline design. Several parameters based on complex fluid behaviour such as wall slippage, yield stress, particle size etc. and their impacts, which have been previously considered less important in previous studies, need to be investigated in pressure drop calculation and the accuracy with which these parameters are determined is essential in the design and control of sludge-pumping operations.
 
This study aims to develop pipe flow theory for the viscoplastic behaviour of highly concentrated sanitised sludge as a model fluid suitable for pumping design of concentrated wastewater sludge. This will pioneer laboratory investigation which can circumvent use of biohazardous wastewater sludge in pipe flow study and ensure reliable data collection. The first objective involved developing safe, stable and non-biohazardous simulants to carry out pipe loop experimental study. Furthermore, benchtop rheological measurement to predict pipe loop laminar flow data was used to discern the fluid flow behaviour required for system design. This is very important where pilot test pipe rigs are not available and because the flow behaviour of test fluid is required prior to pump design. Laminar-turbulent transition was then studied using some available predictive models appropriate for HB fluid. The impact of suspended particles in delaying transition was quantified. Finally, turbulent flow prediction from selected turbulent flow equations was carried out and the effect of particle size on turbulent flow was investigated.

Experiments were carried out at the PC2 Lab at both RMIT City and Bundoora campuses, where benchtop rheological measurement and pipe loop testing were studied extensively. Raw sludge (WAS and DS) was collected from the Eastern Treatment Plant and was decontaminated with clinical grade potassium monopersulphate. Then steady shear and oscillatory testing was conducted on the raw sludge and decontaminated samples for different solid concentrations (3–6% TS). Experiments were carried out with an HR3 rheometer equipped with hatched parallel plate and vane geometry to avoid possible slip effects. Furthermore, upon establishing the suitability of the simulant for pipe loop testing, pipe loop experimentation was carried out for different sludge concentrations across three different pipe sizes. Samples were collected after every pipe loop test and subsequent rheometric and compositional tests were carried out on the collected samples.
 
Experimental laminar pipe loop data were separated and analysed by fitting the extended Buckingham equation to the pseudo-shear plots to discern the flow parameters. These parameters were compared with the results of benchtop measurement after appropriate data correction techniques. Flow parameters discerned from fitting extended Buckingham equations to the laminar flow data were used subsequently in predicting transient and turbulent flow.
Experimental results of comparative study on the rheological properties of simulant sludge and raw sludge showed that the material is safe, stable and suitable for use in pipe loop testing. Steady shear behaviour and viscoelastic properties suggested that the simulant can mimic the behaviour of raw sludge albeit at higher concentrations. Comparison of complex viscosity and steady shear viscosity was largely matched as suggested by the Rutgers–Delaware proposition. Additionally, benchtop rheological measurement using parallel plate largely matched flow parameters from pipe loop compared to vane geometry. This implies that parallel plate geometry better predicts pipe loop laminar flow than vane geometry and is suggested as the geometry of choice in discerning fluid flow properties.
 
Laminar-turbulent transition from different predictive models (𝑅𝑒3, 𝑅𝑒𝑀𝑅, 𝑅𝑒𝐺𝑒𝑛) was analysed by employing the concept of wall viscosity. Contrary to popular belief that transition occurs at 2100, transition was delayed well above this value. Accordingly, the impact of suspended particles in delaying transition was quantified. By implication, this allowed accurate determination of the critical velocity required in system design. Furthermore, following accurate material characterisation and succinct prediction of transition, prediction of turbulent flow was assessed employing four selected turbulence models. Performance of the models was estimated statistically, and they were ranked accordingly. The model appropriate for sludge pump design was developed by correlating the experimental turbulent flow data.

Finally, for practical application, case studies employing either of the developed turbulence models in design were investigated. Techno-economic analysis suggested the importance of appropriate selection of the best model even though very minimal error ranges were estimated between these models. Hence, it is important that material flow properties are accurately determined, transition predicted well, and the appropriate turbulence model is selected for an efficient system design for transportation of highly concentrated sludge displaying viscoplastic behaviour.