New approaches to flocculator design in water treatment

Efficient coagulation and flocculation are crucial for the optimal performance of surface water treatment plants (WTWs). Coagulation alters the stability of small particles, causing them to become unstable, while flocculation promotes the agglomeration of these particles through gentle mixing, forming irregularly shaped, loosely bound flocs. Inadequate coagulation or flocculation can lead to poor-quality water entering clarifiers and filters, which may compromise the final water quality and increase operational costs.

The design process for rotating paddle flocculators is largely based on empirical criteria established by Camp [1955] using the average velocity gradient, G, (s−1), (i.e. the square root of average power per unit volume divided by the dynamic viscosity of the water) and retention time, 𝑡 (s).

However, G is an imperfect design parameter. Clark [1985] and Graber [1994] demonstrated that the original derivation of the average velocity gradient value was flawed for three dimensional flows, and consequently it should not be universally applied to different types or sizes of mixer. Different impellers will also produce different performance levels at the same average velocity gradient, and thus the parameter can only be used to scale up flocculation results from stirred tanks with extreme care ([Lai, 1975], [Ducoste, 1998a] and [Ducoste, 1998b]). Despite these historic criticisms, the velocity gradient remains in widespread use.

Recently, we have begun to consider flow vorticity (i.e. a measure of the degree of rotation of flow, defined as the curl of the velocity vector) as an alternative design parameter for flocculation processes in water treatment. Initial data indicate differences in floc size at lab and full scale, with lab scale flocs being almost twice the size of those found at water treatment works, despite being exposed to similar average velocity gradient values. However, when considering the vorticity of the flows, we find an enhanced exposure to volume-weighted vorticity in the lab scale experiments compared to full-scale experiences.

This PhD project will consider the role of vorticity in floc growth in water treatment in order to improve the design of flocculators. The project will involve numerical analysis of fluid flow using computational fluid dynamics, as well as laboratory and full-scale experimentation.

The principal aim of the project is to develop further our understanding of the flocculation process in water treatment, through accurate numerical simulation at laboratory and full scales in order to provide a much-needed step change in flocculator design processes. 

The specific project objectives underpinning this aim are to:

1.      investigate and determine at lab scale, the precise mechanisms involved in particle agglomeration, breakage and regrowth, and the interactions between turbulence scales and water chemistry for the broadest range of water types.

2.      determine the most appropriate method of simulating the flocculation process in water treatment using a modelling strategy that will consider the use of computational fluid dynamics, discrete element modelling and population balance modelling for laboratory scale applications.

3.      simulate flocculation processes at full scale and to validate these models with appropriate field data.

4.      develop criteria for successful, optimised flocculation for a wide range of raw waters, coagulant types and doses, and flocculators that will be universally applicable and will facilitate a reduced water treatment carbon footprint.

The laboratory work will make use of start-of-the-art simultaneous 2D3C Particle Image Velocimetry and Planar Laser Induced Fluorescence to study turbulent mixing, and a laser diffraction system for accurate floc size characterisation. Computational work will make use of the University of Liverpool high performance computing resource, the parallel Linux cluster, Barkla.

We want all of our staff and Students to feel that Liverpool is an inclusive and welcoming environment that actively celebrates and encourages diversity. We are committed to working with students to make all reasonable project adaptations including supporting those with caring responsibilities, disabilities or other personal circumstances. For example, If you have a disability you may be entitled to a Disabled Students Allowance on top of your studentship to help cover the costs of any additional support that a person studying for a doctorate might need as a result.

We believe everyone deserves an excellent education and encourage students from all backgrounds and personal circumstances to apply.

Applicant Eligibility

Candidates will have, or be due to obtain, a Master’s Degree or equivalent from a reputable University in an appropriate field of Engineering. Exceptional candidates with a First Class Bachelor’s Degree in an appropriate field will also be considered.

Application Process

Candidates wishing to apply should complete the University of Liverpool application form [How to apply for a PhD - University of Liverpool] applying for a PhD in Civil Engineering and uploading: Degree Certificates & Transcripts, an up-to-date CV, a covering letter/personal statement and two academic references.