Author: Rajina K C
K C, Rajina, 2023 Experimental and numerical study of the flow field in a High-Rate Algal Pond (HRAP), Flinders University, College of Science and Engineering
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The constant growth of the world population by 1.2% on average since 2000 has created pressure on water, food, and energy resources, increasing the need for a sustainable future that includes cost-effective and power-efficient systems. Water stress is related to climate change, focusing concerns on source availability and protection, and water reuse. Emerging technology is microalgae for wastewater treatment, reuse, and biofuel production due to the system's ability to break down organic matter in the presence of sunlight with bacterial interaction. The open raceway, or high-rate algal pond (HRAP), has been adopted as a sustainable technique for wastewater treatment as it is cost-effective, energy efficient, and requires minimal maintenance. The wastewater in the HRAP, of 0.3-0.5 m depth, is mixed by a slowly rotating paddlewheel.
Although there has been an increased understanding of algal productivity and wastewater treatment efficiency there have been few advances in understanding the hydrodynamics of HRAPs (High-Rate Algal Ponds) since 2010. To analyse the system performance, wastewater treatment and algal productivity, it is important to understand the flow field since efficient algal culture requires mixing, which is possible by ensuring a turbulent flow and flow maintenance throughout the pond. The wastewater mixing and circulation ensures microalga exposure to the sunlight penetrating the water column to support the photosynthesis process, nutrient, and CO2 distribution throughout the pond.
The primary aim of this thesis was to develop a 3D-computational model to assess the velocity distribution pattern of an HRAP of 5000 m2 at Peterborough, South Australia, operating at 0.3 m depth and a design surface velocity of 0.2 m s-1 mixed by one paddle wheel. In operation since 2019 treating 425 m3 d-1 of wastewater (design capacity 475 m3 d-1) the treated wastewater is used for irrigation of urban green space. Most computational fluid dynamic studies (CFD) have been either small-scale laboratory-based or pilot-scale; few studies have been validated on physical systems.
A numerical study was carried out using ANSYS FLUENT. Additionally, the numerical approach was taken using the inlet velocity method and the contour of velocity and turbulent intensity was obtained to study the flow field and hydrodynamic character of a HRAP. Experimentally at the Peterborough HRAP, the flow velocity profile was determined at 9 points on the channel cross section at 26 sites along the 1km channel using a Flo-Mate 2000. The measurements were carried out at 3, 15 and 30 cm depth at 3 points across the channel width, centre (2.125 m) and left and right (at 1.35 m on either side from the centre). Statistical data analysis was done to obtain the graphical presentation of velocity distribution to observe the flow pattern. Furthermore, the numerically predicted data were validated against the experimental measurements using EXCEL and SPSS software obtaining correlation values and regression models.
The results of the research presented here are unique since the large, serpentine Peterborough HRAP comprises 6 channels (total length 1.1 km) interconnected by five 180° bends. Both the experimental and numerical studies captured the flow patterns in the straight sections and around bends. It was shown that the average experimental velocity in 30 cm, 15 cm, and 3 cm is 0.256 m s-1, 0.253 m s-1 and 0.200 m s-1, and the pond was 0.23 m s-1. The standard deviation showed 99.7% of the velocity sample taken lies between 0.17- 0.31 m s-1. Similarly, the average numerical velocity in 30 cm, 15cm and 3 cm is 0.259 m s-1, 0.247 m s-1 and 0.200 m s-1, respectively and the overall pond was 0.23 m s-1. The standard deviation showed 99.7% of the velocity sample taken lies between 0.19- 0.28 m s-1. Another apparent result was for the bends that showed the higher velocity and flow directed towards the outer curve and low flow towards the inner wall depicting the dead zone (velocity < 0.1 m s-1). The average lowest velocity in an experiment and CFD observed was 0.01 m s-1 and 0.04 m s-1 respectively, whereas the average highest data captured in the experiment was 0.48 m s-1 and the predicted CFD was 0.45 m s-1.
Furthermore, the CFD model of Peterborough HRAP was experimentally validated with the Flo-Mate data, which showed a strong agreement between the measured and predicted data with correlation (R) and R2 values of 0.844 and 0.720, respectively. The R2 for the bend velocity was 0.8397, showing the strong agreement between experimental and CFD simulation. The model processed using commercial CFD, ANSYS FLUENT code has represented the hydrodynamics (velocity and turbulence) of HRAP validating the model. The CFD results have captured the phenomena of flow separation, recirculation, and reattachment of flow to the normal flow direction along with the dead zone's location and how far they have extended along the channel. Therefore, the HRAP at Peterborough is operating well with required velocity and turbulence maintenance throughout the pond facilitating algal activity and ensuring the wastewater treatment process.
Keywords: High-Rate Algal pond, Computational Fluid Dynamics, Hydrodynamics, Flow field, Validation
Subject: Engineering thesis
Thesis type: Masters
Completed: 2023
School: College of Science and Engineering
Supervisor: Prof. Howard Fallowfield