Achieving appropriate hydrodynamic mixing in the design and operation of large-scale High Rate Algae Pond (HRAP) ponds can be a delicate task which depends on a number of sometimes competing factors. Dealing with living cells require knowledge of their sensitivity to external conditions (shear stress, light, CO2, etc.). For example, vertical mixing is used to control the amount of light reaching the micro-organisms but must account for the organism sensitivity to excessive light exposure. The design and operation of mixing systems must also consider potential damage due to excessive strain induced by hydrodynamic shear stresses. Too much mixing can generate cell damage due to the sensitivity of certain strains to shear stress. From an operational efficiency point of view, it is also important to minimize the energy required by the mixing system. In standard HRAP, mixing is often provided by horizontal axis paddlewheels whose primary role is to drive the pond circulation. This project aims to study the full mixing process in a conventional HRAP design with a view to determining the flow conditions needed improve the productivity of the systems. A pilot experiment was built in Arava demo-site to study the fish wastewater treatment with spirulina. Numerical simulations of the fluid flow in a small (3.5m x 1.5m x 0.2 m) and a long (16.5 m x 1.5 m x 0.2 m) HRAP taking into account the paddle wheel using the immersed boundary method implemented by Specklin et al. [1]. A Large Eddy Simulation (LES) model have been used to model turbulence, a multiphase-particle-in-cell (MPPIC) method was included to capture the interaction of flow and moving boundaries with inert particles used to simulate the micro-algae. The results highlight that mixing occurs mostly in the bends and the neighbourhood of the paddlewheel. There is little mixing in the middle of the long pond. The quality of mixing with different paddlewheel geometries is also studied. The effect of the geometry (aligned and non-aligned blades) has previously been investigated by Hreiz et al. [2]. They show that by using the aligned-blades configuration the mixing is enhanced. In this study, we focussed on two different paddlewheel geometries with aligned-blades. A methodology to evaluate the vertical mixing has been developed by computing the average absolute velocity in several transversal sections all along the pond. Also, the particle light/dark cycles have been characterised by following the particle trajectories. Particles are injected in the fluid flow to determine the amount of particles that receive enough light and those that stay in the dark zones. The shear stress in all the pond has also been computed. The results show that maximum shear stress occurs in the bends, the walls and in the neighbourhood of the paddlewheel. The different configurations of paddlewheels are evaluated in terms of their ability to provide a suitable vertical mixing with a proper amount of energy and with minimum shear stress. This study was financed by the European project SaltGae [3] whose goal is to develop a viable solution to treat saline wastewater. [1] M. Specklin, R. Connolly, B. Breen and Y. Delauré, A versatile immersed boundary method for pump design, 3rd international Rotating Equipment Conference (Dusseldorf), Germany (sept. 2016). [2] R. Hreiz, B. Sialve, J. Morchain, R. Escudié, J.-P. Steyer and P. Guiraud, Experimental and numerical investigation of hydrodynamics in raceway reactors used for algaculture, Chemical Engineering Journal, 250 (2014) 230-239. [3] SaltGae, Algae to treat saline wastewater, saltage.eu