Advanced biokinetic and hydrodynamic modelling to support and optimize the design of full-scale high rate algal ponds

High rate algal ponds (HRAP) are known for their suitability to treat wastewater and to produce microalgal biomass, which can be converted into bioproducts. However, full-scale application of HRAP is still limited to few cases, and design procedures are not consolidated or standardized. In this study, a demonstrative-scale HRAP system for secondary wastewater treatment to be implemented in India (treatment capacity of 50 m³·d^-1) has been designed combining conventional dimensioning techniques and advanced modelling tools. The objective of the study was to assist, verify and optimize the conventional dimensioning of the secondary HRAP by means of simulations predicting the behaviour of the system in the specific local conditions under different configurations and operational strategies. Biokinetic modelling and hydrodynamic analysis using Computational Fluid Dynamics (CFD) were carried out. The simulations performed with the biokinetic model showed that the optimal hydraulic retention time to enhance nutrient removal and biomass production is 4 days. For the hydrodynamic modelling, a 3D model of the HRAP was built to simulate the hydrodynamic behaviour of 36 different designs. Simulations allowed quantifying the presence of low velocity zones as well as the land use efficiency of the different designs in terms of the useful area vs. the total occupied area. Two baffles and tear-shapes with a diameter equal to ¼ of the channel width was the most efficient configuration. Moreover, a technical-economic assessment of the system was carried out, resulting in an investment cost of 483 € per population equivalent and an operational cost of 0.19 € per m³ of treated wastewater.