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Appropriate hydrodynamic conditions are crucial in photobioreactors (PBR) in order to prevent sedimentation of microalgal biomass and to ensure the uniform exposure of microalgae cells to light and nutrients. Hydrodynamic conditions are also important to guarantee efficient mass transfer and proper shear stress on the transparent walls of the PBR, which can avoid the formation of undesired biofilm. Numerical simulations based on Computational Fluid Dynamics (CFD) can assist to improve the hydrodynamic design and optimization of PBRs. In this study, CFD was used as a tool to investigate the hydrodynamics of a hybrid horizontal tubular PBR designed for microalgae cultivation and wastewater treatment. The flow regime, average circulation time and shear stress distribution in the tubes were evaluated. To establish the reliability of the simulation study, the CFD model was validated using tracer experimental tests and ultrasonic flow meter measurements. Results showed that the hydrodynamic conditions in the tubes resembled plug flow with small axial dispersion. The simulated velocity profile in the tube corresponded to the analytical velocity profile based on experimental data. Simulations also showed that, even increasing flow velocities, low velocity zones were present in some zones of the PBR. The shear stress distribution in the tubes showed values higher enough to reduce or avoid the formation of biofilm, nevertheless the shear stress value is not sufficient to remove the already formed biofilm. Based on the numerical investigation and practical evaluation, this study demonstrated that CFD is a useful tool to optimize PBR design and operation in order to enhance microalgae production and boost the scale-up of this technology., This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic under OP RDE grant number CZ.02.1.01/0.0/0.0/16_019/0000753 “Research centre for low-carbon energy technologies”. This research was supported by Student Grant Competition of CTU as part of grant no. SGS20/118/OHK2/2T/12. Authors would like to thank the European Commission [project INCOVER, GA 689242] and the Government of Catalonia [Consolidated Research Group 2017 SGR 1029] for their financial support. E. Uggetti and R. Díez Montero would like to thank the Spanish Ministry of Industry and Economy for their research grants [RYC2018-025514-I and IJC2019-042069-I, respectively]., Peer Reviewed, Postprint (published version)

Microalgae-based treatment systems are a suitable solution for agricultural wastewater treatment in order to tackle eutrophication. Valuable biomass can be produced in photobioreactors while removing nutrients from agricultural runoff. However, little is known about experiences of microalgae systems for bioremediation of agricultural wastewater. This study evaluated for one year the performance of a full-scale semi-closed photobioreactor treating agricultural runoff and partially treated domestic wastewater, located in a coastal area in Barcelona (Spain, 41.288 N and 2.043 E, UTM), characterized by Mediterranean climate. The evaluation of the daily variations highlighted specific features of the photobioreactor, such as the cooling capacity of the lateral open tanks, which avoided the extremely high temperatures that would be reached in closed systems, eventually reporting daily variations of temperature similar to those in open systems. The open tanks also allowed for oxygen release, maintaining the dissolved oxygen concentration below inhibiting levels. In turn, the paddle wheels were able to aerate the mixed liquor during night when the dissolved oxygen concentration was lower than the saturation value. Regarding the dynamics of nitrogen removal, ammonium was rapidly removed after feeding. However, nitrate concentration was reduced after feeding but not completely removed in winter. Regarding orthophosphate, its concentration was practically negligible during the whole experimentation due to the removal in the photobioreactor and the low influent concentration. It can be concluded that during the warm seasons, the nutrients removal capability of the system was underused, and in spite of the satisfactory biomass concentration, biomass growth was likely limited by the low concentration of nutrients in the photobioreactor during most part of the day. On the contrary, biomass growth in winter was limited by phosphorus and the low water temperature. The insight gained, The authors would like to thank the European Commission (INCOVER, GA 689242) for their financial support. E. Uggetti, M.J. García-Galán and R. Díez-Montero would like to thank the Spanish Ministry of Industry and Economy for their research grants (RYC2018-025514-I, IJCI-2017-34601 and FJCI-2016-30997, respectively). V. Belohlav would like to thank the Ministry of Education, Youth and Sports of the Czech Republic under OP RDE grant number CZ.02.1.01/0.0/0.0/16_019/0000753 “Research centre for low-carbon energy technologies”. Authors would like to thank Estel Rueda for her help with the microscopy observation and identification of microorganisms., Peer Reviewed, Postprint (author's final draft)

The aim of this work was to investigate the influence of hydrodynamic conditions on the process of microalgae cultivation in a pilot flat-panel photobioreactor (150 L of culture medium). Based on the pulse-input indicator measurement, the numerical model of the hydrodynamic conditions in the photobioreactor was validated. Using this model, it is possible to simulate the hydrodynamic conditions for different operating parameters. For the selected operating conditions, the wall shear stress distribution on the transparent plates of the photobioreactor was created. Simulated data were compared with biofilm formation during microalgae cultivation. According to this, it is possible to specify the required wall shear stress corresponding to the elimination of biofilm formation. Using the developed numerical model, it is possible to simulate the hydrodynamic conditions of the various operating and design parameters. The results of this work can be used for further scale-up of the equipment, for the design optimization, and for the development of novel cultivation system., Postprint (published version)

For modern crop management practices, like precision farming, is crucial information about detailed spatial distribution of soil properties. A study was conducted to evaluate the on-the-go measurement of soil electrical conductivity for mapping of agronomical relevant soil properties. The experimental work was carried out on the eight fields of Rostenice a.s. farm enterprise, located in the South Moravia region of Czech Republic. The measurement of apparent electrical conductivity of soil was done by using CMD-1 and CMD-6L instruments (GF Instruments, Czech Republic) in 2013 (117 ha) and 2016 (359 ha). Soil properties were obtained by soil sampling in irregular grid with the density of 1 sample per 3 ha. Soil samples were taken from the depth of 30 cm and analyzed for soil texture (percentage of clay, silt and sand particles), content of available nutrients (P, K, Mg, Ca) and soil organic matter (SOM) content. The results of correlation analysis showed differences in main sensitivity of EMI to the soil properties across observed fields. Most frequent correlation was found in the percentage of clay particles smaller than 0.002 mm (r = 0.598). The correlation between EMI and nutrients content in soil and pH value was significant only for few fields. These results were obtained for individual fields, the aggregated evaluation showed lower relationships to EC. These outcomes showed, that rather than predictor of soil properties could be on-the-go measurement of soil EC used for identification of main zones within the fields at high spatial level.

The objective of this paper is to present the design, construction and operation (during one year) of 3 full scale semi-closed, horizontal tubular photobioreactors (PBR, 11.7 m3 of volume each) used to remove nutrients of a mixture of agricultural run-off (90%) and treated domestic wastewater (10%). PBRs were located outdoor and have 2 paddlewheels (engines of 0.25 kW) to ensure the movement of the mixed liquor. The microalgal biomass produced in the PBRs was harvested in a static lamella settling tank in which a polyaluminium chloride coagulant is applied. Each PBR treated in average 2.3 m3/d, being the actual mean hydraulic retention time 5 d. PBRs were submitted to strong seasonal changes regarding solar radiation and temperature, which had a direct impact in the activity of microalgae and the efficiency of the system. Higher mixed liquor pH values were registered in summer (daily average > 10). These high values were not observed in the effluents because the system was designed to discharge the mixed liquor (effluent) only at the end of night, when pH reached the lowest daily values (around 8.5). Most of the influent and effluent nitrogen content was inorganic (average of 9.0¿mg¿N/L and 3.17 mg N/L, respectively), and in the form of nitrate (62% and 50%, respectively). Average nitrogen removal efficiency was 65%, with values of around 90% in summer, 80% in autumn, 50% in winter and 60% in spring. Most of the influent and effluent phosphorus content was in the form of orthophosphate. Influent average was 0.62 mg P/L, but with great variations and in a considerable number of samples not detected. Removal efficiency (when influent values were detected) was very high during all the study, usually greater than 95%, and there were not clear seasonal trends for efficiency as observed for TIN. Volumetric biomass production greatly changed between seasons with much lower values in winter (7 g VSS (volatile suspended solids)/m3·d) than in summer (43 g VSS/m3·d). Biomass s, Peer Reviewed, Postprint (author's final draft)