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In the present study, the microalga Spirulina platensis (Arthrospira) was grown in two horizontal photobioreactors (HPBR) under two different irradiances (150 and 270 μmol m-2 s-2). Anaerobically digested cattle wastewater (ACWW) was used as substrate. The experiment was carried out in batches for a period of 8 days. The maximum specific growth rate of 0.347 day-1 and the doubling time of 2.08 days were obtained under the highest illumination of the culture. Dry biomass production reached maximum values between 2.17 g L-1 and 6.52 g L-1, with volumetric biomass productivities between 0.0812 and 0.5578 g L-1 day-1. Productivity per area was equal to 47.97 g m-2 d-1, which is the highest value recorded compared to those found in the literature consulted. As for CO2 biofixation, relevant values for reducing this gas in the atmosphere were obtained, ranging from 128.52 to 882.36 mg L-1 day-1. In terms of organic matter, 16.3-77% of BOD5 and 12.6-61.6% of COD were reduced. In the reduction of TS, TSS and VSS, values of 71.3-78.5%, 79.5-84.4% and 87.0-88.3%, respectively, were reached. NH4+ reduction was 32.5-98.3%, organic nitrogen reduction was 20.3-95.9% and total phosphorus reduction was 33.5-89.9%. The reductions of thermotolerant coliforms were between 71.7% and 99.9%. In view of the results found, it can be considered that the bioremediation of the effluent reached promising efficiencies, with the advantage of producing biomass with potential to obtain bioproducts of relevant economic value. [ABSTRACT FROM AUTHOR]