Influence of hydraulic retention time and carbon loading rate on the production of algae

•Algae production depends on light color, being green illumination the best choice.•The higher the HRT and CLR, the higher is the resulting algae production.•Ratio COD/TSS of the algae produced is ca. 3.13 mg COD mg−1 TSS.•Maximum algae production of 44 mg d−1 L−1 are obtained within the conditions studied.•Quality of product as fuel (as MOSC) improves with longer HRT and for CLR around 10 mg d−1 L−1. This paper is focused on the assessment of the production of algae in batch bioreactors. Hydraulic retention time, carbon loading rate and light color were the inputs of the study and algae production the main output. Bioreactors were operated in semi-continuous mode and tests lasted two months, more than two times the period required to meet a steady-state response. This steady-state was verified with plateau responses in both, soluble parameters and suspended solids. Results points out the great relevance of temperature. Likewise, they show that green light improves the production of algae, as well as long HRT and high CLR. Maximum production rates attained were in the range 4–14 mg d−1 L−1. The ratio COD /TSS for this biofuel was almost constant (3.13 mg COD mg−1 TSS) but the quality of the product obtained in terms of the Mean Oxidation State of Carbon is completely different. Longer HRT leads to lower MOSC and hence to potentially more valuable fuels.