Scale-down of high-rate algae ponds systems for urban wastewater reuse

Abstract

In spite of the important environmental and operative advantages of microalgae-based wastewater treatment systems, they present a low level of implementation. Experimental validations in lab scale should reproduce conditions as close as possible to outdoor conditions. In this sense, two experimental reactors were operated under irradiation and temperature levels corresponding to summer and winter conditions found in temperate climates. The configuration of these reactors resulted in mass transfer rates (gas exchange) in the range of the values reported for industrial scale outdoor facilities. While superficial biomass productivity was in the range of previously reported experiences (between 5.7 and 22 g m−2 d−1, in winter and summer conditions, respectively), pollution removal efficiencies reached the values required for wastewater discharge: >78 % of CODt, >96 % of NH4+ and >79 % of PO43−. Unlike previous experiences, hydraulic retention time (HRT) was maintained constant (3 days) in both seasonal conditions, reaching sufficient pollution removal and biomass productivity. The pathogen concentration reached in the final effluent was compatible with the reclaimed water standards in spite of the absence of ultraviolet light in the illumination system, evidencing the relevance of the environmental conditions created by microalgae metabolism in the disinfection process. The characterization of the microbial population revealed the presence of bacteria responsible of organic matter removal, ammonia oxidation and microalgae commonly found in wastewater systems.