Influence of solar irradiance levels on the formation of microalgae-bacteria aggregates for municipal wastewater treatment

Abstract

Light intensity is considered an important factor in the production of extracellular polymeric substances and the formation of microalgae-bacteria aggregates. The influence of the solar irradiance level on the formation of microalgae-bacteria aggregates was studied, considering the settling properties and the removal of organic matter and nutrients. Three different solar average irradiance levels of 6213, 2741 and 3799Whm−2d−1 were studied in an 80L outdoor high rate algae pond (July to November 2015) operating at a hydraulic retention time of 10days and treating municipal wastewater. The highest irradiance level (6213±1186Whm−2d−1), showed a poor wastewater treatment performance related to low removal efficiencies of TN (36±12%), total COD (50±8%). However, the removal efficiency of P-PO43− evidenced the highest values (92±1%). Furthermore, low settling velocity (Sv) and settleability (4×10−3mh−1 and 26±11%, respectively) were associated with a poor aggregation formation in the system. In contrast, low irradiance levels (<3800Whm−2d−1) promoted the formation of microalgae-bacteria flocs and granules with high settling velocity and settleability (18mh−1 and 85%, respectively). Moreover, under low irradiance levels, high removal efficiencies for TN (60±5%), total COD (89±3%) and P-PO43− (28±7%) were observed. Nitrification mechanism was only detected at low irradiance levels, which contributed, on average, to 30% of the TN removal from the influent. A relevant factor in the overall good performance of the microalgae-bacteria systems was EPS formation.