Settling velocity distribution of microalgal biomass from urban wastewater treatment high rate algal ponds

The aim of this study was to evaluate the settling velocity distribution of microalgal biomass with and without flocculant (Tanfloc SG). Microalgal biomass was obtained from two experimental wastewater treatment high rate algal ponds (HRAPs) operated with 4 and 8 days of hydraulic retention time. Two sets of dynamic sedimentation tests were carried out using a water elutriation apparatus. In the first set, most of the biomass of the 8 days-HRAP (63%) had settling velocities between 16.5 and 4 m/h, while most of the biomass of the 4 days-HRAP (65%) had settling velocities between 16.5 and 1 m/h. In the second set, most of the biomass from both HRAPs (60% from the 8 days-HRAP and 80% from the 4 days-HRAP) had settling velocities between 6.5 and 0.4 m/h. In this second set, settling velocities of < 0.4 m/h were reached by 20% and 40% of the biomass from 4 days-HRAP and 8 days-HRAP, respectively. The addition of flocculant at optimal doses ranging from 20 to 40 mg/L had impressive effects on the settling velocity distribution in this second set. 70% and 84% of biomass reached velocities of > 6.5 m/h, compared to 10% and 14% of microalgal biomass without flocculant for the 8 days- and 4 days-HRAPs, respectively. With flocculant, a very small amount of biomass (3% for the 4 days-HRAP and 8% for the 8 days-HRAP) had settling velocities of < 0.4 m/h. Microscopic examination of samples from sedimentation tests showed how an important amount of microalgae settled in the system. Indeed, < 1500 microalgae individuals/mL were found in all outlet samples from the elutriation apparatus (inlet samples of > 105 microalgae individuals/mL). According to our results, a settler designed with a critical settling velocity of 1 m/h would reach biomass recoveries as high as 90-94% with flocculant compared to 77-88% without flocculant.