An investigation of mechanisms for the enhanced coagulation removal of Microcystis aeruginosa by low-frequency ultrasound under different ultrasound energy densities.

There is a lack of studies elaborating the differences in mechanisms of low-frequency ultrasound-enhanced coagulation for algae removal among different ultrasound energy densities, which are essential to optimizing the economy of the ultrasound technology for practical application. The performance and mechanisms of low-frequency ultrasound (29.4 kHz, horn type, maximum output amplitude = 10 µm) -coagulation process in removing a typical species of cyanobacteria, Microcystis aeruginosa, at different ultrasound energy densities were studied based on a set of comprehensive characterization approaches. The turbidity removal ratio of coagulation (with polymeric aluminum salt coagulant at a dosage of 4 mg Al/L) was considerably increased from 44.1% to 59.7%, 67.0%, and 74.9% with 30 s of ultrasonic pretreatment at energy densities of 0.6, 1.11, and 2.22 J/mL, respectively, indicating that low-frequency ultrasound-coagulation is a potential alternative to effectively control unexpected blooms of M. aeruginosa. However, the energy density of ultrasound should be deliberately considered because a high energy density (≥18 J/mL) results in a significant release of algal organic matter, which may threaten water quality security. The specific mechanisms for the enhanced coagulation removal by low-frequency ultrasonic pretreatment under different energy densities can be summarized as the reduction of cell activity (energy density ≥ 0.6 J/mL), the slight release of negatively charged algal organic matter from cells (energy density ≥ 1.11 J/mL), and the aggregation of M. aeruginosa cells (energy density ≥ 1.11 J/mL). This study provides new insights for the ongoing study of ultrasonic pretreatment for the removal of algae via coagulation.

A comparative study on the use of palm bark as a supplementary carbon source in partially saturated vertical constructed wetland: Organic matter characterization, release-adsorption kinetics, and pilot-scale performance.

In this study, a comparative investigation of palm bark and corncob (a well-investigated material) for enhancing nitrogen removal efficiency in partially saturated vertical constructed wetland (PSVCW) was performed to evaluate an effective and cost-effective supplementary carbon source. The characteristics of the released organic matter and the release processes were analyzed through optical property characterization and a first-order release-adsorption model, respectively, and the nitrogen removal performance was evaluated in a series of pilot-scale PSVCWs. Results showed that the amount of organic matter released per unit mass of corncob was larger than that released per unit mass of palm bark under the same pretreatment conditions (control, heat, and alkaline pretreatment). The organic matter released from corncob has a higher apparent molecular weight and a higher degree of aromatic condensation than those of the organic matter released from palm bark, whereas the organic matter released from palm bark has higher and more stable bioavailability. Moreover, palm bark showed a more significant improvement of release capacity with the heat and alkaline pretreatment methods. Pilot-scale studies revealed that PSVCW using palm bark as the supplementary carbon source has a longer replacement cycle and higher total nitrogen (TN) removal efficiency than that using corncob, indicating that palm bark can be considered an effective and inexpensive supplementary carbon source. This study provides initial guidance for the ongoing research on supplementary carbon sources for improving nitrogen removal efficiency in constructed wetlands.