31 May 2024

pH Pivoting for Algae Coagulation: Bench-Scale Experimentation

Abstract: Harmful algal blooms (HABs) threaten recreational waters and public supplies across the US, causing detrimental economic and environmental effects to communities. HABs can be mitigated with dissolved air flotation (DAF) treatment, which requires addition of pH-sensitive charged chemicals to neutralize algae, allowing them to attach to microbubbles and float to the surface. During HAB events and photosynthesis, algae raise the pH to levels that are not ideal for DAF. Traditionally, pH is reduced with a strong acid; however, this adds operational cost and permanently adjusts the water’s pH. This study assessed an approach that might allow for infusing CO₂ from diesel-powered electricity generators into the water prior to DAF treatment. It was hypothesized that formation of carbonic acid could temporarily reduce the pH. Results showed that 2.5%–5.0% CO₂ mixed within compressed air can achieve pH levels between 6–7 in algal water with an initial pH of 9–11 and alkalinity of 150 mg/L as CaCO₃. Further, dosing CO₂ before chemical addition yielded a 31% improvement in water clarification. Returning the pH back to natural levels was not achieved using ambient air microbubbles; however, coarse bubble air spargers should be tested to provide more volumetric capacity for CO₂ absorption.

5 Oct 2023

Rapid Algae Flotation Techniques

Abstract: Some harmful algae produce mucilage or extracellular polymeric substances useful for flotation. This study evaluated natural polysaccharides to determine effects on algal flotation with DAF. Food-grade gums (xanthan gum, guar gum, gum arabic, gellan gum, and diutan gum) were tested with cyanobacteria cultures singly and in combination with commercial flocculants (including Tramfloc 222 and Tramfloc 300). Gum arabic alone had no effect when evaluated at concentrations between 10 mg/L and 5,000 mg/L. However, the combination of gum arabic and Tramfloc 300 yielded higher algal flocculation than Tramfloc 300 alone. The combination of xanthan gum (anionic) and guar gum (cationic) did not perform at the level of the combined xanthan gum and Tramfloc 222 in either flocculation or flotation of algae. Tramfloc 222 and xanthan gum; however, yielded effective flocculation seemingly resistant to changes in interfering factors such as turbulence, pH, and temperature. Furthermore, the combination of xanthan gum and Tramfloc 222 provided the most effective flotation and flocculation independent of pH effects. The results suggest that anionic polysaccharides can be used to increase the efficacy of cationic coagulants such as Tramfloc 222.