An application of cellular organic matter to coagulation of cyanobacterial cells (Merismopedia tenuissima)
Abstract
Algae affect the performance of drinking water treatment significantly when they decay and release considerable amounts of cellular organic matter (COM). The study describes the cyanobacterium Merismopedia tenuissima and its COM and investigates the effect of their simultaneous coagulation.
As COM is highly complex mixture, we characterised it in terms of hydrophobicity, protein content and molecular weights (MWs). To describe the coagulation mechanisms and molecular interactions in the system, we determined both COM and cell surface charge by means of potentiometric titration and zeta potential analysis, respectively, and performed the jar tests with single components and their mixtures with and without a coagulant (ferric sulphate).
The coagulation tests performed with the individual components or with their mixtures proved efficient cell removals (up to 99%) but relatively low COM removals (37 divided by 57%). This disproportion can be attributed to the prevalence of hydrophilic compounds and to the high portion of low-MW organics in COM.
Coagulation of COM/cell mixtures achieved comparable efficacy with single component tests, using even lower coagulant doses. Furthermore, COM presence substantially deviated the pH optimum for cell removal and thus altered coagulation mechanisms.
While single cells interacted prevailingly through adsorption onto Fe-oxide-hydroxides at about neutral pH (6.0-7.7), the COM/cell mixtures succumbed to charge neutralisation by Fe-hydroxopolymers within moderately acidic pH range (5.0-6.5). Moreover, COM initiated cell flocculation also at acidic pH in both the presence (pH 3.4-3.9) and the absence of a coagulant (pH 3.6-4.6).
This supportive effect is ascribed to relatively high-MW COM (>10 kDa), serving as a natural flocculant through inter-particle bridging mechanism and exhibiting nearly the same COM/cell removals as ferric sulphate.