(49c) Removal of Algal Matter By Coagulation Both in Presence and Absence of Pre-Oxidation
Removal of algal matter by coagulation both in presence and absence of pre-oxidation
Ziming Zhao, Hassan Gomaa and Madhumita B. Rayïª
Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, N6A 5B9, Canada
Keywords: Algal organic matter, coagulation, pre-oxidation
The eutrophication of surface water is a threat to the safety of drinking water supply worldwide. The presence of algae in intake water poses challenges for water treatment facilities in terms of increased coagulant dose due to release of algal organic matters (AOM), membrane fouling, reduced adsorption efficiency in activated carbon, and increased formation of harmful disinfection by-products (DBP). DBP formation increases after the filtration step due to the accumulation of algal matters on the filter and their subsequent release. AOM consists of both extracellular organic matter (EOM) and intracellular organic matter (IOM). Due to their hydrophilic nature, IOM is not easily removed in coagulation causing formation of downstream membrane foulants. There remains a critical lack of understanding on the type and concentration of IOM which cause adverse effects on water treatment performance, as well as DBP formation. Our initial research with four different algae indicates that AOM release (measured as dissolved organic carbon (DOC) and subsequent DBP formation significantly depend on the type of algae. For example, P. Tricornutum (diatom) (with cell density 2.4 ± 0.2 × 106 cells/mL) released AOM up to 32 mg/L as compared to only 5 mg/L of C. Vulgaris (Green algae) (with cell density 2.5 ± 0.1 × 107 cells/mL), with much higher DBP formation potential for diatom. Typical natural organic matter (NOM) in surface water is in the range of 5 mg/L highlighting the impact of AOM on treatment plants. The objective of present work is to determine the optimum coagulation schemes in presence and absence of pre-chlorination and pre-ozonation for algal matters (both AOM and IOM) produced by six different algae namely, Chlorella vulgaris, Scenedesmus quadricauda, Merismopedia sp, Microcystis aeruginosa, Cyclotella meneghiniana, Phaeodactylum tricornutum.
The effect of background water quality parameters such as turbidity, dissolved organic carbon (DOC), and alkalinity on the coagulation will be measured. Central composite design of experiments involving pH, dosage of coagulant, algal cell density, dosage of ozone/NaOCl, and concentration of background NOM are being used to determine the optimal coagulation conditions. Standard jar tests are conducted using a Phipps & Bird programmable jar tester, with rapid mixing for 2 min at 150 rpm, followed by mixing at 25
rpm for 20 min for flocculation, followed by settling at different time. The supernatant is collected for pH, turbidity, DOC and UV254 measurement.
Preliminary results for Chlorella Vulgaris show that the maximum cell density removal
(99.72 ± 0.04 %) and turbidity removal (92.27 ± 0.84 %) were obtained with initial cell density of 4.8 × 106 cell / mL, alum dosage of 50 mg /L, 1.5 mg / L chlorine as pre-oxidation at pH value of 5.5. However, the DOC increased after coagulation due to the lysis of algal cell in presence of chlorine.
Figure 1. DOC and UV 254 absorbance removal performance of EOM and IOM for
The DOC and UV 254 absorbance removal rate for EOM and IOM is enhanced with the increase of alum dose as shown in Figure 1. Interestingly, DOC removal rate for EOM of Chlorella Vulgaris was less than that of IOM probably due to higher amount of hydrophilic compounds present in the EOM solution, which is difficult to remove by tradition coagulation. On the contrary, limited research in literature on this topic indicates that it is IOM which is typically more hydrophilic. However, the UV 254 absorbance removal rate for EOM was higher than that of IOM indicating the greater removal of aromatic and double-bond compounds in EOM than that in IOM. These results indicate the variability of coagulation performance of different species of algae indicating the need for comprehensive research on this topic, which is the objective of the present study.