Control of algae in drinking waters by coagulation and oxidation
Algae in drinking water supplies can cause problems with coagulation and production of disinfection byproducts (DBPs). Stringent regulations imposed by the Disinfectants/Disinfection Byproduct Rule have prompted many water utilities to consider using ozone as an alternative disinfectant to chlorine. The effects of ozone and chlorine on the coagulation of algae and the production of disinfection byproducts from algae were investigated. Two species of algae were cultivated: Scenedesmus quadricauda and Cyclotella sp.
Initial experiments examined oxidant effects on algae cells and extracellular organic matter (EOM). Scanning electron micrographs indicated substantial alterations to the cell wall of Scenedesmus after oxidation, which was accompanied by a decrease in the particle charge. Particle size distributions of Scenedesmus showed a reduction in cell numbers after ozonation alone (no settling) when the cell concentration was low (20,000 cells/mL). In addition, the EOM was shifted to lower apparent molecular weights after oxidation. Cyclotella was less affected by oxidation: cells experienced little alteration.
The effects of ozone on DBP production were determined. For both algae, preozonation increased the rate of formation and the formation potential (FP) of both trihalomethanes (THMs) and haloacetic acids (HAAs). THM yields were increased by up to 50% following 1 mg/L ozone, while HAA concentrations were up to 20% greater than samples that did not receive preozonation. For Cyclotella, it was additionally found that the cells accounted for about 70% of the total yield, while the EOM accounted for the remaining 30%. Comparing the algae species, DBP yields from Cyclotella were double those from Scenedesmus when normalized to the total organic carbon concentration. Comparing DBP species, it was found that HAAFPs were approximately twice as high as THMFPs for both algae.
Bench scale jar tests were performed to evaluate the effects of ozone and chlorine on coagulation of algae cells and EOM. Coagulation experiments with a polyaluminum. chloride showed that oxidation with either ozone or chlorine improved the coagulation and settling of Scenedesmus. This was evidenced by lowered settled water particle counts and turbidity. However, dissolved organic carbon concentrations were increased by oxidation, and this additional material was difficult to remove by coagulation. Under the same conditions, removals of Cyclotella without pretreatment were superior to removals of Scenedesmus. Coagulation of Cyclotella did not benefit from preoxidation with ozone or chlorine. Additional coagulation experiments examined the coagulation of algae-clay and EOM-clay suspensions. In both cases, increased calcium concentrations improved coagulation; however, preoxidation did not significantly impact the settled water quality.