Use of a high resolution photographic technique for studying coagulation/flocculation in water treatment

Abstract

The coagulation/flocculation process is an important part of surface water treatment. It has direct impact on the reliability of plant operations and final water qualities together with cost control. Low water temperature has a significant impact on the operation of drinking water treatment plants, especially on coagulation/flocculation processes. A microscopic image technique has been used to study the coagulation and flocculation process in recent years, but it requires sample handling that disturbs the floc characteristics during measurement. A high resolution photographic technique was applied to evaluate flocculation processes in the present work. With this technique, the images of the flocs were obtained directly while the flocculation process was taking place. In combination with camera control software and particle size analysis software, this procedure provided a convenient means of gathering data to calculate size distribution. Once the size distribution was calculated, the floc growth and floc size change in the aggregation process could be analyzed. Results show that low water temperature had a detrimental impact on aggregation processes. A water temperature of 0 °C resulted in a slow floc growth and small floc size. Although the floc growth rates at 4 °C and 1 °C were less than those at 22 °C, they were higher than at 0 °C. To improve aggregation processes at low water temperature, adding the coagulant aid of anionic copolymer of acrylamide into the water was found to be effective when the temperature was not less than 1 °C. However, it made only a slight impact on aggregation when the temperature approached 0 °C. At water temperatures of 22 °C, 4 °C and 1 °C, the polymer caused the formation of large floc (larger than 0.5 mm2 in projected area). The polymer significantly shortened the required time of flocculation and sedimentation. Three minutes of flocculation and 20 minutes of sedimentation were sufficient for the polymer to achieve good treatment performance, while the flocculation time and sedimentation time had to be 20 and 60 minutes, respectively, without using the polymer. On the other hand, when the temperature was close to 0 °C, the polymer did not cause the formation of the large floc, nor did it shorten the time of flocculation and sedimentation. The experimental results in this research agree with the model for flocculation kinetics given by Argaman and Kaufman (1970). With decreasing water temperature, the aggregation constant (KA) decreased and breakup constant (KB) increased. KA and KB with aluminum sulfate was close to those with ferric sulfate, respectively. In treating the South Saskatchewan River water, an aluminum sulfate or ferric sulfate dosage greater than 50 mg/L resulted in marginal gains in treatment efficiency. Decreasing dosages of aluminum sulfate or ferric sulfate caused lower floc growth rates and smaller floc sizes. Extremely low dosages (5 mg/L or less) resulted in poor floc formation and extremely small sizes.