Influence of flocs size and dissolved air concentration on the efficiency of flotation

ABSTRACT

This paper shows the results of a research that aimed to investigate the influence of flocculated particles size on the dissolved air flotation (DAF) efficiency. Special image acquisition method and equipment were developed to characterize the flocculated particles and air microbubbles size distribution in the contact zone outlet of the DAF unit. The trials were carried out with synthetic water (temperature of 26.0±0.5^oC; turbidity of 7.5±0.2 NTU; apparent color of 38±1 uC; alkalinity of 29.8±0.6 mgCaCO₃.L^-1; conductivity of 59.3±0.8 µS.cm^-1; UV_254nm of 0.128±0.004 cm^-1), that it was coagulated by applying 1.9 mg.L^-1 of Al³⁺ (pH 6.6±0.05; 22.5 mg.L^-1alum). The results showed that: i) the flocculation conditions interfered directly on the efficiency of DAF process, and the best flocculation conditions were those that conducted to formation of flocculated particles size with Feret mean diameter above 300 µm; ii) the best results were obtained applying Condition 2 of flocculation (T=17 min/G=60 s^-1) combined to air mass concentration of 4.8 mg.­L^-1, however, DAF was also capable of operating with a satisfactory efficiency when applying flocculation time of 10 minutes; iii) the special image acquisition method and equipment developed proved to be an important tool to characterize the flocculated particles and air microbubbles size distribution.

Keywords:
flocculation; dissolved air flotation; image analysis.