Comparison of floc properties of coagulation systems: Effect of particle concentration, scale and mode of flocculation

Abstract

This study is aimed to ascertain the influence of varying quantum of organic and inorganic colloids on parameters like floc size, fractal dimension and residual particle size, under alkaline conditions for water treatment by coagulation. Synthetic sample waters were prepared by using humic acid and kaolin for simulating organic matter and inorganic turbidity respectively. PACl (Poly aluminium chloride) was used as a coagulant. Image analysis for floc size measurement and box-counting method for fractal dimension measurement has been used. A batch jar test reactor and two continuous flow pilot-scale reactors: 1. Conventional clariflocculator (CC) and 2. Sludge blanket clarifier (SBC) were studied to evaluate the effect of scale-up on floc properties. The pilot systems differed in the mode of flocculation, as CC used a mechanical impeller and SBC used hydraulic mode for flocculation. For CC, the trendlines indicated a decrease in the size of flocs with increasing organic concentration. The order of floc size was: SBC > jar test > CC. The average fractal dimension of flocs in jar test was 1.28 which was lesser than that in CC (1.51) indicating more filament-like structure of flocs in jar test. The floc sizes and fractal dimensions depended on the scale and mode of flocculation. The size of residual particles in the treated water was quite similar irrespective of the scale and mode of flocculation. However, the trends were opposite with increasing input TOC. For jar test, the residual particle sizes decreased and for pilot-scale, it increased with increasing input TOC.