Effects of velocity gradient and mixing time on particle growth in a rapid mixing tank

Abstract

Effects of velocity gradient (G value) and mixing time (t) on the coagulation of particles were investigated by monitoring particle counts in a rapid mixing process. Total particle counts after mixing of 30 s at G value of 200 s(-1) were similar to the summation of clay particles and alum precipitates at alum dose of 30 mg L(-1). Although small particles (microflocs less than 5 microm) were formed within the mixing time of 30 s, macroflocs larger than 8 microm did not increase significantly until the mixing time of 60 s. However, macroflocs larger than 8 microm started to increase after mixing of 75 s and they reached the maximum counts at 150 s. On the other hand, macroflocs larger than 8 microm decreased after mixing time of 180 s due in breaks of the macroflocs, which resulted in resuspension of small particles. The rapid mixing conditions for the maximum growth of macroflocs were the G value of 200 s(-1) and the mixing time of 150 s, which confirmed the best performance of turbidity removal in jar tests. Growth and break of particles and flocs in a rapid mixing step would influence significantly on the sequential flocculation and sedimentation processes for turbidity removal.

The role of rapid mixing condition on picophytoplankton floc growth

“…Nevertheless, the velocity gradient giving the largest floc formation for both picophytoplankton and kaolin system was slightly higher than that suggested in previous studies. In the previous study, researchers suggested that G value ranges between 180 and 400 rpm for kaolin system [22][23][24][25][26]. However, here we observed that G value range giving the maximum floc formation was between 500 and 600 rpm for picophytoplankton system.…”

Effects of hydrodynamics and coagulant doses on particle aggregation during a rapid mixing

“…The raw water samples were kept in suspension by gentle agitation at room temperature until use. Park et al [12] reported that the particle size distribution of synthetic kaolin suspension was comparable to that of natural lake water samples. The characteristics of synthetic water and natural lake water are shown in Table 1.…”

“…They confirmed that high turbidity removal could be achieved using both a short mixing time (approximately 10 s) and a long mixing time (approximately 60-90 s) using highly turbid synthetic water. Recently, Park et al [12] investigated particle growth at different G values and mixing times using standard jar tests. At the predetermined optimum coagulant dose, they showed that the number of particles larger than 8.0 μm was lowest for a G value of 200 s -1 and a mixing time of 150 s. The best turbidity removal was also confirmed under these rapid mixing conditions followed by flocculation and sedimentation processes.…”

Utjecaj Miješanja Na Učinkovitost Procesa Koagulacije I Flokulacije Pri Kondicioniranju Vode Vodocrpilišta „jarčevac“

“…Nekoliko studija je, prateći vrijednosti više parametara kakvoće vode, ispitalo i potvrdilo utjecaj brzine miješanja na učinak koagulacije i flokulacije pri kondicioniranju vode [9,18,19]. Tako Lin i suradnici [9], ispitujući povezanost početne mutnoće (200 NTU, 15 NTU) i brzine miješanja vode pri doziranju koagulanta, navode da je utvrđena značajna korelacija između brzine miješanja vode pri doziranju koagulanta i završne kakvoće vode, naročito kod uzoraka vode s manjom početnom mutnoćom.…”

“…Ukupnu tvrdoću vode čine sve kalcijeve i magnezijeve soli, i to prije svega njihovi hidrogenkarbonati i karbonati te sulfati, kloridi, nitrati i silikati [4]. Kako čestice kalcijeva karbonata također posjeduju negativan naboj, primjenom kationskih koagulanata može doći do djelomičnog mekšanja vode [18].…”