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Coagulation process is highly influenced by solution pH, floc breakage, and coagulant aid. In this work, two-stage addition of alum-based coagulants, polyaluminum chloride (PACl) and aluminum sulfate (Al2(SO4)3), was compared to their one-stage addition with the same total dosage. The dose ratio of primary and secondary addition was optimised. The effects of solution pH, floc breakage and dosing timing of coagulant aid of non-ionic polyacrylamide (NPAM) on coagulation efficiency and floc property were investigated. The results showed that two-stage addition of PACl showed less sensitivity with the variation of pH. Compared to PACl, two-stage addition of Al2(SO4)3 was more sensitive to the solution pH, especially when the pH ranged from 7 to 10. For all cases, the presence of floc breakage improved the removal of colloidal particles. The coagulation mechanism of primary coagulant was charge neutralisation, while other mechanism was involved with the secondary addition of coagulant. Compared to one-stage addition, the settling performance of aggregates and particulates removal in two-stage addition was more effective with the variation of dosage or dosing timing of NPAM. The present work provides detailed operating parameters for this coagulation strategy.

The mechanism of removal of humic acid, phosphate and kaolin particles by coagulation with alum and PACl or adsorption by their pre-formed precipitates was investigated, and it was found that the coagulation mechanisms for monomeric Al at neutral pH and polymeric Al13 at alkaline pH were very similar. The removal of phosphate and humic acid by coagulation with alum or PACl did not change with stirring time (between 1 min and 15 min), independent of the dose and species of coagulants. However, for adsorption of these impurities by pre-formed precipitates, the results were significantly different. Both Al3+ and nano-sized Al13 could precipitate and form aggregates at pH 7 and pH 9, respectively, and their precipitates became less active (fewer binding sites on the surface of precipitate) with the increase of shear time or shear rates before adsorbing pollutants. Thus, although the total surface area increased (the average size of flocs became smaller) at higher applied shear rates or longer shear time, the removal efficiency of humic acid and phosphate decreased. Also, from the MW distributions, it was confirmed that less humic acid was removed by the adsorption on alum precipitate pre-formed with longer shear time. Chemical groups (OH2and OH) on the surface of precipitate determined the removal efficiency of phosphate and humic acid, and the activity of precipitate become lower as a result of higher applied shear and longer shear time. This is confirmed be due to some crystallization of the amorphous precipitate, forming inactivated hydroxyl. When kaolin was added 10 min after the alum or PACl precipitate formed, the precipitates captured kaolin particles only on their surface, whereas when alum was added to kaolin suspensions particles were trapped within the growing flocs. When alum/kaolin flocs were broken at high shear rate re-growth of flocs decreased with increasing shear time, but after a short breakage period, long aging of broken flocs had little effect on floc regrowth.

This study was aimed to study the effect of solution pH on the aid effect of compound bioflocculant (CBF) in dye wastewater treatment combined with widely used alum coagulants. The color removal performance of aluminum sulfate (AS) and polyaluminum chloride (PAC) was significantly improved with the aid of CBF. In addition, the effective pH range of alum salts was also broadened by CBF. Floc properties including size, growth, breakage, re-growth and sedimentation natural were determined by PDA2000. The results showed that floc size was enlarged by CBF, especially at acidic conditions and that floc grew faster when CBF was added, especially under neutral and alkaline conditions. Floc sedimentation rate, floc breakage and recovery factor were also enhanced by CBF throughout the studied pH range. CBF was characterized by FT-IR on which there is abundant –COO − and –NH. It was suggested that adsorption and bridging effect of CBF performed a positive role in dye wastewater treatment.

Made available in DSpace on 2019-10-06T17:02:00Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-07-01. Added 1 bitstream(s) on 2019-10-09T18:35:44Z : No. of bitstreams: 1 S0104-66322018000301029.pdf: 928475 bytes, checksum: ed4f66466d12137d7a27d19a978fc734 (MD5) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The reversibility of aggregates during flocculation was investigated. The stable diameter (d) and the power law slope coefficient of the particle size distribution (β) were applied to follow re-formation after breakage. A non-intrusive image-based technique was used for monitoring flocs. Aggregates were formed by adding alum [Al 2 (SO 4 ) 3 ·18H 2 O] by the sweep-coagulation mechanism to two synthetic waters, prepared from kaolin (Fluka) and humic acid (Aldrich Chemical). Velocity gradients (G) varied from 20 to 120 s -1 during experiments, and the rupture occurred under controlled conditions. After rupture, the initial condition was reinstated and reversibility analyzed. Results pointed out the irreversibility of breakage for both Al-kaolin and Al-humic flocs. The stable diameter of aggregates after breakage (d 2 ) varied from 157 to 132 µm for Al-humic and from 233 to 123µm for Al-kaolin aggregates, using G from 20 to 120 s -1 . β 2 values varied from 1.2 to 4.6 for Al-humic and from 0.6 to 7.7 for Al-kaolin. Universidade Estadual Paulista (Unesp) Instituto de Geociências e Ciências Exatas, Avenida 24-A, nº1515, C. P. 178 Universidade Estadual Paulista (Unesp) Instituto de Geociências e Ciências Exatas, Avenida 24-A, nº1515, C. P. 178 FAPESP: 2013/25641-9

The effect of polyacrylamide (PAM) dosage and mixing time on the formation, breakage, and regrowth of alum flocs was explored by Mastersizer 2000. The result showed that the coagulation efficiency and regrowth ability of broken flocs were determined by the mixing time, PAM dosage, and pH value. When PAM was dosed simultaneously with alum, the broken flocs could not regrow, independent of PAM dosage, while the regrowth ability of broken flocs increased as the dosing time of PAM delayed. Broken flocs could fully regrow when PAM was dosed 5 min after the addition of alum. Both flocs’ size and regrowth ability enhanced with the increase in PAM dosage when PAM was dosed 5 min or more later than alum. Although the size of alum-HA flocs could be slightly improved by PAM, the addition of PAM significantly improved both the growth ability of flocs and the regrowth ability of broken flocs at pH 7 and 8. This might be caused by the variation of chemical bonds with pH for PAM and alum precipitate. Scanning el...