The impact of k-value-based aluminium speciation and basicity of coagulants on flocs resistance to breakage

The paper presents the results of the research on flocs resistance to breakage. In the study coagulants of various basicity and aluminium speciation were tested. It was noted that with the increase of mixing energy at flocs rupture stage, impurities concentration in water after flocs breakage and further settling of broken flocs increased. However, the values of water quality indicators decreased with the increase of coagulant basicity and the decrease of k₁/k₂ ratio which reflected the share of polymeric Al species. For aluminium chloride after flocs breakage at a rotational speed of 200 rpm followed by clarification, the values of TSS, turbidity, TOC and absorbance UV₂₅₄ (in non-filtered samples) increased by 170%, 203%, 34% and 72%, respectively, in comparison to quality indicators measured in water after settling without flocs rupture stage before clarification. The coagulant with the highest share of polymeric Al species (B4 = 1.47) produced the strongest and the most resistant to shear flocs structure. The lowest increase of TSS, turbidity, TOC and absorbance UV₂₅₄ after flocs breakage and settling was noted then in comparison to the coagulants of lower basicity. TSS, turbidity, TOC and absorbance UV₂₅₄ increased by 42%, 93%, 19% and 27%, respectively. The results revealed the linear relationships between the increase of TSS, turbidity, TOC and absorbance UV₂₅₄ in non-filtered samples after settling of broken flocs and the k₁/k₂ ratio values (k₁, k₂ rate constants stated in ferronometry assay indicate the presence of specific forms of aluminium). The linear relationships were also noted considering the coagulant basicity. However, k₁/k₂ ratio was a better indicator to estimate flocs resistance to stress. R-squared values (R²) were higher compared to basicity values. Independently from the coagulant Al speciation, flocs rupture did not cause the desorption of dissolved organic pollutants adsorbed during the flocculation stage.