Treatment of printing ink wastewater using high liquid carryover mode function

The aim of this project is to investigate the feasibility of the use of a novel process of high liquid carryover (HLC) mode flotation to treat wastewater from a cardboard printing and coating plant. This is conducted by characterising the process conditions with the purpose of achieving a set condition that have a low susceptibility to variability in the wastewater composition. Information pertaining to the surface and physical characteristics of the solid contained in the wastewater sample were used to explain the flotation outcomes. Fe(II) was found to be more suited for industrial use as coagulant over the more commonly known coagulants, Al(III) and Fe(III), as it has the advantages of having a constant optimum flotation pH (6.5) regardless of wastewater characteristics, as well as having a much lower detrimental effect on the flotation outcome when present in excess of the minimum requirement for flocculation of wastewater solids. This allows the setting of a coagulant dose (5 mM) that will treat both high and low solids content wastewater samples. The use of cationic surfactant (CTAB) was found to require lower coagulant dosages compared to those required for anionic (SDS) or non-ionic (TericG12A8) surfactants in order to achieve good flotation outcomes. A CTAB concentration of 300 ppm was required to achieve a stable foam that can sustain for a prolonged period to allow solid (in foam)/liquid separation. The optimum batch mode conditions for HLC flotation were found to be pH 6.5, 5 mM Fe(II) and 300 ppm CTAB. This was found to correspond to a small and negative electrophoretic mobility at the wastewater solid surface and large floc size. Batch mode flotation of individual ink components under the optimum conditions for industrial wastewater treatment revealed that the presence of large quantities of either blue or yellow inks in the wastewater can lead to poor removal. Their combined presence, however, did not have detrimental effects. Continuous mode flotation at 5 mM Fe(II), 300 ppm CTAB and pH 6.5 was found to remove above 96% of contaminants at cell residence times of 6 minutes and 88% of contaminants at cell residence times of 1.5 minute. Greater than 87% of the initial wastewater volumes were recovered as treated effluent (i.e. < 13% disposed as waste foam sludge), yielding a waste foam sludge containing between 12 and 17% solids. The best flotation outcomes were achieved when all the wastewater volume entering the flotation cell leaves the cell with the foam.