Use of flocculants in processes water softening

Abstract

Deterioration of the quality of natural waters every year causes increasing concern of specialists and ordinary citizens. The shortage of water in certain regions and the disappointing forecasts for the supply of water for humanity in the future transfer the problems of the hydrosphere into the category of catastrophic. Mineralization of natural waters as a result of anthropogenic and natural factors is one of the most acute problems of water supply today. Water softening by removing calcium and magnesium ions allows you to partially solve this problem. Today, reactive methods of softening, in which phosphates are used as a precipitant, are able to form compounds with calcium and magnesium ions that are poorly soluble in water. Due to the high dispersion of solid phase particles formed as a result of such processing, there are problems of effective separation of liquid and solid phases. The use of flocculants makes it possible to intensify the filtering process, prevent clogging of the filter pores with solid particles, and reduce the duration of the technological process. During the research, we studied the effect on phase separation of different types of flocculants - polyacrylamide - as a nonionic flocculant, Magnofloc - 336 from Ciba - as an anionic flocculant, Zetag - 7692 from Ciba - as a cationic flocculant. The model solutions contained approximately equal amounts of calcium and magnesium ions with a total hardness of 27 mg-eq/dm3 and were treated with an equivalent amount of sodium phosphate mixed with a flocculant dose of 1 – 80 mg/dm3. Conducted studies have shown that the addition of polyacrylamide in concentrations of 1 - 10 mg/dm3 is not accompanied by a positive effect during advocacy. After settling for 1 hour, the apparent volume of the solid phase is 30-40 % higher than the similar indicator without flocculant treatment. A noticeable effect is observed only at flocculant doses of 30 - 80 mg/dm3. Moreover, in the latter case, illumination occurs quite quickly and within 10 min the imaginary volume of the solid phase reaches its minimum and changes very little during further settling. During filtration, the addition of a flocculant only slows down the process of separating the solid phase and significantly prolongs the production of a softened liquid phase. Addition of flocculant even in concentrations of 1 mg/dm3 negatively affects the parameters of the filtration process. Therefore, it is possible to unequivocally state that polyacrylamide cannot be used as an intensifier of the phase separation process for systems of low and medium productivity (in which it is intended to use filtering). Under the same conditions, the efficiency of Magnofloc - 336 anionic flocculant during sedimentation is somewhat higher compared to polyacrylamide. At a flocculant dose of 30-80 mg/dm3, the bulk of the solid phase settles within 5 minutes and forms a volume of about 30 cm3. Without a flocculant, softened water of such hardness clears very slowly. Therefore, it is obvious that during settling this flocculant can be used under the condition of the appropriate ratio in the initial solution of calcium and magnesium ions. Flocculant of this type turned out to be more effective during filtration. Traditionally, significant doses of flocculant (30-80 mg/dm3) inhibit the filtration process. At lower concentrations, the effect is more significant. Thus, at flocculant concentrations of 1 - 10 mg/dm3, an increase in filtration speed is observed and its value approaches the filtration parameters of distilled water. Such a result can be considered positive both from the point of view of increasing the efficiency of the process and from the point of view of the lower consumption of reagents. Zetag - 7692 cationic flocculant did not have a positive effect during settling. Compared to other flocculants, Zetag - 7692 does not provide a difference in settling rates for different concentrations. In fact, the effect of the flocculant on the formed solid phase is quite insignificant. The same can be said about the effect of this flocculant on the filtration rate. In the flocculant concentration range of 1 – 80 mg/dm3, all points of the graphs differ by 5 – 10 % from the filtration speed curve without flocculant and are far from the filtration curve of distilled water. Thus, flocculant of this type does not provide a positive effect in the process of separating liquid and solid phases, and its use in softening processes is not very promising. Thus, in softening processes, it is recommended to use Magnofloc - 336 anionic flocculant in concentrations of 1 - 10 mg/dm3 to intensify the phase separation process.