2010

Grönroos, Antti

There are a great variety of potential applications of high-intensity ultrasonic energy. Of these, cleaning, plastic pounding, and at present also sludge disintegration and the remediation of contaminated soil are probably the best known and offer the most general market for high-intensity ultrasonics. All developments within the area of ultrasound applications lead to the creation of environmentally friendly processes and compounds, emphasizing the role of ultrasound in "green chemistry". Ultrasound technology is considered not easy to use in industrial processes, since devices providing high sonic energy are not easy to construct. This thesis investigates on a semi-pilot scale if it is possible to enhance the disintegration of three quite different samples: polymers, sludge, and contaminated soil by using ultrasound. The results indicate that it is possible to enhance the disintegration of polymers by means of ultrasonic power only when the cavitation threshold is exceeded. Above the cavitation threshold, the most extensive degradation took place at the lowest ultrasonic frequency used. The biggest decrease (from 115,000 g/mol to 30,000 g/mol) in relative molecular mass (RMM) was observed when the concentration of polyvinyl alcohol (PVA) was the lowest (1.0%). However, in the case of carboxymethylcellulose (CMC) it was observed that there is an optimum polymer concentration (1.5-2.0%) where degradation is most efficient. The thesis shows that the extent of ultrasonic depolymerization decreases with decreasing molecular mass of the cmC polymer. The study also reveals that ultrasonic irradiation causes narrowing of the molecular mass distribution. The degradation of cmC polymer proceeded linearly and the rate of ultrasonic depolymerization decreased with decreasing molecular mass. In cases where the initial dynamic viscosities of polymer solutions were not the same, the sonolytic degradation of cmC polymer mainly depended on the initial dynamic viscosity. The higher the initial dynamic viscosity, the faster the degradation. This work confirms the general assumption that the shear forces generated by the rapid motion of the solvent following cavitational collapse are responsible for the breakage of the chemical bonds within the polymer. The effect of polymer concentration could be interpreted in terms of the increase in viscosity with concentration, causing the molecules to become less mobile in solution with smaller velocity gradients around collapsing bubbles. Ultrasonic disintegration of sludge increased the amount of soluble chemical oxygen demand (SCOD) and the production of methane. Multivariate data analysis suggested that ultrasonic power, sludge dry solids (DS), sludge temperature, and ultrasonic treatment time significantly affect sludge disintegration. It was also found that high ultrasound power together with a short treatment time is more efficient than low ultrasound power with a long treatment time. When using high ultrasound power, the ultrasound propagation is an important factor both in cavitation erosion prevention and reactor scale-up. Ultrasound efficiency rose linearly with input power in sludge at small distances from the transducer. On the other hand, ultrasound efficiency started even to decrease with input power at long distances from the transducer. When using oxidizing agents together with ultrasonic disintegration there was no increase in SCOD and only a slight increase in total organic carbon (TOC) compared to ultrasonic treatment alone. However, when using oxidizing agents together with ultrasound, no enhancement in methane production was observed. Ultrasound improved the remediation results of both products (sink and float products) in heavy medium separation. This phenomenom was based on the fact that the amount of ultrafine metal fraction was diminished