Design and application of a hydrodynamic cavitation system in textile wastewater treatment

Abstract

The industrial sector has been growing in South Africa and in Africa in general. This has caused an increase in generation of wastewater which needs to be treated to avoid polluting the environment and those living in it. Textile industry is one of the major polluting industries and textile dyes such as azo dyes are among dyes that are hard to degrade due to their low biodegradability. The azo dyes are among persistent organic pollutants (POPs) which are hard to treat using conventional treatment methods namely biological methods, coagulation/flocculation, chlorination, adsorption, reverse osmosis etc. This issue has created a need for the development of advanced treatment methods. Advanced oxidation processes (AOPs) are among advanced treatment methods that are effective in the removal of POPs from wastewater. In recent years, one form of AOPs has emerged as a simple method, that is energy-efficient and that has proved to be successful in the degradation of textile dyes. The technology is cavitation, which is a technique that involves the generation, growth and collapse of cavities/bubbles caused by rapid pressure changes. The collapse of the cavities/bubbles produces high amount of energy releasing highly reactive free radicals (hydroxyl radicals) that are generated through the dissociation of water molecules. Cavitation can be generated in different ways and the names of the different types of cavitation are based on the way the cavitation is generated. The common types of cavitation are hydrodynamic cavitation and acoustic cavitation. In this study, a hydrodynamic cavitation (HC) jet loop system with different cavitating devices was designed and his performance in the treatment of simulated textile wastewater was investigated. A 10 L of 20 ppm of Orange II sodium (OR2) dye solution was used as simulated textile wastewater and its decolouration was monitored. The cavitating devices used were a 10 mm throat diameter venturi and 5 orifice plates with different hole diameters (2 mm, 3 mm, 4 mm, 5 mm, and 6 mm). The different cavitating devices were each used in the HC jet loop system to determine their efficiency in decolourising OR2 dye. The combination of the venturi and the best performing orifice plate in the HC jet loop system was also investigated. In the study different inlet pressures (200 kPa, 300 kPa, and 400 kPa) were also investigated to determine how they affect the performance of the HC jet loop system. Lastly, the energy consumption and the cost of using the HC jet loop system as an extension on to an existing plant was also determined. Results have shown that the HC jet loop system performs best at an inlet pressure of 400 kPa. For the cavitating devices, the orifice plate having a 2 mm orifice plate was found to be the best cavitating device among all the cavitating devices tested. It has allowed for a 91.11% decolouration in 10 min when using 400 kPa inlet pressure. The combination of the 10 mm throat diameter venturi and the 2 mm hole diameter orifice plate provided an intermediate performance with a 74.08% decolouration while the 10 mm throat diameter venturi provided the poorest performance with a 58.73% decolouration in 10 min at an inlet pressure of 400 kPa. With regards to the consumption of energy, the HC jet loop system was found to use 0.548 kW/m3 for dye decolouration which is a relatively low energy consumption compared to the energy consumption incurred by the UV for instance for dye decolouration. Regarding costs, the investment costs was estimated to be R 104 843.00 for an HC jet loop system with a capacity to decolourise 500 L of dye contaminated wastewater. The operating costs were estimated at R 127.24 for the decolouration of 500 L of dye contaminated water when considering that the system will be an extension to an existing plant. Overall the designed HC jet loop system was found to be an effective technique to use for the decolouration of textile dyes found in textile wastewater such as Orange II sodium salt.