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Pilot plant for ozone sonolysis

Ozone sonolysis is a combination of acoustic cavitation (sonolysis, ie. ultrasound) with ozonation. Use of ultrasound for wastewater treatment is a relatively new technology that is mainly applied for treatment of wastewater from textile industry.

Ultrasound is generated by ultrasonic transducers by converting electrical energy into mechanical oscillation at corresponding frequency. The effect of ultrasound is a result of physical and/or chemical changes in the substance being treated. In the case of living cells the main effect is a damage of cell membranes by cavitation. In the low intensity ultrasonic processes physical processes are the most important. Low intensities are achieved in e.g. ultrasonic cleaners used to remove physical particles from surface of various objects. Low intensities remove dissolved gases from the liquid which is undesirable process because the dissolved ozone is then wasted (i.e. not being part of chemical reactions). For this reason the reactor is designed with high intensity of ultrasound. At high ultrasonic intensity the pollution is degraded by oxidation by hydroxyl radicals, pyrolysis inside cavitation bubbles, by hydrodynamic erosion and by supercritical wet oxidation on the surface of the cavitation bubbles. In addition the high intensity sonolysis decomposes ozone forming hydroxyl radicals. All this leads to faster decomposition of many substances compared to solely ozonation.

Ozone sonolysis allows reduction of ozone doses by one third while maintaining the removal efficiency. This technology is capable of reduction of chemical oxygen demand, colour, total nitrogen, total phosphorus, microbial pathogens and many micropollutants below limits of quantification.

The main components of ozone sonolysis pilot plant are:

  • biologically treated wastewater pump;
  • hydrogen peroxide dosing pump;
  • ozone generator;
  • gaseous oxygen supply;
  • gas dispersion system;
  • ozone contact tank (volume 200 L);
  • ultrasonic reactor with generators;
  • decay tank (volume 2 100 L);
  • system for destruction of waste ozone;
  • measurement and regulation.

A submersible pump is used to pump biologically treated wastewater from the storage tank to the pilot plant. The discharge of the pump is passed through the inductive flowmeter and into the contact tank. The pump is controlled by a frequency converter used for changing the pump speed and hence the flow of the water. Dosing of hydrogen peroxide is ensured by the peristaltic pump in order to ensure uniform flow of hydrogen peroxide. The pump discharge is connected down flow to the inductive flowmeter.

Ozone is generated from pure oxygen in the ozone generator through the dielectric discharge. Pure oxygen is supplied from the liquid oxygen tank equipped with reducing valve. The ozone generator is rated at 100 g O3•h 1. The generator is equipped with water cooling and ozone detector used to shut down the generator in case of leakage of the ozone. Essential component required for ozonation is a ozone destructor used for destruction of unused ozone. The destructor used is catalytic with heating and with suction blower.

Gas dispersion system is used for the purpose of dissolution of gas in the treated water. Dispersion of gas is accomplished by injector and injector pump ensuring constant flow through the injector. Whole system is design to maximize dissolution of ozone in contact tank, i.e. the ozone contact tank is pressurized to improve dissolution of ozone.

Ultrasonic unit consists of ultrasonic generator and ultrasonic transducers mounted on ultrasonic reactors. Ultrasonic generator produces sine wave electric signal at frequency of 25 kHz and is connected to transducers outside the reactor. Output power of the ultrasonic generator can be controlled locally or remotely (by value of redox potential). The ultrasonic reactor is located down flow to the contact tank. Inside the reactor the remaining ozone is decomposed and the concentrations of pollutants are further reduced. The concentrations of ozone down flow the ultrasonic reactor will be low enough to ensure safe discharge of water to surface water.

Electric consumption of individual components is measured to allow assessment of electricity consumption and economic evaluation of the pilot operation. The redox potential is measured and logged on the inflow and outflow of the decay tank. The actual flow and total volume of treated water is measured by inductive flow meter. Two automatic samplers are installed to sample the treated and untreated water for further analytic examinations. Ozone gas analyser is the used for determining quantity of ozone produced and quantity of ozone wasted to the destructor (i.e. determination of gas dispersion system efficiency).