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In recent years, there has been an increase in the application of ultraviolet (UV) light as an alternative to chemical disinfection technologies. However, in the case of poor quality effluents, the practical limit of UV disinfection of wastewater is dictated by disinfection-resistant, particle-associated bacteria. Although these particles may be removed by filtration, an alternative method to reduce the impact of suspended particles on disinfection efficiency is to decrease particle size using ultrasound technology. Mechanical forces exerted on particles due to the collapse of cavitation bubbles created by sonication break suspended particles into small fragments. In this paper, a critical review of ultrasound application for wastewater treatment is presented with emphasis on disinfection. Much of the work in this area remains at the laboratory scale. As a result, there is a need for fundamental information regarding the effect of sonication on the kinetics of disinfection and interaction of ultrasound with suspended particles. Such information is necessary for process engineering, design, and scale-up of ultrasound systems.

Municipal wastewater contains bacteria, viruses, and other pathogens that adversely affect the environment, human health, and economic activity. One way to mitigate these effects is a final disinfection step using ultraviolet light (UVL). The advantages of UVL disinfection, when compared to the more traditional chlorine, include no chlorinated by-products, no chemical residual, and relatively compact size. The design of most UV reactors is complex. It involves lamp selection, power supply design, optics, and hydraulics. In general, medium pressure lamps are more compact, powerful, and emit over a wider range of light than the more traditional low pressure lamps. Low pressure lamps, however, may be electrically more efficient. In UV disinfection, the fraction of surviving organisms (e.g. E. coli) will decrease exponentially with increasing UV dose. However, the level of disinfection that can be achieved is often limited by particle-associated organisms. Efforts to remove or reduce the effects of wastewater particles will often improve UV disinfection effectiveness. Regrowth, photoreactivation, or dark repair after UV exposure are sometimes cited as disadvantages of UV disinfection. Research is continuing in this area, however there is little evidence that human pathogens can photoreactivate in environmental conditions, at doses used in wastewater treatment. The UV disinfection of combined sewer overflows, a form of wet weather pollution, is challenging and remains largely at the research phase. Pre-treatment of combined sewer overflows (CSOs) with a cationic polymer to induce fast settling, and a low dose of alum to increase UV transmittance, has shown promise at the bench scale.

An alternative design of the membrane aerated biofilm reactor (MABR) was developed to overcome some of the current technical and economical limitations preventing full scale applications of the process. The ZeeLung TM system uses a new dense hollow-fibre membrane with an unprecedented thin diameter. Two pilot units treating a synthetic high-strength industrial wastewater (4700 mgCOD/L, 145 mgTKN/L) operated successfully for 16 months. They performed simultaneous COD removal, nitrification and denitrification. The very high specific surface area (810 m 2 /m 3 ) allowed the surface loading rate to be kept low enough (3.6 gCOD/(m 2 .d)) to maintain a relatively thin biofilm (200 to 350 mm) and use low-pressure air (41 kPa) instead of high-pressure pure oxygen. Intermittent air and liquid mixing at high frequency and low shear were compared: they were equally effective in enhancing substrate transfer, but failed to stabilize biofilm accumulation. Air sparging additionally prevented the acidification of the bulk by stripping CO 2 . Water Environ. Res., 83, 3 (2011).

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