Martina Oder

Use of hydrodynamic cavitation in (waste)water treatment.

The use of acoustic cavitation for water and wastewater treatment (cleaning) is a well known procedure. Yet, the use of hydrodynamic cavitation as a sole technique or in combination with other techniques such as ultrasound has only recently been suggested and employed. In the first part of this paper a general overview of techniques that employ hydrodynamic cavitation for cleaning of water and wastewater is presented. In the second part of the paper the focus is on our own most recent work using hydrodynamic cavitation for removal of pharmaceuticals (clofibric acid, ibuprofen, ketoprofen, naproxen, diclofenac, carbamazepine), toxic cyanobacteria (Microcystis aeruginosa), green microalgae (Chlorella vulgaris), bacteria (Legionella pneumophila) and viruses (Rotavirus) from water and wastewater. As will be shown, hydrodynamic cavitation, like acoustic, can manifest itself in many different forms each having its own distinctive properties and mechanisms. This was until now neglected, which eventually led to poor performance of the technique. We will show that a different type of hydrodynamic cavitation (different removal mechanism) is required for successful removal of different pollutants. The path to use hydrodynamic cavitation as a routine water cleaning method is still long, but recent results have already shown great potential for optimisation, which could lead to a low energy tool for water and wastewater cleaning.

Can rapid pressure decrease induced by supercavitation efficiently eradicate Legionella pneumophila bacteria

The presence of Legionella pneumophila bacteria in engineered water systems can pose a significant health risk. Current prevention and outbreak treatments are cost and environmentally ineffective. Moreover they do not prevent rapid bacteria recolonization. Although cavitation was already suggested as a possible water treatment technique a systematic study has not yet been performed. In the present experimental campaign we set out to evaluate the efficiency of removal of L. pneumophila by three types of cavitation—the most commonly used acoustic cavitation, the aggressive developed hydrodynamic cavitation, and the supercavitation. We show that it is probably not the pressure peaks or the high local temperatures that cause the eradication of the bacteria, but the rapid pressure decrease which was initiated in supercavitating flow regime. Results of the study show promising ground for further optimization of a methodology for L. pneumophila removal by cavitation.

Microbial adhesion capacity. Influence of shear and temperature stress

Environmental parameters dictate the conditions for both biofilm formation and deconstruction. The aim of this study is to analyse the impact of hydrodynamic and thermodynamic effects on bacterial detachment. Escherichia coli grown on two stainless steel metal surfaces with different roughness (brushed with roughness of 0.05 μm and electropolished with roughness of 0.29 μm) are exposed to laminar and turbulent (shower) flows of phosphate buffered saline media at temperatures of 8, 20 and 37 °C. Results show that the turbulent flow removes significantly more bacterial cells than laminar flow (p <0.05) on both materials. This indicates that the shear force determines the rate of detached bacteria. It is also observed that detachment of cells is more efficient on brushed than on electropolished contact surfaces because on the latter surface, fewer cells were attached before exposure. Moreover, we demonstrate that the temperature of the washing agent has an impact on bacterial detachment. At the same flow conditions, the exposure to higher temperature results in greater detachment rate.

The impact of material surface roughness and temperature on the adhesion of Legionella pneumophila to contact surfaces.

The adhesion of bacterial cells to various surfaces is based on physical and chemical interactions between the micro-organisms and the surfaces. The main purpose of this research is to determine the effect of material roughness and incubation temperature on the adhesion of bacteria. To determine the adhesion of the bacterial strain of Legionella pneumophila ATCC 33153 to the glass coupons, a spectrophotometric method of measuring the optical density of crystal violet dye that is released from pre-stained bacterial cells attached to the test surface was used. The intensity of adhesion is in positive correlation to the increase in surface roughness (p < 0.05). The adhesion is the greatest at an optimal temperature of 36 °C, whereas the temperature of 15 °C has a bacteriostatic effect and the temperature of 55 °C a bactericidal effect.

Escherichia coli biofilm formation and dispersion under hydrodynamic conditions on metal surfaces

Abstract The aim of this study was to analyze the impact of hydrodynamic forces on the multiplication of E. coli, and biofilm formation and dispersion. The experiments were provided in a flow chamber simulating a cleaning-in-place system. Biofilm biomass was measured using a crystal violet dye method. The results show that hydrodynamic forces affect not only biofilm formation and dispersion but the multiplication of E. coli in the first place. We found more biofilm biomass on the rough surface than on the smooth one. The results of the biofilm formation test show that laminar flow promotes the biofilm growth over 72 h, meanwhile turbulent flow after 48 h causes decrease in biomass. The results of the biofilm dispersion test, in contrast, show that laminar flow removed less biofilm from both materials that turbulent flow did. Therefore, taking into account these findings in cleaning-in-place technology can substantially reduce E. coli multiplication and biofilm formation.

Bacterial adhesion capacity on food service contact surfaces

Abstract The aim of this study was to analyse the adhesion of E. coli, P. aeruginosa and S. aureus on food contact materials, such as polyethylene terephthalate, silicone, aluminium, Teflon and glass. Surface roughness, streaming potential and contact angle were measured. Bacterial properties by contact angle and specific charge density were characterised. The bacterial adhesion analysis using staining method and scanning electron microscopy showed the lowest adhesion on smooth aluminium and hydrophobic Teflon for most of the bacteria. However, our study indicates that hydrophobic bacteria with high specific charge density attach to those surfaces more intensively. In food services, safety could be increased by selecting material with low adhesion to prevent cross contamination.

The influence of shear stress on the adhesion capacity of Legionella pneumophila

Abstract Bacterial adhesion is a complex process influenced by many factors, including hydrodynamic conditions. They affect the transfer of oxygen, nutrients, and bacterial cells in a water supply and cooling systems. The aim of this study was to identify hydrodynamic effects on bacterial adhesion to and detachment from stainless steel surfaces. For this purpose we observed the behaviour of bacterium L. pneumophila in no-flow and laminar and turbulent flow conditions simulated in a fluid flow chamber. The bacterial growth in no-flow and laminar flow conditions was almost identical in the first 24 h, while at 48 and 72 h of incubation, the laminar flow stimulated bacterial growth. In the second part of this study we found that laminar flow accelerated bacterial adhesion in the first 48 h, but after 72 h the amount of bacterial cells exposed to the flow dropped, probably due to detachment. In the third part we found that the turbulent flow detached more bacterial cells than the laminar, which indicates that the strength of shear forces determines the rate of bacterial removal.

Efficacy of sodium dodecyl sulphate and natural extracts against E. coli biofilm

Abstract The aim of this study was to determine and compare the efficacy of a standard cleaning agent, sodium dodecyl sulphate, and natural extracts from pomegranate peel grape skin and bay laurel leaf against E. coli biofilm. The biofilm was exposed for 10 minutes to three different concentrations of each tested compound. The results show that bay laurel leaf extract is the most efficient with 43% biofilm biomass reduction, followed by pomegranate peel extract (35%); sodium dodecyl sulphate and grape skin extract each have 30% efficacy. Our study demonstrated that natural extracts from selected plants have the same or even better efficacy against E. coli biofilm removal from surfaces than the tested classical cleaning agent do. All this indicates that natural plant extracts, which are acceptable from the health and environment points of view, can be potential substitutes for classical cleaning agents.