High-pressure jet-induced hydrodynamic cavitation as a pre-treatment step for avoiding cyanobacterial contamination during water purification

Highlights

• High pressure jet submerged into water causes hydrodynamic cavitation.

• Cyanobacteria are sensitive to HC conditions showing enhanced sedimentation.

• HC itself or combined with H₂O₂ pre-treatment limits photosynthesis temporarily.

• Already one run of HC is able to evoke effects without damage to cells.

• HPJS as pre-treatment step could improve water purification process.

Abstract

Due to specific physical properties, hydrodynamic cavitation (HC) is assigned to the powerful technologies for treating the biotic contamination in water including cyanobacteria. Contaminated water stream (CWS) can be cavitated directly by passing through some HC device, or indirectly when high-pressure jet stream (HPJS) is directed against its flow. Relatively small HPJS stream can thus treat a big volume of CWS in a short time or even work in continuous mode. Cyanobacteria floating in the CWS are forced to flow through the mixing cavitation zone. Within 2 h after single HC treatment, cyanobacterial cell suspensions showed disintegration of larger colonies and enhanced biomass sedimentation. Additional pre-treatment of CWS with low amounts of hydrogen peroxide (H₂O₂; 33, 66 and 99 μmol/L) enhanced the effect of HC and led to further inhibition of cyanobacterial photosynthesis (maximum quantum yield of photosystem II decreased by up to 60%). The number of cyanobacterial cells in the treated CWS decreased continuously over 48 and 72 h, though some cells remained alive and were able to recover photosynthetic activity. The technique proposed (direction of a HPJS against a CWS and pre-treatment with low H₂O₂ concentrations) provides (i) effective removal of cells from the water column, and (ii) reduced contamination by organic compounds released from the cells (especially cyanotoxins) as the cell membranes are not destroyed and the cells remain alive. This process shows potential as an effective pre-treatment step in water purification processes related to cyanobacterial contamination.

Introduction

Surface freshwaters are subject to pollution from with a wide range of contaminants, including inorganic compounds, manufactured organic chemicals (Malaj et al., 2014) and biotic pollutants from microorganisms. Cyanobacteria, for example, are an integral part of natural phytoplankton communities; however, under eutrophic conditions, they over-propagate and cause huge cyanobacterial blooms, with both environmental and health risks due to the release of toxic cyanobacterial metabolites, cyanotoxins (Li et al., 2014). Such events are becoming common in water bodies with heavily polluted raw water (Huisman et al., 2018), and this has greatly increased the demand for new technologies to purify it, especially in case of drinking water. Common treatments today include aeration, flocculation, sedimentation, filtration and disinfection with chlorine (Cl) or ozone (O₃) (Bai et al., 2019, Odlare, 2014). Physical or chemical pre-treatment steps are often incorporated to enhance the purification process, e.g. ferrous-activated peroxymonosulphate has been proposed as an efficient pre-treatment for enhancing purification capacity and alleviating membrane fouling in nano-filtration processes (Kim et al., 2007, Lin et al., 2019). Much research is now being directed toward ensuring that these new technologies are safe, easy-to-use, cost-effective and sustainable for the future (Ciriminna et al., 2017).

One such method utilises the erosive potential and oxidative ability of hydrodynamic cavitation (HC). In any flowing liquid, HC causes high incidence of vapour cavities, whose violent collapse induces flash changes in local pressure and temperature (Dular et al., 2016). These physical characteristics, along with the natural occurrence of reactive oxygen species (ROS; for instance OH·), produce an environment hostile to living microorganisms such as bacteria or yeasts, and may even affect some organic pollutants (Arrojo and Benito, 2008, Arrojo et al., 2008, Gogate, 2002). The oxidative force of HC can be even further increased through the addition of O₃ or H₂O₂ (Bai et al., 2019, Jyoti and Pandit, 2003). As such, HC has the potential to act as a non-thermal (no external heating source required) and non-chemical (no addition of chemical sterilising agents) method for sterilising liquids, and perhaps even as a tool for water pre-treatment.

Owing to its oxidative properties, H₂O₂ has many industrial applications (e.g. bleaching, aseptic packaging, advanced oxidation processes) and is known as an “ecological” chemical due to its rapid and complete decomposition to water and oxygen (Ciriminna et al., 2016). In nature, H₂O₂ and other ROS are common metabolic by-products of all cells. In cyanobacteria, H₂O₂ may even be the main electron donor during photosynthesis instead of water (Samuilov et al., 2001). ROS concentrations exceeding the capability of the cell's anti-oxidative defence mechanism cause oxidative stress, with negative impacts on cell membrane structure and other compartments. Application of H₂O₂ at a concentration of ca. 1 mg/L has previously been shown to act as a potentially selective anti-cyanobacterial agent, displaying direct toxicity to cyanobacterial cells (Drabkova et al., 2007a) and an ability to decrease transcription of the genes responsible for cyanotoxin transport (Mikula et al., 2012).

The aim of the present work was to assess the potential of using a high-pressure jet stream (HPJS) directed against a stream of water contaminated with cyanobacteria to generate HC effects as pre-treatment step in drinking water production. We hypothesise that a single treatment should be sufficient to cause i) at least temporal metabolic inactivation of cyanobacteria, and ii) damage to the cell gas vesicles, resulting in biomass sedimentation, thereby removing cyanobacteria.