Power ultrasound decontamination of wastewater from fresh-cut lettuce washing for potential water recycling

Highlights

• Power ultrasound decontamination efficacy increased with increasing specific energy.

• Ultrasound associated with in situ generated heat disinfected wash water efficiently.

• Synergy between power ultrasound and heat was found.

• Ultrasound-heat combination can be used for recycling water from fresh-cut industry.

Abstract

The decontamination effect of pulsed and continuous power ultrasound, provided at either controlled or uncontrolled temperature regimes, was studied with reference to native microflora and inoculated pathogenic bacteria in wastewater obtained by fresh-cut lamb's lettuce washing. Results showed that decontamination efficacy increased with increasing specific energy and was higher when ultrasound treatment was provided under uncontrolled temperature regime. Continuous ultrasound supplied without temperature control allowed to achieve 3.2 Log reductions of native microflora during 20 min treatment, while 5 Log reductions of inoculated Listeria monocytogenes, Escherichia coli and Salmonella enterica were attained within 5 min of ultrasonication. The heat generated during continuous ultrasound accounted for approximately 58% of the total decontamination effect against L. monocytogenes, while it contributed for 100% to E. coli and S. enterica inactivation.

Industrial relevance

The application of power ultrasound combined with in situ generated heat could represent an effective tool for water decontamination and recycling in the fresh-cut industry. In addition, besides safety requirements, this technology would also meet cost-effectiveness criteria and existing standards.

Introduction

Nowadays, water scarcity is a major issue at global level. It has been estimated that in the next 15–20 years, the water supply-to-demand gap will be approximately 40%. Tackling the water gap is a challenge for (Horizon 2020) EU research programme. The food sector greatly contributes to water scarcity. It has been estimated that about 20–50% reduction in water consumption in the food sector can be achieved by recycling and reuse of water (Hiddink, Schenkel, Buitelaar, & Rekswinkel, 1999).

The fresh-cut vegetables market has grown considerably in the last few decades in response to an increased demand for fresh-like, healthy and convenient foods. Fresh-cut vegetables production requires intensive use of water to both wash and move vegetables along the production line. In order to secure water supply and protect the environment from the adverse effects of the wastewater discharges (EEC, 1991), water recycling in the fresh-cut industry has to be improved. Recycling of water that is intended to re-enter the washing step, implies wastewater disinfection. As well known, a 5 Log reduction of pathogenic bacteria is the generally accepted requirement for safe water disinfection. Wastewater decontamination may be accomplished by means of chemical and physical interventions (Casani et al., 2005, Olmez and Kretzschmar, 2009). Among these, sodium hypochlorite is the most used due to its low cost and easy use (Gil et al., 2009, Olmez and Kretzschmar, 2009). However, not only wastewater containing chlorine has a great environmental impact, but also chlorination disinfection by-products are known to represent a potential risk for human health (Itoh, Gordon, Callan, & Bartram, 2011). Consequently, there is great effort to find suitable technologies to allow wastewater recycling (Artés et al., 2009, Casani et al., 2005, Olmez and Kretzschmar, 2009). Power ultrasound has been suggested as a technology alternative to chlorination for wastewater decontamination (Neis and Blume, 2002, Piyasena et al., 2003). Ultrasound frequencies higher than 20 kHz are actually considered safe, non-toxic and environmentally friendly (Kentish & Ashokkumar, 2011). During ultrasound treatment, cavitation phenomena occur into the liquid medium causing a rapidly alternating compression and decompression zones, which are in turn responsible for generating shock waves with associated local very high temperatures and pressures, as well as free radicals and hydrogen peroxide (Leighton, 1994, Mason et al., 2003). Ultrasound effectiveness in wastewater decontamination was found to increase with the power input and exposure time, and to depend on microorganism type (Elizaquivel et al., 2012, Gao et al., 2014, Hulsmans et al., 2010, Joyce et al., 2003, Scherba et al., 1991). Improved efficiency of ultrasound technology can be obtained by its combination with other biocidal treatments, such as chlorination (Ayyildiz et al., 2011, Drakopoulou et al., 2009), organic acids (Gómez-López, Gil, Allende, Vanhee, & Selma, 2015) and ultraviolet irradiation (Blume and Neis, 2004, Gómez-López et al., 2015, Mason et al., 2003, Naddeo et al., 2009). An increase of microbial sensitivity to ultrasound in combination with temperature increase, experienced with ultrasonic treatment, for wastewater disinfection has been also reported (Gómez-López et al., 2014, Madge and Jensen, 2002, Salleh-Mack and Roberts, 2007). It has been estimated that the heat generated during ultrasound processing accounted for approximately 52% of the resulting disinfection (Madge & Jensen, 2002).

In contrast with the huge number of studies in the literature dealing with ultrasound decontamination of municipal wastewater and effluents as well as model fluids, very few studies investigated ultrasound effectiveness for water decontamination deriving from fresh-cut vegetable production (Elizaquivel et al., 2012, Gómez-López et al., 2014, Gómez-López et al., 2015). It has been demonstrated that power ultrasound was effective in inactivating pathogenic bacteria inoculated in fresh-cut lettuce wash water (Elizaquivel et al., 2012), especially in the presence of the residual peroxyacetic acid concentration that can be found in the wash water (Gómez-López et al., 2015). In these studies, ultrasonic treatments were carried out with temperature control, allowing the inactivation effects of ultrasound only to be evaluated. In another study, Gómez-López et al. (2014) showed that ultrasound disinfection against Escherichia coli O157:H7 inoculated in fresh-cut lettuce wash water can be increased by combination with heating. Reductions of 6 Log of this microorganism were actually achieved after 60 and 20 min of ultrasonication with and without temperature control, respectively.

In light of this, there is a lack of knowledge on the efficacy of power ultrasound in combination with in situ generated heat against naturally occurring microflora and foodborne pathogens, other than E. coli, potentially contaminating fresh-cut vegetable wash water.

In this study, the efficacy of power ultrasound in decontaminating wastewater deriving from fresh-cut vegetable washing was investigated. To this aim, wastewater obtained by washing fresh-cut lamb's lettuce was subjected to power ultrasound, provided in pulsed or continuous modality, with or without temperature control. The decontamination efficacy of the treatments was evaluated on both the native microflora and inoculated pathogenic bacteria, i.e., Listeria monocytogenes, E. coli and Salmonella enterica. These microorganisms were chosen due to their natural occurrence in a water environment and because they are generally considered indicators of fecal contamination (Szewzyk, Szewzyk, Manz, & Schleifer, 2000). The final goal was to find the potentiality of combined ultrasound with in situ generated heat in the attempt to implement strategies for efficient management of water resource in the fresh-cut industry. To this regard, the decontamination efficacy was related to the ultrasound cavitation and heat contributions.