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Ultrasonics - Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Application of (super)cavitation for the recycling of process waters in paper producing industry Janez Kosel a, ⁎ , Matej Š u š tar š i č b , Martin Petkov š ek c , Mojca Zupanc c , Mija Se ž un b , Matev ž Dular c a Institute for the Protection of Cultural Heritage of Slovenia, Slovenia b Pulp and Paper Institute of Ljubljana, Slovenia c Faculty of Mechanical Engineering, University of Ljubljana, Slovenia
ARTICLE INFO
A B S T R A C T
In paper production industry, microbial contaminations of process waters are common and can cause damage to paper products and equipment as well as the occurrence of pathogens in the end products. Chlorine omission has led to the usage of costly reagents and products of lower mechanical quality. In this study, we have tested a rotation generator equipped with two sets of rotor and stator assemblies to generate developed cavitation (unsteady cloud shedding with pressure pulsations) or supercavitation (a steady cavity in chocked cavitation conditions) for the destruction of a persistent bacteria Bacillus subtilis. Our results showed that only super- cavitation was e ff ective and was further employed for the treatment of waters isolated from an enclosed water recycle system in a paper producing plant. The water quality was monitored and assessed according to the chemical (COD, redox potential and dissolved oxygen), physical (settleable solids, insolubles and colour in- tensity) and biological methods (yeasts, aerobic and anaerobic bacteria, bacterial spores and moulds). After one hour of treatment, a strong 4 logs reduction was achieved for the anaerobic sulphate reducing bacteria and for the yeasts; a 3 logs reduction for the aerobic bacteria; and a 1.3 logs reduction for the heat resistant bacterial spores. A 22% reduction in COD and an increase in the redox potential (37%) were observed. Sediments were reduced by 50% and the insoluble particles by 67%. For bacterial destruction in real industrial process waters, the rotation generator of supercavitation spent 4 times less electrical energy in comparison to the previously published cavitation treatments inside the Venturi constriction design.
Keywords: Rotational cavitation generator Hydrodynamic cavitation Paper mill industry Bacillus subtilis Anaerobic sulphate reducing bacteria COD Redox potential
1. Introduction
bleached without chlorine- and chlorine derivatives (TCF or Totally Chlorine Free) and to label these products as ecologically friendly (Ecolabel fl ower; www.ecolabel.eu). However, the pulp that ful fi ls these standards has lower mechanical properties, requires larger quantities of wood and uses costly reagents, such as ozone. Therefore, alternative methods are being developed that are safe, easy to perform, inexpensive, less labour intensive but e ff ective, and hydrodynamic ca- vitation is one of such options [7]. Cavitation is a physical phenomenon caused by the formation of vapour bubbles in an initially homogeneous liquid due to the decrease of local pressure at an approximately constant temperature [8]. It is composed of various physical (pressure pulses, shear forces, high local temperatures) and chemical side e ff ects (decomposition of H 2 Omainly to % OH and other radicals). Ultrasonic cavitation has been proven to be e ffi cient for the intensi fi cation of oxidation reactions (H 2 O 2 and % OH) [9,10] and for the destruction of bacteria [11 – 13]. It provides sig- ni fi cantly higher rates for the Weissler reaction (oxidation of iodide to
Water is essential for the paper producing process, because it per- mits the fi bres to be transported from the apparatus which de- fi bres the wood-pulp down to the manufacturing wire of the sheet of paper [1]. However, the continuously recycled process water (white water) con- tains organic substrates (starch), has favourable temperatures and a neutral pH, which is all in all an optimal environment for microbial growth [2]. Additionally, the development of bacteria gives rise to the accumulation of slime, which causes holes and spots or even breakage of the continuous paper sheet leading to expensive delays [3]. For disinfection purposes, chlorine is most commonly applied, however chlorination has several shortcomings among them the formation of carcinogenic organochlorines and the need for careful control of chlorine dosing [4,5]. Consequently, the Directorate-General for the Environment [6] issued guidelines for the elimination of all chemical molecules containing chlorine atoms in any form to produce cellulose
⁎ Corresponding author at: Poljanska 40, SI-1000 Ljubljana, Slovenia. E-mail address: janez.kosel@zvkds.si (J. Kosel).
https://doi.org/10.1016/j.ultsonch.2020.105002 Received 26 August 2019; Received in revised form 3 February 2020; Accepted 3 February 2020
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