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2212-8271 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of the 24th CIRP Conference on Life Cycle Engineering doi: 10.1016/j.procir.2016.11.175

Procedia CIRP 61 ( 2017 ) 558 – 563

ScienceDirect

The 24th CIRP Conference on Life Cycle Engineering

Functional and environmental evaluation of alternative disinfection

methods for cutting fluids

Nadine

Madanchi

a

*, Sebastian Thiede

a

, Christoph Herrmann

a

aInstitute of Machine Tools and Production Technology (IWF), Sustainable Manufacturing & Life Cycle Engineering Research Group, Technische Universität

Braunschweig, Langer Kamp 19b, 38106 Braunschweig, Germany

* Corresponding author. Tel.: +49-531-391-7639; fax: +49-531-391-5842. E-mail address: n.madanchi@tu-bs.de

Abstract

Cutting fluids are widely used to ensure high production quality and process stability in machining processes. However, especially in case of water based cutting fluids microbial contamination leads to a reduced performance and thus, shortened service life. To control the growth of microorganisms biocides are used in practice. Biocides, however, can be toxic to humans and the use is officially regulated, whereby alternative disinfection methods e.g. ultraviolet radiation or ozone are getting more in the focus. This paper describes and performs a functional and an environmental evaluation of alternative disinfection methods.

© 2017 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the scientific committee of the 24th CIRP Conference on Life Cycle Engineering. Keywords: Cutting Fluid; Microbial Contamination; Disinfection Methods; UV Radiation; Ozone

1. Introduction

The application of cutting fluids in machining processes is of great importance to maintain a high productivity. The use of cutting fluid has a crucial influence on the workpiece quality, tool wear and power demand of the cutting process and the machining system [1]. Thus, keeping up the cutting fluid quality and ensuring a long service life are critical issues from technological but also economic and environmental perspective. Particularly water based cutting fluids are susceptible to microorganisms and this leads to a reduction in service life [2]. In order to control and to reduce the microbial contamination of cutting fluids biocides are widely used as preservatives in practice. However, due to their toxicological properties biocides can be toxic to humans and can create health-related problems. As a consequence, the use of biocides is limited by regulations and might be further restricted in the future. Besides the use of biocides also other physical and chemical methods exist that can possibly be applied to reduce microbial contamination [3], [4]. These methods differ greatly in terms of applicability and related

environmental impacts. Therefore, this paper investigates the functionality of alternative disinfection methods on microorganisms in cutting fluids. Based on the results of an experimental study and additional studies described in the literature the environmental impact of different disinfection methods is analyzed and discusses.

2. Research Background

2.1. Cutting fluids and the development of microorganisms

Cutting fluids are used in different machining processes to cool, to lubricate and to transport metal ships. By the fulfillment of these tasks it is possible to increase the efficiency of the process [1]. However, due to different requirements regarding workpiece materials, tools and type of processes a wide range of different cutting fluids exists. Generally, cutting fluids consist of a base fluid and an additive package. This package includes on the one hand additives such as extreme-pressure-additives, anti-wear-additives, anti-foam-additives or anti-corrosion-additives [5].

© 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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The base fluid on the other hand can be distinguished, according to DIN 51385 between water miscible and non-water miscible fluids [6]. The non-non-water based fluids consist of 80-95 % mineral oil as a base fluid, while water based fluids contain a significant lower share of base oil. They can further be distinguished between emulsions and dilutions and normally consist of 85-98 % water. The other share comprises the base oil and the additive package, which includes emulsifiers or solublizers [5].

As non-water based cutting fluids are usually not affected by microorganisms they are not within the scope of this study. In contrast water based fluids provide conditions that support the growth of microorganisms and they are mainly contaminated by bacteria and fungi. Bacteria are simple organisms and can be distinguished by different aspects. In this case it is reasonable to differentiate anaerobic and aerobic bacteria. While aerobic bacteria need oxygen for reproduction, anaerobic bacteria can exist without oxygen [7]. Fungi, however, are more complex organisms and can be classified in a group related to animals and plants [7]. Different research studies analyzed the types of bacteria and fungi that could be detected in cutting fluids. Most of the bacteria belong to the genus Pseudomonas, but also other bacteria were detected such as Comamonas, Staphylococcus and Achromobacter [2], [8], [9] and [10]. In case of fungi species such as Aspergillus and Fusarium could be detected [2], [9]. However, Rabenstein et al. described that the types of detected microorganisms vary over time as there are pioneer and secondary species [2].

2.2. Impact of contaminated cutting fluid

Microorganisms have an impact on the condition and performance of cutting fluids [11]. The components of the cutting fluid (additives and base fluid) serve as nutrients and enable a growth of microorganisms. However, these components are metabolized gradually and not simultaneously. Microorganisms prefer readily biodegradable substances, while substances with complex molecules have a low bioavailability and are later metabolized. Thus, the microbial degradation of cutting fluids depends on the composition of the fluid as well as the existence of different species of microorganisms [12].

This demonstrates the interdependencies between cutting fluid and a microbial contamination. Koch and Rabenstein have demonstrated in their research that emulsifiers and solubilizers influence the microbial degradation rate of the base oil. Adding emulsifiers leads to a higher bioavailability of the base oil and thus an accelerated microbial degradation [13]. Further, Koch and Rabenstein demonstrated the microbial degradation of corrosion inhibitors as well. Most corrosion inhibitors contain nitrogen which is an important nutrient for microorganisms. Thus, their bioavailability and microbial degradation is even higher compared to emulsifiers [13]. A reduction of corrosion inhibitors leads to corrosion issues on machine parts or workpieces. Additionally, microbial degradation processes lead to an enrichment of acidic metabolites into the cutting fluid resulting in a decreasing pH value that enhances the corrosive effect [14].

By the microbial degradation of important components such as base oil, emulsifier and corrosion inhibitors there is a decreasing stability of the water based emulsion and a decreasing technical performance of the cutting fluid. This leads to quality issues such as higher surface roughness and tool wear and as a result a shortened service life of the cutting fluid. Brinksmeier et al. has demonstrated the influence of contaminated cutting fluids on the technical performance within a research project. For a drilling process it could be verified that microorganisms have an impact on the process quality expressed as the surface quality and tool wear [11].

However, a shortened service life due to microbial contamination also has an economical and environmental impact. Since cutting fluids have to be renewed frequently it results in costs for the disposal of used fluid and acquisition of new fluid. Additionally, every change of cutting fluid also means a machine downtime and thus, unproductive time. Besides the costs for disposal and acquisition a shortened service life results in a higher demand of resources as well.

Another important aspect concerning contaminated cutting fluid focuses on occupational health and safety. The hygienic and toxicological risks for machine operators that have to work with cutting fluids are not to be neglected. Especially by inhalation, by skin injuries or through the mucous membranes in the facial area microorganisms can enter the body [9], [14]. This may cause skin irritation or dermatitis and in some case it may also lead to allergic reactions [9].

2.3. Possible disinfection methods

The described impacts of contaminated cutting fluid require the use disinfection methods in order to control the growth of microorganisms. The aim of these methods is to maintain the characteristics and technical performance of cutting fluids as long as possible and thus to increase service life. Therefore, possible chemical and physical disinfection methods are discussed in the following.

Biocides

Biocides are defined as chemical agents, which are used individually or in combination to eliminate microorganisms and are used among other applications for the disinfection of cutting fluids [15]. Biocides may either be used preventively within the additive package of the cutting fluid or they can be added continuously or only added in case of microbial contamination [16]. There are different biocides available that can be used and due to different microorganisms these biocides are sometimes also combined. Selvaraju et al. analyzed the effect of formaldehyde and non-formaldehyde biocides at various concentrations (100 – 100,000 ppm (parts per million)) against two different species of microorganisms in cutting fluids. It was revealed that the type of biocide and its concentration have a significant influence [17]. However, the maximal concentration of biocides is often officially regulated, since the toxicological properties of the biocides may have negative impact on the health of the machine operator [9]. Therefore, a compromise has to be reached between type and concentration of biocides that are still effective against microorganisms but at the same time

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harmless to humans. For this reason the use of alternative disinfection methods became more and more attractive.

Ultraviolet radiation

Ultraviolet (UV) radiation is defined as the part of the electromagnetic spectrum with wavelength from 100 to 380 nm [18]. For disinfection purposes the UVC-radiation with wavelength from 200 to 280 nm is usually used [19]. The ultraviolet light is absorbed and subsequently leads to damage to the genetic material of bacteria and fungi. Thus, reproduction of the microorganisms is affected and they are eliminated as a long term result [4]. This effect, however, depends on the UV concentration that is affected by radiation power, irradiated area and treatment time. In case of cutting fluid the effective penetration depth of the UV radiation is also reduced due to its turbidity [20]. However, the application of UV radiation as a disinfection method is further restricted. Organisms have the ability to repair UV radiation induced damages. In the case of an insufficient or discontinuous radiation, the microorganisms can regenerate and lead to recontamination of the fluid [4]. Moreover, it was found that surviving species are even more resistant to UV radiation [19]. Nevertheless, the application as an effective disinfection method for cutting fluids was verified in different research studies. Saha et al. analyzed the effect of different parameters such as treatment time and radiation power on the disinfection performance. Based on the results it is recommended to apply high radiation power and to keep the fluid moving in order to reduce treatment time [21]. Merschbrook determined a positive effect of UV radiation on contaminated cutting fluid as well. Microorganisms were partially reduced by 100 %. However, in case of a high initial contamination of 106- 107CFU/ml (colony forming unit per

milliliter) no reduction could be detected [22].

Ozone

Ozone is a natural gas which exists to 0.01-0.04 ppm in the atmosphere. Due to oxidation of organic compounds ozone can lead to a reduction of microorganisms. Furthermore, the living conditions for anaerobic microorganisms can be significantly deteriorated by the formation of aggressive radicals. Ozone is already successfully used in drinking water treatment [7]. The application as disinfection method for cutting fluid, however, has not been examined in detail so far. Bromley described in 1977 that 10 grams of ozone per day are sufficient to prevent the growth of microorganisms [23]. Thanomsub, et al. analyzed the effects of ozone treatment on different bacteria at different concentrations. It is concluded that ozone at 0.167 mg/min/l can be used to sterilize water which is contaminated with up to 105CFU/ml [24].

Pasteurization

Pasteurization describes a brief heating of fluids to eliminate the vegetative cells of microorganisms. The required temperature depends on the particular type of microorganisms. A brief exposure to a temperature of 61-63 °C inactivates most common pathogenic bacteria, but a prolonged heat treatment above 120 °C is needed to eliminate many of the species commonly recovered from cutting fluids

[25]. Thus, the application of this procedure to cutting fluids requires a re-cooling of the fluid in order to fulfill its cooling task. The need for heating and re-cooling is therefore connected with high energy consumption. Further, other components and additives of the cutting fluid might be affected by the heating process and lead to a changing performance of the cutting fluid and affect it adversely [4], [22]. This method, however, has been successfully applied for many years in the liquid food industry in order to kill potentially pathogenic bacteria [3]. Elsmore and Hill examined the use of microwaves as a method of pasteurizing cutting fluids. In case of a continuous treatment a reduction of bacteria was achieved within 45s at temperature of 60-70 °C [26]. Another research project funded by Deutsche Bundesstiftung Umwelt analyzed the thermal treatment of cutting fluids as well. Experiments within the project identified a required treatment time of 3 minutes at 85 °C to eliminate 99-99.99 % of the bacteria [27].

Ultrasonic

Ultrasonic describes sound waves in the frequency range of 20 kHz to 10 MHz, which are above of the frequency are of human hearing. For cleaning purposes ultrasonic frequencies between 20 MHz and 40 kHz are used, while sterilization requires ultrasonic frequencies of 200 kHz and 350 kHz [22]. Eucaryotic microbes, such as fungi and algea are generally more labile to ultrasonic than bacteria [3]. Therefore, ultrasonic is for example used in sewage treatment plants to eliminate algea but also to treat the surface of workpieces or parts in medical technology [28]. In case of cutting fluids Merschbrook analyzed a combined use of ultrasonic with UV radiation. It was found that using ultrasonic reduces the requirement for biocides, but a brief treatment of 30 KHz has no significant impact on the bacteria [22]. Buchal conducted further research on the impact of ultrasonic on different cutting fluids. The study reveals that not only the frequency but also the power is important. Thus, the parameters of 352 kHz und 400 W/l were suggested to improve the disinfecting performance [29].

2.4. Research demand

The literature review shows that different chemical and non-chemical disinfection methods exist. Most of them were tested as a treatment for cutting fluids, but always under different conditions within different experimental setups and without considering the environmental impacts. However, to allow a fair comparison the functional performance of two methods, ozone and UV radiation, is analyzed in the following within a similar experimental setup. Further, a screening LCA is performed to compare the environmental impact as well.

3. Functional Evaluation of Disinfection Methods

3.1. Experimental setup

For the functional evaluation a cutting fluid circulation system was developed on a laboratory scale. The system is

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shown in Figure 1. It actually consists of two circuits, one as a reference circuit and another one, where the disinfection method is applied. Thus, the effectiveness of the applied method can be compared to the reference system and neglects external influences. For the experiments a mineral oil based emulsion and a polymer based dilution [30] were used both with a concentration of 5 %.

Figure 1: Circulation system on a laboratory scale

For the functional evaluation of UV radiation a commercial 60 W UV-System from UVECO GmbH was used. In this case cutting fluid passes a bypass where the system is installed (see Figure 1). By using the valves it is possible to adjust the cutting fluid volume flow. The treatment time within the UV-system was set at 1.07 s and with a radiation power of 6 mW/cm². Thus, a radiation concentration of 6.42 mJ/cm². In both systems, reference and treated system, the cutting fluid is continuously circulated with an index of around 30 times per hour. Over 24 hours of continuous UV treatment the cutting fluid is periodically analyzed at 0, 0.25, 0.5, 1, 2, 4, and 24 hours. Additionally, the fluid was also tested 24 hours after the treatment.

In case of the ozone exposure experiments, a commercial ozone generator (Water Ozonator YL-A300n) was used with an ozone output of 250 mg/h. Thus, for the experiments an ozone output of 0.52 mg/min/l was achieved. The cutting fluid was treated over a period of 2 hours and analyzed periodically at 0, 0.25, 0.5, 1, 2 hours and additionally 24 hours after the treatment. In contrast to UV radiation, the bypass was not needed for this method (see Figure 1).

The microbial contamination was analyzed using dip-slides, as they are widely used in practice and research. Additionally, the ph-value, temperature, nitrite concentration and water hardness were tested.

3.2. Functional results

The results for the application of ozone to the mineral oil based emulsion and polymer based dilution are presented in Figure 2 and Figure 3. In case of emulsion the development of microbial contamination over time shows only a minor effect of the ozone. The contamination decreased slightly from 104.5 to 104 CFU/ml. In case of the dilution a similar effect is shown. However, 24 hours after the application the contamination of the treated dilution has significantly decreased to 10² CFU/ml. It is striking that in this case the

contamination of the reference dilution has decreased to 103.5

CFU/ml as well. This might be due to the fact that the fluid circulated continuously over 24 hours and that this has affected the fluid as well.

Figure 2: Effect of ozone on contamination of mineral oil based emulsion (treatment time of 2 hours)

Figure 3: Effect of ozone on contamination of polymer based dilution (treatment time of 2 hours)

The results for the application of UV radiation to the mineral oil based emulsion and polymer based dilution are presented in Figure 4 and Figure 5. In this case the disinfection method shows a more significant impact. For the emulsion the development of microbial contamination decreased from 106.5 to 104.5 CFU/ml and for the polymer no bacteria could be detected anymore after a treatment of 24 hours. The better results for the dilution could be explained by its transparency. In a clear fluid the effective penetration depth of the UV radiation is higher and better results can be achieved [20]. However, due to external circumstances the initial contamination for both cutting fluids was different as well. The emulsion had been stronger contaminated and therefore the different results might also be due to other reasons. Moreover, it is striking that in case of the emulsion 24 hours after the treatment the microbial contamination is back to the same level or even stronger compared to the initial situation. However, in literature it is described that organisms have the ability to repair UV radiation induced damages (see section 2), which might be the reason for this effect. Further, in case of the dilution it is again striking that the contamination of the reference dilution changed as well. After 24 hours the contamination has decrease from 104.5 to 102.5 CFU/ml, but after 48 hours it is back to 103.5 CFU/ml. This development might be due to the fact that in this case cutting fluid was circulated for 24 hours during the treatment time and not further circulated for the remaining 24 hours after the treatment. Thus, similar to the experiments before this demonstrates that circulation has an impact. However, since this effect can only be determined in case of the dilution it can further be assumed that the type of cutting fluid has an impact as well. Pumps Piping system Reference System Valve for bypass Treated System Ozone system 2. Method 1. Method Cutting fluid UV-system 105 104 103 102 101 100 Co nta mi nat io n [C FU /ml] 0.25 0.5 1 2 24 Time [hours] 0 without treatment Treated Reference 105 104 103 102 101 100 Co ntamina ti on [CFU/ml] 0 0.25 0.5 1 2 24 Time [hours] without treatment Treated Reference

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Figure 4: Effect of UV on contamination of mineral oil based emulsion (treatment time of 24 hours)

Figure 5: Effect of UV on contamination of polymer based dilution (treatment time of 24 hours)

4. Ecological Evaluation of Disinfecting Methods

4.1. Goal and Scope

A screening LCA according to the ISO 14040 methodology [31] was performed in order to compare the environmental impact of the different disinfection methods. Although the application of alternative disinfection methods has been discussed for many years (see section 2), the environmental impact has not been analyzed so far. Besides ozone and UV radiation, the use of biocides, pasteurization and ultrasonic is considered as well. The focus of this analysis is on the production and application phase of the disinfection methods, without considering additional infrastructure or transportation systems. The functional unit describes the complete disinfection of 1 liter water based cutting fluid with an initial microbial contamination of 105CFU/ml. However,

the results are then projected to 250 m³ cutting fluid for a more realistic comparison. For the impact assessment the ReCiPe midpoint method was used, but the results of this study focus on climate change and human toxicity as relevant output oriented midpoint indicator and agricultural land occupation as an input oriented mid-point indicator.

4.2. Life Cycle Inventory (LCI) Analysis

Within the inventory analysis all input and output energy and material flows are analyzed with regard to the functional unit. Generally, the analyzed system is based on the circulation system described in Figure 1. The first step describes the production of the disinfection method by using a specific energy demand. The actual amount of disinfectant required to disinfect 1 liter of cutting fluid is considered in the next step. Additionally, the energy of the pumps is included in this step. Similar to the UV radiation some disinfection methods require a bypass in order to be applied effectively

and thus, the additional pump energy for this process is included as well. In case of pasteurization the cooling process is also considered.

LCI data sets for general processes like energy supply were taken from Ecoinvent 2.2 database [32] as well as the data for the biocides. For this study the biocides formaldehyde and phenol were analyzed and their performance was modeled based on [17]. Similarly, the other disinfection methods were modeled based on literature as well (UV radiation: based on specific project results, [21] and [22]; Ozone: based on specific project results and [24]; Pasteurization: based on [27]; Ultrasonic: based on [29]).

4.3. Life Cycle Assessment Results

Based on the life cycle inventory analysis an impact assessment was conducted. The results of the selected impact categories are presented in Figure 6, Figure 7 and Figure 8. The comparison of the environmental impacts shows that the relevant results for each category show similar trends. In each case ultrasonic and pasteurization have the highest impacts. In case of ultrasonic this is due to the energy intensive production phase and longest treatment time. The results for the pasteurization method are mainly caused by the energy intensive heating and cooling processes.

Figure 6: Impacts assessment results for “climate change”

Figure 7: Impacts assessment results for “human toxicity”

Figure 8: Impacts assessment results for “agricultural land occupation”

108 106 104 102 100 Conta minat io n [CFU/ml] 0 0.25 0.5 1 2 4 24 48 Time [hours] without treatment Treated Reference 0 0.25 0.5 1 2 4 105 104 103 102 101 100 Con tamination [CFU/ml] 24 48 Time [hours] without treatment Treated Reference 1 10 100 1,000 10,000 100,000 1,000,000

Ozone Ultras. Past. UV Form. Phenol

climate change [k g C O2 -E q ] 1 10 100 1,000 10,000

Ozone Ultras. Past. UV Form. Phenol

human toxicity [k g a .4 -D C B -E q ] 1 10 100 1,000 10,000

Ozone Ultras. Past. UV Form. Phenol

agricultural land occupation

[m

² a

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5. Conclusion and Outlook

This paper examines the application of different disinfection methods to control microbial contamination of cutting fluids. Based on a theoretical description of each method, the functional performance and environmental impact were analyzed. The functional evaluation basically demonstrated a successful application of ozone and UV radiation. In comparison significantly better results could be achieved by using UV radiation. Additionally, the research identified that a circulation of the cutting fluid and the type of cutting fluid have an influence as well. In terms of environmental evaluation biocides, pasteurization and ultrasonic were analyzed additionally. The results show the highest impacts for ultrasonic and pasteurization in each impact category, while the other methods are mostly equal. Overall it can be concluded that especially UV radiation shows the highest potential to be used in addition to biocides or even substitute biocides in order to control microbial contamination without increasing the environmental impact.

In future studies this approach will be extended by considering the functional performance of biocides, pasteurization and ultrasonic within the same experimental setup. This allows an adjustment of the environmental assessment to the same database as well in order to increase the comparability. To extend the ecological evaluation the recontamination should also be considered in future studies. Further, it might be interesting to consider the disposal phase as well and to compare the results of the disinfection methods in relation to a more frequent exchange of cutting fluids.

Acknowledgements

This project ViLAr was funded by the Federal Ministry of Economics and Technology (BMWi) upon a decision of the German Bundestag. The authors would like to express their thanks to UVECO GmbH and Frederike Stolz for their support.

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