Abstract

This work aimed to select materials capable of favouring biofouling build-up in order to develop plain coupons as alternative to expensive commercial biofouling mesh coupons. Plain coupons of copper, stainless steel (SS), polyvinyl chloride (PVC) and high density polyethylene (HDPE) were dipped and tested in a cooling water from a food industry. PVC and HDPE coupons showed promising responses and appear to be preferable since they are corrosion-free. Moreover, an experimental vibration sensor monitored biofilm adhesion on SS and PVC tubular coupons (simulators of the respective sensor tubes), inside which flowed the water aforementioned. The SS sensor tube and tubular coupons displayed the most satisfactory results, i.e. the highest vibration amplitude and the highest adhered biofilm mass, respectively. Biofilm adhesion onto the materials tested depended on their surface shear stress, effective roughness and hydrophobicity, as determined by scanning electron microscopy and goniometry.

SYMBOLS LIST

     
  • ɛ

    absolute roughness (m)

  •  
  • ρ

    density (kg·m−3)

  •  
  • μ

    dynamic viscosity (Pa·s)

  •  
  • τo

    surface shear stress (Pa)

  •  
  • D

    internal diameter (m)

  •  
  • f

    friction factor

  •  
  • k25°C

    electrical specific conductivity (S·m−1)

  •  
  • L

    length (m)

  •  
  • T

    temperature (°C)

  •  
  • v

    average velocity (m·s−1)

  •  
  • V

    volume (m3)

  •  
  • Pabs

    absolute pressure (Pa)

  •  
  • Re

    Reynolds number

  •  
  • Q

    volumetric flow rate (m3·s−1)

ABBREVIATIONS LIST

     
  • Amp

    vibration amplitude (V)

  •  
  • AmpNC

    normalized corrected vibration amplitude (V)

  •  
  • CIP

    cleaning-in-place

  •  
  • HDPE

    high density polyethylene

  •  
  • PVC

    polyvinyl chloride

  •  
  • SEM

    scanning electron microscopy

  •  
  • SS

    stainless steel

  •  
  • TSA

    tryptic(ase) soy agar

INTRODUCTION

In industrial applications, water is commonly used to refrigerate products and/or processes due to its high heat capacity, in order to maintain constant operating conditions (e.g. temperature and pressure) crucial to the processes' efficiencies. On the other hand, it is very well-known that water is a universal solvent. This feature is not always desirable in industry because water may dissolve many substances, including gases such as oxygen and carbon dioxide which favour metal corrosion. As water also dissolves minerals, their concentrations may exceed their solubilities and lead to scaling. Furthermore, water carries nutrients providing ideal conditions for microbiological growth, i.e. biofouling (Bott 1988).

The designation ‘fouling’ stands for the undesirable formation of inorganic, organic and biological deposits on the materials' surfaces. These deposits decrease heat transfer across surfaces, increase pressure losses, as well as corrosion rates on the surfaces. Each of these phenomena causes energy losses. There are several types of fouling such as scaling due to minerals precipitation, corrosion due to leaching, corrosion due to microorganisms, organic fouling, and biofouling (Strathmann et al. 2013).

Biofilm build-up was described by Characklis (1981): (1) reversible adsorption of cells onto a surface due to electrostatic attractions; (2) binding of adsorbed cells by extracellular matter and medium nutrients, promoting new cells adhesion; (3) cells reproduction, creating micro colonies, and turning adsorption irreversible; (4) biofilm maturation and complexity rise; (5) attainment of biofilm critical mass and outer layers cells release to colonize new surfaces.

In general, microorganisms' growth depends on the type of substrate for colonization, the physical conditions (temperature, pressure, pH, solar exposure, etc.) and the nourishment conditions (water, O2, CO2, nitrogen, phosphorus, etc.). Usually, bacteria grow at about 20–40°C but some species develop at 4.5–70°C (Samimi 2013). The optimal pH for most microorganisms is around 7 (Guo et al. 2012; Samimi 2013).

The biofilm growth is also influenced by several other factors such as the materials properties, and the medium and bacteria characteristics. The materials properties that most influence the microorganisms' adhesion and proliferation are the chemical composition due to the hydrophobicity and electrical charges of the functional groups, as well as the surface roughness. For instance, the greater the surface irregularities are, the higher the biofilm growth rate is, due to the higher surface area available for biofilm adhesion (Grenho 2010).

Biofilms formed on low-shear surfaces show a low tensile strength and break easily, whereas biofilms formed on high-shear surfaces are remarkably strong and resistant to mechanical breakage. Turbulent flow also enhances bacterial adhesion and biofilm formation (Donlan & Costerton 2002). High flow rates increase biofilm formation and, for average velocities higher than 1 m·s−1, the effect of the medium nutrients content becomes negligible (Mattila-Sandholm & Wirtanen 1992). In sum, the higher the velocity is, the higher the surface shear stress (friction) becomes, retarding the microorganisms at the surface vicinity and promoting biofouling.

Generally, microorganisms prefer to adsorb onto hydrophobic nonpolar surfaces, like plastics, but their adhesion also depends on the microorganisms' hydrophobicity, as well (Donlan 2002; Grenho 2010). As for the materials for biofouling detection, polymeric materials have been preferred, mainly polyvinyl chloride (PVC) and polyethylene (PE). Niquette et al. (2000) showed that biofilm growth on PVC and PE was lower than on iron matrices, and also that the viable counts on these plastics were similar. Conversely, the research of Cloete et al. (2003) revealed that the biofilm formation on PVC surfaces was higher than on stainless steel (SS). Lehtola et al. (2004) observed that biofilms grew faster on PE pipes than on copper pipes, and that the biofilm was less influenced by the material surface than by the chlorine content. Schwartz et al. (1998) studied biofilm development on high density polyethylene (HDPE), PVC, and copper. They concluded that the density of viable cells on these materials was 35–38%, except for copper which was less than 10%. In hospitals, the use of copper (antibacterial agent) surfaces aid current hygiene practices (Grass et al. 2011).

Although many studies have shown that physical-chemical interactions between the substrates and the biofilms affect their adhesion, most of these studies have been conducted in potable water distribution systems. In the last two decades, extensive studies have been carried out by van der Kooij, van der Wielen, and co-workers on biofouling growth onto metals, plastics and membranes in contact with unchlorinated tap water and model waters (e.g. addition of polysaccharides and proteins to unchlorinated tap water) (van der Kooij & Veenendaal 2001; Sack et al. 2014; Hijnen et al. 2016). Recent monitoring techniques for online, real-time detection of biofouling on pipes have been developed (Tan et al. 2014; Bruchmann et al. 2015).

Commercial plain coupons of SS, carbon steel, and copper are available in the market for corrosion monitoring. Likewise, plain biofouling mesh coupons for biofouling monitoring are also available in the market but they are rather expensive.

The main objective of this work was the selection of plain coupons materials that favour biofilm growth for easy biofouling detection, as alternative to the expensive biofouling mesh coupons. Moreover, tubular coupons were used to quantify the mass of adhered biofilm onto their surfaces and thereby assess the reliability of an experimental vibration sensor to early detect biofouling on the sensor tubes.

The materials selected for the plain coupons were two classic materials used for corrosion detection, i.e. copper and SS, as well as two plastics, namely PVC and HDPE. The tubular coupons and the vibration sensor tubes were in SS and PVC, materials commonly used in the food industry and water distribution systems.

MATERIALS AND METHODS

Experimental set-up

In order to assess the performance of several materials for biofouling detection, the experimental set-up shown in Figure 1 was used.

Figure 1

Experimental set-up scheme. Photographs of coupons for biofouling detection: plain coupons on top left-side and tubular coupons on bottom right-side.

Figure 1

Experimental set-up scheme. Photographs of coupons for biofouling detection: plain coupons on top left-side and tubular coupons on bottom right-side.

A test fluid was recirculated for 36 days in a closed loop by a pulsating-free pump (centrifugal Grundfos UPS 25–60 N), from reservoir 1 (plastic, V = 25 L) to two parallel vertical sensor tubes (upwards flow) of SS (D = 8 mm) and PVC (D = 10 mm), followed by a U-turn and a set of four tubular coupons (downwards flow) of the same materials and diameters.

The volumetric flow rates were daily measured using a graduated cylinder and a stopwatch. They were manually adjusted by valves (QSS = 90 ± 1 L·h−1; QPVC = 140 ± 3 L·h−1), such that the average velocities in the sensor tubes and in the tubular coupons were 0.5 m·s−1. The test fluid was heated by an electrical resistance (TETRA HT 100) to promote biofouling growth. The temperature was measured by a type T thermocouple connected to the data acquisition system (display error ≤ 0.00001°C). It ranged in between 18°C and 28°C due to the small electrical power of the resistance and the absence of air conditioning. The effect of the temperature variation throughout the biofilm adhesion assay was studied, as described in the end of the experimental procedure.

An aquarium pump recirculated the fluid test with a flow rate of 35 L·h−1 into reservoir 2 (plastic, V = 6 L) where four plain coupons of each material selected were located. Reservoirs 1 and 2 were meant to test simultaneously the plain and tubular coupons. Reservoir 2 prevented obstructions of reservoir 1 outlet by loose plain coupons. The fluid heat in reservoir 2 was dissipated by natural convection, thus the temperature therein was lower than in reservoir 1.

Vibration sensor

An experimental non-invasive on-line vibration sensor was used to detect biofouling on the sensor tubes surfaces (fully described in Pereira et al. 2008; Teixeira et al. 2014).

The adhesion, growth and removal of deposits cause variations on the vibration amplitude of a monitored surface. The vibration sensor comprised a sensor and an actuator attached to the outer surface of each sensor tube. The actuator forced the tubes to vibrate and the vibration responses (expressed in Volt) were captured by the sensor. Both actuation and sensing processes were carried out automatically at constant time intervals.

In general, vibration sensors should be protected against external vibrations, moisture and atmospheric harsh conditions. The experiments were performed in a facility in-doors at an industrial park with controlled traffic.

Tubular and plain coupons

Four tubular coupons of SS (D = 8 mm, L = 47 mm) and four others of PVC (D = 10 mm, L = 59 mm) were cut from tubes of these materials (homemade) and inserted in two distinct pipelines to quantify the mass of adhered biofilm (Figure 1).

Furthermore, four plain coupons of each of the following materials, copper and SS (acquired), HDPE and hard PVC (homemade), were placed in reservoir 2 for biofouling detection. The dimensions of the plain coupons were 75.5 mm × 12.2 mm × 1.5 mm and their orifice diameter was 5 mm (Figure 1).

The commercial plain coupons of copper and SS were already polished by the manufacturer. The tubular coupons of SS and PVC, and the plain coupons of HDPE and PVC were polished with a cotton pad and toothpaste followed by a water rinse to remove traces of oils present on the metals and waxes present on the plastics, contaminants generated throughout their manufacture.

Test fluid

The test fluid consisted of a water sample collected from a cooling water system of a food industry, with no added biocide. The characterization of this water, presented in Table 1, comprised the temperature, electrical specific conductivity at 25°C, pH, total alkalinity, calcium hardness, phosphates, viable bacterial count and bacterial summary description.

Table 1

Test fluid characterization

T (°C)k25°C (mS/cm)pHTotal alkalinity (mg/L CaCO3)Calcium hardness (mg/L CaCO3)Phosphates (mg/L PO4)Viable bacterial count (CFU × 108/100 mL)
Bacterial summary description
(1)(2)(1)(2)
30–45 1.570 7.31 390 80 2.46 0.67 20% Gram positive coccus
80% Gram negative bacillus 
24% Gram positive coccus
76% Gram negative bacillus 
T (°C)k25°C (mS/cm)pHTotal alkalinity (mg/L CaCO3)Calcium hardness (mg/L CaCO3)Phosphates (mg/L PO4)Viable bacterial count (CFU × 108/100 mL)
Bacterial summary description
(1)(2)(1)(2)
30–45 1.570 7.31 390 80 2.46 0.67 20% Gram positive coccus
80% Gram negative bacillus 
24% Gram positive coccus
76% Gram negative bacillus 

(1) Before the biofilm adhesion assay.

(2) After the biofilm adhesion assay.

The membrane filtration method was used for the viable microorganisms count. The medium was Tryptic(ase) soy agar (TSA) and the incubation elapsed for 120 h at 30°C. The Gram's method was used for the bacterial summary description (positive coccus and negative bacillus). The microbial analyses were made by an accredited laboratory, the Chemical and Biological Analysis Laboratory of Instituto Superior Técnico.

Bearing in mind the relatively high microbial count in the test fluid and the relatively short assay time period (36 days), only biofouling was significant in this work, whereas scaling and corrosion were negligible. The release of toxins and/or biofouling promoters from the metals and plastics that could affect the biofouling growth was also negligible for such a short assay time period.

Experimental procedures

The protocols/procedures described below were followed for the characterization of the coupons materials (viz., scanning electron microscopy (SEM) and goniometry), the biofilm adhesion and cleaning-in-place (CIP) assays, and the temperature effect throughout those assays.

Scanning electron microscopy

The plain coupons of copper, SS, PVC, and HDPE were cut and washed with alcohol to remove any grease present. For the metallic coupons, conductive tape was used to fix the samples onto the sample holder and to guarantee electron conduction, as well. Besides conductive tape, the plastic coupons were coated with a thin layer of chromium (conducting material), using a sputter coater (QUORUM-Q150TES, Pabs = 0.04 Pa).

After these pre-treatments, the samples were inserted in a SEM microscope (JEOL JSM-7001F) and their surfaces were observed and photographed.

Goniometry

The contact angle of a solid material depends on its surface energy, roughness and heterogeneity (Coutinho 2007).

The samples of copper, SS, PVC, and HDPE were cut, washed and dried in a vacuum furnace (Lab Line Duo-Vac Oven, Tambient, Pabs = 9.5 × 104 Pa). The contact angle determinations were carried out using a microscope (Wild Heerbrugg M3Z), a focus light (Leica Type: MTR31), a sample chamber (Ramé Hart Inc, 1000700) and an image analysis software (Axisymmetric Drop Shape Analysis-Profile: ADSA-P).

The sessile drop method was used for the contact angles determinations at room temperature (Restolho et al. 2009). A droplet (average volume = 0.0055 mm3) was deposited by a vertical syringe onto the sample surface, and the high resolution microscope captured the images, which were analysed by the analysis software. The contact angle evolution along time was recorded until a plateau was achieved.

Biofilm adhesion assay

Before the biofilm adhesion assay, reservoir 1 was filled with the test fluid and all valves were opened. The pumps were switched on and the flow rates were set such that the average velocities were 0.5 m·s−1 in both pipes. The system was monitored until air was absent. Finally, the acquisition software was turned on and set to collect data every 30 min. The system was automated, thus the daily procedure only consisted of saving the data acquired by the software on the eve, as well as controlling the flow rates in both pipes.

The virgin plain and tubular coupons were rinsed with water, dried in an oven at 103°C until constant mass was achieved and pre-weighted (Mettler Toledo AB204 analytical scale). The days selected for the tubular coupons withdrawal (one of each material at a time) were based on the vibration sensor data. The plain coupons were withdrawn (one of each material at a time) preferably on the same days as the tubular ones. After the coupons withdrawal, they were dried in an oven at 103°C until constant mass was attained and weighted. The subtraction of the coupons pre-weights yielded the biofilm dry masses.

To investigate the coupons weight losses due to corrosion and/or biocorrosion (Pizarro & Vargas 2016), and the weight gains due to scaling, the coupons were brushed with water and Fairy washing detergent, dried in an oven at 103°C until constant mass was reached and weighted once again. It was checked that the masses of the virgin and cleaned coupons after the biofouling assay were practically constant with deviations around 3%.

Cleaning-in-place assay

A chemical cleaning was carried out by using tap water, acid and basic solutions to remove any deposits in the experimental set-up. Firstly, tap water circulated in open mode. Then, a solution of nitric acid, ACS 30% (VWR) was poured into reservoir 1 until pH 3 was reached to remove any scaling. After 30 min, the acid solution was drained and tap water circulated, until a neutral pH was attained. To remove organic and biological deposits, a solution of caustic soda, reagent grade ≥ 98% (Sodacasa) was added into reservoir 1, until pH 14 was reached. After 1 h, the basic solution was drained and tap water circulated, until neutral pH was achieved. Meanwhile, reservoir 2 was emptied and washed separately through a similar procedure.

Temperature effect throughout biofilm adhesion and CIP assays

A distinct assay was conducted to correct the temperature effect on the vibration sensor data by varying the tap water temperature, as follows: an increase from 18°C up to 28°C (6 h), a plateau at 28°C (10 h), and a decrease from 28°C to 19°C by natural convection (10 h). The relationships between the vibration amplitudes on SS and PVC sensor tubes and the tap water temperature (R2 = 0.94) were respectively:  
formula
 
formula

The amplitudes registered by the vibration sensor throughout the biofilm adhesion and CIP assays were thereby corrected to 25°C, and further normalized by subtracting the initial vibration amplitudes (t = 0). The final results were named AmpNC (NC stands for normalized corrected).

RESULTS AND DISCUSSION

SEM and goniometry

The photographs of plain coupons samples (Figure 2) were taken throughout SEM procedure. Copper showed a high surface roughness most likely due to oxides (Figure 2(a)). Analogously, SS also revealed a high surface roughness probably due to the presence of oxides (Figure 2(b)). HDPE presented a smooth surface without any porosity (Figure 2(c)). Similarly, there was no significant surface roughness on PVC, but there were plenty of surface pores with sizes up to 10 μm (Figure 2(d)).

Figure 2

SEM photographs: (a) copper (×3,000), (b) stainless steel (×3,000), (c) HDPE (×3,000), (d) PVC (×3,000).

Figure 2

SEM photographs: (a) copper (×3,000), (b) stainless steel (×3,000), (c) HDPE (×3,000), (d) PVC (×3,000).

As for the goniometry, five water drops were analysed for each material and the data presented herein are the average of the thresholds obtained for each drop. SS, HDPE, and PVC showed similar contact angles, respectively 68°, 68°, and 67°, each one with an uncertainty of ±2°, whilst copper presented a contact angle of 84 ± 3°, revealing a more hydrophobic behaviour.

Biofouling monitoring

Biofilm adhesion on the vibration sensor tubes

For 36 days, the vibration sensor registered the evolution of the normalized corrected vibration amplitudes on the sensor tubes, AmpNC, as depicted in Figure 3(a). Each experimental point plotted corresponds to an average of nearly 48 daily measurements, with a standard deviation of ±0.003 V.

Figure 3

(a) Evolution of the normalized corrected vibration amplitude on SS and PVC sensor tubes; (b) dry mass of biofilm adhered per surface area unit of SS and PVC tubular coupons; (c) vibration amplitude vs. dry mass of biofilm adhered onto SS and PVC tubular coupons.

Figure 3

(a) Evolution of the normalized corrected vibration amplitude on SS and PVC sensor tubes; (b) dry mass of biofilm adhered per surface area unit of SS and PVC tubular coupons; (c) vibration amplitude vs. dry mass of biofilm adhered onto SS and PVC tubular coupons.

The vibration amplitude of SS sensor tube presented a steep rise until day 6, reached a plateau by day 14, slowly decreased up to day 29, and it vanished from days 30 to 36, in close agreement with the biofilm development theory (Characklis 1981).

As for the vibration amplitude of PVC sensor tube, apparently the biofilm attachment occurred only on day 29, and the biofilm critical mass attainment took place on day 33, meaning that PVC sensor tube was less sensitive to the biofilm growth than SS sensor tube.

Biofilm adhesion on tubular coupons

Figure 3(b) depicts the biofilm dry mass per surface area unit of the tubular coupons in SS and PVC throughout the biofilm adhesion assay. Similar to Figure 3(a), there was a significant growth of biofilm mass onto SS coupons in between days 3 and 31. Between days 31 and 36, the biofilm mass decreased, meaning that the biofilm critical mass was reached and the cells on the outer layers migrated to form colonies elsewhere. Unlike Figure 3(a), there was also a considerable growth of biofilm mass onto PVC tubular coupons throughout the biofilm adhesion assay, denoting that PVC sensor tube was unsuitable for monitoring biofilm development.

In Figure 3(c), the vibration amplitudes of the sensor tubes were plotted against the adhered biofilm dry mass on the corresponding tubular coupons. Assuming a relationship crossing the plot origin is definitely valid, because the vibration sensors were calibrated when the sensor tubes and the tubular coupons were clean. Despite the reduced experimental data, it appears that the vibration amplitudes of both sensor tubes varied linearly with respect to the biofilm mass adhered onto the tubular coupons, in agreement with the work of Pereira et al. (2008). Moreover, the straight lines slopes were practically identical, i.e. the relationship AmpNC=f(biofilm mass) was unique, irrespective of the material surface where the biofilm attached.

Assuming the density and viscosity of water at 25°C for the test fluid and using the definitions of Reynolds number (Re) and friction factor (f), as well as Moody's diagram and a table of absolute roughnesses, the surface shear stress (τo) was estimated by an overall momentum balance to the tubular coupons of SS (ɛ = 45 μm, Re = 4,500, f = 0.011), and PVC (ɛ = 1.5 μm, Re = 5,600, f = 0.009), yielding 1.37 and 1.12 Pa, respectively.

The adhered biofilm mass was generally higher on SS coupons than on PVC coupons, i.e. mSS > mPVC. This matches the order of the surface shear stresses, τo,SS > τo,PVC, in agreement with the data obtained by Mattila-Sandholm & Wirtanen (1992) and Donlan & Costerton (2002). Although the surface shear stress on SS coupons was only 22% higher compared to PVC coupons, the biofilm mass on SS coupons was 50% higher than on PVC coupons. Thus, the surface shear stress (involving the relative roughness) may be a relevant factor whenever selecting tubular materials for biofilm detection in water piping.

Biofilm adhesion on plain coupons

Figure 4 depicts the dry mass of biofilm adhered to the plain coupons. The biofilm masses order was mSS > mPVCmHDPE > mcopper, similarly to the observations of Niquette et al. (2000) for iron, PVC and PE.

Figure 4

Biofilm dry mass per surface area unit of the plain coupons.

Figure 4

Biofilm dry mass per surface area unit of the plain coupons.

For SS plain coupons, the biofilm development occurred until day 31, and between days 31 and 36 there was a mass decline, indicating that the biofilm attained the critical mass and the outer layers migrated. The alternate ascents and descents of the biofilm masses adhered to the copper, PVC and HDPE coupons were likely due to the attainment of biofilms critical masses and cells release on day 17, followed by biofilms growth once again. The highest biofilm mass on SS coupons was surely due to its highest absolute roughness of 45 μm, i.e., 30-fold higher than the roughnesses of PVC, HDPE and copper, 1.5 μm. This fact was corroborated by SEM photographs (Figure 2), except for copper. Despite the substantial response of SS plain coupons, they are not recommended for biofilm detection as they are prone to corrosion.

HDPE and PVC coupons, with equal absolute roughnesses and similar contact angles, 68° and 67°, respectively, presented very similar biofilm dry masses. These polymeric coupons showed moderate masses of biofilm adhered, thus they may be used for biofouling detection. Besides, they are advantageous because they are corrosion-free.

As for the copper, a much higher biofilm mass was expected for two distinct reasons: it was the most hydrophobic material (84°), and it revealed a high effective roughness (Figure 2(a)), both factors favouring microorganisms adhesion (Donlan 2002; Grenho 2010). Nevertheless, the mass of biofilm adhered to copper coupons was the lowest likely due to copper bactericide effect (Grass et al. 2011). The unexpected behaviour of copper coupons is analogous to the data reported by Schwartz et al. (1998) and Lehtola et al. (2004).

The effective roughness and hydrophobicity appear to be pertinent factors in the selection of plain materials for biofouling detection, as referred by Grenho (2010). As a matter of fact, the order of the adhered biofilm masses on the plain coupons based solely on the effective roughness and hydrophobicity would be mcopper > mSS>mPVCmHDPE, as obtained in this work, except for copper.

Cleaning-in-place

By the end of the biofilm adhesion assay, CIP assay was performed to assess the vibration sensor response to the biofilm detachment from the sensor tubes.

Throughout CIP assay, immediately after the addition of each solution the vibration amplitude of SS sensor tube revealed a steep scattering (Figure 5(a)). The signal vanished by the end of the third water washing, indicating an effective removal of the biofilm previously adhered. As for PVC sensor tube (Figure 5(b)), variations of the vibration amplitude were also noticed, although data scattering was lower compared to SS sensor tube data.

Figure 5

Cleaning-in-place: (a) SS sensor tube, (b) PVC sensor tube.

Figure 5

Cleaning-in-place: (a) SS sensor tube, (b) PVC sensor tube.

The vibration amplitude scattering detected whenever the cleaning solutions were added might have been due to air inlet with the cleaning solutions, microbubbles formation during the acid solution addition, and/or biofouling swelling under severe conditions (Cogan & Keener 2004).

CONCLUSIONS

The goals of selecting materials that favour biofilm formation and growth on tubular and plain coupons, as well as the assessment of a vibration sensor for the early detection of biofouling, were fulfilled. For such purposes, an assay of biofilm adhesion on vibration sensor tubes, tubular coupons of SS and PVC, and plain coupons of copper, SS, HDPE and PVC was carried out.

The vibration sensor tubes of SS were more effective than the ones of PVC for the detection of biofilm adhesion/removal. For the tubular coupons of the same materials and diameters of the vibration sensor tubes, the same conclusion was drawn, i.e. SS coupons yielded a higher mass of biofilm adhered. The preferential adhesion of biofilm on SS should be due to the higher surface shear stress (higher relative roughness) on this material in comparison to PVC.

Concerning the plain coupons developed for easy detection of biofilm growth, SS yielded the highest mass of biofilm adhered, but it is not recommendable for biofouling detection because it is prone to corrosion. The polymeric coupons of HDPE and PVC presented reasonable masses of biofilm adhered, thus they may be used for biofilm detection. The effective roughness and hydrophobicity are surely relevant factors in the selection of plain materials for biofouling detection.

ACKNOWLEDGEMENTS

Professors Patrícia Almeida de Carvalho and Benilde Saramago are deeply acknowledged for the determinations in the scanning electron microscope and goniometer, respectively.

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