Algae growth

Microorganisms, plants, algae, or small animals can accumulate on wetted surfaces. It is shown as appearance of stains, readily recognizable on the material surfaces, since they form patinas varying in extent, thickness, consistency, and colour. Depending on the type of microorganism and on their life cycle phase, dark green, brown, grey and pink coloured patinas may occur (Figure 4–15).

The biological colonization of external façades by microorganisms can change the aspect of the surfaces and can even compromise the durability of materials. Algae and cyanobacteria are the main colonizers of building façades, and later they can favour the growth of mould, lichens, fungi and other microorganisms.

Figure 4–15: Examples of algae growth on external walls: a) and b) façades exposed to wind driven rain; c) and d) fed by drain water from windows overhangs.

Table 4‑8 sums up what to look for, and where to pay special attention with regard to algae growth.

Table 4‑8: What to look for, and where with regard to algae growth.

Algae and cyanobacteria are the main colonizers of building façades, and later they can favour the growth of mould, lichens, fungi and other microorganisms (Maury-Ramirez, Muynck, Stevens, Demeestere, & Belie, 2013)

Algae and cyanobacteria can develop on a large variety of façades (i.e. on stone, brick, plaster and mortar), whenever suitable combination of relative humidity, temperature and light occurs. The presence of water is fundamental for algae growth. The main causes for wetting of façades are mainly given by wind driven rain, leaks from rainwater drainage systems and dew water (Flores-Colen, Brito, & Freitas, 2008).

In addition to the aesthetic deterioration, algae and cyanobacteria may also cause a biochemical and a biophysical deterioration of the substrate (Tiano, 2001). The production of organic and inorganic acids by the microbial layer and the mechanical pressure exerted by the growing microbial structures, caused by the shrinking/swelling phenomena during the cycles of drying and moistening, induces different types of damages:

• aesthetical (coloured patinas and encrustations);

• physical (mechanical stress and loosens mineral grains especially on stone surface);

• chemical (solubilisation of essential minerals, excretion of organic acids or enzymes, pH reduction, change of the electrical conductivity).

Algae and cyanobacteria are often widespread on façades, and several are the factors that influence their growth, such as climate, building design and façade materials. Considering that building envelope is characterized by extreme fluctuations of temperature, repeated desiccation and high UV-radiation, algae and cyanobacteria can tolerate these variations, maintaining a metabolic activity. Different algal species have very different life-styles and tolerance of variations in temperature, moisture etc. and they are metabolically active only when appropriate combinations of relative humidity, temperature and light are present. Temperature and free water availability are the most important environmental conditions that affect the growth of these microorganisms (D’Orazio, et al., 2014), (Ortega-Calvo, Ariño, Hernandez-Marine, & Saiz-Jimenez, 1995).

For most algae and cyanobacteria an optimal temperature for growth is estimated within the range 20 °C - 30 °C, while the range of suitable growth is usually considered between 5 °C and 40 °C (Singh & Singh, 2015).

Wind driven rain, leaks from rainwater drainage systems and dew water are the main causes for wetting of façades, providing liquid water for algae. However, algae and cyanobacteria can survive in dry periods and can restart their growth when enough water is available. Therefore, the drying of façades during the day is not enough to prevent algae colonisation. Hence, the presence of algae and cyanobacteria on the surface indicates a high moisture content of the substrate (Miller, Dionísio, Laiz, MacEdo, & Saiz-Jimenez, 2009).

The growth of algae and cyanobacteria is also influenced by the orientation of the façade. Indeed, the north-facing walls, which are wetter for longer time and less irradiated by the sun, are faster colonized. Similarly, a façade exposed to dominant winds could be colonized more easily than the other sides of the same building: the wind could transport both rain and biological contaminants. A façade which is often wet by rainfall promotes the growth of algae. However, a high temperature caused by direct irradiation induces water evaporation by heating the material surfaces (Ortega-Calvo, Ariño, Hernandez-Marine, & Saiz-Jimenez, 1995).

The geometry of the building may offer preferential routes, where water could stagnate after a rain event, creating the ideal conditions for the proliferation of algae and cyanobacteria. If balconies or roof overhangs reduce the wind driven rain on the walls, a light inclination of the façade increases the surface exposed to the water. Once the algae have grown, the run-off rain water contributes to replace the old cells with new cells and favours the spread of algal spots to further building components not already contaminated. External parts of building, which are often moistened for long periods or easily covered by biological propagules, are highly sensitive to the biological colonization. Biofouling often increases at the foot of walls, junctions of different coatings, and overhanging elements (cornices, mouldings, balconies, etc.) (Barberousse, 2007).

The biological colonization on building surfaces is also highly dependent on the material substrate: porous building materials (e.g. bricks, stones, mortar) can contain a large quantity of water, which becomes available to the microorganisms. Since historic masonry walls are generally built by porous bricks and natural stones, they can contain high quantity of moisture, and thus be greatly exposed to algae colonisation. Moreover, roughness affects the flow of water on the surface and favours the adhesion of organic material blown by the wind or brought by water flow on the substrate. Consequently, rough building façades are more subject to biofouling than smooth ones (Tran, et al., 2014). Finally, the influence of pH of the substrate on algae is not well known, but, like most microorganisms whose growth is satisfactory between pH values equal to 6.0 and 9.0, algae and cyanobacteria found on building façades can normally develop at a pH equal to 8.0 (Singh & Singh, 2015).

To date, only few mathematical models are able to describe and predict algae and cyanobacteria biofouling on building surfaces.

A main algae failure model, recently developed based on the Avrami’s Theory, predicts the algae coverage of a certain surface, depending on specific material properties and boundary conditions (Quagliarini, u.c., 2019). Material properties are related to the porous structure of the material: indeed, open porosity and pore distribution affects the retention of water and nutrients important for the growth of algae. A relevant effect is also given by the roughness, which is not an intrinsic property of the material, but depends on the production process (surface smoothing procedure). Environmental conditions, as temperature and relative humidity, are considered in the model and affect the growth rate of algae and cyanobacteria over the time when exposed to optimal and non-optimal environmental conditions.

As a result of the failure model assessment, a specific index expresses the percentage of the surface area covered by the microorganisms.

An in-situ evaluation of the deterioration caused by the proliferation of algae on façades can be made with both quantitative and qualitative analyses (Graziani, u.c., Evaluation of inhibitory effect of TiO2 nanocoatings against microalgal growth on clay brick façades under weak UV exposure conditions, 2013), by measuring:

• the chromatic variation of the materials’ surface

• the extension of the covered surface during the time.

Qualitative analyses consist on chromatic investigations on façades. Colorimetric measurements for the evaluation of the chromatic variation (ΔE) can be carried out using a spectrophotometer, in accordance with UNI EN 15886:2010 (UNI EN 15886, 2010) and UNI 1602371:2018 (UNI 11721, 2018). On each investigated area, measurements should be periodically repeated on the same points. Results are expressed in CIELab colour space and averaged to obtain a representative value for each investigated area. Colour variations is calculated in terms of total colour difference ∆E, following Eq (4.1):

where L0*, a0* and b0* indicate the colour coordinates of samples at the first measurement, and L*, a*, b* the coordinates periodically measured during the monitoring. According to standard methods (UNI EN 15886, 2010), (UNI 11721, 2018), a total colour difference ΔE<1 is considered not visible by naked human eyes, while a ΔE ranging between 1 and 2 is detectable only with a close observation. From an engineering point of view, a ΔE=1 can be assumed as the acceptable lowest limit for algae growing. In case of average ΔE>1, the term a0*-a* should be evaluated to verify if the colour change is due to the presence of algae. In fact, the variation Δa indicates a colour difference in a red/green scale. That way, it permits to associate the colour variation to the appearance of algae as green stains: the amount of red is indicated by positive values (Δa>0), while a green toning by negative values (Δa<0).

The colorimetric analyses should be associated to the quantification of the biofouling extension, evaluated by a (quantitative) Digital Image Analysis (DIA) of the growth rate of algae (Graziani, et al., 2014). To calculate the extension of colonized area in-situ the so-called “threshold method” can be adopted (Tiago & Wayne, 2011). The colonized surfaces need to be periodically digitalized with an adequate image resolution, and elaborated to calculate the algal coverage, expressed as a percentage of the total sampled area. Once set the threshold values in CIELab colour space, the acquired images are binary converted to consider only the pixels related to the contaminated surface by algae (Figure 4–16). The covered area by microalgae is represented as the percentage of the black pixels on the total area of the sample (Muynck, Ramirez, Belie, & Verstraete, 2009). Measurements should be carried out periodically in accordance to the estimated growth rate. Results are averaged on at least three samples for each investigated area.

Remedial actions if algae growth is identified

Remedial actions should be performed by specialists to avoid damages of the material surfaces and to ensure that a specific treatment is feasible.

Once a façade is deteriorated by the presence of algae and cyanobacteria, mechanical, physical and chemical measures can be adopted to face the problem. Mechanical methods can be used to remove stains and patinas from contaminated elements either by hand or with tools, and a preliminary biocide treatment (applied before the mechanical intervention) is advantageous to facilitate the removal of biofilm. The most widespread physical interventions against algal and cyanobacteria growth is surface treatment using ultraviolet (UV) radiation. Furthermore, chemical methods can give satisfying results, but it should be remembered that the use biocide agents of synthetic origin like pesticides and disinfectants, have a side problem related to the use of pesticides that persist in the soil or water.

After the removal of biological contaminations, a hydrophobic treatment on the material surface is recommended to prevent or minimize a future algal development. Operations should be performed by specialized practitioners to avoid damages of the material surfaces, and experts should evaluate case-by-case and test the products on small portions of the building before the application (Tiano, 2001).

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