Evaluación de diferentes tecnologías para la inactivación de organismos fitoplanctónicos en aguas de lastre

Debido a que la mayor parte del transporte internacional se realiza a través de buques y el tráfico marítimo anual cada vez es mayor, el intercambio de aguas de lastre se ha convertido en uno de los principales problemas ambientales, principalmente por el intercambio de especies transportadas en el agua de lastre que pueden convertirse en especies invasoras. En torno a 10 000 millones de toneladas de agua de lastre son intercambiadas anualmente provocando una nueva invasión de especies marinas cada nueve semanas según datos de la Organización Marítima Internacional (OMI). Para solventar este problema, la OMI aprobó en el año 2004 el Convenio internacional para el control y la gestión del agua de lastre y los sedimentos de los buques (BWMC), entrando en vigor en el año 2017 y representando el conjunto de los 94 países firmantes a principios de 2023 más del 92% de la flota mercante internacional. El BWMC, en su regla D-2 establece unos estándares para la concentración máxima de organismos viables en el agua de lastre descargada. Para cumplir con los estándares establecidos por el BWMC, los buques deberán contar, antes del 8 de septiembre de 2024, con un sistema de gestión del agua de lastre (BWMS). En la presente tesis doctoral, se han evaluado y optimizado algunas tecnologías de tratamiento de aguas de lastre, con aplicación a la inactivación de organismos fitoplanctónicos (microalgas y cianobacterias), debido a que la mayoría de las investigaciones realizadas hasta la actualidad han estado focalizadas en bacterias. Las tecnologías estudiadas han sido la radiación ultravioleta (UV) aplicada con lámparas de mercurio de baja presión (BP) y con LEDs, ozonización, procesos de oxidación avanzada (POAs) y tratamientos térmicos. En el caso de la radiación UV también se ha evaluado la posibilidad de evitar la fotoreactivación (mecanismos de reparación del daño al ADN dependientes de la luz visible) mediante un postratamiento de cinco días de oscuridad. Finalmente se ha realizado un estudio energético y económico de los distintos tratamientos como orientación para la selección de una determinada tecnología a la hora de implantar un BWMS a bordo de un buque. La radiación UV combinada con un postratamiento de cinco días de oscuridad y la ozonización son tratamientos competitivos económicamente. A pesar de que la ozonización es más económica los bajos costes de ambas tecnologías hacen más interesante la radiación UV por ser más sencillos los procesos de homologación de los BWMS sin el uso de sustancias activas y también por ser un tratamiento más simple, seguro y respetuoso con el medio ambiente. Aunque los LEDs UVC son más eficaces en cuanto a la inactivación que las lámparas UV de mercurio de BP, actualmente el tratamiento con estas últimas sigue siendo más económico. Como organismos estudiados en los diferentes experimentos se han utilizado las microalgas Phaeodactylum tricornutum y Tetraselmis suecica y las cianobacterias Anabaena sp. y Synechococcus sp. Actualmente el BWMC, en la regla D-2 sobre las concentraciones máximas de organismos en el agua de lastre descargada no considera, más allá de tres bacterias especificadas nominalmente, los organismos menores a 10 µm en dimensión mínima, a pesar de representar estos una proporción mayor a los organismos por encima de ese tamaño y tampoco propone un organismo de prueba normalizado (STO) dentro de las categorías sí contempladas (entre 10 y 50 µm y mayores a 50 µm de dimensión mínima). En la presente tesis se ha propuesto la cianobacteria Synechococcus sp. como STO para los tratamientos basados en radiación UV en la categoría de organismos menores a 10 µm y a la microalga Tetraselmis suecica para la categoría entre 10 y 50 µm de dimensión mínima.As most international transport is carried by ships and annual maritime traffic is increasing, ballast water exchange has become one of the major environmental problems, mainly due to the exchange of species carried in ballast water that can become invasive species. Some 10 billion tonnes of ballast water are exchanged each year, resulting in a new invasion of marine species every nine weeks, according to the International Maritime Organization (IMO). To address this problem, the IMO adopted the International Convention for the Control and Management of Ships’ Ballast Water and Sediments (BWMC) in 2004, which entered into force in 2017 and whose 94 signatory countries together represent more than 92% of the international merchant fleet at the beginning of 2023. The BWMC, in its regulation D-2, establishes standards for the maximum concentration of viable organisms in discharged ballast water. To meet the standards established by the BWMC, ships must have a Ballast Water Management System (BWMS) in place by 8 September 2024. In this doctoral thesis some ballast water treatment technologies have been evaluated and optimized, with application to the inactivation of phytoplanktonic organisms (microalgae and cyanobacteria), since most of the research carried out to date has focused on bacteria. The technologies studied have been ultraviolet (UV) radiation applied with low-pressure (LP) mercury lamps and LEDs, ozonation, advanced oxidation processes (AOPs) and thermal treatments. In the case of UV radiation, the possibility of avoiding photoreactivation (visible light-dependent DNA damage repair mechanisms) by a five-day dark post-treatment has also been evaluated. Finally, an energy and economic study of the different treatments has been carried out as a guide for the selection of a particular technology for the implementation of a BWMS on board a ship. UV radiation combined with a dark post-treatment of five days and ozonation are economically competitive treatments. Although ozonation is cheaper, the low costs of both technologies make UV radiation more interesting, as it is simpler for the approval process of BWMS without the use of active substances and because it is a simpler, safer, and more environmentally friendly treatment. Although UVC LEDs are more effective in terms of inactivation than LP mercury UV lamps, treatment with the latter is currently still more economical. The microalgae Phaeodactylum tricornutum and Tetraselmis suecica and the cyanobacteria Anabaena sp. and Synechococcus sp. have been used as study organisms in the various experiments. Currently the BWMC, in regulation D-2 on maximum concentration of organisms in discharged ballast water does not consider, beyond three nominally specified bacteria, organisms smaller than 10 µm in minimum dimension, even though they represent a higher proportion of organisms above that size, nor does it propose a standard test organism (STO) within the categories it does consider (between 10 and 50 µm and larger than 50 µm minimum dimension). In this thesis, the cyanobacterium Synechococcus sp. has been proposed as STO for UV-based treatments in the category of organisms smaller than 10 µm and the microalga Tetraselmis suecica for the category between 10 and 50 µm minimum dimension

Evaluation of three photosynthetic species smaller than ten microns as possible standard test organisms of ultraviolet-based ballast water treatment

The Ballast Water Management Convention (BWMC) establishes limits for viable organisms in discharged ballast water. However, organisms smaller than 10 μm are not considered in this regulation although they represent, in some regions, the majority of the phytoplankton organisms in marine water. The objective in this study is to assess three photosynthetic species smaller than 10 μm as potential standard test organism (STO) in experimentation focused on the inactivating efficacy of ultraviolet treatments (UV). A growth modelling method was employed to determine the reduction of the viable cell concentration under either light or dark post-treatment conditions to evaluate the importance of the photoreactivation. In spite of its moderate growth rate, the high UV resistance in combination with the abundance and worldwide distribution of Synechococcus sp. and the environmental importance of this species constitute important reasons for considering Synechococcus sp. as a valuable STO for ballast water treatment

On the Efficacy of H2O2 or S2O82- at Promoting the Inactivation of a Consortium of Cyanobacteria and Bacteria in Algae-Laden Water

Harmful algal blooms in coastal areas can significantly impact a water source. Microorganisms such as cyanobacteria and associated pathogenic bacteria may endanger an ecosystem and human health by causing significant eco-hazards. This study assesses the efficacy of two different reagents, H2O2 and S2O82-, as (pre-)treatment options for algae-laden waters. Anabaena sp. and Vibrio alginolyticus have been selected as target microorganisms. With the objective of activating H2O2 or S2O82-, additional experiments have been performed with the presence of small amounts of iron (18 mu mol/L). For the cyanobacterial case, H2O2-based processes demonstrate greater efficiency over that of S2O82-, as Anabaena sp. is particularly affected by H2O2, for which >90% of growth inhibition has been achieved with 0.088 mmol/L of H2O2 (at 72 h of exposure). The response of Anabaena sp. as a co-culture with V alginolyticus implies the use of major H2O2 amounts for its inactivation (0.29 mmol/L of H2O2), while the effects of H2O2/Fe(II) suggests an improvement of similar to 60% compared to single H2O2. These H2O2 doses are not sufficient for preventing the regrowth of V alginolyticus after 24 h. The effects of S2O82- (+ Fe(II)) are moderate, reaching maximum inhibition growth of similar to 50% for Anabaena sp. at seven days of exposure. Nevertheless, doses of 3 mmol/L of S2O82- can prevent the regrowth of V alginolyticus. These findings have implications for the mitigation of HABs but also for the associated bacteria that threaten many coastal ecosystems

UV-LEDs combined with persulfate salts as a method to inactivate microalgae in ballast water

The Ballast Water Management Convention (BWMC) establishes limits for viable organisms in discharged ballast water. UV-based ballast water management systems (BMWS) are among the most common, especially those with low pressure (LP) and medium pressure (MP) mercury lamps. An interesting alternative to mercury lamps could be UV LEDs that have been developing over recent years. UVA, UVB, and UVC LEDs have been tested as a method to inactivate microalgae in ballast water. For this study, the diatom Phaeodactylum tricornutum was selected as a target organism. Comparing the D2 (dose required to achieve two log reductions) for P. tricornutum from different UV treatments, it was observed that UVC LEDs were 74.2 % more efficient than UVB LEDs and, compared with previous studies, 48.1 % more efficient than UVC LP mercury lamps. If a five day dark post-treatment was combined with the UV irradiation to avoid photoreactivation, UVC LEDs were 90 % more efficient than UVB LEDs and, compared with previous studies, 36.8 % more efficient than UVC LP mercury lamps. No damage with or without photoreactivation was caused by UVA irradiation with doses up to 4·104 mJ cm−2. The combination of peroxymonosulfate (PMS) with UVA, UVB and UVC LEDs did not significantly increase the inactivation, and the combination of the peroxydisulfate (PDS) with UVC LEDs slightly decreases the inactivation compared with UVC irradiation alone. In conclusion, UVC LEDs were the most efficient for inactivating P. tricornutum, and the combination of PMS and PDS with UV LEDs did not notably improve it. © 2022 The Author

Semicontinuous and batch ozonation combined with peroxymonosulfate for inactivation of microalgae in ballast water.

The Ballast Water Management Convention (BWMC) establishes limits regarding the permissible number of viable organisms in discharged ballast water. Ozone as a ballast water treatment is interesting because it can be generated in-situ and has strong oxidant power. Additionally, some oxidants can be formed in reaction with seawater, especially brominated compounds, that assist in inactivating microorganisms. The objective of this study is to assess the efficacy of semicontinuous and batch ozonation as well as their combination with peroxymonosulfate salt (PMS) as methods to be used to ensure compliance with regulation D2 of the BWMC using Tetraselmis suecica as a standard microorganism. Growth modeling method was employed to determine the inactivation achieved by the treatments. The results show that ozone is an effective treatment for accomplishing the D2 of the BWMC. Batch ozonation is more efficient than semicontinuous ozonation probably because of the brominated compounds formed during the ozone saturation of the water. The oxidants that are developed during the ozonation of seawater prolong the residual effect of the treatment throughout the days of storage with practically no presence of them in the ballast tanks at 72 h. The addition of the PMS increases the inactivation in the semicontinuous ozonation, but a threshold concentration of ozone is needed to observe the synergistic effect of both oxidants. No increase is associated with the combination of O3 and PMS in the case of batch ozonation

On the Efficacy of H2O2 or S2O82− at Promoting the Inactivation of a Consortium of Cyanobacteria and Bacteria in Algae-Laden Water

Harmful algal blooms in coastal areas can significantly impact a water source. Microorganisms such as cyanobacteria and associated pathogenic bacteria may endanger an ecosystem and human health by causing significant eco-hazards. This study assesses the efficacy of two different reagents, H2O2 and S2O82−, as (pre-)treatment options for algae-laden waters. Anabaena sp. and Vibrio alginolyticus have been selected as target microorganisms. With the objective of activating H2O2 or S2O82−, additional experiments have been performed with the presence of small amounts of iron (18 µmol/L). For the cyanobacterial case, H2O2-based processes demonstrate greater efficiency over that of S2O82−, as Anabaena sp. is particularly affected by H2O2, for which >90% of growth inhibition has been achieved with 0.088 mmol/L of H2O2 (at 72 h of exposure). The response of Anabaena sp. as a co-culture with V. alginolyticus implies the use of major H2O2 amounts for its inactivation (0.29 mmol/L of H2O2), while the effects of H2O2/Fe(II) suggests an improvement of ~60% compared to single H2O2. These H2O2 doses are not sufficient for preventing the regrowth of V. alginolyticus after 24 h. The effects of S2O82− (+ Fe(II)) are moderate, reaching maximum inhibition growth of ~50% for Anabaena sp. at seven days of exposure. Nevertheless, doses of 3 mmol/L of S2O82− can prevent the regrowth of V. alginolyticus. These findings have implications for the mitigation of HABs but also for the associated bacteria that threaten many coastal ecosystems

Effect of the length of dark storage following ultraviolet irradiation of Tetraselmis suecica and its implications for ballast water management

Meeting the recent biological standards established by the Ballast Water Management Convention requires the application of ballast water treatment systems; ultraviolet irradiation is a frequently used option. However, organisms can repair the damage caused by ultraviolet irradiation primarily with photo-repair mechanisms that are dependent on the availability of light. The objective of this study is to quantify the impact of dark storage following ultraviolet irradiation on the viability of the microalgae Tetraselmis suecica. Results showed that one day of dark storage after ultraviolet irradiation enhanced the inactivation rate by 50% with respect to the absence of dark storage and increased up to the 84% with five days of dark storage. These results are consistent with photorepair, mostly in the first two days, prevented in the dark. The dose required to inactivate a determined ratio of organisms was correlated with the length of the dark post-treatment according to an inverse proportional function. This correlation may help to optimize the operation of ultraviolet ballast water treatment systems. Further, the results show that growth assays can detect organisms that are capable of repair after treatment with UV.This work was supported by the Spanish Ministry of Economy and Competitiveness-FEDER through the R+D AVANTE Project [CTM2014-52116-R]

Improving the microalgae inactivating efficacy of ultraviolet ballast water treatment in combination with hydrogen peroxide or peroxymonosulfate salt

Due to the increasing number of ecosystem invasions with the introduction of exogenous species via ballast water, the International Maritime Organization adopted the Ballast Water Convention (BWMC). The BWMC establishes standards for the concentration of viable organisms in a ballast water discharge. Ultraviolet (UV) irradiation is commonly used for treating ballast water; however, regrowth after UV irradiation and other drawbacks have been reported. In this study, improvement in UV treatment with the addition of hydrogen peroxide or peroxymonosulfate salt was investigated using the microalgae Tetraselmis suecica as the target organism. Results reported that each of these reagents added in a concentration of 10 ppm reduced the concentration of initial cells by more than 96%, increased the UV inactivation rate, and enabled reaching greater level of inactivation with the treatment. These improvements imply a reduction of the UV doses required for a consistent compliance with the BWMC standards.This work has been co-funded by the 2014–2020 ERDF Operational Programme and by the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia (Spain). Projects Ref.: FEDER-UCA18 - 108023 and FEDER-UCA18-105151