Open recirculating cooling systems have been vital elements in industry since the early 20th century. Their purpose is to release excess heat from processes through water evaporation so that production can be carried out at optimal temperature. With the global development of industrial activities such as manufacturing, electricity and chemical production, the demand for cooling capacity keeps increasing. Biofouling is one of the main phenomena negatively affecting the performance of wet cooling systems. This phenomenon leads to: (i) loss of heat transfer efficiency, (ii) clogging, (iii) microbiologically influenced corrosion, and (iv) health risk associated to the development of pathogens. Controlling bacterial growth in open recirculating cooling systems is very challenging, and is generally performed via dosage of disinfectants such as sodium hypochlorite. Due to biofouling and the increasing concerns linked to chemical consumption and water discharge, attention has been given to investigate more sustainable approaches to cooling systems operation. Knowledge on the bacterial communities, disinfection impact and biofilm composition is however limited and a more in-depth characterization of biofouling is required to effectively establish new control strategies.
The objectives of this thesis were to contribute to the available knowledge on biofouling in cooling systems and to investigate sustainable and predictable alternative operations, providing results of direct relevance to practice. Attention was given to the identification of factors selecting the microbiome of cooling water subjected to conventional operation, characterization of cooling tower biofilms without disinfection, and assessment of the impact of temperature on biofilm composition. Then, alternative approaches to biofouling control were tested at pilot-scale. Phosphorus depletion was investigated as a solution for limiting biofilm growth and high pH was considered as a Legionella pneumophila control method. These studies were performed with the use of recent analytical methods such as next generation amplicon sequencing (NGS), quantitative polymerase chain reaction (qPCR), and flow-cytometry, which allowed collection of valuable data and a better characterization of biofouling...

Global freshwater demand has significantly increased over the past century and continued growth is expected in the coming century. Since more than 97 percent of the water in the world is seawater, desalination technologies have the potential to solve the fresh water crisis. Currently, the most used desalination technology is reverse osmosis, where a semipermeable membrane is used to separate the salt from the water. The driving force of reverse osmosis desalination is hydraulic pressure, which has to be greater than the osmotic pressure of the seawater. Due to the high hydraulic pressure reverse osmosis has a high energy demand. Lately, hybrid desalination systems, e.g. indirect desalination with forward osmosis combined with low pressure reverse osmosis are getting more importance. Forward osmosis is also a membrane based process that uses the osmotic pressure difference as driving force. One of the main advantages of forward osmosis is the limited amount of external energy requirement compared to reverse osmosis. The major problem of membrane desalination process is fouling, the accumulation of unwanted material on the membrane surface, causing performance decline and increase of costs. Several types of fouling can occur in membrane processes, biofouling (microbial biofilm formation), scaling (mineral salt precipitation), organic fouling (deposition of organic macromolecules) and colloidal fouling (deposition of particulate matter). In practice biofouling is considered as the major problem in membrane systems.