Reverse osmosis (RO) is widely applied to remove salts and other substances from process water. Bacterial growth on RO membranes can greatly reduce the efficiency of this process, so large amounts of chemicals (known as biocides) are dosed in the water, to prevent the proliferation of bacteria. Many biocides are oxidizers, and contact with RO membranes should be avoided.
For this reason, a tradeoff has to be achieved between the efficiency of chemical treatment and the prevention of damages caused by such substances.
In the Figure 1 example, this power plant used reverse osmosis to produce demineralized water for cooling of heat exchangers. Raw water was treated with a biocide, then passed ultrafiltration and was stored in a tank.
RO was then applied. One ALVIM Sensor was installed before RO, in the feedwater line, and another one in the concentrate line. The signal from the two ALVIM Sensors was acquired by the data acquisition system of the plant, along with the other data from the process.
Laboratory analyses (CFU count) were periodically carried out on water samples taken from different points. While this kind of analysis provides a rough indication of the number of free-floating (planktonic) bacteria in the water, the ALVIM Sensor monitors the growth of biofilm.
After the settlement of the first bacteria coming from water bulk, biofilm growth is no longer related to the presence of free-floating bacteria. The microorganisms settle on surfaces, duplicate themselves, growing independently from the planktonic ones.
More than 90 per cent of bacteria live in biofilms, and these structures provide better protection from external agents, including biocides and other chemicals. This makes biofilm up to 1000x more resistant to sanitation than free-floating bacteria.
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