Estudo da microbiota associada a torres de resfriamento de uma refinaria de petróleo

Abstract

Biofilms are complex communities of microorganisms attached to surfaces. Cooling water systems provide ideal aquatic environment for the microorganism multiplication, increasing the risk of occurrence of microbiologically influenced corrosion, and decreasing the efficiency of a heat exchanger. Therefore, evaluation of biofilms on metal surfaces is essential to understand the corrosion process and to develop alternatives that will minimize this problem. The current study aimed to compare biofilm formation in carbon steel, stainless steel and glass surfaces. Those materials were inserted vertically into water basins in a cooling tower of a petroleum refinery, and removed after different time intervals to quantify total aerobic heterotrophic bacteria, nitrifying bacteria, iron bacteria, Pseudomonas aeruginosa, sulfate-reducing bacteria and fungi. Three stainless steel coupons, carbon steel bio coupons and glass slides were removed from the basin, dip-rinsed in distilled water to remove unattached cells and transferred into flaks containing sterile saline. The flasks were subjected to ultrasonic bath for biofilm removal. The resulting suspensions were serially diluted and inoculated culture media to isolate the microorganisms mentioned before. Samples of water from the water basins also were examined. Also, three coupons and glass slides were utilized to verify the dry biomass of the biofilm. Those materials were too dip-rinsed in distilled water to remove unattached cells and next in methanol to fix the cells, dried in a heater, weighed on an analytical balance, washed, dried and weighed again. The planktonic populations of AHB and P. aeruginosa varied between 1.45x103 and 3x104 CFU/mL. The analysis demonstrated that AHB counts were higher on the carbon steel surface, followed for glass and stainless steel. The materials were rapidly colonized by microorganisms (3 days) The biofilms on surfaces reached steady state at 21 days, except for carbon steel, where microbial populations reached maximum level at 10 days. There was no growth of SRB, and IB and NIT occurrence were punctual and at low levels. The biomass tests demonstrated maximum microbial growth at the 7th day to both materials (0.0006g to glass slides and 0.0003g to coupons), decreasing on the next samples. It could be concluded that biofilms do not attach in the same way in different materials, being necessary the selection of the suitable material to minimize the possibility of biofilm development associated with the operation of cooling tower systems and new methodologies simpler to be used in the industry.