The use of biocides in water cooling towers, specially in open recirculating systems to prevent problems arising from the microorganisms is an important subject. With the aim of removing non oxidizing biocides in these towers, a research project was started . During the 16 months period, sodium hypochlorite was injected to the system. Production of sodium hypochlorite in the complex made its selection as a substitute for biocides, logical and cost effective. According to a regular program, three times a day, sodium hypochlorite was injected into the basins of cooling towers so that the extent of free residual chlorine was between 0.3 - 0.6 ppm. The results of microbial and chemical tests during the control period indicate that TBC reduced to less than 30 cell /ml most times(less than allowable level). The existence of SRB was mostly negative.

Corrosion Results & Ryznar Stability Index

The test conducted by using corrosion coupons indicated that corrosion rate was less than 1MPY most times. The highest corrosion rate, 3MPY , belonged to just one plant . Apart from coupon test , Ryznar Stability Index ,RSI, Which determines corrosive or scaling tendency of water was calculated in 13 months. Using the available sources and the conducted studies, the optimal range of RSI was found to be between 5.5 to 6.5, in that case water has neither corrosive nor scaling tendency. Results show that seven out of the eleven plants, the RSI monthly mean index had constantly remained at the optimal level of 5.5-6.5 during the control period. It is important to note that no month's average was within the corrosive or scaling limit. In one plant 92 percent (12 months) of monthly mean indices were inside the appropriate limit.LD plant with a monthly average RSI of 70 percent within the optimal range, had the lowest standing among plants. PP and HD plants, 77 percent of the time were in that range. Generally , considering that every plant achieved more than 70 percent of the suitable level on Ryznar Stability index , also, with a view of coupon test results , it seems that the results of corrosion and scaling control in water cooling towers had been successful, with changing the method of microorganisms control.