| It is one of key measures using municipal reclaimed water as makeup water of circulating cooling water to alleviate the urban water crisis in China. Compared with the natural water, a larger number of microorganisms and more abundant nutrients lie in municipal reclaimed water. Thus, circulating cooling water system makeup by municipal reclaimed water faces more complex microbial problems.Water quality, microbial characteristic, microbiologically induced corrosion (MIC) and microbial control technology were researched comprehensively and systematically by DNA technology, electrochemical method, scanning electron microscopy and conventional water quality analysis method in laboratory and on-site for power recycling cooling water system makeup by municipal reclaimed water.Main conclusions were as follows:(1) Based on water quality analysis, conductivity, total dissolved solid (TDS), chloride and surfactant in municipal reclaimed water were2to4times than those in surface water. CODMn, total phosphorus, dissolved total phosphorus, soluble orthophosphate and total nitrogen were all1.5to2times than those in surface water. Total number of bacteria was higher an order magnitude than that in surface water. Results showed that more serious corrosion and higher microbiological risk would be faced when power recycling cooling water was replenished by municipal reclaimed water rather than surface water.(2) Chloride, conductivity, hardness and sulfate increased linearly, turbidity increased slightly and iron ion kept unchanged with the concentration factor rose in the power circulating cooling water system makeup by municipal reclaimed water. More crystallization fouling, particulate fouling and sediment would be occurred and more serious corrosion would be happened in the system. At the same time, CODMn increased and ammonia nitrogen decreased with the concentration factor growth, which meant that metabolic rates of nitrification and nitrosification bacteria were higher than that of heterotrophic bacteria. Hardness, turbidity and orthophosphate in water decreased when temperature increased, showing that crystallization and particulate fouling increased in the system. Moreover, with the temperature growth, CODMn declined continuously but ammonia nitrogen tended to stable, which meant nitrification and nitrosification bacteria growth restricted due to decline of alkalinity.(3) Foulant weight in municipal reclaimed water was44%lower than that in surface water. However, total number of bacteria and iron bacteria were8.6times and2.7times higher than those in surface water respectively. These showed that less foulant and more serious MIC would be happened if replacing surface water by municipal reclaimed water to replenish power recycling cooling water. Pitting corrosion in municipal reclaimed water and uniform corrosion in surface water were found by scanning electron microscope.(4) More fouling occurred and thus higher heat transfer resistance and energy loss happened when increasing concentration factor and temperature of the power circulating cooling water system makeup by municipal reclaimed water. Foulant weight was unchanged with the variation of flow rate, but heat transfer resistance and adhersion of foulant on the surface increased. Higher density and stronger adhesion were obtained due to the role of shear, which meant it was difficult to remove foulant by mechanical method. Most unfavorable operating conditions of the cooling water system were the concentration factor4.0, temperature35℃and flow rate0.8m/s～1.0m/s. In addition, Foulant weight and microorganisms would increase40%and80%when scale and corrosion inhibitors were added into the system. High TOC and TP caused by scale and corrosion inhibitors was main reason.(5)There were28kinds of suspended microorganisms detected by DNA technology in municipal reclaimed water, in which dominant strain belonged to Bacteria Proteobacteria Betaproteobacteria Methylophilales Methylophilaceae. For the recycling cooling water system,18kinds of suspended microorganisms and44kinds of fixed microorganisms were detected, in which dominant strain belonged to Bacteria Proteobacteria Betaproteobacteria. After adding chemical agents, suspended microorganisms increased to38kinds and distribution of bacteria species was more evenly. Thus dominant strain changed and belonged to Bacteria Firmicutes Bacilli Bacillales Staphylococcaceae Staphylococcus, which was caused by TOC and TP increased sharply due to scale and corrosion inhibitors.(6) MIC on stainless steel was the biggest though corrosion of carbon steel was most serious in three typical metals of the power circulating cooling water system makeup by municipal reclaimed water. According to average corrosion rate of15days, the proportion of MIC in stainless steel corrosion was36.36%, which was1.6times and2.4times more than the values in carbon steel and brass corrosion respectively. Electron microscope scanning result showed that serious MIC may be caused by biofilm uneven distribution on the surface of stainless steel.(7)Compared with isothiazole morpholinone and bio-dispersant DREWSPERSE738, sterilization efficiencie of chlorine ingot and sodium hypochlorite on suspended and fixed microorganisms were the higher in the power circulating cooling water system makeup by municipal reclaimed water. Chlorine ingot was selected as microbial control agent due to its stronger UV resistant ability in outdoor environment. At the same time, dosing period was determined for3days according to effect of chlorine ingot on total number of bacteria, iron bacteria and sulfate reducing bacteria. Orthogonal test showed that sterilizing rate of chloride ingot increased when dosages of scale and corrosion inhibitor, carbon steel corrosion inhibitor and brass corrosion inhibitor increased. By the dynamic experiment insite, chemical agent control technology was optimized:chlorine150mg/L, scale and corrosion inhibitor8mg/L, brass corrosion inhibitor6mg/L and carbon steel corrosion inhibitor6mg/L. |
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