Research On Characteristics And Control Of Biofouling In Recirculating Cooling Water At Different Nutrient Levels

The quality of industrial recirculating cooling water is characterized by the conditions of relatively high temperature, optimum pH value, abundant dissolved oxygen and nutrients, which are beneficial to the growth of ubiquitous microorganisms and consequent biofouling. Biofouling is a common but serious problem in recirculating cooling systems. It damages equipments through biocorrosion, and causes clogging and increased energy consumption through decreased heat transfer. Essentially, the quality of cooling water, which would fluctuate within a certain range according to the conditions of makeup water, was the major factor determining nature and control of biofouling. So the primary aim of this study was to determine the effects of necessary nutrients, common inorganic ions and suspended particles concentration on chemical composition and activity of problematic biofouling. Then the impacts of bulk heterotrophic bacteria on physicochemical properities of attached biofouling were investigated at three representative nutrient levels. The behavior and growth processes of biofouling were also studied and simulated by mathematical models. A new expression of the generalized logistic equation was proposed to interpret biological population growth data for a better undersanding of the biofouling accumulation and its optimization. Different dominant bacterial communities within the biofouling at three gradient nutrient levels were confirmed by analysis of 16S rDNA sequences of the polymerase chain reaction (PCR) products separated by denaturing gradient gel electrophoresis (DGGE) techniques. By means of specialized gel electrophoresis profile and statistical softwares, the species diversity and similarity between different biofouling could be acquired and compared. In order to minimize the dosage of biocides on the premise of effective biofouling treatment, the inhibitory effects of oxidizing and non-oxidizing biocides on the biofouling at gradient nutrient levels were evaluated combining with scanning electron microscopy (SEM) analysis. Finally, the relationship between recirculating cooling water quality and effective biocidal action time and dosage was established. The following conclusions were drawn from the results obtained.The extracellular polymeric substances (EPS) composition and dehydrogenase activity (DHA) of biofouling would vary with the water quality of recirculating cooling water, among which, the carbon source was found to be the most significant determinant of biofouling development. There was no evident correlation between EPS concentration and DHA of biofouling, which indicated that both removal and killing efficiencies should be concerned when dealing with biofouling. Consequently, based on an overall consideration of EPS and DHA inhibition, the COD, NH4+-N and TP concentrations should be limited to no more than 25mg/L, 5mg/L and 1mg/L respectively. Meanwhile, the CaCO3, Ca2+, Mg2+, Na+ and Fe3+ concentrations should also be kept below 40mg/L, 50mg/L, 25mg/L, 50mg/L and 0.5mg/L respectively.There existed a heterogeneous relation between adherent biofouling and bulk heterotrophic bacteria at different nutrient levels. The growth processes and competitions between attached and bulk bacteria varied greatly even at the same amount of bulk heterotrophic bacteria, which indicated that the nutrient level was an important factor affecting the growth of biofouling bacteria.The simulation results applying general logistic model indicated that both the maximum and specific growth rates of biofouling decreased with increasing nutrient levels. On the other hand, the critical lag phase time and stable growth time of biofouling formation were reduced and prolonged respectively with the elevated COD concentration. However, unstable structure of biofouling during mature phase resulted in the detachment of biofouling easily, which significantly disturbed the fitting accuracy of general logistic growth model. Thus, a piecewise analysis method and consequent parameters were presented with a particular emphasis on the role of biofouling detachment and reattachment under fluidized state. The improved model was demonstrated to be more practical and suitable for biofouling growth prediction throughout the entire process, even in the dynamically fluctuating mature stage.The PCR-DGGE analysis showed that the diversity of biofouling bacteria community decreased with increasing nutrient levels. Furthermore, the resemblance of bacterial community structure in biofouling at abundant nutrient level was demonstrated to significantly differ from the other two samples by Jaccard coefficient and clustering analysis. In addition, 19 main bands in DGGE profile were further selected, cloned and sequenced. Based on similarity matching, all the sequences showed higher than 97% similarity to relative registered DNA sequences of bacteria strains in GenBank. Phylogenetics analysis revealed that there were two distinct groups of preponderant bacteria strains. Although pair groups of them were closely related to each other, they showed different predominant position in biofouling at different nutrient levels.The removal rates of both ClO2 and quaternary ammonium compounds were enhanced for attached biofouling at increasing nutrient levels. However, as for biocidal efficacy, the minimum inhibitory time (MIT) and minimum inhibitory concentration (MIC) of both ClO2 and quaternary ammonium compounds were least needed for biofouling at scarce nutrient level. By SEM observation, differences in biofouling conformation were revealed among the microbial species. The efficacy of the biocides tested varied with not only the initial DHA but also the structure of biofouling formed.