Optimization on Microbial Anti-Scaling and Corrosion Inhibition Technology for Low Alkalinity and Low Hardness Circulating Cooling Water in Thermal Power Plants

This paper analyzed the potential effectiveness of microbial anti-scaling and corrosion inhibition technology for low alkalinity and low hardness circulating cooling water in thermal power plants and used response surface models to study the effects and optimal conditions of key technical parameters. The results showed that when the nutrient solution dosage, bacterial agent dosage and aeration rate were set at 0.1-0.3 ‰, 0.1-0.5 ‰, and 1-3 L/min, respectively, the calcium hardness, corrosion rate of brass and stainless steel 304 in the water could be controlled within 9.77-15.30 mmol/L, 0.77-6.58 μm/a, and 1.20-4.50 μm/a, respectively. The variance analysis results indicated that the response surface linear model was the most suitable for evaluating the effects of key operation parameters on the application efficiency of the technology. The obtained model furtherly revealed that the nutrient solution dosage and bacterial agent dosage were extremely significant explanatory variable factors affecting the application of microbial technology in inhibition and corrosion control (P < 0.01). Still, the aeration rate did not reach a significant effect (P > 0.05). Based on the regression equation of actual factors, the study further revealed through the hill climbing algorithm that the optimal comprehensive effect of microbial technology in inhibition and corrosion control was achieved when the nutrient solution dosage, bacterial agent dosage, and aeration rate were set at 0.3‰, 0.5‰, and 1 L/min, respectively.


Introduction
The descaling treatment of circulating cooling water is a necessary management link in the operation of thermal power plants.Currently, chemical and physical treatments are the main methods for treating circulating cooling water in thermal power plants.Although both methods have shown certain effects, they face the common challenge of high operating costs, and the chemical method carries the risk of secondary pollution [1][2].In this context, the biological method based on the metabolic processes of microorganisms is gradually gaining attention as a new treatment method in this field.Microbial technology for treating circulating cooling water treats the water as a slightly polluted water body.It utilizes functional microorganisms to address scaling, corrosion, and microbial proliferation issues in the circulating cooling water system [3].In recent years, microbial anti-scaling and anti-corrosion technology has been proven to apply to the treatment of circulating cooling water [4].However, current research mainly focuses on microbial anti-scaling and anti-corrosion technology for high-alkaline and high-hardness thermal power plants circulating cooling water.There is no research on microbial antiscaling and anti-corrosion technology for low-alkaline and low-hardness thermal power plant circulating cooling water.Researching and implementing microbial anti-scaling and anti-corrosion technology before the cooling water reaches a high concentration ratio is more meaningful for extending the service life of thermal power plant equipment.Therefore, based on the previously selected and prepared functional bacterial agents [4], the research team analyzed the potential efficacy of microbial anti-scaling and anti-corrosion technology in treating low-alkaline and low-hardness thermal power plant circulating cooling water.They used a response surface model to study the effects and optimal setting conditions of key technical parameters such as nutrient solution quantity, bacterial agent dosage, and aeration volume, aiming to provide technical support for the further improvement of circulating cooling water treatment in thermal power plants.

Experimental Water
The circulating cooling water used in the experiment was taken from the effluent of the circulating cooling system of Zhangjiakou Power Plant.The water had a calcium hardness of 16.75±0.15mmol/L, total alkalinity of 0.71±0.20 mmol/L, and chloride ion content of 186.85±2.2mg/L.Compared to previous research reports, this water exhibits characteristics of low-alkaline and low-hardness circulating cooling water in thermal power plants [4][5].

Antiscaling Bacterial Agent
The antiscaling bacterial agent mainly consists of Denitrobacillus licheniformis EM1, with a bacterial liquid concentration of approximately 10 -6 cells/mL.The China Center for Type Culture Collection preserves the bacterial strain, with preservation number CCTCC No. M2017678.The nutrient solution composition mainly includes: ammonium chloride 0.1 g/L, sodium sulfate 0.1 g/L, zinc sulfate 0.01 g/L, magnesium sulfate heptahydrate 0.01 g/L, potassium dihydrogen phosphate 0.01 g/L, calcium chloride 0.01 g/L, peptone 0.001g/L, yeast extract 0.001g/L, threonine 0.001 g/L, leucine 0.001 g/L, with pH controlled within 7.0-7.5.

Antiscaling Performance Test.
The experiment used the static antiscaling method to test the antiscaling performance.In this process, researchers first added 5 liters of experimental circulating cooling water to a beaker, then added a quantitative amount of anti-scaling bacterial agent and nutrient solution.An aeration pump and a gas flow meter controlled the aeration volume.The system was then placed in a constant temperature water bath (50±5 °C) for evaporation and concentration, and the system was kept uniform by a stirrer (rotation speed of 150 rpm).To maintain the total volume of circulating water in the experimental system during the experiment, researchers controlled continuous water supply to compensate for evaporation.The antiscaling performance of microbial technology was evaluated by measuring the calcium hardness in the water, and the testing endpoint was set based on the final carbonate hardness testing method.Three replicates were set for each system in the experiment, and the average of the three sets of data was used as the model analysis data.

Corrosion Rate Test.
According to common materials used in thermal power plant equipment, brass and stainless steel 304 were selected as experimental objects.The weight loss method tested the sample corrosion rate [6].Three sample pieces were suspended in each system, and the experiment was repeated three times.The average of the three data sets was used as the model analysis data.

Response Surface Model Analysis
The quantity of nutrient solution, the dosage of the bacterial agent, and aeration volume are key parameters of microbial anti-scaling and anti-corrosion technology.To systematically analyze the impact of the three factors on cooling water treatment, this study, based on the above experimental method, constructed 10 sets of systems through central composite design, thereby testing the influence of different factors on the extreme calcium hardness, brass, and stainless steel 304 corrosion rates.Subsequently, based on the above data, this study will use linear function, 2FI model, second-order model, and third-order model for data fitting.Significance testing (P<0.05) and lack-of-fit testing (P>0.05) will be conducted for various models.The study will analyze the response patterns of the investigated process by selecting the most suitable model.Meanwhile, based on the fitted model, the study will use a hill-climbing algorithm to determine the optimal operating conditions [7].The full factorial experimental design conditions for this paper are shown in table 1.
Table 1.Design information of experimental system parameter factors.

Technical Implementation Performance
The average levels of calcium hardness in the water and the average corrosion rates of brass and stainless steel 304 in the experimental system are shown in figure 1.In the stable stage of each system, the average levels of calcium hardness in water for systems R1-R10 were 15.30 mmol/L,

Regression Model and Statistical Analysis
Based on the experimental system operating parameter settings and the results of different effect indicator measurements, this study found that the fitting of the 2FI, second-order, and third-order models to the data did not reach a significant level (P>0.05).The response surface linear model is the most suitable for evaluating the impact of key operating parameters on the technical application efficiency.The variance analysis (ANOVA) results presented in table 2 indicate that for the three response surface linear models, their fitting effects have reached a significant level (P < 0.05).In addition, the precision and lack-of-fit detection results also meet statistical requirements, i.e., precision values are greater than 4, and lack-of-fit detection results are insignificant (P > 0.05).

Scale Inhibition Efficiency Control Law
The analysis results of the response surface linear model show that the nutrient solution quantity has the most significant impact on the application of microbial technology in scale inhibition, and the bacterial agent dosage is also a highly significant explanatory variable factor affecting the application of microbial technology in scale inhibition (P < 0.01).However, the aeration volume did not achieve a significant effect (P > 0.05).Based on the linear regression model obtained in the study, response contour heat maps for the water calcium hardness level in nutrient solution quantity and bacterial agent dosage were plotted under aeration volumes of 1 L/min, 2 L/min, and 3 L/min conditions, as shown in figure 2. The results indicate that under different aeration conditions, the patterns of the effects of nutrient solution quantity and bacterial agent dosage remain consistent.The water calcium hardness level increases with both, with the effect of nutrient solution quantity slightly greater than that of bacterial agent dosage.The water calcium hardness level reflects the dissolution capacity of calcium ions in water, and a higher value means a higher dissolution capacity of calcium ions in water, suggesting a lower scaling potential for circulating cooling water [1].Therefore, the research results indicate that under the experimental conditions, the nutrient solution quantity and bacterial agent dosage are the effective control factors dominating the scale inhibition efficiency of microbial technology.Gradual additions of nutrient solution quantity and bacterial agent dosage can significantly increase the water calcium hardness level, reducing the potential scaling tendency of circulating cooling water.This phenomenon indirectly indicates that microorganisms dominate this process, consistent with previous research findings [2,3].At the same time, this result further demonstrates that microbial scale inhibition and corrosion inhibition technology can also play a significant scale inhibition role in the treatment of low-alkalinity and low-hardness thermal power plant circulating cooling water.Liu et al. [4] found in the process of using microbial technology to treat high-alkalinity and high-hardness thermal power plant circulating cooling water that the nutrient solution quantity was the only significant explanatory variable affecting the water calcium hardness level (P < 0.05).However, bacterial agent dosage in this experiment also became a key control parameter affecting the technical scale inhibition efficiency (P < 0.05).This may be attributed to two main reasons: firstly, high-alkalinity and highhardness thermal power plant circulating cooling water usually contains higher residual organic and nutrient salt components, leading to higher microbial abundance in such water, making it difficult for the bacterial agent invested in microbial technology to establish a dominant position in the system, reducing the impact of this parameter [5]; secondly, the toxic inhibitory substances for the functional bacterial agent used in this experiment are lower in low-alkalinity and low-hardness thermal power plant circulating cooling water, and the environmental factors are more friendly, such as low free ammonia in low-alkalinity environments, making it difficult to produce significant toxic effects on microorganisms [6].
The research results indicate that the nutrient solution quantity is the dominant factor influencing the scale inhibition efficiency of microbial technology (impact coefficient = 10.21).At the same time, the aeration volume has the smallest impact (impact coefficient = 0.06).Nutrients play a crucial role in the growth and metabolism of microorganisms and are essential to ensure the logarithmic growth phase of microorganisms [7].Since the ammonia nitrogen content is very low in low-alkalinity and low-hardness thermal power plant circulating cooling water used in this experiment, the nutrient solution mainly composed of ammonia nitrogen can provide necessary elements for the synthesis of substances such as proteins and nucleic acids in microorganisms.Therefore, the nutrient solution quantity becomes a key adjusting parameter (impact coefficient = 10.21)dominating the enrichment of functional microorganisms, and its impact is greater than that of bacterial agent dosage (impact coefficient = 8.04).Even if the bacterial agent dosage is high, without the necessary nutrients, the functional bacterial agent cannot achieve high-density enrichment in the system [8].Therefore, although the amount of aeration can provide the necessary dissolved oxygen environmental requirements for the experimental functional bacterial agents, due to the low alkali and low hardness of the thermal power plant circulating cooling water in the low oxygen-consuming substances, the microbial oxygen depletion rate by the atmospheric oxygen enrichment rate of the limitation, resulting in the amount of aeration in the microbial technology scale inhibition efficacy of microorganisms is difficult to play a significant role.However, it is worth noting that aeration measures not only provide dissolved oxygen but also provide the necessary shear force to ensure the stability of system homogeneity in the circulating cooling water [9].Given that the mixing equipment has been used in this experiment to control the uniformity of the system, the experimental results may have underestimated the impact of the aeration parameters on the system, and the operational settings of the equipment and the design of the aeration conditions should be considered comprehensively during the actual application of the technology.

Material Corrosion Inhibition Efficiency Control Law
The analysis results of the response surface linear model show that the influence of the three control factors on the corrosion inhibition efficiency of brass and stainless steel is similar to the law of scale inhibition efficiency.The nutrient solution quantity and bacterial agent dosage remain highly significant explanatory variable factors (P < 0.01), with the nutrient solution quantity having the greatest impact.At the same time, the aeration volume still did not achieve a significant effect (P > 0.05).
Based on the linear regression model obtained in the study, the effects of the other two parameters on the corrosion rates of brass and stainless steel under aeration volumes of 1 L/min, 2 L/min, and 3 L/min conditions are shown in figure 3 and figure 4. The study found that the corrosion rate of brass shows a significant negative correlation with the nutrient solution quantity and bacterial agent dosage.As the nutrient solution quantity and bacterial agent dosage increase, the corrosion rate of brass gradually decreases.However, the regression equation of the linear model reveals that the impact coefficients on the corrosion rate of brass are 14.80 and 5.29, respectively, with the impact of the nutrient solution quantity far greater than that of bacterial agent dosage.The law of the effects of the nutrient solution quantity and bacterial agent dosage on the corrosion inhibition efficiency of stainless steel 304 is consistent with that of brass; that is, with the increase of the nutrient solution quantity and bacterial agent dosage, the corrosion rate of stainless steel 304 gradually decreases.In addition, the study also found that the corrosion rates of brass and stainless steel 304 under different aeration volumes exhibit significant changes in response values for the other two factors.With the increase of the aeration volume, the corrosion rate of brass shows a noticeable increasing trend.Therefore, based on the above results, the study believes that microbial technology can enhance brass's corrosion inhibition efficiency in circulating cooling water.In practical applications, the nutrient solution quantity, bacterial agent dosage, and aeration volume should all be considered as control factors for the corrosion inhibition efficiency of microbial technology.The optimal predicted values are nutrient solution quantity of 0.3 (‰), aeration volume of 1 L/min, and when the bacterial liquid dosage is 0.5 (‰), the corrosion rate of brass is the minimum; when the bacterial liquid dosage is 0.1 (‰), the corrosion rate of stainless steel SS304 is the minimum.Due to their good thermal conductivity and corrosion resistance, brass and stainless steel, 304 are often used as heat exchange materials in the circulating cooling systems of thermal power plants [10].Therefore, this paper conducted an optimization study on microbial scale inhibition and corrosion inhibition technology using these two materials.Pearson two-tailed correlation results show a significant correlation in brass and stainless steel 304 corrosion rates under the differential setting of microbial scale inhibition and corrosion inhibition technology parameters (R = 0.842, P < 0.01).This result indicates that the response patterns of corrosion rates for brass and stainless steel 304 to technical parameters are similar.Literature reviews also indicate that the corrosion of brass and stainless steel in circulating cooling systems of thermal power plants is mainly influenced by inorganic ion scaling, direct corrosive effects of chemical substances, and the effects of microbial acidic metabolic products.Therefore, under corrosion prevention measures, these two materials usually exhibit similar corrosion inhibition responses [11].
Research results on the significant inhibitory effects of nutrient solution quantity and bacterial agent dosage on the corrosion rates of brass and stainless steel 304 suggest that microorganisms do play a role Through linear model regression equations, it is found that compared to the impact of aeration volume on scale inhibition efficiency, this parameter has a significantly increased impact on material corrosion inhibition.This is partly because aeration volume does provide a favorable environment for the enrichment of functional bacterial agents, inhibiting the enrichment of anaerobic acid-producing bacteria.However, it is also crucial to consider the physical effects generated by this measure.Recent studies report that bubbles from aeration can provide a passivation layer for materials, reducing the contact area between corrosion ions and metal surfaces [12].Considering the relatively small impact of aeration volume on corrosion inhibition effects for brass and stainless steel 304, the study suggests that in the microbial scale inhibition and corrosion inhibition technology for treating low-alkalinity and lowhardness circulating cooling water of thermal power plants, the mechanism of action of the aeration volume factor in corrosion inhibition is more related to physical effects.Therefore, in subsequent research, it may be beneficial to further optimize and control the technology from the perspective of aeration volume bubble control.

Optimization of Technical Parameters
According to the experimental set intervals and the application goals of the technology, this paper set the control ranges of nutrient solution quantity, bacterial agent dosage, and aeration volume, which serve as independent variables, to 0.1-0.3‰,0.1-0.5‰,and 1-3 L/min, respectively.The target for solving water calcium hardness was set to maximize, while the expected targets for brass corrosion rate and stainless steel 304 corrosion rate were set to minimize.Numerical optimization research results indicate that when nutrient solution quantity, bacterial agent dosage, and aeration volume are set to 0.3‰, 0.5‰, and 1 L/min, respectively, the comprehensive effect of microbial technology on scale inhibition and corrosion inhibition is optimal (expected utility value = 0.939), representing the best operating conditions for this technology.Under these operating conditions, the predicted values for water calcium hardness level, brass corrosion rate, and stainless steel 304 corrosion rate are 14.62 mmol/L, 1.10 μm/a, and 0.68 μm/a, respectively, with the expected values for brass and stainless steel 304 corrosion rates significantly lower than the relevant national standards in China (< 5 μm/a) [13].Therefore, microbial scale inhibition and corrosion inhibition technology provide an efficient and feasible technical solution for treating low-alkalinity and low-hardness thermal power plant circulating cooling water, and the results of this study can provide theoretical references for optimizing and controlling the technical application process.

Conclusions
(1) In low-alkalinity and low-hardness circulating water, the biological method (0.1‰ nutrient solution + 0.5‰ bacterial strain) has higher total hardness and calcium hardness maximum and limit values than the chemical method (15 mg/L chemical agents), indicating conditions for replacing the chemical method.The research results show that microbial scale and corrosion inhibition technology is suitable for treating low-alkalinity and low-hardness thermal power plant circulating cooling water.Nutrient solution quantity and bacterial agent dosage are highly significant variables affecting technology application effectiveness, with nutrient solution quantity having the greatest impact.Increasing nutrient and bacterial liquid addition can effectively increase the limit calcium hardness, playing a certain scale inhibition role, highlighting the advantages of the pure microbial method in low-alkalinity and low-

Figure 1 .
Figure 1.Implementation efficiency of microbial scale and corrosion inhibition technology.

Table 2 .
Linear model analysis of variance results.
in corrosion inhibition in low-alkalinity and low-hardness circulating cooling water of thermal power plants.Combining the research results on scale inhibition, the study believes that the main mechanisms of microbial technology in corrosion inhibition include both direct and indirect effects: (1) Microbial direct effects manifest in the potential inhibitory effects of functional bacterial agents on locally produced acid-producing microorganisms.The significant negative correlation between bacterial agent dosage and material corrosion rate validates this inference.(2) Microbial indirect effects arise from the scale inhibition effects produced by microorganisms, indirectly reducing the corrosion rates of brass and stainless steel 304.The changes in water calcium hardness level provide effective evidence for the existence of this mechanism.