How to measure biofilm thickness?

Please refer to this article "Research on Biofilm Thickness in Three-Phase Biological Fluidized Bed". What projects need to study biofilm thickness?

This post was last edited by cdpulin at 2009-9-8 13:09 Summary of research on biofilm thickness in three-phase biological fluidized beds: Through the industrial-scale three-phase biological fluidized bed test, the inevitable relationship between the biofilm thickness on the carrier surface and traditional parameters such as organic matter removal rate, volume load, and sludge concentration was confirmed. It was confirmed that the biofilm thickness is a key parameter to describe the reactor behavior. It was revealed that the essence of the high treatment efficiency of the three-phase biological fluidized bed is high microbial concentration, and the optimal film thickness was found to be 90 to 110 μm. keywords: Wastewater treatment three-phase biological fluidized bed biofilm thickness removal efficiency Three-phase biological fluidized bed is an emerging sewage aerobic treatment process that was only developed in the 1970s. Compared with other biochemical treatment processes, high treatment efficiency is its most competitive feature. 　　Based on field tests, this article reveals the essence of high treatment efficiency of biological fluidized beds by investigating the inevitable relationship between biofilm thickness (referring to wet biofilm thickness, the same below) and treatment efficiency. 1 Test principle and method When the material and particle size of the carrier are determined, the thickness of the biofilm on the surface of the carrier determines the settling characteristics of the carrier particles in water, thus determining the expansion height of the bed [1]. On the other hand, when the particle size and quantity of the carrier are determined, the thickness of the biofilm determines the concentration of microorganisms in the reactor, and the concentration of microorganisms is closely related to the treatment efficiency. Therefore, biofilm thickness is a key parameter linking the hydrodynamic properties of biological fluidized beds and the kinetic properties of biochemical reactions. In the design, when the water quality and quantity of the sewage are known, it is necessary to determine a suitable biofilm thickness so that it can meet the requirements for treatment efficiency, and then determine the expansion height of the bed. 　　The biofilm grown on the surface of the carrier generally consists of two parts: The part close to the surface of the carrier is called the inert biolayer. This part of the microorganisms has poor activity due to difficulty in obtaining food, and basically does not participate in biochemical reactions. ； Wrapped outside the inert layer is called the active biological layer, and the removal of organic pollutants mainly relies on the microorganisms in this layer. The rate at which the matrix in the liquid phase body enters the active biological layer through the water film and diffuses within this layer directly affects the rate of biochemical reactions, which also affects the treatment efficiency of the fluidized bed. Andrews believes [2] that there is no accumulation of matrix in the biofilm under steady state. He assumes that the biofilm is a flat film and ignores the liquid phase mass transfer resistance, so that D(d2S/dω2)-r'=0 (1) where D - the diffusion coefficient of the matrix in the biofilm S - the concentration of the matrix in the film ω - the distance from the carrier surface r' - the rate at which the biofilm consumes the matrix per unit volume. This leads to the mathematical model of the matrix concentration distribution in the biofilm and the following conclusion: The rate of substrate absorption per unit volume of biofilm first increases and then decreases with the increase of biofilm thickness. There is a maximum value in between. The biofilm thickness corresponding to the maximum absorption rate is the optimal film thickness. 　　Intuitively, when the biofilm thickness is small, all biofilms are active. At this time, an increase in the amount of biofilm will certainly increase the treatment efficiency. When the film thickness increases beyond the optimal film thickness, although the total amount of biofilm is still increasing, the activity decreases rapidly, resulting in a decrease in treatment efficiency. It can be seen that the bigger the biofilm thickness is, the better. In a two-phase biological fluidized bed, the biofilm thickness is generally controlled through specialized membrane removal equipment. Since the film thickness determines the bed expansion height, in actual operation, controlling the bed height achieves the purpose of controlling the film thickness. In a three-phase bed, due to the agitation of bubbles in the reactor, the hydraulic turbulence is violent and the biofilm surface is updated quickly. When the concentration of the incoming water is not very high, special defilming equipment is generally not needed. Instead, a sedimentation zone is set up in the reactor to remove remaining sludge. In this case, the stable biofilm thickness in the bed is usually no greater than the optimal film thickness. The so-called stable biofilm thickness refers to the film thickness when the growth rate of the biofilm is equal to the biofilm reduction rate caused by factors such as endogenous respiration and hydraulic erosion. 　　 In the experiment, raw water with different water quality and quantity was used to obtain different stable film thicknesses to examine the relationship between treatment efficiency and film thickness. The experiments were carried out on an industrial unit with a diameter of 1.4 m and a height of 6.5 m, using jet aeration (Fig. 1). The carrier is quartz sand with a diameter of 0.3 to 0.5 mm and a volume mass of 2.63 g/cm3, and the filling height is 0.6 m. The apparent liquid velocity in the bed is 0.954 cm/s, the apparent gas velocity is 0.42 cm/s, and the return water volume is 14.7 L/s. The change of incoming water quantity and quality is achieved by adjusting the ratio of clean water and sewage. The water quality changes from low concentration to high concentration. http://www.lunwentian * a.com/images_lunwen_free/2007112417494582995.gif 　　 It was found in the experiment that when the amount and quality of the incoming water changed (it took about 10 d), the expansion height of the bed stabilized at a fixed value, indicating that the thickness of the biofilm also reached stability at this time, so the time interval between each test was selected to be about 15 d. 　　 Biofilm thickness is obtained by measuring the diameter of particles using a graduated microscope [3]. Randomly select 50 carriers wrapped with biofilm from the sample, and measure the diameter di of each particle using a microscope. The average particle size is calculated by the following formula: 　　d=Σdi3/Σdi2 (2) and the biofilm thickness is: 　　δw=(dp-dm)/2 (3) In the formula, dp——the average particle size of the carrier wrapped with biofilm dm——the average particle size dp of the quartz sand carrier, dm is measured by the same method. 2 Test data The test results are shown in Table 1. It should be noted that when calculating each parameter in the table, the effective volume of the reactor refers to the bed expansion height HB multiplied by the bed cross-sectional area. The microbial concentration Table 1 Three-phase biological fluidized bed treatment efficiency test date 1995-11-05 1995-11-23 1995-12-07 1995-12-20 Wet biofilm thickness δw (μm) 52 80 115 137 Inlet water flow Q (L/s) 4.23 6.35 6.29 5.08 BOD5 inlet water concentration (mg/L) 47.6 63.9 70.0 77.2 BOD5 outlet water concentration (mg/L) 12.5 18.6 17.6 * * BOD5 removal rate (%) 74 71 75 90 COD inlet water concentration (mg/L) 102 130 140 152 COD effluent concentration (mg/L) 29.3 29.3 52.3 38.1 COD removal rate (%) 71 77 63 75 Dry biofilm thickness δd (μm) 4.50 * * 0 10.5 11.5 Dry biofilm density ρfd (g/cm3) 1.44 1.41 1.46 1.34 Bed expansion height HB (m) 1.85 2.59 3.86 4.80 Microbial concentration X (g/L) 11.4 16.2 17.1 14.1 BOD5 removal rate r (kg/d) 12.8 24.8 28.5 30.6 Volume load Fv 0.395 0.386 0.281 0.294 Reflux ratio R (%) 348 231 234 289 Residence time HRT (min) 11 10 16 24 3 Conclusion ① Three-phase biological fluidized bed is an efficient sewage treatment process. In this experiment, when the sewage flow rate was 4.23-5.08 L/s, the concentration was 47.6-77.2 mgBOD5/L, the removal rate reached 71%-90%, the volume load was 4.14-6.25 kgBOD5/(m3·d), and the residence time was only 10-24 min. 　　② The relationship between pollutant removal rate and film thickness can be seen from Figure 2. When the film thickness δW increases, the reaction rate r also increases. However, when the film thickness increases to about 140 μm, the reaction rate no longer increases. The highest reaction rate occurs when the film thickness is 120 to 140 μm. http://www.lunwentian * a.com/images_lunwen_free/2007112417494536518.gif 　　③ The relationship between volume load and film thickness: When the film thickness δW90 μm, the FV begins to decrease sharply. Although the removal rate r is still increasing at this time, it is obvious that its increase is at the expense of a greater increase in reactor volume. Therefore, when the film thickness is optimal relative to the removal rate (δW=130~150 μm), it is not optimal from an investment perspective. Based on these two aspects, we believe that in the case of the carrier used in this experiment, δW is preferably 90 to 110 μm, and the corresponding r should not exceed 30 kg/d. 　　 In fact, limiting the film thickness also limits the amount and quality of the incoming water. When the incoming water volume is large and the concentration is high, the particle size of the carrier should be increased to reduce the bed height, thereby saving investment. For quartz sand carriers of 0.3 to 0.5 mm, the applicable inlet water volume does not exceed 6.3 L/s (544 m3/d), and the inlet water BOD5 does not exceed 75 mg/L. When the water volume is small, the concentration can be appropriately increased, but r should be used

Contact oxidation method refers to the thickness of the biofilm adsorbed on the filter material in the aerated biological filter. The filter material used is volcanic rock.

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