Research On Plastic Optical Fiber Sensor For Accurate Online Measurement Of Algae Biofilm Thickness

Microalgae can synthesize carbon dioxide and water into organic matter through photosynthesis.At the same time,microalgae can also use the carbon source（such as phenol）in organic wastewater to achieve microalgae biomass production.In addition,microalgae can produce proteins,vitamins,oils,antibiotics and other substances needed in human life.Therefore,microalgae have received extensive attention in research fields such as carbon sequestration,organic wastewater treatment,and biomass energy production.At present,the cultivation methods for microalgae mainly include a suspension method and a biofilm method.Among them,the microalgae biofilm technology has become a research hotspot because of its advantages of convenient biomass harvesting,small floor space and high utilization efficiency of light energy.Although the microalgae biofilm technology has many of the above advantages,its growth and metabolism process is significantly affected by the microalgae biofilm thickness.The reason is that the microalgae biofilm thickness is a key parameter that relates the hydrodynamics and biochemical reaction kinetics of the reactor.The biofilm thickness parameter directly affects the efficiency and quality of microalgae production of nutrients such as protein and fat,and the utilization of light energy and substrate.Efficiency,microalgal biomass production and the efficiency of microalgae in degrading CO₂ or organic pollutants.Therefore,real-time monitoring of microalgae biofilm thickness is of great significance for humans to carry out large-scale cultivation of microalgae biomass energy,coping with future energy crisis,CO₂ fixation and degradation of organic pollutants.At present,methods for measuring the thickness of microalgae biofilms mainly include microscopy,electrochemical methods,and fiber optics.Among them,microscopic and electrochemical methods are difficult to achieve real-time online in situ measurement of microalgae biofilm thickness.Although fiber optic sensors have the advantages of high sensitivity,small size,anti-electromagnetic interference,and biocompatibility,they can be used for the measurement of biofilm thickness.However,it is difficult to accurately measure the thickness of microalgae biofilms by fiber optic sensors.During the growth and metabolism of algae biofilm,not only the biofilm thickness（biofilm volume）will change,but also the liquid phase substrate and product concentration will change;the liquid phase change information will interfere with the sensor output,which makes the sensor output signal difficult to real-time.Reflects changes in biofilm thickness.Therefore,in order to realize real-time online accurate measurement of microalgae biofilm thickness,firstly,a high-sensitivity plastic fiber refractive index sensor is constructed by using plastic optical fiber.Secondly,a high sensitivity phenol solution fiber sensor was developed.Thirdly,a high-sensitivity fiber optic sensor was developed to construct a dual-probe fiber evanescent wave sensor for on-line accurate measurement of microalgae biofilms.One of the probes serves as a bio-thickness sensor probe for responding to biofilm thickness and liquid phase.Concentration change information;another probe as a biofilm thickness reference probe for responding to changes in liquid phase concentration during biofilm growth and metabolism.In order to analyze the double probe fiber sensor to accurately measure the microalgae biofilm thickness,a sensor theoretical analysis model was established.Finally,the built-in dual-probe fiber optic sensor was used to measure the growth process of microalgae biofilm in phenol solution in real time.The main research contents of this paper are as follows:（1）Research on high sensitivity plastic fiber refractive index sensor.The experiment firstly used mechanical and chemical etching methods to remove the cladding or part of the core of the fiber into D-type fiber sensors of different diameters,and explored the difference in sensitivity of fiber-optic sensors of different diameters.Secondly,the surface of the fiber was coated with Canada.Resin and nano-film were used to investigate the effect of different coating thickness on sensor sensitivity.Finally,D-type fiber evanescent wave sensor was used to measure different refractive index of glucose and different concentrations of mercury（II）ions.（2）Research on fiber-optic sensors with high sensitivity phenol solution response.In order to obtain a high-sensitivity response fiber-optic sensor with phenol response,on the basis of（1）,a Canadian resin film is coated on the surface of the D-type fiber sensor,and then TiO₂,TiO₂/SiO₂,Er³⁺:YAlO₃/SiO₂/is further coated on the surface.Three photocatalytic films of TiO₂ were used to investigate the effect of the thickness of three photocatalytic films on the sensitivity of phenol solution response.（3）Based on the above research,a dual probe type D plastic optical fiber evanescent wave biofilm for the on-line detection of microalgae biofilm thickness in phenol solution was constructed for the microalgae biofilm photobioreactor.Thickness sensor and its detection system.The microalgae biofilm thickness sensor is composed of a double probe fiber（sensing arm and reference arm）,and a sensitive area of one sensor is coated with a nuclear pore membrane for separating and also mixing the liquid phase solution and the microalgae cells.Only the concentration-related information in the liquid phase is recorded as the reference arm;the other sensor sensitive area is not processed,and is used to respond to the microalgae biofilm thickness and liquid phase concentration change information,and is recorded as the sensing arm.At the same time,this paper constructs a sensor theoretical model.Finally,the sensor system was used to detect the thickness change information of the microalgae biofilm during the liquid phase in real time.