Operation And Online Monitoring Of Anaerobic Reactors

Anaerobic process could be used to degrade pollutants and generate energy (hydrogen or methane)simultaneously.It is cost-effective and environmentally friendly,and therefore becomes a favorable technique in the fields of both environmental and energy engineering.However,anaerobic reactors sometimes become unstable after the changes of environmental conditions or operating factors, which limits its application.Therefore,it is important to find out appropriate ways to monitor anaerobic reactors.In the present study,the technical aspects of fermentative biohydrogen production were investigated with an upflow anaerobic sludge blanket(UASB) reactor.The main operating parameters were optimized,and the thermodynamic analysis of biohydrogen production process was also performed.To monitor methanogenic reactors,a system for determining the liquor products and another for gas products were developed.The factors to cause the running errors of the monitoring system were investigated systematically and in depth.Main contents and results of the present work are as follows:1.The influences of pH,one of the main operating parameters,on biohydrogen production in a UASB reactor were studied.As the pH was increased from 6.1 to 9.5, the main aqueous products were butyrate and acetate,while the concentrations of caproate and valerate were significant at several pH levels.The maximum H₂ partial pressure was observed at pH 7.5,while both maximum H₂ production rate and H₂ yield were found at pH 7.0 and 9.0.This indicates that some microbes in the reactor preferred basic culture conditions.With a thermodynamic analysis,possible metabolic pathways for the formation of caproate and valerate were proposed.Results indicate that H₂ might be a reactant and could be consumed in the formation of valerate and caproate.2.In a similar way,the influences of hydraulic retention time(HRT),another important operating parameter,on biohydrogen production were also evaluated.After the HRT was decreased from 30h to 3h,the H₂ pressure changed slightly,while the max H₂ yield was found at an HRT 30h.The main aqueous product was butyrate,and the substrate degradation efficiency kept above 97%.The experimental results demonstrated that the influence of HRT on biohydrogen production was not significant.3.After the substrate was shifted from 10g/L sucrose to 10g COD/L lactose, both H₂ partial pressure and H₂ yield dropped.The caproate concentration decreased, while the acetate and propionate concentrations increased.Acetate became the main aqueous product,but the H₂ partial pressure and H₂ yield dropped to 0.07 atm and 0.008 mol-H₂/mol-sugar,respectively,which indicates that the homoacetogenic bacteria might exist in the reactor.4.Based on single chip microcomputer,amplifier and data acquisition chip,a circuit board was designed and printed to acquisition pH signals and control titration actions.With a stepping motor,burette and precision linear stage,a titration apparatus was fabricated to dose titrating solution quantitatively.A user interface program was compiled for controlling the titration process,and an online titration system to measure VFA and carbonate was assembled with the three parts above.This system had a good precision and repeatability,and was used to verify the 2-step and 5-point titration methods.The VFA titration results had a close correlation with those from the GC method.A 6-point titration model was developed according to the acid and alkali equilibrium theory.This model was used to measure VFA,bicarbonate and phosphate.The 6-point titration model was verified by the titration system,and had achieved satisfying results with the measurement of VFA and bicarbonate.5,With Infrared methane/carbon dioxide sensors,data acquisition unit and programming with Visual Basic language,an online CH₄/CO₂ monitoring system for anaerobic reactors was developed.This system was applied to a methanogenic UASB reactor.The monitoring data show that this monitoring system could record and display the gas partial pressure in situ.It was appropriate for biogas monitoring with a good precision. 6.The VFA titration system and CO₂/CH₄ online monitoring system were integrated to monitor a lab-scale UASB methanogenic reactor.The COD concentration of the influent and the hydraulic retention time(HRT)were changed respectively and the reactor responses were monitored.After the influent COD reached 25g/L,the VFA concentration increased gradually and finally the reactor failed.The increase in VFA concentration from 96 mg/L to 1505 mg/L was recorded by the titration system before the effluent pH and the CH₄ partial pressure decreased suddenly,which demonstrates that the VFA titration system worked well and could give an earlier warning of the VFA-increasing risk.Fluorescence of coenzyme F₄₂₀ could also be used for predicting acidification of the reactor.The UASB reactor had a low VFA concentration and stable CH₄/CO₂ partial pressure when the HRT was changed from 41h to 8h,indicating that the reactor had a better adaptability to an HRT shock than to a COD concentration shock.