Study On The Pattern And Control Of Water-heat Transformation In Biofiltration System

Volatile organic compounds?VOCs?is another air contaminant apart from particulate matter and sulfur dioxide,because of its carcinogenicity and inducement to atmospheric environment pollution,such as photochemical smog,ozone layer depletion,green-house effect,etc.Research into the new treatment technology of VOCs has been gaining more and more concern with an increasing demand on environment quality as VOCs pollution going worse.Biofiltration,compared with combustion,catalytic oxidation,adsorption,scour and inspissation,is a relative appropriate way to control the low concentration and large flow rate of VOCs gas.An effective control over the temperature and water content of packed bed is the key to develop and stabilize the elimination capacity of biofiltration for dealing with VOCs.This dissertation discusses the pattern of water-heat transformation in biofiltration system using wood chips as a major packing material?peat as a mixture?and toluene as a VOCs model,and as a consequence,an improvement and optimization of current controlling technology of temperature and water content in packed bed on that basis is obtainted.First of all,the contributory factors in temperature and water content of packed bed,Empty Bed Residence Time?EBRT?,toluene concentration,inlet temperature and relative humidity,environmental temperature and packing material composite,etc.,were confirmed through analyzing water-heat transformation path.Furthermore,influences of these factors on BF elimination capacity and variation of temperature and water content had also been taken into consideration.The elimination capacity increased as inlet load grew when inlet load was less than 24 g?m^-3h^-1.The removal efficiency of toluene reached the peak,which was 85%,while inlet temperature lay between 30-35?.The removal efficiency of toluene falled down below 70%,when inlet relative humidity was less than 70% or environmental temperature was lower than 20?.Compared with wood-chips,a lot of peat would not significantly improve the biofilter performance,and a small amount of peat would lead to a reduction of toluene removal efficiency to around 60%.Packed bed temperature changed a little at a low level toluene concentration(<2.0 g?m^-3);EBRT and inlet relative humidity had a greater influence on water content of packed bed,while inlet and environmental temperature had a greater influence on packed bed temperature.A lot of peat stabilized water content and increased press drop;a small amount of peat could lead to packing material decomposition and exothermic reaction.A comparatively high packed bed temperature accelerated bed dehydration(up to 200 g?m^-3h^-1).Second,the spacial water-heat transformation and the interrelation among eliminating capacity,microflora and temperature and water content in packed bed were investigated.Results showed that the difference of water-heat transformation was remarkable in axial space due to the airflow.However,this difference was more random than marked in radial space.Elimination capacity correlated quite closely to bed dehydration.Maximum capacity appears when water content was 2.3 g_water·g^-1_dry-material.The critical water content of wood chips?Point C?was 1.7 g_water·g^-1_dry-material and the water content of completely vaporized free water?Point D?was 1.1 g_water·g^-1_dry-material,respectively.When water content was higher than D,packed bed temperature changed dramatically,and vice versa.At room temperature?20-30??,the toluene metabolic process matched the pseudo first-order model in Monod equation only when water content of packed bed was higher than C and the reaction rate constant k was 23h^-1.Activated sludge used in the experiment was mostly mesophilic bacteria,within which those bacteria prefer wet environment while fungi were endurable in an arid environment.Again,on the basis of discussion about the variation pattern of temperature and water content in packed bed,current water-heat controlling strategies were improved and optimized.The air humidification strategy was suitable for room temperature,and the optimum operating conditions of saturated gas kept inlet temperature at 33? and EBRT at 2.2 minutes.Results demonstrated that an increase in inlet temperature would extend leachate generation while a reduction in EBRT would decrease elimination capacity.On the other side,circulating spray strategy was more appropriate for low temperature?<20 ^??,and the optimum operating conditions kept circulating spray rate at 0.3 L?min^-1,water/nutrient solution temperature at 35?,spray cycle shorter than 24 hours.The experimental result showed that a high water content of packed bed(3.5 g_water?g^-1_dry-material)would not degrade elimination capacity.Finally,a mathematical model has been established to illustrate biofiltration reaction and water-heat transformation,which was mainly used for dehydration process of packed bed under the condition of unsaturated inlet.The Simulated data of water content,temperature and elimination capacity were accordant with experimental data.The model parameters of toluene degradation capacity?jb?and water evaporation rate?Re?which were fitted by experiments and theory reflected the microbial metabolism and evaporation commendably.