Research On Three-phase Flow Characteristics Of Super-high-rate Spiral Anaerobic Bioreactor

Water pollution and energy shortage are two major environmental problems in China. Application of the anaerobic digestion to the wastewater treatment not only control the pollution but also recycle the energy. In order to enhance the technology of anaerobic digestion and promote the engineering of wastewater treatment, we developed the super-high-rate spiral anaerobic bioreactor (SSAB). In this paper, we investigated the three-phase flow characteristics of SSAB, such as energy dissipation, bed expansion, dynamic behaviors of sludge and flow patterns of wastewater, for the optimization of design and operation and accelerating the engineering applcaction of SSAB. The main results were as follows:(1) Revealing the energy dissipation(△E) characteristics in SSAB①We established the△E models in SSAB. The△E model for separation unit was△E3-5= 2.79×10-9u13. The△E models for reaction unit included two different submodels. One was△E2-3=1.1216×10-7u12Vp+0.02798×10-7u13Vp under unfludization state, and the other was△Ef2-3=0.5096×10-7u1Vp under fluidization state. The△E model for water distribution unit was△E1-2=3.06×10-4u13. The predicted values calculated from the△E models in SSAB agreed well with the experimental values. According to the above models, we could optimize and control the△E of the same type of anaerobic bioreactor②We analysed the AE characteristics in SSAB. The AE under gas-liquid-solid three-phase fluidization state was more than△E under liquid-solid two-phase fluidization state for reaction unit. The△E for reaction unit was more than△E for water distribution unit at low superficial liquid velocity. However, the△E for reaction unit was less than△E for water distribution unit at high superficial liquid velocity. The maximum△E values for reaction unit under unfludization state, liquid-solid two-phase fluidization and gas-liquid-solid three-phase fluidization and granular sludge agglomerate state were 1.13×10-4W,4.54×10-4W,12.00×l0-4W and 91.75×l0-4 W, respectively. The maximum△E value for water distribution unit was 18.81×10-4 W. The maximum△E value in SSAB was 110.56×10-4 W, in which the△E for reaction unit accounted for 83.0% and△E for water distribution unit accounted for 17.0%. The△E value for reaction unit was so low as to be neglected. From parametric sensitivity analysis,△E for SSAB was significantly influenced by pp, u1, Vp, ug and dp in turn. The maximum△E value of 110.56×10-4 W was the basic parameter for matching power in SSAB and the parametric sensitivity could provide reference to optimize the operation for SSAB.(2) Presenting the bed expansion characteristics for SSAB①The bed expansion models in SSAB were established. The maximum bed contact time between sludge and liquid model (τmax) under unfludization state wasτmax=(380-186.74u1-098ug0.7)/u1. The bed expansion ratio (E) under fludization state was E= (0.435u10.29-0.38)/(1-0.435u10.29)×100%. The maximum bed sludge content (Vpmax) was Vpmax=7850εs. The minimum fluidization velocity (umf) was umf=ε03dp2 (ρp-ρ1)g/150μ(1-ε0). The minimum transportation velocity (umt) was ust=(1-εs)ut. The predicted values calculated from the△E models in SSAB agreed well with the experimental values. Hence, we could use the aboved models to design and optimize the same type of anaerobic bioreactor.②We investigated the bed expansion characteristics in SSAB. Under nonfluidization state, u was less than 0.45 mm·s-1; E was 0; Vpmax was 4,867 mL, andτmax approached 860 s [with hydraulic retention time (HRT),2,222]. Under fluidization state, u ranged from 0.45 mm·s-1 to 6.88 mm·s-1, and E Vpmax andτmax were 5.28%-255.69%,1368-4559 mL and 104-732 s (with HRT between 145 s and 2,222 s), respectively. Under transportation state, u was larger than 6.88 mm·s-1 and then washout of granular sludge occurred. Under bed fluidization state, E correlated positively with ug and u1, while Vpmax andτmax correlated negatively with ug and u1, and the values for E, Vpmax and Tmax approached 160%,1,860 mL and 104 s, respectively. For E and Vpmax, the sensitivities of ug and u1were close to each other. But forτmax, the sensitivity of u1was more than that of ug.(3) Researching the dynamic behaviors characteristics of sludge in SSAB①We established the physical and mathematical models of sludge dynamic behavior in SSAB. The sludge dynamic behavior could be described as follows:upward transport of sludge resulted from the upgoing wake stream, and downward transport of sludge was caused by the back-mixing stream, which led to the sludge concentration changing along bed height. The ratio of sludge transport efficiency for downflow liquid and upfloat biogas(Kt,n/Kt,n-1) between the upper part and the middle part (△V3-△V2) and the middle part and the lower part (△V2-△V1) in reactor bed were 0.8259 and 0.7511, respectively. The sludge transport efficiencies of upfloat biogas (Kt,n-1) of△V3-△V2 and△V2-△V1, were 0.102-0.315m3/m3 (granular sludge/biogas) and 0.0856-0.2532 m3/m3, respectively.②We analyzed the parametric sensitivity of sludge dynamic behaviors for SSAB. Judged from parametric sensitivity, Kt,n-1 was significantly influenced by spiral angle (a), outer diameters of the spiral zone (R), superficial settling velocity of the sludge (vs) and superficial linear fluid velocity upwards of influent (v1), which correlated positively with vs and v1. Based on the parametric sensitivity analyses, reducing the a and R can optimize the configuration of bioreactor, and shortening the HRT can improve the volume removal efficiency of the bioreactor.(4) Studying the flow patterns characteristics in SSAB.①We studied the flow patterns in SSAB. The back-mixing in SSAB was relatively weak at low loading rate and the flow pattern approached to plug flow (D/uL≤0.2, N→∞). The back-mixing at medium and high loading rate were between plug flow and completely mixed flow (0.35≤D/uL≤0.467, 1.82≤N≤2.71). The back-mixing was relatively strong at super-high loading rate and the flow pattern tended to be completely mixed flow (D/uL≥0.2, N→1).②We studied the flow patterns characteristics in SSAB. The mean value of total dead spaces (Vd,%) in SSAB was 27.99%, in which the dead spaces caused by biomass and hydraulic behaviors accounted for 6.98%and 21.01%, respectively. The relationship among Vh, volumetric hydraulic loading rate(L) and volumetric biogas production rate(G)was:Vh=0.7603L+0.1627G-4.0620 (correlation coefficient R2=0.968), and Vh was greatly influenced by G than by L.③We studied the optimum flow patterns in SSAB. When N≤3.01, SSAB had the optimum flow patterns. Considering both mass transfer and volume efficiency,N can be adjusted by optimizing the configuration and operation parameters of bioreactor in design and operation of bioreactor.