PREDICTING DISSOLVED OXYGEN CONCENTRATION IN A MOBILE BED BIOFILM REACTOR

Transcription

1 IWM 2008 ONFERENE PREDITING DISSOLVED OGEN ONENTRATION IN A MOBILE BED BIOFILM Diana ROBESU 1, rina MOGA 2, Dan ROBESU 3* The paper presents a mathematical mdel fr disslved ygen cncentratin prfiles in mbile bed bilgical reactr. General ygen dispersin equatin was slved numerically. The purpse f the study is t find the ptimum dimensin, psitin f the aeratin pipes and the percentage f the ttal vlume f the tank ccupied by the carrier media fr maintaining the ygen cncentratin in the limits indicated in the literature. The theretical results apprved the reality in the bireactr filled with tap water. Further wrks is nw underway t test this mdel n the pilt installatin. Keywrds: bilgical wastewater treatment, disslved ygen cncentratin, mdeling, simulatin, bifilm 1. Intrductin The chief bjective f the bilgical wastewater treatment stage is the remval f the nn-settleable rganic slids frm the wastewater. Bilgical wastewater prcess is a particularly cmple prcess t which cntribute phenmena f different natures such as physical, chemical, bichemical. Underlying these prcesses is the metablic activity f sme micrrganisms capable t degrade rganic substances, cnverting them t carbn diide and water. The feasibility f the bilgical prcess resides in the pssibility f maintaining the degrading phenmena ver a perid f time in the specific cnditins f the reactr in which the prcess takes place. There are a large variety f technlgies fr bilgical wastewater treatment. Technlgy with MBBR (mbile bed bifilm reactr) was develped by The Nrwegian University f Science and Technlgy and Kaldnes Miljteknlgi A/S f Nrway, [8]. This system cnsists f a reactr vessel cntaining mied liqur suspended slids with specially designed small flating carrier media suspended and kept in cnstant circulatin. A screen is prvided at the utfall end f the reactr t keep media frm clgging the effluent sput r passing ut f the reactr. The media are engineered in a wheel shape and is slightly psitively buyant, allwing a small amunt f water flw (created by adding air t the prcess) t circulate the media thrughut the vessel. Oygen and fd (ammnia and nitrite) gives the bacteria the 1 Assc.Prf., Hydraulics, Hydraulic Machinery and Envirnmental Engineering Dept., University POLITEHNIA f Bucharest, Rmania 2 PhD. Student, Hydraulics, Hydraulic Machinery and Envirnmental Engineering Dept., University POLITEHNIA f Bucharest, Rmania 3 * Prf., Hydraulics, Hydraulic Machinery and Envirnmental Engineering Dept., University POLITEHNIA f Bucharest, Rmania 187

2 means t grw, whilst the Kaldnes media prvides maimum surface area fr the bacteria t clnize and prduce bifilm. This technlgy is used fr new r fr upgrading eisted plants. Als, there are cmpact systems fr wastewater treatment that used this technlgy. Disslved ygen cncentratin is very imprtant in the aerbic bilgical wastewater treatment prcesses. There are a lt f studies in this field, sme f them t predict the aeratin efficiency f varius equipments [5], [6], thers t cntrl the aeratin stage using disslved ygen cncentratin [3]. Sme mdels fr ygen masstransfer are presented in [2], [4] and [7]. This paper prpses a mdel fr determining ygen cncentratin prfiles in the bireactr using the dispersin equatin. The theretical simulatin study takes int accunt the gemetry f a labratry installatin. A custmize prgram, using FlePDE sftware, is written fr numerically slving the dispersin equatin. Varius scenaris is taken int accunt in rder t btain the ptimum dimensin and psitin f the aeratin pipes fr maintaining ygen cncentratin in the limits 5-7 mg/l in the whle bireactr, as the literature indicates, [10]. 2. Basic equatins A parallelepiped shaped bilgical tank with lngitudinal flw and with aeratin frm the bttm is cnsidered (fig. 1). The air is dispersed in the water using pneumatic equipment cnsisting f tw pipes with perfratins, cnnected t a blwer. The fllwing hyptheses are taken int accunt: (I) hydraulically permanent mtin; (II) unidirectinal hrizntal mtin; (III) n statinary system as t mass transfer; (IV) perfect miing in bilgical tank. B z Q H h y l L Fig.1 Bilgical reactr Q- water flwrate; B- width f the reactr; H depth f the reactr; L length f the reactr; h- psitin frm the bttm f the aeratin equipment; l- length f the aeratin pipes The general ygen dispersin equatin is: 188

3 ( u) + ( v) + ( w) = ε ε y t y z y y D ε z + m S(, y, z, t) (1) z z y z where the ygen cncentratin, ε the aial dispersin cefficient, ε y - the transversal dispersin cefficient, ε z the vertical dispersin cefficient, S the cnsumptin f ygen, Dm the mass diffusin cefficient. The upper bar means that the respective quantities are averaged due t the turbulence. Obtaining a cmplete slutin f the Eq.1 seems hpeless, s that the fllwing additinal mdeling hypthesis can be frmulated: - planar mvement prcess (water flws n the O-ais with the cnstant velcity Q u 0 =, Q water flwrate, B wide f the tank, A hrizntal area f the tank; gas BA bubbles flw alng Oy frm nw n standing fr the vertical ais instead f Oz); - the mlecular diffusin is negligible against the turbulent cnvective ne, s the term D m Δ can be drpped; - the terms fr the transversal turbulent dispersin are neglected because their very small values against similar phenmena n O and Oy; -due t upward mvement f air bubbles ccurs a gas lift phenmenn that enhances miing and ygen transfer int water; - in the Eq.1, the cmpnent v f velcity is replaced with the raising velcity f air bubbles, w; - the aial dispersin ccurs due t the multiphase flw alng O ais and upward mvement f gas bubbles; - there is perfect miing f phases. The dispersin equatin in these cnditins becmes: ( u ) + ( w ) = ε S(, y, t) + t y ε y + + y y (2) Since the air is intrduced at the bttm f the tank by the pneumatic equipment, it is cnsidered that ε <ε y. The ygen cncentratin in water is the result f tw prcesses: a) the ygen mass transfer frm the air int the water due t air bubbles mvement; b) the ygen cnsumptin fr the bichemical idatin f the rganic matter. The last right term f the equatin takes int accunt the ygen cncentratin decay because f the cnsumptin reactins. It is cnsidered the first-rder decay term fr the ygen cnsumptin: + t ( u) + ( w ) = ε k y ε y + y y (3) 189

4 3. Numerical integratin results Fr the numerically integratin f the Eq. (3) a custmized prgram is written using FlePDE sftware, [4]. FlePDE is a "scripted finite element mdel builder and numerical slver". It means that frm a script written by the user, FlePDE perfrms the peratins necessary t turn a descriptin f a partial differential equatins system int a finite element mdel, slve the system, and present graphical and tabular utput f the results. The user can edit the script, run the prblem and bserve the utput, then re-edit and re-run repeatedly withut leaving the FlePDE applicatin envirnment. FlePDE has n pre-defined prblem dmain r equatin list. The chice f partial differential equatins is ttally up t the user, [9]. The cnstants in the equatin (3) are determined frm the design relatinship fr a bireactr with gemetrical dimensins H=B=L =2 m and water flwrate Q = 100 m 3 /h: u=003 m/s, w= m/s, ε = m 2 /s ε y =2 m 2 /s, k=2 s -1. Initially the disslved ygen cncentratin in the liquid is =2 mg/l. The pipes fr air dispersin are put n the bttm f the tank and they have rifices f 2 mm diameter, hrizntally distributed at 7 mm. The aeratin prduces medium bubbles which prvide the necessary miing and maintain the disslved ygen level at 5-7 mg/l. Bubbles start t flw upwards frm the bttm f the tank at time zer. A lt f simulatins is made t determine the ptimum length f aeratin pipe and the psitin f the pneumatic equipment s that the ygen cncentratin in the whle vlume f bireactr t be arund 5 7 mg/l. Sme theretical results are presented in fig.2 1 Mbile Bed Bifilm Reactr - ygen prfiles :05:23 10/22/08 FlePDE MBBR ygen var 2: ycle=80 Time= 2500 dt= p2 Ndes=4756 ells=2227 RMS Err= 6.8e-5 Integral= Fig.2. Disslved ygen prfiles fr l=1 m, h=02 m 190

5 Mbile Bed Bifilm Reactr - ygen prfiles :22:05 10/22/08 FlePDE MBBR ygen var 2: ycle=70 Time= dt= p2 Ndes=5939 ells=2780 RMS Err= 7.4e-5 Integral= Fig.3. Disslved ygen prfiles fr l=1.3 m, h=02 m Mbile Bed Bifilm Reactr - ygen prfiles :31:14 10/22/08 FlePDE MBBR ygen var 2: ycle=70 Time= dt= p2 Ndes=6124 ells=2885 RMS Err= 1.e-4 Integral= Fig.4. Disslved ygen prfiles fr l=1.3 m, h=05 m 191

6 Mbile Bed Bifilm Reactr - ygen prfiles :57:36 10/22/08 FlePDE MBBR ygen var 2: ycle=70 Time= dt= p2 Ndes=5029 ells=2370 RMS Err= 9.4e-5 Integral= Fig.5. Disslved ygen prfiles fr l=1 m, h=05 m Mbile Bed Bifilm Reactr - ygen prfiles :53:09 10/22/08 FlePDE MBBR ygen var 2: ycle=10 Time= 0104 dt= e-3 p2 Ndes=5146 ells=2421 RMS Err= 1.5e-4 Integral= Fig.6. Disslved ygen prfiles fr l=1.3 m, h=02 m intermediary time step 192

7 Mbile Bed Bifilm Reactr - ygen prfiles 5 zm(5,f-05,5,5) 13:53:09 10/22/08 FlePDE MBBR ygen var 2: ycle=20 Time= 0658 dt= 0133 p2 Ndes=5142 ells=2419 RMS Err= 1.3e-4 Integral= Fig.7. Disslved ygen prfiles fr l=1 m, h=05 m, zm fr l (5, 1) Fig.8. Disslved ygen prfiles fr h=05 m- transversal sectin f bireactr 193

8 Fig.9. Disslved ygen prfiles fr h=05 m- transversal sectin f bireactr 10% filled with carrier media Fig.1 Disslved ygen prfiles fr h=05 m- transversal sectin f bireactr 40% filled with carrier media 194

9 Frm the simulatin results, fig. 2 7, it culd be bserved that the ptimum length f the aeratin pipes is arund 1.3 m. Fr a smaller length there are znes in bireactr where ygen cncentratin is belw limits. The ptimum distance frm the bttm where the pipes can be placed is 05 m, therwise the ygen cncentratin belw the aeratin pipes is t lw. Fig are shwn that filling the bireactr with carrier media leads t smaller ygen cncentratin near surface as the cncentratin f carrier media increase. As a cnsequence, the air flwrate shuld be increase t, in rder t maintain necessary ygen level in the whle tank. 4. nclusins In this paper the disslved ygen cncentratin prfiles fr varius hyptheses is predicted. Mathematical mdeling f disslved ygen cncentratin allws identifying the znes in bireactr with ygen cncentratin belw the limits and chsing the apprpriate dimensin and psitin fr aeratin pipe which leads t a wellperated tank. Als, the air flw rate supplied by the blwer shuld be fitted with the percentage f the tank ccupied by the carrier media. Further wrks is nw underway t test this mdel n the pilt installatin. The results can be used in the ptimizatin and cntrl f the bilgical wastewater treatment peratin and in the design stage fr chsing the apprpriate dispsal f the aeratin equipment in the tank. R E F E R E N E S 1. Jia-Ming., heng-fu., Oygen Transfer Mdeling f Diffused Aeratin Systems, Eng. hem. Res., 36 (12), pp , Lia.., Lee D. J., Estimatin f ygen transfer rate in sequencing batch reactr, Water Science and Technlgy, Vl. 34, N. 3-4, pp , Lindberg.-F., arlssn B., Estimatin f the respiratin rate and ygen transfer functin utilizing a slw d sensr, Water Science and Technlgy, Vl 33, N 1, pp , Rbescu Diana, Lanyi S., Veresty A., Rbescu D., Mdeling and simulatin f wastewater prcesses (in Rmanian), Technical Publishing Huse, Bucharest, Rdgers M., Zhan in-min, asey A.,Water, Air, & Sil Pllutin, Vlume 151, Numbers 1-4, pp , Wagner M. R., Pöpel H. J., Oygen transfer and aeratin efficiency - influence f diffuser submergence, diffuser density, and blwer type, Water Science and Technlgy, Vl. 38 N. 3, pp. 1 6, Wiesmann U., Su hi I., Dmbrwski E.M., Fundamentals f bilgical wastewater treatment, Wiley-VH,

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