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8/7/2019 AA-V5-I2-Moving-Toward-Sustainable-Watewater-Treatment

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a afe material. At the bottom of oe of the bai, legth

of pipe with mall pargig ozzle blow out air bubble

that trafer oxge ito the ludge to accelerate decom-

poitio of the wate. The decompoitio proce iaother bai i aided b the tirrig actio of floatig

aerator, whoe impeller puh liquid wate together

with oxge bubble dowward toward the bai floor to

dicourage ettlig ad promote a more eve ditributio

of bacterial actio throughout the mixture.

Dow reearcher had to overcome formidable

challege i modelig thee procee. A extremel

wide-ragig dimeioal cale had to be accommodated

i the model becaue large ludge bai are tpicall a

few hudred feet log ad wide ad cotai thouad of

millimeter-ized ga pargig ozzle. Turbulet flow of

ga ad liquid i the WWT procee i highl complex

ad deped o factor uch a the wa bubble

are ejected ito the liquid, oxge ma trafer a

bubble diolve ito the liquid, the ize ad hape of

opherical bubble, ad the biochemical reactio

of oxge i degradig the wate. The umerou floatig

aerator are fairl complex aemblie, a each cotai

a flotatio aembl with a electric motor drivig a

crew-tpe impeller that force liquid dow a draft tube.I the face of thee complexitie, the team had to

complete the tud quickl ad attai reult that met

acceptable level of accurac. Moreover, procedure had

to be tadardized o that egieer ubequetl uig

the approach would ot have to re-ivet the procedure

with each applicatio.

To model the effect of multiple pipe ad ga

pargig ozzle, egieer firt imulated the two-phae

ga-liquid actio of a igle ozzle to determie the ga

ditributio ad the velocit of liquid from bubble diplac-

ig it to the ide ad forcig ome of it upward. Thi mall

domai wa the ued a a bai to repreet the com-

bied behavior profile for the thouad of ozzle i the

etire ludge bai.

I repreetig the floatig aerator, the team avoided

modelig the complex impeller geometr ad itead

ued upplier data o the aerator pumpig capacit ad

draft tube volume to calculate liquid velocit from the tube

for differet cocetratio of bubble i the bai.

Modelig the complex ga-liquid flow i the ludge

ivolved firt determiig bubble ize ad urface area

from experimetal meauremet or etimate baed o

the turbulet eerg diipatio rate. A iterphae matrafer coefficiet (the rate at which oxge i diolved

i the liquid) wa the calculated uig the sherwood cor-

relatio. Baed o thi ma trafer coefficiet, fluid

damic imulatio determied total oxge diolved

alog with the reultig biochemical reactio rate i the

bai. Calculatio for diolved oxge ad biochemical

reactio rate were accomplihed through the Ansys

FLUEnT uer-defied fuctio (UDF) feature that allow

Blue isosurfaces for near-floor liquid velocity of 0.2 feet per second

to prevent sludge solids from settling in the basin (top). Similar flow

can be achieved using only 13 aerators (bottom).

Inlets

Outlet

Simulation shows that the initial position of four inlets to the sludge

basin produce significant areas of insufficient dissolved oxygen, called

edge effects (top). Repositioning the inlets reduces these effects

significantly (bottom). Red indicates high oxygen demand, while blue

indicates low oxygen demand.

Edge effect

NormalizedDOObtainedfromCFD

Normalized DO Obtained from Measurement

1

0.9

0.8

0.7

0.6

0.5

0.40.400 0.500 0.600 0.700 0.800 0.900 1.000

There was good correlation between measurement and fluid dynamics

calculations of dissolved oxygen in the basin.

ChemiCal ProCess

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cutomizatio of the olutio i performig pecific calcu-

latio through DEFInE macro provided for thi purpoe.

All thee variable icludig ga-liquid flow,

turbulece, diolved oxge ad biochemical reactio

kietic were combied ito a large-domai model

consisting of 10 million computational cells representing

the etire ludge bai. subequet compario of

the bai diolved oxge meauremet with fluid

damic calculatio howed good correlatio, thu

validatig the accurac of the approach.

Thi validatio gave egieer cofidece i uig the

model i a wide rage of imulatio that provided iight

ito deig chage for optimizig the performace of the

ludge bai. Uig the fluid damic model, egieer

could chage a few ke parameter to quickl tud

variou alterative cofiguratio i arrivig at the mot

efficiet deig for meetig performace criteria. Ma of

the imulatio ue Ansys FLUEnT feature to createiourface that defie boudarie eparatig regio of

variou oxge level or liquid velocitie, for example.

such dipla are helpful i udertadig the character-

itic of a complex tem udergoig multiple behavior.

Oe area of ivetigatio wa the optimal poitioig

of the floatig aerator to miimize area of low ear-floor

liquid velocitie i which olid are likel to ettle ad

prevet uiform mixig of the ludge. Uig color-coded

iourface dipla of cotat ear-floor liquid velocitie,

researchers focused on three settling velocities (0.1, 0.2

and 0.3 feet per second) for particles ranging in size from

500 microns to 5,000 microns in diameter. The teamimmediatel aw area of low ear-floor liquid velocit

with the baseline uniform arrangement of 15 aerators

poitioed i five row of three uit each. A erie of

iteratio idicated that the mot cot-effective

arragemet to miimize thi low mixig actio wa toelimiate two of the aerator i row cloet to the bai

outlet. This reduction in the number of aerators from 15 to

13 was counter-intuitive, with simulation providing the

directio eeded i modifig the deig.

Using this new arrangement of 13 floating aerators,

egieer ext ivetigated the poitio of ilet pipe,

with three at the ed of the bai oppoite the outlet ad

a fourth o a bai idewall. simulatio howed that

ludge eterig the bai from the igle ide-wall ilet

pipe produced igificat edge effect that i, bad of

iufficiet diolved oxge widig their wa through-

out the bai. Re-poitioig the ilet baed o the

imulatio reult helped avoid thee effect.

Baed o the three-ilet cofiguratio, imulatio

alo idicated that diolved oxge progreed liearl

from the ilet ide of the bai to the outlet ide, with

all aerator ruig at the ame peed. seeig thi

ditributio, egieer cocluded that a erie of diolved

oxge probe could be paced aroud the perimeter of

the bai for WWT operator to mot accuratel meaure

diolved oxge cotet i the bai. such meauremet

are critical to optimal performace of the ludge bai,

allowig the oxge flow rate to be adjuted accordig to

varig oxge demad throughout the bai.

Thi elected project how that WWT performace

ca be improved igificatl through imulatio-baed

optimi-zatio of aerator poitioig ad flow rate, ilet

ludge placemet, ad poitioig of diolved oxge

probe. The tud demotrate that the ue of fluid

damic provide Dow with iight ito the deig ad

operatio of WWT tem ad cotiue to ait the

compa i ogoig iovatio. n

Dissolved oxygen in the horizontal center plane of the basin increases

linearly with the distance from the sludge inlet, so engineers were able

to position dissolved oxygen probes around the perimeter of the basin

to get accurate measurements.

Reducing the flow rate 50 percent for the bottom aerators resulted in a

near-uniform dissolved oxygen concentration throughout the sludge

basin. Red indicates 1 and blue indicates 0.

NormalizedDO

1

0.8

0.6

0.4

0.2

0

ChemiCal ProCess