Last Updated 06 Apr 2021

Design Of Baffle In Upflow Anaeorobic Engineering Essay

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The Upflow Anaerobic Sludge Blanket procedure was developed by Lettinga in the Netherland during the early 1980s, as a comparatively simple effluent intervention system, in which no moving parts are present ( Lettinga et al, 1980 ) . It was foremost proposed for the intervention of high strength industrial waste, but shortly research for its application besides within domestic sewerage intervention was initiated. The UASB reactor is now going a popular intervention method for industrial H2O, because of its effectivity in handling high strength effluent. From the Seghezzo et Al. ( 1998 ) , the characteristics which make UASB reactor to be popular:

High efficiency. Availability of farinaceous or woolly sludge, leting it to accomplish high chemical O demand ( COD ) remotion efficiencies without the demand of support stuff. Furthermore, the natural turbulency caused by caused by lifting gas bubbles which buoy the sludge, provides efficient effluent and biomass contact.

Simplicity. The building and operation of the reactor is comparatively simple.

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Flexibility. Anaerobic intervention can easy be applied on either a really big or a really little graduated table. Besides, due to the granulation/blanketing in a UASB reactor, the solids and hydraulic keeping clip can be manipulated independently and efficaciously, therefore allowing the design to be based upon the degradative capacity of the biomass, ensuing in the decrease of intervention times from yearss to hours. ( Hickey et al. 1991 )

Low energy ingestion. Equally far as no warming of the influent is needed to make the on the job temperature and all works operations can be done by gravitation, the energy ingestion of the reactor is about negligible. Furthermore, energy is produced during the procedure in the signifier of methane.

Low sludge production. The sludge production is low, when compared to anaerobic methods, due to the slow growing rate of anaerobiotic bacteriums. The sludge is good stabilized for the concluding disposal and has good dewatering features. It can be preserved for long periods of clip without a important decrease of activities, letting its usage a inoculant for the start-up of new reactors.

Low foods and chemical demand. Particularly in the instance of sewerage, an equal and stable pH can be maintained without the add-on of chemicals.

The UASB has been successfully used in the recent yesteryear to handle a assortment of industrial every bit good as domestic effluent. The applications for this engineering are spread outing to include intervention of chemical and petrochemical industry wastewater, fabric industry effluent, landfill leachates, every bit good as applications directed at transitions in the sulfur rhythm and remotions of metals. Furthermore, in warm climes the UASB construct is besides suited for intervention of domestic effluent.

Problem statement

The design and optimisation of Upflow Anaerobic Sludge Blanket ( UASB ) reactor units required cognition of bio dynamicss, commixture, chemical reaction and else. However, the hydrokinetics within a UASB reactor is a critical importance to the public presentation of the system. Current effluent intervention design methods make premises of the commixture conditions and it is hence hard to foretell how vessel design for illustration, place of recess, baffles or dimensions which could impact hydrokineticss, therefore overall public presentation. Besides, the applications of the experimental techniques to look into flow Fields and mass concentration Fields are highly dearly-won and besides extremely limited in application. Thus, an appropriate factor which is design of baffles had been chosen ; and investigated the influences of hydrokinetics and public presentations of UASB reactor in this survey.

Computational Fluid Dynamics ( CFD ) provides a mathematical method for anticipation of the consequence that effluent intervention procedure design features on the hydrokinetics from a cardinal degree. Progress in CFD have provided an efficient, economical and clip salvaging tool to look into the hydrokinetics and reaction transition happening in a UASB reactor.


The thesis has two chief aims ; foremost is to carry on the public presentation survey of the designed Upflow Anaerobic Sludge Blanket ( UASB ) reactor which can be applied to full graduated table systems. The Computational Fluid Dynamics ( CFD ) theoretical account presented here has the ability to pattern multiple stages ( in this instance the sludge mixture with H2O and air ) . Besides, the CFD theoretical account was developed and applied for the building of existent scale theoretical account.

The 2nd aim is to analyze the effects of baffles to the hydrokineticss for illustration the fluid commixture form, flow field, matching and of the 3 stages ( liquid, solid and air ) of waste H2O intervention procedure. In this survey, the experiment would carry on with befuddled UASB reactor and the other is un-baffled UASB reactor to further analyze the influences to the hydrokineticss and overall public presentations.

Treatment principle of upflow anaerobic sludge blanket ( Uasb ) Reactor

The Upflow Anaerobic Sludge Blanket ( UASB ) procedure consists of an upflow of effluent through a dense sludge bed with high microbic activity. In the reactor, the solids profile varies from really heavy and farinaceous atoms with good settee ability near to the underside ( sludge bed ) , to more spread and light sludge atoms near to the top ( sludge cover ) .

The UASB reactor can be divided into four constituents: sludge bed, sludge cover, gas-solid-liquid centrifuge and secondary compartment above the centrifuge. The sludge bed is situated at the underside of the reactor and consists of a dense sludge with exceeding subsiding features ; it is hence kept in the reactor. Above the sludge bed is the sludge cover, with solid showing lower concentration and settling speeds. The sludge cover consists of sludge atoms in a mixture with the biogas formed, and is therefore held in suspension. It is in these two compartments, the sludge bed and the sludge cover, that the entrance effluent is biologically degraded ( Chernicharo, 2007 ) .

The effluent flows upward in a perpendicular reactor through a cover of granulated sludge and bacteriums in the sludge interrupt down organic affair by anaerobiotic digestion, transforming it into biogas. The biogas-production and the influent flow cause natural turbulency in the reactor, which provides a good wastewater-biomass contact in the UASB reactor system ( Heertjes al. , 1978 ) . The upflow government and the gesture of the gas bubbles allow blending without mechanic aid. To avoid sludge washout, the 3 stage centrifuge is installed in the upper portion of the reactor. The gas formed is separated from the liquid, which allow sludge keeping and return. Above the centrifuge, a gas free zone is formed which is settling compartment, for deposit of solid atoms, and most of the atoms that have entered this zone will settle back to the reactor, whereas the smallest atoms will be washed out with the wastewater ( Angelidaki et al, 2007 ) . Baffles at the top of the reactor allow gases to get away and forestall the escape of the sludge cover.

The 3-phase centrifuge, or the gas-liquid-solid ( GLS ) centrifuge, enables a high keeping capacity of big sums of high-activity biomass in the reactor. Through this characteristic, a solids abode clip ( SRT ) much higher than the hydraulic keeping clip can be achieved. Consequently, the care of high SRT is the major point of involvement in practical application of UASB procedure. This ability to develop and keep high-activity sludge within the reactor is the most of import facet of the UASB construct ( Chernicharo, 2007 ) .

Anaerobic process in the uasb reactor

In UASB reactor, anaerobiotic micro-organisms in the sludge cover digest the organic pollutants in the entrance effluent. Anaerobic digestion produces biogas ( a mixture of methane CH4, C dioxide CO2 and hints gases ) . After some hebdomads of ripening, farinaceous sludge signifiers and this is the chief outstanding feature of UASB reactor named phenomenon of granulation. The formation of granules is really of import because bacteriums in granules are more efficient for biogas production than the flocculated biomass ( Wendland2008 ) .

The anaerobiotic farinaceous sludge consists of microbic communities, with 1000000s of micro-organisms per gm biomass. Normally the granules are grouped dumbly together and hold first-class settling ability. The size wise each granule ranges from 0.1to 5mm. The microstructure of each granule will be dictated by the substrate features of the influent, for simple substrates merely methanogens are needed for complete debasement. For complex substrates, by and large the different bacterial populations will group together selectively in beds on top of each other ( Tiwari et al, 2006 ) .

The farinaceous sludge enables the keeping of a really high figure of micro-organisms in the reactor, which means that a rapid debasement of organic affair can be obtained. In bend, a big volume of waste can be treated within a volume that takes up merely little sum of infinites. Besides, anaerobic sludge has or acquires good deposit belongings, and is automatically assorted by the upflow forces of the entrance effluent and gas bubbles being generated in the reactor. For that ground, mechanical commixture can be omitted from an UASB reactor and therefore cut downing capital and care costs. This commixture besides encourages the formation of sludge granules.

Design considerations of uasb reactor

The UASB reactor can be designed as handbill or rectangular. It is necessary to choose proper scope of operating parametric quantities for design, such as organic lading rate ( OLR ) , SLR, superficial liquid upflow speed and hydraulic keeping clip ( HRT ) .

By and large, there are two ways to plan UASB reactor which are based on HRT or OLR. In the instance of low strength effluent, such as sewerage, it is the HRT instead than the OLR that determines the design method of UASB reactor. In position of the instead low organic tonss that can be applied in the intervention of dilute effluent, and the lower blending ensuing from the gas production, it is evident that more recess points are needed, in comparing with the same reactor under high organic burden rates condition ( Lettinga et al, 1983 ) .

Low strength effluent - Hydraulic Retention Time

For low strength effluent with COD input less than 5000mg/l, the design method should be calculated based on the HRT which can be controlled by volumetric hydraulic burden. It is note that HRT means the step of the mean length of clip that a soluble compound remains in the reactor. Anaerobic digestion depends on the biological activity of comparatively slowly reproducing methanogenic bacteriums. These bacteriums must be given sufficient clip to reproduce, so that they can replace cells loss with the wastewater sludge, and adjust their population size to follow fluctuations in organic burden. If the rate of bacteriums loss from the digester with the wastewater slurry exceeds the growing rate of the bacteriums, the bacterial population in the digester will be `` washed out '' of the system. This washout is avoided by keeping a sufficient HRT for guaranting that the bacterial cells remain in optimum concentration within the digester. The longer a substrate is kept under proper reaction conditions the more complete its debasement will go. However, the reaction rate will diminish with increasing HRT. Thus, the sum of effluent applied day-to-day to the reactor, per unit volume, is termed the volumetric hydraulic burden:

( 2.1 )


VHL = volumetric hydraulic burden ( d-1 )

Q = flow rate ( m3/d )

V = entire volume of reactor ( M3 )

The hydraulic keeping clip ( HRT ) , given in yearss, is expressed as

( 2.2 )

which gives that,

( 2.3 )

For tropical climes and semitropical climes experimental consequences showed that a HRT of six hours was sufficient to accomplish satisfactory consequences in a one compartment UASB. In table 1 nowadayss some guidelines for the constitution of HRTs in design of UASB reactors handling domestic effluent.

Table 1: Applicable Hydraulic detainment clip for natural domestic effluent in a 4m tall UASB reactor at assorted temperature ranges. ( adopted from Lettinga et Al, 1991 )

Sewage temperature ( A°C )

HRT ( H )

Daily Average

Minimum ( during 4 to 6 H )


10 - 14

7 - 9


6 - 9

4 - 6




High strength effluent - Organic burden rate

In the COD input between 5000 - 15000mg/l or more, the design method should be calculated based on OLR. Bacteria have a maximal production rate depending on the type of reactor and substrate. The OLR is one of parametric quantities used to depict this production rate. Bacteria and micro-organisms have their specific growing rate that will accomplish a maximal production rate when they degrade substrate. Therefore, different OLR give different impacts to the reaction rate and efficiency every bit good. By definition, the volumetric OLR is the sum of organic affair applied daily to the reactor, per volume unit:

( 2.4 )


OLR = organic burden rate ( kgCOD/m3d )

S = influent substrate concentration ( kgCOD/m3 )

COD intervention efficiency can be calculated by:

( 2.5 )

For COD concentration in the scope of 2 to 5g/L. the public presentation of the reactor depends on the hydraulic burden rate and is independent of inflowing substrate concentration. For COD concentration greater than 5g/L it is recommended to thin the effluent to about 2g COD/L during primary start up of the reactor. Once the primary start - up of the reactor is over with the granulation of sludge, lading rates can be increased in stairss to convey the existent COD concentration of the effluent. The lading above 1-2kg COD/m3d is indispensable for proper operation of the reactor.

Upflow speed, reactor tallness and volume

Higher upflow speed, favours better selective procedure for the sludge and better commixture in the reactor. However, excessively high upflow speed may do the incolumn acquire washed out during start up. Besides, during normal operation granules may acquire disintegrated and the ensuing fragments can easy be washed out from the reactor. Therefore, design the optimal liquid upflow speeds ensuing favourable for granule growing and good fluid blending with the activated sludge. The upflow speed, V, is calculated from the relation between the inflowing flow rate and the cross subdivision of the reactor:

( 2.6 )


V = upflow speed ( thousand / H )

A = cross sectional country of the reactor ( M2 )

Alternatively, the upflow speed can besides be calculated from the ratio of the tallness and the hydraulic keeping clip:

( 2.7 )


H = tallness of the reactor ( m )

The pick of appropriate tallness of the reactor depends on the needed public presentation and economic considerations. Another of import facet is the place of the underside of the reactor, comparative to anchor degree. Construction costs can be reduced if the reactor underside can be placed at such degree that no pumping system of influent is required. The reactor tallness besides has importance for the efficiency of the organic affair remotion, as the upflow speed must non transcend the bound where the sludge washed out. The upflow speed, and reactor tallness are closely related in Equation 2.7.

Based on the higher suited value of OLR, for given COD concentration, the volume of the reactor required is to be worked out as:

( 2.8 )

The volume of sludge should be less than 50 % of the reactor volume, worked out based on OLR, to avoid overloading of the reactor with regard to SLR. If the volume is non run intoing the demands, the OLR can be reduced to increase the volume.

Influent Distribution System

It is of critical importance that the influent substrate is equally distributed in the lower portion of the reactor. Otherwise a close contact between biomass and substrate can non be obtained. The gas production will ever lend well to the commixture of the sludge bed, and hence the commixture within the digestion compartment will typically be hindered when handling effluent. Poor blending can take to the creative activity of discriminatory tracts through the sludge bed. For illustration, hydraulic short circuits, which in the long term will give a shorter sludge bed height and the formation of dead zones in the sludge bed ( Lettinga et al, 1991 ) . To avoid this job, the influent should be introduced at several points from the reactor underside. A particular influent distribution system can vouch equal distribution over the full reactor surface country. Therefore, the influent so passes a dense and expanded anaerobiotic farinaceous biomass bed and the biological intervention expeditiously.

The figure of distribution pipes needed depend on the country of the cross subdivision of the reactor. Chernicharo ( 2007 ) suggests that Equation 2.9 be used to find the figure distribution pipes:

( 2.9 )


Nx = figure of distribution tubings

A = country of cross subdivision of the reactor ( M2 )

Ax = influence country of each distributer ( M2 )

Fluid Mixing In UASB Reactor

The flow form in the UASB reactor is one of the most of import factors to be considered for design to ease an efficient intervention. The efficiency of all bioprocesses is closely connected with commixture and conveyance phenomena, as an even blending form will supply good conditions for substrate conveyance to and from the microbic sums. Therefore, the transition of organic affair in the UASB reactor is governed by non merely the public presentation of the microbiological procedures, but besides the hydrokineticss of the reactor. However, the behaviour of the UASB procedure is non to the full understood.

The commixture inside a UASB reactor is related to several parametric quantities, such as the type of influent-feeding device, upflow speed and biogas production rate, and different surveies have used different theoretical accounts to depict its hydrokineticss. Heertjes et Al ( 1978 ) assumed the flow to be wholly assorted within the sludge bed and sludge cover, although the sludge bed could besides hold dead infinites and returning flows.

The more accurate theoretical accounts of the UASB fluid mechanicss where late highlighted by both Zeng et Al ( 2005 ) and Lou et Al ( 2006 ) , saying that the bing mathematical theoretical accounts of anaerobiotic digestion in UASB reactors mostly assume ideal commixture, therefore pretermiting concentration gradients. To make a more right theoretical account of the reactor fluid mechanicss, Zeng et Al ( 2005 ) alternatively used a two-compartment theoretical account, with the sludge bed and liquid zones described by a two-zone axially spread system. The survey showed that in a UASB reactor there is a strong dependance of the scattering coefficient on both reactor tallness and upflow speed.

Computational Fluid Dynamics ( CFD )

With visual aspect of general intent codifications, such as FLUENT, CFX and others, Computational Fluid Dynamics ( CFD ) has become progressively popular in environmental engineering. CFD codifications besides can be used to visualise elaborate flow phenomena, a important benefit for the measuring of parametric quantities such as force per unit area, speed, phases volume fraction and else. The work mentioned above chiefly concentrated on using CFD codifications to obtain UASB reactor hydrokineticss informations, therefore doing good suggestions for UASB reactor design and optimisation. The theoretical accounts used were simplified two stages or individual stage systems. Related UASB reactor simulation based on gas-liquid-solid three stage theoretical accounts and flow procedure related reaction dynamicss theoretical accounts widely studied. For the first clip, the focal point lies on set uping hydrodynamics-reaction dynamicss coupled theoretical account of a gas-liquid-solid three stage waste H2O intervention system utilizing CFD simulation followed by experimental confirmation in this paper.

Although UASB reactor has been used in environmental engineering applications for many old ages, lithe research has been published on UASB reactor mold. The chief aims of this survey are to develop an easy to utilize of CFD theoretical account of the important procedure parametric quantities, based on cardinal scientific discipline and to formalize the theoretical account by usage of experiment consequences. Due to non much researching on baffled UASB reactor, our conjugate theoretical account was applied and validated on a waste H2O intervention procedure and look into the overall public presentation. Once developed and assessed with the all-out test consequences, the theoretical account can be employed to analyse the consequence of waste H2O quality features on the public presentation of the procedure. It is expected that this survey will turn out utile in using UASB engineering.

Computational Fluid Dynamics ( CFD ) simulation

The commercial Computational Fluid Dynamics ( CFD ) codification ANYSYS FLUENT was used to imitate the two and three dimensional flow field before building of the Upflow Anaerobic Sludge Blanket ( UAB ) reactor. A conceptual theoretical account was developed by the package and this proposed CFD theoretical account is composed of the nucleus hydrokinetics theoretical account for the liquid and gaseous stages, and coupled with the sludge. CFD simulation helps to depict flow of the liquid and gas constituents of the multiphase flow. The uninterrupted stage is the effluent and sludge and the spread stage is air or biogas. The premises made for the spread stage are:

  • The bubbles are spherical
  • The bubbles have changeless diameter
  • No hits, coalescency or break-up of bubbles

The gas stage physical belongings for illustration denseness and viscousnes were air belongings. For the liquid stage, the denseness was considered to be that of H2O, while in footings of viscousnes.

Eulerian-Eulerian theoretical account

This theoretical account is help to work out a set of impulse and continuity equations for each stage. Applications of Eulerian multiphase theoretical account include bubble column, risers, atom suspensions and fluidized beds ( Saurel and Abgrall 1999 ; Mathisesen et al.2000 ) . In this survey, two dimensional Eulerian-Eulerian three stage fluid theoretical account has been employed to depict the flow behaviour of each stage, so the biogas, effluent and sludge granules are wholly treated as different continua, with effluent as a primary stage, and the gas and sludge granules as the secondary stages. This theoretical account was chosen because of the high proportion of gas bubbles and granules particulates ( Bin et al. 2003 ) .

Species Transport and Reaction theoretical account

CFD codifications can pattern the commixture and conveyance of chemical species by work outing preservation equations depicting convection, diffusion and reaction beginnings for each constituent species ( Sivertsen and Djilali, 2005 ) . Multiple coincident chemical reactions can be modeled, with reactions happening in the majority stage ( volumetric reaction ) , on inside wall of the reactor or atom surfaces.

Numeric solution

The complete geometry of the UASB reactor have analyzed by a computational planar mesh. For efficiency usage of computational clip, simulation of the UASB reactor exploits the symmetric geometry of the reactor in a planar surface. The meshes were created in the ANYSYS Fluent as a preprocessor plan and exported into the ANYSYS Fluent CFD flow patterning package bundle to work out the continuity and impulse equations.

In Eulerian-Eulerian theoretical account, each stage was assumed incompressible. The effluent was regarded as assorted liquid, ab initio incorporating pure H2O and some chemical wastes and the denseness was determined by utilizing a volume weighted mixing jurisprudence. The sludge granules took up approximately 35 % of the volume in the bed part and were considered to be 1mm diameter spherical solid granules. The biogas was assumed to hold a denseness by the incompressible-ideal-gas jurisprudence ( FLUENT 6.0 Users ' usher, 2001 ) . The gas stage volume fraction was related to gas production in reaction and the gas bubbles were assumed to hold a diameter of 0.1 millimeter.

The simulation consequences vary small with grid denseness so truncation mistakes in the numerical simulation can be neglected. An analysis independent of the grid was performed to extinguish mistakes in simulation truth, numerical stableness, convergence and computational stairss related to grid saltiness ( Ait-Ali-Yahia et al.2002 ; Lu et al,2009 ) .

Experimental Design

The research is conduct with two different types of UASB reactor theoretical account which are baffled and un-baffled to further analyze the influences of fluid blending form in the reactor. Figure 1 shows the conventional diagram with dimensions of 1m ten 0.2m.

One of the UASB reactors is baffled and the other one is un-baffled. Both of these reactors operated with the same hydraulic keeping clip, organic burden and composing of waste H2O which are the changeless variables in this experiment. The differences of the chemical waste substances removal efficiency between both reactors would be the consequence of this survey.

Composite sample of the reactors influent and wastewater were collected on a day-to-day footing and analyzed for COD, BOD, sulphate and others chemical waste substances. Sludge sampling was carried out through side ports in the sludge zone of the reactor. The flow rate was control by the valve and continuously regulated by a pump.

3.2.1 BOD trial

Biochemical O demand ( BOD ) is the sum of dissolved O by aerophilic biological beings in a organic structure of H2O to breakdown organic stuff nowadays in a given H2O sample at certain temperature. BOD besides can be used as gage of the effectivity of UASB reactor. The process of the BOD trial:

The dilution H2O was prepared by 1ml each of phosphate buffer, Mg sulphate, Ca chloride, ferrous chloride solution into 1L distilled H2O.

1ml effluent sample was added into a 500ml beaker.

Dilution H2O was added up to 300ml into same beaker.

The pH value was adjusted to 6.5 to 7.5 by added acid or base.

300ml dilution H2O was prepared as control.

All prepared samples and control put in 300ml-incubation bottle.

The DO for each sample was measured by utilizing Dissolved Oxygen Meter.

All the bottles put in BOD brooder for 5 yearss. The temperature was set at 20A°C.

The BOD5 was calculated harmonizing to the expression below:


D1 = DO value in initial sample

D2 = DO value in concluding sample

P = denary volumetric fraction of sample used

Or ;

Dilution factor = Bottle volume ( 300ml ) / sample volume

COD trial

The chemical Oxygen Demand ( COD ) trial measured the O equivalent consumed by organic affair in a sample during strong chemical oxidization. It can assist to foretell the O demands of the wastewater and is used for monitoring and control of discharges, and for accessing reactor public presentations. The trial method:

The effluent sample was oxidized by digesting in a certain reaction tubing with sulfuric acid and K bichromate in the presences of a Ag sulfate accelerator for 2 hours at a temperature of 150A°C.

The sum of bichromate reduced is relative to the COD.

A reagent space was prepared for each batch of tubings in order to counterbalance for the oxygen demand of the reagent itself.

Over the scope of the trial a series of colours from xanthous through green to blue are produced. The colour is declarative of the chemical O demand and its measured by utilizing photometer.

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