effects of cationic polymer on start-up and granulation in upflow anaerobic sludge blanket reactors

Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 79:219–228 (online: 2004)DOI: 10.1002/jctb.961

Effects of cationic polymer on start-up andgranulation in upflow anaerobic sludge blanketreactorsYing Wang, Kuan-Yeow Show,∗ Joo-Hwa Tay and Kwee-Hock SimSchool of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Ave, Singapore 639798

Abstract: The upflow anaerobic sludge blanket (UASB) has been used successfully to treat a variety ofindustrial wastewaters. It offers a high degree of organics removal, low sludge production and low energyconsumption, along with energy production in the form of biogas. However, two major drawbacks are itslong start-up period and deficiency of active biogranules for proper functioning of the process. In thisstudy, the influence of a coagulant polymer on start-up, sludge granulation and the associated reactorperformance was evaluated in four laboratory-scale UASB reactors. A control reactor (R1) was operatedwithout added polymer, while the other three reactors, designated R2, R3 and R4, were operated withpolymer concentrations of 5 mg dm−3, 10 mg dm−3 and 20 mg dm−3, respectively. Adding the polymer ata concentration of 20 mg dm−3 markedly reduced the start-up time. The time required to reach stabletreatment at an organic loading rate (OLR) of 4.8 g COD dm−3 d−1 was reduced by more than 36% (R4)as compared with both R1 and R3, and by 46% as compared with R2. R4 was able to handle an OLR of16 g COD dm−3 d−1 after 93 days of operation, while R1, R2 and R3 achieved the same loading rate onlyafter 116, 116 and 109 days respectively. Compared with the control reactor, the start-up time of R4 wasshortened by about 20% at this OLR. Granule characterization indicated that the granules developed inR4 with 20 mg dm−3 polymer exhibited the best settleability and methanogenic activity at all OLRs. Theorganic loading capacities of the reactors were also increased by the addition of polymer. The maximumorganic loading of the control reactor (R1) without added polymer was 19.2 g COD dm−3 d−1, while thethree polymer-assisted reactors attained a marked increase in organic loading of 25.6 g COD dm−3 d−1.Adding the cationic polymer could result in shortening of start-up time and enhancement of granulation,which may in turn lead to improvement in the efficiency of organics removal and loading capacity of theUASB system. 2004 Society of Chemical Industry

Keywords: UASB; granulation; start-up; polymer; granule characteristics

INTRODUCTIONAnaerobic digestion is essentially a process ofmicrobial decomposition of organic matter in asystem devoid of molecular oxygen. Compared withaerobic treatment, it leads to a higher degree ofwastewater stabilization, lower sludge productionand lower energy requirement along with methaneproduction, and is a well-recognized and matureindustrial wastewater treatment technology.

The upflow anaerobic sludge blanket (UASB) pro-cess has become a popular treatment for industrialwastewaters. It exhibits positive features such as highorganic loadings, low energy demand, short hydraulicretention time (HRT) and easy reactor construc-tion. Important parameters affecting the treatmentefficiency of UASB reactors include granulation inthe reactor, the characteristics of the wastewater tobe treated, the selection of inoculum, the influence

of nutrients and several other environmental factors.Among these parameters, the granulation process isbelieved to be the most critical one.1 The success ofthe UASB process relies on the formation of active andsettleable sludge granules, which results from micro-bial self-immobilization and, subsequently, aggregateformation and growth. There is a close correlationbetween the efficiency of a UASB reactor and devel-opment of granular sludge.2 Granulation not onlysignificantly enhances the settleability of biomass lead-ing to effective bacterial retention in the reactor, butalso improves physiological conditions making themfavorable for bacteria and their interactions, especiallysyntrophs in the anaerobic system.3

However, as anaerobic bacteria are slow-growingmicroorganisms, major problems encountered withUASB are the typical long start-up time andspontaneous development of biogranulation. Start-up

∗ Correspondence to: Kuan-Yeow Show, School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Ave,Singapore 639798E-mail: [email protected] or [email protected](Received 22 January 2003; revised version received 18 August 2003; accepted 9 September 2003)Published online 23 January 2004

2004 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2004/$30.00 219

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times for UASB reactors treating sewage sludge areusually several months, and the reduction of start-up time is one of the key parameters to increase thecompetitiveness of high rate anaerobic reactors.4–6

In anaerobic reactors, polymers have been usedeither to immobilize the anaerobic sludge within gelbeads or to reinforce the strength of the already existinggranules, by coating the granule surfaces with a thinlayer of polymer.7,8 This study was initiated to examineaccelerated start-up and enhanced granulation ofbiomass in UASB reactors by this incorporation ofa cationic polymer. The associated effects on thecharacteristics of granules and reactor operation andperformance under the influence of the polymer werealso investigated.

MATERIALS AND METHODSReactor systemFour Plexiglass UASB reactors were used. The internaldiameter of each reactor was 100 mm with a heightof 680 mm and a total effective volume of 4.4 dm3.The reactors were placed in a walk-in temperature-controlled room set at 35 ± 1 ◦C. A refrigerator,set at 4 ◦C, housed the substrate storage tanks toprevent premature degradation. Synthetic substratewas pumped into the reactor inlet by peristaltic pumps(Cole-Parmer, MasterFlex L/S, Vernon Hills, IL,USA). Wet gas meters (Ritter TG 05, UK) wereinstalled to measure the gas production. The biogasproduced from each reactor was first passed through awater trap before being channeled to the gas meter, toavoid severe corrosion and possible damage to the gasmeter. The experimental set-up is shown in Fig 1.

Seed sludgeThe reactors were inoculated with a digestedsludge obtained from an anaerobic digester of theJurong Water Reclamation Plant, Singapore, treating

industrial and domestic sewage. The characteristicsand inorganic composition of the seed sludge areshown in Tables 1 and 2 respectively.

Synthetic substrateA synthetic substrate having the composition listedin Table 3 was used throughout the study. Thechemicals used were of technical grade supplied bySigma-Aldrich, Singapore. Trace components weresimilar to those used for cultivating anaerobic bacteriaby Yan.9 Buffer capacity was provided by sodiumbicarbonate at levels sufficient to maintain the pH inthe system between 6.5 and 7.3. To sustain an active

Table 1. Characteristics of seed sludge

Suspended solids (SS) 72.5 g dm−3

Volatile suspended solids (VSS) 30 g dm−3

Sludge volume index (SVI) 41.2 cm3 (g SS)−1

Specific methanogenic activity(SMA)

0.5 g CH4-COD (g VSS d)−1

Median particle size (diameter) 101 µm

Table 2. Elemental content (mg g−1) in seed sludge

Element Content (mg g−1)

Ca 39.04Al 16.55Mg 3.55Mn 0.43K 2.47Na 4.95Mo 0.075Fe 28.89S 143.67Ni 4.56Zn 21.26Cu 24.45P 79.88

gas meter

biogas

effluent

three-phase separator

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substrate in refrigerator (4°C)

pump

water trap

Figure 1. Schematic diagram of experimental set-up.

220 J Chem Technol Biotechnol 79:219–228 (online: 2004)

Start-up and granulation in UASB reactors

Table 3. Synthetic substrate composition in mg dm−3 (based on COD

of 4000 mg dm−3)

Component Amount (mg dm−3)

Carbon sourcePeptone 800Glucose 2720Meat extract 560

Macro-nutrientsCaCl2.H2O 38.4MgSO4.7H2O 43.2NH4Cl 640FeSO4.7H2O 32KH2PO4 160

Micro-nutrientsH3BO3 0.08ZnCl2 0.08CuCl2 0.08MnSO4.H2O 0.08(NH4)6Mo7O24.4H2O 0.08AlCl3 0.08CoCl2.6H2O 0.08NiCl2 0.08Conc HCl (36%) 1.6 cm3

Alkalinity bufferNaHCO3 2400

microbial culture, a COD: N: P ratio of 100: 10: 1was adopted.10–12

Polymer usedThe cationic polymer solution (A 184 H) was suppliedby courtesy of Nalco Pacific Pte Ltd, Singapore, andhas been used extensively as a coagulant aid in watertreatment systems. Details of its nature were notavailable because of trade confidentiality.

Analytical methodsParticle/granule sizeThe particle/granule size was measured by a laserparticle size analysis system (Malvern Master SizerSeries 2600, Malvern Instruments Inc, Southborough,Massachusetts, USA) or an image analysis system(Quantimet 500 Image Analyzer, Lerca CambridgeInstruments, Cambridge, UK), depending on theparticle size range. The Master Sizer is based onthe principle of laser diffraction and the range ofmeasurable particle size is 0.05–550 µm, whereas theImage Analyzer was used to measure larger particles.

Sludge volume index (SVI)After 30 min settling of the suspended solids, SVI wasmeasured according to the APHA standard method.13

Specific gravity (SG)The SG was determined by the APHA standardmethod.13

Settling velocitySettling velocity was determined according to themethod proposed by Thaveesri et al.14 The granules

were transferred to a 1.25 m high glass cylinder(diameter 4 cm) filled with water. The time needed forhalf of the total biomass volume to settle at the bottomof the cylinder is defined as the static sludge-settlingrate.

Granule strengthThe strength of granules is defined as the capacity ofthe granules to resist disintegration during operation.15

The sample sludge was diluted 10 times with tapwater and 25 cm3 of the diluted sample was takenfor the test. This sample was subjected to abrasionby placing it on a shaker with a circular stroke ofradius 20 mm at 200 rpm for 5 min. It was allowedto settle and the granules settled in a column within1 min were collected for the volatile suspended solids(VSS) test. The VSS of the two portions (ie the settledagitated particles and supernatant with unsettled anddisintegrated particles) of sludge respectively weredetermined. The granule strength is expressed interms of integrity coefficient (%), which is definedas (residual granule VSS: total VSS) ×100. A higherintegrity coefficient implies higher strength of thegranules.

Specific methanogenic activity (SMA)The measurement of SMA followed the method usedby Yan.9 The test was carried out in a temperature-controlled room maintained at 35 ± 1 ◦C. Onehundred cm3 of the UASB reactor effluent was addedto a 250 cm3 Kimax flask (Voigt Global Distribution,Millville, NJ, USA). Then 25–50 cm3 (0.5–1.0 g VSS)of sludge sample was added, followed by 5.0–12.5 cm3

(0.50–1.25 g COD) of the synthetic substrate, and thevolume was made up with the effluent to a predefinedlevel (eg 200 or 250 cm3) for ease of calculation ofVSS. In this study, a predefined volume of 200 cm3

was used. During the preparation and transferring ofthe effluent and sludge sample, special care was takento reduce exposure of both the effluent and sludge toair as much as possible. The initial pH was within therange of 6.5–7.2 in each test.

The flask containing the biomass and culturewas placed on a platform shaker with a horizontalstroke of 25 mm at 20 rpm in order to enhancecontact between the biomass and the culture (Fig 2).The biogas produced was passed through a 2 N

NaOH solution saturated with NaCl in a 500 cm3

bottle to remove CO2. The remaining gas (methane)accumulated in the bottle, and was measuredby an auto-pipette with graduations of 0.1 cm3

through the liquid displacement technique. Basedon the cumulative methane production rate curveover time and a theoretical conversion of CODto CH4 at 395 dm3/(g COD)−1, the SMA of thesludge, expressed as g CH4-COD (g VSS d)−1, wasdetermined based on the steepest gradient plotted.

Experimental procedureTo evaluate the UASB start-up, performance, andcharacteristics of the granules developed when

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Biogas

Platform shakerAutopipette

Waterbath

NaOH

Figure 2. SMA set-up.

different polymer concentrations were used, fourlaboratory-scale UASB reactors designated as R1,R2, R3 and R4 were operated. During start-up,the reactors were all fed at the same rate. R1was started up without adding polymer and servedas a control reactor. El-Mamouni et al16 found anoptimal dose of cationic polymer of 2 mg (g SS)−1

for OLRs increasing from 0.2 to 0.6 g COD(g VSS d)−1. To examine the effects of polymerdose, reactors R2, R3 and R4 were dosed withpolymer concentrations of 5 mg dm−3, 10 mg dm−3

and 20 mg dm−3, respectively. The loading andoperating conditions of the reactors are summarizedin Table 4.

Successful operation of UASB reactors dependson careful and thorough routine monitoring of gasproduction, system pH, alkalinity, COD, volatile fattyacids (VFAs), SS and VSS. The pH, alkalinity, COD,SS and VSS tests were conducted in accordance withAHPA standard methods.13 Samples for VFA testswere obtained after filtration through 0.2 µm cellulosenitrate membrane filters (Whatman, Maidstone,UK) and measured by high-performance liquidchromatography (HPLC) (Perkin Elmer, Series 200,Wellesley, MA, USA). The column used was polyporeH (220 × 4.6 mm) and the detector was UV at 210 nmH2SO4 (0.005 N) was chosen as the mobile phase andthe flow rate was 0.15 cm3 min−1. Gas production ofeach reactor was recorded daily by a wet gas meter(Ritter TG 05). The gas composition was analyzed bya gas chromatograph (Hewlett Packard HP 5890 A,Albertville, MN, USA) for methane, carbon dioxideand nitrogen.

Table 4. Loading and operating conditions

CODloading(g dm−3 d−1)

InfluentCOD conc(mg dm−3)

HRT(h)

0.8 4000 1201.6 4000 603.2 4000 304.8 4000 206.4 4000 159.6 4000 10

12.8 4000 7.516.0 4000 619.2 4000 5

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RESULTS AND DISCUSSIONStart-up timeA major benefit of enhanced granulation is reducedstart-up period to reach a specified organic loading.The OLRs applied versus operation time are plottedin Fig 3. The effect of polymer on reducing start-uptime at OLRs lower than 4.8 g COD dm−3 d−1 wasnot obvious. The operation times required to reachan OLR of 1.6 g COD dm−3 d−1 were similar forall reactors at about 12 days, and about 27 days toreach 3.2 g COD dm−3 d−1 except for reactor R3,which took 36 days to reach the same loading. Thetime required to reach stable operation in R4 at anOLR of 4.8 g COD dm−3 d−1 was reduced by morethan 36% as compared with the control reactor R1,and 46% and 36% as compared with R2 and R3,respectively. At an OLR of 6.4 g COD dm−3 d−1,the time required to reach steady-state performancein R4 was reduced by 27% as compared with R1,and 36% and 27% as compared with R2 and R3,



respectively. It has been reported that 120 days wereneeded to reach steady-state performance at an OLRof 14.4 g COD dm−3 d−1.17 Enhanced with polymer,reactors R2, R3 and R4 were able to reach an OLRof 16 g COD dm−3 d−1 in periods of 116, 109 and93 days respectively, while the control R1 achieved thesame loading rate on day 116. At an OLR of 19.2 gCOD dm−3 d−1, the time required to reach steady-state performance in R3 and R4 was reduced by 7 and24 days respectively as compared with R2. For R2 andR3 with polymer concentrations of 5 and 10 mg dm−3,the start-up time was not shortened as much as inR4. These results indicated that adding 20 mg dm−3

of the cationic polymer could significantly reduce thestart-up time of a UASB reactor.

Reactor performanceThe influent substrate concentration was maintainedat 4000 mg COD dm−3 in all reactors. Steady-stateperformance was considered to be attained when theCOD of the effluent and the biogas production wererelatively constant (within 5%) for several days. Theorganic loading rate was subsequently step-increasedto the next higher rate through shortening of the HRT.

Figure 4 shows the COD removal efficiencies ofall the reactors without experiencing major systemupset. The COD removal efficiencies in all reactorswere between 890 g kg−1 and 950 g kg−1 at OLRslower than 12.8 g COD dm−3 d−1. At an OLR of16 g COD dm−3 d−1, COD removal efficiency inR4 was the highest at 890 g kg−1, compared with850 g kg−1 in R1 and 860 g kg−1 in both R2 and R3.At a higher loading rate of 19.2 g COD dm−3 d−1,the efficiency of R4 of 890 g kg−1 was still higherthan the 730, 820 and 810 g kg−1 in R1, R2 andR3, respectively. The OLRs in the three polymer-assisted reactors were further increased to 25.6 gCOD dm−3 d−1 while R1 was terminated at 19.2 gCOD dm−3 d−1 upon reaching its loading capacity. Atthis OLR, the COD removal efficiency of 750 g kg−1

in R4 remained superior compared with those in

R2 and R3 (710 and 700 g kg−1, respectively). Thehigher COD removal efficiencies in the three polymer-assisted reactors demonstrated that adding cationicpolymer improved the performance though there wasno obvious difference between the COD removals ofreactors R2 and R3. The results also indicated that thepolymer concentration of 20 mg dm−3 appeared to bethe best dose for removal efficiency.

A significant benefit of system enhancement inany anaerobic process is the increased capacity toremove COD. After the OLR reached 19.2 g CODdm−3 d−1 on day 120, the COD removal efficiency ofR1 dropped substantially, from 850 to 730 g kg−1. Thegas production dropped significantly, indicating thatthe 19.2 g COD dm−3 d−1 was the maximal capacityfor R1, while for R2, R3 and R4, the operations werecontinued until they were terminated on days 132, 125and 110 at an OLR of 25.6 g COD dm−3 d−1 when theremoval efficiencies dropped sharply. The increasesof organic loading capacity in the polymer-assistedreactors were likely attributable to the enhancedgranulation caused by the cationic polymer.

Development of granulesThe bioparticle sizes in all the reactors reached theirpeaks at a loading rate of 12.8 g COD dm−3 d−1

(Fig 5).The effect of polymer enhancement on granule

development is shown in Fig 6. The onset ofgranulation was subjectively defined as the formationof bioparticles having diameters larger than 0.5 mm.18

In R4, this occurred on day 50 and for R1, R2 and R3,on days 70, 88 and 70 respectively. The time needed toform granules in R4 was shortened by 29%, 43% and29% as compared with R1, R2 and R3 respectively.Comparing with a study by Yoda et al using ananaerobic expanded micro-carrier bed reactor, whichhas the ability to cultivate granular sludge similarto that formed in the UASB process, granules witha diameter of 2.0 mm were formed 200 days afterstart-up.19 With sufficient cationic polymer, granules

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Figure 4. COD removal efficiency.


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Figure 6. Granule development since start-up.

of similar size were formed much earlier, at around80 days in R4 in this study.

Granule characteristicsStrengthThe strength of the granules was expressed in terms ofthe integrity coefficient (ζ ) defined as (residual granule

VSS: total VSS) ×100, as mentioned above. Althoughthe integrity coefficient does not indicate an absoluteshear strength, it is assumed to represent the relativeresistances of granules to hydraulic abrasion andshear, which granules often undergo during reactoroperation.

The data in Table 5 indicate that, generally, thestrength increased along with granule size and settlingvelocity. Development of granules with adequatemechanical strength is a prerequisite for a successfulUASB operation. It should be noted that thecharacteristics of the granules in the control reactorwere not determined at the highest OLR of 19.2 gCOD dm−3 d−1, since steady-state operation couldnot be attained at the limiting OLR.

Figure 7 shows that the granules in R4 consistentlydemonstrated higher strength than those in R1, R2and R3 at the same OLR. The strengths of granulesin R1, R2, R3 and R4 at 4.8 g COD dm−3 d−1 weremeasured respectively at 70, 78, 78 and 80%. At thehigher OLR of 16 g COD dm−3 d−1, granule strengthincreased slightly to 72, 78, 83 and 88%, respectively.Comparing with R1, the granules developed in R2and R3 also demonstrated higher strengths, but theimprovements were not as significant as in R4. Thegranule strength measured showed slight differencesamong the reactors; it was noticed that it increasedwith polymer addition, especially in R4. But the slightdifferences in granule strength among the polymer-added reactors might not have statistical significance.It can be observed that the granule strength peaked atan OLR of 9.6 g COD dm−3 d−1 for all the reactors.

Settling velocityRetention of a high density of anaerobic activatedsludge can be achieved by the formation of settleableaggregates of microorganisms as granules. Thedevelopment of granules with high settleability andhigh degradation activity for organic substances iscrucial for efficient operation of UASB processes.Granular sludge can be divided into three categoriesbased on the observed settling velocities: poor,satisfactory and good, with settling velocities of upto 20 m h−1, from 20 to 50 m h−1, and over 50 m h−1,



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Table 5. Granule characteristics

OLR (gCOD dm−3 d−1)

Meandiameter

(mm) Strength (%)

Settlingvelocity(m h−1)

R1(control)4.8 0.57 70 28.16.4 1.23 82 44.99.6 1.91 85 50.0

12.8 1.83 81 46.716 1.82 72 39.9

R2 (5 mg dm−3 polymer)4.8 1.21 78 30.86.4 2.07 81 29.69.6 2.07 90 58.2

12.8 2.38 84 42.416 2.05 78 53.719.2 1.86 71 25.1

R3 (10 mg dm−3 polymer)4.8 0.52 78 23.66.4 0.76 85 44.99.6 1.35 90 37.9

12.8 1.94 90 63.416 1.93 83 42.919.2 1.10 72 40.0

R4 (20 mg dm−3 polymer)4.8 0.62 80 40.96.4 1.53 87 48.79.6 1.89 90 51.7

12.8 2.62 90 64.216 1.86 88 45.919.2 1.20 80 49.4

respectively.20 As shown in Table 5, all the granuleslarger than 0.5 mm in all the reactors exhibitedsatisfactory settling velocities of over 20 m h−1, withR4 showing consistently higher settling velocitiesthan R1, R2 and R3. Settling velocities of granulesranging from near 0 to 52 m h−1 were reportedby Blaszczyk et al.21 All the granules examined inthe present study exhibited settling velocities in or

above this range (Table 5). The settling velocities ofgranules correspond well with the results of strengthdeterminations discussed previously. The fact thatgranules in R4 demonstrated both higher settlingvelocity and strength may have resulted in R4 beingless susceptible to biomass wash-out, leading to higherprocess stability.

Specific methanogenic activity (SMA)The metabolic activities of granules can be expressedin terms of SMA. As an important characteristic ofgranular sludge, SMA was measured at each steady-state in all the reactors. Sharp increases of SMA wereobserved in all the reactors immediately after start-up, but reasons for this were not investigated in thisstudy. As shown in Fig 8, the metabolic activities ofgranules in R3 and R4 were higher than those inR1 and R2 at most of the OLRs. This demonstratedthat the addition of cationic polymer enhanced themethanogenic activities. When the OLR exceeded 16 gCOD dm−3 d−1 in all the reactors, the SMA decreased,indicating that methanogenic activity may have beeninhibited by accumulation of fatty acids, although thisis a tentative suggestion since there was no directdetermination of VFAs.

Sludge volume index (SVI)Reduction of SVI, which is an important physi-cal parameter, is generally considered as an indi-cator of improvement in granule settleability. TheSVIs of sludges in the four reactors are shownin Fig 9. SVI values of the control R1 in therange of 21.7–43.32 cm3 (g SS)−1 were generallyhigher than R2 (12.63–35.06 cm3 (g SS)−1), R3(22.25–30.24 cm3 (g SS)−1) and R4 (18.97–37.88cm3 (g SS)−1) for all OLRs, but the differencesbetween the three polymer-assisted reactors were notmarked. The SVI in R1 varied rather randomly withchanges in the COD loading, while that in R2 gen-erally fell steadily but slowly as the COD loading


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increased. In R3 and R4, the SVI fell more sharplyas COD loading increased initially but rose again atthe higher levels of COD loading. Enhanced granulesettleability in R2, R3 and R4 as indicated by theSVI results could be due to improved bacterial adhe-sion caused by addition of the cationic polymer. Thepositively charged polymer may form bridges amongthe negatively charged bacterial cells through elec-trostatic charge attraction. The resulting chain-likestructure may lead to a complex network structure forenhanced bacterial aggregation.22 Settleability as mea-sured by SVI generally deteriorated with increase ofOLR beyond 12.8 g COD dm−3 d−1.The SVI resultscorresponded well with those for settling velocity dis-cussed earlier, indicating better settleability of thegranules in the three polymer-assisted reactors.

Suspended solids and volatile suspended solids (SS/VSS)The SS and VSS of the reactor contents increased withincreasing OLR (Fig 10), indicating a correspondingincrease in the biological solids inventory. The SSand VSS in R1 decreased when the OLR reached12.8 g COD dm−3 d−1. Decreases of SS and VSSwere recorded in R2, R3 and R4 when the OLRexceeded 16 g COD dm−3 d−1. The decreases insolids could be attributed to biomass wash-out tothe effluent streams as discussed in the previoussection. The results indicated that the amount of

biomass wash-out had likely exceeded the amountof biological growth at 12.8 g COD dm−3 d−1 in R1and 16 g COD dm−3 d−1 in the three polymer-assistedreactors. These data also indicated that biomass wash-out was reduced by adding the polymer, which inturn, increased the organic loading capacities of thepolymer-assisted reactors. However, there was noobvious difference among the three polymer-assistedreactors in this aspect. The better retention of sludgein the polymer-assisted reactors could be attributed tohigher settleability and strength of granules developedin the reactors.

CONCLUSIONThe effect of polymer on the start-up time at OLRslower than 4.8 g COD dm−3 d−1 was not obvious. Theresults obtained at higher loadings up to 25.6 g CODdm−3 d−1 indicated that adding the cationic polymercould significantly reduce the start-up time of UASBreactors. The time for the reactor with 20 mg dm−3

polymer to reach stable treatment was reduced at anOLR of 4.8 g COD dm−3 d−1 by 36% and by 20% atan OLR of 16 g COD dm−3 d−1 compared with thereactor without polymer.

The polymer-assisted reactors generally showedbetter characteristics of the granules and betterreactor performance, R4 with 20 mg dm−3 added



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40

50

60

0.8 1.6 3.2 4.8 6.4 9.6 12.8 16 19.2

SS

/VS

S (

g l−1

)

R2 (5 mg l−1 polymer)



COD Loading Rate (g/L.d)

VSS (g l−1)SS (g l−1)

Figure 10. SS/VSS profiles.

polymer exhibiting the best settleability and the highestmetabolic activity.

The COD removal efficiency in all the reactorsranged between 890 and 950 g kg−1 at OLRs lowerthan 12.8 g COD dm−3 d−1. At higher loadings, thereactor with a cationic polymer concentration of20 mg dm−3 achieved higher COD removal efficienciesthan the control reactor (without added polymer)and the reactors with 5 and 10 mg dm−3 polymerconcentrations. The higher COD removal efficiencyin the three polymer-assisted reactors demonstratedthat adding cationic polymer could improve the CODremoval efficiency in UASB reactors and the extent ofimprovement was related to the concentration of thepolymer.

The maximum organic loading capacity of thecontrol reactor without adding polymer was 19.2 gCOD dm−3 d−1, and the polymer-dosed reactorsattained a maximum organic loading capacity of 25.6 gCOD dm−3 d−1.

The laboratory results obtained so far demonstratedpositive effects of cationic polymer addition on

granulation, start-up and reactor performance inUASB operation. Adding the polymer during start-upresulted in improvement in granule characteristics andshortening of the start-up period, with improvementin the organics removal efficiency and loadingcapacity. It is hypothesized that the positivelycharged polymer forms bridges among the negativelycharged bacterial cells through electrostatic chargeattraction. The bridging effect would enable greaterinteraction between biosolids resulting in preferentialdevelopment and enhancement of biogranulation inUASB reactors. More studies are needed to test furtherthe effects of polymer dosage beyond 20 mg dm−3.Since increases in COD loading were achieved byincreasing the feed rate, resulting in correspondingdecreases in hydraulic retention time, it is not possibleto say which of these factors would be dominantas the feed rate varied. To separate these effects, itwould be necessary to vary the input concentrationat constant feed rate or to vary the concentration inproportion to increases in feed rate. But it is generallyaccepted that the OLR should be used as a controlparameter.

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effects of cationic polymer on start-up and granulation in upflow anaerobic sludge blanket reactors

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