~ Pergamon Waz. Sci. Tech. Vol.34. No. ,-6.pp,44S4'2. 1996.Copyright=19961AWQ.Published by Elsevier ScienceLId

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PIT: S0273-1223(96)00677-4

STARTUP OF THERMOPHILIC (55°C)UASB REACTORS USING DIFFERENTMESOPHILIC SEED SLUDGES

Herbert H. P. Fangand IvanW. C. Lau

Environmental Engineering Research Centre.Department of CivilandStructural Engineering, TheUniversity of Hong Kong.Pokfulam Road, Hong Kong

ABSTRACf

Performances during startup of three 2.8-litre UASB (upflow anaerobic sludge blanket) reactorsoperatedunderthermophilic condition were investigated. All reactors were seededwith mesophilic sludges: one withflocculent digester sludge (Reactor-F). another with UASB granules (Reactor-G). and the third withdisintegrated granules (Reactor-D). The reactors wereoperated in parallel at SS'C and 24 hoursof retentiontime.usingsucrose and milkas substrate at COD(chemical oxygendemand) loadings up to 10g-CODlloday.Immediately afler temperature was step-increased from 37'C to 55·C. all reactors encountered sludgewashout and deterioration of COD removal efficiency; however, the impact of temperature increase wasmore severe on Reactor-F. Sludge granulation took place in all reactors; first granules became noticeableafter 45 days in Reactor-D. and after 90 days in Reactor-F. Reactor-G and Reactor-D were capable ofremoving 9S% of soluble COD after 7S days. whileReactor-F after 110days. Throughout this study, therewas littledifference in performance between Reactors G and D. The thermophilic granule wereestimated tohave a yield of 0.099g-VSSlg-COD, and a methanogenic activity of 0.71-1.55 g-methane-COD/g-VSSoday.comparable to thatof mesophilic granules. Copyright ~ 1996IAWQ. Published by Elsevier ScienceLtd.

KEYWORDS

Anaerobic; bioactivity; granulation; granule; sludge; startup; thermophilic; UASB; yield.

INTRODUcnON

Due to the development of high-rate reactors. such as anaerobic filter (Young and McCarty, 1969) andupflow anaerobic sludge blanket (UASB) reactor (Lettinga et al.• 1980). and a better understanding ofmicrobiology, anaerobic technology has advanced considerably to become viable in industrial wastewatertreatment (Speece, 1983). UASB technology has been successfully applied to the treatment of variouswastewaters (Lettinga and Hulshoff Pol. 1991), including those containing concentrated organic pollutants.such as sugars (Fang and Chui, 1993), starch particulates (Fang and Kwong. 1995). proteins (Fang et al••1994). and even aromatic chemicals (Li et al.• 1995). However nearly all of these studies were conductedunder the conventional mesophilic condition (3S"C to 4Q"C).

Many industrial effluents. such as those from pulp/paper and food processing, are often discharged atelevated temperatures. Treating these effluents under conventional mesophilic condition requires pre­cooling, which is costly, and has the risk of losing the biomass activity should the cooling system breaks

445

446 H.H. P. FANG andI. W.C.LAU

down. It is thus natural to treat these effluents under thermophilic conditions. Furthermore, at elevatedtemperatures, degradation efficiency and killing of pathogens are presumably more effective (Wiegant et 01.,1985; Lettinga et al., 1991). Although thermophilic UASB process has been investigated in both bench-scale(Wiegant et al., 1985, 1986; Van Lier et al., 1992; Shi and Forster, 1993) and pilot-scale reactors (Souza etal., 1992; Ohtsuki et al., 1994) in the past decade, it seems no attempt has been made to compare the startupperformance of UASB reactors using different seed sludges. Since there are very limited number ofthermophilic reactors being operated, it is difficult to start a thermophilic UASB reactor using sludge froman existing reactor as seed. Most likely one will seed it with mesophilic sludges which are much easier toobtain .

In this study, thermophilic reactors were seeded with three individual mesophilic sludges. flocculentdigester sludge is readily available in many municipal wastewater treatment plants, while UASB granulescould be obtained from hundreds of full-scale plants worldwide. On the other hand, Macario et 01. (1991)and Visser et al. (1991) found that bacterial composition of UASB granules was drastically changed overtime after the reactor was transformed from mesophilic to thermophilic condition, even though the integrityof the granules remained intact. One would thus speculate that, during the transformation some bacteria inthe granule, such as the filamentous Methanothrix, might provide the network structure to agglutinate thethermophilic biomass. Thus, disintegrated UASB granules were chosen as the third seed sludge in this studyto see if the dispersed biomass would re-granulate during the transformation to thermophilic condition.Performance of each reactor, such as sludge washout, removal efficiency of chemical oxygen demand(COD), sludge yield, bioactivity of biomass, etc., was closely monitored, after the reactor temperature wasraised to thermophilic condition .

MATERlAlS AND METIIODS

Three identical UASB systems (Fang et 01., 1995), each with 2.8 litres of reactor volume and an internal 2.0­litre gas-liquid-solid separator, as illustrated in Figure I, were used in this study. Reactor-F was seeded with2 litres of flocculent sludge containing 1.3% of VSS (volatile suspended solids) from the anaerobic digesterof Shatin Sewage Treatment Works, Hong Kong. Granular sludges containing 2.8% of VSS used to seed theother two reactors were obtained from a 4O-litre UASB reactor operated at 37"C and a constant COD loadingof 10 g-CODn·day. Reactor-G was seeded with 1.4 litres of granular sludge as is, while Reactor-D wasseeded with same amount of sludge, but after the granules had been disintegrated first by a Waring blenderand then by an ultrasonic homogenizer (Cole-Parmer 4710 Series) .

The reactors were fed continuously with synthetic wastewater comprising milk powder and sucrose asorganic substrates with a constant hydraulic retention time of 24 hours. The composition of the wastewater,including concentrations of trace metals and nutrients, followed those reported previously (Fang and Chui,1993). The effluent pH of all reactors was kept at pH 7.1-7.6, due to the chemical buffers in the wastewater.Prior to starting up the thermophilic operation, all reactors were operated at 37"C and 2 g-CODn-day for 10days. On Day I, the reactor temperature was step-increased to SS"C and kept at that temperature throughoutthe rest of this study. The temperature was controlled by circulating heated water inside tbe water jacket ofthe reactors. The loading was raised from the initial 2 g-CODn-day to 2.5 g-CODn-day on Day 80, and thento 5 g-eODn-day on Day 85, and finally on Day 91 to 10 g-CODn-day, and was kept at that loading for therest of the study.

The biogas production rate and the effluent pH were measured daily. The COD, VSS, and total suspendedsolids (TSS) of both wastewater and effluent were measured twice a week. The sampling strategies and theanalytical procedures followed those reported previously (Fang and Chui, 1993). Unless specified otherwise,the analytical procedures followed those in the StandardMethods (APHA, 1985). Sludge was sampled fromeach reactor on Day 150 for specific methanogenic activity (SMA) analysis , using the method of Dolfingand Bloemen (1985) adapted from Owen et al. (1979). The SMA analyses were conducted in triplicate usingsucrose and acetate as individual substrate .

Startup of thermophilic UASB reactors

0 ~I~

Gascounlet

0Emuenl.,.,

8Sampling

pons

§ Sludge

~blanke t

1.0 . = 84

Sludge bed

447

Pump Dimensions in mm

Figure I. The UASB reactor system.

RESULTS AND DISCUSSION

Initial sludKe wa.~hout and eranylatjon

To start up the experiment. Reactor-F. Reactor-G and Reactor-D were respectively added with 26. 38 and 38grams of VSS of seed sludge. Fig. 2 illustrates that. as soon as the temperature was raised to SS'C on Day I.the sludge bed in Reactor-F began to expand from the initial level of 290 nun to 350 nun in ten days. Thiscould be caused by the changes of some undetermined sludge surface characteristics. such as extracellularpolymers. surface charge. etc . On Day 30. the sludge washout in Reactor-F became worse. and the bedheight started to decrease. reaching the level of 190 nun on Day 80. It began to further decrease on Day 93.due to the turbulent action caused by the increasing biogas production as a result of loading increase . FromDay 110 on, the sludge bed in Reactor-F was maintained at the constant level of 100 nun . On the other hand,the sludge bed heights in Reactor-G and Reactor-D did not expand as the temperature was raised to 55'C;instead , the sludge washout took place inunediately and respectively reached the constant levels of 120 mmand 135 nun after about 30 days .

Dispersed biomass in Reactor-D and Reactor-F gradually agglutinated to form granules. However. those inthe former reactor were easier to develop into granules than those in the latter reactor. Granulation becamevisible on Day 45 in the Reactor-D. but not until Day 90 in Reactor-F. When the experiment terminated onDay ISO, granules in both reactors had an average size of about 0.5 nun. In Reactor-G. on the other hand. theseed granules were partially disintegrated soon after the temperature increase to 55 'C, reducing the averagesize from about 2 nun to the order of 0.1 nun. However, the disintegrated sludge soon began to re-granulateand developed to granules of 0.5-1.0 nun on Day ISO.

The vssrrss ratio of biomass decreased from the initial 0.74 to 0.59 on Day 150 in Reactor-F. and from0.65- 0.69 to 0.46-0.47 for the other two reactors. The decrease of Vssrrss was due to the increase ofcalcium content in the sludge (Fang and Chui, 1993; Doffing , 1987). which may have enhanced the sludgesettleability. At 10 g-CODIl.day. effluents of Reactor-F. Reactor-G and Reactor-D respectively contained anaverage of 470, 320 and 440 mg-vssn.

H. H. P.FANG andI. W. C. LAU448

500

400

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time (Day)

Figure2. Variations of heightof sludge bed.

COD remoyal efficiency

Prior to the temperature increase. Reactor-F. Reactor-D. and Reactor-G respectively removed 37.5%. 96.9%,and 95.8% of soluble COD from the wastewater at 37·C and 2 g-CODlloday. Figure 3 illustrates the solubleCOD removal efficiency of the three reactors at 55·C. Immediately following the temperature increase.soluble COO removal efficiency of both Reactor-D and Reactor-G decreased drastically to the level of 60%.It took about 75 days for both reactors to recover their soluble COD removal efficiency to 95%. Subsequentincreases of COD loading did not affect the COD removal efficiency.

100

~ 80 I

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o 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

time (Day)Figure3. SolubleCODremoval efficiency.

On the other hand, the soluble COD removal efficiency of Reactor-F increased inunediately after thetemperature increase. reaching 75% in about one week. It then decreased drastically. due to the severesludge washout. to the level of 44% by Day 28. Van Lier et al. (1993) reported a similar observation thatthe initial thermophilic activity of digested organic fraction of municipal solid waste was higher than that ofmesophilic granules. As the sludge began to develop into granules. the soluble COD removal efficiency in

Startupof thermophilic UASB reactors 449

Reactor-F gradually improved. reaching over 95% after Day llO. The increase of COD loading also did notsignificantly affect the reactor performance.

Methane production and sJud~e yield

Anaerobic degradation is a multi-step process (Gujer and Zehnder, 1983; Thiele and Zeikus, 1988).Complex organic substrates. such as proteins and carbohydrates, are first hydrolyzed by enzymes formingsoluble amino acids and sugars, which are then degraded by acidogenic bacteria into volatile fatty acids(VFA). These intermediate acids are further degraded by acetogenic bacteria forming acetate, formate,carbon dioxide and hydrogen. all of which are ultimately converted to methane by the methanogenicbacteria. In this study. when the temperature was raised to 55'C. biomass in all reactors did not completelylose their methanogenic activities; methane was continuously produced. although at lower rates as reflectedby the decrease of COD removal efficiency. On the other hand, in a similar experiment, van Lier et al.(1992)reported that, as the reactor temperature was raised from 3S'C to 55'C. biomass in granules ceased toproduce methane for about 5 days before gradually regaining its bioactivity.

The net sludge yield in a reactor is usually estimated by monitoring the COD removal as well as the VSScontents in both the reactor and the effluent. The amount of VSS accumulated inside the reactor plus thosewashed out divided by the total COD removed in the same period equals the net sludge yield. However. theaccuracy of this conventional means of estimation is strongly dependent on the reliability of the VSS data. Inreality. obtaining accurate data of VSS in both reactor and effluent is not easy. However. the net sludge yieldin an anaerobic reactor can be estimated by another means without any VSS measurements. One mayassume that during anaerobic degradation all the COD removed are converted to either methane, carbondioxide or biomass. The COD equivalent for the biomass inside the reactor can be directly measured, whilethe COD equivalent is 4 g-COD/g for methane, and nil for carbon dioxide. Thus, the net sludge yield canthus be estimated from three sets of data, i.e, methane production. the total COD removed, and the CODequivalent of the biomass, all of which can be accurately measured. This method has been recently used toestimate the net sludge of various fatty acids (Chui et al., 1994; Fang et al.• 1995) and hydrolyzed proteins(Fang et al.• 1994b).

Figure 4 illustrates that. in Reactor-G and Reactor-D. the specific methane production rate increased linearlywith the specific substrate utilization rate with an average slope of 0.859. The slope indicates that. of theCOD removed. an average of 85.9% was converted into methane. and the remaining 14.1% was presumablyutilized for the bacterial growth. Since the average CODNSS ratio in the reactors Reactor-D and Reactor-Gwas 1.43, the average yield of thermophilic granules was estimated as 0.099 g-VSS/g-eOD. This figure iscomparable to the 0.10 g-VSS/g-eOD for the mesophilic starch-degrading granules (Fang and Kwong,1995) and the 0.13 g- VSS/g-COD for mesophilic sucrose-degrading granules (Hulshoff et al., 1989). Thedata of Reactor-F was not included in Fig. 4, because small amount of scum was formed and trapped in gas­liquid-solid separator inside the reactor; the COD equivalent of the scum. and thus the specific substrateutilization rate, could not be accurately estimated.

Specific methano~enic activity

Table 1 summarizes the SMA of the granular sludges sampled from each reactor on Day 150, using sucroseand acetate as individual substrates. Also included are corresponding data in litreature for comparison. Ingeneral, thermophilic granules developed in this study exhibited methanogenic activity ranging from 0.71­1.55 g-methane-COD/g-VSSeday, which is comparable to those of mesophilic granules. Granules inReactor-F exhibited higher SMA than those of granules in the other two reactors for reasons yet to beidentified.

450 H. H.P. FANG andI. W. C. LAU

1.61.40.4 0.6 0.8 1 1.2

specific substrate utilization rate (g-COD/g·vss •day)

0.2

+ Reactor 0

o Reactor G ~~_=IIlI!!S!II_i~

f 1.4 r;:=====::;-------------------+~:1en~ 1.201aoo2~ 0.8r:;,gs 0.6'02Q,

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Figure4. Relation between specific methane production rateandspecific substrate utilization rate.

Table 1. Specific methanogenic activity of granules using acetate and sucrose as individual substrate

Reactor SMA Specific Methanogenic ActivitySource ofsludge! temp temp (g-methane-COD/(g-VSS'day»substrate eC) (0C) acetate sucrose References

----Reactor-F 55 55 U5 1.07 present studyReactor-D 55 55 0.82 0.83 present studyReactor-G 55 55 0.97 0.71 present studyMixedVFA 36 55 0.37' N/A Van Lier, et al., 1992MixedVFA 46 55 0.66' N/A Van Lier, et al., 1992MixedVFA 55 55 0.91' N/A Van Lier, et 01., 1992Brewery 5S 55 1-2 N/A Ohtsuki, et aI., 1992Starch 37 37 2.26 0.99 fang and Kwong, 1995Sucrose 37 37 1.20 0.85 fang. et 01., 19941Brewery 37 37 0.40 0.32 fang. et 01., 1994aSugar 37 37 0.90 N/A Dolfing. etal.• 1985Sucrose 37 37 0.30 N/A MacLeod, et 01., 1990

.--.-_..._._.._------.._....._----_....-acetotrophic activity in g-acetate-COD/(g-VSS'day)

N/A : not available

CONCLUSION

Results of this experimental study indicate that various forms of mesophilic sludge could beused as seed forthermophilic UASB reactors. Immediately after the temperature increased to 55·C. all reactors experiencedvarious degrees of sludge washout and decrease of COD removal efficiency; the impact was, however, moresevere to the flocculent digester sludge. Granular sludges were eventually developed in all three reactors,although flocculent digester sludge required a longer time to develop . After 100 days, all reactors werecapable of removing over 95% of soluble COD at 10 g-CODn-day of loading and 24 hours of hydraulicretention . The thermophilic granules were estimated to have a yield of 0.099 g-VSS/g-COD, and a

Stanup of thermophilic UASBreactors 451

methanogenic activity ranging from 0.71 to 1.55 g-methane-COD/g-VSSoday, comparable to that ofmesophilic granules.

ACKNOWLEDGEMENTS

The authors wish to thank Taikoo and Nestle for the generous supply of feedstock, and the CRCG researchgrant from the University of Hong Kong for the financial assistance.

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