TECHNICAL PAPER Effect of Inoculum Concentration on Start-up of UASB Reactor

Transcription

1 TECHNCAL PAPER Effect of noculum Concentration on Startup of UASB Reactor Three identical laboratory upflow anaerobic sludge blanket reactors were operated under various initial sludge concentrations (1, 3 and 6 gvsslreactor) to determine the effect of sludge concentration on the startup rate. t was hypothesised that increasing the sludge concentration would increase the formation of rapid settling granules or floes. A sludge concentration of 3 gvsslreactor gave better COD removal of 88% (at COD loading of 5 gil) than 1 gvsslreactor (58%). ncreasing the sludge concentration to 6 gvsslreactor resulted in greater washout with a slight improvement in COD reduction (9%). When the COD loading was increased to 2 gil (week 4) removal of 79% was still achieved in the reactor with high initial sludge concentration compared to low initial sludge concentration (51% removal). Washout detracted from reactor performance, so of the three concentrations tested 3 gvsslreactor appeared to be optimal. The upflow anaerobic sludge blanket (U ASB) process has gained wide acceptance for the treatment of medium to high strength industrial wastewaters. Lettinga and Hulshoff P (1991) reported that as of September 199 there were at least 25 full scale UASB reactors in operation. Advantages ofthe UASB process include simple reactor construction without internal moving parts, the retention of rapidsettling dense floes or granules of microorganisms and hence its ability to operate as a high rate process. A major disadvantage of UASB process is reactor startup, since it can take several months to build the required biomass in the reactor. Startup is ideally carried out using aninoculumfrom an operating UASB reactor treating the same type of waste. The ideal situation is not always possible because of lack of available granular sludge in many places or due to cost factors (US$ 6 for 1 ton (dry basis) granular sludge, Weiland and Rozzi, 1991). Sludge from anaerobic digestion of sewage or other wastes, e.g. abattoir waste, can be used as inoculum, although startup time can be up to 6 months. The use of concentrated sludge as the inoculum for startup of laboratory U ASB reactors has indicated thatthe startup period can be reduced (Tenorio and Ho, 1993). The basis for this is postulated to be that the number of suitable microorganisms at the start of operation is increased, i.e. there are more microorganisms that can form rapidsettling granules/ dense floes when concentrated sludge is used. This hypothesis has been further tested by carrying out experiments using a wider range ofinitial sludge concentrations, and the results are reported in this paper. Sludge samples were clected at a sheep and cattle abattoir wastewater stabilization pond in Harvey, Western Australia. The sludge was centrifuged at 8G for 15 minutes to concentrate the vatile suspended sids (VSS). Centrifugation of the sludge appeared to result in the agglomeration ofthe biomass into small sid forms (2 to 5 mm). The centrifuged sludge was found to have 56% vatile sids (192 g TS /Land 18 g TVS /L) and methanogenic activity of 58 ml methane (CH4) g VSS. The design of the laboratory reactors was based on UASB reactors described in the literature (Souza, 1986 and Lettinga et al., 1987) and has been described previously (Tenorio and Ho, 1993). Each reactor had a working vume of approximately 1.9 L. A water jacket was employed to maintain a temperature of 37 oc. Substrate was fed into the reactors by means of peristaltic pumps. Gas was clected from the top of the reactor and measured by water displacement using an inverted graduated cylinder. A water seal was placed between the gas storage cylinder and the reactor, to maintain a slight positive gas pressure in the reactor. A mixture of vatile fatty acids (VFA) (acetic, butyric and propionic) was used as the substrate in order to establish the optimum startup parameters. The conversion of VF A to methane is usually one of the limiting factor in anerobic decomposition of organic wastes. Equal amounts of VFA (12 mg/l) was mixed with 2 ml trace metal sution (TMS), 1 ml basal nutrients (BN) and.2 g yeast to make a liter of synthetic substrate (COD value of approximately 5 mg/l). The preparation of TMS and BN was adapted from Kosaric et al. (199) and Wiegant and Letting a (1985). The synthetic substrate was adjusted to ph 7 with 1M NaOH. Three reactors (Rl), (R2) and (R3) were operated in parallel with different initial sludge concentrations (1,3 and 6 g VSS/L of reactor respectively). Each was operated in six stages of one week each over a

2 period of six weeks, with decreasing hydraulic residence time (HRT) from 22 h to 5 h. nfluent chemical oxygen demand (COD) concentration was maintained constant at 5 g/l while the COD loading was increased from 5 g/l to 22 g/l of reactorday (Table 1) to test the reactors to loading rates higher than 15 g/ L of reactorday, normally achieved in a UASB reactor (in this paper the amount of organic loading and COD or VF A removed per liter of reactor per day is expressed as g/l of reactorday or mg/l of reactorday). 1. Stage (no. of days) ( 7) 5.1±. ( 714) 9.6±.9 (15.:.21) 15.5±.7 V (2228) 2.2±1.2 v (2935) 23.4±.9 V (3642) 22.4±.3 COD loading (g/l of reactorday) Total sids (TS) and total vatile sids (TVS), of the initial concentrated sludge were measured in duplicates flowing Standard Methods (APHA, 1985). Methanogenic activity of the sludge was determined using the serum bottle technique (Chiang and Dague, 1989; Hashimoto, 1989). Gas Daily analysis was conducted for biogas production, methane (CH4) concentration, effluent ph, VFA, COD and sid washout (as VSS). All the analyses were done in duplicate. Both VFA and methane (CH4) concentrations were determined by gas chromatography (GC) using a VarianModel37 equipped with a flame ionisation detector. The gas cumn was packed with Porapak QS (81 mesh) and the cumn temperature was 1oC for methane and 19oC for VF A. Samples for COD analysis were digested by closed reflux and measured by using the procedure describedinstandardmethods (APHA,1985). Particle size of the sludge was determined using an inverted microscope. The daily methane production was computed from the vume of biogas produced and CH4 concentration based on the equation modified from Viega et al., 199 (Tenorio and Ho, 1993). The vume of biogas was corrected to standard temperature ( C). Figure 1 shows the amounts of propionic, butyric acid and acetic acid in the effluent. The amounts of VF A in the effluent were initially high flowing any incease in VFA loading. The amounts gradually decreased indicating that a certain period of time was needed for the acclimatisation of the biomass to the changes in operating conditions. The results indicated that at the end of one week, the degradation of VFA was close to steady state. The amount of propionic acid degraded was higher in R2 and R3 compared to that in R1. The propionic acid degradation in stage 1 for R1 to R3 was 34, 84 and 1% respectively. The amount of propionic acid degraded increased up to stage 3 which attained a degradation of 49, 8 and 73%. A futher increase in loading in stage 4 did not change the amount of propionic acid degraded with the exception of sludge in R1, where the degradation of propionic acid in stage 6 seems to improve. A similar trend was observed in the degradation of acetic acid, although the degradation of propionic and butyric acid increased the total acetic acid input and hence a higher amount of acetic acid was degraded in comparison to that of propionic acid. The acetic acid degradation attained in stage 1 for R1 to R3 was 78, 94 and 99% respectively, then in stage 4 it decreased to 9% for R3 and R2 and 72% for Rl. Butyric acid was fully degraded in all reactors by the end of each stage. There was a strong relationship between COD reduction, VFAdegradationandmethaneproduction, with the COD reduction (Table 2) reflecting the overall degradation of VFA. The COD reduction indicated the better performance of sludge in R2 and R3 in comparison to that in Rl. The relationship between methane production (ml) g COD removed (Table 2 and Figure 2) is close to but below the theoretical value of 35 ml CH4 produced/ g of COD removed (Souza, 1986). Some of the high values in the graphs could be due to errors in biogas measurement but could also be related to the lower feed input at the particular time of sampling. The methane content of the biogas was relatively high (Table 2). This was associated with the high effluent ph (7.88±.18) and absorption of the carbon dioxide in the biogas to the liquid phase. The conversion of VFA into methane decreased acid concentration which in turn increased the ph of the effluent. The relationship between CH4 production and COD removal did not differ between reactors indicating that the conversion rate of COD to methane was similar in all reactors. Thus the differences in performance of the reactors were due to the differing amount of microbial biomass present in the reactors.... VOLUME 5 NUMBER 5 OCTOBER 995

3 < : r (a) Oo Poo poo b Q \ \ b c b \ \,, Q, Q,1\... J;JJq " \ Y 1 ()6, 1\ p 1,, C2t:i CO : A. f,lo "_.1!1, or,r :: a Oi.: C 1\ (J \ Q l!j P,l A :. li..qi!!o () 1 i:).. u t S) /.: g,.,&, "f ooooo6 f,bt.6 c o.. :,..: o 61Pr:/i ir (b) o Rl o R2 6 R3 o Rl o R2 6 R3 2 1 OHmm&BH 4 o Rl 3 o R2 6 R Time (days) Figure 1. Comparison of VFA in the effluent during the period of operation (mg!l of reactorday) (a) Propionic acid (HPr), (b) Butyric acid (HBu) and (c) Acetic acid (HAc)

4 Stage %COD %CH 4 mlch 4 produced reduction in biogas gcod removed Rl Rs Rl 58± 88±1 9± 67±1 65± 87± 86±3 75±8 58±1 78±1 8± 75±4 V 51±1 79± 79±2 85± v 54±3 7± 65±2 77± V 63± 63±1 6±1 79±2 Rz Rz 68±2 62± ±1 8± ±3 72± ±1 84± ±2 77± ±2 69± Q u b1} ::r: u 8 Table 3 shows the sids (as VSS) in the reactor and washout during the period of operation. The calculated negative yield might be due to sampling error because a high yield was obtained after the negative values. The samples were taken from different vertical positions of the reactor and it was observed that a high concentration ofthe sids tended to accumulate at the very bottom of the reactor which was not accessible for sampling. The data obtained show that the washout was proportional to the initial amount of sludge in the reactor. The increase in COD loading also increased the washout, due to the increase in upward liquid flow in the reactor and increased gas production. A high sids washout occurred until steady state conditions were reached and well settling sludge selected by the operating conditions c t,,,, Time (days) Figure 2. Methane produced (ml)/g of COD removed during the period of operation The assessment of sludge performance was based on COD reduction, VFA degradation and methane production. The results show that the performance of sludge in R 2 and R 3 was better than R 1 and the better performance was associated with the higher initial amounts of sludge in the reactors. Although a high washout of sids occurred in R 2 and Ry the high initial amounts of sids at the beginning sustained the high performance, even though the COD loading was increased. The propionic acid degradation was a good indicator of reactor performance because of its lower degradation compared to other acids used. The increase in COD loading in stage 4 clearly affected the degradation of propionic acid not only in but also in R 2 and R 3 A further increase in loading did not proportionally increase the propionic acid degraded implying that the microbial biomass had reached substrate saturation level. o R1 The results indicate that the COD c R2 loading must be 6 R3 limited to 15 g COD /L of reactorday to prevent overloading of the propionic acid degrading bacteria, and further increases in loading should only be attempted at a slower pace The advantage of a high initial amount of sids was clearly shown in the results ob... OLUME 5 NUMBER 5 OCTOBER 1995

5 Stage a nitial VSS htotal VSS washout ccalculated vss at the end of phase dcalculated VSS yield Rl R3 Rr R2 R m 36.7m 29.9m R V v V *d = c (ab); m= measured at the end of experimental run tained, but the amount of sids will be limited by the capacity of the reactors to retain the sids. A severe loss of sids occurred in R 3 and to a lesser extent in The expansion of sids up to the gas sids separator resulted in a heavy carry over of sids out of the reactor. The severe loss of sids in resulted in a similar final amount of sids to that of R 2. n order to avoid severe washout, the initial amount of sids should be limited to 3 g VSS/L of reactor. To take advantage of an initial sids concentration of greater than 3g VSS /L of reactor, the reactor configuration needs to be modified, e.g. by having an enlarged section at the top of the UASB. n summary, the results confirm the potential of using concentrated sludge in UASB startup. The high initial amount of sludge had the advantage of attaining a high loading condition of 15 g COD /L of reactor with 8% COD reduction, within a period of three weeks. Further increases in COD loading were limited by propionic acid degradation. The optimum sludge concentration, as well as other operating factors (variable HRT or constant HRT) that will affect the startup when using concentrated sludge are the subject of further investigation. APHA (1985). Standards Methods for the Examination of Water and Wastewater 16th ed.washington, DC Chiang, C. F. and Dague, R. R. (1989). Determination of acetoclastic methanogenic activity in anaerobic systems. 43rd Purdue ndustrial Waste Conference Proceedings, p Hashimoto, A.G. (1989). Effect of inoculum/substrate on methane yield and production rate from straw. Bioi. Waste 28, Kosaric N., Blaszczyk, R., Orphan, L., and Valladares, J. (199). The characteristics of granules from upflow anaerobic sludge blanket reactors. Wat. Res. 12, Lettinga, G. and Hulshoff P, L.W. (1991). UASBprocess design for various types of wastewater. Wat. Sci. Tech. 24(8),8717. Lettinga, G., DeZeeuw,W., Wiegant, W. and Hulshoff P, L. (1987). Highrate anaerobic granular sludge UASBreactors for wastewater treatment. Bioenvironmental System, Vl. Wise, D. ed., Florida, CRC Press p Souza, M.E. (1986). Criteria for the utilization, design and operation of UASB reactors. Wat. Sci. Tech. 8(12),5569. Tenorio, D.O. and Ho, G. E. (1993). Startup of UASB reactor with concentrated abattoir stabilization pond sludge. Proceedings of the 15th Australian Water and Wastewater Association Federal Convention V3,61462 Viega, M.C., Soto, M., Mendez, R. and Lema, J. M. (199). A new device for measurement and contr of gas production by bench scale anaerobic digesters. Wat. Res. 24(12), Weiland, P. and Rozzi, A. (1991). The startup, operation and monitoring of highrate anaerobic treatment systems: Discussers report. Wat. Sci. Tech 24(8), Wiegant, W. M. and Lettinga, G. (1985). Thermophilic anaerobic digestion of sugars in upflow anaerobic sludge blanket reactors. Biotechn. Bioeng. 27, t s11m AMRAD PHARMACA BOTECH AMRAD Pharmacia Biotech, exclusive distributors of the Pharmacia Biotech BioProcess products, is pleased to support the Australian Biotechnogy Association Pharmacia Biotech