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Accelerated start-up and enhanced granulation in upflow anaerobic sludge blanket reactors
ARTICLE IN PRESS
Water Research 38 (2004) 2293–2304
Accelerated start-up and enhanced granulation in upflow anaerobic sludge blanket reactors Kuan-Yeow Show*, Ying Wang, Shiu-Feng Foong, Joo-Hwa Tay School of Civil and Environmental Engineering, Nanyang Technological University, Block N1#1A-29, Nanyang Ave., Singapore 639798, Singapore Received 1 July 2003; received in revised form 6 January 2004; accepted 30 January 2004
Abstract In the present study, the effects of a cationic polymer on reactor start-up and granule development were evaluated. A control reactor R1 was operated without adding polymer, while the other five reactors designated R2, R3, R4, R5 and R6 were operated with different polymer concentrations of 20, 40, 80, 160 and 320 mg/L, respectively. Experimental results demonstrated that adding the polymer at a concentration of 80 mg/L markedly accelerated the start-up time. The time required to reach stable treatment at an organic loading rate (OLR) of 4 g COD/L.d was reduced by approximately 50% in R4 as compared with the control reactor. The same reactor with 80 mg/L polymer was able to achieve an OLR of 12 g COD/L.d after 59 days of operation, while R1, R2, R3, R5 and R6 achieved the same loading rate at much longer period of 104, 80, 69, 63 and 69 days, respectively. Comparing with the control reactor, the start-up time of R4 was shortened markedly by about 43% at this OLR, while other reactors also recorded varying degree of shortening. Monitoring on granule development showed that the granule formation was accelerated by 30% from the use of the appropriate dosage of polymer. Subsequent granules characterization indicated that the granules developed in R4 with 80 mg/L polymer exhibited the best settleability, strength and methanogenic activity at all OLRs. The organic loading capacities of reactors were also increased by the polymer addition. The maximum organic loading of the control reactor was 24 g COD/L.d, while the polymer-assisted reactor added with 80 mg/L polymer attained a markedly increased organic loading of 40 g COD/L.d. The laboratory results obtained demonstrated that adding the cationic polymer could result in shortening of start-up time and enhancement of granulation, which in turn lead to improvement in organics removal efficiency and loading capacity of the UASB system. r 2004 Elsevier Ltd. All rights reserved. Keywords: UASB; Granulation; Start-up; Polymer; Granule characteristics
1. Introduction Over the past two decades, upflow anaerobic sludge blanket (UASB) technology has been employed for wastewater treatment [1–6]. More than 900 UASB units are currently being operated all over the world [7]. It exhibits positive features such as high organic loadings, low energy demand, short hydraulic retention time *Corresponding author. Tel.: +9-65-6791-5282; fax: +9-656791-0676. E-mail address: [email protected] (K.-Y. Show).
(HRT) and easy reactor construction. Important parameters affecting the treatment efficiency of UASB reactors include the granulation process in the reactor, the characteristics of the wastewater to be treated, the selection of inoculum material, the influence of nutrients and several other environmental factors. Among these parameters, the granulation process is believed to be the most critical one [8]. In UASB reactors, the biomass is retained as granules formed by the naturally self-immobilization of the bacteria. Sludge granules are those tightly structured aggregates, which have a clear-cut shape and some
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.01.039
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mechanical strength. They do not break up or disintegrate easily even under fairly high hydraulic and gaseous shear stresses. However, loosely structured sludge flocs can easily fall apart when subjected to a moderate shear stress [9]. Granular sludge is the main prominent characteristic of UASB reactors as compared to other anaerobic technologies [10]. There is a close correlation between efficiency of an UASB reactor and development of granular sludge [1]. Granulation not only significantly enhances the settleability of biomass leading to effective bacterial retention in the reactor, but also improves physiological conditions making them favourable for bacteria and their interactions, especially syntrophs in the anaerobic system [11]. The formation and stability of the granules are essential for successful operation [5]. Reactor start-up is a very important economic process step, because during this period the productivity of the wastewater supplier must be adapted to the capacity of the wastewater treatment plant. This leads in all cases to a dramatic reduction in the production capacity and can result in a strong deterioration of the product quality, when the adaption of the productivity can be achieved only by change from continuous to discontinuous operation of the production facilities. Inadequate start-up causes poor subsequent treatment and often requires expensive system maintenance and effluent post-treatment. Start-up is often considered to be the most unstable and difficult phase in anaerobic digestion. Its main task is to develop a highly active settleable sludge as quickly as possible. While as anaerobic bacteria are slowgrowing microorganisms, major problems encountered with UASB are the typical long reactor start-up and spontaneous development of biogranulation. Start-up times for UASB reactors with digested sewage sludge usually take several months [12,13]. Hence, long start-up period is the major deterrent to the use of UASB system. The reduction of start-up time is one of the key parameters to increase the competitiveness of high-rate anaerobic reactors [14]. Recent advances in understanding the fundamentals of the biochemistry and microbiology of granulation in UASB system have led to successful applications, which show a great deal of promise in overcoming its long start-up time limitation. Some approaches have been developed to enhance the granulation and shorten the start-up period. Physical– chemical and biological factors promoting granulation have been documented [15–18]. To remedy the drawback of long start-up period and extend the application of UASB reactors, the present study has been conducted in order to accelerate the start-up and natural granulation process by incorporation of cationic polymer. In anaerobic reactors, polymers have been used either to immobilize the anaerobic sludge within gel beads or to reinforce the strength of the already existing granules, by coating the granule
surfaces with a thin layer of polymer [19,20]. This project aims to measure the optimum dosage of cationic polymer by evaluation of the effects of polymer on UASB start-up and development of granules. The effects on the characteristics of granules and reactor operation and performance under the influence of the polymer were investigated.
2. Materials and methods 2.1. Materials and reactor system Six 4.4 L plexiglass UASB reactors with an internal diameter of 100 mm and a height of 680 mm were used in this study. The reactors were placed in a walk in temperature-control room set at 3571 C. A refrigerator, set at 4 C, housed the substrate storage tanks to prevent premature degradation. Synthetic substrate was pumped into the reactor inlet by peristaltic pumps (ColeParmer, MasterFlexs L/St). Wet gas meters (Ritter TG 05) were installed to measure the gas production. To avoid severe corrosion and possible damage to the gas meter, the biogas produced from each reactor was first passed through a water trap before being channelled to the gas metre. The experimental system set-up is shown in Fig. 1. All reactors were inoculated with 50% of digested sludge obtained from an anaerobic digester of the Jurong Water Reclamation Plant treating industrial and domestic wastewater. The characteristics and elements of the seed sludge are shown in Table 1. The components of a synthetic substrate (Table 2) were mixed together in a tank after adding the necessary nutrients and buffering chemicals. Trace components were similar to that used for cultivating anaerobic bacteria by Yan [9]. Buffer capacity was provided by sodium bicarbonate at levels sufficient to maintain pH in the system between 6.5 and 7.3. Two essential
Fig. 1. Schematic diagram of experiment setup.
ARTICLE IN PRESS K.-Y. Show et al. / Water Research 38 (2004) 2293–2304 Table 1 Characteristics of seed sludge Characteristics Suspended solids (SS) Volatile suspended solids (VSS) Sludge volume index (SVI) Specific methanogenic activity (SMA)
Median particle size (diameter)
72.5 g/L 30 g/L 41.2 mL/ g SS 0.5 g CH4COD/g VSS.d 101 mm
Elemental contents Element
Content (mg/g)
Element
Content (mg/g)
Ca Al Mg Mn K Na Mo
39.04 16.55 3.55 0.43 2.47 4.95 0.075
Fe S Ni Zn Cu P
28.89 143.67 4.56 21.26 24.45 79.88
Table 2 Synthetic substrate composition in mg/L (Based on COD of 5000 mg/L) Carbon source Peptone Glucose Meat extract Macro-nutrients CaCl2 H2O MgSO4 7H2O NH4Cl FeSO4 7H2O KH2PO4
1000 3400 700
48 54 800 40 200
Micro-nutrients H3BO3 ZnCl2 CuCl2 MnSO4 H2O (NH4)6Mo7O24 4H2O AlCl3 CoCl2 6H2O NiCl2 Conc.HCl (36%)
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2 mL
Alkalinity buffer NaHCO3
3000
2295
components for biomass synthesis, namely nitrogen and phosphorus, were added in the form of NH4Cl and KH2PO4, respectively. To sustain an active microbial culture, a COD:N:P ratio of 100:10:1 [21–23] was adopted. The substrate was prepared once every two days and stored below 4 C to prevent premature degradation. The commercially available polymer with exclusive chemical compositions has been used extensively as a coagulant aid in water treatment systems. Coagulation action of the cationic polymer is promoted by charge neutralization on colloids by the ionized sites as well as bridge formation. ‘‘AA 184 H’’ cationic polymer was supplied by courtesy of Ondeo Nalco Pacific Pte. Ltd. and its vast application in the wastewater treatment industry as a sludge conditioner had prompted its use in this study. This polymer has a surface charge density of 16% and its molecular weight ranges between 1 and 10 million. 2.2. Experimental procedure and analytical methods Six laboratory-scale UASB reactors designated as R1, R2, R3, R4, R5 and R6 were operated to evaluate the UASB start-up, performance and characteristic of the granules developed when different polymer concentrations were added. During the start-up, all reactors were fed with the same COD concentration. R1 was operated without polymer addition serving as a control reactor. Based on a study by El-Mamouni et al. [24], an optimum dose of cationic polymer of 2 mg/g SS with OLR increased from 0.2 to 0.6 g COD/g VSS.d was used for reference. To examine the effects of polymer concentration, reactors R2, R3, R4, R5 and R6 were dosed with polymer of 20, 40, 80, 160 and 320 mg/L, respectively, together with the inoculum during the seeding. The reactors were started up simultaneously with an influent COD of 5000 mg/L and a corresponding organic loading rate of 2 g COD/L.d, which was computed based on the concentration of COD loaded into the reactor per unit hydraulic retention time. The reactors were operated until steady-state conditions were reached (evidenced by constant gas production and COD removal). Evaluation of the system was carried out during the steady-state conditions. The OLR was then step increased by shortening the hydraulic retention time (HRT) and maintaining the influent COD at 5000 mg/L throughout the study. The reactors were operated until the next steady-state conditions were reached. pH, alkalinity, COD, SS, VSS, sludge volume index (SVI) and specific gravity (SG) of sludge were conducted in accordance with Standard Method [25]. Gas production of each reactor was recorded daily from a wet gas meter (Ritter TG 05). The gas composition was analyzed by a Gas Chromatograph (Hewlett-Packard HP 5890 A) for methane, carbon dioxide, and nitrogen. The particle/ granule size was measured by laser particle size analysis system (Malvern Master Sizer Series 2600, Malvern
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Instruments Inc., Southborough, Massachusetts) and image analysis system (Quantimet 500 Image Analyzer, Lerca Cambridge Instruments, Cambridge, UK) when the particle size was beyond the Maser Sizer’s limitation (0.05–550 mm). The resistant capacity of granules against disintegration during the operation can be expressed with integrity coefficient (%) described by Ghangrekar et al. [26]. The volatile suspended solids concentration of settled granules and its disintegrated supernatant were measured after a period of mechanical agitation and settling. The integrity coefficient (%) was then expressed as the ratio of residual granule VSS and total VSS. A higher integrity coefficient implies higher strength of the granules. Settling velocity of granules was determined according to the method proposed by Thaveesri et al. [27]. Measurements of specific methanogenic activity (SMA) were conducted according to the method reported by Yan [9] using a similar complex proteincarbohydrate synthetic substrate (Table 2).
3. Results and discussion 3.1. Start-up The influent substrate concentrations were maintained at 5000 mg COD/L in all reactors. Pseudo-steady-state 90
condition was considered attained when COD removal and the biogas production were relatively consistent within 5% for three consecutive days. As indicated in Fig. 2, steady gas production was used to ascertain that reactors could be loaded to the next OLR. The organic loading rate was subsequently step increased to the next higher rate through shortening of HRT. A major benefit of enhanced granulation is shortening of start-up period to reach a specified organic loading. The operation time required to reach steady-state treatment at each OLR and the corresponding COD removal efficiency are shown in Fig. 3 and Table 3. At all OLRs, the 80 mg/L polymer added in R4 provided the shortest start-up periods. Compared with the other concentrations, the polymer concentration at 80 mg/L could significantly accelerate the start-up time. To reach stable treatment of 8 g COD/L.d, the time needed in the control reactor R1 was 92 days, while the times required were 67, 52, 44, 48 and 50 days in R2, R3, R4, R5 and R6, respectively. The start-up time in R4 was markedly reduced by 52% compared with R1. The time required to reach stable treatment in R2, R3, R4, R5 and R6 at OLR of 16 g COD/L.d was reduced by 20%, 23%, 36%, 34% and 29% as compared with R1. Reactors R3, R5 and R6 reached stable treatment at OLR of 32 g COD/ L.d on 140, 122 and 132 days, respectively, while R4 achieved the same OLR on shorter time of 118 days. 35
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Fig. 2. Gas production rates in each reactor.
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Fig. 3. Operation time and COD removal efficiency of each reactor.
These results demonstrated the polymer concentration at 80 mg/L added in R4 provided the optimum dosage for acceleration of start-up time of UASB reactor. This could be a result of the formation of a protective gelatinous layer around cells from the addition of polymer. The protective layer would act like a shield
for bacterial cells when they are subjected to organic shocks or sudden increment in hydraulic abrasive forces due to the changes in OLR. An overdose of polymer addition would prolong the time acquired to achieve steady state of designed OLRs in comparison with the reactor added with the optimum concentration of
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2298 Table 3 Operating time at each OLR (days) Reactors
R1 R2 R3 R4 R5 R6
(control) (20 mg/L polymer) (40 mg/L polymer) (80 mg/L polymer) (160 mg/L polymer) (320 mg/L polymer)
OLR (g COD/L.d) 2
4
8
12
16
24
32
26 26 14 14 14 14
57 35 33 29 29 29
92 67 52 44 48 50
104 80 69 59 63 69
115 92 88 74 76 82
127 103 113 95 99 105
110 140 118 122 132
cationic polymer. This might have resulted from the formation of a thick polymeric gel around cells that inhibited substrate diffusion into and out of the microbial mass. Similarly, an underdose of polymer as reflected in R2 and R3 could be a result of the formation of loose biomass flocs that were unable to shorten diffusion distances significantly. 3.2. COD removal efficiency Fig. 3 shows the COD removal efficiency of all reactors without experiencing major system upset. At 2 g COD/L.d, COD removal efficiencies of 83% and 80% in R5 and R6 added with 160 and 320 mg/L polymer, respectively, were lower than the control reactor R1 (91%), and the polymer-assisted reactors R2 (92%), R3 (93%) and R4 (90%) with 20, 40 and 80 mg/L polymer supplement. The higher concentrations of polymer in R5 and R6 had not provided satisfactory performance at low OLRs. When OLRs were raised to 4 g COD/L.d, the efficiency in all reactors reached 90%. This could be due to system adaptation of higher doses of polymer in R5 and R6. Floating flocs were observed in R5 on day 49 at 12 g COD/L.d, and the same phenomenon was observed on day 40 in R6 at 8 g COD/L.d. The bioparticles floatation led to the decreases of COD removal efficiency at 12 g COD/L.d in R5 and 8 g COD/L.d in R6, respectively. At higher OLR of 16 g COD/L.d, the COD removal efficiency in all polymer-assisted reactors were almost similar ranging between 93% and 95%, attributed to the flocs in R5 and R6 settling back to the sludge blanket. At high OLRs of 32 g COD/L.d, the efficiency of R4 was 83%, which was higher than 75%, 77%, 80% and 74% in R2, R3, R5 and R6, respectively. The OLRs in R4 and R5 were further increased to 40 g COD/L.d after the other reactors were terminated at 32 g COD/L.d due to low organic removal efficiency. At 40 g COD/L.d, the organic removal efficiency in R4 was 76%, which was higher than 72% in R5. At most OLRs, COD removal efficiency in R4 was the highest among the reactors. The results indicated the optimum dose of polymer of 80 mg/L in improving the reactor treatment performance.
40
131 135
3.3. Reactor loading capacity A significant benefit of system enhancement in any anaerobic process is the increased capacity to remove COD. After the OLR reached 24 g COD/L.d on day 116, the COD removal efficiency of R1 dropped substantially from 89% to 73%. The gas production dropped significantly while the VFA increased drastically, indicating that the 24 g COD/L.d was the maximum capacity for R1. For R2, R3 and R6, the operations were continued until they were terminated at 32 g COD/L.d, at which loading the removal efficiencies dropped sharply. While at 32 g COD/L.d, COD removal efficiency in R4 and R5 still could reach 83% and 80%, then the OLR in R4 and R5 were further increased to 40 g COD/L.d after the other reactors were terminated. Both reactors were terminated at 40 g COD/L.d, which seemed to be the maximum capacity for R4 and R5. Compared with the control reactor, the organic loading capacity was improved to 32 g COD/L.d by adding 20 mg/L polymer, and further improved to 40 g COD/L.d by adding higher polymer dosages of 80 and 160 mg/L. 3.4. Formation of granules The bioparticle sizes at each OLR of all reactors versus operation time were shown in Fig. 4. The diameters of particles in all reactors rose steeply and reached their peak diameters and subsequently declined beyond the peak OLRs. At high OLRs of 24 and 32 g COD/L.d, the granules in R5 and R6 added with the highest dosages of polymer of 160 and 320 mg/L, respectively, showed larger size compared with the other reactors. The successful operation of UASB reactor depends on the formation of granules with good settleabilities and degradation activities. A granule could not be judged as a ‘‘better granule’’ only according to its ‘‘larger size’’. It was observed that the ‘‘big granules’’ in R5 and R6 showed loose structure and lots of white and black fluffy flocs attached on the surface. The characteristics of these granules were not as good as those polymer-assisted reactors with lower dosage of polymer.
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Fig. 4. Particle size in each reactor.
Onset of granulation is adopted here as the formation of bioparticle having diameter larger than 0.5 mm [28]. The solids in all polymer-assisted reactors began to form granules with diameter larger than 0.5 mm around day 50, while the granule formation took place in the control reactor around 70 days. The granule formation was accelerated by 30% from the polymer application. The fact that granules were formed earlier in the polymerassisted reactors could be attributed to the fact that biomass aggregation took place at a much shorter period with the incorporation of polymer. The result also indicated that the time required for the biomass aggregation could not be shortened any further by overdosing of polymer. The superior effects resulting from the cationic polymer could be hypothesized that the cationic polymer essentially forms a bridge among negatively charged bacterial cells through electrostatic charge attraction, which results in agglomeration of dense and active biogranules enhancing the reactor performance. 3.5. Granule characteristics 3.5.1. Suspended solids and volatile suspended solids (SS/ VSS) Table 4 shows the amounts of the SS and VSS retained in the reactors. The VSS in a range of 60–80% represented the volatile portion of the solids. In R1, R2, R3 and R4, the SS generally increased along with OLRs
due to the increase in biological solids inventory. In R5 and R6, the SS decreased sharply from 47.1 to 11.2 g/L and from 39.6 to 15.2 g/L, along with the OLR increased from 2 to 8 g COD/L.d, likely attributed to the occurrence of bioparticles flotation. Large amount of solids was washed out when the bioparticles floated apart from the sludge blanket. The flocs in R5 and R6 settled to the sludge blanket upon 12 g COD/L.d, and the SS of the system increased to 21.8 and 27.7 g/L subsequently. Compared with R3 and R4, the lower concentration of SS in R5 and R6 at OLRs higher than 12 g COD/L.d demonstrated that the overdose of polymer may lead to poor solids settleability. The decreases of SS values at high OLRs could be attributed to the biomass wash-out when reactors were operated near their loading capacity. The SS in R3, R4 and R5 decreased, respectively, from 62.0 to 56.6 g/L, from 60.8 to 50.2 g/L and from 50.0 to 46.3 g/L after the OLR reached 24 g COD/L.d. Decrease of SS from 49.8 to 47.5 g/L was also recorded in R6 after the OLR achieved 16 g COD/L.d. These data indicated the extent of biomass wash-out had been reduced by adding appropriate dosage of polymer, which in turn, increased the organic loading capacities of the polymer-assisted reactors. 3.5.2. Specific methanogenic activity (SMA) Metabolic activities of granules can be expressed in terms of specific methanogenic activity (SMA). As an
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Table 4 Granule properties at each OLR Parameters
R1 (Control)
R2 (20 mg polymer/L)
R3 (40 mg polymer/L)
R4 (80 mg polymer/L)
R5 (160 mg polymer/L)
R6 (320 mg polymer/L)
SS (g/L) at each OLR 2 4 8 12 16 24 32
21.4 38.7 43.1 34.3 50.7 44.6
37.3 38.2 42.3 46.0 41.3 55.4 30.3
34.9 36.3 38.5 45.7 51.3 62.0 56.6
37.3 33.4 41.3 45.4 59.6 60.8 50.2
47.1 31.1 11.2 21.8 42.8 50.0 46.3
39.6 20.9 15.2 27.2 49.8 47.5 44.4
SS (g/L) at each OLR 2 4 8 12 16 24 32
12.7 25.9 32.5 26.8 41.4 36.6
21.0 21.0 29.0 32.0 27.8 43.9 24.2
25.4 22.7 25.4 40.1 45.1 53.7 49.4
20.9 22.8 28.1 31.2 52.3 54.4 43.6
24.6 20.3 7.5 16.7 35.0 44.9 41.8
23.6 17.6 12.5 23.1 43.5 40.2 38.7
2.4 2.0 1.6 1.4
2.0 2.4 2.3 2.1 1.8
1.7 2.1 2.4 2.1 2.1
1.8 2.2 2.5 2.2 2.1 1.9
1.6 1.6 2.4 2.1 1.9 1.7
1.6 1.3 2.4 2.0 2.0
44.9 50.0 46.7
48.7 51.7 58.2 49.4
35.1 40.2 55.5 52.8 42.4
38.3 54.8 64.2 62.4 47.3 36.3
29.2 49.5 46.2 52.2 45.3 36.2
26.7 30.1 42.6 44.9 38.3
23.2 29.2 21.7
28.8 23.5 19.0 29.7
31.2 29.0 23.5 32.2 36.3
24.2 23.3 19.8 26.9 28.3 30.1
30.3 25.7 25.6 35.6 33.3 36.3
33.5 34.7 30.3 36.3 36.5
SMA (g CH4-COD/g VSS.d) at each OLR 8 12 16 24 32 40 SV (m/h) at each OLR 8 12 16 24 32 40 SVI (mL/g SS) at each OLR 8 12 16 24 32 40
important characteristic of granular sludge, SMA was measured at each steady-state in all reactors. SMA of bioparticles in all reactors was shown in Table 4. Low SMA values of R5 and R6 at 8 and 12 g COD/ L.d were noted, which was attributable to the fact that flocs were formed and began to float. The SMA of granules in R1, R2, R3 and R4 at 12 g COD/ L.d were, respectively, 2.04, 2.37, 2.06 and 2.16 g CH4COD/g VSS.d, while SMA of bioparticles in R5 and R6 were, respectively, only 1.59 and 1.3 g CH4-COD/g VSS.d at the same OLR. After OLRs reached 16 g COD/
L.d, SMA of granules in all reactors dropped slowly and the differences between them were not obvious. Beyond 12 g COD/L.d, the highest SMA values in R4 indicated that the granules with the highest methanogenic activity was developed at 80 mg/L, which was the optimum concentration of polymer addition for the enhancement of the granular sludge methanogenic activities. The SMA results corresponded with the study by ElMamouni [24] reporting that the specific activity of granules could be enhanced in reactors supplemented with polymers.
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3.5.3. Granule settleability A main advantage of UASB process is that no support material is required for retention of active sludge, which could be achieved by the formation of settleable granules. Hence, the development of granules with high settleabilities is a prerequisite for successful operation of the UASB process. Settleability of granules could be indicated by measurement of settling velocity (SV). Granular sludge can be divided into three fractions based on the reported settling velocities: a poor settling fraction, a satisfactorily settling fraction, and a good settling fraction, with settling velocities up to 20 m/h, from 20 to 50 m/h, and over 50 m/h, respectively [5]. As shown in Table 4, all granules with size larger than 0.5 mm in all reactors exhibited satisfactory settling velocity over 20 m/h. At low OLR of 8 g COD/L.d, granules in R2 dosed with 20 mg/L polymer demonstrated the highest settling velocity. This could be due to the fact that granules in the reactors were formed at different OLRs. Biomass in R1 and R2 began to form granules at 8 g COD/L.d. While in R3, R4, R5 and R6, granulation took place at higher OLR of 12 g COD/L.d. At high OLRs beyond 8 g COD/L.d, granules in R4 with 80 mg/L polymer achieved the highest settling velocity at each OLR. At 16, 24 and 32 g COD/L.d, the effects of 80 mg/L polymer on granule settleability ranging from 47.29 to 64.21 m/h were superior compared with the other reactors. This gave a good indication that 80 mg/L polymer was the optimum dosage for improving the bioparticles settleability. Settling velocities of granules ranging from near 0 to 52 m/h were reported by Blaszczyk et al. [29]. All granules examined in the present study exhibited settling velocities in the range reported by Blaszczyk and his co-workers, with several measurements even exceeding the range reported. The positive effect of cationic polymer can be observed based on the higher settling velocity in the polymer-assisted reactors especially in R4 dosed with a concentration of 80 mg/L. 3.5.4. Sludge volume index (SVI) Reduction of SVI, which is an important physical parameter, was generally considered as an indicator of improvement in granule settleability. SVI of granules in all reactors, which has inverse ratio with settling velocity, were also shown in Table 4. At 12 g COD/L.d, the SVI of 23.26 mL/g SS in R4 was lower than 29.20, 23.45, 29.00, 25.66 and 34.65 mL/g SS in R1, R2, R3, R5 and R6, respectively. At high OLRs, granules in R4 consistently demonstrated better SVI values than the other reactors. SVI of granules in R4 was 9% lower than R2, 16% lower than R3, 24% lower than R5 and 26% lower than R6 at 24 g COD/L.d. At 32 g COD/L.d, SVI of granules in R4 was 22%,
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15% and 22% lower than granules in R3, R5 and R6, respectively. At all OLRs, SVI values of bioparticles in R5 and R6 were obviously higher than R4. This result further demonstrated the overdose of polymer had not enhanced particle settleability. The SVI results corresponded with the settling velocity discussed previously, which indicated that 80 mg/L was the optimum dosage of polymer in providing positive effect on granule settleability. Enhanced granule settleability as indicated by settling velocity and SVI results could be due to improved bacterial adhesion possibly caused by addition of the cationic polymer. The positively charged polymer may form a bridge among negatively charged bacterial cells through electrostatic charge attraction. The resulting chain-like structure may lead to a complex network structure for enhanced bacterial aggregation [30,31]. Settling velocity and SVI of granules from those reactors terminated were not shown at OLRs of 24, 32 and 40 g, COD/L.d, as these OLRs were beyond the reactors’ loading capacity.
3.5.5. Granule strength Development of granules with adequate mechanical strength is a prerequisite for a successful UASB process operation. The strength of granules was expressed in terms of integrity coefficient (z) defined as the ratio of solids in the residue to the total weight of the granular sludge. Although the integrity coefficient does not indicate an absolute shear strength, it is assumed to represent relative strength of granules against hydraulic abrasion and shear, which granules often undergoes during reactor operation. The integrity coefficient representing strength of granules cultivated under different organic loading rate is depicted in Fig. 5. The strength increased with the OLR up to 16 g COD/L.d and subsequently declined beyond this OLR. At OLRs beyond 16 g COD/L.d, the strength of granules generally increased along with the increase of polymer concentration at each OLR. The strength of granule in R4 with 80 mg/L polymer was 93% at 16 g COD/L.d, which was higher than 81%, 90%, 90%, 81% and 85% in R1, R2, R3, R5 and R6, respectively. When the OLR reached 24 g COD/L.d at which R1 was terminated, the strength of granules in R4 was 92% indicating better granule strength in R4 as compared with 80%, 89%, 85% and 84% in R2, R3, R5 and R6, respectively. These results indicated that granules in R4 exhibited the highest granule strength among the reactors at OLRs higher than 12 g COD/L.d. The polymer concentration of 80 mg/L in R4 appeared to be the optimum concentration for formation of strong granules. This result corresponded well with the results of settling velocity and SVI discussed previously.
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Fig. 5. Strength of granules.
4. Conclusions The results of operation at loadings up to 40 g COD/ L.d indicated that incorporation of cationic polymer during seeding could significantly accelerate the start-up of UASB reactor. The reactor added with the optimum
dosage of 80 mg/L polymer could reduce the start-up time by more than 52% at an OLR of 8 g COD/L.d when compared with the control reactor without applying polymer. Compared with the control reactor, the polymerassisted reactors exhibited better reactor performance
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and better granule characteristics throughout the operation. Characterization of granules showed that the granules in reactor incorporating 80 mg/L polymer exhibited the best settleability at all OLRs. The polymer had not caused inhibition to biomass activity, and the results showed that granules’ specific activities were enhanced in the reactors supplemented with polymers. The strength of granules in the reactor with the optimum dosage of polymer was also significantly enhanced by the cationic polymer. The higher COD removal efficiency in the polymerassisted reactors demonstrated that incorporating the cationic polymer could improve the COD removal efficiency and the extent of improvement depended on the concentration of the polymer. Results had demonstrated that the concentration of cationic polymer at 80 mg/L appeared to be the optimum dose in enhancing the UASB organics removal efficiency. From the results obtained, the reactors added with 80 and 160 mg/L polymer were able to achieve the highest organic loading capacity of 40 g COD/L.d, as compared with all the other reactors. The result demonstrated that the organic-loading capacity of UASB reactors could be significantly increased by incorporating appropriate dosage of cationic polymer as the coagulant aid during seeding. The laboratory results obtained demonstrated positive effects of cationic polymer on granulation, start-up and reactor performance in UASB operation. Adding the cationic polymer could result in improvement in granule characteristics and shortening start-up period, which leads to improvement in organics removal efficiency and loading capacity of UASB system. The cationic polymer enhancement in reactor with 80 mg/L concentration demonstrated the most significant effects on biogranulation and reactor start-up, possibly due to the optimum concentration of polymer.
References [1] Lettinga G, Van Velsen AF, Hobma SW, Zeeuw Wde, Klapwy A. Use of the upflow sludge blanket (USB) reactor concept for biological wastewater treatment especially for anaerobic treatment. Biotechnol Bioeng 1980;22: 699–734. [2] Lettinga G, Hulshoff Pol LW, Koster IW, Wiegant WM, Zeeuw WJ, de Rinzema A, Grin PC, Roersma RE, Hobma SW. High-rate anaerobic wastewater treatment using the UASB reactor under a wide range of temperature conditions. Biotechnol Genetic Eng Rev 1984;2:253–84. [3] Alphenaar PA, Visser A, Lettinga G. The effect of liquid upflow velocity and hydraulic retention time on granulation in UASB reactors treating wastewater with a highsulphate content. Bioresources Technol 1993;43:249–58. [4] Fang HHP, Chui HK. Maximum COD loading capacity in UASB reactors at 37 C. J Environ Eng 1993;119:103–19.
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[5] Schmidt JE, Ahring BK. Granular sludge formation in upflow anaerobic sludge blanket (UASB) reactors. Biotechnol Bioeng 1996;49(3):229–46. [6] Tay JH, Yan YG. Influence of substrate concentration on microbial selection and granulation during start-up of upflow anaerobic sludge blanket reactors. Water Environ Res 1996;68:1140–50. [7] Alves M, Cavaleiro AJ, Ferreira EC, Amaral AL, Mota M, da Motta M, Vivier H, Pons MN. Characterization by image analysis of anaerobic sludge under shock conditions. Water Sci Technol 2000;41:207–14. [8] Fang HHP, Chui HK, Li YY. Microbial structure and activity of UASB granules treating different wastewaters. Water Sci Technol 1994;30(12):87–96. [9] Yan YG. Characterization of microbial granulation process in upflow anaerobic sludge blanket (UASB) reactor. PhD thesis, Nanyang Technological University, Singapore, 1996. [10] Liu Y, Xu HL, Show KY, Tay JH. Anaerobic granulation technology for wastewater treatment. World J Microbiol Biotechnol 2002;18:99–113. [11] McCarty PL, Smith DP. Anaerobic wastewater treatment. Environ Sci Technol 1986;20:1200–6. [12] Versprille AT. Anaerobic treatment of potato-production wastewater. Report No. 16, IBVL, Wageningen, Netherlands, 1978 [in Dutch]. [13] Pette KC, Vletter Rde, Wind E, Gils Wvan. Development of an industrial anaerobic treatment plant for beet wastewater. Report Min of Public Health and Environmental Protection, No. 33, 1979 [in Dutch]. [14] Weiland P, Rozzi A. The start-up, operation and monitoring of high-rate anaerobic treatment systems: discusser’s report. Water Sci Technol 1991;24(8):257–77. [15] Mahoney EM, Varangu LK. The effect of calcium on microbial aggregation during UASB reactor start-up. Water Sci Technol 1987;19:249–60. [16] Guiot SR, Gorur D, Kennedy JK. Nutritional and environmental factors contributing to microbial aggregation during upflow anaerobic sludge bed filter (UBF) reactor start-up. In: Hall ER, Hobson P, editors. Fifth International Symposium on Anaerobic Digestion. Italy: Bologna; 1988. p. 47–53. [17] Grotenhuis JTC. Structure and stability of methanogenic granular sludge. PhD thesis, Agricultural University of Wageningen, Netherlands, 1992. [18] Arcand Y, Guiot SR, Desrochers M, Chavarie C. Impact of the reactor hydrodynamics and organic loading on the size and activity of anaerobic granules. J Chem Eng 1994;56:23–35. [19] Araki N, Kawai N. Surface immobilized anaerobic bacteria granule and treatments of wastewater. Patent No. 62279887, Japan, 1987. [20] Chigusa K, Matsumoto J. Gel-reinforced self-granulated sludge. Patent No. 3254895, Japan, 1991. [21] Tay JH, Show KY, Jeyaseelan S. Effects of media characteristics on the performance of upflow anaerobic packed-bed reactors. J Environ Eng ASCE 1996;122(6): 469–76. [22] Tay JH, Show KY. Media-induced hydraulic behavior and performance of upflow biofilters. J Environ Eng ASCE 1998;124(8):720–9.
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[23] Show KY, Tay JH. Influence of support media on biomass growth and retention in anaerobic filters. Water Res 1999;33(6):1471–81. [24] El-Mamouni R, Leduc R, Guiot SR. Influence of synthetic and natural polymers on the anaerobic granulation process. Water Sci Technol 1998;38(8–9):341–7. [25] APHA. Standard methods for the examination of water and wastewater. Washington, DC: American Public Health Association (APHA); 1998. [26] Ghangrekar MM, Asolekar SR, Ranganathan KR, Joshi SG. Experience with UASB reactor start-up under different operating conditions. Water Sci Technol 1996;34(5–6):421–8. [27] Thaveesri J, Gernaey K, Kaonga B, Boucneau G, Verstraete W. Organic and ammonium nitrogen and
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oxygen in relation to granular sludge growth in lab-scale UASB reactors. Water Sci Technol 1994;30(12):43–53. Lettinga G, Zehnder JTC, Gretenhuis LWen, Hulshoff Pol. Granular anaerobic sludge; microbiology and technology. Proceedings of the GASMAT-Workshop, Netherlands; 1987. p. 25–7. Blaszczyk R, Gardner D, Kosaric N. Response and recovery of anaerobic granules from shock loading. Water Res 1994;28(3):675–80. Wirtz RA, Dague RR. Enhancement of granulation and start-up in the anaerobic sequencing batch reactor. Water Environ Res 1996;68(5):883–92. Yoda M, Kitagawa M, Miyaji Y. Granular sludge formation in the anaerobic expanded micro-carrier bed process. Water Sci Technol 1989;21:109–20.
Water Research 38 (2004) 2293–2304
Accelerated start-up and enhanced granulation in upflow anaerobic sludge blanket reactors Kuan-Yeow Show*, Ying Wang, Shiu-Feng Foong, Joo-Hwa Tay School of Civil and Environmental Engineering, Nanyang Technological University, Block N1#1A-29, Nanyang Ave., Singapore 639798, Singapore Received 1 July 2003; received in revised form 6 January 2004; accepted 30 January 2004
Abstract In the present study, the effects of a cationic polymer on reactor start-up and granule development were evaluated. A control reactor R1 was operated without adding polymer, while the other five reactors designated R2, R3, R4, R5 and R6 were operated with different polymer concentrations of 20, 40, 80, 160 and 320 mg/L, respectively. Experimental results demonstrated that adding the polymer at a concentration of 80 mg/L markedly accelerated the start-up time. The time required to reach stable treatment at an organic loading rate (OLR) of 4 g COD/L.d was reduced by approximately 50% in R4 as compared with the control reactor. The same reactor with 80 mg/L polymer was able to achieve an OLR of 12 g COD/L.d after 59 days of operation, while R1, R2, R3, R5 and R6 achieved the same loading rate at much longer period of 104, 80, 69, 63 and 69 days, respectively. Comparing with the control reactor, the start-up time of R4 was shortened markedly by about 43% at this OLR, while other reactors also recorded varying degree of shortening. Monitoring on granule development showed that the granule formation was accelerated by 30% from the use of the appropriate dosage of polymer. Subsequent granules characterization indicated that the granules developed in R4 with 80 mg/L polymer exhibited the best settleability, strength and methanogenic activity at all OLRs. The organic loading capacities of reactors were also increased by the polymer addition. The maximum organic loading of the control reactor was 24 g COD/L.d, while the polymer-assisted reactor added with 80 mg/L polymer attained a markedly increased organic loading of 40 g COD/L.d. The laboratory results obtained demonstrated that adding the cationic polymer could result in shortening of start-up time and enhancement of granulation, which in turn lead to improvement in organics removal efficiency and loading capacity of the UASB system. r 2004 Elsevier Ltd. All rights reserved. Keywords: UASB; Granulation; Start-up; Polymer; Granule characteristics
1. Introduction Over the past two decades, upflow anaerobic sludge blanket (UASB) technology has been employed for wastewater treatment [1–6]. More than 900 UASB units are currently being operated all over the world [7]. It exhibits positive features such as high organic loadings, low energy demand, short hydraulic retention time *Corresponding author. Tel.: +9-65-6791-5282; fax: +9-656791-0676. E-mail address: [email protected] (K.-Y. Show).
(HRT) and easy reactor construction. Important parameters affecting the treatment efficiency of UASB reactors include the granulation process in the reactor, the characteristics of the wastewater to be treated, the selection of inoculum material, the influence of nutrients and several other environmental factors. Among these parameters, the granulation process is believed to be the most critical one [8]. In UASB reactors, the biomass is retained as granules formed by the naturally self-immobilization of the bacteria. Sludge granules are those tightly structured aggregates, which have a clear-cut shape and some
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.01.039
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mechanical strength. They do not break up or disintegrate easily even under fairly high hydraulic and gaseous shear stresses. However, loosely structured sludge flocs can easily fall apart when subjected to a moderate shear stress [9]. Granular sludge is the main prominent characteristic of UASB reactors as compared to other anaerobic technologies [10]. There is a close correlation between efficiency of an UASB reactor and development of granular sludge [1]. Granulation not only significantly enhances the settleability of biomass leading to effective bacterial retention in the reactor, but also improves physiological conditions making them favourable for bacteria and their interactions, especially syntrophs in the anaerobic system [11]. The formation and stability of the granules are essential for successful operation [5]. Reactor start-up is a very important economic process step, because during this period the productivity of the wastewater supplier must be adapted to the capacity of the wastewater treatment plant. This leads in all cases to a dramatic reduction in the production capacity and can result in a strong deterioration of the product quality, when the adaption of the productivity can be achieved only by change from continuous to discontinuous operation of the production facilities. Inadequate start-up causes poor subsequent treatment and often requires expensive system maintenance and effluent post-treatment. Start-up is often considered to be the most unstable and difficult phase in anaerobic digestion. Its main task is to develop a highly active settleable sludge as quickly as possible. While as anaerobic bacteria are slowgrowing microorganisms, major problems encountered with UASB are the typical long reactor start-up and spontaneous development of biogranulation. Start-up times for UASB reactors with digested sewage sludge usually take several months [12,13]. Hence, long start-up period is the major deterrent to the use of UASB system. The reduction of start-up time is one of the key parameters to increase the competitiveness of high-rate anaerobic reactors [14]. Recent advances in understanding the fundamentals of the biochemistry and microbiology of granulation in UASB system have led to successful applications, which show a great deal of promise in overcoming its long start-up time limitation. Some approaches have been developed to enhance the granulation and shorten the start-up period. Physical– chemical and biological factors promoting granulation have been documented [15–18]. To remedy the drawback of long start-up period and extend the application of UASB reactors, the present study has been conducted in order to accelerate the start-up and natural granulation process by incorporation of cationic polymer. In anaerobic reactors, polymers have been used either to immobilize the anaerobic sludge within gel beads or to reinforce the strength of the already existing granules, by coating the granule
surfaces with a thin layer of polymer [19,20]. This project aims to measure the optimum dosage of cationic polymer by evaluation of the effects of polymer on UASB start-up and development of granules. The effects on the characteristics of granules and reactor operation and performance under the influence of the polymer were investigated.
2. Materials and methods 2.1. Materials and reactor system Six 4.4 L plexiglass UASB reactors with an internal diameter of 100 mm and a height of 680 mm were used in this study. The reactors were placed in a walk in temperature-control room set at 3571 C. A refrigerator, set at 4 C, housed the substrate storage tanks to prevent premature degradation. Synthetic substrate was pumped into the reactor inlet by peristaltic pumps (ColeParmer, MasterFlexs L/St). Wet gas meters (Ritter TG 05) were installed to measure the gas production. To avoid severe corrosion and possible damage to the gas meter, the biogas produced from each reactor was first passed through a water trap before being channelled to the gas metre. The experimental system set-up is shown in Fig. 1. All reactors were inoculated with 50% of digested sludge obtained from an anaerobic digester of the Jurong Water Reclamation Plant treating industrial and domestic wastewater. The characteristics and elements of the seed sludge are shown in Table 1. The components of a synthetic substrate (Table 2) were mixed together in a tank after adding the necessary nutrients and buffering chemicals. Trace components were similar to that used for cultivating anaerobic bacteria by Yan [9]. Buffer capacity was provided by sodium bicarbonate at levels sufficient to maintain pH in the system between 6.5 and 7.3. Two essential
Fig. 1. Schematic diagram of experiment setup.
ARTICLE IN PRESS K.-Y. Show et al. / Water Research 38 (2004) 2293–2304 Table 1 Characteristics of seed sludge Characteristics Suspended solids (SS) Volatile suspended solids (VSS) Sludge volume index (SVI) Specific methanogenic activity (SMA)
Median particle size (diameter)
72.5 g/L 30 g/L 41.2 mL/ g SS 0.5 g CH4COD/g VSS.d 101 mm
Elemental contents Element
Content (mg/g)
Element
Content (mg/g)
Ca Al Mg Mn K Na Mo
39.04 16.55 3.55 0.43 2.47 4.95 0.075
Fe S Ni Zn Cu P
28.89 143.67 4.56 21.26 24.45 79.88
Table 2 Synthetic substrate composition in mg/L (Based on COD of 5000 mg/L) Carbon source Peptone Glucose Meat extract Macro-nutrients CaCl2 H2O MgSO4 7H2O NH4Cl FeSO4 7H2O KH2PO4
1000 3400 700
48 54 800 40 200
Micro-nutrients H3BO3 ZnCl2 CuCl2 MnSO4 H2O (NH4)6Mo7O24 4H2O AlCl3 CoCl2 6H2O NiCl2 Conc.HCl (36%)
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 2 mL
Alkalinity buffer NaHCO3
3000
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components for biomass synthesis, namely nitrogen and phosphorus, were added in the form of NH4Cl and KH2PO4, respectively. To sustain an active microbial culture, a COD:N:P ratio of 100:10:1 [21–23] was adopted. The substrate was prepared once every two days and stored below 4 C to prevent premature degradation. The commercially available polymer with exclusive chemical compositions has been used extensively as a coagulant aid in water treatment systems. Coagulation action of the cationic polymer is promoted by charge neutralization on colloids by the ionized sites as well as bridge formation. ‘‘AA 184 H’’ cationic polymer was supplied by courtesy of Ondeo Nalco Pacific Pte. Ltd. and its vast application in the wastewater treatment industry as a sludge conditioner had prompted its use in this study. This polymer has a surface charge density of 16% and its molecular weight ranges between 1 and 10 million. 2.2. Experimental procedure and analytical methods Six laboratory-scale UASB reactors designated as R1, R2, R3, R4, R5 and R6 were operated to evaluate the UASB start-up, performance and characteristic of the granules developed when different polymer concentrations were added. During the start-up, all reactors were fed with the same COD concentration. R1 was operated without polymer addition serving as a control reactor. Based on a study by El-Mamouni et al. [24], an optimum dose of cationic polymer of 2 mg/g SS with OLR increased from 0.2 to 0.6 g COD/g VSS.d was used for reference. To examine the effects of polymer concentration, reactors R2, R3, R4, R5 and R6 were dosed with polymer of 20, 40, 80, 160 and 320 mg/L, respectively, together with the inoculum during the seeding. The reactors were started up simultaneously with an influent COD of 5000 mg/L and a corresponding organic loading rate of 2 g COD/L.d, which was computed based on the concentration of COD loaded into the reactor per unit hydraulic retention time. The reactors were operated until steady-state conditions were reached (evidenced by constant gas production and COD removal). Evaluation of the system was carried out during the steady-state conditions. The OLR was then step increased by shortening the hydraulic retention time (HRT) and maintaining the influent COD at 5000 mg/L throughout the study. The reactors were operated until the next steady-state conditions were reached. pH, alkalinity, COD, SS, VSS, sludge volume index (SVI) and specific gravity (SG) of sludge were conducted in accordance with Standard Method [25]. Gas production of each reactor was recorded daily from a wet gas meter (Ritter TG 05). The gas composition was analyzed by a Gas Chromatograph (Hewlett-Packard HP 5890 A) for methane, carbon dioxide, and nitrogen. The particle/ granule size was measured by laser particle size analysis system (Malvern Master Sizer Series 2600, Malvern
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Instruments Inc., Southborough, Massachusetts) and image analysis system (Quantimet 500 Image Analyzer, Lerca Cambridge Instruments, Cambridge, UK) when the particle size was beyond the Maser Sizer’s limitation (0.05–550 mm). The resistant capacity of granules against disintegration during the operation can be expressed with integrity coefficient (%) described by Ghangrekar et al. [26]. The volatile suspended solids concentration of settled granules and its disintegrated supernatant were measured after a period of mechanical agitation and settling. The integrity coefficient (%) was then expressed as the ratio of residual granule VSS and total VSS. A higher integrity coefficient implies higher strength of the granules. Settling velocity of granules was determined according to the method proposed by Thaveesri et al. [27]. Measurements of specific methanogenic activity (SMA) were conducted according to the method reported by Yan [9] using a similar complex proteincarbohydrate synthetic substrate (Table 2).
3. Results and discussion 3.1. Start-up The influent substrate concentrations were maintained at 5000 mg COD/L in all reactors. Pseudo-steady-state 90
condition was considered attained when COD removal and the biogas production were relatively consistent within 5% for three consecutive days. As indicated in Fig. 2, steady gas production was used to ascertain that reactors could be loaded to the next OLR. The organic loading rate was subsequently step increased to the next higher rate through shortening of HRT. A major benefit of enhanced granulation is shortening of start-up period to reach a specified organic loading. The operation time required to reach steady-state treatment at each OLR and the corresponding COD removal efficiency are shown in Fig. 3 and Table 3. At all OLRs, the 80 mg/L polymer added in R4 provided the shortest start-up periods. Compared with the other concentrations, the polymer concentration at 80 mg/L could significantly accelerate the start-up time. To reach stable treatment of 8 g COD/L.d, the time needed in the control reactor R1 was 92 days, while the times required were 67, 52, 44, 48 and 50 days in R2, R3, R4, R5 and R6, respectively. The start-up time in R4 was markedly reduced by 52% compared with R1. The time required to reach stable treatment in R2, R3, R4, R5 and R6 at OLR of 16 g COD/L.d was reduced by 20%, 23%, 36%, 34% and 29% as compared with R1. Reactors R3, R5 and R6 reached stable treatment at OLR of 32 g COD/ L.d on 140, 122 and 132 days, respectively, while R4 achieved the same OLR on shorter time of 118 days. 35
30
60
25
50
20
40
15
30
10
20
5
10
0
0
R2 (20 mg/LPolymer)
R1 (Control)
80
30
70 60
25 20
50 40
15
30 20
10 5
10 0 0
20
40
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80
100
120
0
140 35
80
30
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50
70 R3 (40 mg/LPolymer) 60
30
15
20
10
10
5
0
0 0
20
40
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100
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140
90
160 45
R5 (160 mg/LPolymer)
80
40
70
35
0
20
40
60
80
100
120
140
45 R4 (80 mg/LPolymer)
40 35 30 25
40
20
30
15
20
10
10
5 0
0 0
20
40
60
80
100
120
140
80
160 35
R6 (320 mg/LPolymer)
70
30
60
25
60
30
50
25
40
20
40
30
15
30
20
10
20
10
10
5
10
5
0
0 160
0
0
20
40
60
80
100
120
140
50
20 15
0
0
20
40
60
80
100
Operation Time (day) Gas P r o d uctio n Rate
OLR
Fig. 2. Gas production rates in each reactor.
120
140
160
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Fig. 3. Operation time and COD removal efficiency of each reactor.
These results demonstrated the polymer concentration at 80 mg/L added in R4 provided the optimum dosage for acceleration of start-up time of UASB reactor. This could be a result of the formation of a protective gelatinous layer around cells from the addition of polymer. The protective layer would act like a shield
for bacterial cells when they are subjected to organic shocks or sudden increment in hydraulic abrasive forces due to the changes in OLR. An overdose of polymer addition would prolong the time acquired to achieve steady state of designed OLRs in comparison with the reactor added with the optimum concentration of
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2298 Table 3 Operating time at each OLR (days) Reactors
R1 R2 R3 R4 R5 R6
(control) (20 mg/L polymer) (40 mg/L polymer) (80 mg/L polymer) (160 mg/L polymer) (320 mg/L polymer)
OLR (g COD/L.d) 2
4
8
12
16
24
32
26 26 14 14 14 14
57 35 33 29 29 29
92 67 52 44 48 50
104 80 69 59 63 69
115 92 88 74 76 82
127 103 113 95 99 105
110 140 118 122 132
cationic polymer. This might have resulted from the formation of a thick polymeric gel around cells that inhibited substrate diffusion into and out of the microbial mass. Similarly, an underdose of polymer as reflected in R2 and R3 could be a result of the formation of loose biomass flocs that were unable to shorten diffusion distances significantly. 3.2. COD removal efficiency Fig. 3 shows the COD removal efficiency of all reactors without experiencing major system upset. At 2 g COD/L.d, COD removal efficiencies of 83% and 80% in R5 and R6 added with 160 and 320 mg/L polymer, respectively, were lower than the control reactor R1 (91%), and the polymer-assisted reactors R2 (92%), R3 (93%) and R4 (90%) with 20, 40 and 80 mg/L polymer supplement. The higher concentrations of polymer in R5 and R6 had not provided satisfactory performance at low OLRs. When OLRs were raised to 4 g COD/L.d, the efficiency in all reactors reached 90%. This could be due to system adaptation of higher doses of polymer in R5 and R6. Floating flocs were observed in R5 on day 49 at 12 g COD/L.d, and the same phenomenon was observed on day 40 in R6 at 8 g COD/L.d. The bioparticles floatation led to the decreases of COD removal efficiency at 12 g COD/L.d in R5 and 8 g COD/L.d in R6, respectively. At higher OLR of 16 g COD/L.d, the COD removal efficiency in all polymer-assisted reactors were almost similar ranging between 93% and 95%, attributed to the flocs in R5 and R6 settling back to the sludge blanket. At high OLRs of 32 g COD/L.d, the efficiency of R4 was 83%, which was higher than 75%, 77%, 80% and 74% in R2, R3, R5 and R6, respectively. The OLRs in R4 and R5 were further increased to 40 g COD/L.d after the other reactors were terminated at 32 g COD/L.d due to low organic removal efficiency. At 40 g COD/L.d, the organic removal efficiency in R4 was 76%, which was higher than 72% in R5. At most OLRs, COD removal efficiency in R4 was the highest among the reactors. The results indicated the optimum dose of polymer of 80 mg/L in improving the reactor treatment performance.
40
131 135
3.3. Reactor loading capacity A significant benefit of system enhancement in any anaerobic process is the increased capacity to remove COD. After the OLR reached 24 g COD/L.d on day 116, the COD removal efficiency of R1 dropped substantially from 89% to 73%. The gas production dropped significantly while the VFA increased drastically, indicating that the 24 g COD/L.d was the maximum capacity for R1. For R2, R3 and R6, the operations were continued until they were terminated at 32 g COD/L.d, at which loading the removal efficiencies dropped sharply. While at 32 g COD/L.d, COD removal efficiency in R4 and R5 still could reach 83% and 80%, then the OLR in R4 and R5 were further increased to 40 g COD/L.d after the other reactors were terminated. Both reactors were terminated at 40 g COD/L.d, which seemed to be the maximum capacity for R4 and R5. Compared with the control reactor, the organic loading capacity was improved to 32 g COD/L.d by adding 20 mg/L polymer, and further improved to 40 g COD/L.d by adding higher polymer dosages of 80 and 160 mg/L. 3.4. Formation of granules The bioparticle sizes at each OLR of all reactors versus operation time were shown in Fig. 4. The diameters of particles in all reactors rose steeply and reached their peak diameters and subsequently declined beyond the peak OLRs. At high OLRs of 24 and 32 g COD/L.d, the granules in R5 and R6 added with the highest dosages of polymer of 160 and 320 mg/L, respectively, showed larger size compared with the other reactors. The successful operation of UASB reactor depends on the formation of granules with good settleabilities and degradation activities. A granule could not be judged as a ‘‘better granule’’ only according to its ‘‘larger size’’. It was observed that the ‘‘big granules’’ in R5 and R6 showed loose structure and lots of white and black fluffy flocs attached on the surface. The characteristics of these granules were not as good as those polymer-assisted reactors with lower dosage of polymer.
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15
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Operation Time (day) OLR
Mean Particle Diameter
Fig. 4. Particle size in each reactor.
Onset of granulation is adopted here as the formation of bioparticle having diameter larger than 0.5 mm [28]. The solids in all polymer-assisted reactors began to form granules with diameter larger than 0.5 mm around day 50, while the granule formation took place in the control reactor around 70 days. The granule formation was accelerated by 30% from the polymer application. The fact that granules were formed earlier in the polymerassisted reactors could be attributed to the fact that biomass aggregation took place at a much shorter period with the incorporation of polymer. The result also indicated that the time required for the biomass aggregation could not be shortened any further by overdosing of polymer. The superior effects resulting from the cationic polymer could be hypothesized that the cationic polymer essentially forms a bridge among negatively charged bacterial cells through electrostatic charge attraction, which results in agglomeration of dense and active biogranules enhancing the reactor performance. 3.5. Granule characteristics 3.5.1. Suspended solids and volatile suspended solids (SS/ VSS) Table 4 shows the amounts of the SS and VSS retained in the reactors. The VSS in a range of 60–80% represented the volatile portion of the solids. In R1, R2, R3 and R4, the SS generally increased along with OLRs
due to the increase in biological solids inventory. In R5 and R6, the SS decreased sharply from 47.1 to 11.2 g/L and from 39.6 to 15.2 g/L, along with the OLR increased from 2 to 8 g COD/L.d, likely attributed to the occurrence of bioparticles flotation. Large amount of solids was washed out when the bioparticles floated apart from the sludge blanket. The flocs in R5 and R6 settled to the sludge blanket upon 12 g COD/L.d, and the SS of the system increased to 21.8 and 27.7 g/L subsequently. Compared with R3 and R4, the lower concentration of SS in R5 and R6 at OLRs higher than 12 g COD/L.d demonstrated that the overdose of polymer may lead to poor solids settleability. The decreases of SS values at high OLRs could be attributed to the biomass wash-out when reactors were operated near their loading capacity. The SS in R3, R4 and R5 decreased, respectively, from 62.0 to 56.6 g/L, from 60.8 to 50.2 g/L and from 50.0 to 46.3 g/L after the OLR reached 24 g COD/L.d. Decrease of SS from 49.8 to 47.5 g/L was also recorded in R6 after the OLR achieved 16 g COD/L.d. These data indicated the extent of biomass wash-out had been reduced by adding appropriate dosage of polymer, which in turn, increased the organic loading capacities of the polymer-assisted reactors. 3.5.2. Specific methanogenic activity (SMA) Metabolic activities of granules can be expressed in terms of specific methanogenic activity (SMA). As an
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Table 4 Granule properties at each OLR Parameters
R1 (Control)
R2 (20 mg polymer/L)
R3 (40 mg polymer/L)
R4 (80 mg polymer/L)
R5 (160 mg polymer/L)
R6 (320 mg polymer/L)
SS (g/L) at each OLR 2 4 8 12 16 24 32
21.4 38.7 43.1 34.3 50.7 44.6
37.3 38.2 42.3 46.0 41.3 55.4 30.3
34.9 36.3 38.5 45.7 51.3 62.0 56.6
37.3 33.4 41.3 45.4 59.6 60.8 50.2
47.1 31.1 11.2 21.8 42.8 50.0 46.3
39.6 20.9 15.2 27.2 49.8 47.5 44.4
SS (g/L) at each OLR 2 4 8 12 16 24 32
12.7 25.9 32.5 26.8 41.4 36.6
21.0 21.0 29.0 32.0 27.8 43.9 24.2
25.4 22.7 25.4 40.1 45.1 53.7 49.4
20.9 22.8 28.1 31.2 52.3 54.4 43.6
24.6 20.3 7.5 16.7 35.0 44.9 41.8
23.6 17.6 12.5 23.1 43.5 40.2 38.7
2.4 2.0 1.6 1.4
2.0 2.4 2.3 2.1 1.8
1.7 2.1 2.4 2.1 2.1
1.8 2.2 2.5 2.2 2.1 1.9
1.6 1.6 2.4 2.1 1.9 1.7
1.6 1.3 2.4 2.0 2.0
44.9 50.0 46.7
48.7 51.7 58.2 49.4
35.1 40.2 55.5 52.8 42.4
38.3 54.8 64.2 62.4 47.3 36.3
29.2 49.5 46.2 52.2 45.3 36.2
26.7 30.1 42.6 44.9 38.3
23.2 29.2 21.7
28.8 23.5 19.0 29.7
31.2 29.0 23.5 32.2 36.3
24.2 23.3 19.8 26.9 28.3 30.1
30.3 25.7 25.6 35.6 33.3 36.3
33.5 34.7 30.3 36.3 36.5
SMA (g CH4-COD/g VSS.d) at each OLR 8 12 16 24 32 40 SV (m/h) at each OLR 8 12 16 24 32 40 SVI (mL/g SS) at each OLR 8 12 16 24 32 40
important characteristic of granular sludge, SMA was measured at each steady-state in all reactors. SMA of bioparticles in all reactors was shown in Table 4. Low SMA values of R5 and R6 at 8 and 12 g COD/ L.d were noted, which was attributable to the fact that flocs were formed and began to float. The SMA of granules in R1, R2, R3 and R4 at 12 g COD/ L.d were, respectively, 2.04, 2.37, 2.06 and 2.16 g CH4COD/g VSS.d, while SMA of bioparticles in R5 and R6 were, respectively, only 1.59 and 1.3 g CH4-COD/g VSS.d at the same OLR. After OLRs reached 16 g COD/
L.d, SMA of granules in all reactors dropped slowly and the differences between them were not obvious. Beyond 12 g COD/L.d, the highest SMA values in R4 indicated that the granules with the highest methanogenic activity was developed at 80 mg/L, which was the optimum concentration of polymer addition for the enhancement of the granular sludge methanogenic activities. The SMA results corresponded with the study by ElMamouni [24] reporting that the specific activity of granules could be enhanced in reactors supplemented with polymers.
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3.5.3. Granule settleability A main advantage of UASB process is that no support material is required for retention of active sludge, which could be achieved by the formation of settleable granules. Hence, the development of granules with high settleabilities is a prerequisite for successful operation of the UASB process. Settleability of granules could be indicated by measurement of settling velocity (SV). Granular sludge can be divided into three fractions based on the reported settling velocities: a poor settling fraction, a satisfactorily settling fraction, and a good settling fraction, with settling velocities up to 20 m/h, from 20 to 50 m/h, and over 50 m/h, respectively [5]. As shown in Table 4, all granules with size larger than 0.5 mm in all reactors exhibited satisfactory settling velocity over 20 m/h. At low OLR of 8 g COD/L.d, granules in R2 dosed with 20 mg/L polymer demonstrated the highest settling velocity. This could be due to the fact that granules in the reactors were formed at different OLRs. Biomass in R1 and R2 began to form granules at 8 g COD/L.d. While in R3, R4, R5 and R6, granulation took place at higher OLR of 12 g COD/L.d. At high OLRs beyond 8 g COD/L.d, granules in R4 with 80 mg/L polymer achieved the highest settling velocity at each OLR. At 16, 24 and 32 g COD/L.d, the effects of 80 mg/L polymer on granule settleability ranging from 47.29 to 64.21 m/h were superior compared with the other reactors. This gave a good indication that 80 mg/L polymer was the optimum dosage for improving the bioparticles settleability. Settling velocities of granules ranging from near 0 to 52 m/h were reported by Blaszczyk et al. [29]. All granules examined in the present study exhibited settling velocities in the range reported by Blaszczyk and his co-workers, with several measurements even exceeding the range reported. The positive effect of cationic polymer can be observed based on the higher settling velocity in the polymer-assisted reactors especially in R4 dosed with a concentration of 80 mg/L. 3.5.4. Sludge volume index (SVI) Reduction of SVI, which is an important physical parameter, was generally considered as an indicator of improvement in granule settleability. SVI of granules in all reactors, which has inverse ratio with settling velocity, were also shown in Table 4. At 12 g COD/L.d, the SVI of 23.26 mL/g SS in R4 was lower than 29.20, 23.45, 29.00, 25.66 and 34.65 mL/g SS in R1, R2, R3, R5 and R6, respectively. At high OLRs, granules in R4 consistently demonstrated better SVI values than the other reactors. SVI of granules in R4 was 9% lower than R2, 16% lower than R3, 24% lower than R5 and 26% lower than R6 at 24 g COD/L.d. At 32 g COD/L.d, SVI of granules in R4 was 22%,
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15% and 22% lower than granules in R3, R5 and R6, respectively. At all OLRs, SVI values of bioparticles in R5 and R6 were obviously higher than R4. This result further demonstrated the overdose of polymer had not enhanced particle settleability. The SVI results corresponded with the settling velocity discussed previously, which indicated that 80 mg/L was the optimum dosage of polymer in providing positive effect on granule settleability. Enhanced granule settleability as indicated by settling velocity and SVI results could be due to improved bacterial adhesion possibly caused by addition of the cationic polymer. The positively charged polymer may form a bridge among negatively charged bacterial cells through electrostatic charge attraction. The resulting chain-like structure may lead to a complex network structure for enhanced bacterial aggregation [30,31]. Settling velocity and SVI of granules from those reactors terminated were not shown at OLRs of 24, 32 and 40 g, COD/L.d, as these OLRs were beyond the reactors’ loading capacity.
3.5.5. Granule strength Development of granules with adequate mechanical strength is a prerequisite for a successful UASB process operation. The strength of granules was expressed in terms of integrity coefficient (z) defined as the ratio of solids in the residue to the total weight of the granular sludge. Although the integrity coefficient does not indicate an absolute shear strength, it is assumed to represent relative strength of granules against hydraulic abrasion and shear, which granules often undergoes during reactor operation. The integrity coefficient representing strength of granules cultivated under different organic loading rate is depicted in Fig. 5. The strength increased with the OLR up to 16 g COD/L.d and subsequently declined beyond this OLR. At OLRs beyond 16 g COD/L.d, the strength of granules generally increased along with the increase of polymer concentration at each OLR. The strength of granule in R4 with 80 mg/L polymer was 93% at 16 g COD/L.d, which was higher than 81%, 90%, 90%, 81% and 85% in R1, R2, R3, R5 and R6, respectively. When the OLR reached 24 g COD/L.d at which R1 was terminated, the strength of granules in R4 was 92% indicating better granule strength in R4 as compared with 80%, 89%, 85% and 84% in R2, R3, R5 and R6, respectively. These results indicated that granules in R4 exhibited the highest granule strength among the reactors at OLRs higher than 12 g COD/L.d. The polymer concentration of 80 mg/L in R4 appeared to be the optimum concentration for formation of strong granules. This result corresponded well with the results of settling velocity and SVI discussed previously.
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Fig. 5. Strength of granules.
4. Conclusions The results of operation at loadings up to 40 g COD/ L.d indicated that incorporation of cationic polymer during seeding could significantly accelerate the start-up of UASB reactor. The reactor added with the optimum
dosage of 80 mg/L polymer could reduce the start-up time by more than 52% at an OLR of 8 g COD/L.d when compared with the control reactor without applying polymer. Compared with the control reactor, the polymerassisted reactors exhibited better reactor performance
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and better granule characteristics throughout the operation. Characterization of granules showed that the granules in reactor incorporating 80 mg/L polymer exhibited the best settleability at all OLRs. The polymer had not caused inhibition to biomass activity, and the results showed that granules’ specific activities were enhanced in the reactors supplemented with polymers. The strength of granules in the reactor with the optimum dosage of polymer was also significantly enhanced by the cationic polymer. The higher COD removal efficiency in the polymerassisted reactors demonstrated that incorporating the cationic polymer could improve the COD removal efficiency and the extent of improvement depended on the concentration of the polymer. Results had demonstrated that the concentration of cationic polymer at 80 mg/L appeared to be the optimum dose in enhancing the UASB organics removal efficiency. From the results obtained, the reactors added with 80 and 160 mg/L polymer were able to achieve the highest organic loading capacity of 40 g COD/L.d, as compared with all the other reactors. The result demonstrated that the organic-loading capacity of UASB reactors could be significantly increased by incorporating appropriate dosage of cationic polymer as the coagulant aid during seeding. The laboratory results obtained demonstrated positive effects of cationic polymer on granulation, start-up and reactor performance in UASB operation. Adding the cationic polymer could result in improvement in granule characteristics and shortening start-up period, which leads to improvement in organics removal efficiency and loading capacity of UASB system. The cationic polymer enhancement in reactor with 80 mg/L concentration demonstrated the most significant effects on biogranulation and reactor start-up, possibly due to the optimum concentration of polymer.
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[5] Schmidt JE, Ahring BK. Granular sludge formation in upflow anaerobic sludge blanket (UASB) reactors. Biotechnol Bioeng 1996;49(3):229–46. [6] Tay JH, Yan YG. Influence of substrate concentration on microbial selection and granulation during start-up of upflow anaerobic sludge blanket reactors. Water Environ Res 1996;68:1140–50. [7] Alves M, Cavaleiro AJ, Ferreira EC, Amaral AL, Mota M, da Motta M, Vivier H, Pons MN. Characterization by image analysis of anaerobic sludge under shock conditions. Water Sci Technol 2000;41:207–14. [8] Fang HHP, Chui HK, Li YY. Microbial structure and activity of UASB granules treating different wastewaters. Water Sci Technol 1994;30(12):87–96. [9] Yan YG. Characterization of microbial granulation process in upflow anaerobic sludge blanket (UASB) reactor. PhD thesis, Nanyang Technological University, Singapore, 1996. [10] Liu Y, Xu HL, Show KY, Tay JH. Anaerobic granulation technology for wastewater treatment. World J Microbiol Biotechnol 2002;18:99–113. [11] McCarty PL, Smith DP. Anaerobic wastewater treatment. Environ Sci Technol 1986;20:1200–6. [12] Versprille AT. Anaerobic treatment of potato-production wastewater. Report No. 16, IBVL, Wageningen, Netherlands, 1978 [in Dutch]. [13] Pette KC, Vletter Rde, Wind E, Gils Wvan. Development of an industrial anaerobic treatment plant for beet wastewater. Report Min of Public Health and Environmental Protection, No. 33, 1979 [in Dutch]. [14] Weiland P, Rozzi A. The start-up, operation and monitoring of high-rate anaerobic treatment systems: discusser’s report. Water Sci Technol 1991;24(8):257–77. [15] Mahoney EM, Varangu LK. The effect of calcium on microbial aggregation during UASB reactor start-up. Water Sci Technol 1987;19:249–60. [16] Guiot SR, Gorur D, Kennedy JK. Nutritional and environmental factors contributing to microbial aggregation during upflow anaerobic sludge bed filter (UBF) reactor start-up. In: Hall ER, Hobson P, editors. Fifth International Symposium on Anaerobic Digestion. Italy: Bologna; 1988. p. 47–53. [17] Grotenhuis JTC. Structure and stability of methanogenic granular sludge. PhD thesis, Agricultural University of Wageningen, Netherlands, 1992. [18] Arcand Y, Guiot SR, Desrochers M, Chavarie C. Impact of the reactor hydrodynamics and organic loading on the size and activity of anaerobic granules. J Chem Eng 1994;56:23–35. [19] Araki N, Kawai N. Surface immobilized anaerobic bacteria granule and treatments of wastewater. Patent No. 62279887, Japan, 1987. [20] Chigusa K, Matsumoto J. Gel-reinforced self-granulated sludge. Patent No. 3254895, Japan, 1991. [21] Tay JH, Show KY, Jeyaseelan S. Effects of media characteristics on the performance of upflow anaerobic packed-bed reactors. J Environ Eng ASCE 1996;122(6): 469–76. [22] Tay JH, Show KY. Media-induced hydraulic behavior and performance of upflow biofilters. J Environ Eng ASCE 1998;124(8):720–9.
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