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Start-up of UASB reactors treating municipal wastewater and effect of temperature/sludge age and hydraulic retention time (HRT) on its performance
⁎Corresponding authors. Tel.: +92 300 9639574. farhat@gcuf.edu.pk (Farhat Abbas), pdiftikhar@yahoo.com (Iftikhar Hussain Bukhari)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The upflow anaerobic sludge blanket reactors seeded with cow dung manure (UASBCD) and activated sludge of a dairy wastewater treatment plant (UASBASDIT) were used to treat raw domestic wastewater of medium strength. The UASBCD reactor required a period of 120 days to start up. In case of UASBASDIT reactor, sludge bed was stabilized in a period of 80 days. The performance of both reactors to treat wastewater was enhanced with an increase in the sludge age and temperature. Under psychrophilic temperature (17 °C) and at early sludge age (60 days), chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal by the reactors were in the range of 57–62% and 61–66%, respectively. However, chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal efficiency of the reactors elevated to the range of 79–81% and 77–83%, respectively at sludge age of 150 days and temperature of 30 °C. In short, overall performance of both reactors was optimum at sludge age ranging from 120 to 150 days and temperature varying between 25 and 30 °C. At hydraulic retention time (HRT) of 9 h the chemical oxygen demand (COD), total suspended solids (TSS) and sulfate removal efficiency of UASBCD reactor reached 81%, 75% and 76%, respectively and 77%, 74% and 69%, respectively for UASBASDIT. The rate of removal of these parameters however, gradually declined with increasing hydraulic retention time. The UASB technology provides a low-cost system for the direct treatment of municipal wastewater and can be applied in small communities.
Keywords
UASB reactors
Municipal wastewater
Sludge
Hydraulic retention time (HRT)
Chemical oxygen demand (COD) and biochemical oxygen demand (BOD)
1 Introduction
Among the various treatment technologies, anaerobic treatment systems are being encouraged because of several advantages, including low construction costs, small land requirements, low excess sludge production, plain operation and maintenance, energy generation in the form of biogas (Singh et al., 2013) and robustness in terms of COD removal efficiency (Conceição et al., 2013), pH stability and recovery time (Abma et al., 2010; Elmitwalli et al., 2002; Hernández and Rodríguez, 2013; Kongjan et al., 2013; ; Lettinga et al., 1992; Leitao, 2004; Van Haandel and Lettinga, 1994). A number of researchers have recommended anaerobic technology like UASB reactor for the treatment of sewage in tropical and subtropical regions (Cavalcanti, 2003; Banihani and Field, 2013; Banu et al., 2007; Halalsheh, 2002; Leitao, 2004; Mgana, 2003; Seghezzo, 2004). The start-up of UASB reactors is a complicated process and a number of factors, including wastewater characteristics, acclimatization of seed sludge, pH, nutrients, presence of toxic compounds, loading rate, upflow velocity (Vup), hydraulic retention time (HRT), liquid mixing and reactor design affect the growth of sludge bed (Aydinol and Yetilmezsoy, 2010; Barbosa and Sant’ Anna, 1989; Foresti, 2002; Souza, 1986; Zhang et al., 2012).
The temperature considerably influences the growth and survival of microorganisms. Although anaerobic treatment is possible at all three temperature ranges (psychrophilic, mesophilic and thermophilic), low temperature usually leads to a decline in the maximum specific growth rate and methanogenic activity (Azbar et al.,2009; Bodik et al., 2000). Methanogenic activity at this temperature range is 10–20 times lower than the activity at 35 °C, which requires an increase in the biomass in the reactor (10–20 times) or to operate at higher sludge retention time (SRT) and hydraulic retention time (HRT) in order to achieve the same COD removal efficiency as that obtained at 35 °C (Foresti, 2001; Kalogo and Verstraete, 2001; Mahmoud, 2002).
Hydraulic retention time (HRT) is one of the most important parameters affecting the performance of a UASB reactor when used for the treatment of municipal wastewater (Vieira and Garcia, 1992). The upflow velocity (Vup) is directly related with HRT and plays an important role to entrap suspended solids. A decrease in Vup entails an increase in HRT, which boosts suspended solids’ (SS) removal efficiency of the system (Liu et al., 2010; Rajakumar et al., 2011; van Haandel and Lettinga, 1994). The COD removal efficiency of a UASB reactor also decreases at elevated upflow velocity because higher Vup reduces the contact time between sludge and wastewater in addition to smashing of sludge granules, and resultantly higher washout of solids (Gonclaves et al., 1994; Kalogo and Verstraete, 1999; ; Leitao, 2004; Mahmoud, 2002; Nkemka and Murto, 2010; Van Haandel and Lettinga, 1994). However, some scientists reported no distinct effect of HRT on the treatment efficiency of UASB reactor (Halalsheh, 2002; Vieira and Garcia, 1992). The difference of opinion in scientific community is may be due to the difference in the reactor design, operating procedures and range of HRT.
In the present study the growth of sludge bed in UASB reactors initially seeded with cow dung and activated sludge of dairy industry treatment plant were investigated. The effect of process conditions (hydraulic retention time, sludge age and temperature) on the performance of these reactors was then examined.
2 Materials and methods
2.1 Physicochemical properties of wastewater and sludge
Composite samples of domestic sewage were collected from the Garden Town municipal wastewater pumping station of Metropolitan Lahore, Pakistan. Physico-chemical properties of samples were determined which included chemical oxygen demand (COD), biochemical oxygen demand (BOD), conductivity, turbidity, total dissolved solids (TDS), total suspended solids (TSS), total hardness, chlorides, sulfates, oil and grease, color and pH. All the parameters were determined following standard methods for the examination of water and wastewater (APHA et al., 1998).
2.2 Analytical techniques
Turbidity, conductivity, dissolved oxygen (DO) and pH were determined by the metric method according to standard methods (APHA et al., 1998).
Standard Methods (4500-SO42−E, Turbidimetric method), (4500-Cl C Titrimetric method), (5220 B Open reflux method) and (5210 B) were used for the determination of Sulfate, Chloride, Chemical oxygen demand (COD) and Biochemical oxygen demand (BOD), respectively.
Standard Methods (4500-N C), (4500-P Vanadomolybdophosphoric acid colorimetric method) and (5310 B) were used for the determination of Total Nitrogen, Total Phosphorous and Total Organic Carbon (TOC), respectively. Standard Methods (2540 B to E) were applied for the determination of Total Solids, Total Dissolved solids (TDS), Total Suspended solids (TSS) and Volatile suspended solids. Standard Methods (5520 B Gravimetric method) and 2120 C were used for the determination of oil and grease contents and color, respectively.
2.3 Design of UASB reactor assembly
A bench scale anaerobic UASB (Upflow Anaerobic Sludge Blanket) reactor was used in this study. The setup consisted of a pair of UASB reactors, peristaltic pump, influent tank, effluent collection tank and gas trapping system. The UASB reactor was made of Perspex material, comprising of a tubular section at the bottom and an expanded section termed as gas–liquid–solid separator (GLSS) at the top. Tubular section was a 120 cm long column with 7 cm internal diameter (ID) and a volume of 4.6 L. The length of the gas–liquid–solid separator was 40 cm and volume was 10.2 L. The GLSS section was further divided into two parts; bottom half was tapered with a slope angle (Ø) of 60° and top half was a 20 cm long column with an internal diameter of 22 cm. An inverted canopy was also attached with the top lid of GLSS in order to promote coagulation of suspended/colloidal particles, boost the collection of suspended particles to enhance the collection of biogas and to control the washout of particles (Yasar, 2006).
3 Results and discussion
3.1 Start-up of UASB reactors
The cow dung seed sludge comprised of predominantly organic matter and heavy population of microbes. Total solids (TS) and volatile suspended solids (VSS) concentrations in the seed sludge were 50.2 and 31.5 g/L, respectively. During the acclimatization of seed sludge, nutrients (COD:Nitrogen:Phosphorus in ratio of 300:5:1) were supplied to boost sludge growth by the addition of Sucrose (C12H22O11) and diammonium hydrogen phosphate (NH4)2HPO4. The Biochemical oxygen demand (BOD)/total organic carbon (TOC) ratio was 1 (Metcalf and Eddy, 2003). The sludge in the UASBCD reactor was an inhomogeneous suspended mass during first three months (Plate 1a). After that granulation started and sludge bed was stabilized in a period of 4 months (Plate 1b) and the quality of sludge was comparable with the well mature sludge of a digester.
Development of sludge bed in the reactors (a) inhomogeneous suspended mass in UASBCD, (b) granulation in UASBCD at day 120.
Similarly, seed sludge in UASBASDIT reactor was fed with nutrient-rich water (C:N:P ratio of 300:5:1) to accelerate its growth. The color of seed sludge changed from light gray to dark gray in a period of 1 month, which demonstrated the initiation of seed sludge stabilization. The sewage wastewater (Table 1) was then introduced to the reactor to acclimatize with sludge. The sludge bed was merely a suspended biomass up to a period of sixty days. After that granulation of biosolids became visible, which indicated successful start-up of the reactor. However, sludge granulation fully appeared after 80 days. These results are in accordance with the findings reported in the literature. For instance, Louwe Kooijmans and van Velsen (1986) reported that start-up of UASB reactor seeded with digested cow dung manure for domestic wastewater treatment at 25 °C required a period of 6 months. Another study Vijayaraghavan and Ramanujan (1999) described that sludge in anaerobic contact filter seeded with cow dung slurry was stabilized in a period of 160 days. Yasar (2006) inoculated UASB reactor with activated sludge of diary wastewater treatment plants, and reported 78 days start-up time for the reactor. Similarly a period of 147 days was required by Rajakumar et al. (2011) to start a UASB filter. The superscript alphabets stand for average of five values.
Parameter
Sewage
pH
7.39a ± 0.27
COD (mg/l)
474.39a ± 36.51
BOD5 (mg/l)
245.9b ± 28.15
Conductivity (mS/cm)
1.39 a ± 0.51
Turbidity (FTU)
69.38a ± 7.95
TSS (mg/l)
379a ± 38.29
Chlorides (mg/l)
69.48b ± 7.75
Color (absorbance)
0.0612b ± 0.02
The start-up of UASB reactors is a complicated process and a number of factors, including wastewater characteristics, acclimatization of seed sludge, pH, nutrients, presence of toxic compounds, loading rate, upflow velocity, hydraulic retention time, liquid mixing and reactor design affect the growth of sludge bed (Barbosa and Sant’ Anna, 1989; Foresti, 2002; Souza, 1986). Variation in time period required for the stabilization of the sludge may owe to several factors, including dissimilarities in wastewater composition, seed sludge, type of reactor, sludge temperature, nutrient content, and sludge pH (Barbosa and Sant’ Anna, 1989; Singh et al., 1997).
Table 2 shows the effect of age on the sludge composition in terms of volatile suspended solids (VSS), total solids (TS), total fixed solids (TFS) contents in UASBCD reactor. The TS and VSS contents were 55.2 and 34.5 g/L, respectively at the sludge age of 30 days, whereas the values of TS and VSS for UASBASDIT reactor were 42.2 and 23.0 g/L, respectively at the same sludge age (Table 3). There was an overall increase in TS and VSS contents as the sludge age increased. The increase in TS and VSS contents continued up to sludge age of 150 days, and TS and VSS contents were elevated to 80.5 and 62.3 g/L, respectively for UASBCD reactor. In case of UASBASDIT, TS and VSS values were 75.4 and 51.7 g/L, respectively. A significant increase in the VSS concentration as compared to TS was clearly an indication of the active biomass growth in the reactors as more than 90% of VSS contents are due to active biomass, and remaining 10% are attributed to non-biodegradable volatile solids and dead cell debris (Metcalf and Eddy, 2003). At sludge of 180 days, a decline in TS and VSS contents of sludge in both reactors appeared, which could be due to the liquidation of granules (Plate 2a and b).
Parameters (g/l)
Sludge age (days)
30
60
90
120
150
180
VSS
34.5
40.9
49.0
57.4
62.3
59.5
TFS
20.7
16.5
18.8
18.3
18.2
18.9
TS
55.2
57.4
67.8
75.7
80.5
78.4
VSS/TS
0.625
0.71
0.72
0.75
0.77
0.76
Parameters (g/l)
Sludge age (days)
30
60
90
120
150
180
VSS
23.0
31.6
39.7
47.6
51.7
49.2
TFS
19.2
23
21.6
23
23.7
14
TS
42.2
54.7
61.3
70.7
75.4
63.3
VSS/TS
0.55
0.58
0.65
0.68
0.66
0.62
Microscopic view of liquidation of sludge granules at day 180 (a) UASBCD reactor and (b) UASBASDIT reactor.
The VSS/TS ratio is important in determining the sludge characteristics and reflects biomass growth and its quality. The VSS/TS ratio in UASBCD reactor gradually increased (from 0.63 to 0.77) up to a sludge age of 150 days followed by a drop in this ratio at sludge age of 180 days. The VSS/TS ratio in UASBASDIT was lower than the VSS/TS ratio in UASBCD. However, it gradually increased (from 0.55 to 0.68) uptill sludge age of 120 days and decreased afterward. The difference in VSS/TSS ratio may owe to a difference in the contents and settleability of digested cow manure and activated sludge (Barbosa and Sant’ Anna, 1989).
3.2 Performance of UASB reactors to treat municipal wastewater
3.2.1 Influence of temperature and sludge age
The influence of temperature on the performance of a UASB reactor is very important because it affects significantly the hydrolysis process, substrate utilization rate, settling of solids and gas transfer rates (van Haandel and Lettinga, 1994; Lettinga et al., 2001). The rate of anaerobic digestion is rapidly decreased as the temperature of sludge bed is dropped below the mesophilic temperature range (30–38 °C) (Bogte et al., 1993; Van Haandel and Lettinga, 1994). That is why, the UASB technology has been applied to a lesser extent in cold climates despite the fact that some successes of high-rate anaerobic treatment under psychrophilic conditions (low temperature) are reported in the literature (Aydinol and Yetilmezsoy, 2010; Kaparaju et al., 2010; Rebac et al., 1995; Seghezzo, 2004).
The performance of UASBCD and UASBASDIT reactors at different temperatures varying between 17 and 38 °C and sludge age ranging from 60 to 180 days is shown in Table 4. There is an increase in the efficiency of the reactors with an increase in the temperature and sludge age. At a temperature of 17 °C and a sludge age of 60 days, COD removal efficiency of UASBCD and UASBASDIT reactors was 62 and 57%, respectively. Whereas TSS removal efficiency of these reactors at the same temperature and sludge age was 44 and 41%, respectively. However, their BOD5 removal efficiency was slightly better (66% and 61%, respectively). Low removal efficiency under psychrophilic conditions (17 °C) and early sludge age may be attributed to incomplete sludge granulation and insufficient volume of settled solids and biomass, which consequently reduced the methanogenic activity of sludge microorganism and slowed down the hydrolysis and substrate consumption rate (Hulshoff Pol, 1989; Lehtomäki et al., 2008; Lettinga et al., 2001; Van der Last and Lettinga, 1992).
Parameters
Removal efficiency (%) with respect to sludge age (days)/ temperature (°C)
60 days/17 °C
90 days/20 °C
120 days/25 °C
150 days/30 °C
180 days/38 °C
UASBCD
UASBASDIT
UASBCD
UASBASDIT
UASBCD
UASBASDIT
UASBCD
UASBASDIT
UASBCD
UASBASDIT
COD
62
57
68
61
77
75
81
78.7
82
80.5
BOD5
66
61
72
68
78
76
83
77
85
77.5
Conductivity
9.6
9.4
12.4
11.9
14.9
14.4
15.3
15
16.5
16.2
Turbidity
66.7
60.3
76.7
71.4
84.6
77.2
88
85.4
89
84.6
TSS
44
41
53
48
68
64
72.7
65.4
73
63.4
TDS
7.8
5.4
10.5
8.5
16.6
13.5
19
14.8
20
15.5
SO42−
48
40.3
55
49.4
68.6
62.6
71.3
66.8
74.3
65.1
Chloride
41
30.6
47
43
62
54.6
66.2
55
68.7
54.8
Oil and grease
70
67
82
78.6
89.4
88.4
93
91.5
93.2
92.8
The COD, BOD5, and TSS removal efficiency of UASBCD reached 68%, 72% and 53%, respectively at 20 °C temperature and sludge age of 90 days. The efficiency of UASBASDIT reactor for the removal of these parameters was 61%, 68% and 48%, respectively at the same temperature and sludge age. The improvement in the efficiency may owe to an increase in the digestion rate due to favorable temperature and relatively better developed sludge bed (Chinnaraj and Venkoba Rao, 2006; van Lier and Lettinga, 1999). These results also agree well with the findings of Lew et al. (2003), who reported 4% increase in the COD removal efficiency of a UASB reactor employed to treat domestic wastewater when the sludge temperature was increased from 14 to 20 °C.
At 25 °C temperature and sludge age of 120 days, UASBCD reactor obtained COD and TSS removal up to 77% and 68%, respectively. These findings are also in agreement with the results of other research workers (Banihani and Field, 2013; Lettinga et al., 1987; Louwe Kooijmans and van Velsen, 1986) who reported COD and TSS removal efficiency of a UASB reactor inoculated with digested cow manure up to 78% and 75%, respectively at an operational temperature of 25 °C. Slightly lower TSS removal efficiency of the UASB reactors used in this study may owe to relatively less volume of sludge bed, which is insufficient to entrap non-settleable suspended solids (Brown, 1998).
At sludge age of 150 days and 30 °C temperature, the UASBCD reactor showed COD, BOD5, TSS and oil and grease removal up to 81%, 83%, 73% and 93%, respectively. The removal of these parameters by UASBASDIT reactor was 79%, 77%, 65% and 91%, respectively. The better performance of the reactors at this stage may be attributed to favorable sludge temperature, well developed sludge granulation and increased growth of biomass which ultimately resulted in accelerated degradation of organic matter and entrapment of suspended solids (Agrawal et al., 1997; Kalogo and Verstraete, 2001; Rajakumar et al., 2011; Uellendahl and Ahring, 2010; Yasar, 2006). At sludge age of 180 days and 38 °C temperature, the UASBCD reactor showed COD, BOD5, and TSS removal of up to 82%, 85% and 73%, respectively. The removal of these parameters by UASBASDIT reactor was 80.5%, 77.5%, 65% and 63.4%, respectively. It was evident from the results that the performance of both reactors was optimal at sludge age ranging from 120 to 150 days and temperature varying between 25 and 30 °C. Beyond sludge age of 150 days, the removal of pollution parameters by the reactors was marginal though the sludge temperature was favorable for anaerobic digestion. The decline in efficiency may be due to liquidation of sludge, disengagement of entrapped solids and/or reduced rate of hydrolysis because enzymes involved in the hydrolysis are very sensitive to temperature (Mahmood, 2002; Rajakumar et al., 2011; Uellendahl and Ahring, 2010).
A comparison of results also revealed that overall performance of UASBCD reactor was better than UASBASDIT reactor, which may be explained due to better development of sludge bed in terms of granulation, biomass growth and settling characteristics of cow dung sludge with time in the former reactor (Barbosa and Sant’ Anna, 1989).
3.2.2 Influence of hydraulic retention times (HRTs)
The hydraulic retention time (HRT) is directly related to upflow velocity (Vup) of influent in a UASB reactor. Hence, an adequate Vup and accordingly HRT provides sufficient contact between sludge and wastewater, reduces the formation of gas pockets, disengages the biomass from gas and resultantly enhances TSS removal efficiency of the system (van Haandel and Lettinga, 1994; Gonclaves et al., 1994; Mahmoud, 2002; Uellendahl and Ahring, 2010; Rajakumar et al., 2011). Table 5 shows the effect of HRT on the performance of both reactors at an operational temperature of 20 °C. The results revealed that the COD, TSS and sulfate removal efficiency of UASBCD reactor was 73%, 65% and 60%, respectively and the removal efficiency of these parameters by UASBASDIT was 70%, 63% and 56%, respectively at a hydraulic retention time of 3 h. The COD, TSS and sulfate removal efficiency of former reactor was 84%, 77% and 76%, respectively and 80%, 75% and 71%, respectively for later one at HRT of 12 h. However, rate of removal of these parameters gradually declined with increasing hydraulic retention time (Fang, 2000; Hernández and Rodríguez, 2013; Leitao, 2004; Nkemka and Murto, 2010; Zhang et al., 2012). Similar findings have also been reported in the literature (Kalogo and Verstraete, 2000; Lettinga et al., 1993; Rajakumar et al., 2011). For instance, Ruiz et al. (1998) reported that COD and TSS removal efficiency of a laboratory scale UASB reactor treating domestic wastewater at 20 °C was increased from 53% to 73% and 63% to 80%, respectively with an increase in HRT from 4 to 8 h. Similarly, Nkemka and Murto (2010) reported 81% COD removal efficiency at HRT of 12 h in a UASB reactor treating seaweed leachate and Zhang et al. (2012) reported 92% COD removal efficiency at HRT of 10 h in a UASB reactor treating sewage.
Parameters
Removal efficiency (%) at different hydraulic retention times (hr)
3
6
9
12
UASBCD
UASBASDIT
UASBCD
UASBASDIT
UASBCD
UASBASDIT
UASBCD
UASBASDIT
COD
72.6
69.8
78.4
75.2
81.7
77.3
84.2
80.0
BOD5
74.3
71.5
80.7
77.5
85.5
80.6
86.6
82.4
Conductivity
12
11.5
15
14
17.2
16.4
17.6
17.3
Turbidity
75
70
82.7
76
87.1
82.4
88.2
84
TSS
65
63
72.8
70.2
75.8
74.3
76.5
74.8
TDS
15
12
18
14
20.8
18.5
21.5
20.3
SO42-
60.1
56.4
69.7
62
75.9
69
76.2
69.7
Chloride
61
54
66
56
68.8
59.2
70.2
60.9
Oil and Grease
91
90
93.5
94
98.7
95
97
95
4 Conclusion
Following conclusions are drawn from this study. The start-up of UASBCD reactor required a period of 120 days. In case of UASBASDIT reactor sludge bed was stabilized in a period of 80 days. The performance of both reactors to treat wastewater was enhanced with an increase in the temperature and sludge age. Overall performance of both reactors was optimal at sludge age ranging from 120 to 150 days and temperature varying between 25 and 30 °C. The UASB technology provides a low-cost system for the direct treatment of municipal wastewater and can be applied in small communities where the wastewater flow variation is high due to rainy season or population load increases during the tourist season or due to seasonally operated food industries.
Acknowledgments
The authors are highly thankful to the Higher Education Commission (HEC), Islamabad Pakistan for the financial support under the indigenous Ph.D. scholarship scheme.
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