Q IWA Publishing 2009 Water Science & Technology—WST | 59.1 | 2009
185
Upgrading of the anaerobic digestion of waste activated sludge by combining temperature-phased anaerobic digestion and intermediate ozonation T. Kobayashi, Y. Y. Li, H. Harada, H. Yasui and T. Noike
ABSTRACT Upgrading of the anaerobic digestion of waste activated sludge (WAS) by the combination of temperature-phased two-stage digestion and intermediate ozonation was investigated by a continuous experiment with two processes, TM and TOM. The TM process is a temperaturephased two-stage system, which consists of a thermophilic digester and a mesophilic digester in series. The TOM process is a temperature-phased two-stage process with the intermediate ozonation. Two processes were operated at hydraulic retention times of 30 days for over 123 days. Waste activated sludge taken from wastewater treatment plant was fed as a substrate. Microbial community structure in each digester was analysed with molecular tools. Despite of less amount of ozone dose in TOM than ozone pre-treatment process, better effect of ozonation on performance improvement was obtained in TOM. TOM had the highest methane yield and CODCr reduction among comparative processes. Furthermore, flocculation efficiency of TOM
T. Kobayashi Y. Y. Li H. Harada Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan E-mail:
[email protected] Y. Y. Li Department of Urban and Environmental Engineering, Tianjin Institute of Urban Construction, Jinjinggonglu 26, Tianjin 300384, China
followed that of mesophilic digestion. Quality of dewatered supernatant is comparable to mesophilic digestion. Key words
| anaerobic digestion, intermediate ozonation, temperature phase, waste activated sludge
H. Yasui Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu, Kitakyushu-city, Fukuoka 808-0135, Japan T. Noike ARISH, Nihon University, 2-1 Kudan-kita 4-chome, Chiyoda-ku, Tokyo 102-0073, Japan
INTRODUCTION Huge amount of waste activated sludge (WAS) is produced
WAS, many efforts by mechanical, chemical and thermal
through the wastewater treatment system, which contains
pretreatment have been achieved (Kang et al. 2000; Kim
activated sludge process. Reduction of WAS with recover-
et al. 2003). Many researches focused on the optimum
ing of energy by anaerobic digestion has been of interest.
method of pretreatment and optimisation of pretreatment
However, it is well recognised that the degradation
condition. But it is also important to propose an optimum
efficiency of WAS is at lower level than those of other
process by combining known unit operations although
organic solids such as primary sewage sludge (Lafitte-Trou-
there is far less researches on the latter subject than that of
que & Forster 2002; Arnaiz et al. 2006). Anaerobic digestion
the former.
process consists of several stages: hydrolysis, acidogenesis,
Anaerobic digestion has well-known two optimum
methanogenesis. For WAS digestion, hydrolysis step is
temperature, one of which ranges around 358C (mesophilic)
rate-remitting (Li & Noike 1992). To promote hydrolysis of
and another around 558C (thermophilic). It is reported that
doi: 10.2166/wst.2009.510
186
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
thermophilic digestion has higher hydrolysis rate of WAS than mesophilic digestion despite of worse dewaterability and the accumulation of soluble COD (Sangsan et al. 2007). However, in resent years, the development of temperaturephased anaerobic digestion (TPAD) system (Streeter et al. 1997) enabled to make an improvement of dewaterability with high hydrolysis rate of thermophilic digestion. So far several studies on TPAD process have been reported (Schmit & Ellis 2001; Vandenburgh & Ellis 2002; Sung & Santha 2003), but no study was reported for treating WAS. Ozonation is one of the pre-treatment techniques to improve hydrolysis of sewage sludge for anaerobic digestion (Weemaes et al. 2000). Generally, the amount of ozone dose for a treatment is determined on the basis of TS or SS concentration of sludge. In order to economize energy consumption, the amount of ozone dose should be as small as possible. The previous study revealed that the ozone post-treatment and recycling process, in which half of anaerobic digested sludge was ozonized and brought back to the digester, had higher effect on improvement of biologic degradability despite of less amount of ozone dose compared to ozone pretreatment process. More effective oxidation of difficult degradable substances was carried out on digested sludge than on WAS because the former consists of difficult degradable substances while the latter
Water Science & Technology—WST | 59.1 | 2009
METHODS Operation of two anaerobic digestion processes Continuous experiment with two processes (Figure 1), TM and TOM was carried out over 123 days. The TM process is a temperature-phased two-stage process, which consists of a thermophilic digester (558C) and a mesophilic digester (358C) in series. The TOM process is a temperature-phased two-stage process with the intermediate ozonation, which consists of a thermophilic digester (558C), an ozone treatment reactor and a mesophilic digester (358C) in series. The digested sludge from thermophilic digester was ozonized in batch in an ozone treatment reactor with 5 L volume. Ozone dose was set at 0.02 g/g-TS. Digesters used in this study were completely stirred tank reactors with a working volume of 5 L. Each digester were operated at a hydraulic retention time (HRT) 30 days respectively. A substrate tank kept at 48C fed WAS to thermophilic digesters of two processes by a time-controlled pump. Seed sludge for thermophilic digester was anaerobic sludge treating WAS at 558C, and that for mesophilic digester was sludge treating WAS at 358C. The WAS used in this study was taken from Sen-en WWTP every 3 –4 week period and then stored at 48C.
consists of both easy and difficult degradable substances. For further improvement in performance of TPAD process, we investigated the effect of the temperaturephased anaerobic digestion process with intermediate ozo-
Chemical analysis TS, VS, SS, VSS and PO32 4 ZP were measured in accordance
nation. In our study, the thermophilic digested sludge was
with the testing method for wastewater (JSWA 1994),
ozonized and fed to the mesophilic digester. Like post
and CODCr was measured with APHA Standard Methods
treatment and recycling, intermediate ozonation of digested
(APHA 1995). Carbohydrates were measured by Phenol
sludge would improve anaerobic digestablity more effi-
H2SO4 method, Proteins were measured by Lowry
ciently than pre-treatment with less amount of ozone dose.
method and Lipids were measured by Bligh-Dyer method.
But unlike that, intermediate ozonation treats all of digested
The biogas production was measured with a wet gas
sludge from thermophilic digester.
meter (SINAGAWA). Gas composition was detected
TM
Substrate tank (4°C)
Thermophilic digester (55°C)
Mesophilic digester (35°C)
TOM
Substrate tank (4°C)
Thermophilic digester (55°C)
Ozonation (0.02g/g-TS)
Figure 1
|
Schemes of two anaerobic digestion processes, TM and TOM.
Mesophilic digester (35°C)
187
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
by the gas chromatograph (Shimadzu, GC-8A). VFAs were measured by the gas chromatograph with FID (Agilent, 6890). Ammonium concentration was determined by the capillary electrophoresis (OHTSUKA, Photal CAPI-3200). Flocculation efficiency was measured by using
polyamidine-base
high
molecular
flocculants
for dewatering of sludge (Yonemoto & Sakano 1995). The value of flocculation efficiency was based on the addition of flocculants until the sludge appeared solidliquid separation.
Water Science & Technology—WST | 59.1 | 2009
RESULTS AND DISCUSSION Characteristics of operation of the two processes The time courses of biogas production rate, VFA and pH in the thermophilic digester and mesophilic digester of TM were shown in Figure 2, and those of TOM were shown in Figure 3. After day 90, the operating conditions of all digester appeared stable, so that each digester was considered to be in a steady state condition. Table 1 indicates the performance of each digester during the steady state condition. As shown in Figure 2(b) and Figure 3(b), the pH was in the suitable range 7– 8 for anaerobic digestion in
Clone analysis and real-time quantitative PCR
the all digesters. The pH in thermophilic digesters remained
Digested sludge samples for clone analysis were taken from
at higher level than that in mesophilic digesters. VFA had
each digester after 100 days of operation. Genomic DNA,
an average of 244 mg-HAc/L at the thermophilic digester of
extracted from samples with Ultra Clean Soil DNA
TM, 173 mg-HAc/L in the thermophilic digester of TOM
Isolation Kit (MO-BIO), followed amplification with the
while little VFA was detected in the mesophilic digesters
primers EUB338-mixF (Amann et al. 1990; Daims et al. 1999) and UNIV1500R (Weisburg et al. 1991) for bacteria. Thermal cycling of PCR consisted of 30 s denaturing
range 2,200 – 2,300 mg/L in each digester. Mesophilic digesters of two processes showed higher flocculation efficiency than thermophilic digesters. 90% of the total
2 min. PCR products were cloned with TOPO TA Cloning
(a)
kit (invitrogen). Cloned DNA fragments obtained from
Biogas production rate (L /d.L)
at 948C, 40 s annealing at 508C and extraction at 728C for
in both TM and TOM. NHþ 4 ZN concentration was in the
randomly selected recombinants served as templates for sequencing analysis with a CEQ8000 (BECKMAN COULTER). Operational Taxonomic Units (OTUs) was classified according to Hae III cleavage patterns. Representative
1.5 1.2 0.9 0.6
0.0
sequences were searched by using the BLAST program Genomic DNA for Real-time quantitative PCR was extracted from digested sludge taken from digesters at day 95, 100 and 120 with Ultra Clean Soil DNA Isolation
(b)
0
20
40
Thermophilic (TM) 6
0
20
40
for 30 s. The DNA fragments amplified by using the primer set EUB338F and UNIV1500R from affiliate clones with
100
120
1,500 Thermophilic (TM)
Mesophilic (TM)
1,000 500 0
each phylogenetic group respectively used as a template for the standard curve.
60 80 Time (days)
Mesophilic (TM)
(c) TVFA (mg-HAc/L)
ing at 948C, 10 s annealing at 608C and extraction at 728C
120
7
primer sets for Coprothermobacter spp. were COP1029f
The amplification followed 40 cycles PCR with 5 s denatur-
100
8
performed with LightCycler (Roche Diagnostics). The
the binding of the SybrGreen to double stranded DNA.
60 80 Time (days)
9
Kit (MO-BIO). Amplification of 16S rRNA gene was
and COP1179r (Kobayashi et al. 2008). The signal was
Mesophilic (TM)
pH
(Altschul et al. 1997) to classify OTUs based on phylogeny.
Thermophilic (TM)
0.3
Figure 2
|
0
20
40
60 80 Time (days)
Time course of operating condition.
100
120
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
188
(a)
methane production rate in TM process was attributed to
Biogas production rate (L/d.L)
1.5
Thermophilic (TOM)
Mesophilic (TOM)
the thermophilic digester, and 78% of that in TOM process
1.2
was produced by the thermophilic digester.
0.9
Table 2 shows the comparison of the system perform-
0.6
ance in TM and TOM process during the steady state
0.3 0.0
condition. Two processes were evaluated in terms of 4 view 0
20
40
60 80 Time (days)
100
points: (1) Methane production yields, (2) Organic matter
120
reduction, (3) Flocculation efficiency of mesophilic digested
9
(b)
sludge, (4) Water quality of dewatered supernatants.
pH
8
Methane production yield through the process in TOM had an average of 0.307 ^ 0.029 L/g-VS, which was 1.13
7 Thermophilic (TOM) 6
0
20
40
(c) TVFA (mg-HAc/L)
Water Science & Technology—WST | 59.1 | 2009
Mesophilic (TOM)
60 80 Time (days)
100
times higher than that in TM. Focused on the organic matter reduction all items in TOM were superior to those in TM
120
excepting the lipids reduction. The reduction efficiency of proteins, which is a major organic component of WAS, was
1,500 Thermophilic (TOM)
Mesophilic (TOM)
higher than any other items. The flocculation efficiency was
1,000
1.16 g-flocculants/L in TM and 0.97 g-flocculants/L respect500 0
ively. This indicates that the dewaterability of digested sludge from TOM was higher than that from TM. Dewa0
20
40
60
80
100
120
tered supernatant was obtained by 10min centrifugation (3,000 rpm) of dewatered sludge from the flocculation
Time (days) |
Figure 3
efficiency test. There is no significant difference between
Time course of operating condition of TM of TOM.
the TM and the TOM in terms of the water quality of
Table 1
|
Performance of each digesters during the steady state condition
TM
TS
g/L
TOM
Thermophilic
Mesophilic
Thermophilic
Mesophilic
30.6 ^ 2.2
26.9 ^ 2.2
30.5 ^ 0.5
22.3 ^ 0.8
VS
g/L
20.2 ^ 2.0
16.5 ^ 2.6
19.7 ^ 0.6
13.3 ^ 1.1
SS
g/L
25.1 ^ 1.2
19.9 ^ 1.6
25.0 ^ 0.4
16.9 ^ 1.0
VSS
g/L
14.9 ^ 1.0
13.6 ^ 1.4
15.2 ^ 1.4
12.1 ^ 1.1
T-CODCr
g/L
33.0 ^ 0.9
26.3 ^ 1.6
32.7 ^ 1.9
21.4 ^ 1.4
S-CODCr
g/L
13.7 ^ 2.0
9.0 ^ 0.8
13.6 ^ 0.4
7.5 ^ 0.5
NHþ 4 -N
mg/L
2410 ^ 64
2460 ^ 210
2320 ^ 51
2270 ^ 95
–
7.78 ^ 0.07
7.66 ^ 0.07
7.84 ^ 0.09
7.69 ^ 0.14
mg-HAc/L
244 ^ 115
27 ^ 54
173 ^ 51
N.D.
pH TVFA Flocculation efficiency
g/L
1.64 ^ 0.21
1.16 ^ 0.22
1.71 ^ 0.30
0.97 ^ 0.18
Gas production rate
L/L.d
0.93 ^ 0.10
0.09 ^ 0.02
0.91 ^ 0.09
0.20 ^ 0.05
N2
%
1.6 ^ 0.8
1.8 ^ 0.7
0.9 ^ 0.3
4.8 ^ 3.1
CH4
%
61.3 ^ 2.9
69.8 ^ 4.0
62.5 ^ 1.9
71.4 ^ 4.1
Gas content
CO2
%
36.9 ^ 2.4
30.5 ^ 3.1
36.7 ^ 1.2
27.7 ^ 3.1
H2S
ppm
740 ^ 141
140 ^ 57
600 ^ 30
250 ^ 71
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
189
Table 2
|
Water Science & Technology—WST | 59.1 | 2009
The comparison of the system performance
(1) Methane production yield
(2) Organic matter reduction
(3) Flocculation efficiency
TOM
CODCr
L/g-VS
0.272 ^ 0.026
0.307 ^ 0.029
VSS
%
56.5 ^ 2.2
63.6 ^ 1.7
%
58.2 ^ 5.3
62.5 ^ 3.3
Carbohydrates
%
51.8 ^ 5.4
58.7 ^ 4.4
Proteins
%
59.5 ^ 4.0
64.4 ^ 5.3
Lipids
%
53.3 ^ 6.6
52.9 ^ 3.6
CODCr
(4) Dewatered supernatant
TM
g/L
1.16 ^ 0.22
0.97 ^ 0.18
g/L
4.2 ^ 0.1
4.3 ^ 0.1
NH2 4N PO32 4 P
mg/L
2260 ^ 102
2120 ^ 87
mg/L
1150 ^ 42
1074 ^ 73
TVFA
mg-HAc/L
N.D.
N.D.
dewatered supernatant. From what has been discussed
of accumulated soluble COD in the thermophilic digester
above, we can conclude that the TOM showed higher
were removed in the mesophilic digester, but little particle
performance than TM from the three view points containing
COD (P-COD) removal was observed. On the other hand,
methane production yield, organic matter reduction and
in the TOM process (Figure 4(b)), half of accumulated
flocculation efficiency.
soluble COD and 28% of particle COD in the thermophilic digested sludge were removed in the mesophilic digester respectively. These results suggest that intermediate ozona-
CODCr mass balances among digesters in
tion improve degradability of both soluble and particle
TM and TOM process
COD of thermophilic digested sludge.
In order to understand the bioconversion process of organic matter in the digesters, COD mass balance in TM and TOM were illustrated in Figure 3(a) and (b) respectively.
Microbial community in digesters
Approximately 40% of influent COD was converted into
TOM process had higher P-COD and VSS reduction than
methane in the thermophilic digesters while in which the
TM. These results suggest that the intermediate ozonation
soluble COD accumulated to approx. 24% of influent total
promoted the hydrolysis of digested sludge from primary
COD. In the TM process shown in Figure 4(a), portion
digester, which consists of residue of thermophilic digestion
(b) 100 25.7
80
39.8 45.1
60 24.3
40
74.3
20
33.5
0
WAS P-COD
Figure 4
|
15.7 30.2
Thermophilic Mesophilic S-COD
CODCr mass balances.
Methane
Recovery from influent (%)
Recovery from influent (%)
(a)
100 80
25.7
40.8 53.8
60 40
23.7 74.3
20 0
13.1 33.5
WAS P-COD
24.3
Thermophilic Mesophilic S-COD
Methane
190
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
Water Science & Technology—WST | 59.1 | 2009
and thermophilic bacteria increased in primary digester. In this study, to understand the behaviour of thermophilic bacteria from primary digester in the mesophilic digester, the bacterial community structure was analysed with molecular approach including 16S rRNA gene (rDNA) clone analysis and real-time quantitative PCR. Results of clone analysis were shown in Figure 5. In the previous study, we reported the clone library constructed from the thermophilic anaerobic digested sludge treating WAS (Kobayashi et al. 2008), so that only approx. 20 clones from thermophilic digested sludge were analysed in this study. As shown in previous study, the predominance of
Figure 6
|
16S rRNA gene concentration of Coprothermobacter spp.
the clones affiliated with Coprothermobacter proteolyticus, which was thermophilic proteolytic bacteria, was observed in both thermophilic digesters of TM and TOM. In the mesophilic digester of TOM, no clone affiliated with Coprothermobacter was detected although in the mesophilic digester of TM, 15.3% of total clones affiliated with Coprothermobacter proteolyticus. Results of Real-time quantitative PCR analysis were shown in Figure 6. As well as results of clone analysis, TOM showed the less rDNA concentration of Coprothermobacter than TM. Judging from this, thermophilic bacteria increased in the primary digester might be killed by intermediate ozonation. For this reason, TOM would have higher P-COD and VSS reduction.
Performance comparison Tables 3 and 4 show comparison of system performance between this study and previous studies. First of all, we will focus our attention on the effect of temperature-phased digestion. TM and TOM have better flocculation efficiency and quality of dewatered supernatant than thermophilic digestion. The most likely cause for this is that the temperature phased two-stage digestion enabled the decrease of soluble COD by secondary mesophilic digester as shown in Figure 5. Furthermore, TM and TOM have the higher CODCr reduction than other processes. These results indicates that the temperature phased digestion, which was the combination of higher rate of hydrolysis by thermophilic digestion and the decrease of soluble COD by secondary mesophilic digester, resulted in the higher CODCr reduction with improvement of the flocculation efficiency and water quality of dewatered supernatant compared to the thermophilic digestion. We will now discuss the effect of intermediate ozonation. Amount of ozone dose for TOM, Ozone pretreatmentmesophilic and Ozone pretreatment-thermophilic shown in Table 4 reached 0.61, 1.16, 1.16 g/L respectively. However, as shown in Table 4, CODCr reduction in TOM was 1.13 times higher than that in TM while CODCr reduction in Ozone pretreatment-mesophilic and thermophilic digestion was 1.07 and 0.96 times higher than those in mesophilic and thermophilic digestion respectively. These results suggest that despite of less amount of ozone dose in TOM than two ozone pretreatment processes, better effect of ozonation on
Figure 5
|
Bacterial 16S rRNA gene clone libraries.
performance improvement was obtained. TOM had the
191
No.
|
Process flow diagrams
Processes
Flow diagrams of each processes
Reference
1 TM
WAS
Thermophilic digester
Mesophilic digester
This study
2 TOM
WAS
Thermophilic digester
Mesophilic
WAS
Mesophilic digester
Sangsan et al. (2007)
Thermophilic
WAS
Thermophilic digester
Sangsan et al. (2007)
Ozone pretreatment-Mesophilic
WAS
Ozonation
Mesophilic digester
Sangsan (2006)
Ozone pretreatment-Thermophilic
WAS
Ozonation
Thermophilic digester
Sangsan (2006)
Ozonation
Mesophilic digester
This study
3
4
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
Table 3
5 Water Science & Technology—WST | 59.1 | 2009
6
T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge
Water Science & Technology—WST | 59.1 | 2009
–
processes. Furthermore, flocculation efficiency of TOM
1.16 ^ 0.01
–
1.78 ^ 0.31
48.2 ^ 3.1
30
0.256 ^ 0.037
6
highest methane yield and CODCr reduction among these Thermophilic†
Ozone pretreatment-
192
followed that of mesophilic digestion. The quality of dewatered supernatant is comparable to mesophilic digestion. These results indicate that TOM is the best process in
–
1.16 ^ 0.01
–
0.73 ^ 0.10
50.3 ^ 2.3
CONCLUSIONS The overall conclusions obtained from this study are
–
1. From the three terms comparison of methane production yield, organic matter reduction and flocculation effi-
N.D.
8.0 ^ 0.3
1.85 ^ 0.37
summarised as: 50.3 ^ 6.5
0.263 ^ 0.029
30 30
0.262 ^ 0.031
5 4
Mesophilic† Thermophilicp
Ozone pretreatment-
terms of total performance.
ciency, the system performance of TOM was higher than the TM process and the TOM process for the water –
quality of dewatered supernatant. 2. In the temperature phased two-stage digestion, the
N.D.
4.0 ^ 0.1
0.65 ^ 0.06
46.8 ^ 2.8
30
0.257 ^ 0.022
3
Mesophilicp
that of TM. There was no significant difference between
combination of higher rate of hydrolysis by thermophilic
N.D.
0.61 ^ 0.01
4.3 ^ 0.1
0.97 ^ 0.18
63.6 ^ 1.7
with improvement of the flocculation efficiency and water quality of dewatered sludge compared to the thermophilic digestion.
– N.D. mg-HAc/L
4. Results of clone analysis and real-time quantitative PCR
pre-treatment. suggest that the mesophilic digester of TOM showed less g/L
rRNA gene concentration of Coprothermobacter, which was predominant bacteria in the thermophilic digesters,
TVFA
than that of TM.
†
Sangsan et al. (2007). Sangsan (2006). ‡ At standard state. § 0.02 g per 1 g-TS of sludge.
REFERENCES
p
Flocculation efficiency
Dewatered supernatant
ozonation had better effect of ozonation on performance improvement in terms of COD reduction than ozone
Amount of ozone dose§
4.2 ^ 0.1 g/L
1.16 ^ 0.22
ozonation than ozone pre-treatment, the intermediate
CODCr
g/L
56.5 ^ 2.2 CODCr reduction
Methane production yield‡
HRT
No.
mesophilic digester resulted in higher COD reduction
3. Despite of less amount of ozone dose in the intermediate
%
30
0.307 ^ 0.029 0.273 ^ 0.026
30 Days
L/g-VS
2 1
Unit Item
Table 4
|
Comparison of the performance of six processes
TM
TOM
digestion and the decrease of soluble COD by secondary
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