Accepted Manuscript Evaluating sedimentation problems in activated sludge treatment plants operating at complete sludge retention time Elisavet Amanatidou, Georgios Samiotis, Eleni Trikilidou, George Pekridis, Nikolaos Taousanidis PII:
S0043-1354(14)00759-3
DOI:
10.1016/j.watres.2014.10.061
Reference:
WR 10980
To appear in:
Water Research
Received Date: 31 July 2014 Revised Date:
24 October 2014
Accepted Date: 28 October 2014
Please cite this article as: Amanatidou, E., Samiotis, G., Trikilidou, E., Pekridis, G., Taousanidis, N., Evaluating sedimentation problems in activated sludge treatment plants operating at complete sludge retention time, Water Research (2014), doi: 10.1016/j.watres.2014.10.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
State point analysis in complete mix extended aeration activated sludge system with pre-anoxic denitrification, at critical point of 15 kgMLSS m-3 and return activated sludge rate of 600% with complete solids retention time is given. The intersection of overflow and underflow line is the state point. State point’s vertical projection on X axis shows the MLSS concentration in the aeration tank. The underflow line is well below the lower limb of gravity flux
AC C
EP
TE D
curve insuring good effluent quality.
ACCEPTED MANUSCRIPT 1
Evaluating sedimentation problems in activated sludge treatment plants operating
2
at complete sludge retention time
3
Elisavet Amanatidou1*, Georgios Samiotis1, Eleni Trikilidou1, George Pekridis1,
5
Nikolaos Taousanidis2, 1
6
RI PT
4
Environmental Chemistry and Waste Water Treatment Lab, Environmental
Engineering Pollution Control Department, Technological Education Institute of
8
Western Macedonia, Koila, Kozani, 50100 Greece 2
Mechanical Engineering Department, Technological Education Institute of Western
M AN U
9
SC
7
Macedonia, Koila, Kozani, 50100 Greece
10
Abstract
12
Zero net sludge growth can be achieved by complete retention of solids in activated
13
sludge wastewater treatment, especially in high strength and biodegradable wastewaters.
14
When increasing the solids retention time, MLSS and MLVSS concentrations reach a
15
plateau phase and observed growth yields values tend to zero (Yobs ≈ 0). In this work, in
16
order to evaluate sedimentation problems arised due to high MLSS concentrations and
17
complete
18
slaughterhouse
EP
TE D
11
retention
AC C
sludge
operational
wastewater treatment
conditions,
plants
two
identical
were studied.
innovative
Measurements
of
* Corresponding author: Environmental Chemistry and Waste Water Treatment Lab, Environmental Engineering Pollution Control Department, Technological Education Institute of Western Macedonia, Koila, Kozani, 50100 Greece Tel. +30 24610 68015, e-mail: [email protected]
1
ACCEPTED MANUSCRIPT wastewaters’ quality characteristics, treatment plant’s operational conditions, sludge
20
microscopic analysis and state point analysis were conducted. Results have shown that
21
low COD/Nitrogen ratios increase sludge bulking and flotation phenomena due to
22
accidental denitrification in clarifiers. High return activated sludge rate is essential in
23
complete retention systems as it reduces sludge condensation and hydraulic retention
24
time in the clarifiers. Under certain operational conditions sludge loading rates can
25
greatly exceed literature limit values. The presented methodology is a useful tool for
26
estimation of sedimentation problems encountered in activated sludge wastewater
27
treatment plants with complete retention time.
M AN U
28
SC
RI PT
19
29
Keywords
30
Activated Sludge, Complete Sludge Retention, Zero Net Biomass, Sedimentation
31
Problems, Sludge Flotation, Floc Trapped Nitrogen
33
TE D
32
Highlights •
Zero net biomass growth can be achieved by complete sludge retention
35
•
Low C/N ratios increase sludge flotation in clarifier
36
•
SLR can exceed literature values, when limited denitrification occurs in
38 39
AC C
37
EP
34
clarifier
•
Stirred and unstirred velocity tests indicate gas production in clarifier
•
State point analysis estimates and verifies sedimentation process critical points
40 41
1. Introduction
42
In activated sludge (AS) wastewater treatment plants (WWTPs), sludge production
43
depends on different factors such as biodegradability of the organic pollutants, mass 2
ACCEPTED MANUSCRIPT loading of the treatment plant, degradation rate of microbial cells by endogenous
45
respiration or cellular lysis and existence of predator bacteria (Rocher et al., 1999). Due
46
to the nature of AS treatment, a large amount of excess sludge is generated daily,
47
proportional to influent substrate load. Recently, many investigations are oriented to
48
reduce the sludge production in AS WWTPs, because management and treatment of
49
sludge accumulate more than 50% of the construction and operating cost (Liu and Tay,
50
2001; Metcalf and Eddy, 2003; Foladori et al., 2010; Guo et al., 2013). Minimization of
51
observed sludge yields (Yobs) can be brought about by amplifying microbial cell lysis
52
and generating biomass growth on the lysis products, which is defined as the cryptic
53
growth (Rocher et al., 1999; Liu and Tay, 2001). Extensive microbial cell lysis occurs in
54
high strength biodegradable wastewaters (EC, 2005). AS treatment operating at high
55
sludge retention time (SRT) enhances cryptic growth, low food to microorganism ratios
56
(F/M), high recycle activated sludge (RAS) rate and low waste activated sludge (WAS)
57
that is the controlling parameter for regulating the SRT (Grady et al. 1999). Complete
58
SRT means that no solids are wasted from the system, resulting biomass age almost
59
equal to WWTP operation days (Henze, 2008).
60
When treating highly biodegradable wastewater, such as slaughterhouse effluents, in
61
complete sludge retention WWTPs (SRT = days of operation), problems in the settling
62
process may arise due to increased MLSS concentration and alterations of biomass
63
settling properties. The efficiency of the activated sludge treatment process is correlated
64
to an efficient solid-liquid separation, which is strongly depended on the biomass
65
settling properties (Govoreanu et al., 2003). Furthermore, efficient solid-liquid
66
separation results from the aggregation of microbes and solids into activated sludge
67
flocs (bioflocs). Poor bioflocculation in a wastewater treatment plant can result in poor
68
settling in the clarifiers, turbid effluent and adverse effects on WAS dewatering (Grady
AC C
EP
TE D
M AN U
SC
RI PT
44
3
ACCEPTED MANUSCRIPT et al., 1999; Sanin et al., 2006; Nguyen et al., 2007). There are two factors that most
70
affect bioflocculation, SRT and substrate loading rate (Nirupa, 2010). SRT greatly
71
affects the floc structure by alternating the proportion of floc forming bacteria and
72
filamentous bacteria present in the biofloc and sludge settling properties. Excessive
73
filaments lead to the bulking of sludge, causing decrease in settleability and
74
compactability of bioflocs, while inadequate filamentous bacteria results in pin point
75
and easily sheared flocs formation (Metcalf and Eddy, 2003; Gerardi, 2006; Nirupa,
76
2010; Ye et al., 2011). Consequently, SRT is the key for controlling biomass production
77
in a WWTP, elevating SRT to one of the most critical design and operational parameter
78
in AS process. Substrate loading rate and nutrient analogy directly influence the growth
79
of filamentous and floc forming bacteria, the extracellular polymeric substances (EPS)
80
content and therefore the sludge nature and the effluent quality. In starvation conditions
81
where enhanced cell lysis occur, bacteria produce EPS that alter mixed liquor viscosity
82
and, depending on the conditions, EPS may help floc bonding or take part in the
83
creation of foam and sludge bulking phenomena (Liu and Fang, 2003;). For typical
84
C/N/P ratios, high substrate loading rates favor the growth of floc forming bacteria
85
while low substrate loading rates favor the growth of filamentous bacteria. Other factors
86
that affect the growth of filamentous microorganisms are pH and dissolved oxygen
87
(DO) content. Changes in nutrient conditions, hydrodynamics and substrate
88
concentration have been shown to affect the biofilm structure (Wimpenny and
89
Colasanti, 1997; Stoodley et al., 1999). Furthermore, the availability of carbon sources
90
and nutrients, such as nitrogen and phosphorus, are known to affect the EPS synthesis in
91
biofilms (Sutherland, 2001). At low C:N ratios (carbon-limitation) carbon is utilized
92
solely for synthesis and energy, while at a high C:N ratios (nitrogen-limitation) excess
93
carbon is used to produce MLVSS or EPS (Durmaz and Sani, 2001; Liu and Fang,
AC C
EP
TE D
M AN U
SC
RI PT
69
4
ACCEPTED MANUSCRIPT 2003). Thus, there is disagreement on the C:N ratio allowing maximum EPS production.
95
However, there is agreement that nitrogen-limitation can affect EPS production.
96
Additionally, floc sizes were shown to increase at high C:N ratios and decrease at low
97
C:N ratios.
98
According to literature, when operating at high SRT, sedimentation problems may
99
appear due to sludge bulking phenomena. Sludge bulking usually occurs when sludge
100
hydraulic retention time and nitrate concentrations in settler are high, resulting in the
101
release of denitrification gases which lower the density of floc. In such cases, RAS rate
102
adjustment or better denitrification prior to the settling tank, resolves these problems.
103
The sedimentation problems encountered in WWTP settlers are usually sludge bulking,
104
floating sludge, pin point floc, straggler floc etc. These problems can be distinguished in
105
two categories, problems caused by gasses entraining sludge on the surface of the
106
clarifier and no gas related sedimentation problems in the clarifier. Additionally,
107
different temperature layers in a sedimentation tank may cause a short-circuit between
108
incoming mixed liquor and RAS stream or effluent stream. Common operational
109
sedimentation problems are summarized in Fig.1.
112
SC
M AN U
TE D
EP
111
Fig. 1. Sedimentation problems and their possible cause
AC C
110
RI PT
94
113
The clarification efficiency of a secondary clarifier is a critical factor in determining
114
the efficiency of the entire wastewater treatment system. Consequently, monitoring of
115
sludge properties and controlling of operational parameters in WWTPs with complete
116
SRT is a necessity in order to avoid sedimentation problems.
117
The target of this work is the study of sedimentation problems occurring because of the
118
increase in MLSS concentration towards the goal of zero net biomass production. Full5
ACCEPTED MANUSCRIPT scale AS WWTPs for high strength wastewaters containing high concentrations of
120
biodegradable organic compounds were studied. The WWTPs examined were two
121
different slaughterhouses in Northern Greece, operating under highly aerobic
122
conditions, with complete sludge retention and a high sludge recycle ratio. The
123
investigation offers new aspects of sedimentation tank design and operation, contributes
124
in the minimization of space required, of engineering investment, of operating cost, and
125
ensures efficient treatment.
RI PT
119
SC
126
2. Materials and Methods
128
Two identical WWTPs, installed in two different slaughterhouses one in Prosotsani
129
municipality of Drama Prefecture (WWTP-1) and another one in Almopia municipality
130
of Pella Prefecture (WWTP-2) in Northern Greece, operating with complete sludge
131
retention were examined. The systems integrate two treatment stages, a preliminary,
132
simultaneous nitrification/denitrification (SNdN) process with high sludge retention
133
time (SRT) and high hydraulic retention time (HRT) and a complete mix extended
134
aeration activated sludge (PCMAS) system with pre-anoxic denitrification. PCMAS
135
was operating under highly aerobic conditions, with a complete retention of sludge
136
(SRT = days of operation), high HRT and a high return activated sludge (RAS) rate.
137
The flowchart of these patented WWTPs (Bellos, 2012) is shown in Fig. 2. The influent,
138
after screening and decanting is stored into the flow and wastewater characteristics
139
equalization tank and then flows in the SNdN system at a rate of QF = 80m3 d-1 and 35
140
m3 d-1 for the WWTP-1 and WWTP-2 respectively. The effluent from SNdN biological
141
pre-treatment system feeds the PCMAS system where sludge and nitrate recirculate
142
from the sedimentation tank into the pre-anoxic tank, following a pattern similar to
143
Ludzack-Ettinger process. The PCMAS enhanced process performance is achieved not
AC C
EP
TE D
M AN U
127
6
ACCEPTED MANUSCRIPT only by complete SRT, high HRT and high DO concentration but mainly due to the
145
high biomass concentration (Lubbeke et al., 1995).
146
Six sampling points were selected in order to measure the operational parameters on
147
both systems. The sampling points were: a) SNdN’s anoxic/oxic tank influent
148
(equalization tank effluent) b) PCMAS system influent (SNdN effluent) c) WWTP
149
effluent d) three sampling points for monitoring MLSS and MLVSS concentration in
150
WWTP reactors (in SNdN anoxic/ oxic tank, in PCMAS’s aeration tank and in sludge
151
recycle stream). All samples were taken according to ISO 5667-10:1992 (Water quality
152
sampling, part 10: Guidance on sampling of wastewaters). WWTP-1 and WWTP-2
153
influent and effluent characteristics, sludge condensation (Xu), Yobs, sedimentation
154
velocity and SVI were monitored for 425 and 370 days after startup respectively and
155
state point analysis was performed during that period. In that period the slaughterhouses
156
were in full operation and the WWTPs were operating under steady state conditions.
M AN U
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144
158 159
TE D
157
Fig. 2. Schematic representation of WWTPs under investigation
The approach of sedimentation efficiency evaluation is based on the solids flux theory,
161
using state point analysis, to maximize secondary clarifier performance under complete
162
sludge retention conditions. Furthermore, the role of sludge recycle rate (QR), in relation
163
to sedimentation efficiency, was studied. The parameters analyzed were COD, BOD5,
164
BOD20, TKN, TSS, VSS, sludge volume index (SVI), stirred sludge volume index
165
(SSVI), pH and total nitrogen (TN). Additionally, biological characteristics of sludge
166
were periodically studied by using phase contrast microscope Leica DM1000. All
167
samples were analyzed at the accredited Environmental Chemistry & Waste Water
168
Treatment Laboratory of Technological Education Institution of Western Macedonia,
AC C
EP
160
7
ACCEPTED MANUSCRIPT Greece, by applying standard methods (APHA, 2012) and using calibrated and certified
170
equipment.
171
The state point analysis was performed by in-situ settling velocities measurements (Vi).
172
Settling column was filled with mixed liquor from recycle stream and WWTP’s effluent
173
was used for the sample dilutions. In order to evaluate interferences in Vi measurement
174
due to possible entrapment of gases in sludge flocs, settling velocity tests were
175
performed at both gentle and intense stirring.
176
Influent and effluent characteristics of WWTP-1 and WWTP-2 are presented in Table 1
177
and Table 2 respectively, and operational parameters are presented in Table 3. The
178
influent quality and quantity differs between the two slaughterhouses studied. WWTP-2
179
is characterized by higher organic and nitrogen load and lower COD/Total-Nitrogen
180
ratio than WWTP-1.
181
Table 1. Measured influent and effluent quality characteristics in WWTP-1
M AN U
SC
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169
WWTP-1 influent
PCMAS-1 influent
WWTP-1 effluent
COD (mgO2 L-1)a
4150
2835
36
BOD5 (mgO2 L-1)a
2380
1720
20
BOD20 (mgO2 L-1)a
3790
2650
34
TKN (mgN L-1)a
250
165
S0043-1354(14)00759-3
DOI:
10.1016/j.watres.2014.10.061
Reference:
WR 10980
To appear in:
Water Research
Received Date: 31 July 2014 Revised Date:
24 October 2014
Accepted Date: 28 October 2014
Please cite this article as: Amanatidou, E., Samiotis, G., Trikilidou, E., Pekridis, G., Taousanidis, N., Evaluating sedimentation problems in activated sludge treatment plants operating at complete sludge retention time, Water Research (2014), doi: 10.1016/j.watres.2014.10.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
SC
RI PT
ACCEPTED MANUSCRIPT
M AN U
State point analysis in complete mix extended aeration activated sludge system with pre-anoxic denitrification, at critical point of 15 kgMLSS m-3 and return activated sludge rate of 600% with complete solids retention time is given. The intersection of overflow and underflow line is the state point. State point’s vertical projection on X axis shows the MLSS concentration in the aeration tank. The underflow line is well below the lower limb of gravity flux
AC C
EP
TE D
curve insuring good effluent quality.
ACCEPTED MANUSCRIPT 1
Evaluating sedimentation problems in activated sludge treatment plants operating
2
at complete sludge retention time
3
Elisavet Amanatidou1*, Georgios Samiotis1, Eleni Trikilidou1, George Pekridis1,
5
Nikolaos Taousanidis2, 1
6
RI PT
4
Environmental Chemistry and Waste Water Treatment Lab, Environmental
Engineering Pollution Control Department, Technological Education Institute of
8
Western Macedonia, Koila, Kozani, 50100 Greece 2
Mechanical Engineering Department, Technological Education Institute of Western
M AN U
9
SC
7
Macedonia, Koila, Kozani, 50100 Greece
10
Abstract
12
Zero net sludge growth can be achieved by complete retention of solids in activated
13
sludge wastewater treatment, especially in high strength and biodegradable wastewaters.
14
When increasing the solids retention time, MLSS and MLVSS concentrations reach a
15
plateau phase and observed growth yields values tend to zero (Yobs ≈ 0). In this work, in
16
order to evaluate sedimentation problems arised due to high MLSS concentrations and
17
complete
18
slaughterhouse
EP
TE D
11
retention
AC C
sludge
operational
wastewater treatment
conditions,
plants
two
identical
were studied.
innovative
Measurements
of
* Corresponding author: Environmental Chemistry and Waste Water Treatment Lab, Environmental Engineering Pollution Control Department, Technological Education Institute of Western Macedonia, Koila, Kozani, 50100 Greece Tel. +30 24610 68015, e-mail: [email protected]
1
ACCEPTED MANUSCRIPT wastewaters’ quality characteristics, treatment plant’s operational conditions, sludge
20
microscopic analysis and state point analysis were conducted. Results have shown that
21
low COD/Nitrogen ratios increase sludge bulking and flotation phenomena due to
22
accidental denitrification in clarifiers. High return activated sludge rate is essential in
23
complete retention systems as it reduces sludge condensation and hydraulic retention
24
time in the clarifiers. Under certain operational conditions sludge loading rates can
25
greatly exceed literature limit values. The presented methodology is a useful tool for
26
estimation of sedimentation problems encountered in activated sludge wastewater
27
treatment plants with complete retention time.
M AN U
28
SC
RI PT
19
29
Keywords
30
Activated Sludge, Complete Sludge Retention, Zero Net Biomass, Sedimentation
31
Problems, Sludge Flotation, Floc Trapped Nitrogen
33
TE D
32
Highlights •
Zero net biomass growth can be achieved by complete sludge retention
35
•
Low C/N ratios increase sludge flotation in clarifier
36
•
SLR can exceed literature values, when limited denitrification occurs in
38 39
AC C
37
EP
34
clarifier
•
Stirred and unstirred velocity tests indicate gas production in clarifier
•
State point analysis estimates and verifies sedimentation process critical points
40 41
1. Introduction
42
In activated sludge (AS) wastewater treatment plants (WWTPs), sludge production
43
depends on different factors such as biodegradability of the organic pollutants, mass 2
ACCEPTED MANUSCRIPT loading of the treatment plant, degradation rate of microbial cells by endogenous
45
respiration or cellular lysis and existence of predator bacteria (Rocher et al., 1999). Due
46
to the nature of AS treatment, a large amount of excess sludge is generated daily,
47
proportional to influent substrate load. Recently, many investigations are oriented to
48
reduce the sludge production in AS WWTPs, because management and treatment of
49
sludge accumulate more than 50% of the construction and operating cost (Liu and Tay,
50
2001; Metcalf and Eddy, 2003; Foladori et al., 2010; Guo et al., 2013). Minimization of
51
observed sludge yields (Yobs) can be brought about by amplifying microbial cell lysis
52
and generating biomass growth on the lysis products, which is defined as the cryptic
53
growth (Rocher et al., 1999; Liu and Tay, 2001). Extensive microbial cell lysis occurs in
54
high strength biodegradable wastewaters (EC, 2005). AS treatment operating at high
55
sludge retention time (SRT) enhances cryptic growth, low food to microorganism ratios
56
(F/M), high recycle activated sludge (RAS) rate and low waste activated sludge (WAS)
57
that is the controlling parameter for regulating the SRT (Grady et al. 1999). Complete
58
SRT means that no solids are wasted from the system, resulting biomass age almost
59
equal to WWTP operation days (Henze, 2008).
60
When treating highly biodegradable wastewater, such as slaughterhouse effluents, in
61
complete sludge retention WWTPs (SRT = days of operation), problems in the settling
62
process may arise due to increased MLSS concentration and alterations of biomass
63
settling properties. The efficiency of the activated sludge treatment process is correlated
64
to an efficient solid-liquid separation, which is strongly depended on the biomass
65
settling properties (Govoreanu et al., 2003). Furthermore, efficient solid-liquid
66
separation results from the aggregation of microbes and solids into activated sludge
67
flocs (bioflocs). Poor bioflocculation in a wastewater treatment plant can result in poor
68
settling in the clarifiers, turbid effluent and adverse effects on WAS dewatering (Grady
AC C
EP
TE D
M AN U
SC
RI PT
44
3
ACCEPTED MANUSCRIPT et al., 1999; Sanin et al., 2006; Nguyen et al., 2007). There are two factors that most
70
affect bioflocculation, SRT and substrate loading rate (Nirupa, 2010). SRT greatly
71
affects the floc structure by alternating the proportion of floc forming bacteria and
72
filamentous bacteria present in the biofloc and sludge settling properties. Excessive
73
filaments lead to the bulking of sludge, causing decrease in settleability and
74
compactability of bioflocs, while inadequate filamentous bacteria results in pin point
75
and easily sheared flocs formation (Metcalf and Eddy, 2003; Gerardi, 2006; Nirupa,
76
2010; Ye et al., 2011). Consequently, SRT is the key for controlling biomass production
77
in a WWTP, elevating SRT to one of the most critical design and operational parameter
78
in AS process. Substrate loading rate and nutrient analogy directly influence the growth
79
of filamentous and floc forming bacteria, the extracellular polymeric substances (EPS)
80
content and therefore the sludge nature and the effluent quality. In starvation conditions
81
where enhanced cell lysis occur, bacteria produce EPS that alter mixed liquor viscosity
82
and, depending on the conditions, EPS may help floc bonding or take part in the
83
creation of foam and sludge bulking phenomena (Liu and Fang, 2003;). For typical
84
C/N/P ratios, high substrate loading rates favor the growth of floc forming bacteria
85
while low substrate loading rates favor the growth of filamentous bacteria. Other factors
86
that affect the growth of filamentous microorganisms are pH and dissolved oxygen
87
(DO) content. Changes in nutrient conditions, hydrodynamics and substrate
88
concentration have been shown to affect the biofilm structure (Wimpenny and
89
Colasanti, 1997; Stoodley et al., 1999). Furthermore, the availability of carbon sources
90
and nutrients, such as nitrogen and phosphorus, are known to affect the EPS synthesis in
91
biofilms (Sutherland, 2001). At low C:N ratios (carbon-limitation) carbon is utilized
92
solely for synthesis and energy, while at a high C:N ratios (nitrogen-limitation) excess
93
carbon is used to produce MLVSS or EPS (Durmaz and Sani, 2001; Liu and Fang,
AC C
EP
TE D
M AN U
SC
RI PT
69
4
ACCEPTED MANUSCRIPT 2003). Thus, there is disagreement on the C:N ratio allowing maximum EPS production.
95
However, there is agreement that nitrogen-limitation can affect EPS production.
96
Additionally, floc sizes were shown to increase at high C:N ratios and decrease at low
97
C:N ratios.
98
According to literature, when operating at high SRT, sedimentation problems may
99
appear due to sludge bulking phenomena. Sludge bulking usually occurs when sludge
100
hydraulic retention time and nitrate concentrations in settler are high, resulting in the
101
release of denitrification gases which lower the density of floc. In such cases, RAS rate
102
adjustment or better denitrification prior to the settling tank, resolves these problems.
103
The sedimentation problems encountered in WWTP settlers are usually sludge bulking,
104
floating sludge, pin point floc, straggler floc etc. These problems can be distinguished in
105
two categories, problems caused by gasses entraining sludge on the surface of the
106
clarifier and no gas related sedimentation problems in the clarifier. Additionally,
107
different temperature layers in a sedimentation tank may cause a short-circuit between
108
incoming mixed liquor and RAS stream or effluent stream. Common operational
109
sedimentation problems are summarized in Fig.1.
112
SC
M AN U
TE D
EP
111
Fig. 1. Sedimentation problems and their possible cause
AC C
110
RI PT
94
113
The clarification efficiency of a secondary clarifier is a critical factor in determining
114
the efficiency of the entire wastewater treatment system. Consequently, monitoring of
115
sludge properties and controlling of operational parameters in WWTPs with complete
116
SRT is a necessity in order to avoid sedimentation problems.
117
The target of this work is the study of sedimentation problems occurring because of the
118
increase in MLSS concentration towards the goal of zero net biomass production. Full5
ACCEPTED MANUSCRIPT scale AS WWTPs for high strength wastewaters containing high concentrations of
120
biodegradable organic compounds were studied. The WWTPs examined were two
121
different slaughterhouses in Northern Greece, operating under highly aerobic
122
conditions, with complete sludge retention and a high sludge recycle ratio. The
123
investigation offers new aspects of sedimentation tank design and operation, contributes
124
in the minimization of space required, of engineering investment, of operating cost, and
125
ensures efficient treatment.
RI PT
119
SC
126
2. Materials and Methods
128
Two identical WWTPs, installed in two different slaughterhouses one in Prosotsani
129
municipality of Drama Prefecture (WWTP-1) and another one in Almopia municipality
130
of Pella Prefecture (WWTP-2) in Northern Greece, operating with complete sludge
131
retention were examined. The systems integrate two treatment stages, a preliminary,
132
simultaneous nitrification/denitrification (SNdN) process with high sludge retention
133
time (SRT) and high hydraulic retention time (HRT) and a complete mix extended
134
aeration activated sludge (PCMAS) system with pre-anoxic denitrification. PCMAS
135
was operating under highly aerobic conditions, with a complete retention of sludge
136
(SRT = days of operation), high HRT and a high return activated sludge (RAS) rate.
137
The flowchart of these patented WWTPs (Bellos, 2012) is shown in Fig. 2. The influent,
138
after screening and decanting is stored into the flow and wastewater characteristics
139
equalization tank and then flows in the SNdN system at a rate of QF = 80m3 d-1 and 35
140
m3 d-1 for the WWTP-1 and WWTP-2 respectively. The effluent from SNdN biological
141
pre-treatment system feeds the PCMAS system where sludge and nitrate recirculate
142
from the sedimentation tank into the pre-anoxic tank, following a pattern similar to
143
Ludzack-Ettinger process. The PCMAS enhanced process performance is achieved not
AC C
EP
TE D
M AN U
127
6
ACCEPTED MANUSCRIPT only by complete SRT, high HRT and high DO concentration but mainly due to the
145
high biomass concentration (Lubbeke et al., 1995).
146
Six sampling points were selected in order to measure the operational parameters on
147
both systems. The sampling points were: a) SNdN’s anoxic/oxic tank influent
148
(equalization tank effluent) b) PCMAS system influent (SNdN effluent) c) WWTP
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effluent d) three sampling points for monitoring MLSS and MLVSS concentration in
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WWTP reactors (in SNdN anoxic/ oxic tank, in PCMAS’s aeration tank and in sludge
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recycle stream). All samples were taken according to ISO 5667-10:1992 (Water quality
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sampling, part 10: Guidance on sampling of wastewaters). WWTP-1 and WWTP-2
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influent and effluent characteristics, sludge condensation (Xu), Yobs, sedimentation
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velocity and SVI were monitored for 425 and 370 days after startup respectively and
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state point analysis was performed during that period. In that period the slaughterhouses
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were in full operation and the WWTPs were operating under steady state conditions.
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TE D
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Fig. 2. Schematic representation of WWTPs under investigation
The approach of sedimentation efficiency evaluation is based on the solids flux theory,
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using state point analysis, to maximize secondary clarifier performance under complete
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sludge retention conditions. Furthermore, the role of sludge recycle rate (QR), in relation
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to sedimentation efficiency, was studied. The parameters analyzed were COD, BOD5,
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BOD20, TKN, TSS, VSS, sludge volume index (SVI), stirred sludge volume index
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(SSVI), pH and total nitrogen (TN). Additionally, biological characteristics of sludge
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were periodically studied by using phase contrast microscope Leica DM1000. All
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samples were analyzed at the accredited Environmental Chemistry & Waste Water
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Treatment Laboratory of Technological Education Institution of Western Macedonia,
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ACCEPTED MANUSCRIPT Greece, by applying standard methods (APHA, 2012) and using calibrated and certified
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equipment.
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The state point analysis was performed by in-situ settling velocities measurements (Vi).
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Settling column was filled with mixed liquor from recycle stream and WWTP’s effluent
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was used for the sample dilutions. In order to evaluate interferences in Vi measurement
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due to possible entrapment of gases in sludge flocs, settling velocity tests were
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performed at both gentle and intense stirring.
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Influent and effluent characteristics of WWTP-1 and WWTP-2 are presented in Table 1
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and Table 2 respectively, and operational parameters are presented in Table 3. The
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influent quality and quantity differs between the two slaughterhouses studied. WWTP-2
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is characterized by higher organic and nitrogen load and lower COD/Total-Nitrogen
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ratio than WWTP-1.
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Table 1. Measured influent and effluent quality characteristics in WWTP-1
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WWTP-1 influent
PCMAS-1 influent
WWTP-1 effluent
COD (mgO2 L-1)a
4150
2835
36
BOD5 (mgO2 L-1)a
2380
1720
20
BOD20 (mgO2 L-1)a
3790
2650
34
TKN (mgN L-1)a
250
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