Chapter-V

5. Evaluation of sludge yield in activated sludge process

During biological treatment of wastewater, the organic substrate is decomposed to
simple end product and energy is liberated. A part of this generated energy is utilized by
microorganisms for new cell growth. The production of new biomass depends on nature of
the substrate, operating parameters and type of process. To evaluate the sludge yield in
activated sludge process treating pulp and paper mill wastewater, one lab scale bioreactor
having ASP configuration was run for more than 6 months. The characteristics of wastewater
fed to the bioreactor during the study are given in Table 5.1.

Table 5.1: Characteristics of the wastewater used as feed in the experimental bioreactor
CODs BOD:CODs AOX Colour
Sample
(mg/l) ratio (mg/l) (Pt-Co)

CD wastewater 1150±225 1:1.9 90±20 600±30

EOP wastewater 1225±235 1:3.8 55±15 1500±90

Weak black liquor 149667±3512 1:5.7 - 471400±10824

The bioreactor was run under ideal operating and environmental conditions as given
in Table 5.2. The pH of the feed was neutral and temperature during the study was 36.0±0.3
°C. The dissolved oxygen was maintained near to 1.5±0.3 mg/l and HRT was 8.6±0.3 h. The
average sludge retention time was 18 days during the study. The concentration of CODs in
the feed was 536±11 mg/l and reduction of CODs was 68±3%. Based on CODs removal, the
organic load and F/M ratio were 0.98±0.06 kg/m3/d and 0.28±0.02 d-1 respectively. The
biomass was flocculating in nature and SVI was 39±14 ml/g throughout the study. The free
filamentous organisms were not observed and higher organisms like protozoa and rotifer
were present in good number. The average sludge yield during the study was 0.31±0.02 g/g
of CODs removal, the sludge yield varied from 0.27 to 0.33 g/g of CODs removal (Figure
5.1).

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 0.35 0.32 0.33 0.31
0.31 0.31
Sludge yield (g/g CODs removal)

0.3 0.29
0.27
0.25

0.2

0.15

0.1

0.05

0
1 2 3 4 5 6 Avg.
Month
Figure 5.1: Sludge yield in lab scale ASP treating pulp and paper mill effluent

5.1 Effect of varying load of organochlorine compounds on operation of ASP

The various chlorophenolic compounds present in pulp and paper mill wastewater
have been reported as metabolic uncoupler. Bacterial anabolism is coupled with catabolism of
substrate through rate limiting respiration. In the presence of these compounds excess free
energy would be directed away from anabolism so that the production of biomass can be
reduced (Liu et al., 1998; Low et al., 2000). The current study is aimed at evaluating the
sludge yield and performance of the process at varying load of organochlorine compounds.

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Table: 5.2: Operating and environmental conditions, and performance of bioreactor during the study
Parameter Month-I Month-II Month-III Month-IV Month-V Month-VI

pH 7.41±0.12 7.36±0.09 7.71±0.14 7.28±0.21 7.47±0.12 7.39±0.19

Temperature, (°C) 36.0±0.7 35.9±1.8 35.6±1.1 35.9±1.4 36.4±0.2 36.2±0.2

DO, (mg/l) 1.91±0.48 1.86±0.57 1.71±0.58 1.44±0.69 1.16±0.51 1.26±0.50

HRT, (h) 9.21±0.98 8.27±0.53 8.64±0.55 8.40±0.47 8.41±0.39 8.59±0.17

Organic load, (kg/m3/d) 0.96±0.12 1.0±0.11 0.95±0.12 0.94±0.10 1.09±0.12 0.96±0.09

F/M ratio,( d-1) 0.27±0.04 0.30±0.05 0.27±0.04 0.27±0.03 0.31±0.04 0.28±0.03

SVI, (ml/g) 39±5 29±8 42±10 26±4 23±7 24±3

CODs reduction, (%) 67.3±3.7 65.9±4.0 67.5±5.0 69.7±2.8 72.7±3.3 66.4±3.4

AOX reduction, (%) 38.7±3.3 33.0±1.2 33.8±2.6 39.7±6.4 37.4±12.4 41.2±9.2

Influent CODs: 536±11 mg/l, AOX: 13.1±0.3 mg/l

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5.1.1 At normal AOX load

Under normal AOX load, the three bioreactors (6 liter capacity) were run for 20 days
under the specified environmental and operating conditions (Table 5.3). The conditions were
used to grow flocculating biomass. Though the pH of feed was neutral, it was higher (7.7 to
8.1) in treated wastewater in all the three bioreactors due to increase in alkalinity.
Temperature and DO were maintained at 34-37 °C and 0.9-1.5 mg/l respectively. SVI of the
sludge in three bioreactors was in the range of 13-28 ml/g. Reduction in CODs and AOX was
68-72 and 39-43% respectively in the bioreactors. Dense flocs were developed with a few
filamentous organisms coming-out of flocs in all the three bioreactors. Abundance of higher
organisms like protozoa, rotifer and nematode was quite significant in all the bioreactors.

Table 5.3: Operating conditions and performance of bioreactors under normal AOX load
Parameter R1 R2 R3

pH 7.91±0.20 7.92±0.22 7.90±0.12

Temperature (°C) 35.8±1.2 35.6±1.1 36.0±0.9

DO (mg/l) 1.2±0.3 1.1±0.2 1.3±0.3

HRT (h) 8.7±0.2 8.5±0.3 8.9±0.2

MLSS (g/l) 4.12±0.25 4.09±0.35 4.00±0.27

MLVSS (g/l) 3.44±0.21 3.42±0.27 3.34±0.22

F/M ratio (d-1) 0.23±0.02 0.25±0.02 0.22±0.05

Organic load (kg/m3/d) 0.80±0.06 0.82±0.02 0.84±0.03

SVI (ml/g) 20±3 16±3 24±4

CODs reduction (%) 69.8±1.3 70.1±1.2 69.9±1.4

AOX reduction (%) 41.3±1.2 40.1±1.3 41.7±0.9

Influent pH: 7.01±0.01, CODs: 482±10 mg/l, AOX: 12.2±0.4 mg/l

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5.1.2 At moderate, low and high AOX load

The first bioreactor (R1) in the previous run was maintained as control (Rc) throughout
the study (moderate AOX load), whereas the second (R2) and third (R3) bioreactors were run
at reduced (Rrl) and higher (Rhl) AOX load respectively (Table 5.4). The other operating and
environmental conditions were similar in all the three bioreactors (Table 5.5). The study on
varying load of AOX compounds was conducted in three phases to observe the impact of its
concentration with time.

Table 5.4: AOX load in bioreactors at moderate, low and high concentrations
Bioreactor Rc Rrl Rhl

AOX (mg/l) 11.64 4.51 29.30

Phase I
(First 10 days) AOX load (g/m3/d) 34.9 13.5 87.9

Phase II
AOX (mg/l) 11.36  0.18 4.59  0.10 28.69  0.40
(Next 20 days)
AOX load (g/m3/d) 34.1  0.5 13.8  0.4 86.1  1.2

Phase III
AOX (mg/l) 10.94 0.42 4.70  0.57 27.61  0.95
(Next 30 days)
AOX load (g/m3/d) 32.8  1.3 14.1  1.7 82.8  2.9

During phase I (PI), MLVSS to MLSS ratio in Rhl decreased to 81.8 from 87.0%
(Table 5.5) due to higher proportion of CD and EOP wastewaters in the feed and accumulation
of salts present in wastewater from bleaching streams. During phase II (PII) and phase III
(PIII), MLVSS content in bioreactor Rhl further decreased to 76.9 and 71.9 % respectively,
whereas the same increased to some extent in Rrl (Table 5.5).

The average removal of CODs in control bioreactor remained in the range of 67.6 to
71.5% throughout the study (Table 5.6). In Rrl reduction of CODs also was decreased to 64-
65 from 70.1±1.2%. The lower removal in CODs in Rrl was due to (i) relatively higher
amount of low biodegradable weak black liquor in the feed to make-up the required CODs
concentration and (ii) relatively lower amount of biodegradable substrate (CD and EOP) to
have low AOX content.

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Table 5.5: Operating conditions of bioreactors during moderate, low and high AOX load
Operating parameter Rc Rrl Rhl

PI 4.06  0.11 3.96  0.16 4.23  0.10

MLSS (g/l) PII 3.97  0.16 3.86  0.16 4.42  0.19

PIII 3.89  0.13 3.85  0.19 4.62  0.19

PI 3.52  0.09 3.48  0.14 3.46  0.11

MLVSS (g/l) PII 3.49  0.13 3.46  0.15 3.39  0.14

PIII 3.44  0.12 3.50  0.18 3.32  0.13

PI 86.7  0.5 87.9  0.7 81.8  1.2

MLVSS/MLSS (%) PII 87.9  0.9 89.6  1.5 76.7  2.1

PIII 88.4  1.0 90.9  1.4 71.9  1.9

PI 0.27  0.02 0.26  0.02 0.30  0.05

F/M ratio (d-1) PII 0.30  0.04 0.28  0.04 0.29  0.03

PIII 0.27  0.04 0.27  0.03 0.26  0.04

PI 8.7  0.3 8.9  0.4 8.5 0.1

HRT (h) PII 8.4  0.5 8.5  0.8 8.6  0.3

PIII 8.4  0.5 8.5  0.4 8.7  0.8

PI 0.94  0.07 0.91  0.07 1.04  0.19

Organic load
PII 1.03  0.14 0.98  0.14 0.97  0.09
(kg CODs removal/m3/d)
PIII 0.94  0.13 0.94  0.11 0.85  0.13

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 90
Ref. P-I P-II P-III
80
70
CODs reduction (%)

60
50
40
30
20
10
0
Rc Rrl Rhl
Figure 5.2: Effect of AOX load on reduction of CODs

60
Ref. P-I P-II P-III
50
AOX reduction (%)

40

30

20

10

0
Rc Rrl Rhl

Figure 5.3: Effect of AOX load on reduction of AOX compounds

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 The lowering in CODs removal was observed in P-I for both Rrl and Rhl and it
remained almost same during the study in Rrl (Figure 5.2). In Rhl, the removal of CODs was
decreased to 66.0±4.9 from 69.9±1.4% in P-I and further to 64.2±3.5 and 60.3±5.7 % during
P-II and P-III respectively.

The performance of bioreactors for reduction of AOX was comparable (Figure 5.3); it
varied between 39-47% irrespective of influent AOX concentration. The final discharge
concentration of AOX was highest in Rhl followed by Rc and Rrl due to higher concentration
of the same in the feed in Rhl. Lower reduction in CODs and higher SVI value indicated that
increase in AOX compounds (i) resulted in diffused flocs and, (ii) hampered the metabolism.
However, The degradation of AOX was not affected at higher AOX load during the study
period.

Table 5.6: Performance of ASP during moderate, low and high AOX load
Parameter Rc Rrl Rhl

PI 48014 52219 52312

CODs (mg/l) PII 50534 52529 54022

PIII 48525 51628 51534

PI 70.92.5 64.02.7 66.04.9

CODs removal (%) PII 71.5 4.9 65.5 2.7 64.23.5

PIII 67.65.3 64.24.4 60.35.7

PI 43.7 39.2 44.8

AOX removal (%) PII 46.81.3 43.51.7 43.80.5

PIII 44.63.6 47.44.6 44.03.3

PI 355 192 291

SVI (ml/g) PII 4411 2812 4614

PIII 349 3712 11425

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 During P-I, the morphology of organisms in all the bioreactors remained similar. The
motility of higher organisms in all the bioreactors was also comparable. The morphology of
microorganisms started to change during the second phase (P-II) with a change in AOX load.
In Rrl there were good dense flocs with a few diffused one. Though a few filamentous
organisms were observed, the motility of the higher organisms was good. In Rhl dense flocs
started to disintegrate with the development of thin and pinpoint flocs (Figure 5.4). The
motility of higher organisms was stressed to some extent.

Figure 5.4: Morphological characteristics during moderate, low and high AOX load

79
 In Phase III, there was a drastic change in structure of sludge in Rhl; most of the same
was in the form of diffused flocs though free forms of filamentous organisms were not
detected. Motility of higher organisms was quite comparable to Rc. A few new flocs started to
grow at the end due to gradual adaptation in the changed conditions. Due to stable
morphology of biosludge in Rc and Rrl, the SVI value remained similar to that of reference
(Figure 5.5), whereas in Rhl the SVI values were more than 100 ml/g in the last phase (P-III)
of the study due to change in morphology.

140

120

100
Ref. P-I P-II P-III
SVI (ml/g)

80

60

40

20

0
Rc Rrl Rhl

Figure 5.5: Effect of AOX load on SVI

5.1.3 Effect of AOX load on sludge yield

In biological treatment processes, growth of cell is well linked with the oxidation of
substrate. Sludge generation at both moderate and low AOX load was 0.29±0.05 and
0.30±0.02 g/g of CODs removal respectively, whereas at high AOX load the same was
reduced to 0.18±0.06 g/g of CODs removal (Table 5.7). The increased concentration of
organochlorine compounds affected both the growth of microorganisms and the oxidation
efficiency. The organochlorine compounds worked as metabolic uncouplers and resulted in
low sludge generation.

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Table 5.7: Sludge generation at moderate and high AOX load
Parameter Moderate load High load

AOX conc. in influent (mg/l) 9.351.87 23.675.41

AOX load (g/m3/d) 28.15.6 71.016.2

AOX reduction (%) 40.86.0 41.33.7

Sludge yield (g/g CODs removal) 0.290.05 0.180.06

5.2 Mode of removal of AOX compounds

Three set of samples were analyzed as a prelude to find the mechanism of removal of
AOX compounds. At moderate load, 45.0±4.1% AOX compounds were dechlorinated
during biological degradation, whereas 2.3±0.6% of the same accompanied with waste
activated sludge in adsorbed form (Table 5.8). The overall AOX removal was 47.3±3.7%.
Similarly, at high load, 44.7±3.3% AOX compounds were dechlorinated, whereas 1.6±0.2%
of the same accompanied with waste activated sludge. The overall AOX reduction was
46.3±3.3%. The concentration of AOX compounds in sludge was dependent on concentration
of the same in influent; it was 2690±969 and 7659±750 mg/kg at moderate and high AOX
load respectively. The major mode of AOX removal was dechlorination (~95 % of the AOX
removal) in both the bioreactors. During biological treatment, 43-51% AOX compounds was
removed and rest (49-57%) compounds were released with treated effluent; only 1.6-2.3%
AOX compounds were adsorbed on waste activated sludge.

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Table 5.8: Removal of AOX compounds at different AOX loads
Parameter Moderate load High load

AOX in sludge (mg/kg) 2690969 7659750

HRT (h) 8.60.2 8.90.5

CODs in influent (mg/l) 45022 44531

CODs in effluent (mg/l) 1523 17134

Biomass (g/g CODs removal) 0.290.05 0.180.06

AOX conc. in influent (mg/l) 9.951.77 23.434.57

AOX conc. in effluent (mg/l) 5.291.30 12.632.93

AOX removal (%) 47.33.7 46.33.1

AOX dechlorination (%) 45.04.1 44.73.3

AOX adsorbed on sludge (%) 2.30.6 1.60.2

Though van der Waals forces, chemical and hydrogen bonding were considered to be
ways of adherence, biosorption was the most important factor in the removal of
organochlorines in secondary treatment system of pulp and paper mill effluent (Gloria et al.,
1994).

To confirm the sorption of organochlorine compounds in biosludge, dichloromethane

(DCM) was used as spiking agent. DCM is volatile organochlorine compound and used as
standard for estimation of POX compounds. 0.1g sludge was dispersed in 80 ml water and
spiked with dichloromethane (DCM) solution containing 50 µg/l as POX compounds; the
recovered concentration of DCM was 49.96 µg/l. Similarly 0.2, 0.35, 0.5 and 1.0 g biosludge
was dispersed and spiked with the same amount of dichloromethane solution (Table 5.9). As
the concentration of biosludge was increased, there was higher sorption of dichloromethane.
The stripping of the same was decreased with increasing concentration of biosludge (Figure
5.6). The increase in sorption of DCM with increase in biosludge concentration was due to its
equilibrium distribution between the solid sludge and aqueous phase. Recovery of the
standard from water containing the same amount of DCM was 50.28±0.09 µg/l.

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 Table 5.9: Effect of biosludge concentration on desorption of dichloromethane (DCM) as
POX compounds
Sludge amount (g/80 ml) Recovered POX Recovery of POX (%)
concentration (µg/l)

0.1 49.96 99.9

0.2 49.41 98.8

0.35 38.67 77.3

0.5 25.62 51.2

1.0 24.01 48.0

Further to check the sorption behavior of biosludge, 0.35 and 1.0 g sludge were
dispersed in 80 ml of water, spiked with DCM as described above and shaked for 1.0 h at 200
rpm. In case of 0.35 g sludge sample, recovery of DCM was 29.81 µg/l and sorption of DCM
increased from 22.7 to 40.4%. For the second case, the sorption of DCM increased from 52.0
to 62.2 % and recovery was 18.9 µg/l. Higher sorption of DCM is due to increased
concentration of biosludge, agitation and time of exposure (Leuenberger et al., 1985).

60

50

40
Sorption (%)

30

20

10

0
0 0.2 0.4 0.6 0.8 1 1.2
Sludge (g/80 ml)

Figure 5.6: Effect of biosludge concentration on sorption of dichloromethane (DCM)

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Summary

The growth of biomass in the biological treatment of wastewater depends on the

nature of substrate, operating parameters and type of process. As the wastewater of pulp and
paper mills is rich in organochlorine compounds, it affects the growth of organisms in the
process. The higher load of AOX compounds had a bearing on the performance as well as
morphology of biomass. Well dense structure of microbial flocs started breaking and resulted
in diffused and pin point flocs when operated at higher AOX load of 84 g/m3/day. Though the
AOX reduction remained more or less same in the range of 39-46 % at moderate and higher
AOX load, there was 5-7 % lower CODs removal at higher AOX load. The sludge volume
index (SVI) was above 100 ml/g at higher AOX load. During biodegradation, the major mode
of AOX removal was dechlorination and only 1.6-2.3% AOX compounds were adsorbed on
waste activated sludge. The concentration of AOX compounds in sludge was dependent on
the AOX concentration in influent. The sludge yield was 0.29 and 0.18 g/g of CODs removal
during low to moderate and higher AOX load respectively. The sorption with
dichloromethane revealed that biosludge from pulp and paper mill was a good adsorbent of
volatile organochlorine compounds.

84