An evaluation of the adsorption process in removing organic substances in a pilot flow-type surface water treatment system

Abstract

This article presents the results of the adsorption process effectiveness in treating surface water, especially in removing organic substances. The effectiveness of the adsorption system was evaluated at different levels of adsorption capacity exhaustion of the activated carbon bed, which was possible due to replacement of the beds during the study period. Studies have shown that among the removed substances, chlorinated disinfection byproduct precursors dominated, and during the period preceding bed replacement, the biological activity of microorganism populating the activated carbon ensured a reduction in not only organic but also non-organic food substrates. In such a adsorption bed populated with microorganism a nitrification process took place, indicating a high degree of process stability. Replacing the adsorption beds provided a significant increase in the effectiveness of removing organic substances, especially those absorbing UV light, therefore removing chlorinated organic disinfection by-product precursors.

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* Corresponding author: wolska@pwr.edu.pl
An evaluation of the adsorption process in removing organic
substances in a pilot flow-type surface water treatment system
Małgorzata Wolska1,2,*, Marek Mołczan1,2, Sławomir Szerzyna1, Wojciech Adamski1, Jacek Wiśniewski1, Zbigniew Ferenc1,
Aleksandra Sambor1, and Małgorzata Żółtowska2
1The Department of Water and Sewage Treatment Technology, Wrocław University of Science and Technology, Wybrzeże
Wyspiańskiego 27 Street, 50-370 Wrocław, Poland
2 Municipal Water and Sewerage Company in Wrocław, Na Grobli 14/16 Street, 50-421 Wrocław, Poland
Abstract: This article presents the results of the adsorption process effectiveness in treating surface water,
especially in removing organic substances. The effectiveness of the adsorption system was evaluated at
different levels of adsorption capacity exhaustion of the activated carbon bed, which was possible due to
replacement of the beds during the study period. Studies have shown that among the removed substances,
chlorinated disinfection byproduct precursors dominated, and during the period preceding bed replacement, the
biological activity of microorganism populating the activated carbon ensured a reduction in not only organic
but also non-organic food substrates. In such a adsorption bed populated with microorganism a nitrification
process took place, indicating a high degree of process stability. Replacing the adsorption beds provided
a significant increase in the effectiveness of removing organic substances, especially those absorbing UV light,
therefore removing chlorinated organic disinfection by-product precursors.
1 Introduction
The increase in contamination of source surface waters due
to their contamination with household and industrial
wastewater and atmospheric precipitation has caused that
water treatment systems more often make use of
adsorption processes [1,2]. This process ensures the
removal of organic substances of medium and low
molecular mass [3-7], whose elimination is not possible
during the coagulation process, which is commonly used
for removing organic substances from surface waters
before the adsorption process. Additionally, as many
studies show [8,9], adsorption of activated carbon allows
for a elimination of various types of microcontaminants.
More frequently, different groups of microcontaminants
are detected in waters, and within these groups a constantly
increasing number of chemical compounds being
introduced into the environment. The adsorption process
effectively removes some pharmaceuticals [10], pesticides
[11], phenols [12] and non-organic microcontaminants
such as heavy metals [13]. This is one of the reasons of the
increasing use of activated carbon in surface water
treatment technology.
Surface water treatment technology also commonly
makes use of that fact that adsorption beds are easily
populated with microorganisms, which form a biofilm on
the bed surface [14,15]. Biologically activated beds yield
an increase in organic substance removal due to
biodegradation by microorganisms. [16]. An increase in
the effectiveness of biologically active beds in removing
organic substances is also achieved by preceding
adsorption process with ozonation [8] which yields a
change in the structure of organic substances in water
towards one more susceptible to adsorption and
biodegradation.
The effectiveness of adsorption depends on many
factors and decreases along with increasing bed operating
time, and therefore the exhausting of adsorbent potential. .
Generally, in technical-scale systems, the need for bed
replacement/regeneration is decided by, apart from
worsening of filtrate quality, by the iodine number value
The study presented in this paper was conducted at a
pilot scale in a flow-type system supplied by a surface
water treatment plant, which allowed for an evaluation of
the changes of the adsorption process efficiency with
changes in raw water quality in conditions of continuous
and long-term operation.
2 Methods of study
Studies of adsorption process effectiveness in the surface
water treatment system were conducted in a pilot-scale
flow-type system with a throughput of 3 m3/h consisting of
coagulation, sedimentation, filtration, adsorption,
disinfection and pH correction. The treatment system was
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
(http://creativecommons.org/licenses/by/4.0/).
E3S Web of Conferences 59, 00018 (2018) https://doi.org/10.1051/e3sconf/20185900018
CIWT 2017

supplied with the same water as the water treatment plant
treating water for human consumption. The adsorption
process was conducted in the system in pressure filters
with a bed with granulated activated carbon, whose
properties are shown in Table 1.
Table 1. Parameters of fresh WG-12 activated carbon.
No
Parameter
Unit
Value
1.
Bulk density
g/dm3
470
2
Volatile content
%
1.50
3
Ash Content
%
11.6
4.
Specific Surface Area
m2/g
968
5.
Iodine Number *
mg/g
1014
6.
CTC –adsorption CCI4
%
62.30
7.
<0,5 mm subfraction
content
%
0.00
8.
Mechanical strength
%
97.30
*value specified by manufacturer of fresh activated carbon
The beds were filled in June 2016, and were replaced in
September 2016, which makes it possible to evaluate the
effectiveness of the adsorption process in different phases
of bed adsorption capacity exhaustion. During the bed
replacement, an analysis of the iodine number of the bed
being removed was performed, which yielded a value of
660 mg/g, which indicated that the adsorbing potential of
this bed had not yet been exhausted. This same value for
the fresh bed was 1040 mg/g.
Despite operation the system at a constant throughput,
the adsorption filters were operated at varying operating
parameters, which resulted from the necessity of
continuous system operation (Table 2).
Table 2. Adsorption bed operating parameters.
Parameter
Unit
Value
Filtration area
m2
0.59
Bed height
m
1.50
Filtration Rate
m/h
4.71-5.15
Water-Activated Carbon
Contact Time
min
17.40-19.0
The study was conducted during the time period from May
2016 to December 2016, with samples being taken weekly
before and after the adsorption process. The long time
period of the study, and therefore the large variation in raw
water, allowed for assessment of the impact of supply
water quality on the adsorption process effectiveness.
In all water samples, the following parameters were
measured: T, pH, specific conductivity, color, turbidity,
UV absorbance UV254 and UV272, total (TOC) and
dissolved (DOC) organic carbon concentration, non-
organic nutrient concentrations (ammonia ion, nitrites,
nitrates, phosphates) and concentrations of chlorides and
sulfates. In all pre- and post-adsorption water samples, the
total psychrophillic and mesophillic bacteria numbers were
also determined.
Based on UV254 absorbance values, and DOC
concentrations the specific UV absorbance values (SUV-a)
was calculated.
All analysis of water quality indicators were conducted
with current Polish standards, according to accredited
laboratory procedures.
3 Results and discussion
The source water quality during the study period was
characterized by a very large variability in composition,
which consequently resulted in a decrease in quality of
water undergoing adsorption (Table 3). The variability in
composition was however lower in water before adsorption
than that found for raw water. This means that the
coagulation process used before adsorption, along with
sedimentation, ensured an effective elimination of organic
substances and significantly reduced the variability of
water quality reaching the adsorption beds [17].
The greatest variability in water reaching the
adsorption filters, apart from temperature, was found for
indicators relating to organic substance content (TOC,
DOC, UV254 and UV272 absorbances and color). Among
the organic substances in water supplying adsorption beds,
the dissolved fraction dominated (from 96.9% to 99.1%),
which testifies to the large susceptibility of organic
substances to removal by adsorption. Furthermore, the
calculated SUV-a values in the range of 1.65-2.20 m2/g
testify to the limited susceptibility of organic substances to
removal during this process [18].
The presence of non-organic nutrient substrates in
water supplying the beds (Table 3) and microorganisms
which may allow for biofilm development (Fig. 1), along
with the long adsorption bed operating time suggest the
conclusion that, apart from adsorption, biodegradation also
took place within the beds.
The effectiveness of the adsorption process is
confirmed by the elimination of organic substances that
was found, which during the study was in the ranges of
11.5-59.4% and 10.8-61.2% for TOC and DOC
respectively (Fig. 2).
A significant increase in organic substance removal
effectiveness was found after bed replacement, which
confirms the partial exhaustion of bed exhaustion potential
before the replacement. On the other hand, the successive
decrease in organic substance removal effectiveness in the
first two months after bed replacement may be explained
by bed break-in and the development of biofilm on bed
grains. After this time period, a certain equilibrium
developed between adsorption and biodegradation.
2
E3S Web of Conferences 59, 00018 (2018) https://doi.org/10.1051/e3sconf/20185900018
CIWT 2017

Table 3. Ranges of water quality indicators before and after
absorption.
Indicator
Unit
Before
Adsorption
After
Adsorption
T
°C
6.70-23.30
7.70-23.50
pH
6.73-7.72
6.65-7.60
Basicity
val/m3
2.00-3.50
2.00-3.50
Mt
NTU
0.10-0.80
0.00-0.60
B
g/m3
4.60-7.60
0.40-4.30
UV254
M
5.94-8.38
0.44-5.58
UV272
M
4.91-6.87
0.36-4.53
TOC
gC/m3
2.90-4.60
1.82-4.77
DOC
gC/m3
2.86-4.33
1.63-4.23
NO3-
gNO3-/m3
2.68-21.66
2.86-21.75
NO2-
gNO2-/m3
0.00-0.03
0.00-0.03
NH4+
gNH4+/m3
0.00-0.07
0.00-0.07
Cl-
gCl-/m3
24.30-55.90
24.00-55.90
SO4-2
gSO4-2/m3
63.80-129.90
63.80-130.20
PO4-3
gPO4-3/m3
0.00-0.04
0.00-0.03
SUVA
m2/g
1.65-2.29
0.46-1.93
Data
2016.05.10
2016.05.17
2016.05.24
2016.05.31
2016.06.07
2016.06.14
2016.06.21
2016.06.28
2016.07.05
2016.07.12
2016.07.19
2016.08.02
2016.08.09
2016.08.16
2016.09.13
2016.10.04
2016.10.11
2016.10.18
2016.10.25
2016.11.08
2016.11.15
2016.11.22
2016.11.29
Total number of bacteria, cfu/cm3
0
200
400
600
800
1000
psychrophillic
mesophillic
Change of active carbon
Fig. 1. Variation in numbers of psychrophillic and mesophillic
bacteria in water supplying adsorption beds.
Data
2016.05.10
2016.05.17
2016.05.24
2016.05.31
2016.06.07
2016.06.14
2016.06.21
2016.06.28
2016.07.05
2016.07.12
2016.07.19
2016.08.02
2016.08.09
2016.08.16
2016.09.13
2016.10.04
2016.10.11
2016.10.18
2016.10.25
2016.11.08
2016.11.15
2016.11.22
2016.11.29
2016.12.06
2016.12.13
Efficiency of removal, %
0
10
20
30
40
50
60
70
TOC
DOC
Change of active carbon
Fig. 2. Total and dissolved organic carbon removal effectiveness
during adsorption.
The following nearly constant effectiveness
corresponds to that obtained in bends before replacement.
It is likely that the reduction in adsorption effectiveness
was accompanied by an increase of the degree of
biodegradation, which ensured a continued constant
process effectiveness for a long time period. This points to
the fact that exhaustion of the beds did not take place in
any of the beds, which is also confirmed by the iodine
number of the replaced bed.
The obtained effectiveness in reducing TOC and DOC
concentrations before bed replacement testifies therefore to
the simultaneous nature of adsorption and biodegradation.
The biological effectiveness of the adsorption beds before
replacement is also testified to by the simultaneous
removal in ammonia ion content and phosphate
concentrations, i.e. non-organic food substrates for
heterotrophic bacteria [19]. The effectiveness in removing
ammonia ions was in the range of 7.1-75.0% and was
accompanied by an increase in nitrates (found in the time
period before bed replacement), which may point to a
process of nitrification in the absorption bed. This
hypothesis is confirmed by the presence of dissolved in
oxygen in water entering the beds, and the decrease in its
concentration in the range 1.12-1.72 gO2/m3 during flow
through the bed. The nitrification process was also aided
by the small variation in water flow through the system,
yielding stable bed operating conditions.
After bed replacement, however, a significant increase
in phosphate concentrations was found, as phosphates were
probably flushed out from fresh activated carbon. With
respect to non-organic nitrogen compounds, no general
trend was found, which may point to their simultaneous
use by microorganisms and their release from activated
carbon. Such a lack of clear decrease or increase is the
result of these mechanisms. After bed replacement, apart
from an increase in phosphate concentration, a significant
increase of the total number o mesophillic and
3
E3S Web of Conferences 59, 00018 (2018) https://doi.org/10.1051/e3sconf/20185900018
CIWT 2017

psychrophillic bacteria was found, being in the ranges of
799-28944 cfu/cm3 and 1170-44510 cfu/cm3 respectively.
It was found that during the study period, the organic
substance removal effectiveness was proportional to their
content in water reaching the adsorption beds. (Fig. 3).
This indicates that the variability in organic substances
levels in water entering adsorption beds and the source
water contamination levels has a significant impact on
effective organic substance elimination.
Fig. 3. Effect of organic substance content in raw water on their
elimination during the adsorption process.
Among the removed organic substances, UV absorbing
substances at wavelength of 254 nm and 272 nm
dominated, corresponding to disinfection by-products. The
effectiveness in removing dissolved organic carbon was
nearly proportional to the effectiveness in decreasing UV
absorbance at 254 nm and 272 nm (Fig. 4).
Fig. 4. Dependence of dissolved organic carbon removal and UV
absorbance.
According to Korshin et al [20], these precursors
exhibit a significantly larger correlation to absorbance at
272 nm than at 254 nm. An effective elimination of
chlorinated organic substance precursors regardless of the
bed adsorption potential exhaustion ensures a significant
limiting of risks associated with disinfection that is used
after the adsorption process [17,21].
A consequence of the decrease in organic substance
content during adsorption was a decrease in specific UV
absorbance in all filtrate samples. This points to the fact
the adsorption process removed organic substances of
medium molecular mass, and to a lesser degree small
organic substances.
Due to the small variability in system throughput, no
unambiguous relationship was found between organic
substance removal effectiveness and the water flow rate
through the bed. Furthermore, no relationship was found
between the temperature of water supplying the bed and
the adsorption process effectiveness.
4 Conclusions
This study has shown:
1. An effective elimination of organic substances just
after filtration bed replacement and achieving an
equilibrium between adsorption and biodegradation after
two months of operation.
2. Biological activity of filtration beds before replacement
ensured the removal of not only organic substances but
also non-organic food substrates.
3. In the biologically active adsorption bed nitrification
took place, which may testify to biofilm stability on the
surface of activated carbon. .
4. Among organic substances, those most effectively
removed regardless of adsorption bed exhaustion, were
chlorinated organic substance precursors absorbing UV
radiation.
5. A slight increase in water-bed contact time has no
effect on the effectiveness of the adsorption process.
This publication was made possible by the National Center for
Research and Development grant (PBS3/B9/44/2015) "Research
on effectiveness of new water treatment technology as a step
towards a shift in thinking about water utility sector”
(WODTECH), and thanks to involvement of both project
consortium members (Wrocław Municipal Waterworks and
Drainage Company and Wrocław University of Science and
Technology)
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