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Journal of Cleaner Production 200 (2018) 568e577
Contents lists available at ScienceDirect
Journal of Cleaner Production
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Treatment of centrifugal mother liquid of polyvinyl chloride by
internal circulation aerobic biofilm reactor: Lab to plant scale system
udia G. Silva d,
Salma Tabassum a, b, *, Qinhong Ji c, Chunjie Li b, Lina Chi b, Cla
Amin F.A. Ajlouni a, Chunfeng Chu b, Rua Alnoman a, Zhenjia Zhang b, **
Chemistry Department, Faculty of Science, Taibah University, Yanbu Branch, 46423, Yanbu, Saudi Arabia
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
New Energy Research Center, CNOOC Research Institute Ltd., Beijing, 100028, China
Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Faculdade de Engenharia, Universidade do Porto,
Rua Dr. Roberto Frias s/n, 4200-465, Porto, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 September 2017
Received in revised form
21 July 2018
Accepted 27 July 2018
Available online 30 July 2018
Centrifugal mother liquid (CML) is a typical poorly biodegradable organic wastewater generated during
the production of polyvinyl chloride (PVC). A method of treating PVC centrifugal mother liquid by a high
efficiency internal circulation aerobic biofilm reactor (four reactors lined up together) is presented in this
study. The optimized technique was tested and evaluated in a lab and in a plant scale system. The effect
of chemical oxygen demand (COD) removal efficiency was studied under different hydraulic retention
time (HRT): 40, 20, 17, 14 and 12 h, respectively. Considering the efficiency and removal rate, the best HRT
was 14 h. At HRT of 14 h, the average volume loading was 0.326 kg COD/(m3∙d). During the product shift
from TL-1000 to TL-800 (two kinds of PVC products with different polymerization degree), the treatment
efficiency of the system was not affected. Compared with the plant scale system, the results showed that
in the lab setup the removal rates of centrifugal mother liquid of TL-1000 (total organic carbon
(TOC) ¼ 70e75 mg/L) and TL-800 (TOC ¼ 20 mg/L) were 90%e95% and 80%e90%, respectively. While in
the plant scale system, the removal rates of centrifugal mother liquid of TL-1000 and TL-800 were 80%
e90% and 65%e80% (effluent TOC 7.5e12 mg/L), respectively. Biofilm formation by the microbial
reproduction as well as intensive filamentous bacteria settlements were observed.
© 2018 Elsevier Ltd. All rights reserved.
Centrifugal mother liquid
Polyvinyl alcohol
Membrane bioreactor
1. Introduction
Worldwide production of Polyvinyl chloride (PVC) has risen
sharply over recent years (Gravenfors et al., 2015; Wang et al.,
2016). Regardless of the claims that PVC production has many
negative effects on the natural environment and human health
(Bidoki and Wittlinger, 2010), it is still extensively used (Jia et al.,
2016). China has become the world's leading PVC manufacturer
and consumer in recent years with an output of PVC reaching 16.10
million tons (Bing, 2012). Centrifugal mother liquid (CML) is one of
the main sources of wastewater which comes from the production
of PVC resin by suspension polymerization process in PVC
* Corresponding author.
** Corresponding author.
(S. Tabassum), (Q. Ji), (Z. Zhang).
0959-6526/© 2018 Elsevier Ltd. All rights reserved.
production industry. Three to five tons of deionized water is
required for every 1 ton of PVC production (Li and Zhang, 2001),
and the CML accounts for about 70% of the total PVC production
wastewater. An annual output of 100 thousand tons of PVC factory
discharges more than 1000 m3 centrifugal mother liquid per day
(Zhao et al., 2011).
Various treatment methods have been used for Polyvinyl
Alcohol (PVA) wastewater treatment, such as adsorption (Zhang
et al., 2012), biological treatment (Yu et al., 1996), chemical oxidation (Kim et al., 2003) and membrane filtration (Blanco et al., 2015).
However, the conventional biological system is not able to degrade
PVA efficiently due to the low biological oxygen demand (BOD) and
chemical oxygen demand (COD) ratio i.e. BOD5/COD (0.01) of PVA
(Wang et al., 2014). Moreover, due to foam generation, low treatment efficiency and difficult operation were observed during PVA
wastewater treated by biological reactors (Zhang and Yu, 2004).
Clogging of membrane was also observed by PVA, as PVA could pass
through the biotreatment processes during membrane-based
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
wastewater treatment techniques (Blanco et al., 2015).
CML contains relatively low concentrations of refractory organic
pollutants. CML has low biodegradability, low hardness, low
salinity, and high turbidity (Zhao et al., 2011). Its COD value usually
ranges from about 100 to 300 mg/L, while its BOD5/COD value
ranges from about 0.2 to 0.3 (Huang, 2007). Due to the low
biodegradability, the biotreatment efficiency of CML is usually low,
and this can cause problems, such as the bulking of activated sludge
and difficulties in microbe culture, and low treatment efficiency
(Phugare et al., 2011; Zhao et al., 2011). It also contains the dissolved
polymers (mainly PVA), which are used as dispersing agents in the
polymerization process. CML may also contain highly toxic residual
vinyl chloride monomer (VCM) (Labunska et al., 2008).
Various conventional CML treatment methods have been used
such as coagulation/flocculation (AlMubaddal et al., 2009), filtration (Zhang et al., 2003), biotreatment (Song and Li, 2009) and
membrane technologies (Yang et al., 2005), etc. Coagulation/flocculation or filtration can effectively remove suspended solids but
cannot remove dissolved pollutants effectively (Huang, 2007).
Other disadvantages of filtration include low filtration speed, high
electricity consumption, etc. (Zhao et al., 2011). These techniques
have certain advantages such as simple operation and high treatment efficiency, but the membrane can be easily clogged by residual PVA, and therefore they need more periodic reverse cleaning
and replacement, which lead to additional cost (Huang, 2007; Yang
et al., 2005).
Recently (Ye et al., 2017), the environmental impact generated
from the PVC industry was reported. Due to the strong surface
activity of PVA, it poses a large threat to the environment by
forming high amounts of foam. Because of the water-solubility of
PVA, its impact on environment is not so obvious, unlike other
polymers, such as solid plastic. Yet, with the increasingly amounts
of industrial wastewater containing PVA being discharged, environmental consequences have to be taken into account, as PVA
usually takes about 900 days to be decomposed completely in
aquatic environment (Zhang and Zhou, 2003). This creates a serious
threat to the sources of fresh water supply and environment (Avio
et al., 2017; Halima, 2016). Therefore, it is necessary to develop
efficient and improved alternative processes for treating this type
of waste (Quartey et al., 2015).
According to many research works, PVA is degradable both by
aerobic and anaerobic methods. Currently, the treatment of
wastewater containing PVA mainly uses techniques combining
biochemical technology with AOPs (advanced oxidation processes)
or physicochemical methods (Punzi, 2015). Combining biochemical
technology with physicochemical technology is the most popular
way and also a growing trend, because of its low cost and good
treatment result.
Moreover, it is necessary for wastewater treatment systems to
be designed by the engineers on the natural ways of cleaning
wastewater through the use of microorganisms. Compared to suspended growth systems, wastewater treatment with biofilm systems have several advantages such as low space requirement,
operational flexibility, reduced hydraulic retention time, increased
biomass residence time, resilience to changes in the environment,
high active biomass concentration, enhanced ability to degrade
recalcitrant compounds as well as lower sludge production (Sehar
and Naz, 2016).
Over time, the biofilms in the suitable environment will grow by
feeding off the organic matter and nutrients in the wastewater that
flow over them and become strongly attached to the surface they
lez, 2012). The surface area and geometry of supgrow on (Gonza
port materials affect the hydrodynamic conditions in the reactor
and thus affect biofilms formation, which in turn affects wastewater treatment (Matos et al., 2011; Yu et al., 2008). Nowadays,
different synthetic and natural materials have been proposed by
researchers as bio-filter media in fixed biofilm reactors for wastewater treatment, such as polystyrene (Naz et al., 2013), polypropylene (Khatoon et al., 2014), tire-derived rubber (Naz et al.,
2014) and pebbles (Khan et al., 2015).
Tianjin LG Dagu Chemical Co., Ltd. (LG-DAGU), located in Tanggu
District in Tianjin, China, is a Sino-Korean joint venture, which is set
up by LG Chemical (the largest chemical company in South Korea)
and Tianjin Dagu Chemical Co., Ltd.
LG-DAGU is one of the largest PVC manufacturer in China, which
uses chlorethylene to produce two kinds of PVC (TL-800 and TL1000) by suspension polymerization process. With the increasing
production, water usage has also increased, which stands at around
2 million tons per year. For the further development, LG-DAGU
needs sufficient water, which is a difficult task, because of water
shortage in Tianjin and strict municipal government control over
industrial water usage. Project collaboration was setup between
LG-DAGU and Shanghai Jiao Tong University to figure out ways to
reduce water consumption and improve water efficiency. In the
current study a laboratory was setup in the waste water treatment
plant that utilizes the BioAX water treatment technology patented
by our group (a novel environmental biotechnological aerobic
process with internal circulation or internal circulation aerobic
biofilm reactor) (Zhang, 26 December 2002e9 November 2005).
A great amount of work is needed for implementation. But our
research group has effectively and practically worked on achieving
the best result from BioAX in last 4 years based on the research
performance not only in laboratories but also employing successfully the installation and operation in the pilot scale. For instance,
we have successfully implemented this reactor (Li et al., 2014) in
the treatment of one of the most difficult wastewater around the
world i.e. coal gasification wastewater (CGW). Also, BioAX exhibited
high efficiency during a key in-depth technological research (Ji
et al., 2015) for treatment of highly contaminated and toxic CGW.
Later, Tabassum et al., 2015a used BioAX along with other reactors
for advanced treatment of coal gasification wastewater. Moreover,
the same reactor has been used in the successful running of 5 m3/h
Palm Oil Mills Effluent (POME) zero discharge Pilot plant, built at
MPOB (Malaysian Palm Oil Board) Experimental Palm Oil Mill, Labu,
Malaysia (Tabassum et al., 2015b). The research team conducted an
experiment on the treatment of PVC-CML by internal circulation
aerobic biofilm reactor or BioAX.
The main objectives of this study are (i) the inoculation and
start-up of BioAX reactor system; (ii) the continuous operation
performance of BioAX reactor system when there is a sudden
change in CML; (iii) wastewater pollutant degradation and biofilm
condition in different reaction phases; (iv) comparing the performances from lab to plant scale condition.
2. Materials and methods
Two kinds of PVC (TL-800 and TL-1000) were produced by LGDAGU, which have different degrees of polymerization. Therefore,
the discharged CMLs have different characteristics (Table 1). This
experimental study used the CML samples taken from LG-DAGU. At
the beginning of the experiment, the plant produced two kinds of
PVC periodically, leading to periodical changes of CML in the
apparatus both in the lab and in the plant scale system. Due to the
change of the water-supply pipeline, the lab scale system processed
only TL-1000 CML, while the plant scale system processed the
mixture CML from the two kinds of PVC.
As shown in Table 1, the conductivity of CML is just a third of
that of tap water, because it comes from deionized water. CML has
little amount of organics, among which two are major ones,
extremely small PVC particle and PVA. The PVA is used as
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
Table 1
Characteristics of CML based on two kinds of PVC.
Temperature ( C)
COD (mg/L)
TOC (mg/L)
SS (mg/L)
Turbidity (NTU)
Conductivity (m S/cm)
dispersant by LG-DAGU, which has a degree of polymerization (DP)
as high as 2400, making it as a major pollutant.
2.1. Centrifugal mother liquid treatment process system setup
This experiment used an original internal circulation aerobic
biofilm reactor or BioAX, which is constituted by four reactors
connected in series with roughly the same size and volume, as
shown in Fig. 1a. The four reactors are made of plexiglass column,
with effective volumes of 46.3 L, 46.3 L, 46.2 L and 46.4 L, respectively. Each reactor is 140 cm tall with 22 cm inner diameter. Each
reactor has a wastewater inlet on its lower part, a wastewater outlet
on its upper part and a sludge outlet at its bottom. The vertical
cylindrical tubes are used for aeration, fixed in the bottom of the
reactors. The mechanism of internal circulation in the aerobic
biofilm reactor can be observed in Fig. 1b, which is also well
described in our previous research articles about coal gasification
wastewater treatment (Ji et al., 2015; Li et al., 2014).
The resilient plastic filling is fixed outside the vertical cylinder
tube for biofilm formation. The resilient plastic filling we used was
like brush and the material was polypropylene (PP), relatively
strong and better for biofilm formation (Qureshi et al., 2005). PP is a
popular synthetic filament, which is used in a wide variety of applications. Polypropylene has excellent stiffness when wet, is nonbrittle, is inert to most solvents, such as oils, acids and chemicals,
and is fungus resistant (Maddah, 2016). The polypropylene, which
we used, is natural in colour and 0.5 mm in diameter. It is elastic
and not easy to sag when it is impacted by the downward flow of
wastewater. Therefore, compared with other biological membrane
fillers, this kind of filler is more suitable for the reactor we
The system operates in internal circulation and down-flow type
mode. The picture in Fig. 1c shows the experimental laboratory
device. CML wastewater from the factory flows through a pipeline
to the distributing gutter in the laboratory, which is then pumped
into the experimental system from the lower part of reactor No.1.
After treated by the experimental system, the CML wastewater
flows out from the last reactor (Fig. 1a). Organic matters in the CML
wastewater were degraded by the microorganism which grew on
the biological fillers.
2.2. Reactor start-up
Sludge collected from LG-DAGU's domestic sewage disposal
system was blended before it was put into the reactors. The mixed
liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) value of the internal circulation aerobic
biofilm reactors were 3000 mg/L and 1680 mg/L, respectively, and
the value of Volatile suspended solids (VSS) to total suspended
solids (SS) ratio was 0.56. Each reactor was filled with CML and
Fig. 1. (a) Schematic diagram of the experimental apparatus of plant scale internal circulation aerobic biofilm reactor process; (b) an insight of flow direction inside the reactor; and
(c) Photograph of the laboratory setup of the test system.
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
loaded with 10 g of glucose. DO (dissolved oxygen) was kept at the
level of 4e6 mg/L by the DO analyzer with its detector going below
the baffle (in the gap of the filling). Then the reactors were maintained in this way while being exposed to air. After two days, a thin
layer of grey biofilms appeared on the surface of the filling, which
suggested the completion of first stage inoculation and biofilm
At the beginning of the biofilm forming process, the sludge is
mixed evenly. Later, it is gradually divided into layers, with relatively clear water in the upper layer and the sludge that failed to
form biofilms, sinks to the bottom. The influent was kept at a low
flow rate. After two days, a small amount of sludge was found in the
wastewater, mostly inorganic, and the wastewater turned clear
after settlement. The biofilm formation just took two days, as the
temperature was kept at around 25 C, which was favourable for
the growth and rapid reproduction of the bacteria. Also, there were
abundant microorganisms in domestic sewage, which grew rapidly
with sufficient dissolved oxygen. Moreover, the surface of resilient
filling was favourable for the growth of microorganisms. After two
days of inoculation, wastewater began to be pumped into the
reactor. The condition of CML in the reactor was monitored
2.3. Analytical methods
Certain indicators of the CML and the effluent were monitored
over a long period of time. The condition of aerobic biofilms was
analyzed from time to time. COD, total organic carbon (TOC), pH,
total suspended solids (TSS), volatile suspended solids (VSS),
wastewater temperature, dissolved oxygen (DO), turbidity and
electro-conductibility analyses were carried out by the standard
methods for the examination of water and wastewater (Eaton et al.,
3. Results and discussion
There is a large room for error using potassium dichromate
method for measurement of low-density liquid (Davies, 2005). In
this study, we paid more attention to the degradation of all carboncontaining organic matters in wastewater, not just PVA. Therefore,
based on the conventional practice in the field of wastewater
treatment, we used TOC index to evaluate the concentration of all
carbon-containing organic compounds in wastewater. Therefore,
this study measured TOC instead of COD to analyse the amount of
organisms in the liquid (Kadlec and Wallace, 2008; Mara and
Horan, 2003). Therefore, the specific components of wastewater
carbon-containing organic compounds were not given.
3.1. Performance of system under different hydraulic retention time
HRT is a key factor for the treatment result of the reactor,
because it has a direct impact on volume loading. Ideally, HRT
should be as short as possible while maintaining the quality of the
effluent. TL-800 did not have a significative contribution to the
sewage treatment system because of the small amount of its production. Besides, the COD of the TL-800 CML was just around
60 mg/L. The main contribution was the CML of TL-1000. During the
experiments, the CML of TL-1000 (COD around 200 mg/L) was
continuously pumped into the reactor. Temperature was kept at
25e35 C and DO 4e6 mg/L. Nutrients ((in mg/L): FeSO4∙7H2O 15,
MgSO4∙7H2O 50, MnCl2∙4H2O 0.5, ZnCl2 0.5, CuCl2 0.5,
NaBO2∙10H2O 0.3, AlCl3 0.5, CoCl2∙2H2O 0.5 and NiCl2∙2H2O 0.5)
were added to the reactor with a COD, N and P proportion of
100:5:1. Different HRTs (HRT ¼ 40,20,17,14 and 12 h) were set to
observe the respective COD removal by measuring COD concentration in the influent and effluent of the reactor.
When HRT was 40 h (Fig. 2), the average volume load of the
system was 0.102 kg COD/(m3∙d). The COD concentration of the
effluent in reactor No.2 was not so much different from that in
reactor No.4, which indicated that pollutant degradation just took
about 20 h to complete. Wastewater in reactor No. 1 was turbid,
having rapid biofilms growth. PVC particles were found around the
inside of the reactor and also on the biofilms after about two days.
Wastewater in the other three reactors was clear, with biofilms
turning beige. Biofilms in reactors No. 3 and No. 4 tended to form
very slowly, which was attributed to long HRT and the lack of
When HRT was 20 h (Fig. 2), the average volume loading was
0.233 kg COD/(m3∙d). Effluent was clear without impurity or unpleasant smell. COD removal rate ranged between 75% and 90%,
with COD concentration in the system effluent mostly below
50 mg/L. The reactors’ adaptability improved after the initial stage.
COD removal rate was also on a rise because of longer time in this
period. The results showed that the removal rate of reactor No.1
directly influenced the total removal rate of the entire system. At
the beginning of the experiment, the removal rate of reactor No.1
was relatively low, just 17% on the 4th day and the total system
removal rate was 61% only. This was probably caused by the influence of the CML on reactor No.1. After some time, the removal
rate gradually picked up, which suggested the increasing adaptability of reactor No. 1. Biofilms in the first two reactors tended to
form more quickly than in the other two reactors. In reactor No.1,
PVC particles were found on the biofilms which turned beige and
jelly-like, sticking to the filling in the later period of this experiment. In some areas of the reactor, air flow rate had a clear decrease
due to limited space in the reactor, which might be one of the
reasons for the fluctuation of removal rate. In reactor No. 2, threadlike biofilms were found on the filling. The presence of biofilms in
the last two reactors were more when HRT was 20 h. Reactor No. 4
did very little contribution to the total pollutant removal, with the
COD concentration of effluent below 10 mg/L, which meant that
HRT can be further reduced.
When HRT was 17 h (Fig. 2), the average volume loading was
0.246 kg COD/(m3∙d), which did not increase much because of
relatively low COD in the influent in this period. But due to the
more stable COD concentration of the influent in comparison with
when the system HRT was 20 h, the removal results were also more
stable. Effluent condition of the whole system was steady, with
clear water without impurity or unpleasant smell. The COD removal
efficiency ranged between 70% and 80%, which might further increase experiment time, considering that both the system stability
and biofilm adaptability were enhanced with time.
When HRT was 14 h (Fig. 2), the average volume loading was
0.326 kg COD/(m3∙d). Effluent condition of the whole system was
steady, with clear wastewater without impurity or unpleasant
smell. The COD removal efficiency was between 73% and 80%,
respectively. Suspended substances were sometimes observed in
the effluent, which were caused by the biofilms falling off during
the process of its metabolism. With the lower HRT, the flow rate
was higher, making it easier for the fallen biofilms to be flushed out.
The COD concentration of effluent was 12.5 mg/L, which was
reduced to 3.3 mg/L after the coagulant was added. CML in the first
two reactors was more turbid, while CML in the other two reactors
was clearer. In reactor No. 4, green algae were found near the outlet.
The HRT was further decreased to 12 h (Fig. 2), the average volume
loading was 0.402 kg COD/(m3∙d). Effluent of the whole system was
steady, with clear wastewater without impurity or unpleasant
smell. Fallen biofilms were often seen in the effluent due to the high
water flow rate. Total removal rate was between 60% and 70%. The
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
Fig. 2. Comparison of COD degradation curve and removal efficiency when HRT was 40 h, 20 h, 17 h, 14 h, and 12 h.
COD concentration of effluent was between 55 mg/L and 70 mg/L.
After inoculation and adaptation, biofilms could effectively
degrade organic pollutants in the CML. Fig. 2 illustrated the relationship between COD removal results and HRT. Under the same
HRT, the removal rate picked up with time as the system got
adapted to the CML. As HRT decrease, COD removal rate also
decreased. With the lowest HRT of 12 h, COD concentration in the
effluent no longer met the COD requirement (COD<60 mg/L) of
reused water. Taking both efficiency and removal rate into account,
the most appropriate HRT was 14 h (2/3 of 20 h), with the removal
rate being slightly lower in comparison to the HRT of 20 h. Each
reactor made a different contribution to the total removal rate.
Reactor No. 1 was the greatest contributor under different volume
loading conditions, with the removal rate as high as 40%. Also, the
fluctuation of removal rate in reactor No. 1 directly influenced the
total removal result. A large amount of PVC particles was found on
the biofilms in reactor No. 1, which led to the appearance of beige
jelly-like substances on the filling in the later experimental period.
This showed that reactor No. 1 removed COD through degradation
as well as adsorption of large amount of PVC particles and other
macromolecular organic pollutants, making this reactor to maintain a high removal rate.
The COD concentration of CML of polyvinyl chloride was low,
only 100e200 mg/L, while the HRT of CML of treatment process
system was relatively long. Moreover, the MLSS concentration of
the system was about 5000e6000 mg/L, so the biomass-based
organic loading/sludge organic loading was very low, which leaded to a sludge yield of only 0.01e0.05 kg MLVSS/kg COD. This
meant that sludge yield of internal circulation aerobic biofilm
reactor was only 5%e10% of the sludge yield of activated sludge
3.2. Impact of product shifts on treatment
According to the production plan of LG-DAGU, the two products
TL-800 and TL-1000 were produced alternately. There was a shift of
product every 30 days, with TL-1000 being the main product.
Product shift exerted great influence on the treatment system due
to the change of CML. In order to analyse the impact of product
shift, temperature was kept at 25e35 C, DO 4e6 mg/L and HRT
14 h. Nutrients were added to the reactor with COD, N and P proportion (100: 5:1). Then, TOC concentration of the influent and the
fourth reactor effluent were continuously measured. Removal rate
curve during product shift is shown in Fig. 3a (TOC curve of each
outlet) and Fig. 3b (TOC removal rate curve of each reactor). During
product shift, removal rate and outlet wastewater changed due to
different adaptation of microorganisms to different CMLs. The CMLs
of the two products had different types and amount of organic
pollutants, which required different microorganisms for the reactor
to adapt to them. Therefore, every time the product was shifted,
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
Fig. 3. (a) TOC curve of each outlet; and (b) TOC removal rate of each reactor.
CML changed, which required microorganisms in the reactor to
change. Hence, the treatment results were influenced during this
When production was shifted from TL-1000 to TL-800, the
treatment result was basically not influenced, because TL-800 CML
had a very low TOC level. In addition, 14 h of HRT was sufficient for
organisms in the CML to degrade. During the production of TL-800,
reactor No. 1 maintained a high removal rate, while TOC level in No.
2 outlet was no more than 2 mg/L higher than that in reactor No. 4,
which suggested that degradation was almost completed in the
first two reactors.
A small amount of suspended substances was found in reactor
No. 4 in the first two days after the shift from TL-800 to TL-1000,
probably due to the fallen biofilms during metabolism. Moreover,
removal rate in reactor No. 1 decreased sharply, because it was not
prepared for a sudden increase of TOC level in the CML. It took the
reactor a week to adapt to the new CML. TOC concentration of
another reactor effluent was also influenced by the shift. But
removal rate in these three reactors picked up after a small
fluctuation, which contributed to decrease the system TOC to a
normal level in about four days. The reason was probably that
microorganisms in the latter three reactors were hungry for organisms due to the low TOC concentration of the CML of TL-800,
which allowed them to grow extremely fast once provided with
abundant organics during the shift to TL-1000. But as for reactor
No.1, it already developed a complete microorganism system
adapted to the CML of TL-800, which made it more difficult to adapt
to the shift. According to the observation, biofilms in reactor No.1
turned cotton-like, with outlet wastewater turning turbid in the
first two days after the shift. The process of the CML shift influencing treatment results was not frequently monitored due to
limited conditions. In addition, research was not carried out on the
best HRT for TL-800 CML as well as each reactor's contribution to
pollutant removal, due to the short production period of TL-800.
According to the data, removal rate in reactor No. 2 increased two
days after the shift from TL-1000 to TL-800, probably because the
reactor had formed a microorganism system adapted to the CML.
Nevertheless, more research is needed to confirm it.
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
3.3. Comparing treatment results of lab and plant scale system
In LG-DAGU, there is also an internal circulation system for CML
treatment. It has four reactors connected in a line, just like the lab
scale. The treatment results of the two systems were compared. As
can be seen in Fig. 4, the HRT of lab scale treatment system was set
at 14 h, just 2/3 of the HRT of the plant scale system which was 20 h.
The experiment took place in November and December. Temperature in the lab could be maintained at 25e35 C by heating devices.
But for the plant scale system, temperature was lower. Although,
while entering the plant scale system, the PVC-CML was relatively
warm (30 C) but the CML temperature dropped sharply due to
exposure to air.
The temperature drop was more rapid when influent was
blocked for maintenance during product shift. According to some
studies, when the temperature of sewage is below 13 C, bioactivity
will be reduced, which leads to the decrease of organics removal
rate. The low temperature also caused the decrease of the removal
rate of the plant scale system. Comparing the treatment results of
the two systems, it was clear that the lab scale setup outperformed
the plant scale system when treating two kinds of CML. When
dealing with the CML of TL-1000 (TOC ¼ 70e75 mg/L), the removal
rate of the lab scale system was 90%e95%, and the TOC concentration of effluent was 4e6 mg/L. As for the CML of TL-800
(TOC ¼ 20 mg/L), the removal rate of the lab scale system was
80%e90%, and the TOC concentration of effluent was 2e4 mg/L. The
removal rates of plant scale system for CML of the two products
were 80%e90% and 65%e80% respectively, and the TOC concentration of effluent was 7.5e12 mg/L. The performance of the plant
scale system was also less stable.
Firstly, the reasons for the different pollutant removal rates is
the accurate temperature control, added nutrients and aeration for
the lab scale system, which could help to increase the pollutant
removal rate. Secondly, the biofilms of the lab scale system tend to
form more readily, because a smaller system is better at selecting
and inoculating microorganisms. Biofilms in the lab scale system
were evenly attached to the fillers. The amount of microorganisms
was higher due to the larger density of the filling, but there was a
smaller amount of microorganisms in the plant scale system. In
addition, on the filling of the latter two reactors there were very
few biofilms which fell off easily. Although both systems used internal circulation, they had slightly different ways of aeration and
sewage circulation due to different scales. The lab scale system was
more efficient in oxygen usage and transmission because of better
control over wastewater circulation. But in the later period of the
experiment, biofilms growth and PVC particles attached on the
filling prevented the CML from fully contacting with the biofilms in
some areas of reactor No.1. In comparison, in the plant scale system
the CML was more evenly distributed, but with less efficiency in
oxygen usage.
Thirdly, the lab scale system was more vulnerable to the change
of CML during the shift from TL-800 to TL-1000. From day 9e11,
TOC removal efficiency of both systems dropped due to sudden
increase of TOC, with 7.6% reduction of the removal rate of the lab
scale system. But in this period, the removal rate of the plant scale
system just decreased by 4.5%. On the 31st day, when there was
another product shift, the removal rate of the lab scale system even
dropped below the plant scale system level to 65%. The reason is
that the plant scale system is larger and with more bio-variety than
the lab scale system, which makes it more resilient to the sudden
increase of volume loading. But microorganisms in the lab scale
system are stronger in adaptation and reproduction, which can
bring up the removal rate to its normal level within two or three
days under the favourable condition. The lab scale system was more
vulnerable to the change of CML during the shift from TL-1000 to
TL-800. According to our observation, the removal rate of the plant
scale system dropped to 50% following a sudden decrease of TOC,
with the removal rate of the lab scale system falling to around 80%.
The following are the possible reasons.
Due to TL-1000 has a longer production period, both systems
will struggle to readapt and reconstruct after the production shifted
to TL-800. But the lab scale system was less vulnerable to the shift
because of its more favourable conditions for the optimal growth of
microorganisms, including temperature, nutrients and aeration. In
the plant scale system, more biofilms fell off than that in the lab
scale system, leading to more suspended substances in outlet water, which increased the TOC concentration of this stream. Besides,
during the product shift, more biofilms fell off. In addition, TOC
concentration in CML of TL-800 was relatively low in the first place.
Therefore, the removal rate was even lower.
3.4. Observation of biofilms growth
This experiment used resilient filling was made of PVC fiber and
Fig. 4. TOC removal curves of laboratory and plant scale system.
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
braided into a bundle (diameter 150 mm). The filling was fixed on
the internal circulation tube with thick iron wires as long as
1500 mm coiling around the tube. The resilient filling has many
advantages (Ji et al., 2015; Li et al., 2014) as it is light-weighted with
stable physical and chemical forms. The fiber bundles have a threedimensional structure in the flowing water, which increases the
surface area of the fiber bundles. The biofilms can easily attach to
the filling (Li et al., 2018), making good contact with CML. The fiber
bundles can sway with the water, which can prevent blockage
caused by biofilms. This design is useful for treatment of sewage
with high organics concentration (Li et al., 2016).
There were no significant differences between the growth
conditions of biofilms in one reactor under different HRT (less than
20 h) due to the low system volume loading. As the TOC concentration gradually decreased, biofilms in each of the reactors started
to show a different growth pattern. Biofilms in the former two reactors tend to form faster than that in the other two reactors. After a
month of steady-state operation (HRT 14 h), sludge was taken from
all the four reactors. Sticky clumps of biofilm were observed in
reactor No. 1 (Fig. 5a). The biofilms were jelly-like, difficult to be
peeled off from the filling. PVC particles were attached to the biofilms. The biofilms in reactor No. 1 had a very different look from
those in other reactors. Fig. 5c shows a picture of the filling before it
was placed into the reactor. Biofilms in reactors No. 2 (Fig. 5d) and
No. 3 (Fig. 5b and e) looked quite similar. Biofilms were clearly seen
attached on the PVC fiber. Biofilms in these two reactors were
easier to peel off. Reactor No. 4 (Fig. 5f) had loose biofilm architecture, making it easier to peel off from the filling.
3.5. Microscopic examination of biofilms
Analysing the growth of the bacteria is the best way to evaluate
the process and result of sewage treatment. Yet, it takes a long time
for observation and identification of the bacteria, which makes
difficult to use the analysis results to guide and forecast the future
production. Protozoa and metazoan are interdependent from bacteria (Sapp, 1994). They are also easier to be observed due to the
larger sizes and easier to reflect environmental changes because
they are more sensitive to environmental changes than bacteria
(Madoni, 2011). Therefore, analysis of the types, amount, growth
and changes of protozoa and metazoan can also reflect the growth
and changes of bacteria, which can help in learning about the
treatment process results. The information gathered can facilitate
in setting future production plans.
During the experiment, biofilms were examined by the microscope to observe their growth as well as protozoa. As the inoculation sludge was collected from domestic sewage, organisms on the
biofilms were diversified, with lots of Vorticella, Nematode, Paramecia, Litonotus, Rotifera and sometimes Colpidium and Amoeba.
After five days of operation, Zoogloea was observed including
mainly Vorticella and Nematode, and sometimes Litonotus. Moreover, when the operation went stable, Zoogloea was observed with
Vorticella, Nematode and lots of Potifera. The latter two reactors
hosted fewer Protozoons, probably due to fewer bacteria. After
product shift, some cotton-like sludge was found in reactor No. 1,
which might be the fallen biofilms and also small amount of sludge
was found in the outlet of reactor No. 4. Internal circulation aerobic
biofilm reactor has the advantage of low sludge production, it is
explained in detail in our previous research article about treating
coal gasification wastewater (Ji et al., 2015).
According to microscopic examination, Zoogloea fell apart,
which indicated the renewal and reconstruction of the biofilms.
Similar to the lab scale system, biofilms in the former two reactors
of the plant scale system tended to form better than those in the
latter two, but were also easier to fall off than those in the lab scale
system due to the large influence from air flow, water flow and
gravity. Biofilms in the latter two reactors of the plant scale system
fell off easily, leading to lots of suspended sludge that were found in
the effluent. More Vorticella and Rotifera were found in the first two
reactors. At the beginning of sludge inoculation, treatment result
was very unstable. Later, biofilms tended to form better. Microscopic examination revealed the presence of Vorticella, Rotifera and
Nematode, which indicated a good treatment result. It was also
found that product shift had some impact on the biofilms
(Fig. 6aed).
3.6. Operational problems and countermeasures
Several problems must be acknowledged as a part of this study
along with the remediation's are listed below.
3.6.1. Blocked area in Reactor No.1
In the lab scale system, blocked areas were often formed in
reactor No. 1, by PVC particles (CML) getting attached to the
Fig. 5. (a) #1 MBR effective volume: before packing 45.7 L, after packing 46.3 L; (b) #3 MBR effective volume: before packing 46.7 L, after packing 46.2 L; (c) filler; and (d)e(f)
photographs of biofilm on the surface of filler (lush breeding microbial colonies).
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
Fig. 6. Microorganisms under the Microscope (a) Rotifera; (b) Nematode; (c) Vorticella; (d) nematode and Vorticella; (e) Chironomus larvae growing stages and (f) Chironomus
biofilms. The blockage often got in the way of air flow and wastewater flow. In order to tackle this problem, some measures can be
taken to prevent PVC attachment on the biofilms, including stirring
and more aeration.
In practice engineering design, we periodically wash off the
excess biofilms on the PVC fiber filling in the reactors. It is also used
to remove the PVC particles that are trapped in the biofilms due to
biofilms filtration and ensure the normal operation of the system.
After two months of operation, sludge can be removed by the above
measures for the reactor to be inoculated again. In the plant scale
system, there were no blocked areas due to different ways of filling
and wastewater flow. Sometimes, PVC was found attached to the
biofilms in the plant scale system, but not as much as in the lab
scale system. So, in the plant scale system, sludge can be removed
less frequently.
3.6.2. Chironomus larvae boom
After two months of stable operation, red bugs were found in
reactor No. 2. And later, same bugs were also seen in the third and
the fourth reactors, which were red and 1e2 cm long, sticking to
the biofilms and reactor walls, mostly on the middle and upper
parts. These bugs were confirmed to be chironomus larvae. A week
later, chironomus were found on top of the reactors (Fig. 6e and f).
In recent years, the research focused on adult chironomid taxonomy and benthic macroinvertebrate community help in evaluating water quality (Nicacio and Juen, 2015). In the sewage
treatment process, chironomid larvae appearance is an indication
of stable and effective sewage treatment process. Chironomid larvae's initial appearance in the early stage can be utilised to accelerate the biofilms detachment and renewal (Babu, 2011).
When they reach a large number, they might harm the biofilms.
Precautions should be taken at this stage by increasing the amount
of influent water in the reactor which can lead to the death of the
larvae. Then the reactors can be re-inoculated. This whole process
takes two days.
4. Conclusion
PVC centrifugal mother liquid was treated by a high efficiency
internal circulation aerobic biofilm reactor (four reactors lined up
together) in this present study from lab to plant scale system. The
treatment in the first reactor has a great influence on the efficiency
of the whole system, and the removal efficiency accounts for about
40% of the total removal efficiency. The change in the product shift
greatly influenced the treatment efficiency of the system. Each
change needs 3e4 days for the system to adapt, therefore, uniform
water quality can promote the stable operation of the system. According to observation, there was usually a large amount of foam
floating in reactor No. 1. In reactor No. 2, a small amount of foam
sometimes appeared. No foam was found in the other two reactors.
From this observation, we can conclude that PVA was successfully
degraded in the first two reactors. Considering the high PVA concentration in the CML and from the above results, it can be deduced
that almost all of the PVA molecules were destroyed or transformed. Therefore, it was reasonable to suppose that the subsequent biological treatment would remove most of the remaining
COD or TOC from the CML which gone through the internal circulation aerobic biofilm reactor treatment system. Under optimized
treatment conditions, most of the COD was removed from the CML
and the biodegradability of the CML increased significantly. Though
the sludge production was less in the system, the PVC particles
caused the reduced biofilm activity and chironomid larvae growth
caused the serious damage to the biofilms. Future study will be
carried out by our research group on the adsorption of PVC particles
and the growth regularity of chironomid larvae. This lab to plant
scale study is expected to account to a good foundation for a successful implementation effort for the treatment of poorly biodegradable organic wastewater produced during the production of
polyvinyl chloride.
The authors express their gratitude to the School of Environmental Science and Engineering, Shanghai Jiao Tong University for
providing the research facilities. Dr. Tabassum was financially
supported by Talented Young Scientist Program supported by the
Ministry of Science and Technology, P. R. China as an Assistant
Researcher (IND-15-003) at School of Environmental Science and
S. Tabassum et al. / Journal of Cleaner Production 200 (2018) 568e577
Engineering, Shanghai Jiao Tong University, China. Dr. Tabassum,
also thankful to the Department of Chemistry, Taibah University,
Saudi Arabia. C.G. Silva acknowledges the FCT Investigator Programme (IF/00514/2014) with financing from the European Social
Fund and the Human Potential Operational Programme and
financing though Project POCI-01-0145-FEDER-006984 e Associate
Laboratory LSRE-LCM funded by FEDER through COMPETE2020 ~o
Programa Operacional Competitividade e Internacionalizaça
(POCI) e and by national funds through FCT - Fundaç~
ao para a
^ncia e a Tecnologia.
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