Capacitive Deionization for RO brine recovery in NEWater production

Capacitive Deionization for RO brine recovery in NEWater
production
L.Y. Lee1, H.Y. Ng1, S.L. Ong2, J.Y. Hu1, G. Tao3, K. Kekre3, B. Viswanath3, W. Lay4
and H. Seah4
1
Division of Environmental Science & Engineering; 2 Department of Civil Engineering, Faculty of
Engineering, National University of Singapore, Singapore 119260
3
Centre for Advanced Water Technology, Singapore Utilities International, Singapore
4
Public Utilities Board, Singapore
Abstract: Reverse osmosis (RO) brine from water reclamation facility is a potential untapped water
source, provided a feasible and economical treatment process is available to recover this waste stream.
Organic and inorganic compounds are two major groups of pollutants in the RO brine. In this study, an
integrated treatment scheme consisting of a biological activated carbon (BAC) column and a low
pressure Capacitive Deionization (CDI) process was investigated. BAC was used as a pretreatment to
remove the organic compounds prior to the inorganic removal using the CDI process. Two empty bed
contact times, namely 20 min and 40 min tested in the BAC columns provided similar TOC removal
efficiency within the range of 15–21%. High ions removals of more than 85% from the RO brine were
achieved in the CDI process when operated with water recovery up to 89%. This study has successfully
demonstrated that the integrated BAC with CDI process has high potential to increase water recovery
of a water reclamation plant while gaining the advantage of a reduced volume of RO brine for disposal.
This system could further contribute to enhancement of sustainable water reclamation practice.
Keywords: Biological activated carbon, capacitive deionization, inorganic removal, organic removal,
pre-treatment, RO brine, water recovery.
Introduction
Water reclamation and reuse have been increasingly practised especially in the fast developing urban
cities. Table 1 shows a few of the cities that uses reclaimed domestic wastewater to augment the water
supply for various applications. In 1998, the NEWater initiative was carried out as a joint study by the
Singapore Public Utilities Board (PUB), the Ministry of the Environment (ENV) and the National
University of Singapore (NUS) to determine the feasibility and viability of using reclaimed water
(known as NEWater) as a source of raw water for augmenting Singapore’s freshwater supply.
Two full-scale NEWater plants were commissioned in February 2003 following the favorable results
obtained from the study (MOE and PUB, 2002; PUB, 2003). NEWater produced is used to augment
Singapore’s water resource by indirect potable reuse which a suitable amount of NEWater is being
introducted into the reservoirs. The current reclamation capacity in Singapore has increased to meet
15% of its water demand (MEWR, 2007) which will increased to 30% in 2011 following a planned
addition of the 5th NEWater Factory by 2011. Currently, the RO brine generated from the NEWater
production is disposed to the sea through direct discharge. However, with the increase in NEWater
production, RO brine generation will also increase correspondingly. This waste stream could be a
potential valuable source of water if an efficient and cost-effective solution can be formulated. Hence,
this could reduce the volume of concentrated RO brine for disposal, increase overall water recovery
while having the potential to provide a long term solution to sustainable water reclamation and reuse
practices.
Table 1. Reuse application for reclaimed domestic wastewater reuse in some of the major cities
(extracted from Hu et al., submitted).
City
Reuse application
1
North Barcelona
Singapore2
Tokyo, Japan 1
Windhoek, Namibia
(Goreangab WRP)1
1
2
non-potable reuse
non-potable reuse and in-direct potable reuse
non-potable urban reuse
makes up of 35% of the city drinking water
Asano et al. (2007)
Stedman (2007)
Water Practice & Technology Vol 3 No 4 © IWA Publishing 2008 doi: 10.2166/WPT.2008081
The contaminants in RO brine from domestic wastewater reclamation mainly consist of organic and
inorganic compounds. The RO brine characteristics from a NEWater plant in Singapore is summarized
in Table 2. As noted from Table 2, total dissolved solids (TDS) in the RO brine are in the range of
569 – 996 ppm while having a conductivity ranging from 1500 – 2700 µS/cm (depending on the stage
where the RO brine is obtained). Organic matters are mainly consists of slow- and hard-to-degrade
compounds. A cost-effective technology to treat the RO brine would need to be able to remove the
inorganic and organic compounds to an acceptable level for reuse applications or as RO feed water.
In view of this, a novel treatment process consisting of a biological activated carbon (BAC) as a pretreatment step prior to the capacitive deionization (CDI) process is proposed for the RO brine treatment
and recovery in this study.
Table 2. Characteristics of RO brine from a NEWater plant in Singapore.
Parameter
pH
Conductivity
TDS
TOC
COD
Unit
µS/cm
mg/L
mg/L
mg/L
Concentrations
7.31 - 7.4
1523 - 2700
569 - 996
15 - 30
19 - 47
Cations
Na+
NH4+
K+
Mg2+
Ca2+
mg/L
mg/L
mg/L
mg/L
mg/L
230 - 416
ND - 7
29 - 53
11 - 21
39 - 75
Anions
ClNO3PO43SO42-
mg/L
mg/L
mg/L
mg/L
257 - 1010
38 - 62
7 - 16
131 - 254
Removal of organic matters had shown to reduce the fouling potential in RO membranes in water
reclamation process (Hu et al., 2005). Hence, the biological activated carbon (BAC) process is used as
a pretreatment process which aimed to remove a substantial amount of organic matter prior to CDI
process. BAC has the advantage of combined adsorption and biodegradation effect which the activated
carbon can be partially regenerated by biochemical activities while the carbon bed is in operation
(Rodman et al., 1978; Rice and Robson, 1982). This will reduce the need for regeneration process and
prolonged the useful life-span of the BAC column. Inorganic matters in the RO brine can be further
removed using the CDI process. The CDI process have been tested for removal of inorganic
compounds from different types of water, namely, seawater, groundwater, industrial process water and
wastewater (Weldemoed and Schutte, 2005, Garrido et al., 2006, ENPAR Technologies Inc., 2007).
The main advantage of CDI process over membrane technology for removal of inorganic compounds
mainly lies in its low pressure requirement during purification process.
Materials and Methods
RO brine
Brine from the 2nd stage of RO process was collected once a week from the Bedok NEWater Plant,
Singapore. They were stored at 4oC in the laboratory’s cold room until use.
Experimental set-up
Figure 1 illustrates the schematic diagram of the experimental set-up used in this study for the
treatment of RO brine.
2
CDI unit
Product
water
Concentrate
BAC
column
RO brine from
NEWater plant
BAC effluent
holding tank
Pretreatment
Figure 1. Schematic diagram of the experimental set-up to treat RO brine.
The RO brine was cooled to room temperature (30±3oC) before feeding into the BAC columns.
RO brine was supplied continuously at a flow rate of 49 ml/min and 25 ml/min to the bench-scale
BAC columns, BAC 1 and BAC 2, respectively using peristaltic pumps (Mflex economy digital drive,
10 – 600 rpm). The BAC columns used in this study were constructed using perspex tubes having a
diameter of 50 mm and an effective packing height of 500 mm. Packing material used was granular
activated carbon (Filtrasorb ® F400D, Calgon Carbon Co.). The activated carbon had an effective size
and density of 0.65 mm and 0.47 g/cm3, respectively. BAC 1 and BAC 2 were maintained at an empty
bed contact times (EBCTs) of 20 mins and 40 mins, respectively. An intermediate holding tank was
used to collect the effluent from BAC 2 for further treatment with CDI process.
The bench-scale CDI process (Model: DesEL 400, ENPAR Technologies Inc., Canada) with a flow
rate of up to 250 mL/min and an electrode surface area of 0.7 m2 was operated in batch mode. Each
batch was operated for 6 h at varying water recovery.
Water quality analysis
The water quality of samples from each stage of treatment, namely RO brine, BAC effluent, and CDI
product and concentrate was characterized for pH, conductivity, TDS, total organic carbon (TOC),
anion and cation concentrations. The pH was measured using the Horiba pH meter F-54 BW (Horiba
Ltd, Japan) while conductivity and TDS were quantified using WTW Conductivity meter LF 538
(WTW, Germany). Color was measured using the UV-Vis Spectrophotometer DR 4000U (Program
1660, Hach Company, USA), and TOC was analyzed using the Total Organic Carbon analyzer (TOCVCSH, Shidmadzu, Japan). The anion and cation concentrations were determined using the Dionex LC
20 Chromatography (Dionex Corporation, USA). The anions analyzed were Cl-, NO2-, NO3-, PO4- and
SO4- and cations were Na+, NH4+, K+, Mg+ and Ca2+. Chemical oxygen demand (COD) was quantified
only for RO brine and BAC effluent. All water quality analyses were carried out in accordance with the
Standard Methods for Water and Wastewater Analysis (APHA, 1998).
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Results and Discussion
Pretreatment using BAC at different EBCTs
The TOC performances of the two BAC columns are illustrated in Figure 2. The TOC removal
performances between the two different EBCTs at the initial volume treated did not differ significantly.
BAC 1 achieved its stabilized stage after treating more than 1,440 bed volume while BAC 2 was noted
to stabilize after treating 3,000 bed volume. The stabilized performance and the respective periods are
summarized in Table 2. The results indicated breakthrough was achieved faster at shorter EBCT and
exhaustion of the BAC adsorption capacity took approximately 1,440 and 3,000 bed volume for BAC 1
and BAC 2, respectively. The organic matters in the RO brine were also demonstrated to be highly
resistant to biodegradation and BAC was only able to remove 15 – 21% of the TOC concentration in
the RO brine.
1.00
0.90
Normalized concentration (C/Co)
0.80
0.70
0.60
0.50
0.40
0.30
0.20
BAC 1 - EBCT 20 mins
0.10
BAC 2 - EBCT 40 mins
0.00
0
1000
2000
3000
4000
5000
6000
Bed Volume Treated
Figure 2. Normalized TOC concentrations for BAC 1 and BAC 2.
Table 2. Stabilized performance and the respective periods in BAC 1 and BAC 2.
BAC
columns
Bed
volume
treated (-)
Effluent concentration
(mg/L)
Removal efficiency
(%)
TOC
COD
TOC
COD
1
1440 – 4825
24.0 ± 2.6
52.4 ± 9.4
21.5 ± 6.5
24.2 ± 9.0
2
3000 – 5021
22.2 ± 2.6
49.1 ± 14.0
25.4 ± 5.1
37.2 ± 9.3
CDI process for recovery of RO brine
Effluent from BAC 2 was used a feed to the CDI process. The CDI process was able to achieve stable
operating performance with no significant pressure build up during the 6 h continuous operation.
Process characteristics and product quality of water recovery at 85 and 89% are presented in Table 3.
The results indicated that at higher water recovery, the process rate and the production rate were
reduced by about 22 and 19%, respectively. Lower removal efficiencies in terms of conductivity, TDS,
TOC and TDS were also experienced when operated at higher water recovery. The anion and cation
removal efficiencies at both water recoveries were similar except for approximately 8% lower chloride
ion removal at 89% water recovery compared with water recovery at 85%. Hence, the results indicated
relatively stable performance at the two water recovery conditions tested especially for anions and
cations removal.
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Table 3. Results of CDI operating at 85% and 89% water recovery.
Operating conditions:
Process rate (L/d)
Production rate (L/d)
Water recovery (%)
Removal (%)
Conductivity
TDS
TOC
TN
Cation
Na
NH4
K
Mg
Ca
Anion
Cl
NO3
PO4
SO4
Run 1
265.6
226.8
85
Run 2
207.1
184.7
89
91.5
91.6
67.4
91.0
88.2
88.3
66.3
87.7
92.2
94.7
92.7
91.1
92.4
92.9
94.7
96.5
96.6
96.8
93.3
93.8
82.2
95.1
85.7
91.1
85.3
92.7
Summary
The combined pre-treatment with BAC with CDI process has demonstrated high potential of increased
water recovery while giving the advantage of reduced volume for disposal to further enhance
sustainable water reclamation practices. Research is currently on-going to optimize this process for
brine treatment and recovery.
Acknowledgments
This project is co-funded by the European Commission within the Sixth Framework Programme (20022006) (Contract-No. 018309), Public Utilities Board, Singapore and National University of Singapore
(Ministry of Education’s AcRF Tier 1, Grant no.: R-288-000-044-731). The authors would like to
thank Calgon Carbon Co., Singapore for providing the complimentary Filtrasorb ® F400D and ENPAR
Technologies Inc., Canada for the technical advice on CDI process in this study.
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