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Assessing the Removal of Cyanobacteria Cells and Cyanotoxins by means
of Ultrafiltration Membrane in Pilot Scale Testing
A. Bragança, A. Giani, and M. Libânio*
* Department of Hydraulics and Water Resources Engineering of the Federal University of Minas Gerais
Av Antonio Carlos, 6627 – Bloco I – Pampulha – Belo Horizonte – Minas Gerais - Brazil
(E-mail: [email protected] or [email protected])
Abstract
This paper focuses on the cyanobacteria cells and cyanotoxins (microcystins and saxitoxins) removal by
means of ultrafiltration (UF) membrane technology in a pilot scale. This research took place near an
eutrophicated lake where high densities of the species Cylindrospermopsis raciborskii and Sphaerocavum
brasiliense could be found. The polyethersulfone UF membrane pilot unit was operated during four months
and the water quality parameters monitored were pH, alkalinity, hardness, turbidity, true colour, apparent
colour and phytoplankton counting. It was also detected total coliforms and Escherichia coli in the raw water
and in the filtrate (permeate). Twenty-two cianotoxin analyses in raw water were done by ELISA (Enzime
Linked Sorbent Assay) method for microcystin (14) and saxitoxin (8). The UF membrane achieved 5 log for
C. raciborskii removal and was able to remove completely cells of Sphaerocavum brasiliense during each
cyanobacterial bloom. In addition, the UF membrane pilot unit efficiently removed microcystin at water
temperature nearly 20°C and pressure of 138 kPa (20 psi).
Keywords
Ultrafiltration membrane, cyanobacteria removal, cyanotoxin removal, water treatment.
Many areas near water sources have been occupied due to the increasing of urbanization. This fact
has jeopardized the water quality of and has brought many troubles for cities such as flooding,
diseases and social matters. In addition a large amount of substances such as pesticides, phosphates,
nitrates and others have been carried to water sources and this fact occasions the increase of
eutrophization. In this scenario, the water quality standards have been become very strict by many
regulatory and monitoring agencies. Moreover the water conventional treatment and wastewater
biological one can´t always achieved these current rules (Jacangelo, Trussel & Watson, 1997).
In the USA and Europe the sanitation market industries have developed membrane technology since
the early 1990s. Thereafter the water treatment plants (WTP) have started to produce drinking water
full-scale mainly through microfiltration (MF) and ultrafiltration (UF) membranes. The potable
water quality parameters have achieved great results below the maximum allowed values
established by standards (Schneider & Tsutya, 2001).
Pittsburgh Water and Sewer Authority have operated a Water Treatment System where they have
been working with UF membrane technology like post-treatment because of the strict standards of
USEPA. They supplied more than 500,000 people in early 2000s (State et al., 2000).
The membrane technology has been spreading around the world through different activities. For
instance, a high volume of effluent is generated during oil refining process, which contains a large
of organic and inorganic compounds. The membrane bioreactor (MBR) has been widely applied to
this wastewater treatment due to higher efficiency compared to conventional biological treatment
system. In Brazil, a pilot scale PETROBRAS was operated using effluent after stabilization lagoon.
They studied about the membrane fouling and the sludge concentration into the MBR (Amaral et
al., 2011).
In February 2004, the largest Germany´s reverse osmosis (RO) plant started to produce water for
the pulpy industry Zellstoff Stendal. Since then, the plant has always been operating reliably at
capacity of 2,100 m 3 h -1 (Sehn et al., 2011).
Modified polyacrylonitrile low molecular weight cutt-off UF membrane has been tested for the
arsenic removal. Singh & Agarwal (2011) have developed a study where they obtained high arsenic
removal with the excellent combination of high flux at low pressure with high rejection through this
modified UF membrane.
The ceramic membrane modules are getting so much attention for water treatment plants and
markets such as pharmaceutical and industrial treatment. Most of advantages of these membranes
are related to operation beyond of the high service life and excellent water recovery percentage
(Freeman & Darby, 2011).
The cyanobacteria blooms have become very common around the world due to large amount of
nutrients carried to water sources and it should probably point out to the climate changes. Beyond
the toxic compounds production, the cyanobacteria can jeopardize the water treatment system a lot
(Edzwald, 1993).
In Brazil, the warning systems were devised because the blooms have been hazardous to public
health since the late 1990s. Due to the history of several intoxications caused by cianotoxins and to
the fatal event in Caruaru (1996), when sixty chronic renal patients of a hemodialysis clinic died as
a consequence of microcystin presence in the water, the cyanobacteria were included in the
Brazilian Drinking-water Standards under the Regulation 518 (Healthy Brazil, 2011). The same
norms established by World Health Organisation on this topic were kept in the recent edition
(WHO, 2011). For drinking water, the limits are 1.0 g L-1 of microcystin, and even 10.0 g L-1 can
be acceptable when measured in up to three samples in a period of twelve months.
In terms of water treatment the significant presence of algae and cyanobacteria in water sources has
brought several difficulties for the most of conventional plants, mainly reducing the filter runs or
even provoking taste and odour events. The chemical coagulation is usually inefficient in the
cyanotoxins removal probably because of the inefficacy of the coagulants in the destabilization and
the precipitation of these organic compounds, hampering their removal in the subsequent steps of
the treatment (sedimentation or dissolved air flotation). This fact was confirmed in a survey aiming
to evaluate the perception of taste and odour issues in 826 water utilities across the USA and the
province of Alberta in Canada. Without formal training for taste and odour classification, this
survey pointed out plankton as principal cause for these events for utilities treating surface water
(Suffet et al., 1996). For the conventional water treatment method applied to eutrophic waters, the
use of prior disinfection usually causes a significant increase of cyanotoxin concentration in the raw
water due to the mentioned cell wall rupture. As a consequence the water supply system
management has prioritized the intact cells removal or algae control measures in water source.
Focusing on cyanotoxin removal, there is a consensus about the effectiveness of some disinfectants
(such as, chlorine, ozone) or activated carbon to reduce the concentration of cyanotoxin in the
treated water. However, recent researches have not been focusing on the cyanotoxin removal by
means of ultrafiltration membrane (UF). In a similar context, the role of the natural organic matter
(NOM) in the progress of fouling in UF membrane system has been testified in several researches
(Brinkman and Hozalski, 2011).
Relating to intact cells and cyanotoxin removal, UF membrane can arise as an alternative
technology for eutrophicated water sources presenting some advantages and limitations. Among the
main advantages, depending on the raw water quality, UF membrane will be associated only with
disinfection and final pH correction. In this way, in the absence of chemical coagulation the reduced
amount of solids when compared to conventional water treatment process becomes the disposal less
difficult and expensive.
In UF membrane systems, smaller contaminants, such as suspended and colloidal particles, bacteria,
algae, cysts or oocysts of protozoa and cyanobacteria, are removed via size exclusion. Removal of
Giardia cysts and Crypto oocysts higher than 4 log were achieved in several studies. The UF
membranes commercially available do not remove effectively uncharged species such as hydrogen
sulfide and small, uncharged organic contaminants (Durance et al., 2011). On the other hand, other
mechanisms in UF membrane process, such as charge repulsion or physical retention, can be
effective in removal of some specific cyanotoxins.
OBJECTIVES
According to these concepts, this paper proposes to evaluate an UF membrane system at pilot scale
focusing on cyanobacteria intact cells and cyanotoxins removal. Additionally, the paper proposes:
i) To monitor some water quality parameters in permeate and reject, such as turbidity, true colour
(as an indicator of NOM), apparent colour, alkalinity and hardness;
ii) To detect total coliforms and Escherichia coli in raw water and along the UF membrane system.
METHODS
Experimental apparatus
UF membrane pilot scale unit was installed near a eutrophicated urban lake in Belo Horizonte
(Brazil, 19°55’09’’ S and 43°56’47’’ W), where cyanobacteria blooms have been happening since
1997.
The pilot plant was monitored during 80 hours in four months. The following parameters relating to
UF membrane operation had checked and marked their values down: Permeate Flux Rate, Reject
Flux Rate and Pressure of System. The schematic diagram of experimental apparatus used in UF
membrane testing is depicted in Figure 1.
Figure 1. Schematic diagram of the experimental apparatus used for UF membrane evaluation.
A sand media pre-filter (II) was used and the UF membrane was made from polyethersulfone in
spiral shape, and this organic compound makes the membrane surface very hydrophilic. The
membrane cut-off weight was 10,000 Dalton and its main features, provided by manufacturer, are
shown in Table 1.
Table 1. Main features of polyethersulfone PW4040F UF membrane.
Maximum temperature
pH range for treatment operation
pH interval for cleaning
Filtration area
Flux rate interval
Maximum flow
50° C
3 – 11
2.0 – 11.5
7.9 m2
15 – 40 L h-1 m-2
320 L h-1
Water quality parameters and analytical methods
The samples spots were raw water (I), after sand filter (II), permeate (III) and reject (IV). All
physical, chemical and biological analyses were carried out based on the premises established by
Apha (2005). These analyses included true colour, apparent colour, turbidity, pH, temperature,
hardness, alkalinity, phytoplankton density, total coliforms and E. coli.
The Elisa (Enzime Linked Sorbent Assay) method for Congener-Independent Determination of
Microcystins and Nodulars in Water Samples was used in this research and the HPLC method was
used to determine saxitoxin concentration. The samples were collected during period of the year
that the most remarkable blooms in Pampulha Lake usually take place.
RESULTS AND DISCUSSION
The UF membrane system was monitored from April to August 2008. The system pressure,
permeate and reject flows were monitored during one month before the beginning of the tests. The
UF membrane system has operated beneath 138 kPa (20 psi) most of time. The constant flux was
achieved throughout operation process and under these conditions the system could produce 180 L
h-1.
According to expectations, the UF membrane system presented high turbidity removal during the
four months monitoring. Despite the great variability in raw water turbidity (from 25 to 75 ntu), the
permeate turbidity presented consistently lower than 0.30 ntu. The apparent colour of permeate was
always lower than 15 cu, the same limit established by Regulation 2914 (Health Brazil, 2011) and
WHO (2011). The true colour removal was less significant (at about 33 per cent) mainly due to the
low value in raw water (approximately 6 cu). In same perspective, the hardness removal was
negligible keeping in permeate practically the same value of raw water (approximately 90 mg.L-1
CaCO3).
In order to evaluate the microbiological performance of UF membrane system, ten analyses were
carried out in last two months of monitoring. In raw water, the high densities of total coliforms
(higher than 2,419 MPN 100 mL-1) testified the effect of urbanization in the watershed. E. coli was
not detected in permeate and the maximum density of total coliforms was 31 MPN 100 mL-1,
indicating a high efficiency of this water treatment process dealing with very polluted surface water.
The Sphaerocavum brasiliense (colony) and Cylindrospermopsis raciborskii (form of rods) were
the main cyanobacteria species identified by biological examination. This study was conducted 20
phytoplankton analyses with remarkable prevalence of these cyanobacteria species during four
months. The results were very worthy and important for the consolidation of this membrane
technology research about cyanobacteria cells and cyanotoxins removal. The results related to raw
water and UF membrane influent (after pre-filter) are shown in Table 2. The analyses showed that
all cyanobacteria cells after sand filter barrier were found into membrane reject. So there weren´t
any cyanobacteria species in finished water.
Table 2. Statistics of cyanobacteria densities in raw water and UF membrane influent.
Statistics
Mean
Coefficient
of
variation
Geometric mean
Maximum
Minimum
Raw water (Pampulha Lake)
C. raciborskii
S. brasiliense
(cells. mL-1)
(cells. mL-1)
56,989
21,505
UF membrane influent (after pre-filter)
C. raciborskii
S. brasilience
(cells. mL-1)
(cells. mL-1)
33,871
2,688
0.96
3.34
1.29
2.34
29,300
147,000
710
4,160
239,516
0
21,600
130,107
710
1,720
20,072
0
Limited by the cyanobateria densities shown in Table 2, the efficiency of UF membrane system
reached removal higher than 5 log keeping an average at about 4 log during the experimental tests.
Based on Elisa method, Microcystin was not detected in permeate and its concentration in raw
water and reject reached 11.13 gL-1 and 1.00 gL-1, respectively. These results allow of the
speculation about the susceptibility of microcystin to the mentioned physical retention mechanism
in UF membrane process. Also, the low concentration of microcystins in reject is a reliable
indicator of absence of cell wall rupture under 138 kPa pressure.
On the other hand, four Presence-Absence tests for saxitoxins were carried out in raw water and
permeate, indicating presence of saxitoxins in all of them. Despite the modest number of analyses,
these results seem to show the fragility of UF membrane system for saxitoxin removal and its use in
actual scale will depend on the prevalence of the specific cyanobacteria species in the water source.
CONCLUSIONS
Anchored in four months monitoring and in experimental results, it is possible to conclude that:
i) As expected, the permeate quality could accomplish the current Brazilian Drinking-water
Standards in terms of turbidity, apparent colour and E. coli;
ii) The UF membrane system was operated under 138 kPa pressure all time and had produced
permeate whose turbidity value was lower than 0.30 ntu, nearly 0.15 ntu. However, the low
hardness removal testified the mentioned incapacity of its cut-off weight (10,000 Dalton) to reject
carbonates, calcium bicarbonates, magnesium and other ions;
iii) The UF membrane system removed completely algae and cyanobacteria cells during more than
80 hours that the system had worked. Additionally, the UF membrane system removed efficiently
microcystin and the lyses of cyabacteria cell wall seem do not occur under 138 kPa pressure.
However, the system could not remove saxitoxins, these alkaloid toxins whose cut-off weight is
below 10,000 g mol-1.
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