ORIGINAL PAPER
Preliminary surface topographical characteristics of biofilmsattached on drip irrigation emitters using reclaimed water
Yunkai Li • Bo Zhou • Yaoze Liu • Yinguang Jiang •
Yiting Pei • Ze Shi
Received: 9 December 2011 / Accepted: 24 February 2012
� Springer-Verlag 2012
Abstract Drip irrigation is the most effective and reliable
method for reclaimed water irrigation. Emitter clogging is
the bottleneck to restrain the application and popularization
of reclaimed water drip irrigation technology, and some
researchers have reported that this was tightly related to the
formation of biofilms in the irrigation system. We selected
reclaimed water treated with cyclic activated sludge system
(CASS) and four kinds of labyrinth emitters in cusp-shaped
saw-tooth, rectangular straight-tooth, arc-shaped saw-tooth
and arc-angular straight-tooth and studied the surface
topographical characteristics of biofilms in different posi-
tions of reclaimed water drip irrigation emitters with the
3D white-light scanning interferometer (WLSI). The
results showed that biofilms in different positions of units
were different with each emitter while showing the largest
thickness in water-side tooth-tip zone ([20 lm); the bio-
film thickness in the same monitoring sites inside one unit
segment gradually decreased along the flow direction,
while the flow at the inlets was much larger than that at the
outlets; comparing the head, middle and tail parts, the
biofilm thickness at the inlet and outlet showed the largest
in the tail part, followed by the middle and the head parts.
This can be explained by the equilibrium relation between
hydrodynamic behavior and the transportation of nutrient
and particles inside the emitters. The water-side tooth-tip
zone of the first unit in the last emitter was selected to
monitor surface topographical characteristics of biofilms,
and its biofilm thickness also could be used as the indicator
for evaluating the characteristics of surface topography.
These results were aimed to provide references to explain
the emitter clogging mechanism of reclaimed water drip
irrigation as well as its technological application and
popularization.
Introduction
Irrigating with reclaimed water has been widely used to
alleviate the global water shortage (Asano et al. 2007).
Although the quality of the reclaimed water reaches basic
standards of irrigation, it still contains a certain number of
pollutants. Hence, excessive and inappropriate use of
reclaimed water for irrigation may harm plants, soil, envi-
ronments of surface water and groundwater, and human
health. Drip irrigation was regarded as the most effective
and reliable pattern for reclaimed water irrigation (Adin and
Sacks 1991; Ravina et al. 1992, 1997; Capra and Scicolone
2004, 2005; Liu and Huang 2009). The emitter is one of the
key components of the drip irrigation system. However, due
to its narrow flow path of 0.5–1.2 mm, it can be easily
clogged by pollutants like suspended particles, chemical
deposits and microorganism, which may eventually com-
promise the whole irrigation system (Li et al. 2006).
Reclaimed water contains a large number of suspended
Communicated by T. Trooien.
Yunkai Li, Bo Zhou and Yaoze Liu equally contributed to this paper.
Y. Li (&) � B. Zhou � Y. Liu � Y. Pei � Z. Shi
College of Water Resource and Civil Engineering,
China Agricultural University, Beijing 100083, China
e-mail: [email protected]
Y. Li
State Key Laboratory of Urban and Regional Ecology,
Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, Beijing 100085, China
Y. Jiang
Beiqijia Sewage Treatment Plant, CPWAB,
Beijing 102209, China
123
Irrig Sci
DOI 10.1007/s00271-012-0329-1
particles, ions, algae, organic pollutants and microorgan-
isms. The particles in the water can have dynamic physical,
chemical and microbial interactions with other components,
which greatly increases the risk of clogging in emitters.
Therefore, to promote the application and popularization of
reclaimed water drip irrigation technology, it is critical to
identify the clogging mechanisms of the emitters.
Previous studies have suggested that the emitter clogging
in the drip irrigation system with reclaimed water is tightly
related to the growth of microbes on the internal substrate
surface (Adin and Sacks 1991; Ravina et al. 1992, 1997;
Capra and Scicolone 2004, 2005; Liu and Huang 2009).
Actually, microorganisms in the water, such as bacteria,
rarely exist in free-state form. Instead, over 90 % of the
microbes adhere to the substrate surface and exist in the
form of biofilms (White et al. 1998; Dong et al. 2002; Kang
et al. 2006). Biofilms are coexistence systems composed of
microbial communities, solid particles and organic polymer
matrices (e.g., extracellular polymers that were secreted by
bacteria or humus). In the natural environment, biofilms
exist on almost all of the solid surfaces exposed to water
(White et al. 1998; Dong et al. 2002, 2004; Kang et al. 2006;
Qin 2008) and thus have received close attention from
researchers in the fields of sewage treatment (Bishop 2007;
Simpson 2008) and water supply/drainage (Gouidera et al.
2009; Liu et al. 2008). To date, research on pipeline bio-
films has been focused on the water supply pipeline
(Gouidera et al. 2009; Liu et al. 2008; Manuel et al. 2007),
whereas the biofilms in irrigation systems have been rarely
studied. Although knowledge about biofilms in reclaimed
water drip irrigation systems is limited (Li et al. 2012a, b), it
has been found that biofilm formation was probably the
basic cause of the emitter clogging. At the preliminary
running stage of drip irrigation system, the status of biofilms
in emitters could directly affect the adsorption and accu-
mulation of sediments. Therefore, it is critical to study the
characteristics of biofilms at this stage to elucidate the
mechanisms underlying the clogging of emitters.
The morphology characteristics of biofilms included the
properties of roughness, thickness and pore ratio. They were
the comprehensive expression under the common effects of
the hydraulic power, water quality and temperature (Qin
2008). Based on these, the on-site emitter clogging experi-
ments of reclaimed water after the cyclic activated sludge
system (CASS) process were performed. The surface topo-
graphical characteristics of biofilms inside the units, unit
segments and multilevel unit segments of four labyrinth flow
path emitters were systematically studied with 3D white-light
scanning interferometer (WLSI). The correlation between
different levels of biofilm topographical characteristics and
the microflows in the surrounding areas was analyzed. Our
study advances the understanding of the emitter clogging
mechanisms of drip irrigation systems with reclaimed water
and the establishment of a controlling model.
Materials and methods
Experimental materials
The experiment was performed in the Beiqijia sewage
treatment plant, Beijing, China. In this plant, wastewater
Fig. 1 Water reclamation technological process of CASS
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123
was treated with CASS, which was developed based on the
SBR (sequencing batch reactor–activated sludge process).
The water reclamation technological process is shown in
Fig. 1. The water quality characteristics during the exper-
iment are summarized in Table 1.
Experiment design
As shown in Table 2, emitters with labyrinth flow path
were used in this study. For each treatment, 60 emitters
were selected and were numbered from the inlet of the drip
irrigation lateral (no. 1–60). The 120-micron disk filter was
used at the head controlling part of the experimental sys-
tem, which was shown in Fig. 2. The pressure of the flow
was controlled by a flow-diversion system, and the exper-
imental system was automatically controlled by an
irrigation controller. The experimental system ran 3 9 1 h
cycles every other day, starting at 9:00, 15:00 and 21:00.
Selection of monitoring sites
As shown in Fig. 3, the surface topographical characteris-
tics of biofilms inside the unit, unit segments and multi-
level unit segments at three levels were monitored (Liu
et al. 2010). The unit means the smallest and repeatable
structure of the labyrinth flow path (e.g., site 1–5 in Fig. 3).
And the unit segments represent several units that are
located continuously in the labyrinth flow path (e.g., site 1,
6, 7, 8 in Fig. 3), while multilevel unit segments (e.g., site
1, 12, 14 in Fig. 3b) indicate units that were different in
horizontal but same in vertical. Five monitoring sites in the
structural unit were selected as follows: site 1, the water-
Table 1 Water quality characteristics of reclaimed water in drip irrigation
Ca2? (mg/L) Mg2? (mg/L) SO42- (mg/L) HCO3
- (mg/L) PO43- (mg/L)
37.5–45.7 15.0–18.8 32.1–56.4 283.0–326.2 2.65–5.89
COD (mg/L) NH4?-N (mg/L) SS (mg/L) TP (mg/L) pH
15.8–32.0 0.95–8.14 2.1–19.5 0.37–1.58 7.09–7.27
Table 2 Drip irrigation tube used in the experiment
Number Flow path type Geometric parameter
of flow path
length 9 width 9 depth (mm)
Flow of
emitter
q (L/h)
Producing
area
Emitter types
1 Cusp-shaped
saw-tooth
50.23 9 0.57 9 0.67 1.0 Israel
2 Rectangular
straight-tooth
450.04 9 1.68 9 0.78 3.1 China
3 Arc-shaped
saw-tooth
142.35 9 1.27 9 0.99 2.9 China
4 Arc-angular
straight-tooth
152.23 9 2.40 9 0.83 1.8 China
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side tooth-tip zone; site 2, the water-reverse-side tooth-tip
zone; site 3, the main flow deformation zone; site 4, the
water-side dedendum zone; and site 5, the water-reverse-
side dedendum zone. The measurements of site 1 were used
as comparison inside unit segments and between multilevel
unit segments. For the unit, there was one site that could
not be measured because the cusp-shaped saw-tooth flow
path was destroyed (Fig. 3a).
Biofilm sampling and testing methods
Samples were collected when the average flow rate of
emitters reduced to 95 % of the rated flow (September 19,
day 77 of monitoring) and 90 % (November 14, day 133
of monitoring), and were sealed in bags and stored at 4 �C
for later use. Researchers have reported that the head (no.
1–3), middle (no. 29–31) and tail (no. 58–60) parts of drip
irrigation lateral show similar clogging patterns in the
three neighboring emitters. Thus, we collected a part of
the lateral and one emitter at each position and replaced
them with new ones. We selected the no. 1, 29, 58 emit-
ters as the former samples and the no. 2, 30, 59 emitters as
the latter samples. At the time of sampling, the two sets of
relative flow rates of emitters were 97.4 %, 96.4 %,
94.8 %, 95.3 % and 91.4 %, 92.16 %, 88.2 %, 88.1 %,
respectively, which were consistent with the relative rate
flow.
The biofilm topography analysis was conducted with the
3D WLSI system (Type: Micro-XPM) manufactured by
ADE. The sample size was 0.3 cm 9 0.3 cm. The objec-
tive lens was 509, and the testing scope was about the
rectangular region of 128 9 173 lm.
Evaluation index of biofilm 3D surface topography
SPIP software was used to analyze the images collected by
the 3D WLSI. Except average biofilm thickness (Sd), the
parameters of the surface topography were Sq (roughness
average), Sy (peak height) and Sdr (area ratio of extended
surface to projecting plane). The parameters were calcu-
lated as follows:
Roughness Sq
Roughness was the square root of the average height of the
contour offset distance. Formula (1) shows its calculation:
Sq ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
MN
X
M�1
k¼0
X
N�1
l¼0
zðxk; y
lÞ � l½ �
2v
u
u
t ð1Þ
where M denotes No. M horizontal unit of the region, N
denotes No. N vertical unit of the region, xk denotes No. K
horizontal unit of the region, yl denotes No. L vertical unit
of the region, Z(xk, ylk) denotes the height corresponding to
the No. K horizontal unit of the region and No. L vertical
unit of the region, and l denotes the average height.
Formula (2) was used for the calculation:
l ¼ 1
MN
X
M�1
k¼0
X
N�1
l¼0
zðxk; y
lÞ ð2Þ
Peak height Sy
Sy ¼ Zmax � Zmin ð3Þ
where Zmax and Zmin denote the highest point and the
lowest point in the sampling region, respectively.
Fig. 2 Experimental setting. 1-reclaimed water; 2-water pump pro-
tector; 3-water pump; 4-valve; 5.1, 5.2-water meter; 6.1, 6.2-water
meter protection pipe; 7-filter; 8-pressure fine-tuning valve;
9-pressure gage; 10-distribution box; 11-automatic irrigation control-
ler; 12-magnetic valve; 13-side valve; 14-return pipe; 15-flow testing
unit; 16-drip irrigation tube; 17-terminal return pipe
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Specific surface area Sdr
The specific surface area was the area ratio of extended
surface to projecting plane, which was calculated as
follows:
Sdr ¼PM�2
k¼0
PN�2l¼0 Akl
� �
� ðM � 1ÞðN � 1Þdxdy
ðM � 1ÞðN � 1Þdxdy� 100 %
ð4Þ
Akl ¼1
4
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
d2x þ zðxk; ylÞ � zðxk; ylþ1Þð Þ2
q
�
þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
d2y þ zðxkþ1; ylÞ � zðxkþ1; ylþ1Þð Þ2
q
� ð5Þ
where dx and dy denote the pixel distances along x-axis and
y-axis, respectively.
Results and analysis
Surface topographical characteristics of biofilms
inside the flow path unit
The topographical characteristics of surface biofilm in
these four types of flow path are shown in Figs. 4, 5, 6 and
7, and the analysis of the 3D topographical characteristics
is summarized in Table 3.
As shown in Figs. 4, 5, 6 and 7 and Table 3, the biofilm
thickness was 7.70–29.58 lm in cusp-shaped saw-tooth
flow path, 4.53–23.21 lm in arc-shaped saw-tooth flow
path, 5.29–21.03 lm in rectangular straight-tooth flow path
and 4.47–28.91 lm in arc-angular straight-tooth flow path.
Obvious differences were between the monitoring sites. All
four types of flow path showed that the biofilms thickness at
water-side dedendum zone (site 4, [20 lm) were much
(a)
(b)
(c)
1 2 3
4 5
6 7
1 23
4 5
6 7 8 9 10
11
12
13
14
16
15
17
8 1 2
3 45
6 7 8
(d)
1 23 45
6 7 8 9
10
12
11
13
Fig. 3 Monitoring sites of biofilms. a Cusp-shaped saw-tooth flow path. b Arc-shaped saw-tooth flow path. c Rectangular straight-tooth flow
path. d Arc-angular straight-tooth flow path
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larger compared to other monitoring sites. In addition, site 4
exhibited the largest Sq, Sy and Sdr. Three types of flow
path— cusp-shaped saw-tooth, arc-shaped saw-tooth and
rectangular straight-tooth—showed that the biofilm thick-
ness at main flow deformation zone (site 3) was the smallest,
while the arc-angular straight-tooth flow path had a different
result. The water-reverse-side dedendum zone (site 5)
showed the second largest biofilm thickness. The biofilm
thickness of water-reverse-side dedendum zone (site 5) was
larger than that of water-reverse-side tooth-tip zone (site 2),
while the biofilm thickness of water-side tooth-tip zone (site
1) was between those of sites 3 and 4. The biofilm thickness
of the arc-shaped straight-tooth flow path was obviously
larger than the other three types of flow path.
Fig. 4 Surface topographical characteristics of biofilms in cusp-shaped saw-tooth flow path unit. a Site 1. b Site 2. c Site 3. d Site 4. e Site 5
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Changes in the topographical characteristics of biofilms
in the flow path unit segment
Figures 8, 9, 10 and 11 show the biofilm topographical
characteristics at sites which were analogous to the water-
side tooth-tip zone (site 1), inside the units of the four types
of flow path. In the figures, the unit shown is the closest to
the flow outlet. The analysis of the topographical
characteristics of biofilms inside different units in unit
segment along with the flow direction is shown in Table 4.
The biofilm thickness was 9.68–13.55 lm for cusp-
shaped saw-tooth flow path, 5.52–16.75 lm for arc-shaped
saw-tooth flow path, 5.81–11.89 lm for rectangular
straight-tooth flow path and 4.73–9.56 lm for arc-angular
straight-tooth flow path. In summary, the biofilm thickness
gradually reduced from the inlet to the outlet of the flow
Fig. 5 Surface topographical characteristics of biofilms in arc-shaped saw-tooth flow path unit. a Site 1. b Site 2. c Site 3. d Site 4. e Site 5
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path by 28.6, 66.3, 51.3 and 50.5 %, respectively. However,
we did not observe any obvious regularity in Sq, Sy and Sdr.
Changes in the topographical characteristics of biofilms
inside multilevel unit segments
Figure 3b, d shows the multilevel unit segments of two
flow paths. The topographical characteristics at sites which
were analogous to the water-side tooth-tip zone (site 1) are
shown in Figs. 12 and 13. The biofilm topographical
characteristics are summarized in Table 5, from which we
found that the biofilm thickness was 3.59–11.75 lm for the
arc-shaped saw-tooth flow path and 4.86–8.64 lm for the
arc-angular straight-tooth flow path. These results indicated
that the thickness of biofilms at multilevel unit segments
gradually reduced by 69.4 and 43.8 % from the inlet to the
Fig. 6 Surface topographical characteristics of biofilms in rectangular straight-tooth flow path unit. a Site 1. b Site 2. c Site 3. d Site 4. e Site 5
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outlet of the flow path. However, we did not observe any
obvious regularity in Sq, Sy and Sdr.
Changes in the topographical characteristics of biofilms
at the inlet and outlet of the flow path
Table 6 shows the biofilm topographical parameters at the
inlets and outlets of emitters with different flow paths. At
the inlets, the biofilm thickness was 6.65–9.12 lm for cusp-
shaped saw-tooth flow path, 5.25–9.26 lm for arc-shaped
saw-tooth flow path, 6.50–9.99 lm for rectangular straight-
tooth flow path and 4.71–8.53 lm for arc-angular straight-
tooth flow path, while at the outlets, the biofilm thickness
were 4.11–7.36, 2.34–7.08, 5.15–7.74 and 3.09–7.25 lm
correspondingly. Compared to the inlets, biofilm thick-
nesses at the outlets were reduced by 28.6, 34.5, 23.2 and
Fig. 7 Surface topographical characteristics of biofilms in arc-angular straight-tooth flow path unit. a Site 1 b Site 2. c Site 3. d Site 4. e Site 5
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20.6 %, respectively. Furthermore, the reduction rate of the
biofilm thickness at the head, middle and tail parts of the
experimental system gradually decreased in all types of
emitters except the rectangular straight-tooth one. Mean-
while, the biofilm thickness at the inlets and outlets of the
emitters increased from the head to the middle and to the tail
part of the system. Compared to the head part, the inlet
biofilm thickness at the tail part of the system increased by
27.1, 43.3, 34.9 and 44.3 % for the four types of emitters,
respectively, while the outlet biofilm thickness increased by
44.2, 66.9, 33.5 and 57.4 %. In addition, the inlet biofilm
thickness showed lower levels of Sq, Sy and Sdr compared to
the outlet. However, there was no observable difference in
principle among the emitters at the head, middle and tail
parts of the monitoring system. When the average flow rate
was reduced to 90 %, analysis of the samples increased by
1.4–10.7, 2.3–14, 3.2–12 and 2.5–16 %, respectively,
compared to the first batch of samples, while showing
similar regularities of biofilm changes for the emitters at
different positions of the system and at the inlets and outlets
of the emitters. Furthermore, the increasing rate of biofilm
thickness at the outlets was higher than for the inlets.
Table 3 3D topography parameters of biofilms in different positions of different types of flow path unit
Number Flow path type Site 3D topography parameters
Sd (um) Sq (nm) Sy (nm) Sdr (%)
1 Cusp-shaped saw-tooth 1 13.55 10,651 46,560 462
2 10.89 1,539 16,331 28.5
3 7.70 2,489 22,351 74.2
4 29.58 12,668 55,993 582
5 17.82 11,275 52,709 520
2 Rectangular straight-tooth 1 5.52 6,098 32,758 124
2 9.30 3,406 27,432 106
3 4.53 1,189 9,130 14.7
4 23.21 14,323 63,605 362
5 10.01 5,032 34,892 96.3
3 Arc-shaped saw-tooth 1 6.29 2,066 23,545 29
2 6.55 1,795 16,850 17.3
3 5.29 1,827 28,612 53
4 21.03 11,269 41,747 276
5 6.67 2,666 25,640 48.9
4 Arc-angular straight-tooth 1 4.86 2,271 25,755 70.5
2 4.47 1,736 12,792 46.2
3 7.14 4,879 34,913 159
4 28.91 11,971 58,119 667
5 9.97 3,150 25,819 111
Fig. 8 Surface topographical characteristics of biofilms in cusp-shaped saw-tooth flow path unit segment. a Site 6. b Site 7
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Discussion
A large number of studies (Adin and Sacks 1991; Ravina
et al. 1992, 1997; Capra and Scicolone 2004, 2005; Liu and
Huang 2009) have suggested that drip irrigation is the most
effective and reliable reclaimed water irrigation approach.
However, due to the poor quality of reclaimed water, the
clogging mechanisms of the drip irrigation system and
clogging control technology are very complex (Rowan
2004; Li et al. 2012a, b; Nakayama and Bucks 1991) and
have been a major difficulty in the application of reclaimed
water drip irrigation technology. Although reclaimed water
can reach basic requirements for drainage and irrigation,
the microbial communities, nutrients and particles in
Fig. 9 Surface topographical characteristics of biofilms in arc-shaped saw-tooth flow path unit segment. a Site 6. b Site 7. c Site 8. d Site 9.
e Site 10
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reclaimed water can still penetrate through the filter to the
irrigation system. Actually, biofilms exist on almost all of
the solid surfaces exposed to water in the natural envi-
ronment (White et al. 1998; Dong et al. 2002; Kang et al.
2006; Dong et al. 2004; Qin 2008), and microorganisms in
the irrigation system can form biofilms. To date, the
research on biofilms has been focused on sewage treatment,
water supply/drainage, water environment self-purification
and biological clogging of porous media (Bishop 2007;
Simpson 2008; Gouidera et al. 2009; Liu et al. 2008;
Manuel et al. 2007). In contrast, the knowledge about
biofilms in drip irrigation systems is much less. Li et al.
(2012a, b) studied the structure of biofilms on suspended
particles in reclaimed water treated with the FBR (fluid-
ized-bed reactors) and BAF (biological aerated filter)
technology and analyzed its effects on the clogging of
labyrinth flow path of drip irrigation emitters. Based on this
study, using A16 PE tubes to simulate drip irrigation lat-
eral, the authors further studied the influence of the intra-
lateral average flow rate of the two types of reclaimed
water on the growth and surface topographical character-
istics of biofilms (Li et al. 2012a, b). Also, Yan et al.
(2009) explored the structural characteristics of biofilms in
emitters of reclaimed water drip irrigation system. All
these studies confirmed that biofilm formation and growth
are concerned with emitter clogging. Li et al. (2012a, b)
explanatorily proposed the procedure of emitter clogging
as follows: after the particles entering the irrigation system,
the biofilms on the particles undergo ‘‘attachment–growth–
detachment–death’’ continuously. With the increase in
biofilm thickness, their adhesion capacity reduces, and
biofilms detach from the substrate surface due to the shear
stress and pulse increase of the flow and then enter the
emitter with the flow. Biofilms would accumulate at zones
with low flow shear stress (mainly inlet and outlet),
resulting in reduction in the effective diameter of the flow
route and eventually cause clogging. According to this
model, biofilms are probably the initial condition and
predisposing factors of emitter clogging. It is therefore
critical to study the characteristics of biofilms at the pre-
liminary stage.
The surface topographical characteristics of biofilms,
including the roughness, thickness and cover rate, are an
integrated result of factors such as hydraulics, water
Fig. 10 Surface topographical characteristics of biofilms in rectangular straight-tooth flow path unit segment. a Site 6. b Site 7. c Site 8
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quality, temperature and time (Qin 2008). Hence, these
surface topographical characteristics could be used as
indicators for biofilm formation and emitter clogging. The
development of STM (scanning tunneling microscopy),
AFM (atom force microscopy) and scanning interferometry
has made it possible to investigate the complex topo-
graphical characteristics of biofilms. In the present study,
using a 3D WLSI, we studied the surface topographical
characteristics of biofilms inside the unit, unit segment,
multilevel unit segments, and the inlets and outlets of
emitters. Combining the flow dynamics and the transport of
nutrients and particles, we further analyzed the mecha-
nisms underlying the topographical characteristics of the
biofilms. At the structural unit level, we studied the bio-
films at five monitoring sites as follows: the water-side
tooth-tip zone (site 1), the water-reverse-side tooth-tip zone
(site 2), the flow deformation zone (site 3), the water-side
dedendum zone (site 4) and the water-reverse-side deden-
dum zone (site 5). Among these above, site 4 showed the
largest biofilm thickness, indicating that biofilms can easily
form at this site and subsequently cause emitter clogging,
which is mostly because of lowest flow rate and weakest
flow shear stress, so microbes, nutrients, and particles tend
to accumulate and subsequently promote biofilm growth. In
addition, site 4 cannot easily detach due to the weak flow
shear stress, and exhibited the largest Sq, Sy and Sdr, indi-
cating that biofilms here also had the strongest surface
adhesion capability and thus can easily bind nutrients and
particles in the water. Hence, site 4 is not only important
for monitoring biofilm formation and emitter clogging but
also an important site for the structural optimization of
emitters. Three types of flow path—cusp-shaped saw-tooth,
arc-shaped saw-tooth and rectangular straight-tooth—all
showed smallest biofilm thickness at site 3, and we suppose
this was because site 3 has a high flow rate and strong flow
shear stress, by which biofilms tend to detach, while
microbes, nutrients and particles are easy to deviate.
However, for the arc-angular straight-tooth flow path, the
arc-shaped tooth effectively reduces the shear stress of the
flow around the tooth-tip; thus, the biofilm falloff rate was
much lower. Site 5 showed the second largest biofilm
thickness, indicating that site 5 easily forms biofilms,
which resulted in emitter clogging. Site 5 is located in a
secondary flow vortex region that was deviated from the
Fig. 11 Surface topographical characteristics of biofilms in arc-angular straight-tooth flow path unit segment. a Site 6. b Site 7. c Site 8. d Site 9
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Fig. 12 Surface topographical characteristics of biofilms in cusp-shaped saw-tooth flow path multilevel unit segments. a Site 11. b Site 12. c Site
13. d Site 14. e Site 15. f Site 16. g Site 17
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mainstream; thus, biofilms cannot easily detach and
microbes, nutrients and particles tend to accumulate. In
addition, although site 2 was also in a secondary flow
vortex region, the biofilm thickness of site 5 was larger
than that of site 2, which is mainly because that water flow
passed by site 5 first. For site 1, although the supply of
nutrients and particles was relatively abundant, the high
flow rate and strong shear stress led to not only a high
growth rate of biofilms but also a high detachment rate.
And this is the key controlling zone for growth and
detachment. Comparing the same sites in the same unit
segment and multilevel unit segments, we found that bio-
film thickness always decreased along the flow direction.
The flow dynamics was similar at the similar monitoring
sites, so biofilm growth was mainly determined by the
concentrations of nutrients and particles, which were sig-
nificantly decreased along the flow direction. For the same
reason, the thickness of biofilms at the inlets was much
Table 4 3D topography parameters of biofilms of different types in unit segment along flow path direction
Number Flow path type Site 3D topography parameters
Sd (um) Sq (nm) Sy (nm) Sdr (%)
1 Cusp-shaped saw-tooth 6 13.55 6,229 33,103 175
1 10.86 10,651 46,560 462
7 9.68 11,552 60,344 369
2 Rectangular straight-tooth 6 16.75 5,354 31,016 129
7 10.85 3,432 23,957 66.2
8 10.32 4,689 23,724 154
9 8.55 2,736 22,662 70.8
1 5.52 6,098 32,758 124
10 5.64 2,364 14,689 33.1
3 Arc-shaped saw-tooth 6 11 89 7,262 31,083 168
1 10.21 2,066 23,545 29
7 6.29 9,305 21868 300
8 5.81 1,284 12,287 12.5
4 Arc-angular straight-tooth 6 9.56 3,817 27,173 115
7 6.25 1,484 14,745 36.1
1 4.86 2,271 25,755 70.5
8 4.75 2,339 25,682 83.6
9 4.73 2,251 22,252 71.2
Table 5 3D topography parameters of biofilms of different types in multilevel unit segments
Number Flow path type Site 3D topography parameters
Sd (um) Sq (nm) Sy (nm) Sdr (%)
1 Rectangular straight-tooth 1 5.52 6,098 32,758 124
11 3.59 447 7,411 7.7
12 5.64 1,293 15,600 21.4
13 6.34 2,000 17,959 41.3
14 5.67 6,591 34,234 279
15 6.08 2,260 12,886 52.1
16 6.26 2,606 18,957 89.6
17 11.75 3,892 22,171 112
2 Arc-angular straight-tooth 1 4.86 2,271 25,755 70.5
10 5.05 1,889 17,832 42.8
11 6.58 3,190 35,250 113
12 8.64 3,304 22,460 89.1
13 7.82 3,053 23,590 108
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larger than at the outlets, and the reduction rate of biofilm
thickness also decreased from the head to the middle and
the tail parts of the emitters. However, during these com-
parisons, Sq, Sy and Sdr were failed to show any obvious
changing regularity.
In summary, we selected reclaimed water treated with
CASS in the experiment, using 3D WLSI and found some
preliminary research results on the effects of flow path
types on the growth and topography of biofilms. But there
are still some issues to be solved in the future:
1. This paper was limited to the preliminary stage of the
experimental system, which needs further research in
the later stage;
2. This paper was limited to the study on the topography
of the biofilms. It is necessary to use modern molecular
biology technologies like PCR-DGGE (polymerase
chain reaction–denaturing gradient gel electrophoresis)
to construct the method for analyzing the structure of
microbial communities within the biofilms and ana-
lyzing the dynamic change in the microbial commu-
nity structure;
3. It is necessary to deeply analyze the response the
characteristics (growth and detachment, topography
and structure) of the biofilms to the water movement
and construct the dynamic growth model of laterals
biofilms under common constraints of hydraulic shear
force and nutrient. Moreover, it is necessary to
analyze the topography and formation mechanism of
lateral biofilms and focus on the pressure at inlets and
on pipe length design for controlling the formation of
biofilms.
Conclusions
1. For all types of emitter flow paths, the water-side
tooth-tip zone (site 4) showed the largest biofilm
thickness. Namely, clogging tended to occur here.
Hence, site 4 was an important site for the structural
optimization of emitters.
2. For the same monitoring sites inside one unit segments
and in each unit of the multilevel unit segments, the
Fig. 13 Surface topographical characteristics of biofilms in arc-angular straight-tooth flow path multilevel unit segments. a Site 10. b Site 11.
c Site 12. d Site 13
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biofilm thickness gradually decreased along the flow
direction.
3. The thickness of biofilms at the flow inlets was much
larger than that at the outlets. Comparing the emitters
in the head, middle and tail parts of the irrigation
system, the biofilm thickness of the inlet, outlet and
lateral wall was always the largest in the emitters in the
tail part, followed by the middle part and the head part.
4. The water-side tooth-tip zone (site 4) of the first
structural unit in the last emitter of the irrigation
system could be used as a monitoring site for the
surface topography and dynamic characteristics of
biofilms. The biofilm thickness, but not Sq, Sy or Sdr,
could be used as an indicator for the evaluation of the
surface topography.
Acknowledgments We are grateful for financial support from the
National Natural Science Fund of China (No. 51179190), Beijing
Excellent Researcher Award Program (2010D009007000003) and
New Century Excellent Researcher Award Program of Chinese
Ministry of Education (NETC-10-0780). The 3D white-light scanning
interferometer was provided by the State Key Laboratory of Tribol-
ogy, Tsinghua University.
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