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: liyunkai@126.com

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

Irrig Sci

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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

Irrig Sci

123

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.

References

Adin A, Sacks M (1991) Drip clogging factors in wastewater

irrigation. J Irrig Drainage Eng 117(6):813–826

Asano T, Burton F, Leverenz H, Tsuchihashi R, Tchobanoglous G

(2007) Water reuse: issues, technologies and applications.

McGraw-Hill Professional Publishing, New York

Bishop PL (2007) The role of biofilms in water reclamation and reuse.

Water Sci Technol 55:19–26

Capra A, Scicolone B (2004) Emitter and filter tests for wastewater

reuse by drip irrigation. Agric Water Manage 68:135–149

Capra A, Scicolone B (2005) Assessing dripper clogging and filtering

performance using municipal wastewater. Irrig Drainage 54:71–79

Dong DM, Li Y et al (2002) Relationship between chemical

composition of coating and waters in natural waters. Chem J

Chin Univ 8(23):1507–1509

Dong DM, Ji L, Hua XY, Li Y, Zheng N (2004) Studies on the

characteristics of Co, Ni and Cu adsorption to natural surface

coating. Chem J Chin Univ 2(25):247–251

Table 6 3D topography parameters of biofilms in inlets and outlets of different types of emitter flow path

Number Flow path type Tube

position

Flow path

position

First batch of samples

(95 % rated flow)

Second batch of samples

(90 % rated flow)

Sd

(um)

Sq

(nm)

Sy

(nm)

Sdr

(%)

Sd

(um)

Sq

(nm)

Sy

(nm)

Sdr

(%)

1 Cusp-shaped saw-tooth Head Inlets 6.65 3,290 26,319 58.6 7.33 1,629 17,085 39.5

Outlets 4.11 6,006 30,732 149 4.55 10,803 43,377 358

Middle Inlets 8.42 7,825 39,052 250 8.54 2,790 21,621 28.6

Outlets 5.81 12,386 45,579 610 6.08 1,107 11,188 24.8

Tail Inlets 9.12 1,553 13,579 46.3 9.99 2,130 20,877 43

Outlets 7.36 3,136 37,670 155 7.65 1,190 10,829 26.4

2 Rectangular straight-tooth Head Inlets 5.25 5,205 32,395 75.6 5.73 7,821 43,677 282

Outlets 2.34 1,386 17,588 13.4 2.49 4,699 32,205 121

Middle Inlets 6.60 1,347 17,292 24.6 6.81 12,156 52,668 124

Outlets 4.41 3,358 19,558 132 4.51 9,394 45,196 215

Tail Inlets 9.26 2,997 21,426 48.3 9.95 6,256 34,829 229

Outlets 7.08 4,250 34,051 147 8.07 5,733 31,745 165

3 Arc-shaped saw-tooth Head Inlets 6.50 13,713 68,987 613 7.28 4,043 31,454 95.7

Outlets 5.15 1,623 9,307 13.3 5.62 5,360 28,380 100

Middle Inlets 8.72 6,008 43,149 368 9.00 8,726 63,340 321

Outlets 6.46 3,053 17,164 98 6.71 13,124 50,651 393

Tail Inlets 9.99 5,200 28,510 185 10.56 2,849 23,945 99.8

Outlets 7.74 3,566 23,801 126 8.45 5,670 28,425 180

4 Arc-angular straight-tooth Head Inlets 4.71 6,245 33,874 101 4.83 8,648 38,900 122

Outlets 3.09 5,463 30,214 119 3.18 2,540 20,030 52.5

Middle Inlets 6.31 4,947 30,654 106 6.78 4,831 30,959 191

Outlets 5.18 7,458 40,315 274 6.01 4,308 27,906 65.8

Tail Inlets 8.53 10,612 55,232 233 8.90 3,454 30,273 129

Outlets 7.25 5,343 28,700 201 7.84 7,258 40,001 137

Irrig Sci

123

Gouidera M, Bouzida J, Sayadi S et al (2009) Impact of orthophos-

phate addition on biofilm development in drinking water

distribution systems. J Hazard Mater 167:1198–1202

Kang CL, Su CY, Guo P, Zhao YX, Dong DM (2006) Studies on

Pb2? and Cd2? adsorption by extracellular protein of natural

biofilm. Chem J Chin Univ 7(27):1245–1246

Li YK, Yang PL, Ren SM et al (2006) Hydraulic characterizations of

tortuous labyrinth path drip irrigation emitter. J Hydrodyn Ser B

18(4):449–457

Li GB, Li YK, Xu TW, Liu YZ et al (2012a) Effects of average velocity

on the growth and surface topography of biofilms attached on the

reclaimed wastewater drip irrigation system laterals. Irrig Sci

30(2):103–113. doi:10.1007/s00271-011-0266-4

Li YK, Liu YZ, Li GB, Xu TW et al (2012b) Surface topographic

characteristics of suspended particulates in reclaimed wastewater

and effects on clogging in labyrinth drip irrigation emitters. Irrig

Sci 30(1):43–56. doi:10.1007/s00271-010-0257-x

Liu HJ, Huang GH (2009) Laboratory experiment on drip emitter

clogging with fresh water and treated sewage effluent. Agric

Water Manag 96:745–756

Liu XL, Liu WJ, Jin LY, Gu JN (2008) Microbes in the Beijing water

distribution systems. J Tsinghua Univ 48(9):1458–1461

Liu HS, Li YK, Liu YZ et al (2010) Flow characteristics in energy

dissipation units of labyrinth path in the drip irrigation emitters

with DPIV technology. J Hydrodyn Ser B 22(1):137–145

Manuel CM, Nunes OC, Melo LF (2007) Dynamics of drinking water

biofilm in flow/non-flow conditions. Water Resour 41:551–562

Nakayama ES, Bucks DA (1991) Water quality in drip/trickle

irrigation: a review. Irrig Sci 12:187–192

Qin SY (2008) Study on morphology and biodiversity of biofilm of

river Oder (PhD thesis). Harbin Institute of technology, Harbin

Ravina E, Sofer Z, Marcu A et al (1992) Control of emitter clogging

in drip irrigation with reclaimed wastewater. Irrig Sci 13(3):

129–139

Ravina E, Sofer Z, Marcu A et al (1997) Control of clogging in drip

irrigation with stored treated municipal sewage effluent. Agric

Water Manage 33(2):127–137

Rowan MA (2004) The utility of drip irrigation for the distribution of

on-site wastewater effluent (PhD thesis). Ohio State University,

Columbus

Simpson RD (2008) Biofilm processes in biologically active carbon

water purification. Water Resour 42:2839–2848

White DC, Headley JV, Gandrass J, Kuballa J (1998) Rates of

sorption and partitioning of contaminants in river Biofilm.

Environm Sci Technol 32:3968–3973

Yan DZ, Bai ZH et al (2009) Biofilm structure and its influence on

clogging in drip irrigation emitters distributing reclaimed

wastewater. J Environ Sci 21(6):834–841

Irrig Sci

123