DRIPPER-CLOGGING FACTORS IN WASTEWATER IRRIGATION

By Avner Adin1 and Mollie Sacks2

ABSTRACT: Wastewaters that have undergone treatment and are stored in open reservoirs frequently cause clogging problems while being applied through drip-irrigation systems. This study is aimed at defining the clogging factors and mech­anisms of blockage within three types of drippers as a basis for developing technical measures to overcome the problem. The relevant effluent constituents are defined and physical and chemical properties of the deposits in hundreds of emitters are examined, using both field and laboratory experiments. A gradual clogging mech­anism is proposed: The sediment buildup begins with the deposition of amorphous slimes, to which other particles adhere. The clogging rate is more affected by particle size than by particle-number density. The chemical composition of the deposits in the dripper changes with the season. Filtration prevents immediate clogging by large particles. Clogging potential may be decreased by modifying the emitter structure and by chemical pretreatment.

INTRODUCTION

Wastewater that has undergone treatment and is then stored in a reservoir during the rainy season is being used in arid or semiarid countries as a water resource for irrigation during the dry summer. Effluent reservoirs provide a special aquatic environment in which physical and biological processes affect the water quality and influence the reservoir effluent characteristics (Dor et al. 1987). The effluents are often being applied through drip-irri­gation systems. Among its various advantages, drip irrigation eliminates the hazard of transport of pathogens via aerosols caused by sprinklers, thereby allowing safer use of effluents in irrigation (Sadovski et al. 1978).

The main problem concerning the performance of drip-irrigation systems utilizing wastewater effluents is clogging of the drippers. Each emitter has a certain design flow rate ranging from 1-8 L/hr, which is affected by the operation pressure (between 100-200 kPa), water temperature, and by clog­ging. Emitter flow paths usually have widths of only 0.5-1.0 mm and are vulnerable to clogging (Adin 1987a, 1987b; Gilbert et al. 1982; Gilbert et al. 1979).

The causes of clogging can be divided into three main categories: (1) suspended matter; (2) chemical precipitation; and (3) bacterial growth (Adin and Rubinstein 1989; Bucks et al. 1982; Ford 1978). Roots, sand, rust, microorganisms, and dirt in the irrigation water often appear to be the major sources of clogging. Biological films and slimes, termed biofilms, derived from microorganisms and algae, are among the major biological clogging factors.

In nature, biofilms are to be found on gravel and sand, and in stream

•Assoc. Prof., Envir. Sci. Div., Grad. School Appl. Sci. and Tech., The Hebrew Univ. of Jerusalem, 91904 Jerusalem, Israel.

2Field Instructor, Ministry of Agric., Lachish Extension Service Bureau, South Lachish Post, Israel; formerly, M.S. Student, Envir. Sci., The Hebrew Univ. of Jerusalem, Israel.

Note. Discussion open until May 1, 1992. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on July 12, 1990. This paper is part of the Journal of Irrigation and Drainage Engineering, Vol. 117, No. 6, November/December, 1991. ©ASCE, ISSN 0733-9437/91/0006-0813/$1.00 + $.15 per page. Paper No. 26425.

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beds and aquifers. In engineering systems, they frequently occur in pipes, tanks, and filter beds (Rittman and McCaity 1980).

Some research has been performed relating to the prevention of clogging by filtration (Adin and Elimelech 1989; Adin and Alon 1986, Ben-Harim and Steinhauer 1983) and oxidation (Bucks et al. 1979). However, very little has been done to correlate directly the development of deposits within the emitter with the characteristics of effluent quality.

This study was aimed at defining the clogging factors and mechanisms of blockage within the dripper as a basis for developing technical measures to overcome the problem. For that purpose, effluent constituents were defined and physical and chemical properties of the deposits in the emitter were examined, using both field and laboratory experiments.

METHODS

A drip-irrigation experimental system was set up in the fields of Kibbutz Naan. The effluent source was a 10-m-deep, 700,000-m3 reservoir preceded by two oxidation ponds. The effluent was pumped from the depth of 80 cm below the reservoir water surface. A plastic (polyethylene) main line sup­plied the effluent to the experimental plot. The main line branched into two parallel subsystems: one pretreated by a rapid (7.5 m/hr) gravel filter containing angular basalt media of 0.69 mm effective grain size, followed by a 120-mesh disk filter; and the other left untreated. A water meter was installed in each branch. The filters were used for the purpose of altering the effluent characteristics (the investigation of filtration as a treatment process was beyond the scope of this work). Sets of laterals were connected together to the main supply lines of each subsystem. These connections were also equipped with a valve in order to enable water sampling. The lateral diameters were 20 mm.

The emitters were welded inside the polyethylene-tube laterals during manufacture. Three types of emitters were tested. Every type of emitter occupied four laterals. Each lateral was 60-m long, with emitter spacing at 1-m intervals along the lateral. Each emitter was code marked by its treat­ment (with or without filtration), type, and location. Two types were non-regulated, long-path emitters (Fig. 1 and Table 1): the R-emitter having a flow path of 520-mm in length and the L-emitter having a flow path of 960-mm in length. The flow path in such emitters is in the form of a labyrinth containing teeth-like protrusions. The loss of energy through the emitter pathways results in a uniform discharge when used with clean water. Their initial discharge rates at 141 kPa were 3.2 L/hr and 1.8 L/hr, respectively. The third type—the V-emitter [Fig. 1(b) shows both sides of the emitter] is self-regulating. The discharge is regulated by a small diaphragm at the end of the flow path that reduces the dripper orifice when the pressure rises in the lateral. The flow path of the V-emitter is also tortuous but only 25 mm in length. The initial discharge of the emitter was 2.5 L/hr.

The experiments were carried out over two periods of time: (1) Winter (rainy season) and spring; and (2) summer (dry season). During the first period, irrigation was allowed to run 24 hours per day every other day (three days per week). The duration of this experiment was equivalent to approx­imately three cotton irrigation seasons. A total volume of 4,777 m3 of ef­fluent passed through the system. Towards the summer irrigation period the emitter lines were totally replaced with new lines. The irrigation was increased to five days a week, and 1,475 m3 of effluent passed through the new system.

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FIG. 1. Types of emitters used: (a) type R; (b) type V; and (c) type L

TABLE 1. Basic Characteristics of Emitters

Emitter type (1)

R (nonregulated) L (nonregulated, long) V (self-regulating)

Flow rate (L/hr) (2)

3.2 1.8 2.5

Path length (3)

520 960 25

Path width (mm) (4)

1.09 1.09 1.34

•Nominal flow rate at 141 kPa.

Tests were carried out in the field and in the laboratory where the water quality along with the sediment deposits in the emitters were examined. The following parameters were examined weekly in the field: (1) The pres­sure gradient along the laterals and the filters; (2) the water temperature along the laterals; (3) the flow rate of specific emitters situated in the

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beginning, middle, and end of the laterals; (4) water turbidity before, after and between the filters; and (5) water pll.

Water samples were taken monthly, representing effluents without fil­tration, after filtration, and between the two filters. Grab samples of 2-3 L were taken and stored in plastic bottles. The samples were refrigerated within one-and-a-half hours. The water quality was examined to determine total suspended solids (TSS) concentration, volatile suspended solids (VSS), total hardness, alkalinity, electrical conductivity (EC), biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved organic carbon (DOC), particle-size distribution (PSD), algae concentration, and zeta (elec­trostatic surface) potential of the suspended particles. The latter parameter is a common indicator for coagulation potential of particulates in water.

Emitters were removed systematically from the lines to the laboratory where they were carefully opened and their contents were examined. Most of these emitters were placed in a formaldehyde solution in the field to preserve their contents from decomposing although some were withheld for chemical examination of the sediment. The drippers removed were imme­diately replaced by new ones. The examination of the sampled emitters, after stripping them carefully, included mapping of sediment accumulation within the emitter, determination of the shape and size of the particles forming the sediment using electron microscopy, chemical analysis of the sediment by X-ray, and also microscopic examination of the algae and zooplankton material in the sediment.

RESULTS

Effluent Quality The characteristics of the reservoir effluent before and after filtration are

summarized in Table 2. Water pH ranged from 7.1 to 8.5. Turbidity ranged from 6.5 to 100 nephelometric turbidity units (NTU), and was higher than 30 NTU for most of the samples. BOD ranged from 12-60 mg/L, which is typical for a well-functioning wastewater reservoir. Higher concentrations (65 mg/L geometric mean) of suspended solids after filtration than without filtration (55 mg/L geometric mean) can be explained by occasional de­tachment of deposits from the filters (Adin and Elimelech 1989; Adin and Alon 1986). The dominant algae species identified in the irrigation water were: Ankistrodesmus, Chlorella, Coelastrium, Dictuspheriem, Oocystis, and Tetrahedon. Their sizes ranged from 3-50 u>m. In July the concentra­tions of Chlorella and Oocystis increased by a factor of 10, resulting in total algae concentration of 410,000 cells per milliliter, in the feed water. The zeta potential ranged from —14 to — 30 mv which indicates a stable suspen­sion.

Figs. 2 and 3 demonstrate the particle-size distribution in the filtered effluent in May-July. Until the middle of June, most of the particles mea­sured in the range of 5-300 u,m were under 30 |xm. From the beginning of July until the end of the experiment most of the particles were in the 20-300-u,m range. Table 3 demonstrates the element composition of the sus­pended solids in the irrigation effluent and of the deposits in the drippers. Fe, Al, Si, and Ca were present in the majority of the samples. Higher concentrations of Al and Si were found in June compared to July. The concentrations of P and Ca rose significantly in July.

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TABLE 2. Characteristics of Effluents Used in Experimental System

Parameter

0)

Minimum (mg/L)

(2)

Maximum (mg/L)

(3)

Arithmetic mean (mg/L)

(4)

(a) Without Filtration

BOD BOD of filtrate COD COD of filtrate DOC TSS Turbidity (NTU) VSS

12 6

102 70 15 13 6.5 9

60 30

278 242

33 93

100 84

35 17

165 151 23 55 36 42

(b) After Filtration

BOD BOD of filtrate COD COD of filtrate DOC TSS Turbidity (NTU) VSS

13 6

113 96 15 3 7 7

45 26

232 204

31 163 100 160

31 13

181 151 20 65 39 47

(c) Additional General Parameters

Alkalinity (CaC03) C a + + (CaC03) EC (mmho/cm) Hardness (CaC03) Mg+ + (CaC03) pH Zeta potential mv

350 198

1.08 387

93 7.1

- 1 4

450 347

1.74 445 239

8.5 - 3 0

405 259

1.4 420 160

8 - 2 4

Discharge Variations In this work, a dripper is considered clogged when its discharge becomes

50% of the original rate. Table 4 presents two sets of data pertaining to the summer months: (1) The percent of emitters were discharges of 10% less than their original rate; and (2) the percent of drippers clogged during the summer months. The two most prominent features that can be seen from this data are: (1) The emitters at the end of the lines experienced a higher percentage of clogging than those situated in the beginning or in the middle of the lateral; and (2) the L-emitter had the highest clogging ratio of all the emitters tested. When supplied with filtered effluents, the V-emitter dis­played no evidence of clogging, in contrast with the performance of the other types of emitters that displayed clogging under the same conditions.

During the summer period, the flow-rate measurements from the drippers showed, in general, that each emitter behaved as an independent unit. Two adjacent drippers could perform differently, with one clogged and the other unclogged. There were fluctuations in the discharge rate as time progressed.

Fig. 4 depicts the R-emitter flow rates in the beginning, middle, and end of a lateral in the subsystem without filtration. The range of emitter dis-

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5-7 7-10 10-15 15-20 20-30 30-40 40-50 50-60 60-100 100-300

Particle size ( jum )

FIG. 2. PSD of Irrigation Water from Subsystem with Filtration: May 11-June 10

E

5 150

I I Jul I

UZ\ Jul 8

B l J y l 22

• * /*~ rT^—i U J L I | _ 5-7 7-10 10-15 15-20 20-30 30-40 40-50 50-60 60-100 100-300

Particle size ( jum )

FIG. 3. PSD of Filtered Irrigation Water from Subsystem with Filtration: July 1-July 22

charge rates of the first dripper through the summer period were 1.4-3.5 L/hr. After 10 days of irrigation, the dripper flow rate fell by 50%. A week later the discharge returned to its original flow rate. Twelve days later the flow rate decreased again. The middle dripper behaved in a similar manner although the flow rate dropped to a lower level. At the end of the lateral the discharge dropped and remained low until the end of the experiment.

The aforementioned phenomenon was found to be common to hundreds of drippers throughout the whole system.

Characterization of Sediment in the Emitter The examination of "peeled" drippers in the laboratory revealed that

particulates were frequently entrapped in the water entrance of each of the three types of emitters. The shape of the entrance to the V-emitter enabled a 1-cm-long larva to penetrate the emitter (Fig. 5). This did not occur in the two types of nonregulated emitters v/here the entrance structure pre­vented it. In some of the nonregulated emitters, more sediment was observed in the lower part of the emitter (that faced the ground) than in its upper

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TABLE 3. Element Spectrum of Particle Composition

ELMT

(D Al Ca CI Cu Fe K Mg P S Si Zn

Fractions (%)

June (2)

15.5 8.3 4.6 9.8 4.8 5.2 0.0 0.0 4.3

40.3 7.2

in Effluent SS

July (3)

1.2 12.3 9.8 8.3 0.3 6.0

17.1 35.1 0.0 2.7 6.9

Fractions (%) in

June (4)

18.9 9.1 2.1 3.2 9.1 2.1 0.0 0.0 0.0

52.2 3.2

Dripper Deposits

July (5)

2.1 54.2

1.6 3.7 1.8 1.1 3.7

23.4 0.0 5.8 2.3

TABLE 4. Percentage of Clogged Emitters During Summer Months

Emitter type (D

Without Filtration

Start of line (%) (2)

Middle of line (%)

(3)

End of line (%) (4)

After Filtration

Start of line (%) (5)

Middle of line (%)

(6)

End of line (%) (7)

(a) Percentage of Emitters with 10% Drop in Discharge Rate

R V L

50 40 55

55 40 70

60 60 65

35 5

65

60 0

75

55 5

75

(b) Percentage of Emitters with 50% Drop in Discharge Rate

R V L

25 25 20

25 10 45

60 50 45

0 0

20

25 0

50

25 0

55

part. This could be explained by quiescent settling of particles during the purposely made interruptions in the system operation. Emitters using fil­tered effluents experienced a gradual accumulation of sediment. The dep­osition started in the corners and in the curved sections of the emitter, i.e., where changes in the water flow direction took place [Fig. 5(a)]. In the second stage, material deposited in the areas between the protruding teeth-like structures.

The L-emitter was constructed of four sections attached by seams [Fig. 5(c)], A notable amount of sediment accumulated around each of the seams. No significant decrease in discharge was observed in emitters that were examined after the first and second stages of sedimentation. However, in the third stage the labyrinth itself was filled with sediment, causing a con­siderable decrease in water discharge.

Emitters using unfiltered effluents experienced similar phenomenon as described previously, but were also susceptible to immediate clogging by particles larger than the interval passages, e.g., by egg sacs (ephippium) of Dafnia that settled between the protruding teeth [Figs. 5(b) and 6].

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0 -) 1 1 1 — 1

Jun22 Jul 01 Jul 08 Jul 15 Jul 22

Dat e

FIG. 4. Discharge Rates of R-Emitter at Lateral Beginning (RdS), Middle {Rd31), and End (flc/55)

The form of the sediment in the drippers and the shape of the suspended material in the water were determined using electron microscope photo­graphs. Gelatinous particles with no discernible configuration were observed along with other particles: spherical, oval, rectangular, and filamentous in shape, ranging in size from 0.5 |xm to 30 |xm. Fig. 7 depicts a slimish deposit to which several aggregates of about 10 |xm in size adhere.

In the beginning of June, silica and aluminum were the predominant elements of the suspended particles in the water and in the drippers (Table 3). In July the elemental composition of the suspended particles changed so that phosphorous and calcium were predominant. The main components were filaments, lignine, and cellulose from plant sources and chitin and exuviate from animal sources.

Fig. 8 presents two pie charts illustrating the volume ratio of suspended particles ranging in size from 5 (xm to 300 fjum in filtered reservoir effluents. On June 10, no particles larger than 100 jxm were discernible in the water, correspondingly the discharge rates did not decrease. On July 8, 47% of the volume was composed of particles larger than 100 |xm, and serious clogging took place (Fig. 4). It appears that suspended particles of sizes ranging from 60 u,m to 300 \xm are a major factor in the clogging of emitters.

ANALYSIS

The fluctuations in emitter discharge rates suggest a phenomenon of clogging and self-cleaning of the emitter. This cannot be explained by a loss of pressure along the laterals due to the local topography. The field has a downward slope resulting in a total pressure rise of 5 kPa. The pressure variance was insignificant in respect to its influence on the emitter discharge rate. The cause may be a combined effect of fluctuations of the suspended solids content in the water and the hydraulic operation of the system. Oc­casionally, when the system is turned on, the pressurized water flow may "clean" the dripper of the material clogging it. The very high startup ve­locities in the lines, cleans off the hose inlet area, and flushes accumulated particles to the downstream end. This decreases the potential of the cleansing

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larva

FIG. 5. Sediment Deposits in Emitters

of the end emitters during operation, keeping the drippers' discharge low as reflected in the reported results.

According to various investigators (Zobell 1943; Heukelekian and Heller 1940), biofilms remove organic compounds in low concentrations from the water since:

1. The known accumulation of many organic compounds at solid-liquid in­terfaces facilitates biofilm formation.

2. Owing to the high cell density within biofilms, the activities of various extracellular enzymes derived from different bacteria render metabolism of organic materials more efficient.

3. Biofilms utilize accumulated biomass for survival during periods of sub­strate starvation.

The individual cells in biofilms are surrounded by extracellular polymers,

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FIG. 6. Dafnia egg (Ephippium), 0.5 mm In Size, Entrapped in Dripper Pathway

i

i i I

FIG. 7. Particles Adhering to Slime Deposited onto Emitter Flow Path

polysaccharides, and glycoproteins (Fletcher 1979; Jones et al. 1969). Bio-films primarily utilize organic compounds to provide energy and carbon for cell growth and maintenance.

Such slimy gelatinous deposits of amorphous shape serve as triggers for serious blockage. Particles of a definable shape are found in a matrix of the gelatinous substance and form the primary sediment in the drippers. The

ffigr

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60-100/jm (50.5%)

a. June 10 5-l5jum (4.6%)

15-20/jm (7.0%)

IOO-300/jm(47.3%

20-30>jm (13.4%)

30-40/jm (7.1%)

40-50;um (8.0%)

50-60/jm (9.3%)

b. July 8 5-40/jm (3.0%)

40-50/jm (4.3%)

50-60/jm (6.7%)

60-100/jm (38.7%)

FIG. 8. Volume Distribution of Particles in Irrigation Water from Subsystem with Filtration: (a) June 10; and (b) July 8

primary sediment then acts as a trap for algae and zooplankton limbs that increase the volume of the sediment until they block the water pathway.

The accumulation of deposits in the drippers is associated with a rise in organic matter content and the subsequent decrease in alumino-silicates (e.g., clays) in the water. The decrease in clay minerals in the water in the dry summer months may be attributed to quiescent settling in the reservoir due to decreased turbulence and to the lack of penetration surface water to the influent. The relatively large amounts of phosphorous in the sediment can be attributed to the large quantities of algae in the water due to summer algae blooms. The high concentration of calcium possibly stems from zoo­plankton remains, which have accumulated in the drippers. The fleshless character of the material indicates that the decomposition takes place in the reservoir and not in the drippers. Zooplankton range in size from 0.2 to 5.0 mm and feed on the algae biomass in the reservoir become a major factor in clogging. Algae can be considered a special group of particles of a high

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indirect clogging potential within the size range of a few to 60 u m. One of the prominent findings, with respect to clogging mechanism by algae, is that this type of clogging develops only when initial deposits of minerals or a gelatinous material occurred previously. Algae particles are trapped and compressed into these primary sediments in the dripper.

From analyzing the results concerning the location of sediments accu­mulated within the emitters, it seems that clogging can be decreased by certain modifications in the emitter structure, thus saving considerably on investments in water-quality-control facilities. The performance of the self-regulating emitter (V) compared to the low-path emitters shows that the emitter design has a significant influence on the clogging potential. The shorter and wider flow path prevented the accumulation of material within, while the design permitted the necessary pressure loss for regulating the discharge rate. The gradual sediment deposition in the long path emitters shows that there are superfluous flow spaces, e.g., dead areas in the flow path, which do not affect the discharge rate.

PRACTICAL APPLICATIONS

The clogging of emitters in drip-irrigation systems using wastewater ef­fluents can be decreased by modifying the emitter design to prevent sediment accumulation in the flow path. The emitter internal design can be improved by: (1) Shortening and widening the flow path, taking into consideration the necessary flow rate; (2) rounding the straight edges on the protruding teeth in the flow path; (3) removing superfluous flow spaces, e.g., dead areas in the flow path; (4) designing the orifice entranceway to act as a barrier for large particles; and (5) manufacturing seamless emitters or plac­ing the seams away from the flow path.

Filtration (no flocculant added) can prevent immediate blockage by re­moving particles larger than the width of the emitter flow path. Granular filtration helps remove particles with irregular shapes. Additional means of reducing clogging are: efficient backwash of the filters; and flushing the ends of the lines and installing long laterals when the topography permits. Another suitable treatment process that may be suggested as a result of this work is the use of oxidants such as chlorine or chlorine dioxide to attack the slimes and algae.

Nakayama et al. (1977) and Engliah (1985) pointed out that chemical oxidants, such as chlorine, chlorine dioxide, and ozone, may serve to clean drippers and prevent clogging phenomena by inhibiting growth and accu­mulation of microorganisms and algae. They can also react with humic material, bringing about changes in surface area and absorption capacity (Rav-Acha et al. 1985).

The use of low concentrations of flocculants for improvement of the removal of particulates by granular filtration should also be considered. The latter process seems to be effective in preventing the gradual clogging caused by the adherence of fine particulates to the primary sediments.

Additional research is recommended to develop application methods for the aforementioned processes and to reduce the clogging potential of par­ticles ranging from 60 urn to 300 |xm.

CONCLUSIONS

1. Clogging of emitters using reservoir effluents is caused primarily by sus­pended solids in the water but they do not necessarily initiate the clogging

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process. The clogging rate is more affected by particle size than by particle-number density.

2. The sediment buildup begins with the deposition of amorphous slimes, to which other particles adhere. The algae caused dripper clogging only when they attach themselves to other particles. That happened when their concentrations were above 105 cells/ml for a continuous period of a few weeks.

3. The chemical composition of the sediment in the dripper changes with the season. Winter (wet season) and spring analyses resulted in high percentages of aluminum and silicon, while high percentages of phosphorus and calcium were found in the summer (dry season). The disintegration of the zooplankton that later accumulated in the drippers occurred in the reservoir before entering the drippers. In general, the sediment contained skeletal remains of zooplankton or whole live algae.

4. Filtration prevents immediate clogging by large particles. Rapid granular filtration has an important role in protection from clogging due to effective removal of particles with irregular shapes.

5. Clogging potential may be decreased by modifying the emitter internal design and by chemical pretreatment with oxidants and flocculants.

ACKNOWLEDGMENTS

The work was supported by a grant from the Israeli Water Commission. Kibbutz Naan members are acknowledged for their aid in equipment and field supervision.

APPENDIX. REFERENCES

Adin, A. (1987a). "Clogging phenomenon in irrigation systems reusing pond effluents and its prevention." Proc. International Conference on Waste Stabilization Ponds, IAWPRC Jun. 29-Jul.

Adin, A. (1987b). "Problems associated with particulate matter in water reuse for agricultural irrigation and their prevention." Water Sci. and Tech., 18 (Split), 185-195.

Adin, A., and Alon, G. (1986). "Mechanisms and process parameters of filter screens." J. Irrig. and Drain. Engrg., ASCE, 122(4), 293-304.

Adin, A., and Elimelech, M. (1989). "Particle filtration for wastewater irrigation." /. of Irrig. and Drain. Engrg., ASCE, 115(3), 474-487.

Adin, A., and Rubinstein, L. (1989). "Particle characterization in wastewater ef­fluents in relation to filtration and irrigation." Filtration and Separation (Jul., Aug.), 284-287.

Ben-Harim, L., and Steinhauer, Z. (1983). "A new filtrability test for water quality control." Environmental quality and ecosystem stability, III A/B, Bar Ilan Univ. Press, Israel.

Bucks, D. A., Nakayama, F. S., and Gilbert, R. G. (1979). "Trickle irrigation water quality and preventive maintenance." Agric. Water Mgmt., 2(2), 149-162.

Bucks, D. A., Nakayama, F. S., and Warrick, A. W. (1982). "Principles, practices and potentialities of trickle irrigation." Advances in irrigation, I, Academic Press, San Diego, Calif., 219-298.

Dor, I., Schechter, H., and Bromley, H. J. (1987). "Limnology of a hypertrophic reservoir storing wastewater effluent for agriculture at kibbutz Naan, Israel." Hy-drobiologia, 150, 225-241.

Engliah, D. (1985). "Filtration and water treatment for micro-irrigation." Proc. of the Third Int. Drip/Trickle Irrigation Congress, I, 50-57.

Fletcher, M. (1979). "The attachment of bacteria to surfaces in aquatic environ­ments." Adhesion of microorganisms to surfaces, Academic Press, London, U.K.

825

J. Irrig. Drain Eng. 1991.117:813-826.

Dow

nloa

ded

from

asc

elib

rary

.org

by

RM

IT U

NIV

ER

SIT

Y L

IBR

AR

Y o

n 03

/12/

13. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Ford, H. W. (1978). "Bacterial clogging in low pressure irrigation systems." Proc. brig, Assoc. Annual-Technical Congress on Efficiency in Irrig., (Feb.) 293-244.

Gilbert, R. G., Nakayama, F. S., and Bucks, D. A. (1979). "Trickle irrigation; prevention of clogging." Trans. ASAE., 22(3), 514-519.

Gilbert, R. G., Nakayama, F. S., Bucks, D. A., French, O. F., Adamson, K. C., and Johnson, R. M. (1982). "Trickle irrigation: Predominant bacteria in treated Colorado River water and biologically clogged emitters." brig. Sci., 3(2), 123-132.

Heukelekian, H., and Heller, A. (1940). "Relation between feed concentration and surface fat bacterial growth." /. Bacteriol., 40, 547-558.

Jones, H. C., Roth, I. L., and Sanders, W. M. (1969). "Electron microscope study of slime layer." /. Bacteriol., 99(1), 316-325.

Nakayama, F. D., Bucks, D. A., and French, O. F. (1977). "Reclamation partially clogged trickle emitters." Trans. ASAE, 20(2), 278-280.

Rav-Acha, C , Chosen, E., Serri, A., andLimoni, B. (1985). "The role of formation and reduction of THIX and chlorite concentrations in the disinfection of water with Cl2 and C102." Envir. Pollution, 10(1), 47-60.

Rittman, B. E., and McCarty, P. L. (1980). "Model of steady state biofilm kinetics." Biotechnical and Bioengrg., 22, 2343-2357.

Sadovski, A. Y., Fattal, B., and Goldberg, D. (1978). "Microbial contamination of vegetables irrigated with sewage effluent by the drip method." / . Food Protection, 41(5), 336-340.

Zobell, C. E. (1943). "The effect of solid surfaces upon bacterial activities." J. Bacteriol, 46, 39-56.

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J. Irrig. Drain Eng. 1991.117:813-826.

Dow

nloa

ded

from

asc

elib

rary

.org

by

RM

IT U

NIV

ER

SIT

Y L

IBR

AR

Y o

n 03

/12/

13. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.