Utilization of reclaimed wastewater for irrigation of field-grown melons ...

Israel Journal of Plant Sciences

Vol. 59

2011

DOI: 10.1560/IJPS.59.2-4.159

pp. 159–169

Utilization of reclaimed wastewater for irrigation of field-grown melons by surface and subsurface drip irrigation Mollie Sacksa and Nirit Bernsteinb,* Extension service, Ministry of Agriculture, P.O. Box 30, Beit Dagan 50250, Israel b Institute of Soil, Water and Environmental Sciences, ARO, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel a

(Received May 3, 2010; accepted in revised form June 7, 2010)

Abstract Shortage of water in arid and semiarid areas throughout the world makes utilization of marginal water for agricultural irrigation a necessity. The marginal water most used for irrigation in Israel is secondary-treated urban effluents. In spite of the water treatment process, these waters often contain higher levels of bacterial human pathogens than the potable water from which they were derived. Utilization of the treated effluents for irrigation in Israel is strictly regulated according to the water quality and the irrigated crop. Due to health concerns, and a lack of experimental data, the treated effluents are not yet used for irrigation of vegetables. In the present study we have evaluated safety and agronomic issues involved in irrigation of summer melon with secondary-treated urban effluents, administered to the production field by surface and sub-surface drip irrigation according to the national regulations. Two water qualities were compared, secondary-treated wastewater and potable water. The effluents contained higher levels of EC, pH, Na and Cl, N, P, K, microelements, and heavy metals than the potable water. Potable water was applied by surface drip irrigation, and three irrigation regimes were compared for the treated effluents. These included surface irrigation, and subsurface irrigation at 20 or 40 cm below the soil surface. No differences in yield quantity and quality were found between treatments. Na concentrations and SAR levels of the soil were higher under irrigation with the effluent. Contamination by E. coli, fecal coliforms, and total coliform bacteria were found on the melon peel of all treatments, and the quantity and quality of the contamination did not vary significantly between treatments. E. coli and fecal coliforms were found in the surface 0–2 cm soil samples of treatments irrigated with both water qualities by surface drippers, but no contamination was found in the treatments irrigated by subsurface irrigation. The fact that the microbial contamination of the fruit was not prevented by subsurface drip irrigation or by irrigation with fresh water suggests that environmental factors, rather than an irrigation treatment affect, were the cause for the microbial spread. Further analysis is required concerning effects of environmental factors, such as the interaction between weather conditions and distance from the effluent oxidation ponds on temporal geospatial distribution of the bacterial human pathogens and the potential for subsequent contamination of fresh produce in the field. Keywords: E. coli, effluent, fecal coliforms, irrigation, melon

*Author to whom correspondence should be addressed. E-mail: [email protected] © 2011 Science From Israel / LPPltd., Jerusalem

160 Introduction Due to severe water shortages, maintained and increased agricultural production in many arid and semiarid regions of the world require utilization of marginal water for irrigation. Reclaimed municipal wastewater is the main alternative water source used today in the production fields for agricultural irrigation (Feigin et al., 1991). Although the concentration of human pathogens present in non-treated sewage decrease during the reclamation process (Van der Steen et al., 2000), the secondary treated effluents, which are the effluents most commonly used for irrigation, still contain pathogens that may pose a threat to public health (Armon et al., 1994, 2002; Kinde et al., 1997; Maynard et al., 1999; Bernstein et al., 2008). Therefore, many countries have developed regulations or guidelines for utilization of effluents in agricultural production fields. In Israel, wastewater irrigation is supervised by the Ministry of Health according to purification and chlorination standards set for various agricultural uses (Israeli Ministry of Health, 2001). Due to health concerns, strict recommendations were defined for application of the treated wastewaters for vegetable crops, and treated wastewaters are used today primarily for orchards and field crops cultivation and not for irrigation of vegetables eaten raw. Food and environmental safety issues were the main factors affecting the development of the guidelines for effluent irrigation, and the effect of irrigation with the treated wastewater on the soil and on the plant yield was not addressed in the recommendations. For many crops, including most vegetable crops, agronomic aspects of irrigation with treated effluents are yet to be evaluated. Therefore, the present study aimed to investigate effects of irrigation with high-quality secondary-treated municipal effluents, treated and chlorinated according to the national regulations for non-restricted irrigation of vegetables, on the potential health risks involved with vegetable irrigation and on yield quantity and quality. Summer melons were used as a model system. The plants were grown in a field trial over a three-year period. Two water types were compared, potable water and high-quality secondary-treated wastewater. The secondary-treated wastewater was disinfected by chlorination and distributed by surface drip irrigation or subsurface drip irrigation according to the guidelines for melon irrigation with secondary-treated effluents. Effects of the water type and the irrigation method on yield quantity and quality and on the presence of human pathogen indicators on the produce, in the soil surface, and in the air, along with the effects on soil fertility, were studied. Israel Journal of Plant Sciences 59

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Materials and Methods Plant material and growing conditions Melon (Cucumis melo L. cv Eyal) was evaluated in the study. Twenty-one-day-old plantlets, prepared at the Hishtil nursery (Hishtil, Nehalim, Israel), were planted in mid-June, in two rows per 196-cm-wide bed, at a spacing of 0.4 m within the row. Irrigation was applied via pressure regulated drippers (UniRam; Netafim, Kibutz Hazerim, Israel), one lateral per each row of plants, with a discharge rate of 1.6 L hr–1, at 0.4 m spacing. P2O5 (150 kg/ha) and K2O (1000 kg/ha) were applied pre-plant, and N was applied as CO(NH2)2 +NH4NO3 (200g N ha–1 day–1 during the first month of cultivation and 400 g N ha–1 day–1 during the remaining growing season), in the ratio of 1:1:2 of N–NO3, N–NH4, and N–NH2). The field trial site was located at Kibbutz Revidim, approximately 35 m from the Kibbutz’s oxidation pond and wastewater reservoir. The soil at the experimental site had a clay texture containing 32% sand, 21% silt, and 47% clay. The field was harvested from the end of August through the beginning of September. The experiment was repeated in three consecutive years with similar results. Irrigation treatments The experiment included four irrigation treatments in a random experimental design with five replications. The experimental site accordingly consisted of 20 subplots, each 12 × 5.88 m in size, and consisting of three beds. The applied irrigation treatments were: potable water applied by surface drip irrigation; effluent applied by surface drip irrigation; and effluent applied by subsurface drip irrigation at 20 or 40 cm depth. Subsurface emitters are increasingly used for irrigation of effluents (Adin and Sacks, 1988a,b, 1991; Sacks and Adin, 1990: Oron et al., 1991). The effluents used for irrigation were secondary-treated urban effluents treated at the Hagihon treatment plant (Hagihon Ltd., Jerusalem Water and Wastewater Works Ltd.). Chemical composition of both sources of irrigation water was determined and is detailed in Table 1. According to the Israeli guidelines for crop irrigation with treated effluents, two barriers have to be applied in the production fields for irrigation of melons with secondary-treated effluents (Israel Ministry of Health, 2001). Additionally, the effluents have to be disinfected. At the experimental site the water was disinfected by hypochlorite with the control system (ProMinent Doisetechnik, Heidelberg, Germany), which was set to upgrade the water quality to the required level of 1 ppm chlorine according to the Israeli regulations (Israeli Ministry of Health, 2001).

161 Table 1 Chemical characterization of potable water and secondarytreated municipal effluent utilized for irrigation in the study Parameter pH EC (dS m–1) Cl (mg L–1) Na (mg L–1) Ca +Mg (meq L–1) B (mg L–1) HCO3 (meq L–1) N-NO3 (mg L–1) N-NH4 (mg L–1) P (mg L–1) K (mg L–1)

Potable water

Effluent

7.0–7.3 1.23–1.3 127–231 138–147 5.1–5.5 0.07–0.25 2.6–5.2 0.5–2.3 0 0.04–0.1 0.2–6.4

7.6–7.9 1.7–1.83 323–458 227–241 6.7–9.6 0.18–0.38 5.7–6.4 0.9–8.4 11.2–13.3 3.4–5.2 29.25–33.93

Routine monitoring and sampling procedures The yield was harvested from August 25 until September 10. Average yield per treatment was calculated from five individual replicated plots. Average fruit weight was calculated by dividing the cumulative weight of the fruits per replicated plot by the number of fruits. During each growing season samples of plant tissues, soil, irrigation water, and air in the experimental sight were collected and analyzed for microbiological properties. Fruits were collected for the microbiological analysis during each harvest; the surface soil layer, 0–2 cm, was collected just before the termination of the experiment, and water sources were sampled once a month. Irrigation water and soil samples were also analyzed for their chemical properties. Five replicated plots were analyzed per treatment. Results are presented as average ± standard error, SE. Soils were sampled for mineral analyses from three different layers separately, 0–30, 30–60, and 60–90 cm, from each of the 20 plots. The samples were collected along the irrigation lateral between two drippers. Data are presented as averages ± standard error, SE. The air in the experimental site was sampled for evaluation of E. coli, coliforms, and total aerobic count and with a microbiological air sampler, SAS HiVAC Petri, for. 2000 liters of air were vacuumed onto chromogenic agar plates for each sample. Samples were collected at the plant height near the melon fruits, which were sampled as well for determination of bacterial indicators of human pathogens. Microbiological methods Total aerobics counts: Luria-Bertani agar medium (LB; Difco Laboratories) plates containing 100 µg g–1 cycloheximide (Sigma Chemical Co., St. Louis, MO,

USA) were used for total aerobic bacterial evaluation. Ten-fold dilutions (10 µl drops, with three replicates of each dilution) from the two water sources were plated on Luria broth (LB) agar plates (Hy-labs, Rehovot, Israel) according to the drop method (Hoben and Somesageran, 1982). Plates were incubated for 24–48 h at 30 ºC until small colonies developed. Plates on which 10–100 colonies per replicate (drop) developed, were used to calculate the total aerobic counts. Coliforms and E. coli: Serially-diluted samples (1 ml in saline) from the different monitoring samples were added to Petri plates. Twenty ml of Violet Red Bile (VRB) agar medium (Hy-Lab, Rehovot, Israel), cooled to 40 ºC, was then added to each plate. Agar media were mixed with the saline solutions by gently rotating the plates. Plates were incubated for 24–48 h at 37 ºC until red colonies of coliforms were visible. Eosin Methylene Blue (EMB) agar medium (Atlas, 2006) was used to determine the number of E. coli in samples showing coliforms on the VRB medium. Plates were incubated for 24–48 h at 37 ºC until colonies showing a deep purple with the coppery, metalic sheen of E. coli were visible. Fecal coliforms were analyzed following Paille et al., 1987. For detection and quantification of the microorganisms on the melon fruits, three fruits from each plot were aseptically excised and combined for one sample. Two 4 mm2 samples that included the peel were taken from the periphery of each fruit. Samples from the three sub-sampled fruits per plot were combined for one replicate. The fruits were sampled three times during the summer months. The sampled tissue was aseptically cut into small sections, placed in a pre-weighed plastic stomacher bag, and the plant tissue fresh biomass was determined. Aseptic methods included replacement of gloves between each sample, and disinfection of scissors by flaming with ethanol (Bernstein et al., 2007a). Twenty ml of buffered peptone water (BPW; Becton Dickinson, Sparks, Md.) were added and bags and their content was pummeled twice for 1 min each by a stomacher (Lab-Blender-400, Seward Laboratory, England). In addition, the plant tissue was further mashed manually, by pressing with a thumb through the bag. A sample of the obtained suspension, and of 10-fold dilutions, was used to enumerate the bacteria. Detection of microorganisms in the soil was performed according to (Bernstein et al., 2008). E. coli in air: Coliform Chromogenic agar (Hy Labs. LTD, Rehovot, Israel) was used to monitor total coliform and E. coli numbers in air. Plates were incubated for 24–48 h at 37 ºC until red colonies of coliforms and purple colonies of E. coli were formed. The selective plates were then incubated at 37 ºC for 24 h, before Sacks and Bernstein / Irrigation of melon with treated effluent

162 counting. Tryptic soy agar plates were used for sampling total bacteria and fungi in 100 L of air per sample, and then incubated at 30 ºC for 24 h, before counting. Chemical analysis EC and pH of the soil (as saturated paste extract) and the irrigation solution was determined with conductivity and pH meters, respectively. The soil was analyzed for nitrate, following extraction in 1 M KCl at a 1:5 soil-to-solution ratio. Sodium was determined in the extract of saturated paste by flame photometery and Ca and Mg were analyzed with an atomic absorption spectrophotometer. Sodium adsorption ratio (SAR) was calculated from the soil saturation extracts data (Munshower, 1994). Sodium and K in the irrigation water sources were analyzed by flame photometery; Ca and Mg with an atomic absorption spectrophotometer, N–NH4, N–NO3, P, and Cl with an autoanalyzer (Lachat Instruments, Milwaukee, WI); and Mn, Cu, Zn, Fe and B by inductively coupled plasma atomic emission spectrometry (ICP-AES, Spectro, Kleve, Germany). Statistics Results of yield parameters, soil, air water and plant microbiological composition or chemical parameters are expressed as means ± standard errors (SE), n = 5. The results were subjected to statistical testing with the JMP software package, version 5 (SAS, 2002, Cary, NC). Results Three years of field data were collected and analyzed. Table 1 lists chemical properties of the irrigation waters. Typical of secondary-treated effluents (Feigin et al., 1991) the reclaimed water used for irrigation in the present study contained significantly higher levels of plant nutrients such as K, P, N, and Ca+Mg, as well as higher levels of the damaging ions Cl, Na, and HCO3–, compared to the potable water (Table 1). Boron concentration and the levels of EC and pH of the effluents were higher compared to the potable water, but the increase was small, defining the effluents as a good quality effluent for agronomic use (Feigin et al., 1991). Plant appearance and yield parameters The overall appearance of the plants was monitored regularly, with special attention to mineral deficiency or toxicity symptoms. Throughout the three years of the project, plants from both treatments looked similar. Irrigation with treated effluents and the form of application of the effluents in the field i.e., by surface or subsurface irrigation, had no significant effect on yield quantity and fruit size (Fig. 1). The average yield was Israel Journal of Plant Sciences 59

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3.5 ton per dunam and the average weight was 1.8 kg per melon. Chemical composition of the soil The higher Na concentrations in the treated effluent used for irrigation, compared to the potable water (Table 1), were reflected in higher Na concentrations in the soil (Table 2). Under irrigation with potable water, in the three soil layers evaluated (e.g., 0–20, 20–40 and 40–60 cm) the concentration of Na was below 300 mg Na L–1, while under irrigation with effluents Na concentrations was higher than 300 mg Na L–1 in the soil layer where the treated effluents were administrated (e.g., at 0–20 cm for the surface drip irrigation, at 0–40 cm for the subsurface drip at 20 cm depth, and at 20–60 cm for the subsurface drip at 20 cm depth) and sometimes in adjacent soil layers as well (Table 2). As a consequence, the calculated sodium adsorption ratio (SAR) in the 0–60 cm soil layer was higher under the effluent irrigation treatments compared to the potable water treatment (Table 3). In addition to the effects of effluent irrigation on Na, overall salinity—measured as electrical conductivity, EC—of the saturated soil paste extract was increased as well by effluent irrigation (Table 3). The irrigation treatments did not affect soil pH (Table 3). Nitrate concentration in the soil was also not affected by effluent irrigation, and its concentration was similar throughout the soil profile under the two surface irrigation treatments. Subsurface application of the effluent increased N–NO3– concentration in the soil layers to which the effluent was applied (Table 2). Microbiological parameters The secondary-treated municipal effluent used for irrigation contained a higher bacterial load than the potable water (Table 4). Total aerobic counts in the effluent prior to chlorination was higher by two orders of magnitude compared to the potable water. Chlorination was efficient in reducing the total aerobic bacterial count from 295,000 CFU 100 ml–1 to 3100 CFU 100 ml–1 a concentration similar to the potable water (Table 4). E. coli, fecal coliforms, and total coliform concentrations also were higher in the effluent water, compared to the potable water, prior to chlorination of the effluent. As with the potable water, after chlorination neither E. coli nor fecal coliforms were detected in the effluent even following an enrichment test, and total coliform concentration was greatly reduced (Table 4). The total and fecal coliform counts in the effluent prior to chlorination (Table 4) were higher than the level permitted for unlimited use for irrigation of vegetables eaten raw (10 CFU (colony forming units) fecal coliforms per 100 ml; Halperin, 2002) thus necessitating chlorination.

163

Fig. 1. Effect of the irrigation treatments on cumulative melon yield, A, and average fruit weight, B. The plants were irrigated with potable water (Potable) by surface drip irrigation, and with secondary treated effluent by surface drip irrigation (Effluent), and subsurface drip irrigation at 20 cm (Effluent 20) and 40 cm (Effluent 40) below ground. Data are average ± SE (n = 5). Means followed by a similar lower-case letter are not significantly different according to Tukey’s test at p = 0.05.

Table 2 Effect of the irrigation treatments on chemical characteristics of the soil in the experimental plot, at three soil layers. The plants were irrigated with secondary-treated urban effluent (Effluent) of with potable water. Potable water were administrated to the field by surface drip irrigation, and effluent by surface drip irrigation or subsurface drip irrigation at 20 or 40 cm depth. Data are average of 5 replicates. Within the row, means followed by a different lower-case letter are significantly different according to Tukey’s test at p = 0.05

Surface drip irrigation

Effluent

Subsurface drip irrigation at 20 cm

Subsurface drip irrigation at 40 cm

Potable water Surface drip irrigation

Soil layer (cm)

0–20

20–40 40–60

0–20 20–40 40–60

0–20 20–40 40–60

0–20 20–40 40–60

pH EC (dS m–1) Na (mg Kg–1) NO3 (mg Kg–1)

7.8 a 7.9 a 7.9 a 2.23 a 1.31 b 1.09 b 332 a 233 b 233 b 131 a 36 bc 21c

7.8 a 7.8 a 8 a 2.39 a 1.92a 1.12b 370 a 328 a 264 b 64 b 96 ab 15 c

7.8 a 7.8 a 7.9 a 7.7 a 2.02 a 1.96 a 1.32 ab 1.58 ab 257 b 300 a 306 a 276 b 125 a 118 a 58 b 134 a

7.9 a 8 a 1.2 b 1.0 b 212b 219b 48b 20c

Sacks and Bernstein / Irrigation of melon with treated effluent

164 Table 3 Effects of the irrigation treatments on SAR at two different soil layers. Data are averages of five replicated measurements. Means within each row followed by different lower case letters, are significantly different according to the Tukey test (p < 0.05) Treatment

Effluent surface drip irrigation

Effluent subsurface drip irrigation at 20 cm


Potable water surface drip irrigation

0–30 cm 30–60 cm

4.68 a 4.93 a

4.03 b 4.43 ab

4.24 ab 4.50 a

3.40 c 3.75 c

Soil depth (cm)

Table 4 Microbiological characterization of the potable water and secondary-treated municipal effluent utilized for irrigation in the study. Data are means ± SE (n = 5). Means within each row followed by a different lower-case letter, are significantly different according to Tukey’s test (p < 0.05) Microorganism Potable water (CFU 100 ml–1)

Before chlorination

ND* ND ND 5700 b

E. coli Fecal coliforms Coliforms Total aerobic bacteria count

Effluent

6.7 100 900 a 295,000 a

After chlorination ND ND 21 b 3121 b

*ND, not detected after an enrichment test in any of the replicates.

Table 5 Concentration of E.coli, coliforms, and total bacteria and fungi cells in the air above the plants and 100 m away from the experimental site Location 100 m from the field CFU/1000 liters

E. coli Coliforms Total bacteria & fungi count

0 b 4 a 350 a

Potable water surface drip irrigation



0 b 7 a 450 a

1 a 9 a 405 a

0b 6a 360 a

In order to test whether irrigation with the treated effluent contaminated the air at and around the experimental field, air samples were taken at the plant height in the plots of the various treatments, as well as 100 m away from the field (Table 5). The samples were analyzed for the presence of E. coli, coliforms, and total bacteria and fungi count. The concentration of all the microbiological parameters tested, i.e., E. coli, coliforms, and total bacteria and fungi count, were similar at the locations sampled (Table 5). At the end of the summer, the top 2-cm soil layer was sampled as well for bacteriological analysis. E. coli Israel Journal of Plant Sciences 59

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and fecal coliforms were found in soil from treatments irrigated with surface drippers, by both potable water and treated effluent (Table 7). The soil surface was dry in the plots irrigated by subsurface drippers and no coliforms were found in the surface 0–2 cm soil layer of these treatments. Bacteriological assessment of melon fruit peels revealed that the quality of the irrigation water and the method of application of the effluent in the field did not affect the contamination of the melons (Table 6). E. coli was found on nearly 25% of the samples and fecal coliforms were present on 38% of the samples.

165 Table 6 Analysis of variance between frequencies of melon contamination. The tissue analyzed contained the peel of the fruits. Numbers in parentheses are standard error of mean




Freshwater surface drip irrigation

Prob > f

incidence of E. coli

0.208 (0.109)

0.236 (0.082)

0.319 (0.150)

0.264 (0.102)

f = 0.9118

incidence of fecal coliforms

0.319 (0.131)

0.514 (0.514)

0.458 (0.169)

0.265 (0.102)

f = 0.5547

incidence of coliforms

0.639 (0.174)

0.444 (0.148)

0.347 (0.178)

0.375 (0.161)

f = 0.6045

Analysis of variance all pairs Tukey-Kramer (n = 5).

Table 7 Number of microorganisms in the top soil layer, 0–2 cm, at the experimental site. Data are means ± SE (n = 5). ND = not detected (