Implications of municipal wastewater irrigation on soil health from a ...

SoilUse and Management doi: 10.1111/sum.12056

Soil Use and Management, September 2013, 29, 384–396

Implications of municipal wastewater irrigation on soil health from a study in Bangladesh M. A. M O J I D 1 & G. C. L. W Y S E U R E 2 1

Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh, and Division of Soil and Water Management, Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, 3001 Leuven (Heverlee), Belgium 2

Abstract This study evaluated soil health in fields of wheat (Triticum aestivum L. cv Shatabdi) and potatoes (Solanum tuberosum L.) irrigated by different blends of municipal wastewater (hereafter called wastewater). The crops were grown with and without added fertilizers over three consecutive years. The wastewater contained high concentrations of organic carbon (C), nitrogen (N), phosphorus (P), sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), zinc (Zn) and boron (B). It also contained negligible concentrations of a few heavy metals. Irrigation by wastewater resulted in an increase in the porosity of the surface soil and thus a reduced bulk density. Wastewater enhanced the saturated hydraulic conductivity and water retention capacity of the soils. The organic carbon, total N, available P and S, and exchangeable Na, K, Ca and Mg of the soils increased proportionately with the quantity of applied wastewater. C, N and K increased significantly (a = 0.05) when fields were irrigated using raw wastewater after applied fertilizers; the other elements accumulated in the soil insignificantly under both fertility levels. Electrical conductivity (EC) and pH of the upper 0–20 and 20–40 cm soil layers increased with the application of wastewater; the increase was significant only under raw wastewater irrigation. In the 40–60 cm soil layer, both EC and pH remained unchanged. The applied inorganic fertilizers raised EC but reduced soil pH. The wastewater contained large counts of total coliform (TC: 17.2 9 106 cfu/100 mL) and faecal coliform (FC: 13.4 9 103 cfu/ 100 mL). Irrigation using municipal wastewater is proposed for improving soil fertility as well as for alleviating water scarcity with the exception of some crops whose edible parts come in direct contact with wastewater and/or are eaten uncooked.

Keywords: Municipal wastewater, irrigation, fertilizer, soil health

Introduction The use of municipal wastewater for irrigation can make an important contribution to optimizing water use in periurban areas where water is scarce (Virto et al., 2006; Pedrero et al., 2010). It is also an attractive option, not only for its disposal but also for improving crop yield, physical properties and soil fertility (Mathan, 1994; Siebe, 1998). However, inappropriate use of wastewater may adversely affect crop production and soil health because it generally contains high concentrations of suspended and dissolved solids of organic and inorganic origins. The solids Correspondence: G. C. L. Wyseure. E-mail: [email protected] Received July 2012; accepted after revision May 2013

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accumulate over time in the soil pores, especially in the crop root zone, with possible harmful impacts on soil health and crop yield. Prolonged use of saline and sodiumrich wastewater causes soil structural degradation and loss of productivity. Apart from accumulation in the soil profile, leaching of dissolved chemicals below the root zone may cause groundwater pollution (Bond, 1999). Thus, with respect to the use of wastewater, concerns are often expressed about degradation of soil structure, accumulation of heavy metals and health risks of irrigators and food consumers. However, when studying actual practices in poorer countries, a more complex scenario is apparent. With respect to food contamination, Ensink et al. (2007) found that in Pakistan, unhygienic postharvest handling was a more severe problem than contamination by wastewater irrigation. Smits et al. (2009) report a

© 2013 The Authors. Journal compilation © 2013 British Society of Soil Science

Soil health under municipal wastewater irrigation

participatory study with stakeholders around Rajshahi, a major town in western Bangladesh, and another in Sri Lanka. They state that the concentration of nutrients, pathogens or other contaminants in wastewater was not as severe as originally thought and that the wastewater with other factors had only a limited impact on yields. Results on soil health and structure are contradictory. Soil porosity and hence soil hydrological properties are sensitive to chemical compounds in wastewater. Several studies (Singh & Bahadur, 1998) report reduced porosity, increased bulk density and pores becoming smaller (Coppola et al., 2004) because of the accumulation of organic matter from wastewater (Tarenitzky et al., 1999). There are contradictory results on the impacts of wastewater on soil water retention, with both increases (Tarenitzky et al., 1999) and decreases in water retention reported (Coppola et al., 2004). Raw/ untreated wastewater reduced infiltration (Xantholagis & Wellender, 1991) and soil hydraulic conductivity (Vinten et al., 1991; Vandevivere & Baveye, 1992; Singh & Bahadur, 1998; Tarchitzky et al., 1999; Magesan et al., 2000; Coppola et al., 2004) owing to clogging of pores with biomass and suspended solids. Excessive growth of soil microfauna induced by high C/N ratios in wastewater could cause soil clogging (Magesan et al., 2000). It has been shown that treated water of marginal quality improved infiltration (Ashraf et al., 2004), reduced soil pH (Shahalam et al., 1998; Mohammad & Mazahreh, 2003; Ashraf et al., 2004), elevated pH (Khai et al., 2008) and also caused an inconsistent change (Rusan et al., 2007; Kiziloglu et al., 2008). Raw wastewater increased soil salinity (Mohammad & Mazahreh, 2003; Rusan et al., 2007; Khai et al., 2008; Kiziloglu et al., 2008), but treated wastewater reduced it (Ashraf et al., 2004). Several investigators report increased organic C, N and P and exchangeable Na, K, Ca and Mg in wastewater-irrigated soils (Heidarpour et al., 2007; Rusan et al., 2007; Kiziloglu et al., 2008. The impacts of wastewater on soil depend on a number of factors, such as the sources and quality of the wastewater, crops to be irrigated and soil characteristics. Some case-specific positive and negative aspects of irrigation by wastewater are always associated with irrigated soils. It is therefore necessary to evaluate the impacts of wastewater on soil health before planning wastewater irrigation in the long-term. The aims of this study were to (i) evaluate some important physicochemical properties of soils in wheat and potato fields irrigated by municipal wastewater and (ii) quantify the nutritional and toxic effects of wastewater on soils.

Methodology The study aimed to determine the impacts of municipal wastewater on growth and yield of wheat and potatoes, and to evaluate the implications of wastewater irrigation on soils

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for field crops. Wheat and potatoes were selected as (i) they are widely cultivated in the study area under irrigation, (ii) their irrigation requirements are similar, and, most importantly, (iii) the edible part (grain) of wheat does not come in contact with irrigation water while that of potato comes in direct contact with irrigation water. The last point enabled an evaluation of the dietary risk from eating the edible parts of the crops. This study focuses on soil quality as affected by wastewater irrigation.

Experimental site and climate The study was over three consecutive years (November - March of 2007–2008, 2008–2009 and 2009–2010) at the experimental field of Bangladesh Agricultural University at Mymensingh (24.75o N and 90.50o E longitude) in Bangladesh. The silt loam underlain by a sandy loam is a consequence of the old Brahmaputra flood plain (BARC, 2005). The climate is subtropical with an average annual rainfall of 2420 mm which is concentrated from May to September. Only 131.3 mm of rainfall occurred in three events during the experimental period in 2007–2008. The crops were protected by temporary plastic covers over bamboo frames during rainfall. There was no rainfall during the experiment in subsequent years.

Wastewater collection and analysis Samples of municipal wastewater were collected once a month during the wheat and potato growing seasons (November–March) of 2007–2010 at 1.5 km downstream from the outlet of the main sewage system of Mymensingh town. In the fields around this open drain, farmers irrigated their crops, mainly rice, using the same wastewater with small motor pumps. During the irrigation season, several small earth dams were erected in the drain for pumping water. Water hyacinths grew vigorously, and people even fished in the polluted water of the drain. Samples of freshwater were collected from an existing borehole in the experimental field that extracted groundwater from 100 m depth. The wastewater originated from households, restaurants, educational institutes, offices and hospitals; there was no major industrial source. Major chemical properties of the water samples were measured in the laboratory at a constant temperature of 25 1 °C following the required protocols prescribed for each quality parameter. The pH of the water samples was measured with a glass electrode pH meter, and EC was measured with a digital EC meter. TDS was measured by evaporating an aliquot of filtered water to dryness and weighing the solid residues following the method outlined by Chopra & Kanwar (1980). The dissolved oxygen (DO) of the samples was measured by a DO meter (modelJENWAY 9015) as outlined by APHA (1995). Biological oxygen demand (BOD) and chemical oxygen demand (COD) were measured, also following the method of APHA (1995).

© 2013 The Authors. Journal compilation © 2013 British Society of Soil Science, Soil Use and Management, 29, 384–396

386 M. A. Mojid et al. Ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), iron (Fe), manganese (Mn), Zn and B were measured with a colorimeter (model: DR/890, Hach, USA) following the standard methods suggested by APHA (1995). Total N was measured using the Kjeldhal method by which N-containing organic compounds were digested in sulphuric acid in the presence of potassium sulphate and a copper sulphate catalyst. Total P was measured with the colorimeter using ammonium molybdate/ stannus chloride as a reducing agent. Concentrations of soluble Ca and Mg were measured using the EDTA titration method, and K and Na were measured by flame photometry. Carbonate and bicarbonate (CO23 and HCO3 ) were determined by titration with 0.02N HCl using phenolphthalein and methyl orange indicators, respectively. Total coliform was measured using the viable count method. One millilitre dilution was inoculated in petri dishes with agar and subsequently incubated at 45–48 °C for 24–48 h. Dark red colonies measuring 0.5 mm or greater with a precipitation halo were counted and reported as cfu/100 mL. Faecal coliform was measured using the same viable count method as for TC with the exception that the incubation temperature was 44.5 °C.

Wheat and potato cultivation The experiments, both for wheat and potatoes, consisted of two factors: irrigation water quality as the main factor with five treatments/levels and fertilizer dose as the sub factor with two treatments/levels. The irrigation treatments were - I1: freshwater (groundwater extracted with a borehole tubewell) as control, I2 - I4: blended wastewaters (with the fraction of wastewater 0.25, 0.50 and 0.75) and I5: raw/untreated wastewater. The fertility treatments were - F0: no fertilizer and F1: recommended dose. The experimental land after ploughing and harrowing was divided into three equal blocks, each block into five sub-blocks and each sub-block into two unit plots of 3 9 2 m size. There was a 1.5-m buffer zone between the adjacent blocks, 1.2-m between the sub-blocks and 0.8-m between the unit plots to minimize interference effects of the factors and treatments among the plots. The experiment was set in a split-plot design with three replications of the treatments. The irrigation treatments were distributed to the sub-blocks and fertilizer treatments to the unit plots. The same plots on the same field layout were used for the same treatments and replications in the 3 yr of the experiment. Recommended applications of major fertilizers for wheat and potatoes in Bangladesh (BARC, 2005) were applied in the form of urea, triple super phosphate, muriate of potash, gypsum, zinc sulphate and boric acid. The corresponding fertilizers were at rates of 260, 160, 110, 100, 3 and 1 kg/ha for wheat and at 350, 228, 264, 100, 8 and 6 kg/ha for potatoes. Wheat was grown from the last week of November to the third week of March, while potatoes were grown

during the third week of December to the first week of March. Wastewater was collected and carried to the experimental field in plastic barrels and blended with freshwater in a polyethylene-sealed pit in compliance with the treatments. The irrigation requirement of the crops was calculated based on average root zone depth (60 cm for wheat and 45 cm for potatoes), soil water content and field capacity. Three irrigations totalling 12.5, 13.5 and 13.25 cm in the first, second and third year, respectively, were applied to wheat in the check basin. For potatoes, 13.0 cm irrigation in the first year and 13.5 cm both in the second and third years were applied in furrows. For controlling water management, the crops were protected from rainfall (13.13 cm) that occurred during the first year.

Soil sampling and analysis Soil sampling was carried out four times: before setting up the experiments in the first year and at harvest in each year. The initial soil samples, three at each location, were collected from six representative locations of the experimental field with an auger at 20 cm increments to a depth of 60 cm. Homogeneous composite samples were prepared by mixing the samples of the same depths of the six locations. Nine undisturbed samples from three representative locations, three samples from each location at 0–5, 20–25 and 40–45 cm depth, were collected in 5 9 5 cm core samplers to determine bulk density. At harvest, soil samples were collected at three points in each plot at 20 cm increments to a depth of 60 cm in the wheat field and 15 cm increment to a depth of 45 cm in the potato field. Composite samples were prepared by thoroughly mixing the samples from the same depth in each plot. After removal of grasses, roots and other debris, samples were air-dried in the laboratory and sieved at 2-mm. Approximately 500 g of each sample was sealed in polyethylene bags and stored for subsequent chemical analysis. Undisturbed soil samples, six from each of the freshwater- and raw wastewater-irrigated wheat plots with no added fertilizers in three replications (6 9 3 9 2 = 36 samples), were also collected from topsoil (0–5 cm) in core samplers. The undisturbed samples were collected at harvest in the first year only and were used to determine porosity, bulk density and hydraulic properties. From measures of the sand, silt and clay contents of the initial soil samples by the hydrometer method, textural classes were determined from Marshall’s triangle following the USDA system. Saturated hydraulic conductivity of 18 undisturbed soil samples at harvest (three from each of freshwater- and wastewater-irrigated plots in three replications) was determined by the constant head method. Then, these samples as cores were immersed slowly in water from the bottom and kept for 48 h in a vacuum desiccator under 50-cm suction to achieve maximum saturation. The



saturated samples were weighed followed by drying in an oven at 105 °C for 24 h. The porosity of the samples was represented by their water contents at saturation. The bulk density was determined from the oven dry samples and their volumes; the bulk densities of the nine initial undisturbed samples were also determined after drying in the oven. Soil water retention data were measured on the remaining 18 undisturbed samples using a sand box (at lower suction, ≤100 cm of water) and a pressure plate apparatus (at higher suctions >100 cm of water). Organic C, total N, available P, S, Zn and B and exchangeable Na, K, Ca and Mg of the stored soil samples were determined following standard methods. EC and pH were measured in a saturation extract (air-dry soil to water ratio of 1:2.5 at 25 °C) using an EC/pH meter. The major physicochemical properties of different soil layers prior to start of the experiments are given in Table 1.

Results Soil chemical properties Organic C and total N. Organic C and total N in the soil root zone for wheat and potatoes under wastewater irrigation increased with time. The increase in C and N correlated with the quantity of applied wastewater. After 3 yr of cropping, the average C in raw wastewater-irrigated wheat plots increased from 0.63 to 0.74% with no applied fertilizers (I5F0) and 0.63 to 0.76% with fertilizers (I5F1). In potato plots, the increase in C under the corresponding fertilizer levels rose, respectively, from 0.63 to 0.69% and 0.63 to 0.78%. The average N increased from 0.056 to 0.065% under the no-fertilizer condition and 0.056 to 0.068% with applied fertilizers in wheat plots. In the potato plots, the increase in N under respective treatments was from 0.56 to 0.70% and 0.056 to 0.076%. The increases in C and N owing to wastewater irrigation were insignificant (a = 0.05) on fields with fertilizers but significant for fields with added fertilizers. In the wheat field, ≥75% wastewater in the applied irrigation raised C and N significantly. In the potato field, a significant increase occurred in C owing to raw wastewater irrigation and in N with ≥75% wastewater in the applied irrigation. The total N contents of the soils under I3, I4 and I5 for the wheat field, and under I4 and I5 for the potato field were identical. The accumulation of C and N was the most notable only in the topsoil layer (Figure 1).

Phosphorus, potassium and sulphur. Wastewater application led to increases in P, K and S in the irrigated soils irrespective of fertilizer levels. These nutrients increased consistently over time with applied wastewater (Tables 2 and 3). There seemed to be an increase in P, but this was insignificant in the topsoil layer under both fertility levels (Figure 2). Although K increased significantly under raw wastewater irrigation (I5), it

387

Table 1 Mean values of soil properties prior to sowing for the experimental wheat and potato fields Soil depth (cm) Properties Sand (%) Silt (%) Clay (%) Textural class Bulk density (g/cm3) pH (soil: water = 1:2.5) Electrical conductivity (dS/m) (soil: water = 1:2.5) Organic carbon (%) Total nitrogen (%) Available phosphorus (mg/kg) Exchangeable potassium (me per 100 g soil) Available sulphur (mg/kg) Exchangeable sodium (mg/kg) Exchangeable calcium (mg/kg)

0–20

20–40

40–60

32.58 56.66 10.76 Silt loam 1.26

54.57 40.00 5.43 Sandy loam 1.35

67.91 26.67 5.42 Sandy loam 1.40

6.89 0.12

7.14 0.058

7.18 0.042

0.625 0.056 4.01

0.341 0.029 2.24

0.278 0.023 1.92

0.118

0.081

0.083

6.72 9.41

4.87 7.06

4.05 8.73

742

804

526

decreased under freshwater irrigation (I1) over time owing to uptake by the crops. In general, ≥50% wastewater in the applied irrigation in wheat field and only raw wastewater in potato field significantly increased K. Figure 3 shows a higher accumulation of K in the top and bottom soil layers than in the mid one. The S content of the soil increased, but insignificantly, under wastewater irrigation because of contributions from ZnSO4 and wastewater.

Sodium, calcium and magnesium. Na, Ca and Mg in the wastewater boosted these elements in the irrigated soils (Tables 2 and 3); the trends in their increases were comparable for the 3 yr of cropping. The increases in Na, Ca and Mg were insignificant in the wheat field. The accumulation of Na was more pronounced than Ca and Mg in the potato field. In general, ≥75% wastewater in the applied irrigation significantly raised the soil Na content.

Zinc and Boron. Zn and B in the irrigated soils increased according to the amount of applied wastewater. These elements increased by 3.52 and 13.8%, respectively, in the raw wastewater-irrigated 0–15 cm soil layer in the potato field over 3 yr. Zn increased significantly in the soil with


388 M. A. Mojid et al.

1.0

0–20 cm

20–40 cm

40–60 cm

0.10

0.6 0.4 0.2

0.06 0.04

0.00 F0

F1

F0

Fresh water

F0

F1

Waste water 0.10

0–15 cm

15–30 cm

F1

F0

Fresh water

30–45 cm

0–15 cm

F1

Waste water

15–30 cm

30–45 cm

0.08 Tot-N (%)

0.8 Org-C (%)

20–40 cm 40–60 cm

0.02

0.0

1.0

0–20 cm

0.08 Tot-N (%)

Org-C (%)

0.8

0.6 0.4

0.06 0.04 0.02

0.2

0.00

0.0 F1 F0 Fresh water

F0 F1 Fresh water

F0 F1 Waste water

F0 F1 Waste water

Figure 1 Variation in organic C and total N at different depths owing to irrigation with freshwater and wastewater under nonfertilized (F0) and fertilized (F1) treatments in wheat (top) and potato (bottom) fields at the third year crop harvest.

Table 2 Interaction effects of irrigation water quality and two fertilizer treatments over three wheat seasons for 0–20 cm Treatment I1F0 I1F1 I2F0 I2F1 I3F0 I3F1 I4F0 I4F1 I5F0 I5F1 HSD0.05

C (%)

N (%)

P (ppm)

K (meq/100 g)

S (ppm)

Na (ppm)

Ca (ppm)

Mg (ppm)

0.630bc 0.569c 0.648abc 0.655abc 0.672abc 0.702ab 0.701ab 0.713ab 0.744ab 0.757a 0.1217

0.056ab 0.051b 0.057ab 0.059ab 0.059ab 0.060ab 0.062ab 0.065a 0.065a 0.068a 0.0117

3.697e 4.964abcd 3.744e 5.317abc 4.148de 5.404ab 4.254cde 5.476a 4.341bcde 5.626a 1.1155

0.104d 0.111 cd 0.113bcd 0.123abc 0.114abcd 0.124ab 0.117abc 0.124ab 0.119abc 0.126a 0.0123

6.703b 7.537ab 7.049ab 8.424ab 7.104ab 8.156ab 7.402ab 9.643a 7.377ab 8.779ab 2.5992

143.1a 141.4a 142.4a 139.0a 152.1a 147.5a 149.5a 141.3a 145.0a 157.0a 23.046

757.1a 796.3a 815.9a 801.5a 815.4a 878.1a 922.8a 931.8a 891.2a 889.9a 222.86

126.8a 136.8a 142.0a 125.2a 141.6a 130.8a 148.2a 147.3a 122.7a 125.5a 51.46

Common letter(s) within the same column do not differ significantly at 5% level of significance analyzed by Tukey.

fertilizers compared to soil without these. With freshwater irrigation, B content was greater, although insignificant in the soil without added fertilizers than in the one with such additions. With wastewater irrigation, although Zn increased by 13.29% and B by 3.73% in the soil with applied fertilizers, their contributions to the soil were insignificant. Figure 4 shows a noticeable difference in Zn and B contents of the wastewater-irrigated 0–15 cm soil layer in the potato

field between the two fertility levels. Both water quality and fertility treatments had variable effects on Zn and B contents in the lower soil layers.

pH. Wastewater owing to its Na, Ca and Mg content raised soil pH, while the inorganic fertilizers reduced it (Tables 4 and 5); similar results are also reported by Palma



389

Table 3 Interaction effects on concentrations in the top 0–15 cm soil layer under five irrigation water quality and two fertilizer treatments over three potato seasons Treatment

C (%)

N (%)

P (ppm)

K (meq/100 g)

S (ppm)

Na (ppm)

Ca (ppm)

Mg (ppm)

I1F0 I1F1 I2F0 I2F1 I3F0 I3F1 I4F0 I4F1 I5F0 I5F1 HSD0.05

0.596b 0.578b 0.630b 0.594b 0.658ab 0.664ab 0.658ab 0.712ab 0.694ab 0.777a 0.1333

0.059bcd 0.056d 0.062bcd 0.058 cd 0.065abcd 0.066abcd 0.068abcd 0.071ab 0.070abc 0.076a 0.0134

3.746ab 4.368ab 3.937ab 3.176ab 2.743b 4.578ab 4.413ab 5.161a 4.492ab 5.203a 2.0375

0.110d 0.127bcd 0.118 cd 0.136abc 0.123bcd 0.140abc 0.126bcd 0.145ab 0.137abc 0.153a 0.0226

5.950c 6.777abc 6.124bc 6.932abc 6.226bc 7.180ab 6.677abc 7.513a 6.800abc 7.710a 1.2140

124.00e 132.64de 133.50de 138.78bcde 141.89abcde 134.82cde 155.07abc 148.47abcd 156.26abc 161.72a 21.41

724.29a 761.06a 748.74a 774.42a 753.17a 806.71a 783.98a 811.46a 801.02a 820.82a 132.83

0.940b 1.031ab 0.992ab 1.103ab 1.137ab 1.096ab 1.113ab 1.296ab 1.231ab 1.367a 0.421


(a)

8

0–20 cm

20–40 cm

(b)

40–60 cm

6

0–15 cm

15–30 cm

30–45 cm

P (mg/kg)

P (mg/kg)

6

4

4

2

2

0

F0 F1 Fresh water

0

F0 F1 Waste water

F0 F1 Fresh water

F0 F1 Waste water

Figure 2 Variation in accumulated P at different depths owing to the effects of irrigation by freshwater and wastewater under nonfertilized (F0) and fertilized (F1) treatments for wheat (a) and potatoes (b) fields at the third year crop harvest.

0.15

0–20 cm

20–40 cm

40–60 cm

0.12 0.09 0.06 0.03 0.00

(b)

K (meq/100 g soil)

K (meq/100 g soil)

(a)

F0

F1

Fresh water

F0

F1

Waste water

0.18

0–15 cm

15–30 cm

F0

F1

30–45 cm

0.12

0.06

0.00

Fresh water

F0

F1

Waste water

Figure 3 Variation in K accumulation at different depths owing to the effects of irrigation by freshwater and wastewater under nonfertilized (F0) and fertilized (F1) treatments for wheat (a) and potato (b) fields at the third year crop harvest.

et al. (2005). In the wheat field, pH increased significantly in the 0–20 cm soil layer with no added fertilizers; in the 20–40 cm layer, only the raw wastewater raised pH

significantly. In the potato field, a significant increase in soil pH occurred only under raw wastewater irrigation in the topsoil layer.


390 M. A. Mojid et al.

(a) 1.2

0–15 cm

15–30 cm 30–45 cm

(b) 0.4

15–30 cm 30–45 cm

0.3 B (mg/kg)

Zn (mg/kg)

0.9

0–15 cm

0.6

0.2

0.1

0.3

0.0

0.0 F0 F1 Fresh water

F0 F1 Waste water

F0 F1 Fresh water

F0 F1 Waste water

Figure 4 Variation in Zn and B at different depths as a function of irrigation water quality (freshwater and wastewater) and fertilizer application rate (nonfertilized and fertilized) in the potato field at the third year crop harvest.

Table 4 Interaction effects on pH and electrical conductivity (EC) in the 0–20, 20–40 and 40–60 cm soil layers under five irrigation water quality and two fertilizer treatments over three wheat seasons pH of soil layer (cm) Treatment I1F0 I1F1 I2F0 I2F1 I3F0 I3F1 I4F0 I4F1 I5F0 I5F1 HSD0.05

EC (dS/m) of soil layer (cm)

0–20

20–40

40–60

0–20

20–40

40–60

7.00bcd 6.9d 7.08abcd 6.97 cd 7.18abc 7.04abcd 7.23ab 7.07abcd 7.26a 7.11abcd 0.2452

7.16bc 7.12c 7.25abc 7.15bc 7.27abc 7.23abc 7.28ab 7.26abc 7.33a 7.27abc 0.1528

7.27a 7.24a 7.30a 7.26a 7.30a 7.34a 7.35a 7.36a 7.36a 7.32a 0.1613

0.1087c 0.1109bc 0.1168bc 0.1325abc 0.1317abc 0.1506abc 0.1395abc 0.1622ab 0.1438abc 0.1687a 0.0516

0.0524b 0.0521b 0.0537b 0.0588ab 0.0591ab 0.0578ab 0.0681ab 0.0668ab 0.0645ab 0.0812a 0.0272

0.0362a 0.0399a 0.0361a 0.0409a 0.0392a 0.0400a 0.0405a 0.0425a 0.0440a 0.0464a 0.0127


Table 5 Interaction effects on pH and electrical conductivity (EC) in the 0–15, 15–30 and 30–45 cm soil layers under five irrigation water quality and two fertilizer treatments over three potato seasons pH of soil layer (cm) Treatment I1F0 I1F1 I2F0 I2F1 I3F0 I3F1 I4F0 I4F1 I5F0 I5F1 HSD0.05

EC (dS/m) of soil layer (cm)

0–15

15–30

30–45

0–15

15–30

30–45

7.11ab 6.99b 7.21ab 7.18ab 7.30ab 7.19ab 7.32a 7.26ab 7.40a 7.31a 0.3146

7.01a 6.94a 7.16a 7.11a 7.23a 7.15a 7.26a 7.22a 7.28a 7.21a 0.3683

7.13a 7.11a 7.17a 7.09a 7.13a 7.12a 7.20a 7.14a 7.20a 7.19a 0.3190

0.118f 0.128ef 0.157def 0.168cde 0.182bcd 0.197abcd 0.208abc 0.216ab 0.227ab 0.231a 0.0458

0.044d 0.047 cd 0.052bcd 0.052abcd 0.057abcd 0.057abcd 0.061abc 0.059abcd 0.063ab 0.068a 0.0154

0.033a 0.030a 0.028a 0.031a 0.028a 0.033a 0.038a 0.035a 0.040a 0.040a 0.0128




Electrical conductivity. Wastewater raised the EC of the soils in proportion to the applied amount. The EC increased only in the topsoil layer similar to pH (Tables 4 and 5). The contribution of wastewater to increases in EC in the two upper soil layers was insignificant in the wheat plots without added fertilizers. With such additions, the impact was significantly positive. In the potato field, wastewater increased EC significantly in the two upper soil layers. The impact of wastewater on EC was always insignificant for the bottom soil layers in both crop fields. The EC of the soil with added fertilizers was significantly greater than that of the untreated soil.

Soil physical properties Irrigation by wastewater improved soil porosity and reduced bulk density (Table 6). Opening of the soil by extensive plant roots in the wastewater-irrigated soil also helped to increase the porosity. The average bulk density of the three sampled wheat plots was 1.29 g/cm3 under freshwater irrigation and 1.19, 1.28 and 1.25 g/cm3 for three wheat plots under wastewater irrigation. The porosity of the soils in the plots was 44.6, 45.0 and 42.9% under freshwater irrigation and 48.3, 52.5 and 49.5% under wastewater irrigation. The saturated hydraulic conductivity of the soils was 21.1, 17.6 and 12.4 cm/days in the three freshwater-irrigated plots and 24.1, 19.9 and 14.1 cm/days in the corresponding wastewaterirrigated plots (Table 6). Water retention increased in the wastewater-irrigated soils as demonstrated by the soil water retention curves over 0–1000 kPa suction (Figure 5).

Discussion Quality of wastewater Wastewater and freshwater quality parameters are listed in Table 7 along with the safe limits for irrigation as

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recommended by FAO (1992) and GOB (1997). Total soluble solids (TSS) in wastewater were low as the solids could have settled along the 1.5-km path from the sewage outlet. The concentrations of B, Fe, K, NO3-N, Na, Pb, Cu, Zn and Cd were below their threshold values set by FAO (1992) for safe agricultural use; only PO4-P and Mn exceeded the allowable limits. However, Mn was within the safe limits according to guidelines in Bangladesh. The dissolved oxygen in wastewater was much less than the allowable lowest limit, indicating that the wastewater was not suitable for aquatic life. The EC of the freshwater was 0.39 dS/m and that of the wastewater varied from 0.55 to 1.05 dS/m. The EC, whose impact depends on the sodium adsorption ratio (SAR), often exceeded the recommended threshold value. The wastewater was slightly alkaline with a pH of 7.33. As reported by Crites & Tchobanoglous (1998), the BOD of the wastewater (124 12.76 mg/L) was at the lower end of the typical range for domestic wastewater (110–400 mg/L). According to WHO (2006), municipal wastewater with BOD concentrations in this range can increase crop productivity and soil fertility if used for irrigation; no negative effects were observed until the BOD was 500 mg/L. COD of the wastewater (181 25.66 mg/L) was considered relatively low according to Crites & Tchobanoglous (1998) who report a COD range of 250–1000 mg/L for domestic wastewater. Details on wastewater quality for Mymensingh sewage are given by Mojid et al. (2010). Total coliform (TC) and faecal coliform (FC) of the wastewater were 17.2 9 106 and 13.4 9 103 cfu (colony-forming unit) per 100 mL, respectively. WHO guidelines (1989) for wastewater used in agriculture specify a the maximum FC of