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Intl. J. BioRes.9 (5):14-23 November, 2010 Ferdous and Masud

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IMPACTS OF MUNICIPAL WASTEWATER ON PHYSICO-CHEMICAL PROPERTIESOF SOIL

J Ferdous1* and MB Masud1

1Agricultural Systems and Engineering, Asian Institute of Technology, Thailand* Corresponding author: [email protected]

ABSTRACT

The effects of municipal wastewater on the physico-chemical properties of five different soils were studiedduring the period of December2008 to May2009 at Bangladesh Agricultural University in which threesamples were disturbed and rest two samples were undisturbed. The experiments were conducted both at thelaboratory and field; some experiments were under disturbed and some under undisturbed condition of thesoils. The electrical conductivity, pH, saturated and unsaturated hydraulic conductivity, gravitational water,field capacity, porosity, bulk density, and water retention of the soils were measured by treating the soils withtap water (fresh water) and municipal wastewater. Soil pH increased by 0.04, 0.39 and 0.19 unit and electricalconductivity increased by 0.14, 0.13 and 0.11 dS/m due to the effect of municipal wastewater in the disturbedsoil 1, 2 and 3, respectively with respect to the effect of fresh water. Soil pH increased by 0.070.32 unit due to

municipal wastewater with respect to the fresh water in all undisturbed soils (soil 4 and soil 5). Electricalconductivity of the soils increased by 0.0020.26 dS/m due to the impacts of wastewater on undisturbed soils.The percentage decrease of saturated hydraulic conductivity was 28.58, 20.78 and 42.77 due to the impact ofmunicipal wastewater in the soil 1, 2 and 3. The field capacity of the soils increased due to the effect ofwastewater both in the disturbed and undisturbed soils. Water retention of the soils increased due to theimpact of wastewater both in disturbed and undisturbed soils. In comparison to the soils treated with freshwater, saturated hydraulic conductivity in the soils treated with municipal wastewater was decreased in theundisturbed soils. The unsaturated hydraulic conductivity of the undisturbed soils also decreased due to theeffects of municipal wastewater.

Key words: wastewater, water retention, saturated and unsaturated hydraulic conductivity.

NTRODUCTION

In arid and semi-arid regions, wastewater reclamation and reuse has become an important element in water

resources planning (Abedi-Koupai and Bakhtiarifar, 2003). Virto et al. (2006) described the use of wastewaterfor irrigation as a prime solution in the optimization of water resources in semi-arid areas. The reuse ofwastewater for purposes such as agricultural irrigation reduces the amount of water that needs to be extractedfrom environmental water sources (USEPA, 1992; Gregory, 2000). Bangladesh is endowed with goodgroundwater resources and a major part of irrigation fully depends on this vital source. However, a watercrisis occurs every year in dry season. This is because groundwater levels deplete beyond the pumpingcapacity of suction pumps and surface water sources also become limiting. For all these reasons, a hugechronic shortage of water is felt in many parts of the country during dry season irrigation period. Therefore,additional source(s) of water for irrigation may be an important solution to this problem. According to theEconomic and Social Commission for Asia and the Pacific (ESCAP) (2000), about 725 million cubic meters ofwastewater was produced every year from the urban areas of Bangladesh. Utilization of this wastewater forirrigation can minimize water shortage for irrigation to a considerable extent if managed properly.Wastewater reuse may reduce fertilizer rates and provide a low-cost source of irrigation water. Munir et al.(2006) showed that long-term irrigation with wastewater increased salts, organic matter and plant nutrients inthe soil. Soil pH was not consistently affected. Wastewater is a preferred unconventional water source, sincesupply is increasing due to population growth coupled with augmented awareness of environmental qualityand relatively low cost. The relevant costs of wastewater for agricultural reuse are just the additional costsneeded for application to agriculture (Haruvy and Sadan, 1994). Benefits of agricultural reuse of wastewaterare expressed by maintaining agricultural production while preserving water sources and environmentalquality. Wastewater irrigation may also be hazardous to environment since this water may contain pollutants


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such as macro- and micro-organic and inorganic matters. These constituents may harm environment, health,soil, aquifer and crops (Feigin et al., 1990; USEPA, 1992). In addition to groundwater contamination by waste-derived nutrients, wastewater irrigation has shown to change physical, chemical and biological properties ofsoil (Cook et al., 1994; Mathan 1994; Feigin et al., 1991; Schipper et al., 1996; Gharaibeh et al., 2007). Until now,limited information is available on the effects of irrigation using untreated wastewater on soil physical and

chemical properties.

MATERIALS AND METHODS

Collection of soil samples

Soil samples for experiment under disturbed and undisturbed condition were collected from five differentagricultural field of the department of irrigation and water management (IWM) under the faculty ofAgricultural Engineering and Technology, Bangladesh Agricultural University (BAU), Mymensingh.

Determination of soil texture, electrical conductivity, pH, gravitational water, field capacity, porosity andbulk density

The texture was determined by Hydrometer Method. In this method the percentage value of sand, silt andclay were plotted on Marshalls triangular curve.Electrical conductivity (EC) and pH of the soils were determined by using a combine electrical conductivityand pH meter.

The gravitational water, field capacity, porosity and bulk density of the experimented soils were determinedby using their standard formula. For determining the quantity of gravitational water of the soils, the coresamplers containing soil were submerged into water in a dish and kept them for 48 hours to attain fullsaturation with water. The samples were then removed from water and placed on separate funnels placedover conical flasks. The samples were covered with a polyethylene sheet to prevent evaporation loss of water.The soil samples were kept for 48 hours for drainage. After 48 hours, the volumes of water drained out andstored into the conical flasks due to gravitational force were measured.

Gravitational water =soilofVolume

waterdrainedofVolume 100 %

Field capacity =

soilofVolume

waterretainedofVolume 100 %

After measuring gravitational water and field capacity, porosity was calculated by adding gravitational waterand field capacity, that is,

Porosity = Gravitational water + Field capacityTo determine the bulk density, the mass of the empty core sampler and soil was measured. Bulk density wascalculated by the following formula:

Bulk density = , g/cm3

Where, wt. of dry soil = (wt. of oven dry soil and core sampler wt. of empty core sampler), gThe volume of soil was determined from the internal diameter and height of the core samplers. All the soilproperties described above were determined from the same set of soil samples. In doing so, all necessarymeasurements from the samples were taken first before taking the oven dry weight. The samples were dried

in the oven after all measurements were done.


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Hydraulic conductivity

Saturated hydraulic conductivity of the soil samples was determined by Constant Head Method. Formeasuring saturated hydraulic conductivity, core sampler of 5 cm diameter and 5 cm height (Eijkelkamp,Agrisearch Equipment, Netherlands) was used. Six samples were prepared with each of the 3 soils. The

samplers were filled with sieved air dry soils. For settlement of the soil into the samplers, they were wetted for7 days by intermittent application of fresh water (tap water). For measuring saturated hydraulic conductivity,a PVC pipe (5.1 cm inside diameter and 21 cm height) having two 0.5 cm diameter holes at a distance of 2 cmfrom the top was used. One of these holes was used for applying water and the other one for draining outwater from the PVC pipe. The PVC pipe was then attached with the sampler by using M-seal glue (generalpurpose epoxy compound). The arrangements for this measurement are depicted in Figure 1. A plastic tubewas inserted into one hole of the PVC pipe and a steady flow of water was applied. The flow of water wascontinued for 4872 hours to attain steady state flow of water through the soil samples. At steady-statecondition, water drained out from the samples was collected in conical flasks for a certain time to getmeasurable quantity of water. The volume of water was measured. From the volume of leached water andconstant head of water, the saturated hydraulic conductivity of the samples was calculated from Darcys lawas:

Q = kiA or K =

Ati

V

Where,K = hydraulic conductivity of the soil, cm/h. Q = flow rate of water through the soil, cm 3/hV = volume of water collected in time t, h. A =r2 = area of the soil core, cm2

r = inside radius of the core sampler, cm. i =L

h= hydraulic gradient

h = difference in hydraulic head of water under which water flows through the sample, cmL = length of the soil sample, cm

Fig. 1. Experimental set up for measuring saturated hydraulic conductivity of soils.

For measuring the unsaturated hydraulic conductivity in-situ, a disk infiltrometer was used. The equipmentwas filled with water. The disk and water tower were placed on a flat, clean surface, and the bubble tower wasfilled until 7 cm from the top. Unsaturated hydraulic conductivity was calculated as follows:The inside radius of the water supply tube of the disk infiltrometer, r = 2.225 cm.

The radius of the disks nylon mesh, R = 10 cm. At steady condition, the average rate of water supply Q/

1

cm/h for first suction h1 cm, and so on.


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Infiltration rate, Q1/ = r2Q1 at h1Wooding (1968) proposed the following equation:

Q = r2K [1+r

4]

The exponent of Woodings equation is expressed by

=

12

2

/

2

/ )](/)([ln

hh

hQhQ

Saturatedhydraulic conductivity is given by,

Ksat =

( ) ]4

1[exp

)(

1

2

1

/

rhr

hQ

+

Unsaturated hydraulic conductivity is given byK(h) = Ksat exp (h)

Fig. 2. Experimental setup for measuring unsaturated hydraulic conductivity with a disk infiltrometer.Soil-water retention curve

For the determination of soil-water retention curve of the experimental soils, sand box (Eijkelkamp,Agrisearch Equipment, Netherlands) and pressure plate apparatus (Soil Moisture Equipment Corp., SantaBarbara, Ca., USA.) were used. For low suction (100 cm of water) pressure plate apparatus was used.

RESULTS AND DISCUSSION

Physico-chemical properties of soils were affected by the impacts of wastewater. The results obtained in thisstudy on soil properties due to the impacts of wastewater, have been summarized in Tables and Figures.

Soil textureThe three disturbed soils used in this study were designated as soil 1, 2 and 3 and two undisturbed soils weredesignated as soil 4 and 5. The percentage of sand, silt and clay of the soils are given in Table 1.


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Table 1. Percentage of sand, silt and clay content along with the textural class of five sample soils

Particle size distribution (%)Soil sample no.

Sand Silt Clay

Textural class

Soil 1 22.60 65.96 11.44 Silt loamSoil 2 26.64 51.96 21.40 Silt loamSoil 3 8.68 79.96 11.36 SiltSoil 4 41.82 49.76 8.42 LoamSoil 5 35.68 55.28 9.04 Silt loam

pH and electrical conductivity

The pH and electrical conductivity (EC) of the five different soils are given in Table 2.

Table 2. pH and electrical conductivity of five different soils treated with tap water and municipalwastewater.

pH of the soils treated with EC (dS/m) of the soils treated withSoilsample

no.

Soil condition

Tap waterMunicipal

wastewater

Tap waterMunicipal

wastewater1 7.32 7.36 0.03 0.17

2 5.99 6.38 0.02 0.15

3

Disturbed

7.07 7.26 0.01 0.12

4 5.97 6.22 95 104.1

5Undisturbed

5.68 5.95 73.4 74.5

pH increased by 0.04, 0.39, 0.19, 0.25 and 0.27 unit in soils 1, 2, 3, 4 and 5 respectively due to the treatmentwith municipal wastewater. Concentration of hydrogen ions, expressed by pH and electrical conductivityincreased in all soils after application of wastewater due to some metallic ions, which were present in thewastewater. The metallic ions elevated the solute content of the soils. Mancino and Pepper (1992) found thatirrigation with recycled wastewater resulted in an increase in soil pH by 0.10.2 units in comparison to

irrigation with fresh water. Vogeler (2009) and Schipperet al

. (1996) also found similar results of soil pH dueto the effects of wastewater. Electrical conductivity (EC) increased by 0.14, 0.13, 0.11, 9.1 and 1.1 dS/m in soils1, 2, 3, 4 and 5 respectively for the application of municipal wastewater on these soils. Similar EC increasingtrend was found by David et.al (1993).

Gravitational water and field capacity

The gravitational water of the soils as given in Table 3 for the three disturbed and one undisturbed soilsdecreased due to the impact of municipal wastewater. Decrease in gravitational water due to municipalwastewater was 59, 35 and 65% in soils 1, 2 and 3, respectively. Decrease in gravitational water of soil 4 treatedwith municipal wastewater was 63%. Since field capacity and gravitational water combined constituted thesaturated water content of the soils, the variation of field capacity was in opposite trend to that ofgravitational water of the soils. In case of soil 5, it contained some grass roots that are why volume of drainedwater was increased in comparison to other samples and ultimately value of gravitational water wasincreased. Field capacity of different soils treated with municipal wastewater increased.


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Table 3. Gravitational water and field capacity of five different soils treated with tap water and municipalwastewater.

Gravitational water (%) of thesoils treated with

Field capacity (%) of thesoils treated with

SoilSample

No.

Soil condition Replication

Tap water Municipalwastewater Tap water Municipalwastewater

1 3.72 1.62 51.48 54.86

2 3.35 0.49 53.60 55.47

3 3.06 2.01 56.67 51.421

AVERAGE 3.37 1.37 53.92 53.92

1 1.03 0.64 51.65 53.62

2 1.89 0.99 51.78 52.9

3 0.87 * 50.00 *2

AVERAGE 1.26 0.82 51.14 53.26

1 3.33 1.31 49.63 50.91

2 3.08 0.15 54.78 54.97

3 0.80 1.07 54.92 54.133

Disturbed

AVERAGE 2.40 0.85 53.11 53.34

1 0.68 0.54 49.92 37.60

2 0.22 0.14 53.19 46.42

3 1.02 0.04 26.51 50.164

AVERAGE 0.64 0.24 43.21 44.73

1 0.61 1.98 49.16 50.16

2 0.70 2.13 47.28 55.66

3 0.61 0.99 48.55 49.045

Undisturbed

AVERAGE 0.64 1.70 48.33 51.62* samples were damaged and data could not be measured.

Porosity and bulk density

Porosity and bulk density of the three disturbed and two undisturbed soils are given in Table 4. Wastewatersreduced the porosity except for soil 2 in which the municipal wastewater caused an increase in porosity.

Municipal wastewater caused increase in the porosity for two undisturbed soil. This variation comes fordisturbed and disturbed condition. The reason for decreasing porosity of the soils was the clogging of somepores due to various suspended materials of the wastewater. The reason for increasing porosity is theaccumulation of organic matters in the pore spaces. Dawes and Goonetilleke (2004) also reported thatapplication of wastewater to soils decreased the volume of pores. Tarenitzky et al. (1999) observed increasedporosity of soils due to the accumulation of organic matters in the pore spaces. Municipal wastewater causedan increase in bulk density of the soils except in soil sample 5 as this sample contained some grass roots whichlessen the wt of dry soil.

Hydraulic conductivity

Saturated hydraulic conductivity of the five different soils is listed in Table 5. It is observed that the saturatedhydraulic conductivity of the soils decreased due to the impacts of municipal wastewater. In comparison tothe soils treated with tap water, the percentage decrease in saturated hydraulic conductivity in the soilstreated with municipal wastewater was 29, 21and 43 in soils 1, 2 and 3, respectively. Various organic and

inorganic matters suspended in the wastewaters clogged some of the pore spaces of the soils with theresultant reduction in the saturated hydraulic conductivity. The observed result is in conformity with that ofDawes and Goonetilleke (2004) who found that application of wastewater to soils reduced their saturatedhydraulic conductivity.


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Table 4. Porosity and bulk density of three disturbed and two undisturbed soils treated with tap water andmunicipal wastewater.

Porosity (%) of the soilstreated with

Bulk density (g/cm3) of thesoils treated with

SoilSample

No.


Tapwater

Municipalwastewater

Tapwater

Municipalwastewater

1 55.20 56.48 1.22 1.31

2 56.95 55.96 1.24 1.34

3 59.73 53.43 1.27 1.311

AVERAGE 57.29 55.29 1.25 1.32

1 52.68 54.26 1.33 1.38

2 53.67 53.89 1.35 1.37

3 50.87 * 1.38 *2

AVERAGE 52.41 54.08 1.35 1.37

1 52.96 52.22 1.22 1.39

2 57.86 55.13 1.26 1.33

3 55.73 55.20 1.29 1.353

Disturbed

AVERAGE 55.51 54.18 1.26 1.36

1 50.60 38.14 1.32 1.322 53.41 46.55 1.23 1.33

3 27.53 50.20 1.24 1.344

AVERAGE 43.85 44.96 1.27 1.33

1 49.77 52.13 1.38 1.26

2 47.99 57.78 1.48 1.27

3 49.16 50.02 1.39 1.325

Undisturbed

AVERAGE 48.97 53.31 1.42 1.28

* samples were damaged and data could not be measured.

Table 5. Saturated hydraulic conductivity of three disturbed and two undisturbed soils treated with tapwater and municipal wastewater.

Saturated hydraulic conductivity (cm/h) of soils treated withSoil

SampleNo.


Tap water Municipal Wastewater

1 0.46 0.22

2 0.49 0.451

3 0.69 0.50

AVERAGE 0.55 0.39

1 0.88 0.38

2 0.67 0.33

3 0.51 0.483

Disturbed

AVERAGE 0.69 0.39

1 0.89 1.27

2 0.29 0.22

3 0.56 0.154

AVERAGE 0.58 0.551 0.13 0.22

2 0.34 0.30

3 0.86 0.705

Undisturbed

AVERAGE 0.44 0.41* sample 2 was damaged and data could not be measured.


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The unsaturated hydraulic conductivities of the two undisturbed soils treated with tap water and municipalwastewater are illustrated in Figures 3 and 4. It shows that unsaturated hydraulic conductivity of the soilsdecreased due to the impact of municipal wastewaters.

Soil-water retention curve

Soil-water retention curves of the three disturbed soils are illustrated in Figures 5, 6 and 7 for the soils treatedwith tap water and municipal wastewater. Figures 5, 6 and 7 depicted that water holding capacity of the soilsincreased after treating them with wastewater. Wastewater contained a number of organic and inorganicmatters, which might improve soil structure and, consequently, increased the water holding capacity of thesoils. Dawes and Goonetilleke (2004) observed similar increase in water retention of soils due to theapplication of wastewater. Tarenitzky et al. (1999) showed that addition of organic matters to soils increasedtheir water retention capacity. Soil-water retention curves of the two undisturbed soils both before and aftertreatment with wastewater are illustrated in Figures 8 and 9. These figures show the similar trend in result aslike disturbed soil.

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0 200 400 600 800 1000 1200

Suction (cm)

Soil-watercontent

Before wastewater After wastewater

Fig. 5. Water-retention curves of soil 1 before and

after treatment with municipal wastewater

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0 200 400 600 800 1000 1200

Suction (cm)

Soil-watercontent


Fig.6.Water-retention curves of soil 2. Before and


Fig.3.Variation of unsaturated hydraulic

conductivity of soil 4 before and aftertreatment with municipal wastewater

Fig.4.Variation of unsaturated hydraulic conductivity

of soil 5 before and after treatment withmunicipal wastewater


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0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0 200 400 600 800 1000 1200

Suction (cm)

Soil-waterco

ntent


Fig.7. Water-retention curves of soil 3 before and


0.20

0.25

0.30

0.350.40

0.45

0.50

0.55

0 200 400 600 800 1000 1200

Suction (cm)

Soil-watercontent


Fig. 8. Water-retention curves of soil 4 before and


0.20

0.25

0.30

0.35

0.40

0.45

0.500.55

0.60

0 200 400 600 800 1000 1200

Suction (cm)

Soil-watercontent


Fig.9. Water-retention curves of soil 5 before and after treatment with municipal wastewater

CONCLUSIONS

For all soils, wastewater caused an increase in soil pH, EC, porosity and soil-water content, but it reduced bothsaturated and unsaturated hydraulic conductivity. Wastewater caused variable effects on the bulk density,field capacity and gravitational water of the soils.

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Cook, J.J., F.M. Kelliher and M.S.D. Mahon, 1994. Changes in infiltration and drainage during wastewater

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Dawes, L., and A. Goonetilleke, 2004. Assessing changes in soil physical and chemical properties under long-

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Feigin, A., I. Ravina and J. Shalhevet, 1990. Irrigation with Treated Sewage Effluent. Ecological Series, SpringerVerlag, New York. U.S.A


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