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Journal of Environmental Management 84 (2007) 274–281

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Agricultural recycling of treatment-plant sludge: A case study for avegetable-processing factory

Deniz Dolgena,�, M. Necdet Alpaslana, Nafiz Delenb

aDepartment of Environmental Engineering, Faculty of Engineering, Dokuz Eylul University, Buca, 35160 Izmir/TurkeybEge University, Faculty of Agriculture, Bornova, 35100 Izmir/Turkey

Received 15 March 2005; received in revised form 13 March 2006; accepted 5 June 2006

Available online 24 August 2006

Abstract

The present study evaluated the possibility of using the sludge produced by a vegetable-processing factory in agriculture. The sludge

was amended with a soil mixture (i.e., a mixture of sand, soil, and manure) and was applied at 0, 165, 330, 495 and 660 t/ha to promote

the growth of cucumbers. The effects of various sludge loadings on plant growth were assessed by counting plants and leaves, measuring

stem lengths, and weighing the green parts and roots of the plants. We also compared heavy metal uptake by the plants for sludge

loadings of 330, 495, and 660 t/ha with various recommended standards for vegetables. Our results showed that plant growth patterns

were influenced to some extent by the sludge loadings. In general, the number of leaves, stem length, and dry weight of green parts

exhibited a pronounced positive growth response compared with an unfertilized control, and root growth showed a lesser but still

significant response at sludge loadings of 165 and 330 t/ha. The sludge application caused no significant increase in heavy metal

concentrations in the leaves, though zinc (Zn) and iron (Fe) were found at elevated concentrations. However, despite the Zn and Fe

accumulation, we observed no toxicity symptoms in the plants. This may be a result of cucumber’s tolerance of high metal levels.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Sludge disposal; Land application; Agricultural recycling; Biosolid

1. Introduction

Sludge is one of the principal byproducts of wastewatertreatment plants. It may be deposited in landfills, in the sea(ocean disposal), under the ground, or (to a certain extent)in the air as a consequence of incineration. In addition,sludge can be used (recycled) in various ways, including itsuse as fertilizer, as a soil conditioner in farmland, in forests,and in home gardens, as well as in concrete and inbituminous mixes in buildings and roads (Ødegaard et al.,2001). Oceanic disposal of sludge is currently forbidden.Landfilling of sludge has hitherto been an inexpensivemethod of disposal, but in the future, the use of landfillswill have the lowest priority in the waste-handlinghierarchy (i.e., waste minimization first, then reuse;

e front matter r 2006 Elsevier Ltd. All rights reserved.

nvman.2006.06.013

ing author. Tel.: +90 232 4127139; fax: +90 232 453 1143.

esses: deniz.dolgen@deu.edu.tr (D. Dolgen),

@deu.edu.tr (M. Necdet Alpaslan),

e.edu.tr (N. Delen).

recovery; recycling, composting, and energy production;and disposal last of all), and will only be chosen when thereare no other ways to dispose of the sludge. The currenttrend is towards agricultural use (recycling) and incinera-tion. Incineration reduces the sludge to ash, which can thenbe used for landfilling, but in most cases, supplementaryfuel is needed to burn the sludge, which makes this methodless economical. For these reasons, recycling of sludge foragricultural purposes seems to be an appealing solution forthe sustainable management of sludge in the coming years.Recycling of sludge for agricultural purposes has many

beneficial effects. These benefits include supplying nutrients(nitrogen, phosphorus, and micronutrients) to the crops,improving soil physical properties, and increasing soilorganic matter content (USEPA, 1983). On the other hand,there are concerns about the presence of certain toxicelements in the sludge. The application of sludges inagriculture may lead to a risk for humans and theenvironment as a result of heavy metals and toxic organiccompounds accumulating to levels high enough to cause

ARTICLE IN PRESSD. Dolgen et al. / Journal of Environmental Management 84 (2007) 274–281 275

damage (Mamais et al., 2000). Such utilization of sludgecould result in soil contamination, phytotoxicity, and theaccumulation of trace elements in the food supply. In orderto prevent buildup of these compounds to unhealthy levelsin soils and plants, extensive scientific research has beenconducted to understand the potential risk. In addition,environmental hazards caused by the potentially toxiccompounds have been controlled by setting limits on theamounts of such compounds in the sludge as well as in thesoils.

The beneficial effects of using sewage sludge inagriculture have been proven by numerous studies. Thesehave shown that the application of sewage sludge improvesthe physical, chemical, and biological properties of the soil(Mantovia et al., 2005; Benitez et al., 2001; Aggelides andLondra, 2000) and increases crop production through theaddition of nutrients and organic matter (Mantovia et al.,2005; Akrivos et al., 2000; Du Preez et al., 2000; Vasseuret al., 1999; Marx et al., 1995). Wei and Liu (2005) usedsewage sludge compost in their experiments. They haveshown that the compost application was beneficial at lowerapplication rates (i.e. o150 t ha�1). The yields of barleyand Chinese cabbage generated positive response to thesewage sludge compost application.

Although agricultural recycling of sewage sludge hasbecome a common practice over the past several decades,the application of sludge generated by industrial waste-water treatment plants is limited. This may be due to fearsof the accumulation of toxic matter in soils and plants.However, the accumulation of heavy metals and theiravailability to plants depend strongly on the compositionof the sludge, the application rate, soil properties, and thecrop species. Therefore, certain industries such as food(vegetable) processing, the tobacco industry, pulp andpaper production, fermentation industries, and flourproduction have considerable potential due to the lowheavy metal content and high organic matter content oftheir sludges. Bowen et al. (1995) conducted a field trial inRhinelander (Wisconsin) using relatively nitrogen-richpaper mill sludge to grow potatoes. Their results demon-strated that potato plots amended with paper-mill sludgeproduced crop yields equivalent to or greater than thoseobtained with commercial fertilizer. Different levels ofpaper mill sludge as a soil amendment for the productionof corn (Zea mays L.) were evaluated by O’Brien et al(2002). Addition of paper mill sludge to soil improvedmedia organic matter and P content. The optimal amend-ment rates for domestic and industrial sewage sludge wereinvestigated under limed and unlimed soil conditions byWong et al. (2001). The optimal sludge amendment rate forthe growth of Brassica chinensis in soils amended with limewas found to be 25% domestic sludge and 10% industrialsludge. The effect of the sludge from agro-industrytreatment plants on the growth of Lactuva sativa cv Salinas

was recently reported by Dolgen et al. (2004). They foundthat patterns of plant growth were influenced to someextent by the sludge loadings; the 25% and 50% sludge

treatments yielded lower plant growth due to a nitrogendeficiency in the raw sludge, but maximum plant growthwas obtained with the 75% sludge treatment. These studiesreveal that the type of industry and the application rate areboth important factors in determining effective applicationpractices for sludge from industrial treatment plants.The objectives of the present study were to investigate

the potential for recycling industrial sludge in agricultureand to examine the effects of different sludge loadings onplant growth. In this context, we selected a vegetable-processing industry as a case study. The soil used to growthe plants was amended with sludge from a wastewatertreatment plant at various ratios for cucumber (Cucumis

sativus) plants grown in pots in a greenhouse. Thecucumber plants were the raw material for the industrythat generated the sludge. The effects of various sludgeloadings on plant growth and the optimal sludge loadingrate were investigated by counting the plants and thenumber of leaves, weighing the green plant parts, and bymeasuring the stem length of the plants. We also examinedthe heavy metal content in the green plant parts to confirmthat they conformed with regulations governing thiscontent and thus, posed no risk to human health. Theresults indicated that agricultural recycling of the sludgefrom vegetable-processing industry wastewater for cropproduction is a promising alternative, since characteriza-tion of the sludge suggests that it is appropriate forsupporting plant growth; moreover, it is an easy, cost-effective, and environmentally friendly method for sludgedisposal by the selected industry.

2. Materials and methods

2.1. Sludge generation and characterization

The sludge used in the experiments was obtained fromthe wastewater treatment plant of a vegetable-processingfactory. Since the characteristics of the sludge depend uponthe nature of the wastewater being treated in the treatmentplant, we characterized the raw wastewater and the effluentwater (Table 1). The wastewater was derived fromfermentation processes (vinegar production, cucumberpickling), processing of fresh vegetables (slicing andpacking of iceberg lettuce and spinach), and domesticsources (cafeteria, toilets, showers). The composition of thewastewater was about 15% from vinegar production, 65%from pickling, 10% from the processing of fresh vegetables,and 10% from domestic wastewater. The wastewater fromthe pickling process is usually characterized by highchloride content (a salinity of around 12–15%), organicmatter content (a chemical oxygen demand, COD, of 2500to 3000mg/L), low pH (4.0–4.4), and high suspended solids(5000mg/L) (Dolgen et al., 1998). In contrast, the vinegarprocess and the iceberg lettuce and spinach washingprocesses are typified by high organic matter content (aCOD of 4800 to 5000mg/L) and relatively low chlorideconcentrations (i.e., around 3%) (Dolgen and Alpaslan,

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Table 1

Influent (raw wastewater) and effluent (treated wastewater) characteristics for the vegetable-processing factory’s wastewater

Parameter

PH Salinity (%) Turbidity (mmhos/cm) BOD5 (mg/L) COD (mg/L) NH4–N (mg/L)

Influent 4.0–4.5 3.8–8.1 7.0–14.0 1400–1600 2500–3000 1.7–3.1

Effluent 7.4–7.9 4.9–5.4 8.9–9.7 80–95 110–150 0.01–0.05

Table 2

Physical and chemical properties of the sludge and soil mixture used in the

experiments

Parameter Units Sludge Soil mixture

pH — 6.7 7. 5

Salinity (%) 0.2 0.1

Conductivity mmhos/cm 1500 500

Organic matter % 25.2 17.4

Total nitrogen mg/L 863 2402

Nitrate–nitrogen (NO3–N) mg/L 180 1827

Ammonium–nitrogen (NH4–N) mg/L 470 450

Magnesium (Mg) mg/L 5841 4674

Calcium (Ca) mg/L 5762 3842

Phosphate–phosphorus (PO4–P) mg/L 4200 709

Potassium (K) mg/L 5273 2157

Sodium (Na) mg/L 13903 3596

D. Dolgen et al. / Journal of Environmental Management 84 (2007) 274–281276

2000). In the wastewater characterization study, wecollected samples of the raw wastewater from the equal-ization tank used to homogenize the abovementionedwastewaters.

The treatment facility was capable of treating a flow of90m3/d and included primary, secondary, and sludgetreatment stages. Primary treatment includes a bar screen,plus equalization and neutralization units to remove largesolids, homogenize the wastewater in terms of flow rate andpollutants, and neutralize the acidic wastewater by theaddition of alkaline materials. Secondary treatment in-cludes a biological process known as extended-aerationactivated sludge, which utilizes aerobic microorganisms tohelp degrade the organic matter. Sludge that settles out inthe aeration tanks is pumped into a thickener that increasesthe dry matter content. After the thickening process, sludgeis dewatered using drying beds. The sludge’s dry mattercontent is raised from 3% to between 25% and 30% in thedrying beds before disposal in a landfill. The amount ofsludge disposed of in the landfill is about 10–15 t/year(annual sludge production). We used the dewatered sludgeobtained from the drying beds in our study. The physicaland chemical properties of this sludge are presented inTable 2. The last column of the table describes thecharacteristics of the soil mixture (a mixture of soil, sand,and manure) that was mixed with the sludge in our study.

The heavy metal concentrations in the sludge and soilmixture are presented in Table 3, together with the limitsestablished by the Solid Waste Control Regulations(Republic of Turkey, 1991) and Soil Pollution ControlRegulations (Republic of Turkey, 2001) issued by Turkey’sMinistry of the Environment and Forestry.

2.2. Analysis of sludge

Our analyses of the sludge and soil mixture followed theAPHA-AWWA-WEF (1992) methodology. Organic mat-ter was measured gravimetrically (Method 505A) using afurnace. Nitrogen in the form of nitrates (NO3–N) andammonia (NH4–N), and phosphorus in the form ofphosphate (PO4–P), were analyzed colorimetrically usinga Nova 60 spectrophotometer (Merck, Darmstand, Ger-many), Salinity (Method 2520) and conductivity (Method2510) were measured using a DC 144 69 DR conductivitymeter (HACH, Iowa, USA), and pH was measured usingan NEL 890 pH meter (NEL, Ankara, Turkey). Totalextractable heavy metals (Ni, Zn, Cu, Pb, Cd, Cr, Mn, andFe) were measured by means of the direct air-acetylene

flame method (Method 3111 B) by means of atomic-absorption spectrophotometry (AAS) using a UNICAM9229 spectrophotometer (ATI, Cambridge, England). Wealso used Method 3111 B to measure K, Mg, and Nacontents. We used the EDTA titrimetric method (Method3500-Ca D) to measure Ca contents.

2.3. Plant growth experiments

In our experiment, we used five sludge treatments (SetsS-1 to S-5, including a control), with five replications (i.e., atotal of 25 pots) in a greenhouse at Ege University’sFaculty of Agriculture (Bornova-Izmir, Turkey). Weapplied sludge to the pots at rates of 0, 165, 330, 495,and 660 t/ha before cultivation of the cucumber plants. Thesludge amendments used in the experiments are given inTable 4. Set S-1, the control, included only the soil mixture,which was created from a 1:1:1 mixture of soil, sand, andmanure. The soil was taken from a vegetable grown field.Texture of the used soil and sand was sandy loam andmedium, respectively. The characteristics of the resultingsoil mixture are presented in Table 2. No fertilization wasprovided before or during the study.We sowed five seeds of C. sativus cv. ‘Villaset’ in each

pot, and grew the seedlings under controlled conditions inthe greenhouse. The pots were watered to field capacity atthe same time whenever the soil mixtures in the potsbecame dry. We grew the seedlings for 2 months (betweenMarch and May) under controlled conditions, withconstant temperature (21 1C) and humidity (60%). Duringthis period, we supplied no additional sludge or soil

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Table 3

Heavy metal concentrations in the sludge and soil mixture used in the experiments (nd ¼ not detectable)

Parameter Sludge Soil mixture

Measured value (as mg/kg

dry solids)

Limits in the regulationsa Measured value Limits in the regulationsa

Nickel (Ni) 40 200 4 75

Zinc (Zn) 62 3000 5 300

Copper (Cu) 42 1200 nd 140

Lead (Pb) nd 1200 nd 300

Cadmium (Cd) 0.7 20 nd 3

Chromium (Cr) 30 1200 nd 100

Mercury (Hg) nd 25 nd 1.5

Manganese (Mn) 36 no limit nd no limit

Iron (Fe) 367 no limit 63 189

aSources: Republic of Turkey (1991, 2001).

Table 4

The sludge treatments used in the experiments

Sludge

treatment

Sludge (g) Soil Mixture

(g)

Application

rate (t/ha)

S-1 0 1500 0

S-2 375 1125 165

S-3 750 750 330

S-4 1125 375 495

S-5 1500 0 660

D. Dolgen et al. / Journal of Environmental Management 84 (2007) 274–281 277

mixture, but took measures to control insects andpathogens after emergence of the plants and 20 days later:two applications of 400 ppm endosulfan (Thiodan ConcEC 35, 360 g/L endosulfan, Bayer Crop Science, Turkey)and 700 ppm of thiram (Pomarsol Forte, 80% thiram,Bayer Crop Science, Turkey) were applied to the leaves andthe soils of the plants as pesticides.

At harvesting, we carefully removed the plants withoutdamaging their roots. We counted the number of liveplants and the number of leaves, and measured the stemlength (from the soil level to the end of the plant) of thecucumber plants. We washed the leaves and roots with de-ionized water to remove any attached particles, and thendried them at 60 1C for 1 week. We then measured the dryweight of the green parts (including the runners) and rootsto determine the effect of various sludge loadings ongrowth of the cucumber plants.

2.4. Statistical analysis

We performed our statistical analysis (SPSS program)using analysis of variance, with Duncan’s multiple-rangetest used to assess the significance of the effects of thesesludge loadings on plant growth.

3. Results

We evaluated the growth of the cucumber plants bycounting the numbers of plants and leaves, measuring the

stem length of the plants, and weighing the green parts androots. The range in plant growth and mean values for thefive treatments are summarized in Table 5 and graphed inFig. 1. The highest mean values for plant growth wereobtained in the S-2 (165 t/ha) and S-3 (330 t/ha) treatments,whereas the lowest plant growth was observed in thecontrol (S-1) and S-5 (660 t/ha) treatments. Althoughtreatments S-2, S-3, and S-4 yielded more plants per potthan the control (S-1) and S-5 treatments (Table 5, Fig. 1),the differences were not significant. In other words, none ofthe sludge loadings decreased seed germination or earlyseedling survival.The lowest mean number of leaves per pot (1.3 leaves/

pot) was found in the control (S-1) and S-5 treatments, andthis represented a significant decrease in the number ofleaves compared with the three intermediate sludgeloadings. These treatments (S-2, S-3, and S-4) significantlyincreased the number of leaves per pot compared with thecontrol (1.6 leaves/pot). Among the S-2, S-3, and S-4treatments, the maximum number of leaves (5.1 leaves/pot)occurred in S-3 (330 t/ha). The mean values for S-2 and S-4were slightly lower (4.9 and 4.6 leaves/ pot, respectively),but these values were not significantly different from thevalue for S-3 (Table 5, Fig. 1). However, the mean numberof leaves per pot was significantly higher in treatments S-2and S-3 than in treatments S-1 and S-5.The highest sludge loading (S-5, 660 t/ha) yielded the

minimum stem length (4.2 cm), which was slightly but notsignificantly less than in the control (5.2 cm). Both theselengths were significantly lower than the maximum stemlength, which we observed in the S-2 and S-3 treatments,and lower (but not significantly) than the value in the S-4treatment (19.4, 19.5, and 15.9 cm, respectively). Thesethree treatments did not differ significantly (Table 5).The range and the mean values for the dry weights of

green plant parts and roots are shown in Table 6 andgraphed in Fig. 2. The highest mean weight of green plantparts was obtained in the S-3 treatment (147mg), whichwas slightly but not significantly higher than the value inthe S-2 treatment (145mg). The weight of green plant parts

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Table 5

The effects of the five treatments in Table 4 (sludge loadings) on the plant growth parameters

Treatment Number of plants/pot Number of leaves/pot Stem length of the plants (cm)

Range Mean Range Mean Range Mean

S-1 0–3 2.0a 0–2.6 1.6a 0–8.3 5.2ab

S-2 1–5 3.8a 1.5–7.2 4.9c 4.2–27.1 19.4c

S-3 1–5 3.0a 1.6–8.8 5.1c 5.6–32.1 19.5c

S-4 3–5 3.8a 2.6–7 4.6bc 9.8–22.4 15.9bc

S-5 0–3 1.6a 0–2.6 1.3ab 0–8.3 4.2a

Means in any column followed by the same letter do not differ significantly (Po0.05, Duncan’s multiple-range test).

0

1

2

3

4

5

6

0 165 330 495 660Sludge Treatments (t/ha)

Num

ber

of P

lant

s/L

eave

s pe

r po

t

0

5

10

15

20

25

Stem

Len

gth

of t

he P

lant

s (c

m)

Number of plants/pot Number of leaves/pot Stem length of the plants (cm)

Fig. 1. The effect of sludge loading on the numbers of plants and leaves per pot and on the stem length of the plants.

Table 6

The effects of the five treatments in Table 4 (sludge loadings) on the dry weights of green plant parts and roots

Treatment Dry weight of green plant parts (mg) Dry weight of roots (mg)

Range Mean Range Mean

S-1 0–34 36a 0–4.4 2.6a

S-2 44–278 145b 2–8.6 5.6ab

S-3 43–202 147b 2.8–7.8 5.2ab

S-4 59–170 116ab 7.6–21.2 11.9b

S-5 0–53 29a 0–6 3.72a

Means in any column followed by the same letter do not differ significantly (Po0.05, Duncan’s multiple-range test).

D. Dolgen et al. / Journal of Environmental Management 84 (2007) 274–281278

decreased to 116mg in the S-4 treatment, but this decreasewas not significant (Table 6). The lowest weight of greenplant parts was 29mg in treatment S-5, which was slightlybut not significantly lower than in the control (36mg);these treatments (S-1 and S-5) were not significantlydifferent, but were significantly lower than the weights inthe S-2 and S-3 treatments.

The maximum root weight (11.9mg) was obtained in theS-4 treatment. The next-highest root weights were 5.6 and5.2mg in treatments S-2 and S-3, respectively, which werelower than the weight in the S-4 treatment, but not

significantly. The lowest mean root weight occurred in thecontrol (2.6mg), and the root weight in the S-5 treatment(3.72mg) was slightly but not significantly higher; theweights in both treatments were lower than those in theother three treatments, but this difference was onlysignificant for the S-4 treatment.We also determined the uptake of heavy metals by the

cucumber plants in the green parts of the plants for threesludge loadings (330, 495, and 660 t/ha). The results aregiven in Table 7, which also presents the values given invarious standards. Codex Standard 115 (1981) applies to

ARTICLE IN PRESSD. Dolgen et al. / Journal of Environmental Management 84 (2007) 274–281 279

pickled cucumbers that are intended for human consump-tion, and covers quality criteria such as color, size, texture,food additives, and contaminants. Only Sn and Pb areincluded in this standard, at levels of 250 and 1mg/kg,respectively. For the other heavy metal concentrations, weadopted the maximum limits provided elsewhere in theCodex Alimentarius Commission standards (Codex Ali-mentarius Commission, 2001) and provided by Fisseha(1998) for fresh vegetables. Among the heavy metals, Cd,Pb, and Cr pose the greatest concern in terms of healthrisk, and none were detected in the green parts of thecucumber plants in treatments S-3, S-4, and S-5. Nickelwas also undetectable in the green plant parts. Sludgeapplication did not appear to significantly increase Mncontents in the cucumber plants. Although the Cu contentincreased slightly at the highest sludge loading (S-5), it wasstill present below the acceptable level. We observed highZn and Fe levels, because Zn, Mn, Fe are plantmicroelements and thus plants uptake them for theirgrowth rather than other metals such as Cd, Hg. Moreover,in practice, when those elements are not available in the

0

20

40

60

80

100

120

140

160

0 100 200 300 400 500 600 700Sludge Treatments (t/ha)

Dry

wei

ght

of g

reen

par

ts (

mg)

0

2

4

6

8

10

12

14

Dry

wei

ght

of r

oots

(m

g)

Dry weight of green parts (mg) Dry weight of roots (mg)

Fig. 2. The effect of sludge loading on the dry weights of green plant parts

and of roots.

Table 7

Heavy metal concentrations measured in the green parts of the cucumber plant

respectively) (nd ¼ not detectable)

Parameter Unit Leaves of cucumber p

S-3

Nickel (Ni) mg/kg nd

Zinc (Zn) mg/kg 130

Copper (Cu) mg/kg 49

Lead (Pb) mg/kg nd

Cadmium (Cd) mg/kg nd

Chromium (Cr) mg/kg nd

Manganese (Mn) mg/kg 250

Iron (Fe) mg/kg 1272

aSource: Fisseha (1998).bSource: Codex Alimentarius Standards (2001).cSource: Codex Alimentarius Standards (1981).

media, fertilizers containing Zn, Mn, Fe metals should beused. In this study, although Zn and Fe levels were wellabove the acceptable level, in the green plant parts,however, no toxicity symptoms were observed in theplants. This may be attributable to the ability of cucumberplant to tolerate high metal levels (Kelley et al., 1984;Bingham et al., 1975).

4. Discussion

Our experiments have shown that sludge applicationincreased plant growth in comparison with the control, andthat the difference was often significant. The number ofleaves produced by the cucumber plants increased as aresult of the sludge treatment, except at the highest level ofsludge addition (S-5); intermediate sludge loads (S-2, 165 t/ha; S-3, 330 t/ha) yielded the maximum growth in terms ofnumber of leaves and stem length. The lowest numbers ofleaves were produced in the control (only soil mixture) andS-5 (660 t/ha) treatments, and these values were signifi-cantly lower than in the S-2 and S-3 treatments. Sludgeapplication also significantly increased stem length intreatments S-2 (165 t/ha) and S-3 (330 t/ha), and theminimum stem length was found at the highest sludgeloading (660 t/ha) and in the control. However, there wasno significant difference between treatments in the numberof plants per pot (i.e., in seed germination and earlysurvival). Significant differences between treatments wereobserved in the weights of the green plant parts, whichgrew remarkably larger in the S-3 (330 t/ha) and S-2 (165 t/ha) treatments; the lowest weights were measured in thecontrol and S-5 (660 t/ha) treatments, which did not differsignificantly, but both these weights were significantlylower than in the S-2 and S-3 treatments. The addition ofsludge increased root weights in the S-2, S-3, and S-4treatments, but the value in S-4 was significantly greaterthan in all other treatments. However, root growth did notdiffer significantly among the other treatments, even

s at sludge loadings of 330, 495, and 660 t/ha (treatments S-3, S-4, and S-5,

lants Limits for vegetables

S-4 S-5

nd nd 67.9a

157 166 99.4a

44 54 73.3a

nd nd 1.0b

nd nd 0.2c

nd nd 2.3a

100 100 500.0a

1346 1100 425.5a

ARTICLE IN PRESSD. Dolgen et al. / Journal of Environmental Management 84 (2007) 274–281280

though S-2 and S-3 produced nearly twice the weight ofroots as in the control and more roots than in the S-5treatment. The lowest level of root growth, which occurredin the control, was attributed to inadequate P content inthe soil mixture. In other word, since phosphorus enhancesroot growth, high phosphorus content of the sludge resultsin an increase in root weights. Additionally, nitrogen,especially nitrate compound, is important for leaves. Thus,insufficient nitrate content of the sludge causes lower leafgrowth. In general, we observed a pronounced andfavorable growth response to sludge application in termsof the number of leaves, stem length, and dry weights ofgreen plant parts in treatments S-2 (165 t/ha) and S-3(330 t/ha), and a smaller but still significant response inroot growth in S-4.

Our analyses of heavy metal contents demonstrated thatmetal contents did not change significantly at high sludgeloads. Among the heavy metals, Pb and Cd are mostimportant in terms of health risks because they are subjectto bioaccumulation and may endanger human health whenthey enter the food chain. In our study, neither metal waspresent at detectable levels. The sludge application did notsignificantly increase the Ni, Cu, Pb, Cd, Cr, and Mncontents of the green parts of the cucumber plants. Thismay be explained by the relatively low heavy metalconcentrations in the sludge and the relatively high sludgepH (6.7–7.5). Similar observations were carried out bySpeir et al., 2003. They measured the concentrations,solubility and mobility of certain heavy metals over a 4-year period in soil from a site that had received over1000 t ha�1 wet, undigested, sewage sludge. Soil acidifica-tion, with its consequences of increased metal solubilityenhanced metal uptake by plants, effects on plant healthand germination, and metal leaching could easily becontrolled by application of lime with the sludge, beforeincorporation. Only Zn and Fe levels in the leaves exceededthe proposed maximum levels recommended by Fisseha(1998). Although we observed no toxicity symptoms in ourplants, the relatively lower growth observed at the highestsludge load (S-5) may have resulted from the high Znconcentrations. Even though Zn is an essential micronu-trient for growth, its concentrations must be controlled byuse of an appropriately low sludge application rate or bymaintaining alkaline conditions in the soil (Speir et al.,2003; Shuman et al., 2001).

In addition, since plants differ in their ability to take up,accumulate, and tolerate heavy metals, the selection ofappropriate plant species will be an important factor inplanning the agricultural application of sludge. Forexample, iceberg lettuce accumulates high concentrationsof heavy metals and is very sensitive to certain heavymetals (e.g., Cu, Ni, and Zn), whereas cereals and legumestend to accumulate lower levels of heavy metals than leafyvegetables such as lettuce and spinach (Kelley et al., 1984).In the present study, we selected cucumber because of itslow accumulation of metals and greater tolerance for thesecompounds.

Another important factor in determining the accept-ability of sludge will be its nutrient content. Nitrogen, K,and P are essential nutrients for plant growth, and arerequired in relatively large quantities by plants. During ourexperiments, we observed changes in leaf color from lightgreen to yellow. This situation may have been caused by anutrient deficiency, because we provided no additionalfertilizer or compost to the plants during the study. Inorder to overcome this problem, additional N and K maybe required during the growing period in subsequent fieldtrials. Symptoms of nitrogen deficiency were also reportedby O’Brien et al. (2002) in the leaves of corn after 2 weeksof growth. Supplementing N or delaying seeding wasrecommended to eliminate deficiency symptoms.

5. Conclusion

Sludge derived from a plant that treated the wastewatergenerated by a vegetable-processing (or food production)industry has attributes that distinguish it from the waste-waters generated by other industrial activities. Our experi-mental results demonstrated that the sludge used in thisstudy was appropriate for agricultural purposes. Plantgrowth increased significantly in response to sludgeaddition, without leading to potentially hazardous in-creases in the heavy metal content of the plants. This mighthave resulted from the fact that the sludge from thevegetable processing factory was poor in heavy metals butrich in organic matter, macronutrients, and micronutrients.Consequently, such sludge has considerable potential inagriculture and other applications such as reforestation,and can be used as a partial substitute for inorganicfertilizers and as a soil conditioner.

Acknowledgment

The authors are particularly grateful to the FERSANCompany for their cooperation and help in various steps ofthis research.

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