feasibility of using a mixture of sewage sludge and incinerated sewage sludge as a soil amendment

7/25/2019 FEASIBILITY OF USING A MIXTURE OF SEWAGE SLUDGE AND INCINERATED SEWAGE SLUDGE AS A SOIL AMENDMENT

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FEASIBILITY OF USING A MIXTURE OF SEWAGE SLUDGE AND INCINERATED SEWAGE SLUDGE AS A

SOIL AMENDMENT

Silvana Irene Torri

Ctedra de Fertilidad, Facultad de Agronoma, Universidad de Buenos Aires,

Avda San Martn 4453, Buenos Aires (C1417 DSE), Argentina

[email protected]

Abstract

The accumulation of sewage sludge poses nowadays a growing environmental problem. Incineration is a

feasible means of reducing sewage sludges volume. Public acceptance of this technology is, however,

hampered by concerns about potential adverse environmental impact, mainly due to non-volatile hazardous

constituents that are concentrated in the ash. Application of incinerated sewage sludge ash to agricultural soils

presents the opportunity of recovering nutrients considered essential for plant growth, reducing the need for

commercial fertilizers. However, this practice can contribute to the pollution of agricultural soils by heavy metals.

Combined use of sewage sludge and its incinerated ash may prove to be a beneficial means of disposal,

improving soil quality and crop production. Very little attention has been dedicated to asses the potential of the

application of this mixed waste. This Chapter evaluates the effects of a mixture of sewage sludge and its own

incinerated ash on soil properties when used as a soil amendment. Three typical soils of the Pampas Region

were used in order to predict the feasibility of using similar mixtures in large-scale degraded-land application.

The application of the mixture of sewage sludge and its incinerated ash significantly increased soil organic

carbon, pH and EC in the three amended soils compared to control. An increase in Lolium perenne L aerial

biomass in amended soils compared with plants grown in unamended soils was observed. Cadmium and Pb

concentrations were in all cases below detection limits in aerial part of L. perenne. On the contrary, Cu and Zn

concentration in the above ground tissue was significantly higher in the amended soils than control, indicating a

high Cu and Zn availability. Nevertheless, no significant differences between Cu or Zn concentration in aerial

biomass was observed between soils amended with the mixture of sewage sludge and its incinerated ash

compared to soils amended with sewage sludge. Overall, this assay showed that the use of this mixed waste as

a soil amendment may not pose a significant risk of soil, water or plants contamination. Therefore, the mixture of

sewage sludge and incinerated sewage sludge may play a significant role as a soil amendment in land

reclamation, especially if nonfood chain crops are grown.
mailto:[email protected]:[email protected]:[email protected]


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Sewage sludge Incineration

Incineration is a feasible means of reducing sewage sludges volume and converting this waste in a

practically inert, odorless and sterile ash. This practice has been commonly used in municipalities of other parts

of the world where large quantities of sewage sludge are produced, but potential for land application is limited

(Werther, Ogada 1999). Sludge incineration enjoys a combination of several advantages that are not found in

other alternative treatments, including a large reduction of sludge volume (Torri 2001) and thermal destruction of

toxic organic constituents and pathogens (Vesilind, Ramsey 1996; Porter, Bastian 2005). Usually, an external

energy supply is essential to dry and combust dewatered sewage sludge (Brown 2007). Therefore, incineration

may be considered as a means of waste minimization rather than energy generation. Particulate and gaseous

emissions can be hazardous and require treatment. Different techniques are now available to control gaseous

emissions and to capture, or destroy, potentially harmful substances that are, or may be, released during the

combustion process. Public acceptance of incineration is, however, hampered by concerns about potential

adverse environmental impact, mainly due to non-volatile hazardous constituents that are concentrated in the

ash. Potentially toxic inorganic elements (PTE) are not degraded and therefore can concentrate in the ash or in

the particulate matter that is contained in the exhaust gases generated by the process (Stasta et al, 2006). Inthis way, incinerated sewage sludge requires special consideration for disposal. Technologies have been

developed to make use of the resulting ash, by replacing part of the raw material in brick manufacturing (Hara,

Mino 2008; Liew et al., 2004), cement production (Tomita et al, 2006) and glazed tiles (Lin et al, 2008), among

others. However, the use of incinerated sewage sludge ash in the manufacture of construction materials instead

of land application eliminates a valuable nutrient feedstock.

Disposal of incinerated sewage sludge ash

Land spreading of incinerated sewage sludge ash is a potential means of disposal of this solid waste.

Application of incinerated sewage sludge ash to agricultural soils presents the opportunity of recovering nutrientsconsidered essential for plant growth, reducing the need for commercial fertilizers. It is well known that

phosphates are a limited non-renewable resource. In wastewater treatment plants, phosphate is removed to

meet the limiting effluent concentration, ending in sewage sludge. Thus, the amount of phosphate in incinerated

sewage sludge ash is as high as in natural phosphate ores (Franz 2008). The ash can also improve soil physical

properties because of its silt-size nature (Saikia et al., 2006) and can be an effective liming agent (Zhang et al.,

2002). However, there are some concerns about its high PTE contents (Bagnoli et al, 2005), which can

contribute to the pollution of agricultural soils. It was reported that incinerated sewage sludge ash increased the

soil solution of Cd, Cu and Zn (Bierman et al., 1995) or produced the leaching of high amounts of toxic elements

such as As, Cd, Cr, Pb and Se (Saikia et al, 2006).

Combined use of sewage sludge and its incinerated ash (SSA) may prove to be a beneficial means of

disposal, improving soil quality and crop production. As an organic amendment, sewage sludge improves

physical, chemical, and microbiological properties of soils. It has been postulated that in temperate climates,

where organic matter decomposition is not particularly fast, the protective role of organic matter remains

unaltered decades after sludge application. In this way, PTE can be sorbed onto the remaining non-

decomposable organic fraction (Antoniadis et al, 2008). On the other hand, as inorganic sorption phases are not

altered in the time-scale of a few decades, PTE are also retained by sorption processes onto sludge-borne and


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soil inorganic constituents (Antoniadis et al, 2008). Moreover, it has been suggested that sewage sludges matrix

may act as both a source of and sink for PTE (Corey et al., 1987; Smith, 1996). Soil addition of silt-size particles

present in incinerated ash promotes better aeration, percolation and water retention capacity (Karapanagiotis et

al, 1991; Dollar 2005). Even though sewage sludge tends to increase soil acidity as a result of proton release

from organic matter decomposition and mineralization (Liu et al., 2007), oxides formed during incineration would

buffer pH decrease.

This Chapter presents the results of a greenhouse experimental study on three typical soils of the

Pampas Region. The aim was to evaluate the effects of a mixture of sewage sludge and its own incinerated ash

on soil properties when used as a soil amendment. Availability of some trace elements of concern was also

studied. The final objective of this research is to predict the feasibility of using similar mixtures in large-scale

degraded-land application.

The study area

Buenos Aires City and its outskirts are the major source of sludge production in Argentina, annually

producing about 1.800.000 metric tons. The nearby agricultural region, The Pampas Region, is located between328 to 398S and 56 to 678W, with Mollisols developed from loess-like sediments predominating soils.

The Aeolian sediments from which the soils of the Pampas have developed were brought from the south

west, resulting in a progressively finer texture from south-west to north-east. This, combined with a gradient in

rainfall which increases in the same direction, has produced a geographic sequence of Mollisols, with Entic

Haplustolls (US Soil Taxonomy, USDA, 1999) at the western limit of the region, and the progressive appearance

of Entic Hapludolls, Typic Hapludolls, Typic Argiudolls and Vertic Argiudolls towards the east (Soriano, 1992).

These soils are associated with minor proportions of Aquolls in the drainage ways. The mineralogy of the soils is

dominated by nearly unweathered volcanic material such as plagioclases, glasses and lithic fragments; minor

components include quartz and orthoclase. In the clay-fraction, the predominant mineral is illite, somemontmorillonite and a lesser proportion of interstratified illite/ montmorillonite make up the reminder (Soriano,

1991). The climate of the region is subtropically humid, characterized by long warm summers and mild winters,

an average air temperature of 11 C in July and 25.5 C in January, and a mean annual precipitation of 1147

mm. These weather conditions allow good development and production of forage and crop species typical of

temperate regions.

The pampas Region covers about than 52 million ha of lands suitable for cropping and cattle rearing, the

remaining being either marginally suitable or unsuitable for cropping, mainly as a result of slight differences in

relief. The lack of public acceptance for cropland application of sewage sludge makes these uncropped lands

suitable for sludge application.

Sewage sludge

Sewage sludge from Buenos Aires City was provided by the local water operator Aguas Argentinas S.A.

The aerobically stabilized sludge used in these experiments was previously dried in holding pools in the waste

water treatment plant.

The sludge was oven-dried at 60C, ground and sieved (


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the previously sieved sewage sludge, resulting in a new mixed waste containing 30% DM as ash (SSA).

Analytical data (dry mass basis) for SS and SSA is presented in Table 1.

Figure 1:SEM-EDS Images of sewage sludge (A) and incinerated sewage sludge at 500 C (B)

Crystalline phases present in SS, SSA and in the soils were identified by X-ray diffraction (XRD) using a

Philips PW 1510 diffractometer with Cu radiation, and by SEMEDS. The main crystalline component of SS was

quartz (SiO2, 26.90 2), with a trace of plagioclase [(Na,Ca)(Si,Al)4O8)] (Fig. 2 A). Incineration of SS had little

influence on the overall mineralogy of the sludge components. Comparison of the X-ray diffraction (XRD)

patterns showed that the main effect of incineration was the formation of hematite (-Fe2O3) and calcite (CaCO3)

(Fig. 2 B). There also appears to be a slight increase in the amount of plagioclase relative to the amount of

hematite and quartz (Figure 2).


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A

0

1000

2000

3000

4000

0 10 20 30 40 50 60

counts

B

0

1000

2000

3000

4000

0 10 20 30 40 50 60angle 2

counts

Q

Q

QQ

Q

Q

Q

QQQ

Q

Q

H

H

P P

PPC

Q = quartz (SiO2)

P = plagioclase (Na,Ca)(Si,Al)4O8)]

H = hematite (-Fe2O3)

C = calcite (ACO3)

Q

Figure 2: X-ray diffraction (XRD) patterns of (A) sewage sludge (SS) and (B) 70:30 DMW mixture of sewage sludge

and sewage sludge ash (SSA).

Although trace elements naturally occur in soils, anthropogenic sources may originate hazardous soil

concentrations. The main elements of concern include Cd, Cu, Pb and Zn (Antoniadis et al., 2008; Egiarte et al.,

2008; Wang et al., 2008). Cadmium has been used to prevent corrosion of machinery, and concern for thiselement arises from its possible entry into the food chain (Reeves, Chaney 2008). Copper, zinc, and lead are

among the most heavily used PTE in industries, such as plating, mining, and petroleum refining. Although Cu

and Zn are known to be essential for the normal growth of plants or animals, high soil availability of both

elements can cause phytotoxicity prior to produce plant concentrations that would be toxic to humans (Thomas

et al, 2005). Although Pb is not an essential element for plant growth, it is easily absorbed and accumulated in

different plant parts organs (An 2006). All Pb-compounds are cumulative poisons and normally affect the

gastrointestinal tract and/or the nervous systemof humans.


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Table 1: Selected properties of sewage sludge (SS) and 70:30 DMW mixture of sewage sludge and sewage sludge ash(SSA).

SS SSA

pH 5.82 6.17

Moisture content (%) 5 4.5

Total organic carbon (mg g-1

) 251 176

Total N (mg g-1

) 19.3 22.5

Total P (mg g-1

) 0.052 0.086

Electrical conductivity (dS m-1

) 0.90 0.89

Cation exchange capacity (cmol(c)kg-1

) 11.95 nd

Ca (mg g-1

) 22.5 nd

Mg (mg g-1

) 5.6 nd

K (mg g-1

) 10.7 nd

Total Cd (mg kg-1

) 10.08 13.08

Total Cu (mg kg-1

) 750.8 894.7

Total Pb (mg kg

-1

) 334.2 365.9Total Zn (mg kg

-1) 2500 3150

nd = not determined

The contents of Cd, Cu, Pb and Zn in the SS and SSA of this study did not exceed ceiling concentrations

for land application recommended by Argentine regulation (S.A.D.S. 2001, Res. 97/01), whose values are similar

to USEPAs limits (USEPA, 1993). Moreover, Cd, Cu and Znconcentrations were within the numerical standards

for sewage sludge not subject to cumulative pollutant loading rates (CPLRs) permitted by the USEPA regulations

(Table 2). Because of high Pb concentration, this sludge would not be allowed for use in agriculture without

maintaining records of cumulative applications, 300 kg/ha for Pb. Total Pb loading used in this study for SS and

SSA complies with the Argentine and U.S. regulations on cumulative loadings for sludge-treated soils.

Table 2: Argentine and USEPA acceptable standards of Cd, Cu, Pb and Zn in sludge applied to soils (USEPA 1993).

Cd Cu Pb Zn

mg kg-1

SS 10.08 750.8 334.2 2500

SSA 13.08 894.7 365.9 3150

Argentina 85 4300 840 7500

USEPA - PC Biosolid1

39 1500 300 2800

USEPAMAMC2

85 4300 840 7500

kg ha-1

CPLRs3 39 1500 300 2800

1PC = Pollutant Concentration Biosolid

2MAMC = Maximum Allowable Metal Concentrations

3CPLRs = Cumulative Pollutant Loading Rates


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STUDIES ON PAMPAS SOILS

Greenhouse experiment

The soils selected for this experiment were Typic Hapludoll, Typic Natraquoll and Typic Argiudoll,

sampled near Carlos Casares (35 37' S - 61 22' W), Pila (36 1' S - 58 8' W) and San Antonio de Areco (34

15' S - 59 29' W) towns respectively. Relevant soil properties are presented in Table 3.

Table 3: Main physical and chemical characteristics of the three untreated soils (A horizon, 0-15 cm) used for pot experiment.

Typic

Hapludoll

Typic

Natraquoll

Typic

Argiudoll

Clay (%) 19.2 27.6 30.3

Silt (%) 23.2 43 53.6

Sand (%) 57.6 29.4 16.1

pH 5.12 6.21 5.72

Organic carbon (g kg-1

) 28.6 35.31 24.5

Electrical conductivity (dS m-1

) 0.61 1.18 0.7

Cation exchange capacity (cmol(c)kg-1

) 20.3 22.3 24.5

Exchangeable cations

Ca2+

(cmolckg-1

) 10.2 9.1 12.6

Mg2+

(cmolckg-1

) 2 5.4 4.3

Na+ (cmolckg

-1) 0.3 2.1 0.2

K+ (cmolckg

-1) 2.8 1.6 2.1

Both sludge amendments were homogeneously mixed with each of the three soils at proportions

equivalent to a field application rate of 150 t DM ha-1

. Soil moisture was maintained at 80% of water holding

capacity during the experiment. Unamended soils were used as control. The pots were arranged in completely

randomized blocks and housed in a greenhouse sheltered from rain or direct sunlight. Part of the pots were

sampled on days 1, 30, 60, 150, 270 and 360, air-dried and ground to pass through a 2-mm plastic sieve for

analysis. The rest of the pots were left undisturbed and allowed to settle down over 60 days. After that, 2.00 g

seeds of L. perenne with average germination rate over 95% were sown.L. perenne was harvested 8, 12, 16

and 20 weeks after sowing, by cutting just above the soil surface. Only above-ground parts of the plants were

considered for analysis, since they are more relevant to grazing animals.

Total Organic Carbon

Soil organic matter plays an essential role in the cycle of nutrients (N, P, K), and affects the sustainability

of soil fertility. It has been reported that the non-decomposable organic fraction diminishes PTE toxicity

symptoms, through adsorption and removal from soil solution (Alloway and Jackson, 1991; Alvarenga et al,

2008).


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The application of the mixture of sewage sludge and its incinerated ash (SSA) significantly increased soil

organic carbon in the three amended soils compared to control (Figure 3) in all the studied period. On the other

hand, organic matter content significantly increased (p


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low compared with high clay content (Merckx et al, 1985). Residual substrate and decomposition products may

become stabilized by sorption onto mineral particles and by incorporation into soil aggregates, being physically

inaccessible to microbial turnover (Christensen, 1996). However, sewage sludge carbon mineralization in the

three studied soils did not depend on soil texture. These results suggest that the recently introduced sludge-

organic carbon was located in larger pores and less entangled in aggregates than native soil organic matter.

Thomsen et al. (1999) reported that the turnover of organic matter in differently textured soils was better

explained by soil moisture parameters than by soil texture. As the water content of the three soils studied was

periodically adjusted according to water holding capacity, water availability was high and did not limit microbial

activity. Thus, no relationship between soil texture and sewage sludge mineralization was observed during the

first year of application. In this way, sludge-borne organic matter characteristics and not soil properties would

initially predominate when high doses of sewage sludge are applied to soil, in the zone of sludge incorporation.

Liming effect

The application of the mixture of sewage sludge and its incinerated ash (SSA) significantly increased the

pH-value of the three amended soils (Figure 4). It is usually considered that metals in incinerated sewage sludgeare mainly in the form of oxides, sulfates and phosphates, with metallic oxides being the most abundant. Zhang

et al (2001) explained that the mechanism of pH increase in the incinerated ash can be considered as follows:

(1) formation of alkaline metallic oxides due to decomposition of complicated metal compounds during

incineration (2) losses of acidic anions (SO42

, NO3, Cl

, etc.), accompanied with the emission of acidic gases

(SO2, SO3, NO2, NO, Cl2, etc).

With the passage of time, a decrease in the pH-values of the amended soils was observed, suggesting

that the neutralizing ability of both amendments was not enough to fully inhibit the decrease in pH. The initial and

significant decrease in soil pH could have been the result of a flush in nitrification of ammonium contained in

sludge-borne organic matter, which, according to Stamatiadis et al. (1999) is a likely process shortly after sludgeapplication to soil. The decomposition of organic matter and production of organic and inorganic acids by soil

microorganisms activity is also likely to be responsible for the pH decrease, in agreement with Mathur (1991).


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Typic Hapludoll

bb

bb

a aaaa

cc

ccc

4,0

4,5

5,0

5,5

6,0

6,5

7,0

0 100 200 300 400 days

Typic Natraquoll

b cc

c b

bbb

aa a a

a

4,0

4,5

5,0

5,5

6,0

6,5

7,0

0 100 200 300 400 days

pH

Typic Argiudoll

bb

bb b

aa a

a

a

c c cc

c

4,0

4,5

5,0

5,5

6,0

6,5

7,0

0 100 200 300 400 days

Control

SS treatment

SSA treatment

Figure 4: pH evolution in the three soils amended with 150 dry t. ha-1 of sewage sludge (SS) or a mixture of 70:30

DMW sewage sludge and incinerates sewage sludge ash. Different letters in the same sampling date indicate significant

differences at the 0.05 probability level (Tukey test).

Compared to SS, pH values for SSA amended soils were significantly higher from day 60 onwards,

indicating a slow solubilization of the liming materials produced during the incineration process. It can be

concluded that a potential benefit of mixing sewage sludge with its own incinerated ash is that the liming effect of

the ash can partially offset decreases in soil pH, arising from nitrification or the decomposition of organic matter.

In all cases, the decrease in pH values could be correlated with time ( t), and the amounts of carbon

mineralized (Ecuation 1)

pH (C min-30, t) = - 0.03869 - 0.417 . C min-30 (t) - 9.55 10-4

. t R2

= 0.6161 [Ec 1]

where pH (t) is the difference in pH values between day 30 and day t ; C min-30 (t) is the amount of C mineralized in day t

compared to day 30.

Although a decrease in the pH values was observed for the three amended soils with time, at the end of

the experimental period, all amended soils presented pH values significantly higher than controls. Moreover, pH


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values for SSA amended soils were significantly higher compared to SS, indicating the potential use of

incinerated sewage sludge ash as a soil liming agent.

Electrical Conductivity

Electrical conductivity (EC) is widely used as a reliable indicator of the salinity of soils. It is among the

most useful and easily obtained properties of soil that influences crop productivity (Corwin, Lesch 2003). Under

conditions of excessive soluble salts, the growth reduction or death of a crop is primarily due to reduced root

water absorption, or toxicity or a combination of both (Landschoot, McNitt, 1994). In this study, statistically

significant differences in EC values between soils amended with SS or SSA were measured. The results

indicated that application of both SS and SSA showed a similar pattern of EC changes, a continuous steady rise

until day 360 (Figure 5). This was due to the release of soluble salts in the ash or sludge, and also to the release

of mineral salts such as phosphates and ammonium ions through the decomposition of organic substances,

together with proton-released as a result of microbial nitrification process (Torri, 2001).

The increment in the electrical conductivity of the soils could be correlated to time (Ecuation 2)

EC (t) = m . t + b R2> 0.85 [Ec 2]

where EC (t) is the electrical conductivity (dS m-1

) at day t after amendment

Typic Hapludoll

cc

b

b

aa

a

a

bba

a

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 100 200 300 400 days

Control

SS Treatment

SSA Treatment

Typic Natraquoll

bbbb

a

a

a

a

a

a aa

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 100 200 300 400 days

EC

(dSm-1)

Typic Argiudoll

b

cba

a

a

a

a

ba

aa

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 100 200 300 400 days


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Figure 5: EC evolution in the three soils amended with 150 dry t. ha-1 of sewage sludge (SS) or a mixture of 70:30

DMW sewage sludge and incinerates sewage sludge ash. Different letters in the same sampling date indicate significant

differences at the 0.05 probability level (Tukey test).

Several authors claimed that EC of soil should not exceed the salinity limit value of 1,500 s cm1

in

order to avoid excessive accumulation of salts (Soumare et al. 2003; Amir et al. 2005). In this assay, soil

moisture was maintained at 80% of WHC by daily adding distilled water, so salts were not washed and,

consequently, accumulated in the soil. In the field conditions of the Pampas Region, with a mean annual

precipitation of 1147 mm, these salts can easily be flushed or leached out of the soil if draining conditions are

adequate.

Lol ium PerenneL. biomass

Plant uptake is a major pathway by which potentially toxic metals can enter the food chain. The

availability' of an element in soil is related to its uptake by vegetal species.Perennial ryegrass (Lolium perenne

L.) is a gramineous species that accumulates moderate to high levels of PTE in its biomass from soil reservoirs

in the readily extractable and soluble forms, fitting the definition of a facultative metallophyte (Smith and

Bradshaw, 1979). L. perenne is a cool-season perennial bunchgrass native to Europe, temperate Asia, and

North Africa. It is widely distributed throughout the world, including North and South America, Europe, New

Zealand, and Australia (Hannaway et al, 1999). High palatability and digestibility make this species highly valued

for extensive dairy and sheep forage systems. As a result, it is the preferred forage grass in temperate regions of

the world, like in the Pampas Region, Argentina, where livestock grazing is the predominant farming system in

marginal areas. High growth rates under high fertility and an extensive root system make L. perennealso valued

for use in nutrient recycling systems.

The germination of L. perenne in both sludge amended soils in the pot experiment was delayed for 15

days, conditioned by the phytotoxic potential of SS or SSA (Zucconi et al, 1985). These results are in opposition

with a previous phytotoxic assay on seed germination (Torri et al, 2009), in which no germination delay was

observed among sludge treatments for this species The delay herein observed for both amendments may be the

result of an increase in Zn availability over incubation time (Torri, Lavado 2008 a), the intense mineralization of

the labile organic matter pool of the sludge (Torri et al, 2003), which may have originated ammonia, low

molecular weight organic acids and/or salts, all of which have been shown to have inhibitory effects (Wong et al,1983; Chaney 1983; Adriano et al, 1973). Other studies have also reported that this toxic effect disappears

within 14 to 21 days (Bentez et al , 2001).


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Table 4: Partial and total mean values and standard deviation of aerial dry weight (g) of L. perennegrown in control and sludge-treated pots over four harvests (n = 3,

S.E.). Soils: Typic Hapludoll, Argiudoll and Natraquoll. Treatments: C= control, SS= sewage sludge amended soils, SSA= soils amended with the 70:30 DMW mixture of

sewage sludge and incinerates sewage sludge ash. Groups in a column detected as different at the 0.05 probability level (Tukey test) were marked with different letters (a,

b, c, etc. for partial harvest ; A,B,C, etc. for total yield)

1 harvest 2 harvest 3 harvest 4 harvest Total DM yield

aerial dry weight (in g) of L. perenne / pot

Hapludoll - C 2,43 0,055 ab 2,66 0,116 ab 3,01 0,138 bc 1,73 0,103 b 9,83 BC

Hapludoll - SS 2,45 0,078 ab 3,01 0,108 a 5,49 0,149 a 4,38 0,301 a 15,32 A

Hapludoll - SSA 2,82 0,105 ab 2,99 0,067 a 5,82 0,103 a 3,14 0,072 ab 14,78 A

Natraquoll - C 1,97 0,094 b 1,76 0,133 b 1,11 0,138 c 0,84 0,068 c 5,68 C

Natraquoll - SS 2,22 0,137 ab 2,45 0,136 ab 4,99 0,229 ab 4,65 0,257 a 14,32 A

Natraquoll - SSA 2,32 0,128 ab 2,35 0,095 ab 4,31 0,308 ab 4,17 0,446 a 13,15 AB

Argiudoll - C 2,51 0,163 ab 1,96 0,162 b 1,58 0,161 c 0,78 0,024 c 6,82 C

Argiudoll - SS 3,07 0,070 a 3,28 0,020 a 5,28 0,354 a 4,50 0,264 a 16,13 A

Argiudoll - SSA 2,87 0,134 ab 2,96 0,054 a 4,74 0,220 ab 3,33 0,239 ab 13,90 AB


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After emergence, L. perennegrew uniform in both sludge treatments along the growing period, showing

no visible symptoms of metal toxicity or nutrient imbalances. Partial and total dry matter yields of L perenne

grown in each treatment in the three soils are shown in Table 4. Both sludge amendments resulted in an

increase in plant aerial biomass during the experimental period compared with plants grown in unamended soils.

No significant differences in terms of total or partial dry matter yield were observed between SSA and SS

treatments for each soil. Several factors may have contributed to improve growth in both sludge amended soils,

especially the increased supply of N and P, in agreement with Antolin et al (2005) and Hseu, Huang (2005),

together with an increasing limitation on nutrient supply in control soils with time. In addition, an improvement in

water holding capacity in the amended soils was also observed. Improvement in physical and biological soil

properties rather than in chemical properties (N, P and K content) was reported to be important for the growth of

L. perenne(Villar et al, 2004). On the other hand, organic amendments play an important role in the revegetation

of degraded soils, and were found to be more effective in improving crop yield than inorganic fertilizer (Ye et al.,

1999).

Potentially trace elements concentration in aerial plant tissues of Loluim perenneL.Potentially trace elements concentration of Cd, Cu, Pb and Zn in the first and third harvest of L. perenne

followed the order Zn >> Cu >> Cd, Pb in all treatments.

In all harvests, Cd and Pb concentrations were below detection limits in aerial part of L. perenne. These

results were in agreement with results obtained in a previous study, in which these element were only extracted

from the residual fraction of the amended soils (Torri, Lavado 2008 a), considered inactive in terms of chemical

processes. The results obtained in this research reflect the low availability of sludge-born Cd and Pb in soils

amended with sewage sludge from Buenos Aires City.

Table 5: Accumulation of Cu (mg kg1

DW) in shoots of Lolium perenneL. grown in unamended soils and soils amended with

150 dry t. ha-1

of sewage sludge (SS) or a mixture of 70:30 DMW sewage sludge and incinerates sewage sludge ash.

Groups in a column detected as different at the 0.05 probability level (Tukey test) were marked with different letters.

1 harvest 3 harvest

Treatment Cu (mg kg-1 DW) STD error Cu (mg kg-1 DW) STD error

Hapludoll - C 19.23 0.59 ab 3.70 0.06 c

Hapludoll - SS 26.08 1.174 a 9.63 0.33 ab

Hapludoll - SSA 16.62 1.58 abc 8.61 0.293 b

Natraquoll - C 15.83 0.45 abc 4.14 0.05 c

Natraquoll - SS 16.27 0.15 abc 12.53 0.043 a

Natraquoll - SSA 10.89 0.67 bcd 11.66 0.507 ab

Argiudoll - C 6.64 0.015 d 5.11 0.11 c

Argiudoll - SS 10.11 1.17 cd 11.04 0.341 ab

Argiudoll - SSA 8.004 0.10 d 10.03 0.582 ab


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In the first harvest, Cu concentration in aerial biomass did not seem to depend on soil treatment, for no

significant differences were observed between control and SS or AS treatments for the same soil. On the

contrary, the addition of SS or AS amendments significantly increased Zn concentration in the aerial part ofL.

perenne compared to controls. Nevertheless, Cu and Zn concentrations in shoots in the first harvest were

significantly higher in the coarse textured soil compared to the fine textured soil (Tables 5 and 6). Several

studies indicated that crops grown on sandy, low organic matter status soils are likely to have a greater uptake of

certain PTE compared with crops grown on soils with higher clay and organic matter contents (Alloway 1990).

Other studies on Cu adsorption by individual soil components have indicated relatively strong bonding and high

capacity of silicate minerals to adsorb Cu, whereas the amounts of Cu that can be readily desorbed is very small

(Wu et al, 1999). On the other hand, Egiarte et al. (2006) stated that sludge-borne Zn compounds are relatively

highly soluble and that exchange reactions are the main way of retention for Zn in soils. It can be concluded that

the higher concentration of clay in the Argiudol soil might have supplied more binding sites, reducing Cu and Zn

availability to L. Perenne.

Table 6: Accumulation of Zn (mg kg1

DW) in shoots of Lolium perenneL. grown in unamended soils and soils amended with

150 dry t. ha-1

of sewage sludge (SS) or a mixture of 70:30 DMW sewage sludge and incinerates sewage sludge ash.

Groups in a column detected as different at the 0.05 probability level (Tukey test) were marked with different letters.

1 harvest 3 harvest

Treatment Zn (mg kg-1

DW) STD ERROR Zn (mg kg-1

DW) STD ERROR

Hapludoll - C 64.241 1.80 d 22.84 0.51 b

Hapludoll - SS 378.92 12.95 a 157.77 4.92 a

Hapludoll - SSA 215.64 10.43 abc 121.64 2.13 a

Natraquoll - C 55.968 2.08 d 19.898 0.72 b

Natraquoll - SS 257.58 9.22 ab 156.78 3.17 a

Natraquoll - SSA 178.8 9.29 bc 166.3 3.75 a

Argiudoll - C 27.512 0.17 e 28.048 0.85 b

Argiudoll - SS 177.57 11.28 bc 153.33 2.97 a

Argiudoll - SSA 120.1 3.81 c 122.88 3.51 a

In the third harvest, a significant decrease of Cu and Zn in aerial biomass concentration compared to the

first harvest was observed in both sludge amended soils, irrespective the soil considered. The decrease in Cu

and Zn concentration was probably originated by an initial depletion of available sludge-borne elements.

Nevertheless, Cu and Zn concentrations in the above ground tissue ofL.perenne grown in the amended soils

were still significantly higher than controls, indicating high availability.

L. perennegrown in soils amended with the mixture of sewage sludge and incinerates sewage sludge

ash did not exhibit significantly higher Cu or Zn concentration in aerial biomass compared to SS treatment in the

three soils. These results are in agreement with previous studies, in which incineration was found to reduce the


17/21

availability of Cu and Zn, increasing the percentage of residual fractions (Torri, Lavado 2008 a; Torri, Lavado

2008 b). These findings are in good agreement with previous results reported by Obrador et al. (2001).

Copper and Zn concentration in shoots was in all cases below the range of critical concentration in

plants described by Kabata-Pendias and Pendias (2000). Moreover, the concentration of these elements in

aerial tissue was found to be under the threshold values specified by the NRC (1985) suggesting that

consumption of L.perennegrown on sludge amended soils would pose no risk to grazing animals. This issue is

crucial in order to avoid the threat of transfer of metals to the food chain. Physiological mechanisms that regulate

the internal translocation of PTE have been postulated for this species (Santibez et al, 2008).

CONCLUSIONS

The studies performed over three soil samples of representative soils of the Pampas Region, Argentina,

showed that the use of a mixture of sewage sludge containing 30% DM of its own incinerated ash as a soil

amendment significantly increased soil organic carbon, pH and EC in the three amended soils compared to

control. No significant differences in Cd, Cu, Pb and Zn concentrations in the aerial tissue of L. perenne were

observedbetween both sludge amendments. Cadmium and Pb concentrations were in all cases below detectionlimits in aerial part of L. perenne. On the contrary, Cu and Zn concentration in the above ground tissue was

significantly higher in the amended soils than controls, indicating a high Cu and Zn availability. Nevertheless, the

concentration of these elements in aerial tissue were found below the maximum tolerable levels of daily intake

by cattle.

The results obtained suggest that land application of a mixture of sewage sludge and incinerated ash

does not pose a significant risk of transfer of the studied elements to the food chain compared to land application

of sewage sludge. Therefore, the mixture of sewage sludge and incinerated sewage sludge may play a

significant role as a soil amendment in land reclamation, especially if nonfood chain crops are grown. However,

from an agricultural point of view, the results herein obtained cannot be extrapolated directly for makingpredictions about in situ Cd, Cu, Pb or Zn availability or mobility in sludge amended soils. Nevertheless, the

results obtained in this study provide supporting evidence for the protection theory, which hypothesizes that

mineral components or the stable organic matrix in the sludge may compensate for any loss of metal retention

capacity caused by mineralization of labile organic compounds.

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