Como citar este trabajo: Torri S. 2009. Feasibility of using a mixture of sewage sludge and incinerated sewage sludge as a soil amendment. In: Sludge: Types, Treatment Processes and Disposal. Editor: Richard E. Baily, Nova Science Publishers, Inc., Hauppauge, NY 11788. ISBN: 978-1-60741-842-9. pp. 187-208, 317p.

1. FEASIBILITY OF USING A MIXTURE OF SEWAGE SLUDGE AND INCINERATED SEWAGE SLUDGE AS A
SOIL AMENDMENT
Silvana Irene Torri
Cátedra de Fertilidad, Facultad de Agronomía, Universidad de Buenos Aires,
Avda San Martín 4453, Buenos Aires (C1417 DSE), Argentina
torri@agro.uba.ar
Abstract
The accumulation of sewage sludge poses nowadays a growing environmental problem. Incineration is a
feasible means of reducing sewage sludge’s 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.

2. FEASIBILITY OF USING A MIXTURE OF SEWAGE SLUDGE AND INCINERATED SEWAGE SLUDGE AS A
SOIL AMENDMENT
Silvana Irene Torri
Cátedra de Fertilidad, Facultad de Agronomía, Universidad de Buenos Aires,
Avda San Martín 4453, Buenos Aires (C1417 DSE), Argentina
torri@agro.uba.ar
INTRODUCTION
Disposal of sewage sludge
Wastewater treatment plants usually generate millions of tons of sewage sludge every year. Sewage
sludge results from the accumulation of solids from chemical coagulation, flocculation and sedimentation during
wastewater treatment. Worldwide, sludge production is steadily increasing, driven by the increasing percentage
of households connected to central treatment plants, the increasingly tightening of pollution limits on the effluent
discharged, as well as the availability of technologies capable of achieving higher efficiency of wastewater
treatment.
Sewage sludge contains undesirable hazardous substances such as trace elements, ranging from less
than 1 ppm to over 1000 ppm (Dewil et al, 2006), PCBs, PAHs and dioxins (Abad et al, 2005; Clarke et al, 2008,
Martínez et al, 2007), pesticides and endocrine disruptors (Byrns 2001; Bernhard et al, 2006), pathogens and
other microbiological pollutants (Muga et al, 2008). Therefore, sludge has to be properly treated and disposed to
prevent environmental contamination and health risk. Sludge processing is intended to improve dewatering
characteristics, eliminate disease-causing bacteria, reduce smell and decrease the quantity of organic solids. In
this way, the end product can be treated further or disposed of with less handling problems and environmental
consequences.
Historically, sewage sludge has been disposed of by landfilling or ocean dumping (Bridle and Skrypski-
Mantele, 2000). Ocean disposal of sludge is nowadays forbidden in practice. Alternatively, landfilling is the most
common method for sewage sludge disposal in many parts of the world due to its relatively low cost. However,
due to reduced land availability, increasing compliance costs, public opposition, leachate and greenhouse gas
emission concerns, sludge deposits in landfills are to be soon phased out in many countries (USEPA 1994; CEC
2000; ME 2007). At present, there are other disposal methods for processed sludge, such as agricultural
application as a soil amendment and incineration (Hong et al, 2008; Schmidt et al. 2006; Stasta et al, 2006).
Composting is also recognized as a recycling option in some countries. On the other hand, due to ever
increasing transportation and disposal costs, efforts are being made to reduce the quantity of sludge for
disposal. In Argentina, most of the sludge from wastewater treatment plants are aerobically stabilized and, due
to regulation and jurisdiction problems, discarded in land farming or, to a minor extent, as a soil amendment on
lawns or landfilling. However, as in the rest of the world, production of sewage sludge is expected to continue
increasing in the future. In this context, a likely future option seems to be sludge incineration.

3. Sewage sludge Incineration
Incineration is a feasible means of reducing sewage sludge’s 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). In
this 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 nutrients
considered 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

4. soil inorganic constituents (Antoniadis et al, 2008). Moreover, it has been suggested that sewage sludge’s 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 between
328 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, some
montmorillonite 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 60ºC, ground and sieved (<2 mm) (Figure 1 A), and then split into two
portions. One portion was incinerated at 500º C, and the ash obtained (Figure 1 B) was mixed with a portion of

5. 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 500º 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 SEM–EDS. The main crystalline component of SS was
quartz (SiO2, 26.90º 2θ), 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 (α-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).

6. 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 60
angle 2 θ
counts
Q
Q
QQ
Q
Q
Q
QQQ
Q
Q
H
H
P P
PP
C
Q = quartz (SiO2)
P = plagioclase (Na,Ca)(Si,Al)4O8)]
H = hematite (α-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 this
element 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 system of humans.

7. 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
) 334.2 365.9
Total 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 USEPA´s limits (USEPA, 1993). Moreover, Cd, Cu and Zn concentrations 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 Biosolid
1
39 1500 300 2800
USEPA – MAMC
2
85 4300 840 7500
kg ha
-1
CPLRs
3
39 1500 300 2800
1
PC = Pollutant Concentration Biosolid
2
MAMC = Maximum Allowable Metal Concentrations
3
CPLRs = Cumulative Pollutant Loading Rates

8. 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
Ca
2+
(cmolc kg
-1
) 10.2 9.1 12.6
Mg
2+
(cmolc kg
-1
) 2 5.4 4.3
Na
+
(cmolc kg
-1
) 0.3 2.1 0.2
K
+
(cmolc kg
-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).

9. 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<0.05) in SS amended soils compared to SSA amended
soils, due to its higher organic matter content. This observation indicates a positive impact on improving soil
fertility and soil quality, as reported by Pedra et al (2007). Although physical properties such as bulk density,
water-holding capacity and infiltration rates were not measured in this study, increased soil carbon can lead to
the improvement of these soil properties (Bradford and Peterson 2000).
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
Typic Hapludoll
c c c c c c
a
a a a a ab
b b b a b
10
20
30
40
50
60
0 100 200 300 400 days
Control
SS treatment
SSA treatment
Typic Natraquoll
c c b b b b
a
a ab
b
a a a
10
20
30
40
50
60
0 100 200 300 400 days
mgCg-1
soil
Typic Argiudoll
c c c c c c
a
a a
a a ab
b b b b b
10
20
30
40
50
60
0 100 200 300 400 days
Figure 3: Total Soil Carbon 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).
During the first 60 days after sludge application, a rapid decomposition of the added carbon was
observed. Carbon added through sewage sludge consisted of two fractions of different degrees of
biodegradability: a labile fraction (53-71%) that mineralized quickly and a resistant fraction (29-45%), apparently
not available to soil microorganisms (Torri et al, 2003), that remained in the soils one year after sludge
application. Similar results were observed by other authors in sewage sludge amended soils (Antoniadis 2008;
Ojeda et al, 2008). Carbon mineralization from added substrates has been shown to be more rapid in soils with

10. 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 sludge
are 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 (SO4
2−
, 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 sludge
application 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).

11. 330
340
350
360
Typic Hapludoll
b
b
b
b
a aaaa
c
c
c
cc
4,0
4,5
5,0
5,5
6,0
6,5
7,0
0 100 200 300 400 days
Typic Natraquoll
b c
c
c b
bb
b
a
a 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
b
b
b
b b
a
a a
a
a
c c c
c 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 R
2
= 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

12. 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 R
2
> 0.85 [Ec 2]
where EC (t) is the electrical conductivity (dS m
-1
) at day t after amendment
Typic Hapludoll
c
c
b
b
a
a
a
a
b
ba
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
bbb
b
a
a
a
a
a
a a
a
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
b
a
a
a
0,0
0,5
1,0
1,5
2,0
2,5
3,0
0 100 200 300 400 days

13. 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 μs cm
−1
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.
Lolium Perenne L. 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. perenne also 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 (Benítez et al, 2001).

14. Table 4: Partial and total mean values and standard deviation of aerial dry weight (g) of L. perenne grown 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)
1º harvest 2º harvest 3º harvest 4º 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

15. After emergence, L. perenne grew 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 perenne L.
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 kg
−1
DW) in shoots of Lolium perenne L. 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.
1º harvest 3º 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

16. 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 of L.
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 kg
−1
DW) in shoots of Lolium perenne L. 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.
1º harvest 3º 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 of L.perenne grown in the amended soils
were still significantly higher than controls, indicating high availability.
L. perenne grown 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. 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.perenne grown 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 (Santibáñez 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
observed between both sludge amendments. 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 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 making
predictions 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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