Disposal of Domestic Sludge and Sludge Ashes on Volcanic
Soils
Mauricio Escudeya*, Andrew C. Changb, Juan E. Förstera, Juan P. Becerraa, Magdalena
Quinterosa, Justo Torresa and Gerardo Galindoa
a
Facultad de Química y Biología, Universidad de Santiago de Chile
Av. B. O’Higgins 3363, Santiago, Chile.
E-mail: mescudey@lauca.usach.cl
2
Department of Environmental Sciences, University of California, Riverside CA
E-mail:acchang@ucr.edu
Abstract
Column leaching experiments were conducted to test the ability of volcanic soils in
Chile in retaining the mineral constituents and metals in sewage sludge and sludge ash
that were incorporate into the soils. Relatively small amounts of the mineral constituents
such as Na, Ca, Zn, phosphorus and sulfate in the sludge and sludge ash were readily
soluble. When they were incorporated in to the surface layer of the soils and leaching
with 12 pore volumes of water over a 3 month period of time, significantly small
amounts of the readily soluble constituents were recovered in the drainage water. The
volcanic soils were capable of retaining the mineral constituents, phosphorus, and
metals in applied sewage sludge and sludge ash and gradually release them as nutrients
for plant growth.
Keywords: Sewage sludge, sludge ashes, column studies, volcanic soils.
1. Introduction
Sewage sludge is the inevitable end product of municipal wastewater treatment
processes worldwide. As the wastewater is being purified, the impurities removed from
the water stream are being concentrated. The sludge stream thus contains many
chemical and microbiological constituents usually in concentrated forms that may
become potential sources of pollutants when the material is released. No matter how
many treatment steps it undergoes, at the end, the sludge and/or its derivatives require
the ultimate disposal. For disposal, the sewage sludge may be land applied, land filled,
incinerated, or ocean dumped. There is not an entirely satisfactory solution and all of the
currently employed disposal options have serious draw backs. Land application
however is by far the most commonly used method around the world. Approximately
six million dry tons of sewage sludge is produced annually in the United States.
(Bastian, 1997). Recent report showed that the annual production of sewage sludge in
member countries of the European Union may reach as much as 8 x 10 6 tons (Bonnin, et
al., 2001). Significant amounts of sewage sludge produced in the United States and the
western European nations have been applied on land. Dependent on the regions, 24 to
89% of the sludge produced in the U.S. has been applied on land (Bastian, 1997).
Bonnin (2001) reported that 65% of the sewage sludge in France was land applied. The
situations in other parts of the world are expected to be similar.
As the residue of municipal wastewater treatment, sewage sludge represents the
aggregation of organic matter, pathogens, trace elements, toxic organic chemicals,
essential plant nutrients, and dissolved minerals originally dispersed in the wastewater
and are captured and transformed by the wastewater treatment processes. Properly
managed, the potential pollutants are assimilated via the biochemical cycling processes
of the receiving soils in the land application. The practice provides soils with organic
materials and offers the possibility of recycling plant nutrients, which in turn, improve
the fertility (Walter and Cuevas, 1999) and physico-chemical properties of agricultural
soils (Illera et al., 2000). If not appropriately controlled, the potential pollutants released
through the land application may degrade the quality of downstream water bodies, be
transferred through the food chain to harm the consumers of harvests, and drastically
alter the physical and chemical properties of the receiving soils. It is imperative that
mass input provides adequate amounts of substances that are useful to plant
development and the pollutant inputs are controlled to avert detrimental public health
and environmental effects. Major countries such as the U.S., the European Union
(http://europe.eu.int/comm/environment/sludge), and China (Qiao et al., 2000) had
enacted regulations or issued guidelines that limited the disposal options for a variety of
reasons.
Before the treatment works are gradually being brought online in recent years, the
collected wastewater in Chile was directly discharged and sewage sludge did not exist.
With the commencement of wastewater treatment, sewage sludge and ashes of the
incinerated sewage sludge are accumulating in the metropolitan areas awaiting final
disposals. As the treatment works handles mainly the domestic sewage, the levels of
toxic pollutants were negligible. Land application is the primary option under
consideration.
The agricultural soils in Central Chile where most of the country’s population centers
are situated are derived from parent material of volcanic origin. The predominant
minerals of these soils are allophone and ferrihydrite in the Andisols and kaolinite,
hollysite and amours iron oxides in Ultisols. These soils were rich in iron oxides and
organic matter contents and possessed pH-dependent variable surface charge. However,
the soils have poor fertility. At the indigenous acidic pH range of 4.5 to 5.5, the soil has
low capacity for exchangeable cations. Phosphorus is strongly fixed by the primary
minerals thus is not readily available for plant absorption in these soils. To be
productive, they require frequent adjustments of soil pH, replenishments of
exchangeable Ca2+ and Mg2+ ions, and heavy phosphate applications. Municipal sewage
sludge and ashes of the incinerated sewage sludge appear to possess the essential plant
nutrients and dissolved minerals and the buffering capacity (Eriksson, 1998; Zhang et
al., 2002ab; Pasquini and Alexander, 2004). When land applied, they may replenish the
depleting nutrient reservoirs in these soils under cultivations. If the added constituents
are retained in the soils and absorbed by plants, the risk of contaminating the
downstream water bodies may be minimized. In this study, the capacity of volcanic
soils to retain chemical constituents in the land applied sewage and sewage sludge ash
was investigated.
Methods and Procedures
Soils
The surface 0 to 25 cm depth layers of 5 volcanic soils located in the
agricultural regions of the Southern Central Chile were collected. Namely, they were
Collipulli, Diguillin, Nueva Braunau, Metrenco, and Ralún reflecting the localities from
where soils were extracted. The samples were obtained from well drained and regularly
cultivated fields. By soil classification, Collipulli and Metrenco are Ultisols and Ralún,
Diguillin, and Nueva Braunau are Andosols. General information on the climate and
geography of the soils may be found in Escudey et al. (2001). Soil samples were
screened in the field to pass a screen with 2 mm openings and stored at the field
moisture content in a 4oC cold room until used.
Experiments
Soils were air dried and packed into columns to the depth of 25 cm
according to their respective field bulk densities. Dependent on the treatment, thirty
grams of air dried sewage sludge or the ashes equivalent of 30 grams of air dried
sewage sludge were incorporated into the surface 5 cm of the packed columns. The
experimental controls received neither the sludge nor the ash treatment. Following the
treatments, the columns were irrigated weekly with one pore volume of distilled water
and gravity drained, for a period of 12 weeks. In addition, 30 grams of sludge and the
ash equivalent of 30 grams of sludge were leached in the same manner. The drainages
from each weekly leaching cycle were accumulated for analyses of pH, electrical
conductivity, SO42- and PO43-.
At the end of the leaching experiment, each soil column was cut open lengthwise and
the profile was sectioned into five equal length segments for analyses of the soils’ pH,
electrical conductivity, and organic carbon, exchangeable cations, and phosphorus
contents.
Chemical Determinations The bulk density, exchangeable cations, and organic carbon
content of the soils were determined by methods outlined in the Methods of Soil
Analysis. Part 3 (American Society of Agronomy, Madison, WI). Briefly, the
bulk density was determined by the average air dried weight of soils in
undisturbed soil cores of the 0 to 25 cm soil profile in 5 cm (dia meter) x 5 cm
(height) brass rings; the exchangeable cations were determined as the
concentrations of Na, K, Mg, and Ca in ammonium acetate extracts; and
organic carbon was determined by measuring the chromic acid oxidized
fractions of soils. The pH and electrical conductivity of soils was measured in soil
suspensions with soil to water ratio of 1: 2.5 w/v. The total elemental contents of Na,
K, Mg, Ca, Zn, Cu, Fe, Al, P and S were determined digesting the soils with a
concentrated HNO 3 -HCl-HF mixture in a microwave and measuring the
concentrations by ICP-OES spectroscopy. Comparable components of the sewage
sludge and sludge ash were determined in the same manner. The SO42- and PO43concentrations in the drainage water were measured by ion chromatography.
Results and Discussions
Soils, Sludge, and Sludge Ash
Prior to the sludge and ash treatments, the soils were
acidic with pH varying from 4.5 to 5.9 and low in exchangeable bases contents varying
from 1.5 to 10.4 cmol kg-1 (Table 1). In contrast, the sewage sludge and sludge ash had
pH of 7.7 and 7.4, respectively that were 2 to 3 orders magnitude higher in alkalinity
than those of the soils. The exchangeable base content of the sewage sludge was 34.5
cmol kg-1, 3.5 to 10 times those of the soils. The Na, K, Mg and Ca in the sludge ash
were soluble but not necessary in the exchangeable forms. Judging by their electrical
conductivities, the soluble mineral contents of ash were orders of magnitude larger than
the soils. The total elemental contents of the Ca, Mg, K, and Na in soils follow the same
trends as those in the exchangeable forms and the concentrations are in the same order
of magnitude.
Table 1. Properties of Soils, Sewage Sludge and Sludge Ash
Soil
Collipulli
Metrenco
Ralún
Diguillin
N. Braunau
Sludge
Sludge Ash
pH
5.4
5.5
4.5
5.9
5.5
7.7
7.4
Bulk
Density
(g cm-3)
Organic
Carbon
(%)
Electrical
Conductivity
(S m-1)
Exchangeable Bases
(cmol kg-1)
Na
K
Mg
Ca
1.36
1.33
0.90
1.12
0.82
0.46
-
2.3
1.8
5.0
5.0
11.4
17.8
<0.1
81
29
436
94
20
8520
3890
0.1
0.2
0.1
0.2
0.1
2.7
-
0.2
0.3
0.1
0.7
0.1
1.6
-
1.8
1.5
0.4
1.1
0.2
7.6
-
5.9
4.0
2.5
8.4
1.1
22.6
-
Releases from Sludge and Sludge Ash
When the sludge and sludge ash were
leached, the soluble Na and SO42- were released quickly (Figure 1). Judging from the
shapes of the break through curves (Figure 1), the soluble sodium and sulfate in sewage
sludge were depleted with one pore volume of water used to leach the soils. On the
other hand, the soluble sodium and sulfate in sewage sludge ash gradually release with 5
to 8 pore volumes of water. The quantities of Na and SO 42- released from the sludge and
sludge ash however were comparable, amounted to 19 vs. 16 mg for Na and 342 vs. 319
mg for SO42- (119 vs. 106 mg as S), respectively. The releases were considerably less
than the total amounts in the respective material.
The phosphorus in sludge and sludge ash released slowly and released at constant rates
over time (Figure 2). At the end of 12 leaching cycles, 24 and 7 mg of phosphate were
14
Sodium Release
Na in Leachate (mg/L)
12
Sew age Sludge
10
Sew age Sludge Ash
8
6
4
2
0
0
2
4
6
8
10
12
Cumulative Leachate Volume (L)
350
Slufate in Leachate (mg/L)
Sulfate Release
300
Sew age Sludge
250
Sew age Sludge Ash
200
150
100
50
0
0
2
4
6
8
Cumulative Leachate Volume (L)
10
12
Figure 1. Releases of Sodium and Calcium from Sewage Sludge and Sludge Ash
recovered from the drainages, respectively in contrast to the total amounts of 109 and
121 mg, respectively.
The patterns of zinc releases for the sludge and sludge ash were similar (Figure 3).
However, the amounts released by the sludge, 0.82 mg, were considerably higher than
that of the sludge ash, 0.02 mg. Nevertheless, they were far below the total amounts of
9.6 and 9.8 mg in the sludge and sludge ash, respectively.
30
P Release
Cumulative P Leached (mg)
25
Sew age Sludge
20
y = 2.098x
Sew age Sludge Ash
2
R = 0.99
15
10
5
y = 0.587x
2
R = 0.91
0
0
2
4
6
8
10
12
Cumulative Leachate Volume (L)
Figure 2. Phosphorus Release of Sewage Sludge and Sludge Ash
0.35
Zinc Release
Zn in Leachate (mg/L)
0.30
Sew age Sludge
Sew age Sludge Ash
0.25
0.20
0.15
0.10
0.05
0.00
0
2
4
6
8
Cumulative Leachate Volume (L)
10
12
l
Figure 3. Zinc Release from Sewage Sludge and Sludge Ash
In all, only small amounts of the Na, SO42-, P, and Zn were released when the sludge
and sludge ash were subject to intense leaching for 12 weeks.
Soil Attenuation Even without the applications of sludge or sludge ash, significant
amounts of cations and anions such as Ca2+ such as SO42- maybe leached from the soils
(Figure 4) and the amounts collected in the drainage water were dependent on
conditions of soils. Sludge and sludge ash amendments consistently enhanced the
leaching of minerals. However, the incremental increases were significantly smaller
than the total amounts added through the addition of sludge and sludge ash. Judging
from the break through curves, the releases were spread out over 3 to 4 pore volumes of
drainage water. Soil incorporation further reduced the mobility of the chemical
constituents in the sludge and sludge ash (Figure 5). For phosphorus and zinc, the
amounts found in the drainage water (Figure 5) were 2 to 3 orders of magnitude lower
30
Calcium Leached
Calcium (meq/column)
25
Control
Sludge-treated
Ash-treated
20
15
10
5
0
Collipullli
Metrenco
Diguillin
N. Braunau
Ralún
N. Braunau
Ralún
50
Sulfate Leached
45
Control
40
Sludge-treated
Sulfate (meq/column)
Ash-treated
35
30
25
20
15
10
5
0
Collipullli
Metrenco
Diguillin
Figure 4. Calcium and Sulfate Leached from Sewage Sludge and Sludge Ash Treated Soils
that the amounts present in the added sludge and sludge ash. As a result, nutrients such
as the available phosphorus significantly increased with the application of sewage
sludge and sludge ash for both the Ultisol and Andisol (Figure 6).
7
Zn Leached
6
Control
Zn ( eq/column))
Sludge-treated
5
Ash-treated
4
3
2
1
0
Collipullli
Metrenco
Diguillin
N. Braunau
Ralún
1.4
P Leached
Control
1.2
Sludge-treated
Ash-treated
P (eq/column)
1
0.8
0.6
0.4
0.2
0
Collipullli
Metrenco
Diguillin
N. Braunau
Ralún
Figure 5. Zn and Phosphate Leached from Sewage Sludge and Sludge Ash Treated Soils
Conclusions
Results of column leaching experiments showed that volcanic soils in Chile were
capable of retaining the inorganic mineral constituents, phosphorus, and zinc in sewage
sludge and sludge ash when land applied. These constituents are representatives of
inorganic minerals, nutrients, and metals that often associated with the municipal
sewage sludge. In this regard, the volcanic soils will attenuate the sewage sludge borne
pollutants and provide soils with nutrients that may be slowly released for crop
production.
60
Diguillin
Available P (mg/kg)
50
Control
Sludge-treated
40
Sludge ash-treated
30
20
10
0
0 - 5 cm
5 - 10 cm
10 - 15 cm
15 - 20 cm
20 - 25 cm
Soil Depth
70
Metrenco
Available P (mg/kg)
60
Control
50
Sludge-treated
Sludge ash-treated
40
30
20
10
0
0 - 5 cm
5 - 10 cm
10 - 15 cm
15 - 20 cm
20 - 25 cm
Soil Depth
Figure 6. Available Phosphorus in the Sewage Sludge and Sludge Ash Treated Ultisol (Metrenco)
and Andisol (Diguillin).
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