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Columbia International Publishing Advances in Civil and Environmental Engineering (2014) Vol. 1 No. 1 pp. 27-42

Research Article

______________________________________________________________________________________________________________________________ *Corresponding e-mail: jatosin@tu.kielce.pl 1* Kielce University of Technology, Poland

The Evaluation of the Impact of Sewage Sludge Ash Modification on Leaching of Heavy Metals

Jolanta Latosińska1*

Received 8 October 2013; Published online 8 March 2014 © The author(s) 2014. Published with open access at www.uscip.us

Abstract Incineration of sewage sludge generates ash, the use of which is limited for instance due to concentration of heavy metals. The chemical composition of the sewage sludge ash is substantially similar to the chemical composition of coal ash, which is used to obtain a synthetic zeolite. This paper presents the results of the modification of ash from sewage sludge to produce synthetic zeolite. The study found a positive effect of sewage sludge ash modifications by fusion method. Leaching of heavy metals from the sewage sludge ash before and after the modification procedure was made in accordance with the Extraction Procedure (EP). The increase in the activation temperature resulted in a increase in the leaching level of heavy metals from the sewage sludge ash. Keywords: Sewage sludge ash; Zeolitization; Heavy metals

1. Introduction Sewage sludge is a by-product of municipal wastewater treatment plants. According to the National Waste Management Plan 2014 in Poland the quantity of sewage sludge will be still increasing in future. Main reasons of the increase are the implementation of The Council Directive 91/271/EEC and The National Urban Wastewater Treatment Program. Sewage sludge has the amount of 40 – 80% of organic matter (De la Guardia, 1996; Bezak-Mazur and Dańczuk, 2013). The organic matter, phosphorus, potassium and microelements allow for the usage in the agriculture but the heavy metals content limits its natural usage (Council Directive 86/278/EEC; Pavlíková et al., 1998). Heavy metals may occur in sewage sludge in the states: dissolved, precipitated, co-precipitated with metal oxides, adsorbed or associated on particles of biological remains. They can appear in the form of oxides, hydroxides, sulfides, sulfates, phosphates, silicates, organic combinations in the shape of humic groups and compounds with polysaccharides (Gawdzik and Latosińska, 2010).

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The EU standards limit the deposition of sewage sludge in landfills (Council Directive 99/31/EC; Directive 2008/98/EC) and its application in the natural environment (Council Directive 86/278/EEC). The environmental use of sewage sludge gains no social acceptance, and in addition, it is seasonal (The Law on Waste, 2013). Furthermore, the lack of land suitable for the agricultural application in the vicinity of municipal wastewater treatment plants necessitates the development of thermal methods (Werle and Wilk, 2010). Thermal processes allow for a reduction of sewage sludge volume, which equals 10% of the volume of mechanically dewatered sewage sludge (Werther and Ogada, 1999). The applicability of thermal methods for sewage sludge disposal is determined by its properties, e.g., the heat of combustion (calorific value) and composition (including moisture content). There are thermal technologies available, both in the market and under development, for thermal processing of sewage sludge: combustion (multiple hearth furnace, fluidized bed, smelting and rotary furnaces), co-combustion (with coal, with other fuels, with municipal solid waste, in other process e.g. cement production), alternative processes (pyrolysis, oil from sludge, gasification, hybrid methods) (Gašparovič et al., 2011; Bień et al., 2012). The incineration does not provide a zero-waste disposal method since approximately 30% of the solids remain as ash (Malerius and Werther, 2003). It is estimated that 1.2 million tons of incinerated sewage sludge ash are produced in North America and the EU per year. The sewage sludge incineration contributes to the increase of risks resulting from the landfill deposition of ashes. The concentration of heavy metals in sewage sludge ash determines the method of their utilization. In the case of exceeding the admissible limits of heavy metals, the ash is considered hazardous waste (Donatello et al., 2010b). Sewage sludge ash contains inorganic components such as Al2O3, SiO2 and flux (e.g. Fe2O3, CaO, MgO) (tab. 1). Sewage sludge ash can be used as a partial clay substitute in bricks (Tay and Show, 1992; Anderson et al., 1996; Wiebusch and Seyfried, 1997), partial cement substitute in mortars (Monzo et al., 2003; Pan et al., 2003), in asphalt (Al Sayed et al., 1995), as a raw material in lightweight foamed blocks and aggregates (Wang & Chiou, 2004; Cheesman and Virdii, 2005; Wang et al., 2005; Chiou et al., 2006), in cement clinker (Lin, 2005) and in glass ceramics (Park et al., 2003; Merino et al., 2007). Zeolites comprise a large group of microporous crystalline solids with well-defined structures that contain mainly aluminium, silicon, and oxygen in their regular framework as well as cations and water in the pores (Holler and Wirsching, 1985). There are two kinds of zeolites: natural and artificial. The natural zeolites are mined in many parts of the world, most of them are used at industrial level (Queral and Mereno, 2002). Due to these properties, they can be used in many fields, for instance treatment of waste water. The ratio of Si/Al of natural zeolites ranges from 1 to 6 (Wang et al., 2003). The Si/Al ratio influences the formation of a particular type of zeolite. The content of silicon and aluminium determines the usefulness of raw materials for the synthesis of zeolites. One of the raw materials to synthesize zeolites is fly ash. Nowadays many techniques have been applied for the conversion of fly ash into zeolites (Suchecki, 2005): - classic alkaline conversion, - alkaline fusion stage prior to the conventional zeolite synthesis, - dry or fused salt conversion, - two-stage synthesis procedure. The composition of sewage sludge ashes is significantly similar to the composition of fly ashes (tab.1). It predisposes sewage sludge ashes for the use as raw material for the synthesis of zeolites.

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Table 1 Characteristics of sewage sludge ash and fly ash

Components, % d.m. Sewage Sludge Ash, Suchecki,

2005 Fly Ash,

Suzuku et al., 1997 SiO2 48.2 45.67

Fe2O3 8.5 8.01 Al2O3 18.7 23.47

CaO 4.1 5.09 MgO 1.9 3.69

K2O+Na2O 1.5 2.23 TiO2 1.8 -

MnO2 0.4 - P2O5 5.6 - SO3 2.6 0.61 CuO 0.2 - ZnO 0.5 -

C 0.3 -

The aim of this work was to determine if the modification of sewage sludge ash by the fusion method had an impact on leaching of heavy metals from modified sewage sludge ash.

2. Experimental Sewage sludge ash from Łomża wastewater treatment plant was used in the research. The dried sewage sludge was incinerated in a grate furnace at the temperature of 850oC. Sewage sludge ash was taken from a storage place. The average moisture content of sewage sludge ash was measured by heating samples to 105oC. Loss on ignition was determined by heating sewage sludge ash at the temperature of 550oC. SEM analysis of the sewage sludge ashes (before and after modification) was performed with Microscope – QUANTA FEG 250 (Fig. 1, Figs. 3 and 4). The crystalline phases present in sewage sludge ash were determined by the X-ray diffraction, 32kV/22mA, λ(Kα) - 1.5418 Ǻ, 2Θ step interval 0.03o (10o

-75o). The interpretation of diffraction patterns was performed by comparing the position of the diffraction lines to the database of the International Centre for Diffraction Data, Release 2006 (Fig. 2).

The element contents of sewage sludge ash were determined with spectrometry method (XFR and AAS) (tab.3). Leaching of heavy metals from sewage sludge ash (before and after modification) was prepared according to the EP procedure (EPA Regulation on Land Disposal Restrictions). The EP procedure is based on leaching by deionized water with pH= 5.0 (pH value constant during the research; correction with the use of 0,5 N CH3COOH), shaken for 24 hours, ratio of sewage sludge ash to deionized water 1:10 (by mass).

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The modification of sewage sludge ash was conducted with the fusion method comprising two stages. The first stage: 10 g of sewage sludge ash was mixed and ground with NaOH to obtain a homogeneous mixture. The sample was heated in an electric laboratory furnace at the temperature of 550oC for 1 h. Different ratios of sewage sludge ash to NaOH samples (tab.2) were used to explore the effect of this parameter in the modification. The second stage: the fusion product was ground and dissolved in 85 ml of distilled water, followed by an ageing process with vigorous agitation in a shaking water bath at different temperatures (20oC, 60oC) for a given period of time (tab.2.). Subsequently, the mixture was crystallised under a static condition at different temperatures (40oC, 60oC, 90oC) for 1 hour. Table 2 The experimental conditions of modification of sewage sludge ash

Sample Ratio SSA:NaOH g/g

Activation Crystallization

Temperature, oC Time, h Temperature, oC Time, h SSA1 1:1.0 20 24 40 1.0 SSA2 1:1.0 20 24 60 1.0 SSA3 1:1.0 20 24 90 1.0 SSA4 1:1.2 20 24 40 1.0 SSA5 1:1.2 20 24 60 1.0 SSA6 1:1.2 20 24 90 1.0 SSA7 1:1.4 20 24 40 1.0 SSA8 1:1.4 20 24 60 1.0 SSA9 1:1.4 20 24 90 1.0

SSA10 1:1.6 20 24 40 1.0 SSA11 1:1.6 20 24 60 1.0 SSA12 1:1.6 20 24 90 1.0 SSA13 1:1.8 20 24 40 1.0 SSA14 1:1.8 20 24 60 1.0 SSA15 1:1.8 20 24 90 1.0 SSA16 1:1.0 60 12 40 1.0 SSA17 1:1.0 60 12 60 1.0 SSA18 1:1.0 60 12 90 1.0 SSA19 1:1.2 60 12 40 1.0 SSA20 1:1.2 60 12 60 1.0 SSA21 1:1.2 60 12 90 1.0 SSA22 1:1.4 60 12 40 1.0 SSA23 1:1.4 60 12 60 1.0 SSA24 1:1.4 60 12 90 1.0 SSA25 1:1.6 60 12 40 1.0 SSA26 1:1.6 60 12 60 1.0 SSA27 1:1.6 60 12 90 1.0 SSA28 1:1.8 60 12 40 1.0 SSA29 1:1.8 60 12 60 1.0 SSA30 1:1.8 60 12 90 1.0

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At the end of the modification process the solid was separated by filtration, washed several times with distilled water (until the solution reached pH 10) and then dried for 8 hours at the temperature of 105oC.

3. Results and discussion The examined sewage sludge ash had the moisture of 27.8%, pH 8.6 and loss on ignition 2.17% d.m. The element content of sewage sludge ash from the Łomża wastewater treatment plant was presented in Table 3. According to the elementary analysis, sewage sludge ash consisted of over 10 % of Ca. Therefore, this sewage sludge ash was classified as calcium ash. This sewage sludge ash, compared with the description by Vogel et al. (2011) and Petze et al. (2012) comprised high quantity of phosphorus. The source of phosphorus in sewage sludge ash is phosphorus coming from municipal waste water originating for instance from detergents.

Fig 1. SEM of sewage sludge ash, magnified 2400 x, 10 000 x

The sewage sludge ash had an amorphous phases and locally porous fields (Fig. 1). The concentration of heavy metals in the leaching of sewage sludge ash did not exceed the limits set out in EPA Regulation on Land Disposal Restrictions. The concentration of Pb, Cd and Cr exceeded the limits according to Journal of Laws No. 204, point 178, 2002, Table 4. This sewage sludge ash consisted of complex aluminosilicates and alkali metal salts (Fig. 2). These crystalline phases present in sewage sludge ash are similar to results showed by Pan et al. (2003) and Fontes et al. (2004).

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10 20 30 40 50 60 70

0

20

40

60

80

100

Fe

2(S

O4) 3

·9H

2O

CaS

K2M

g(S

O4) 2

·6H

2O

KC

l

(Mg ,F

e )

3A

l 2S

i 3O

12 p

yro

pe

CaS

1

SiO2

Lic

zba z

liczeń

2 theta

Inte

nsity

Fig 2. XRD data for sewage sludge ash

Table 3 Content of elements in sewage sludge ash, %.

Composition Sewage sludge ash

Al2O3 10.21 SiO2 32.93 P2O5 30.93 Na 0.16 Mg 2.2 K 2.76 Ca 11.8 Ti 0.356

Mn 0.093 Fe 3.34 Ni 0.0024 Cu 0.05 Zn 0.169 Sr 0.0237 Y 0.00071 Zr 0.0278 Nb 0.00061 Mo 0.0001 Pb 0.00625

Figs. 3 and 4 present the results for the impact of modification conditions of sewage sludge ash on the structure. Figs. 3 and 4 show a significant increase in the modified particles surface area

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compared to the sewage sludge ash particle. This is a result of sewage sludge ash modification by fusion. The SEM micrographs of sewage sludge ash after modification show created agglomerates and their interconnection. The samples SSA13, SSA14 and SSA15 are characterised by the smallest quality of agglomerates of samples with activation at ambient temperature, while the sample SSA6 has the biggest crystallisation structure (Fig. 3). The samples SSA17 and SSA23 have the smallest quantity of agglomerates among samples with activation at 60oC (Fig. 4). The samples with activation at 60oC are characterised by higher agglomeration than samples with activation at ambient temperature (Figs. 3 and 4). The modification of sewage sludge ash by fusion method has an impact on leaching of heavy metals (Figs. 5 and 10). The increase of ratio of sewage sludge ash to NaOH from 1:1.4 to 1:1.8 shows the increase of concentration of heavy metals in these extracts (Figs. 5 and 10). The SSA10 sample has the maximum leaching of all researched heavy metals. The high concentration of Pb in the elutes can be explained by the characteristics of this metal. The modification reaction of sewage sludge ash (the presence of H2O, the atmospheric oxygen, CO2, NaOH) and the leaching medium (acetic acid) contributes to the transition of Pb to solution (Bielański, 1981). The relatively low concentration of Cd results from a high volatility of this heavy metal. The transition of metals to the eluates is also due to the amphoteric properties of researched metals. The increase of activation temperature causes the increase of heavy metals leaching from sewage sludge ash (Figs. 5 and 10). Table 4 Leaching of heavy metals from sewage sludge ash and allowable limits in water.

Metal Sewage sludge

ash, mg/dm3

Limitation according to EPA Regulation on

Land Disposal Restrictions,

mg/dm3

Limitation according to Journal of Laws No. 204, point 178, 2002

mg/dm3

Category recommended

Category allowable

Pb 0.061 5 not limited 0.05 Cd 0.009 1 0.001 0.005 Cu 0.008 not limited 0.02 0.05 Zn 0.019 not limited 0.5 3.00 Cr 0.009 5 not limited 0.05 Ni 0.046 not limited not limited 0.05

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SSA1 SSA2 SSA3

SSA4 SSA5 SSA6

SSA7 SSA8 SSA9

SSA10 SSA11 SSA12

SSA13 SSA14 SSA15

Fig 3. SEM of sewage sludge ash after modification – activation 20oC, 24 h, magnified 10000 x

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SSA16 SSA17 SSA18

SSA19 SSA20 SSA21

SSA22 SSA23 SSA24

SSA25 SSA26 SSA27

SSA28 SSA29 SSA30

Fig 4. SEM of sewage sludge ash after modification–activation 60oC, 12 h, magnified 10000x

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Fig 5. Leaching of Cd from sewage sludge ash after modification

Fig 6. Leaching of Cu from sewage sludge ash after modification

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Fig 7. Leaching of Ni from sewage sludge ash after modification

Fig 8. Leaching of Cr from sewage sludge ash after modification

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Fig 9. Leaching of Zn from sewage sludge ash after modification

Fig 10. Leaching of Pb from sewage sludge ash after modification

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Table 5 Electrical conductivity and pH value of water extracts obtained from sewage sludge ash after modification.

Sample Electrical conductivity of water extract

(mS /cm) pH of water extract

(at the end of extraction procedure)

SSF1 5.25 6.3

SSF2 7.19 6.4

SSF3 6.57 6.5

SSF4 5.66 6.7

SSF5 5.73 6.6

SSF6 5.70 7.0

SSF7 9.03 6.3

SSF8 6.53 6.9

SSF9 8.67 6.7

SSF10 11.62 5.4

SSF11 9.76 7.0

SSF12 11.10 5.7

SSF13 9.50 5.6

SSF14 10.25 7.0

SSF15 10.16 6.7

SSF16 8.82 5.24 SSF17 10.16 5.9 SSF18 8.57 5.7 SSF19 9.25 5.59 SSF20 9.63 5.7 SSF21 9.27 6.1 SSF22 9.88 5.64 SSF23 9.47 5.59 SSF24 10.43 5.7 SSF25 9.87 5.67 SSF26 9.59 5.38 SSF27 9.63 5.36 SSF28 9.79 5.56 SSF29 8.26 5.79 SSF30 9.63 5.36

4. Conclusion The research confirms that sewage sludge ash can be used as a potential raw material of artificial zeolites. The ratio of sewage sludge ash to NaOH effected the structure of modified sewage sludge ash. It has been observed that the increase of ratio of sewage sludge ash to NaOH shows the increase of concentration of heavy metals. The activation temperature has disadvantageous impact on reduction of the heavy metals concentrations in the effluent. The activation temperature also

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influenced the change in the structure in the direction of the transformation leading to the formation of zeolites.

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