Comparing different methods of analysing sewage sludge, dewatered sewage sludge and sewage sludge ash

Comparing different methods of analysing sewage sludge, dewatered sewage sludge and sewage sludge ash

Desalination 250 (2010) 399–403 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m ...

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Desalination 250 (2010) 399–403

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Comparing different methods of analysing sewage sludge, dewatered sewage sludge and sewage sludge ash☆ Gaston Hoffmann ⁎, Daniel Schingnitz 1, Bernd Bilitewski Technische Universität Dresden, Institute of Waste Management and Contaminated Site Treatment, Pratzschwitzer Str. 15, 01796 Pirna, Germany

a r t i c l e

i n f o

Article history: Received 27 November 2007 Accepted 25 October 2008 Available online 17 October 2009 Keywords: Sewage sludge Phosphorus Heavy metals Sulphur Transfer coefficients Chemical analysis

a b s t r a c t The following article compares different ways of characterising sewage sludge. Against the background of sludge recycling in agriculture as well as treatment with subsequent phosphorus recovery in mind, the article starts by collating and evaluating the levels of phosphorus, heavy metals, chlorine and sulphur in sludge as reported in the literature. Sewage sludge from the sewage treatment plant at Kaditz in Dresden was analysed using standardised and adjusted methods, which produced different results. In the course of this analysis the results were produced by using elemental analysis, atomic absorption spectroscopy, X-ray fluorescence spectroscopy (X-RFA) and ion chromatography (IC). The second part of the article therefore seeks reasons for the differences in the findings and tries to give solution statements. The article closes by calculating transfer coefficients for selected parameters during the incineration process and solid–liquid separation and weighing up the analysis techniques compared. Basically this article will show significant differences in sludge composition and the effects on the specific elements by ashing or mechanical dewatering of the different sludges. An essential attention obtains the analysis by using the X-RFA. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Knowledge about sludge composition is especially important when it comes to phosphorus recovery and the safe disposal of sewage sludge. However, different analytical methods are known to produce different results. What are the reasons for the differences reported in the literature? During preliminary investigations, the results of phosphorus and heavy metal analysis in sewage sludge found in the literature were collected and evaluated. Thereby significant differences of the concentrations of phosphorus were found. The content of phosphorus was reported to range from 15 wt.% [1] down to 0.01 wt.% [2]. Here the following authors give examples for phosphorus concentrations: - [8]: 4 wt.%, - [9]: 8 wt.%, - [13]: 3 wt.%. Either not all sludge is suitable for phosphorus recovery or the low levels reported are the result of spurious analysis. The content of heavy metals is especially interesting regarding the following aspects: - Compliance with limit values in connection with the use of sewage sludge in agriculture; ☆ Presented at the 1st Conference on Environmental Management, Engineering, Planning and Economics (CEMEPE), Skiathos, Greece, 24-28 June, 2007. ⁎ Corresponding author. Tel.: +49 3501 530032. E-mail addresses: [email protected] (G. Hoffmann), [email protected] (D. Schingnitz). 1 Tel.: +49 3501 530032. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.064

- Enrichment in solid residues during thermal treatment in connection with the use of sewage sludge in agriculture; - Mobilisation in the groundwater after use as fertilizer in agriculture. Here too, analysis of the literature does not provide comparable results regarding heavy metal content in sewage sludge, including compliance with limit values. The results for zinc are found to vary especially greatly. The sludge described in [1,2,4] complies with the German limit value of 2500 mg zinc/kg [3] — whereas that from [5–7] does not. During the investigations, substantial indications regarding the influence of the analytical methods used were found. Furthermore, the effect of the mechanical dewatering carried out was also examined. Besides heavy metal and phosphorus concentrations also interest the sulphur content. Here [12] and [14] published a content of 0,5 wt.%, whereas [13] analysed 1,7 wt.% and [11] 1 wt.%. 2. Materials and methods All investigations were conducted using the quantitative important sewage sludges from the Kaditz sewage treatment plant in Dresden. Primary sludge (PS), surplus sludge (SS), concentrated surplus sludge (cSS) and dried sludge (dS) were analysed. It should be noted that the chemical precipitation of phosphorus was achieved by iron and aluminium salts simultaneously with various compositions. After sampling the sludge samples were centrifuged and filtrated. So it was possible to produce the solid matter out of the sludge. After this treatment step the sludges were dried in a drying chamber up to a constant weight. The dried sludges were pulverised to a size of less than

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100 µm using a Retsch M301 swing mill. The grinding gear was made of tungsten carbide. Table 1 lists the analytical methods used after sample preparation. 2.1. Phosphorus For analysing the contents of phosphorus in the sewage sludge samples on the one hand a photometer was used. On the other hand the analysing of phosphorus was realized by using the X-RFA. With exception of the analysis by the X-RFA a digestion of the dried samples is necessary. The aim of the digestion is the complete transfer of the solid ingredients in a soluble and measureable form. During the present investigation, the extraction with aqua regia was used. Furthermore, the proportion of dissolved phosphate should determine with the IC. Due to high sulfate levels resulting from the digestion of the samples respectively the protection of the anion column of the IC the samples have to be greatly diluted and so the concentration of phosphorus was reduced and reliable results could be achieved. The principle of the IC was used to analyse the content of e.g. sulphur, chlorine and phosphorus. The method of measurement relies on different detention periods of different substances inside the column of the IC. The low affinity of the ions to a stabile phase causes a faster pass through the column and shorter detention period. Before analysing the samples with the IC a digestion of the samples in the calorimetric bomb is necessary. By using the X-RFA the pulverised sludges were analysed as powder and the sludges were also pressed to tablets and then analysed by the X-RFA. By the dint of X-RFA it is possible to analyse all elements from sodium up to uranium. The sample is irradiated by X-ray radiation which is after the polarization animated by targets in the sample. So this measurement allows analysing every single element in the sample depending on the emitted irradiation. 2.2. Heavy metals An alternative to this standardised method was the digestion by a microwave for a subsequent determination of the heavy metals by using the atomic absorption spectroscopy (AAS). AAS is a technique for determining the concentration of a particular metal element in a sample. It can be used to analyse the concentration of more than 62 different metals in a solution. In short, the electrons of the atoms in the atomizer can be promoted to higher orbitals for an instant by absorbing a set quantity of energy. This amount of energy is specific to a particular electron transition in a particular element, and in general, each wavelength corresponds to only one element. This gives the technique its elemental selectivity. By using the flame-AAS it is possible to analyse the content of alkali metals, earth-alkaline metals and heavy metals. Depending on the concentration of the heavy metals the analysis by using the AAS with a graphite tube is necessary.

Phosphorus

Heavy metals

Sulphur was analysed by using the X-RFA and also the IC after the digestion with the calorimetric bomb. Apart from the abovementioned methods, the content of alkali metals, earth-alkaline metals, sulphur and chlorine was also analysed during the investigations. Chemical pulping with a calorimeter bomb (IKA Calorimeter System C700) was used in order to analyse the sulphur and chlorine concentrations with an IC (System Metrohm IC 732). 2.4. Transfer coefficients after the ashing process The produced ashes in the muffle oven were analysed with the AAS after the aqua regia digestion. 3. Results and discussions 3.1. Phosphorus Clear differences resulted when analysing the same sludge with different methods and also, when analysing different types of sludge with the same method (Table 2). Before dealing with the individual types of sludge, the possible drawbacks of the analytical methods need to be considered. Using X-RFA has the disadvantage in that the results depend on the quality of calibration because it is a comparative method. Calibration is important in order to minimise the possible influence of matrix effects (dispersion and absorption of entering X-rays and emerging fluorescence radiation). Only marginal differences were recorded between the results of the phosphate cell test and the use of X-RFA in tablet form. By contrast, the results of using X-RFA in powdered form differed significantly, especially in connection with centrifuged PS, centrifuged cSS and centrifuged SS. This can doubtlessly be attributed to the low number of standards (just 17) used for the calibration when developing the method X-RFA for powder. Alternative disintegration in a microwave oven produced similar results compared to regular disintegration with aqua regia. 3.2. Heavy metals When analysing heavy metals, different results were expected, especially for concentrations below 100 mg/kg. However, as can be seen from the results (Table 3), this was not found in every case. The comparison was based on the assumption that methods using disintegration in a microwave oven and analysis with AAS produce exact results. However, for cadmium and mercury, there are clear differences between the results obtained with the different methods tested. As already stated, the reason for this could be low fluorescence radiation or poor calibration.

Table 2 Results of phosphorus analysis.

Table 1 Analytical methods used. Parameter

2.3. Sulphur

Phosphorus

Disintegration

Measurement

Disintegration

Name

DIN

Name

DIN

Microwave With aqua regia Not essential Not essential Microwave Not essential Not essential

EN 13346 EN 13346

Photometer Photometer X-RFAtabs X-RFApowder AAS X-RFA TQtabs X-RFA TQpowder

EN 1189 EN 1189 – – 38406 – –

EN 13346

X-RFA TQ: X-ray fluorescence spectroscopy with TURBOQUANT evaluation. DIN: Deutsche Industrie Norm. EN: Europäische Norm.

Measurement

PS

cSS

SS

dS

[wt.% DM]

Not essential Not essential Microwave Aqua regia

X-RFA as tabs X-RFA as powder Photometer Photometer

DM: dry mass. av.: average. s.d.: standard deviation. [10].

av.

s.d.

av.

s.d.

av.

s.d.

av.

s.d.

0.5 1.0 0.7 0.6

0.01 0.03 0.02 0.00

2.7 3.9 3.5 2.8

0.03 0.06 0.39 0.19

2.9 3.9 3.5 2.3

0.01 0.05 0.08 0.86

1.1 2.6 1.9 1.9

0.01 0.03 0.37 0.14

G. Hoffmann et al. / Desalination 250 (2010) 399–403

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Table 3 Results of heavy metals analysis. Disintegration

Measurement

PS

cSS

SS

dS

AbfKlärV

[mg/kg DM]

Zn

Ni

Pb

Cd

Cu

Cr

Hg

Microwave Not essential Not essential Microwave Not essential Not essential Microwave Not essential Not essential Microwave Not essential Not essential Microwave Not essential Not essential Microwave Not essential Not essential Microwave Not essential Not essential

AAS X-RFA(tabs) X-RFA(powder) AAS X-RFA(tabs) X-RFA(powder) AAS X-RFA(tabs) X-RFA(powder) AAS X-RFA(tabs) X-RFA(powder) AAS X-RFA(tabs) X-RFA(powder) AAS X-RFA(tabs) X-RFA(powder) AAS X-RFA(tabs) X-RFA(powder)

av.

s.d.

av.

s.d.

av.

s.d.

av.

s.d.

447 350 464 39 12 19 42 18 24 0.9 8.7 4.6 97 87 110 33 10 27 – 1.8 2.3

15.4 2.4 6.1 3.1 1.1 2.1 2.0 0.7 0.6 0.1 1.2 0.4 5.3 3.2 1.4 1.5 0.2 1.9 – 0.0 0.1

610 586 611 41 16 18 37 27 32 7.7 7.0 4.7 212 214 208 71 19 19 0.3 2.4 2.4

0.0 3.8 6.9 3.2 0.2 2.5 0.8 0.5 0.2 0.0 1.2 0.1 5.2 2.9 1.0 1.8 0.6 0.0 0.1 0.1 0.0

635 598 612 43 16 18 37 27 31 9.3 6.5 5.4 208 208 203 59 19 18 0.5 2.6 2.4

21.2 9.9 7.0 2.6 1.8 0.8 2.8 0.6 2.1 0.0 0.5 1.2 2.9 3.5 0.7 1.0 0.0 0.0 0.4 0.1 0.0

722 674 827 39 24 32 49 34 45 0.8 16.6 6.2 228 168 201 66 16 18 0.65 1.8 2.5

15.0 21.0 2.0 2.0 1.5 1.9 3.0 1.2 0.4 0.1 1.8 1.7 8.0 6.8 1.1 5.0 0.0 1.1 0.1 0.1 0.0

2500

200

900

800

900

8

AbfKlärV: sewage sludge ordinance. DM: dry mass. av.: average. s.d.: standard deviation. [3,10].

During the analysis of sulphur the PS contained the highest concentrations of sulphur. That is due to the fact that sulphur is a component of amino acids (predominant in leftovers) and so it deposits as PS. The elemental analysis produced insignificantly higher concentrations of sulphur as the data from the analysis by X-RFA. Table 4 clearly shows that the analysis of sulphur is also possible with the X-RFA in comparison to the chemical pulping with the calorimetric bomb and the analysis by IC. Furthermore, the analysis shows that with increasing weight of the samples during the chemical pulping with the calorimetric bomb there is also a higher variation limit of the samples. The reason is an incremental adsorption of the sample on the wall of the calorimetric bomb.

In contrast, the analysis of the elements zinc, nickel, lead, copper and chromium were highly accurate even though the concentrations of lead and nickel in particular were below 50 mg/kg. X-RFA can be rated as a very quick, simple method for the analysis of heavy metals in dried sewage sludge. In some cases the values obtained need to be verified with standardised methods. Moreover, X-RFA could be improved by a better calibration and the development of a method especially for sewage sludge. Furthermore, if the sewage sludge is to be used in agriculture, analysis with X-RFA could be followed by a verification using some other methods.

3.3. Sulphur 3.4. Transfer coefficients after the ashing process Sulphur is an essential element of all living cells and therefore an important nutrient for flora and fauna. Hence, due to degradation it is also found in sewage sludge. During the thermal treatment of sewage sludge the ratio of sulphur and chlorine affects the high-temperature corrosion. As a result, the concentrations of sulphur are very important.

Thermal treatment seems to be the current trend for the disposal of sewage sludge. Hence, to analyse the behavior of the sewage sludge it is important to analyse the ash of the sewage sludge and define the transfer coefficients. The transfer coefficients specify the concentration

Table 4 Results of sulphur analysis. Method

Sulphur Multi EA 2000

X-RFA (TQ, tabs)

X-RFA (TQ, powder)

Schöniger + IC

Calorimetric bomb + IC

[wt.% DM]

dS PS cSS SS

av.

s.d.

av.

s.d.

av.

s.d.

av.

s.d.

av.

s.d.

0.9 0.3 1.0 0.9

0.07 0.02 0.05 0.05

0.6 0.3 0.8 0.7

0.02 0.002 0.01 0.01

1.0 0.5 1.0 0.9

0.003 0.02 0.01 0.01

0.6 0.3 0.6 0.6

0.01 0.05 0.19 0.05

0,4 0,6 0,5 0,4

0.04 0.01 0.04 0.02

Multi EA 2000: elemental analysis. X-RFA TQ: X-ray fluorescence spectroscopy with TURBOQUANT evaluation. DM: dry mass. av.: average s.d.: standard deviation. [10].

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G. Hoffmann et al. / Desalination 250 (2010) 399–403

Fig. 1. Transfer coefficients of the ashes (potassium, calcium, iron, copper, zinc, lead and copper measured by AAS; sulphur measured by elemental analysis (EA); chlorine measured by IC; and phosphorus measured by photometer).

of an element which transfers in the ash of the sludge. Therefore, a special new calibration for the analysis of the ash using X-RFA has been carried out. By that, it is possible to obtain quicker and more precise results. During the study several ashings at different temperatures were performed (700 °C, 800 °C, 900 °C and 1000 °C). The resulting ash was analysed by AAS. Thereby, a different behavior of special elements was observed. During the ashing the elements calcium, magnesium, potassium, iron, nickel, copper and phosphorus are non volatile. In contrast, the elements zinc, lead, cadmium, chlorine and sulphur degrade with an increasing temperature. It can be noted that cadmium, chlorine and sulphur act most volatile. The elements lead and zinc need a temperature of 800 and 900 °C, respectively, to exchange into the gas phase. Phosphorus concentrates in the ash during the ashing process which is supporting a potential recovery from the ash. With increasing temperatures the amount of ash decreases so that phosphorus becomes a main part in the ash. The determination of the transfer coefficients occasionally showed a coefficient larger than 100% which is assumed to be due to the heterogeneity of the samples. In those cases the transfer coefficient was set to 100%. Fig. 1 shows the transfer coefficients varying with the incineration temperature for the single elements tested. 3.5. Influence of mechanical dewatering The results for the primary sludge indicated a significant accumulation of chromium, copper, lead, cadmium, nickel, zinc, iron and

sulphur in the solid part. In other words, increased dewatering causes the concentration of heavy metals to rise. Potassium, sodium and chlorine were found to behave differently. With increased mechanical dewatering, the concentration of chlorine in the solids drops. These cognitions should not be the purpose in the present article. In spite of everything this aspect needs to be considered in the case of subsequent thermal treatment and the risk of high-temperature corrosion. Fig. 2 shows the analytical results. With sampling being carried out before the precipitation reaction, the discovery of solid phosphorus was unexpected. This phenomenon can be explained by the sewage treatment technology used (the vast majority of the phosphorus stays in a liquid condition and is precipitated in following treatment steps) and the low concentration of phosphorus compared with SS. Consequently, only a small part of the phosphorus is separated as PS by gravitation. 4. Conclusion Our analysis revealed reasons for the at times significant differences in sludge composition. The differences are solely due to aspects of process engineering but can rather be attributed to the analytical techniques used. The investigation also showed how mechanical dewatering or ashing affects the produced sludge and accordingly the produced ash. Furthermore, differences were also found when using standardised methods, most obviously for phosphorus. During the ashing processes of the samples it was possible to show the accumulation of phosphorus and the specific decrease of other elements in the ash. On examination of the standard deviations it badges that if there is less weight of the sample for analysis so it is a raising standard deviation to be expected. This behavior is based on the heterogeneity of the samples and tight-knit influences of the heterogeneity on the results of the measurements. Simple and correct analytical methods for a fast process modification are necessary, especially to develop methods for phosphorus recovery and sludge treatment. X-RFA could be a possibility, although the findings still needed to be validated using standardised methods. References

Fig. 2. Solid/liquid relation after mechanical dewatering.

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