Content uploaded by Luciano A. Gomes
Author content
All content in this area was uploaded by Luciano A. Gomes on Jul 13, 2024
Content may be subject to copyright.
Drying sewage sludge with coal fly ash for producing a soil amendment
L. A. Gomes1,2*, R. Lopes1, J.C. Góis3, M.J. Quina,1
1CIEPQPF, Chemical Process Engineering and Forest Products Research Centre. Department of Chemical Engineering,
University of Coimbra, Portugal; 2IFB - Federal Institute of Education, Science and Technology of Brasília - IFB, Campus
Ceilândia, Brasília - Federal District, Brazil; 3Association for the Development of Industrial Aerodynamics, Department of
Mechanical Engineering, University of Coimbra, Coimbra, Portugal; *luciano.gomes@ifb.edu.br
The high quantities of sewage sludge (SS) and coal fly ash (CFA)
produced require an adequate management for protecting human
health and the environment. This study aims to investigate the
possibility of using CFA as SS drying adjuvant for producing a soil
amendment.
The study involved the characterization of SS and CFA at a physical
and chemical level. The drying process of small cylinders, for several
combinations of SS:CFA (100:0 - control; 95:5; 90:10 and 85:15) at
different temperatures (40, 70, 100 and 130 ºC) was investigated.
The dynamic model of the drying process was based on the Fick law
and an infinite length was assumed for the small cylinders.
The kinetics analysis showed that as the quantity of CFA increases
the rate of drying is faster. The effective diffusion coefficient and
other relevant parameters were determined for each thermal drying
condition.
The results showed that the drying process of SS with small
quantities of CFA may be a route for valorizing these wastes in
agricultural soil.
Introduction
Coal fly ash (CFA) is produced in large quantities in
thermoelectric power plants, that burn coal. CFA is a fine
particulate material, composed mainly by ferroaluminosilicates
and other minor elements, whose quantities depend on the
characteristics of the coal, the operating conditions of the furnace
and the sampling point [1]. In general, CFA contain significant
amounts of Si, Al, Fe, Ca, K and Na. The recycling of the CFA
can be an excellent alternative to manage this waste, allowing
significant economic and environmental benefits. Utilization has
been encouraged in several countries, and the quantities valorised
are about 90% in the European Union, 50% in USA, 60% in India
and 70% in China [2].
Regarding sewage sludge (SS) production in Portugal, data from
the literature shows figures around 300 kt year-1 in dry base [3].
The management of this waste is expensive due to the high
moisture content (greater than 80%), which leads to significant
costs associated with transportation, storage and final disposal.
The main constituents of SS are water, biomass
(microorganisms), inorganic solids and fats. Moreover, important
macronutrients (N, P, K), Ca, Mg and Fe are also present in SS,
while some potentially toxic metals (PTM) may be found [4]. The
application of SS in agriculture soil is an attractive option due to
presence macronutrients and organic matter (OM) [5]. The
combination of these two wastes (SS + CFA) may be an excellent
strategy to produce a soil conditioner, for correction of the pH and
organic matter, which is important for the case of Portuguese soil
(acidic pH<5.5 and poor in OM - around 2%) [4, 6].
Objectives
This work aims at assessing the recovery of two wastes (SS and
CFA) as soil amendment, where CFA is employed as an adjuvant
in the drying process of SS. The production of a soil amendment
to agricultural application, is in line with the circular economy
adopted in Portugal and in other European Countries as well.
Methods
The CFA sample used in this work was collected by a composed
sampling strategy, by collecting 10 subsamples at depth 0.5 – 1
m in a landfill of a power plant in the centre region of Portugal.
The CFA sample was dried at 105 °C for 24 hours, disintegrated
by hand and sieved to obtain particles with a diameter of 100 -
200 mesh. Oxides were analyzed by X-ray fluorescence (XRF)
using a Nex CG Rigaku spectrometer [4]. SS sample was
collected also in the Centre Region of Portugal, in a wastewater
treatment plant (WTP) with primary and secondary treatments,
after mechanical dewatering by centrifugation. pH and electric
conductivity (EC) in CFA and SS samples were measured in a
1:10 (mass:volume) suspension [7]. Initial SS moisture was close
to 80%. For the drying studies, different percentages of SS:CFA
were mixed (Control, 95:5, 90:10 and 85:15). The mixtures of SS
and CFA were extruded as shown in Fig. 1.
Figure 1. Vertical section of the extruder.
The sample fills the upper cylinder, at the bottom of which is
placed the extruder. By extrusion, cylinders of 5 mm in diameter
and a length of 30 mm were obtained, with a mass of 3.5-4 g each.
Afterward, five cylinder units were placed on each Petri dishes.
The drying process was studied at 40, 70, 100 and 130 °C in an
oven. The results were analyzed by the moisture ratio (MR),
which corresponds to the moisture along time normalized by the
initial moisture. Determination of the effective diffusion
coefficient, Deff, and the convective mass transfer coefficient, hm,
during drying process was performed based on the analytical
Sewage sludge
Coal Fly Ash
Drying
Soil
amendment
solutions of the Fick’s second law equation applied to an infinite
cylinder [8].
Results
Table 1 shows some properties of SS and CFA samples. The
initial moisture of SS was around 78%. The use of SS from WTP
in the soil has been encouraged by the European regulations as an
alternative to the management of this waste, mainly due to the
presence of nutrients such as P (4.3% - P2O5) and K (1% - K2O).
The pH of both wastes is alkaline. SS is rich in OM (about 64 %),
whereas CFA is an inorganic material.
Table 1. Properties of SS and CFA used in this study.
Parameters
SS
CFA
Moisture (%)
78
0.07
pH*
7.7
8.6
EC (µS cm-1)*
1 723
487
OM (% TS)
63.7
-
Ash (% db)
36.3
100
P (P2O5) [% TS]
4.3
0.2
K (K2O) [% TS]
1.08
2.4
MgO [% TS]
4.8
1.5
CaO [% TS]
12
1.4
BaO [% TS]
0.1
0.1
TiO2 [% TS]
0.6
1.1
* measured at L/S ratio = 10 L kg-1.
Table 2 reveals the concentration of six metals analyzed for SS
and CFA samples. It is also possible to confirm that the values
found for SS are below the limits established by the Portuguese
Law (Decreto-Lei Nº. 276/2009) for use in agriculture soil.
For agronomic applications, the inactivation of pathogenic
microorganisms is mandatory by law. In this case, thermals
processes may be effective to attain that objective, and thus, in
this study the drying process was considered. As an example,
Figure 2 presents MR as a function of time for the experiments at
100 ºC, for each formulation SS:CFA (control, 95:5, 90:10 and
85:15). The results show that the adjuvant of drying has a positive
effect, since the loss of moisture is faster as the quantity of CFA
increase. This positive effect was also observed for 40, 70 and
130 ºC.
The convective diffusion coefficient at 40 ºC ranged from 1.17 ×
10-7 m2 s-1 (5% CFA) to 9.74 x 10-8 m2 s-1 (15% CFA). At 130 ºC,
these values were 2.62 × 10-7 m2 s-1 (5% CFA) at 2.80 × 10-7 m2
s-1 (15% CFA). The convective mass transfer coefficient varied
slightly with the increase of the adjuvant percentage and the
temperatures studied. For example, at 40 ºC and 5% of CFA, hm
is equal to 8.20 × 10-7 m s-1 and for 15% CFA this parameter is
9.01 x 10-7 m s-1. The values of Deff and hm are found to be close
to those reported in the literature [10].
Figure 2. Moisture ratio as a function of time, for different
percentages of CFA (0, 5, 10 and 15%) at 100 ºC.
Conclusion
In general, the addition of CFA as an adjuvant has a positive effect
on the drying process. This encouraging effect may be observed
through the effective diffusion coefficients. The product obtained
may be further used as soil amendment, which enables to carried
OM and nutrients to poor agriculture soil. The potentially toxic
metals do not present constraints to agronomic applications in
Portugal.
Table 2. Potentially toxic metals (PTM) concentration in this study to SS and CFA and limits to the soil at Portuguese laws.
PTM (mg kg-1)
This work
Literature
Decreto-Lei Nº. 276/2009
SS
CFA
SS (b)
CFA (c)
SS limits
Soil limits (d)
Cd
< LD (a)
< LD (a)
1.3-30
0.7- 130
20
1 - 4
Cr
120.08
162.0
102-200
10– 1 000
1 000
50 - 300
Cu
297.81
84.9
400-625
7-520
1 000
50 - 200
Pb
23.56
51.9
109-158
3.1–5 000
750
50 - 450
Ni
61.88
90.05
49.6-50
6.3–4 300
300
30 - 110
Zn
331.24
188.5
1 300-1 850
10–3 5000
2 500
150 - 450
(a) LD = < 3.85; (b) [9]; (c) CFA generated in electrostatic precipitator (coal F-grade with 40% coal ash) [2]; (d) Soil limits depends on pH of the soil.
Acknowledgements
The authors gratefully acknowledge Federal Institute of Education, Science, and Technology of Brasília - IFB, Campus Ceilândia, for
authorizing the Ph.D studies of L.A. Gomes.
References
[1] J. Skousen, J. E. Yang, J. S. Lee, and P. Ziemkiewicz, Geosystem Engineering 16 (2013), 249-256.
[2] Yao, Z. T., Ji, X. S., Sarker, P. K., Tang, J. H., Ge, L. Q., Xia, M. S., and Xi, Y. Q, Earth-Science Reviews, 141 (2015), 105–
121.
[3] R. J. LeBlanc, P. Matthews, and R. P. Richard, Global Atlas of Excreta, Wastewater Sludge, and Biosolids Management:
Moving Forward the Sustainable and Welcome Uses of a Global Resource. Kenya, 2008. pp. 632.
[4] M.G. Healy, O. Fenton, P.J. Forrestal, M. Danaher, R.B. Brennan, and L. Morrison, Waste Management 48 (2016), 404–408.
[5] P. Alvarenga, M. Farto, C. Mourinha, and P. Palma, Waste and Biomass Valorization 7 (2016), 1189–1201.
[6] M. J. Quina, M. A. R. Soares, and R. Quinta-Ferreira, Resources, Conservation and Recycling 123 (2017), 176–186.
[7] P. Oleszczuk and H. Hollert, Chemosphere 83 (2011), 502–509.
[8] P. P. Tripathy and S. Kumar, Journal of Food Engineering 90 (2009), 212–218.
[9] M. Franz, Waste Management 28 (2008), 1809–1818.
[10] Zhang, X. Y., Chen, M. Q., Huang, Y. W., and Xue, F. Fuel 171 (2016), 108–115.
t (min)
030 60 90 120 150 180
MR
0.0
0.2
0.4
0.6
0.8
1.0 Control
5% CFA
10% CFA
15% CFA
