Accumulation of heavy metals by earthworms from vermicomposting

Marta BOŻYM* – Faculty of Mechanical Engineering, Opole University of Technology, Opole, Poland
Please cite as: CHEMIK 2014, 68, 10, 868–873
Introduction
In Poland, treating of sewage sludge by earthworms was very
popular in the 90’s of the twentieth-century. A pioneer in this field was
a Waste-Water Treatment Plant in Pyrzyce, which began producing
vermicompost for agricultural purposes [1]. Vermicomposting
process is carried out in special areas, which are designed to separate
vermiculture from environment, especially predators such as a mole.
These areas are separated from the sides of the boards by wooden
or concrete elements, while the bottom of the drainage system is
located. In Poland, the most popular species for vermicomposting
of sewage sludge is earthworms Eisenia fetida. In the first phase of
the process, earthworms are introduced into a properly prepared
stratified layer [2], which is pre-composted sludge with plant material
[3, 4]. Vermicomposting of sludge takes place in stages. The precipitate
is distributed in thin layers alternating with green waste (straw, hay).
Consequently, there is no need to fork the compost mass, which is
a routine during normal composting process in piles. Most important
in the breeding of earthworms is to ensure the proper composition
of the substrate, regular feeding earthworms and constant humidity.
Typically, the production process of vermicompost from sewage
sludge takes place from April to October. Before winter vermiculture
should be specially protected with a thick layer of straw. Quality of
vermicompost from sewage sludge depends on the composition of
substrates, contaminants concentration or conducted treatments
[2 – 4]. Under the influence of vermicomposting process improves
the structure of sewage sludge, reduce the odor, dehydrates,
weight decrease and increase of available forms of macronutrients
content. Negative effect of this process may be an increase in heavy
metal content, including mobile forms [5 – 6]. Some metals can be
accumulated in the bodies of earthworms, because these animals
have a high tolerance of heavy metals [6 – 10].
The aim of the study was to evaluate changes of heavy metals
content in sewage sludge during vermicomposting process, examine
their ability to accumulate in the bodies of earthworms and to identify
factors influencing this effect.
Experimental section
Sewage sludges and vermicompost came from the WasteWater Treatment Plant in Lower Ligota, Opolskie Voivodeship.
This Treatment Plant with a capacity of 7000 m3/d, was opened in
1999. The Treatment Plant uses a mechanical-biological treatment
process, which is based on the low-convicted activated sludge
chambers BIOLAK – VOX. Treated wastewater, in accordance
with the permit required by Water Law Act, are discharged into
the river Baryczka. Vermicomposting process of sewage sludge
is used there since 1995. At the Waste-Water Treatment Plant are
located five special places of vermicultures, each one measuring
100 × 3 × 0.55 m. So far vermicompost was used for agricultural
purposes. Because of the complications associated with breeding
Corresponding author:
Marta BOŻYM – Ph.D., Eng., e-mail: [email protected]
nr 10/2014 • tom 68
earthworms, the Board of Water Supply and Sewerage in Kluczbork
had decided to resign from vermicomposting of sewage sludge.
The samples were taken three times at monthly intervals
from September to November 2012, at the end of the process of
vermicomposting. Raw sewage sludge samples were collected from
the dewatering press. Each vermicompost samples were collected
from three vermicompost places, with a depth of about 30 cm. About
7 – 10 primary samples were taken from each vermicultures places,
and then laboratory sample weighing about 0.5 kg was prepared. In
addition, during the sampling of vermicompost, adult earthworms
were collected (with formed clitelium). After delivery of samples
to the laboratory, in sewage sludge and vermicompost a dry matter
content was determined by gravimetric method at 105°C. Organic
matter was determined by gravimetric method at 550°C. The rest
of samples, after drying, were triturated in a mortar and sieved with
a mesh size of 1 mm. After separation from the substrate, earthworms
were transferred to a container of moist lignin, in order to empty the
gastrointestinal tract from coprolite. Then worms were euthanased
with acetone, dried, mineralized in aqua-regia and analysed. In
earthworms and coprolite samples the contents of heavy metals were
analysed. Sewage sludge and vermicomposts were analyzed for dry
matter content and heavy metals, and also pH in 1M KCl. Heavy
metals in all samples were analysed using FAAS method, after aqua
regia digestion. The degree of accumulation of metals by earthworms
was calculated using the formula (1):
(1)
where:
Cumulationearthworms – the degree of accumulation of metals in earthworms
relative to the nourishment (coprolite)
Cearthworms – heavy metals content in earthworms bodies [mg kg-1]
Ccorpolite – heavy metals content in coprolite [mg kg-1]
Obtained results were statistically verified by Statistica 10
programme. Correlation coefficients (r) were calculated for the
relation between concentration of metals in earthworms bodies and
selected properties of sewage sludge and vermicompost samples
(lenear and multivariate regression). For quality control of total metal
content determination in the samples, a certified reference materials
(CRM) „Sewage sludge amended soil” CRM005-050 was analysed. The
recovery of metals content in CRM were respectively: Cd 96%, Pb
95%, Cu 95%, Zn 97%, Cr 95%, Ni 95%.
Discussion
The pH of sewage sludge and vermicompost ranged,
respectively 6.70–6.85 and 6.35–6.71. Organic matter content
ranged 61.2–63.5% d.m. in sewage sludge and 65.4–73.2% d.m. in
vermicompost. The content of heavy metals in raw sewage sludge
and vermicompost was shown in Table 1. The results were averaged
for three times sampling. In addition, the results of vermicomposts
samples from three special places were averaged. The results were
compared with the limit values for sewage sludge applied to the land
in accordance with the Polish law [11].
• 871
XV Conference Environmental
Accumulation of heavy metals by earthworms from
vermicomposting sewage sludges
XV Conference Environmental
Table 1
The content of heavy metals in raw sewage sludge and vermicompost
compared with the limit values for sludge used for agricultural
purposes [11]
Metal
Raw sewage sludge Vermicompost Limit value for agricultural use
Cd [mg kg ]
1.504±0.037
2.141±0.119
20
Pb [mg kg ]
40.7±1.3
56.0±3.4
750
Cu [mg kg-1]
213±13
275±5
1000
Zn [mg kg-1]
767±10
1014±50
2500
Cr [mg kg ]
14.3±0.5
28.5±3.2
500
Ni [mg kg-1]
17.8±0.7
25.4±1.0
300
-1
-1
-1
The results were converted to dry weight, as follows: mean ± SD
The content of heavy metals in sewage sludge and vermicomposts
does not exceed the limit values, defined for sludge used for agricultural
purposes, this allows for the natural use. The total metal content in
sewage sludge was less than in vermicomposts, which occurred due
to the reduction of composted sludge mass. Another reason could be
the fact that storage time of vermicomposts exceeded one year and
mainly the severe winter at the turn of the year 2011–2012 caused
a significant reduction in the population of earthworms. The increase
in the levels of few metals in vermicomposting sewage sludge was
also found in a study conducted on the same Waste-Water Treatment
Plant in 2000 [3]. Then the samples were taken throughout the
period of vermicomposting, from spring to autumn, and the process
proceeds without complications. Similar results were obtained
by other authors, who reported an increase in the share of metals in
the sludge during vermicomposting process [12–13].
Table 2 presents the results of heavy metals in earthworms and
coprolite. In addition, given the degree of accumulation.
Table 2
Content of heavy metals in earthworms (Eisenia fetida) and coprolite
Earthworms
Metal
Coprolite
The degree of accumulation of
metals in earthworms relative to
the nourishment
Based on the obtained results, the statistical analysis was
calculated, in order to determine the impact of certain parameters
of vermicomposts on metals accumulation in earthworms bodies.
The correlation coefficient (r) between heavy metals concentration
in sewage sludge on metal content in earthworms (linear correlation)
was calculated. In addition, multivariate regression analysis was
performed, where the dependent variable was the content of metals in
earthworms, whereas the independent variables were: metal content,
pH and organic matter content (OM) in sewage sludge The results of
the statistical analysis are shown in Table 3.
Table 3
Statistical description of the influence of selected properties of
vermicompost on the heavy metals content in the
earthworms bodies
Metal
Coefficient of
correlation*
Linear and multivariate correlation equation
total
r=0.863
Cd(earthworm)=1.232+0.213(Cd total)
total-pH-OM
r=0.654
Cd(earthworm)=12.1(Cd total)
+0.413(pH)+0.070(OM)+0.614
total
r=0.143
Pb(earthworm)=48.05+0.26(Pb total)
total-pH-OM
r=0.695
Pb(earthworm)=2.730(Pb total)34.2(pH)+2.57(OM)+67.5
Cd
Pb
Cu
total
r=-0.480
Cu(earthworm)=376-0.753(Cu total)
total-pH-OM
r=0.519
Cu(earthworm)=0.890(Cu total)+16.7(pH)
-1.12(OM)+99.2
total
r=0.769
Zn(earthworm)=261.9+0.738(Zn total)
total-pH-OM
r=0.463
Zn(earthworm)=1.70(Zn total)
-12.2(pH)+35.6(OM)-1642
total
r=-0.490
Cr(earthworm)=36.9-0.419(Cr total)
total-pH-OM
r=0.831
Cr(earthworm)=18.6(Cr total)+
-3.5(pH)+1.15(OM)-46.8
Zn
Cr
Cd [mg kg-1]
4.27±0.49 2.12±0.24
2.01
Pb [mg kg-1]
30.7±1.9
56.8±3.2
0.54
Cu [mg kg ]
134±3
278±3.1
0.48
total
r=0.917
Ni(earthworm)=7.226+10.607(Ni total)
Zn [mg kg-1]
1019±54
980±55
1.04
total-pH-OM
r=0.727
Cr [mg kg-1]
2.00±0.28
29.2±3.1
0.07
Ni(earthworm)=-1.60(Ni total)4.3(pH)+0.27(OM)+13.5
Ni [mg kg ]
27.6±1.1
25.0±1.4
1.11
-1
-1
The content of cadmium in the bodies of earthworms was twice
higher than the sewage sludge (accumulation ratio = 2.01) (Tab. 2).
The content of zinc and nickel was at a similar level as in coprolite
(accumulation ratio = 1.04 and 1.11). In contrast, no increased
accumulation of lead and copper in the bodies of earthworms was
observed. The contents of these metals was twice lower than in coprolite
(accumulation ratio = 0.54 and 0.48). The degree of accumulation of
chromium was very low, as indicated by the capacity for accumulate
this metal in earthworms bodies (accumulation ratio =0.07).
Otherwise has been the case in previous studies, which were carried
out on that Treatment Plant, the capacity to accumulate certain metals
by earthworms was observed [3]. In later studies it was found that the
ability of accumulation of metals depends on the phase of development
of earthworms. The highest concentration of metals was determined
in adults (with clitellum) [4].
872 •
Ni
* – r – linear and multivariate correlation coefficient
Nickel (r=0.917), cadmium (r=0.863) and zinc (r=0.769)
contents were correlated positively with this metals content in
earthworms bodies. In the case of these metals, no additional effect
of pH and the content of organic matter (OM) deposits. In contrast,
the content of lead and chromium, also affect the pH and organic
matter of sewage sludge on the accumulation of metals in the bodies
of earthworms. For these metals, taking into account the three
parameters (metal content, organic matter and pH), an increase
of the correlation coefficient values was observed (Tab. 3). Other
authors also confirmed a direct impact of pH and organic matter
content on the accumulation of metals by earthworms [14–15].
Another factor affecting the degree of accumulation of metals in the
body of earthworms can be interaction of various metals in sewage
sludge, for example, Pb, Cd, Zn, and Cu [7]. Accumulation of copper
in the bodies of earthworms dependent slightly on the content in
sewage sludge (r=0.480–0.519).
nr 10/2014 • tom 68
2.
3.
4.
5.
6.
7.
8.
10.
11.
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*Marta BOŻYM – Ph.D., Eng., completed a M.Sc. degree in chemistry,
is a graduate of the Faculty of Mathematics, Physics and Chemistry, Opole
University (1998). Additionally, in 2005, she got a B.Sc. degree in environmental
engineering, is a graduate of the Faculty of Mechanical Engineering, Opole
University of Technology. Ph.D. thesis defended at Wrocław University of
Environmental and Life Sciences (2006). Currently she works at the Faculty
of Mechanical Engineering at Opole University of Technology. Research
interests: migration of heavy metals in the environment, development of
municipal and industrial waste, the use of sewage sludge. She is the author of
over 50 scientific articles and author or co-author over 50 papers and posters
at national and international conferences.
e-mail: [email protected], phone: +48 77 449 8381
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(http://www.plasticseurope.pl, 30.09.2014)
nr 10/2014 • tom 68
Cenny materiał dla energetyki wiatrowej
Starania o uniezależnienie się od energii jądrowej i paliw kopalnych, które podejmują Niemcy, mają miejsce w wielu krajach
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Wersje biegunowe tego materiału, tj. Levapren 500 i Levapren 700,
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wchłonięty olej mógłby osłabić właściwości kabli w przypadku pożaru i zwiększyć gęstość dymu. (kk)
(LANXESS, 15.09.2014)
Dokończenie na stronie 881
• 873
XV Conference Environmental
Conclusions
The results confirmed that during the vermicomposting process of
sludge increased heavy metal content. This is due to reducing the mass
of treated sludge. The metal content in the earthworms bodies was
higher than in sewage sludge and vermicomost. Metal accumulation
by earthworms affected the total content of heavy metals in sewage
sludge, pH and organic matter content.