Application of sewage sludge to agricultural soils can increase the concentration of heavy metals in soil and plants. A study found increasing rates of untreated sewage sludge application from 10 to 80 tonnes/hectare led to a corresponding increase in extractable zinc, copper, lead and cadmium levels in soil. Similarly, Chinese cabbage grown in soils amended with increasing rates of sewage sludge from 5% to 25% showed higher accumulation of arsenic, cadmium, chromium, lead, nickel, copper, and zinc in leaves compared to the unamended control. Several soil properties and plant factors influence the availability and uptake of heavy metals from sludge-amended soils into food crops.

1. Hriday Kamal Tarafder
and
Dr. P. K. Mani

2. Sewage: Sewage is water-carried waste, in
solution or suspension, that is intended to be
removed from a community.*
Sludge: A mixture of solids and water
produced during the treatment of waste water
or sewage.*
*
Central Pollution Control Board(1993)

3. Sewage-sludge
Disposal

4. Sludge in agriculture
Heavy metal
accumulatio
n
Food chain
transfer
Last link of
pollutants

5. Heavy metals:
Specific gravity >5.0
Atomic no. >20

6. Essential heavy metal Non essential heavy metal
Fe, Zn, Cu, Mn, Mo, Ni Pb, Hg, Cd, As, Cr, Se

7. Member
State
Year Sludge
production
(t DS)
Agriculture
t (DS) % of total
Germany 2007 2056486 592552 70
Spain 2006 1064972 687037 65
Sweden 2006 210000 30000 14
UK 2006 1544919 1050526 68
Austria 2006 252800 38400 16
Italy 2006 1070080 189554 18
 In India production of sewage sludge is estimated to be around
1200 tonnes/day
 There exists a potential to produce 4000 tonnes of sludge per
day
Juwarkar et al., 1991

8. Metal contents (mg/kg-1
) in sewage sludge from
different cities in India
Location Zn Cu Cd Pb Ni Cr
Ahmedabad 2147 535 3.5 76.8 32.3 60.4
Delhi 1610 440 5.5 34.5 815 53.5
Nagpur 832 272 1.5 24.3 14.8 49.2
Chennai 935 210 8.3 16.6 60.5 38.5
Jaipur 1720 265 7.3 66.9 37.5 176
Kolkata 1513 188 3.25 157 266 1467
Source : Maity et al (1992).

9. Sample Standards Cd Cu Pb Zn Mn Ni Cr
Soil
(µg g-1
)
Indian Standard
(Awashthi, 2000)
3-6
135-
270
250-500
300-
600
-
75-
150
-
European Union
Standards (EU
2002)
3 140 300 300 - 75 150
Plant
(µg g-1
)
Indian Standard
(Awashthi, 2000)
1.5 30 2.5 50 - 1.5 20
WHO/FAO (2007) 0.2 40 5 60 - - -
Commission
Regulation (EU,
2006)
0.2 - 0.3 - - - -
Guidelines for safe limits of some heavy metals in soils &
plants
Guidelines for safe limits of some heavy metals in soils &
plants

10. Heavy metal availabilityHeavy metal availability
in sewage sludgein sewage sludge
amended soilsamended soils

11. • ‘Available’ heavy metals - fraction of the total
concentration of heavy metals in the soil,
which is present in the soil solution or easily
exchangeable from the soil matric surfaces.
Total heavy
metal (M)
M in soil solution
Exchangeable
M bound to carbonates
M bound to Fe & Al oxides
M bound to organic matter
Residual fraction
Readily available
=
+
+
+
+
+
Naoum et al. (2001)

12. Factors affecting heavy metal availability toFactors affecting heavy metal availability to
plantsplants
A)Soil properties
B)Plant factors
C)Soil-plant transfer coefficient

13. A) Effect of soil propertiesA) Effect of soil properties
a) pH
• Availability of cationic heavy metals decreases with
increase in pH (Alloway and Jackson, 1991).
pH
• Whereas, availability of Mo and elements with anionic
species increases with increasing soil pH (Smith, 1996).
pH

14. b) Organic matter-
Organo-metallic interactions can be
divided into two groups:
1) Ionic interactions (cation exchange)-
includes alkali metals (K, Na, Li) and Group II
elements (Be, Mg, Ca).
2) Non ionic bonds- includes elements like
heavy metals; they tend to interact especially
with groups containing trivalent elements, like
N and P.
Livens (1991)

15. Environmental Protection Capacity (EPC) factor = DxH2
K
Dx - thickness of the soil layer in cm,
H - soil humus content
K - constant depending on the humus quality.
Sludge
Soil humus
EPC Heavy metal
availability
Hargitai (1989)
Contd.

16. c) Clays and hydrous oxides-
2) Specific adsorption to surface hydroxyl groups (Miller et al., 1987)
3) Co-precipitation (Martinez and McBride, 1998)
4) Precipitation as the discrete metal oxide or hydroxide
(Martinez and McBride, 1998).
Increase in clay and hydrous oxide contents in soils
provides more sites for adsorption of metals and reduces the
directly bio-available metal (Qiao and Ho, 1996).
1) Non-specific adsorption (exchange) (Basta and Tabatabai, 1992)

17. d) Redox potential-
Heavy metals expected to precipitate as sulphides
include- Ni2+
, Zn2+
, Cd2+
, Hg2+
,Fe3+
Flooded condition-
SO4
2-
→ SO3 -
→ S2-
S2-
+ M2+
→ MS
Aerobic condition-
M2+
+ SO4
2-
↔ MSO4

18. Reducing conditions may cause the
dissolution of Mn and Fe oxides (Alloway,
1995).
Thus, soil with a fluctuating water table will
often have a lower adsorptive capacity for
metals such as Cd and As which are strongly
sorbed by hydrous oxides of Fe and Mn
(Ghorbani, 2008).
Cont.Cont.

19. e)Effects of other elements-
i) Antagonistic effect-
With high P contents, at neutral to alkaline pH, a
precipitation of Cd3(PO4)2 takes place (Jing and
Logan, 1992).
ii) Synergistic effect-
High concentrations of Cl -
may increase the
availability of Cd to plants ( Sommers and
McLaughlin, 1996).

20. f) CaCO3 content-
At high pH and high pCO2 (partial pressure of CO2)
values, calcite (CaCO3) sorbs Cd, as CdCO3, and
diminishes its availability (Evans, 1989).
Other metals likely to precipitate as carbonates at
high pCO2 and pH values include: Fe2+
, Zn2+
and Pb2+
(Evans, 1989).

21. Soil moisture
Soil temperature
Soil aeration
g) Other factors-

22. Metal High accumulations Low accumulations
Cd Lettuce, Spinach, Celery,
Cabbage
Potato, Maize, French
bean, Peas
Pb Kale, Rye grass, Celery Some barley cultivars,
Potato, Maize
Cu Sugar beet, Certain barley
cultivars,
Leek, Cabbage, Onion
Ni Sugar beet, Rye grass,
Mangold, Turnip
Maize, Leek, Barley,
onion
Zn Sugar beet, Mangold,
Spinach, Beetroot
Potato, Leek, Tomato,
Onion
a) Plant species-
B) Plant factors-B) Plant factors-
Alloway (1990)

23. Other plant factors:
b)Cultivars
c)Genotype

24. E)E) Soil-Plant Transfer CoefficientSoil-Plant Transfer Coefficient
Transfer Coefficient (TC)= ----------------
[M]plant
[M]soil
[M]plant- concentration of an element in the
test plant tissues
[M]soil- total concentration of the same element
in the soil where this plant is grown

25. Transfer coefficients of Cd, Ni, Pb and Zn inTransfer coefficients of Cd, Ni, Pb and Zn in
clay loam soilclay loam soil
Cd Ni Pb Zn
0.40 0.04 0.004 0.13
0.65 0.04 0.005 0.24
1.00 0.20 0.010 0.42
Warm Cool
Control
ss@10 t ha-1
ss@50 t ha-1
Cd Ni Pb Zn
0.45 0.04 0.005 0.15
1.16 0.08 0.010 0.45
1.72 0.21 0.010 0.68
Antoniadis (1998)

26. Heavy metal accumulationHeavy metal accumulation
in soils and plantsin soils and plants

27. Effect of untreated sewage sludge on heavyEffect of untreated sewage sludge on heavy
metal accumulation in soilmetal accumulation in soil
Treatments AB-DTPA extractable 0.1 N HCl extractable
Zn (mg kg-1
) Cu (mg kg-1
) Pb (mg kg-1
) Cd (mg kg-1
)
T0 (NPK: 120:60;60 kg ha-1
) 1.76e
1.66d 0.25 0.042
T1 (ss @ 10 t ha-1
) 2.95d 1.70d 0.31 0.048
T2 (ss @ 20 t ha-1
) 4.76c 2.13c 0.36 0.049
T3 (ss @ 40 t ha-1
) 5.87b 2.76b 0.40 0.056
T4 (ss @ 80 t ha-1
) 6.75a 3.01a 0.42 0.060
LSD 0.057 0.057 NS NS
Khan et al. (2007)

28. Effect of Municipal sewage sludge (MSS) and mixtureEffect of Municipal sewage sludge (MSS) and mixture
ofof
MSS & Yard waste (YS) on heavy metal accumulationMSS & Yard waste (YS) on heavy metal accumulation
MSS MSS+ YS Native soil
Heavy metals in soil and soil mix
Antonious et al. (2010)
Squashyield(lbs/acremgkg-1
drysoil

29. Heavy metal concentration of squash fruitsHeavy metal concentration of squash fruits
grown on MSS amended soilgrown on MSS amended soil
Concentrationin
(mgkg-1
)dryfruit
Heavy metals in squash fruits
Concentrationin
(mgkg-1
)dryfruit
Squash harvest
Heavy metal Maximum permissible
limit in vegetables &
fruits (mg kg-1
dw)*
Cd 0.2
Cu 20
Ni 10
Pb 9
Zn 100
Cr 0.5
* State Environmental Protection Administration, China Antonious et al. (2010)

30. Heavy metal accumulation in ChineseHeavy metal accumulation in Chinese
cabbagecabbage
grown in sewage sludge amended soilgrown in sewage sludge amended soil
Heavy metals Sewage
sludge
Limits for
sewage
sludge usage a
Soil SEPA limits
for soils b
As (mg kg-1
) 322.76±31.77 75 30.12±2.33 30
Cd (mg kg-1
) 5.06±0.65 20 0.57±0.22 0.6
Cr (mg kg-1
) 48.85±5.22 1200 29.07±2.23 250
Pb (mg kg-1
) 41.19±4.78 1000 12.85±1.11 350
Ni (mg kg-1
) 25.32±1.28 200 21.88±1.72 60
Cu (mg kg-1
) 105.08±4.57 1500 18.96±1.22 100
Zn (mg kg-1
) 1872.23±22.7
1
3000 113.44±5.43 300
Total heavy metal concentrations in sewage sludge & soil
a
Permissible limits of sewage sludge usage in agriculture in China
b
State Environmental Protection Administration (SEPA) in China
Wang et al. (2008)

31. Heavy
metal
Control 5% a
10% a
15% a
20% a
25% a
Limits b
As 2.1±0.21 5.8±0.88 5.9±0.97 7.4±1.08 10±0.59 7.9±0.97 0.05
Cd 0.14±0.07
0.15±0.0
7
0.25±0.02
0.25±0.0
7
0.41±0.0
1
0.24±0.09 0.2
Cr 0.7±0.15 2.4±0.47 3.1±0.25 3.2±0.34 5.5±0.53 5.8±0.79 0.5
Pb 0.08±0.01 0.17±0.4 0.24±0.6 0.27±0.2
0.19±0.0
2
0.22±0.5 9
Ni 1.2±0.2 0.6±0.2 1.6±0.4 1.6±0.6 2.1±0.3 3.1±0.1 10
Cu 2.6±0.5 4.7±0.7 5.6±1.1 4.2±0.8 3.6±0.9 4.2±1.1 20
Zn
43.4±5.8 63.3±9.3 65.9±6.6
78.9±11.
6
72.5±11.
1
69.5±10.7 100
Contd.Contd.
a
Percentages of sewage sludge in soil
b
Maximum permissible limits of metal contaminants (SEPA, China)
Concentration of heavy metals (mg kg-1
) in leaves of Chinese cabbage
grown in soil amended with various content of sewage sludge
Wang et al. (2008)

32. Parameters Unamended
soil
6 kg m-2
SSA 9 kg m-2
SSA 12 kg m-2
SSA
pH (1:5) 8.18 ± 0.02 8.06 ± 0.02 8.09 ± 0.06 7.85 ± 0.03
EC (mScm-1
) 0.24 ± 0.01 0.29 ± 0.01 0.32 ± 0.02 0.39 ± 0.01
Organic C (%) 0.77 ± 0.02 1.41 ± 0.01 1.52 ± 0.02 1.74 ± 0.01
Total N (%) 0.18 ± 0.00 0.20 ± 0.00 0.20 ± 0.00 0.21 ± 0.00
P (mg kg-1
) 54.43 ± 3.90 111.81 ± 3.54 124.2 ± 2.59 132.8 ± 3.37
Cu (mg kg-1
) 3.51 ± 0.24 8.50 ± 0.27 10.81 ± 0.26 11.13 ± 0.42
Mn (mg kg-1
) 13.27 ± 0.73 34.36 ± 0.86 41.17 ± 1.13 42.05 ± 1.19
Zn (mg kg-1
) 2.11 ± 0.49 11.95 ± 0.46 22.95 ± 1.29 30.91 ± 1.69
Cr (mg kg-1
) 0.34 ± 0.03 0.77 ± 0.05 1.50 ± 0.07 1.66 ± 0.05
Cd (mg kg-1
) 1.51 ± 0.19 6.39 ± 0.30 6.44 ± 0.34 7.36 ± 0.34
Ni (mg kg-1
) 4.95 ± 0.22 9.69 ± 0.13 10.29 ± 0.16 10.75 ± 0.27
Pb (mg kg-1
) 2.83 ± 0.18 8.49 ± 0.47 10.04 ± 0.25 11.06± 0.15
Effect of sewage sludge on nutrient and heavy metal content inEffect of sewage sludge on nutrient and heavy metal content in
soilsoil
Singh & Agrawal (2010)

33. Treatment Cu Mn Zn Cr Cd Ni Pb
Untreated
soil
0.48 1.43 11.58 0.32 0.23 0.43 0.42
6 kg m-2
SSA
0.77 1.92 20.58 0.83 0.80 1.47 1.88
9 kg m-2
SSA
1.65 2.18 20.62 1.18 1.35 2.85 2.62
12 kg m-2
SSA
2.22 2.82 22.07 1.47 1.62 5.67 3.47
Treatment Yield (g m-2
) Harvest index (g g-1
)
Unamended soil 102.88 0.34
6 kg m-2
SSA 143.34 0.40
9 kg m-2
SSA 180.78 0.41
12 kg m-2
SSA 164.50 0.42
Heavy metal uptake by green mung from sewageHeavy metal uptake by green mung from sewage
sludge amended soilsludge amended soil
Singh & Agrawal (2010)

34. Way outsWay outs
A) Prevention of
heavy metal
contamination
B) Management
of contaminated
soil

35. A) Prevention of heavy metal contaminationA) Prevention of heavy metal contamination
i) Reducing heavy metal content of sewage sludge-
Acid hydrolysis
Alkaline hydrolysis
Fenton’s peroxidation treatment

36. Acid and alkaline hydrolysisAcid and alkaline hydrolysis
Conditions Acid
hydrolysis
Alkaline
hydrolysis
pH 3 10
Temperat
ure
120o
C 100o
C
Time 1 hour 1 hour
Heavy
metals
Untreat
ed
Acid
thermal
hydroly
sis
Alkaline
thermal
hydroly
sis
Cd 2.05 0.83 2.17
Cr 25.5 15.4 14.7
Cu 183 189 45
Pb 158 148 57
Ni 12.7 2.1 13.2
Zn 2144 370 1712
Operating conditions of acid
and alkaline hydrolysis
Concentration (mg kg-1
dry solid) of
heavy metals in the sludge cake after
dewatering for untreated sludge and
sludge subjected to hydrolysis
Dewil et al. (2006)

37. Fenton’s peroxidation treatmentFenton’s peroxidation treatment
Adjusting pH to 3
using H2SO4 + Fe2+
Addition of Ca(OH)2
Addition of
polyelectrolyte
Addition of H2O2 (reaction
time ≈ 1 hour)
Treatment
procedure
Heavy
metal
Untreated
sludge
Fenton’s
peroxidation
Cd 1.44 0.6
Cr 90 74
Cu 284 130
Pb 219 191
Ni 46 20
Zn 859 189
Concentration (mg kg-1
dry solid) of heavy metals in
the sludge cake after dewatering for untreated sludge
and sludge subjected to Fenton’s peroxidation
Dewil et al. (2006))

38. ii) Regulating the rate of application-
Pollutant Pollutant
concentration
in EQ sludge
(mg kg-1
)
Ceiling
concentration in
sludge applied to
land (mg kg-1
)
Annual
pollutant
loading rates
(kg ha-1
yr-1
)
Cumulative
pollutant
loading rates
(kg ha-1
)
As 41 75 2 41
Cd 39 85 1.9 39
Cr 1200 3000 150 3000
Cu 1500 4300 75 1500
Pb 300 840 15 300
Hg 17 57 0.85 17
Mo 18 75 0.90 18
Ni 420 420 21 420
Se 36 100 5 100
Zn 2800 7500 140 2800
US EPA (1993)

39. iii) No application-iii) No application-
 The land is already high in heavy
metal concentrations
 Soil pH < 5.0 or clay content < 10%
 Concentration of any of the heavy
metals in the sludge is beyond
‘ceiling limit’

40. B) Management of contaminated soilB) Management of contaminated soil
Increasing the soil pH to 6.5 or higher
Draining wet soils
Applying phosphate
Careful selection of plants
Application of organic matter
Application of Biochar

41. ConclusionConclusion
ss
 Heavy metal content of both sewage sludge and soil should
be considered during making decisions regarding sewage
sludge use in agriculture.
 Risks of heavy metal contamination of crops grown in
sewage sludge amended soils can be minimized to some
extent by altering various physico-chemical properties of
the soil.
 Use of sewage sludge should be avoided in crops that
accumulate heavy metals in levels toxic to humans without
themselves showing any toxicity symptoms.
 For safe agricultural use of sewage sludge, regular
monitoring of soil and crop edible parts for heavy metal
accumulation is necessary.

42. Future research should be carried out to have a better
understanding of long-term implications of heavy metal
availability to plants grown in sewage sludge amended soils.
Efforts in developing feasible techniques, to reduce heavy
metal content of sewage sludge for agricultural use, should be
made.
Development of standard limit of metals in sewage sludge
under Indian context is needed.
Research Needs