An Evaluation of Dedicated Land
Disposal Practices for Sewage Sludge
Repott No 1209/1/05
This research project forms part of a research program funded by the
Research Commission that started in 2001 to critically assess the South
sludge legislation and revise the guidelines if necessary. This
research program includes the following projects:
The results of the research projects will form the knowledge base to
start a participative process in 2003 to develop Edition 2 of the
Permissible utilization and disposal of sewage sludge guidelines.
- WRC Project Number 1209: An evaluation of dedicated land
disposal practices for sewage sludge
- WRC Project Number 1210: Laboratory and field scale
evaluation of agricultural use of sewage sludge
- WRC Project Number 1240: A technical and financial review
of sludge treatment technologies
- WRC Project Number 1283: A metal content survey of South
African sewage sludge and an evaluation of analytical methods for their
determination in sludge
- WRC Project Number 1339: Survey and methodology for
analyzing organic pollutants in South African sewage sludge
The main aim of this study was to determine the current and future
impact of DLD practices on the environment. The outcome of the study
can serve as input for the revision of the guidelines for sludge
disposal. The objectives of the study were as follows:
Most of the wastewater treatment facilities in South Africa dispose of
their sewage sludge on dedicated land disposal (DLD) sites (sacrificial
lands), since this is the quickest and cheapest way to get rid of the
waste. The sludge is regularly applied at high rates to the surface
soils. No crops are grown and the land is only used for the disposal of
sewage sludge. The impact of this practice on the environment is
believed to be negative, but very little research has been done to
determine the extent of the damage to the soil and water resources.
Negative impact can be caused by erosion and run-off after rainstorms
that will cause surface water pollution. The groundwater may also be
contaminated due to the movement of heavy metals and nitrogen through
the soil. It should also be determined whether the same set of
guidelines for the maximum permissible levels of heavy metals in soils
should be used for DLD areas as for agricultural land.
- To evaluate the extent of the current practice of disposing
of sewage sludge on dedicated land.
- To evaluate the potential pollution risk this practice
poses to the water environment at selected sites.
- To estimate the future impact of dedicated land disposal of
sewage sludge on the water environment.
A survey in conjunction with WRC project number 1283 was executed to
determine to what extent sacrificial land disposal is currently used in
SA. The selection of the sites included sites from all the major
cities, as well as smaller towns with and without industries.
Questionnaires were used to determine amounts of sludge, application
methods, time intervals between applications and other important
information. Topsoil samples were collected at each site and
analysed. The analyses included a semi-quantitative analysis (scan) of
the total metal content (EPA 1050 digestion) of the soil sample to
estimate the heavy metal concentrations, as well as analyses for
essential plant nutrition elements (K, Ca, Mg and Na; total N; Total
P), pH (H2O), organic carbon content and
particle size distribution (7 fractions).
The extent of dedicated land disposal (DLD) practices in South Africa
is widespread. Stockpiling is the practice used by most of the sewage
treatment facilities (40%), either as the only disposal method or a
means to store the dried sludge until it is utilized by farmers and
municipalities, disposed of in landfills or composted. Liquid sludge is
applied to soils by 40% of the remaining disposal sites. This includes
practices like irrigation, flooding, sludge ponds, instant lawn
irrigation and paddies.
Thirty percent of the DLD sites were on sandy soils (<10% clay)
with a high leaching potential while only 11% were on sandy clay and
clay soils (>35% clay) where the adsorption capacity of the
soils may impede groundwater pollution. The majority of topsoil samples
had above average macronutrient and organic carbon contents, and 65% of
the samples had pH(H2O) values <6.5.
Groundwater pollution at these sites with the low pH values is a
possibility because many heavy metals are mobile under acid conditions.
The heavy metal analyses indicated that 88% of the topsoil samples had
at least one element that exceeded the maximum permissible level (MPL)
for soils that are used beneficially (Dept. Nat. Health & Pop.
Dev., 1991). Nickel is the main element that was too high in
most of the samples, followed by Zn and Pb. Other elements
that were present in high concentrations were Cd, Cr and Cu.
It should be kept in mind that the MPL for heavy metals in soils was
set for the beneficial use of sewage sludge for agricultural purposes
and not for DLD practices. A separate set of guidelines should be
considered for DLD practices after the completion of this study.
From the information collected during the initial survey, 40 sites were
selected for further detailed studies. These sites included sewage
works with different soil properties, application techniques, metal
concentrations and period of sludge application. Soil samples were
collected at three different locations and at five depths (to determine
the mobility of metals and other elements) at each of the selected
wastewater treatment facilities. Water samples were collected from
boreholes where possible and analysed for the same elements as the
soil. Four extraction methods were used to determine the metal content
of the soil samples, aqua regia and EPA 3050 digestion (total), NH4EDTA
extractions (potentially bio-available fraction) and NH4NO3
extractions (soil solution fraction). The samples were analysed
according to the information collected during the survey.
The selection of sites for the detailed study consisted of 14 sites
with wet sludge application without beneficial use, 5 sites with sludge
irrigation onto instant lawn (beneficial use) and 21 sites with dry
sludge application (stockpile and belt press dewatered sludge). Seven
of the sampled sites receive only domestic wastewater.
The total P content of 23 sites was above the average for normal soil
(0.1%; Brady, 1984). None of the soil samples had above average total N
(>1.5%; Sparks, 1996) even in the top 100mm of the soil profile.
However, the analysis data of the groundwater samples had high NO3
concentrations, which indicates leaching of nitrate. Sixty percent of
the sampled sites had organic carbon contents higher than 1.2%, due to
high organic matter application.
Correlations for all analytical methods were done for all the samples,
only topsoil samples and subsoil samples (lowest soil layer sampled) to
determine where the strongest correlation would be. The
correlations indicated that the EPA 3050 and aqua regia methods could
be used interchangeably for all the metals, except Ni, because of the
were very strong (r2 >0.82) correlations. The correlation
between the NH4EDTA and NH4NO3
extraction methods were strong for Cr, Ni and Cd. For Zn, Cd and Pb the
correlation between the total methods (EPA 3050 and aqua regia) and NH4EDTA
were also strong.
Of the 40 selected sites, 35 sites had at least one heavy metal that
exceeded the MPL for South African soils (Dept. Nat. Health &
Pop. Dev., 1991). The total topsoil concentration of Cr was above the
MPL for South African soils (80 mg kg-1) in
50-53% of the sampled sites, followed by Ni (45-48% > 50 mg kg-1),
Zn (40-45% > 185 mg kg-1), Pb (35-38%
> 56 mg kg-1), Cu (30-35% > 100 mg
kg-1), Co (25-33% > 20 mg kg-1)
and Cd (25-30% > 2 mg kg-1). The
percentage samples exceeding the MPL for South African soils (Nat.
Dept. Health & Pop. Dev., 1991) decreased in the lower soil
layers indicating accumulation in the surface layers due to surface
application of sludge.
The high total metal concentrations in many of the soils, especially
those that receive only domestic wastewater, may be due to high
background concentrations of these elements in South African soils
(Herselman & Steyn, 2001) and not only sludge application.
Detailed studies of these sites, including surrounding areas to
determine the baseline concentration of the area, is advised.
The NH4EDTA extractable metal fraction gives an
indication of the potentially bio-available or medium term risk of
metals entering the environment. None of the sites (all soil layers)
had NH4EDTA extractable Cr and Pb concentrations
above the NH4EDTA threshold values. The
potentially available topsoil Cd, Zn, Co, Cu and Ni concentrations in
respectively 20% (Cd, Zn), 13% (Co, Cu) and 10% (Ni) of the sites
exceeded the NH4EDTA threshold values (Bruemmer
& van der Merwe, 1989). There is thus a small medium term risk
for environmental pollution.
The exchangeble (NH4NO3
extractable) Ni, Zn and Cd concentrations are reason for concern
because 23-45% of the sites had concentrations above the NH4NO3
guidelines set for groundwater protection (Baden-Wurttemberg, 1993) in
the topsoil and 23-25% sites had elevated concentrations of these
metals in the 400-500mm soil layer, indicating a short term risk for
The lack of groundwater monitoring at most of the wastewater treatment
facilities should be addressed. Seven of the 9 groundwater samples that
could be obtained showed high NO3 concentrations (>6 mg l-1).
This would probably be the case at most of the treatment facilities.
The organic C in the top 200mm of the soil profile may adsorb some
nitrogen, but the rest is mobile and leach through the soil. Therefore,
the soil analyses do not show high total N concentrations. In some
cases, where NH4NO3
extractable metal concentrations in the 300-500mm soil layers are high,
groundwater pollution by heavy metals may even be possible, especially
if the soil has low clay content.
Some degree of leaching of the heavy metals occurred at some of the
sampling sites and the average depth of leaching was 100-200mm. Deeper
than 300mm the metal concentrations in most soil samples reached
background concentrations. The elements that leached in most soils were
Co and Ni. The leaching of the metals, in spite of the high organic
carbon content of the soils, was due to the extremely low soil pH(H2O)
of most sites.
Statistics of the data indicate no significant differences between
sludge type (wet or dry) and leaching, or age of the disposal sites and
leaching. It should be kept in mind that most of the sampled sites
receive industrial effluent, therefore the metal loading of the sludge
at these sites is probably very high. Taking into account the age of
the disposal sites, the frequency of sludge application and the metal
load of the sludge, the depth of leaching is surprisingly shallow in
most soils, in spite of the low soil pH(H2O) and
There is a need to be able to predict the impacts of the practice of
amending soils with sewage sludge on aquifers. The current
investigation explores the feasibility and utility of chemical fate and
transport modelling as a means of predicting the mobility of metals
inherent in sludge-soil mixtures, under a number of specific
A very extensive literature survey revealed that many famous scientists
have attempted the task of modelling the migration of metals in
sludge-amended soils. To date, no satisfactory predictive model has
been developed. Published experimental data is in conflict with respect
to relative strengths of binding of metal ions to sludge-soil matrices,
and with respect to potential mobility of the metals in the
environment. Not only are sludge and soils highly site-specific in
their chemical nature, but so is the nature of biota (species and
community structures) responsible for degrading organic material in the
sludge/soil mixtures into potentially metal-binding soluble material.
Despite the difficulties, some headway has been made in determination
of metal binding constants by thermodynamic means.
A chemical model was constructed using PhreeqC, a geochemical fate and
transport modelling package extensively used in the groundwater field.
The model was constructed from published thermodynamic data, and
calibrated against simple conceptual models of the behavior of soils,
and natural organic matter. A subset of the field results from this
study was analysed statistically to determine a Reference Behavior
Pattern to benchmark the model against. The model was further
calibrated against the extractions of metals from the sludge/soil
samples by NH4NO3 and NH4EDTA.
Literature and modelling studies indicated that the organic carbon
component of the sludge/soil matrix is principally responsible for the
fate of metals. Scenarios were modelled, using cadmium as a
representative metal ion. The scenarios are presented below with the
results of the simulations:
In general only a few of the disposal sites have sound management
practices in place. Most of the disposal sites are not fenced off and
are very close to populated areas. General access by the public occurs
and in some cases the local people harvest edible plants that grow on
the disposal sites.
- Elution with dilute saline solution at pH 7 - metals were
- Elution with dilute saline solution at pH 6.5 - metals were
- Elution with dilute saline solution at pH 5.0 - metals were
- Effect of maintaining carbon content of sludge layer -
microbial decomposition of organic carbon will continuously produce
soluble organic matter, which will bind metals and mobilize the metals
- Effect of maintaining high pH through liming - addition of
lime involves the addition of calcium ion which out-competes toxic
metal adsorption to sludge/soil matrix, resulting in substantial
mobilization of metals
- Effect of cessation of sludge addition - metals would be
mobilized in the short term, due to continuous production of soluble,
metal-binding organic matter, but will cease in the long-term.
Literature suggests that after ten years, metals will cease to be mobile
The majority of the disposal sites are on even terrain, but most of
those that are on a slope have no erosion control measures in place
even though they are near surface water bodies. Surface water
monitoring at these sties is recommended.
Generally the larger cities and metropolitan councils were found to be
knowledgeable in sludge management and legislative requirements but
this was not the case in other towns. Many plant managers
didn’t really care where they put the sludge, as long as it
is disposed of. No systems exist at most wastewater treatment
facilities to manage the disposal of the sludge. At two wastewater
treatment facilities raw sewage was disposed of on the disposal site.
The following recommendations should be considered:
Guidelines should be set for DLD specifically
- Clear demarcation of sludge disposal areas with restrictive
- Continuous groundwater and surface water monitoring should
- Erosion control measures where necessary
- Sound management practices at the disposal site to regulate
- Prerequisites for permit – no disposal on sandy
soils, safe distance from water bodies etc.
- Nitrogen should be considered in the guideline for sludge
disposal because it poses a bigger threat than the metals