Terra Preta sanitation: Re-discovered from an ancient Amazonian civilisation - Integrating sanitation, bio-waste management and agriculture

Abstract

The recent discovery of the bio-waste and excreta treatment of a former civilisation in the Amazon reveals the possibility of a highly efficient and simple sanitation system. With the end product that was black soil they converted 10% of former infertile soil of the region: Terra Preta do Indio (black soil of the Indians). These soils are still very fertile 500 years after this civilisation had disappeared. Deriving from these concepts, Terra Preta Sanitation (TPS) has been re-developed and adopted. TPS includes urine diversion, addition of a charcoal mixture and is based on lactic-acid-fermentation with subsequent vermicomposting. No water, ventilation or external energy is required. Natural formation processes are employed to transform excreta into lasting fertile soil that can be utilised in urban agriculture. The authors studied the lacto-fermentation of faecal matter with a minimum of 4 weeks followed by vermicomposting. The results showed that lactic-acid fermentation with addition of a charcoal mixture is a suitable option for dry toilets as the container can be closed after usage. Hardly any odour occured even after periods of several weeks. Lactic-acid fermentation alone without addition of bulking agents such as paper and sliced-cut wood to raise the C/N ratio is creating a substrate that is not accepted by worms.

Full text

Accepted for Water Science & Technology, 2010
1
Terra Preta Sanitation: re-discovered from an ancient Amazonian
civilisation - integrating sanitation, bio-waste management and
agriculture
H. Factura*, T. Bettendorf*, C. Buzie*, H. Pieplow**, J. Reckin*** and R. Otterpohl*
*TUHH Hamburg University of Technology, Hamburg, Germany
(E-mail: horacio.factura@tuhh.de; otterpohl@tuhh.de )
** Federal Ministry of the Environment, Berlin, Germany
*** Pflanzenvielfalt e.V. i.gr., Berlin, Germany
Abstract The recent discovery of the bio-waste and excreta treatment of a former
civilisation in the Amazon reveals the possibility of a highly efficient and simple sanitation
system. With the end product that was black soil they converted 10% of former infertile soil
of the region: Terra Preta do Indio (black soil of the Indians). These soils are still very
fertile 500 years after this civilisation had disappeared. Deriving from these concepts, Terra
Petra Sanitation (TPS) has been re-developed and adopted. TPS includes urine diversion,
addition of a charcoal mixture and is based on lactic-acid-fermentation with subsequent
vermicomposting. No water, ventilation or external energy is required. Natural formation
processes are employed to transform excreta into lasting fertile soil that can be utilised in
urban agriculture. The authors studied the lacto-fermentation of faecal matter with a
minimum of 4 weeks followed by vermicomposting. The results showed that lactic-acid
fermentation with addition of a charcoal mixture is a suitable option for dry toilets as the
container can be closed after usage. Hardly any odour occured even after periods of several
weeks. Lactic-acid fermentation alone without addition of bulking agents such as paper
and sliced-cut wood to raise the C/N ratio is creating a substrate that is not accepted by
worms.
Keywords Bio-waste treatment; nutrient recovery; terra preta sanitation; urban agriculture
INTRODUCTION
The way to sustainable management of natural resources is one of the greatest challenges of
present time. The sustainability of society depends essentially from secured water and food
supply and an intelligent management of wastes. Food, for example, is produced with
extensive input of energy and resources. Nutrients are detracted from soils and needed to be
replaced. Replacement is often done by artificial fertilisers which are produced with high
energy input. The reduction of organic matter content by the degradation of humus is mostly
not replaced to the required extent. This leads to a slow but steady degradation of soil and loss
of fertility. Urban sanitation systems nowadays, with their sewers and treatment plants, are
not designed to close material cycles and extracted nutrients are not directed back to the soil.
If sludge from mixed wastewater is returned it has a lack of usable nutrients, the phosphate is
to a large extent not accessible to plants because of strong bindings with metal salts from
precipitation (UBA, 2001). Consequently, essential elements, especially carbon, nitrogen and
phosphorus but also trace elements are lost, with consequent over-fertilisation of water bodies
and the seas. Such form of waste disposal is not just water and energy consumptive, but also
very cost intensive due to the necessary infrastructure and structural constructions.
Particularly, the construction and maintenance of sewers demand enormous financial efforts
and do require a high water input to avoid deposits and clogging. This approach of handling
resources is in most situations not sustainable, neither it is feasible for expanding urban areas
where energy and water resources are in short supply.

Accepted for Water Science & Technology, 2010
2
An analysis of a former civilisation in the Amazon, nowadays Brazil, reveals concepts which
enable a highly efficient handling of organic wastes. Terra Preta do Indio is the anthropogenic
black soil that was produced by ancient cultures through the conversion of biowaste and
faecal matter into long-term fertile soils. These soils have maintained high amounts of organic
carbon even several thousand years after they were abandoned (Lehmann et al., 2003b). It was
recently discovered that around 10% of the originally infertile soils in the Amazon region was
converted this way from around 7.000
until 500 years ago (Glaser, 2007). A
hectare of meter-deep terra preta can
contain 250 tonnes of carbon as
opposed to 100 tones in unimproved
soil (Lehmann et al., 2006). Figure 1
shows two soils, one (left image)
having fallowed many years without
charcoal and having a thin dark soil
layer. The other soil (right image),
located in the same region, has been
under fallow for presumably a similar
time period. However, because of the
accumulation of charred biomass and
other organic residues, terra preta
subsequently formed giving it a deep,
distinctly dark and highly fertile soil
layer. Figure 1: Photograph showing terra preta soils
(Adapted from Günther, 2007)
One of the surprising facts is that this soil is highly productive without fertiliser addition still.
From evidence of excavations and lab results it was concluded that this culture had a superior
sanitation and bio-waste system that was based on source separation of faecal matter, urine
and clever additives particularly charcoal dust and treatment steps for the solids resulting in
high yielding gardening. Additives included ground charcoal dust while the treatment and
smell prevention started with anaerobic lactic-acid fermentation followed by
vermicomposting (Pieplow, 2008). Vermicomposting is a decomposition process involving
the combined action of earthworms and microorganisms, and is a recent concept in the field
of sustainable sanitation. An exhaustive review of the theoretical background of the
vermicomposting process, mechanisms and practical application methods of vermicomposting
are provided by Shalabi (2006) and Buzie et al. (2010, in press).
Vermicomposting alone has been proven to be highly efficient for treatment of faecal matter
(Basja, 2002; Shalabi, 2006; Yadav 2008; Buzie et al., 2010, in press) but the initial silage
process makes the combination superior than other combined processes. There is first
qualitative experimental evidence, that it is not only lactic-acid fermentation but also some
other organisms like Bacillus subtilis in combination with the addition of some thin wood
particles that is leading to a better compost quality. Research on practical application of the
principles of modern Terra Preta Sanitation (TPS) has shown that it is possible to convert in a
hygienic and sustainable way biowaste and faecal matter into highly fertile humus-like
material. TPS makes use of lacto-fermentation and vermicomposting in a two-stage process.
One of the main advantages is that the lacto-fermentation works efficiently and stable without
air exchange and produces no offensive odours.
Integration of the anaerobic dry toilet and vermicomposting thus promises to be an ideal
approach for managing wastes even generated by urban households. The product, terra preta,

Accepted for Water Science & Technology, 2010
3
can address the problems of soil degradation and food insecurity common in many areas
across the world. TPS may allow the design of highly resource efficient houses and housing
areas with added value in urban agriculture that can be combined with local grey water reuse.
Thus it can close regional cycles and improve hygienic conditions and soil fertility in a
sustainable manner with the creation of local added value. In addition to macro-nutrients the
issue of micro-nutrients has to be addressed. In important agricultural soils around the world,
micronutrient deficiencies are: Zn (49%), B (31%), Mo (15%), Cu (14%), Mn (10%), and Fe
(3%) (Graham, 2008). These are the essential micronutrients of plants. They are needed by
plants in minute amounts that is why they are called micronutrients but they function equally
important with the macronutrient requirements of plants (e.g. N,P,K). Human health requires
plants for food that have all these substances, too. Especially Zn and Fe deficiencies are very
common and contribute heavily to child death according to WHO.
The objective of this study is to investigate the suitability of the TPS toilet as an alternative
sanitation option. Therefore, we monitored the dry toilet under anaerobic conditions. We
focused on analysing standard chemical and biochemical properties of the toilet products to
evaluate their stability and maturity, and establish nutrient status. The fate of pathogens
during vermicomposting of faecal matter is of much importance in the discussion about reuse-
oriented sanitation concepts. Therefore, in investigating the TPS concept as an alternative
technology for faecal matter treatment, due attention has been given to the hygienic safety of
the product.
MATERIALS AND METHODS
The experiments, which were conducted in the laboratory of the Institute of Wastewater
Management and Water Protection at the Hamburg University of Technology (TUHH),
consisted of three phases: collection, lactofermentation (LF) and vermicomposting. The main
materials used were simple buckets with air-tight lids (without provision for urine diversion),
urine bottles, ground charcoal, stone dust and inoculums (IC). Three independent experiments
were conducted, each involving the three phases as stated above. In the collection phase,
faecal matter (FM) was collected during a two week period with addition of charcoal additive
(CA) (charcoal, stone dust and soil) at each usage. The LF phase involved pre-stabilisation of
the collected material during a 4-week period where the bucket was closed air tight and stored
at a temperature of 20°C, whereas the vermicomposting phase involved aerobic
decomposition of the pre-digested material by the combined action of earthworms and the
waste’s natural flora. The CA consisted of roughly 75% ground charcoal (<1mm), 16%
calcium carbonate (CaCO3) and 9% forest soil (related to dry matter). The moisture content of
CA was adjusted to 16% by addition of water in order to get a dispersible mixture without
dust formation.
Two ICs were investigated, sauerkraut (pickled sour cabbage) juice and a commercial product
‘effective microorganisms’ (liquid EM A). The decomposition was monitored using standard
chemical parameters (moisture content, volatile solids, pH, electric conductivity, soluble total
organic carbon, TOC, as well as N and P compounds). Analyses were conducted using
standard methods as described in the German “Methods Book for the Analysis of Compost”
(BGK, 1994), whereby extractions with CaCl2 solution (for determination of pH, TOC and N
compounds), CAL solution (for phosphorus compounds) and deionised water (for electric
conductivity) were carried out. Mass balances in collected faeces and added CA were also
determined at the end of each phase. A pilot set-up similar to the lab-scale experiments has
been in operation and maintained by one of the authors (H. Pieplow) in his own home for
more than two years. Faeces were collected separately from urine. The composition of the

Accepted for Water Science & Technology, 2010
4
charcoal additive used for faeces during the collection period was the same as in the lab
experiments conducted in TUHH. Using simple bucket with air-tight lid the faeces were
subjected to anaerobic condition with added effective microorganisms (LF phase) for at least
4 weeks and then followed by vermicomposting at his backyard for another 6 months. Results
from the lab-scale analyses were compared with the pilot trial. Additionally, included in the
results and discussion is a data on concentrations of sanitation indicator bacteria for
vermicomposting by Buzie et al. (2010, in press).
RESULTS AND DISCUSSION
During the collection phase of the three experiments no uncomfortable odour was observed
during usage particularly when ICs were incorporated in the CA. This shows that the CA
combined with ICs was effective in odour adsorption.
Table 1. Total mass (TM) in the bucket, used CA, and collected faecal matter (FM) in gram
with their corresponding dry matter (DM) percentage as well as the ratio of CA to FM as
percentage of wet matter (WM) and number of entries (n.o.e.) in experiments 1, 2 and 3.
TM CA FM CA/FM n.o.e.
Exp. 1 4443
(27.9%DM) 859
(83.3%DM) 3584
(14.7%DM) 24.0% WM 23
(155 g FM/usage)
Exp. 2 6650
(31.5%DM) 1040
(84%DM) 5610
(17.1%DM) 18.5% WM 36
(175 g FM/usage)
Exp. 3 6930
(29.1%DM) 1020
(83.5%DM) 5911
(16.8%DM) 17.3% WM 28
(211 g FM/Usage)
As shown in figure 2 only slight changes in DM and organic dry matter (oDM) occured in the
collection (FM) and LF phases in all three experiments. This implies that no remarkable
changes took place in the material during LF phase. In the pilot samples a notably higher
mineral residue was observed (almost equal DM and oDM in terra preta, TP) and a reduction
of 60% to 42% oDM between LF phase and vermicomposting (TP) was measured. Reasons
for the higher mineral content in the LF pilot scale samples were most probably due to a
higher content of stone dust and clay minerals added in the CA. Furthermore, bio-waste was
added to the faeces before LF phase started.
0%
20%
40%
60%
80%
100%
FM LF FM LF FM LF LF TP
Exp. 1 (contr.) Exp.2
(sauerkraut) Exp.3 (EM) Pilot scale
samples
DM [% WM]
oDM [% DM]
Figure 2. Percentage of dry matter and organic dry matter in fresh and lactofermented faeces
in the three experiments as well as in samples from the pilot scale.

Accepted for Water Science & Technology, 2010
5
With regards to soluble N and P (as illustrated in figures 3 and 4) almost all changes from FM
to LF in experiments 1, 2 and 3 showed similar pattern. However a big difference was
observed between the LF and TP pilot scale samples in terms of soluble N (figure 3). The
very low content of soluble N in TP could be interpreted as a result of a strong binding of N to
the humus matrix of the TP material.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
FM LF FM LF FM LF LF TP
Exp. 1 (contr.) Exp.2
(sauerkraut) Exp.3 (EM) Pilot scale
samples
%DM
sol. Norg
NO3-N [%DM]
NH4-N [%DM]
Figure 3. Percentage of soluble N species in fresh and lactofermented faeces in the three
experiments as well as in samples from the pilot scale.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
FM LF FM LF FM LF LF TP
Exp. 1 (contr.) Exp.2
(sauerkraut) Exp.3 (EM) Pilot scale
samples
%DM
PO4-P
Figure 4. Percentage of soluble P in fresh and lactofermented faeces in the three experiments
as well as in samples from the pilot scale.
Testing the lactofermented faeces for vermicomposting, the worms had shown intolerance and
died within 24 hours of exclusive contact with the material. It is assumed that the absence of
suitable co-substrates like bio or garden wastes and the abrupt change of substrate conditions
caused the death of the worms. In contrast to the laboratory trials in TUHH the LF material
(in the pilot scale) were spread on an on-going vermicomposting pile and so the worms
adapted easily. This research will be continued. In the subsequent investigations measures to
eliminate substances toxic to earthworms will be implemented. A possible explanation for the
mortality of earthworms in the lacto-fermented substrate was the release of ammonia (NH )
which is known for its toxicity to living organisms. The release of high levels of NH was
expected owing to the low carbon to nitrogen (C/N) ratio of the substrate. We plan to correct
this problem by adding bulking agents, mainly wood chips to raise the C/N ratio and improve
aeration that facilitates the escape of released NH .
3
3
3
In all samples a pH between 7.5 and 8.1 was measured (Table 2). In experiment 1 and 3 a
slight increase of the pH was observed from FM to LF phase, whereas in experiment 2 the pH
was decreasing. Results imply that release of NH3 was high and nitrification had also occured.

Accepted for Water Science & Technology, 2010
6
In line with the release of NH3 an increase of electrical conductivity (EC) was also observed.
It is remarkable that in experiment 3 (addition of effective microorganisms) negligible nitrate
(NO3-) concentrations were found in the extracts as compared to experiments 1 and 2 which
was also similar with the LF pilot samples where also effective microorganisms were added.
Table 2. Results of the physical and chemical parameters in fresh and lactofermented faeces
in the three experiments as well as in samples from the pilot scale.
Exp. 1 (contr.) Exp.2 (sauerkraut) Exp.3 (EM) Pilot scale samples
Parameter FM LF FM LF FM LF LF TP
DM [% WM] 27.9 26.0 30.2 30.2 29.1 28.0 33.4 41.1
oDM [% DM] 80.6 78.0 79.6 77.1 77.1 28.3 58.6 42.6
Total Soluble N [%DM] 1.34 1.63 1.11 1.32 0.79 n.a. 1.51 0.08
Soluble Organic N 0.83 0.46 0.64 0.38 0.35 n.a. 0.29 0.05
NH4-N [%DM] 0.43 1.10 0.41 0.91 0.38 0.52 1.20 0.01
NO3-N [%DM] 0.09 0.07 0.06 0.04 0.06 0.01 0.02 0.02
Total P [%DM] 0.60 0.73 0.38 0.58 0.52 0.50 0.77 0.59
PO4-P [%DM] 0.57 0.69 0.34 0.51 0.44 0.49 0.72 0.58
pH 7.69 8.01 8.01 7.78 7.52 8.17 8.07 7.58
EC [mS/cm] 2.01 3.03 1.78 2.90 1.99 2.4 4.02 1.7
Based on the amount of CA per DM added to the TPS an amount of 15 kg DM/cap/year is
required to operate the TPS system. It would be between 6.3 and 20.4 kg DM of CA or in
other words 4.7 to 15.3 kg charcoal per capita per year. In comparison with the equivalent
energy input of 39.2 to 127.5 kWh (a heating value of 30 kJ/g for charcoal is assumed) it
seems to be a high energy input but with respect to the corresponding fertiliser value of
deriving TP with its benefits as a soil conditioner it seems more than justified.
Figure 5 shows the initial and final concentrations of six sanitation indicator bacteria (SIB)
monitored in test units (with earthworms) and controls (without worms) during a 60 day
period (Buzie et al., 2010, in press). There was a clear difference in SIB population profiles
between the test and control systems. Although both systems exhibited reductions for all the
SIB analysed, the reduction was more important when earthworms participated in the
stabilisation process: (Escherichia coli (99.98% vs. 45.46% reduction), Faecal coliforms
(99.98% vs. 49.26% reduction), Enterococcus faecalis (99.99% reduction vs. 24.72%
increase), Salmonella spp (99.76 % vs. 74.57% reduction), Shigella spp (99.69% vs. 99.71%
reduction), and Enterobacter spp (99.98% vs. 56.81% reduction).
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
E. Fecalis
E. coli
F. coliforms
Salmonella spp.
Shigella spp.
Enterobacter spp.
Population [CFU/g DS]
Test Initial
Test Fi nal
Control Initial
Contr ol Final
Figure 5: Sanitation indicator bacteria average decrease (shown in logarithmic scale) in the
test and control systems during 60 days of treatment.

Accepted for Water Science & Technology, 2010
7
Similar results were reported by Flack and Hartenstein (1984). In an extensive study on the
growth of E. foetida on microorganisms and cellulose, Buzie et al. (2010, in press)
demonstrated that this earthworm grows similarly on gram-negative and gram-positive
bacteria, and thus concluded that E. foetida is able to destroy human pathogens as well as
non-pathogenic microbes by decomposing bacteria cell wall with a wide variety of
polysaccharides. To meet United States Environmental Protection Agency ( US-EPA) class B
criteria, faecal coliform density must be below 2.0*106 MPN/gTS or 2.0*106CFU/gTS. In the
present study, the density of faecal coliform in the final material (from test units) was
4.80*103 which satisfied the requirement for class biosolids. The finding suggests that
vermicomposting may be a feasible option for elimination of pathogens. Further
investigations of the end-product by variation or prolongation of processing and storage times
may provide improved results.
CONCLUSIONS
The re-discovery of a highly efficient low-cost sanitation system from an ancient civilisation
in the Amazon combined with the latest development in urine diverting dry sanitation opens
new pathways. Research is at the very beginning but results are promising. Application can be
performed right away although there are a lot of open questions to fully understand the whole
process. Lactic-acid fermentation is working so well and stable provided that inoculation is
done. There are few reasons left for the desiccation of faecal matter that is now done in
modern dry sanitation with numbers of installations exceeding a million. Adaptation of
existing dry toilets is feasible where space requirements for the common double vault systems
can be cut to half. The anaerobic, but oxygen-tolerating terra preta sanitation (TPS) offers
smell free operation without ventilation pipes. Besides modification of the existing urine
diverting dry toilets there are three more development lines for implementation: 1.) Upgrading
pit-latrines as used by around two billion people; 2.) Simple bucket toilets with any kind of
urine diversion that are closed air tight after defecation and addition of the charcoal with
lactic-acid bacteria mixture and finally 3.) Well designed and comfortable urine diverting dry
toilets with optional automatic addition of the conditioning/inoculation material. People
suffering inadequate or missing sanitation can be served with one of the three types of
installations. In the worst of situations as in slum areas a simple plastic bag would also be
usable through avoidance of methane-fermentation in the silage process. The inoculation
mixture can be multiplied at very low costs, but ground charcoal is a resource that is required.
As in all on-site systems participatory planning (NETSSAF, 2008), proper utilisation of the
end-products and well organised professional operation and maintenance is crucial for
success. Quality of the end product will depend on avoidance of pollution at the source and in
order to get good results in usage of the soil the dynamics of soil-plant interaction should be
respected (Reckin, 2003). This implies that urine is not applied in a way that results in
imbalances of plant nutrition that results to weaker plants. Research on urine treatment in a
way that results in a balanced rich soil with a proper C/N-ratio will be crucial to achieve a
well-functioning interaction of sanitation, biowaste management and agriculture.
ACKNOWLEDGEMENTS
The authors want to thank the old cultures in the Amazon for their example of highly efficient
ways of dealing with biowaste and sanitation in a way that created very fertile soils. We also
thank the German Ministry of Research, BMBF for their support in the framework of the
IPSWaT-program.

Accepted for Water Science & Technology, 2010
8
REFERENCES
Al-Ghazali M. R. and Al-Azawi S. K. (1990). Listeria monocytogenes contamination of crops grown on soil
treated with sewage sludge cake. J. Appl. Bacteriol. 69:642-647.[Medline]
Basja O., Nair J., Mathew K. and Ho G.E. (2002). Vermiculture as a tool for domestic wastewater management.
In: Proceedings of the International water association 5th specialised conference on small water and wastewater
systems. Instanbul. ISBN 9755612254.
BGK (1994) Bundesguetengemeinschaft Kompost e.V: Methodenbuch zur Analyse von Kompost (Federal
Association on compost: Methods for the analysis of compost).
Buzie C. (2010) Development of a Continuous Flow Vermicomposting Urine Diversion Toilet for On-site
Application. PhD thesis, Hamburg University of Technology, Germany.
Flack, F.M. and Hartenstein, R. (1984). Growth of the earthworm Eisenia fetida
on microorganisms and cellulose. Soil Biol. Biochem. 16: 491–495.
Glaser, B. (2007). Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the
twenty-first century, Phil. Trans. R. Soc. B. 362: 187–196
Graham R. D. (2008). Micronutrient Deficiencies in Crops and Their Global Significance. B. J. Alloway (ed.)
Micronutrient Deficiencies in Global Crop Production. Springer Science + Business Media B.V., p. 41-61.
Günther F. (2007). Carbon sequestration for everybody: decrease atmospheric carbon dioxide, earn money and
improve soil. Submitted to Energy and Environment.
Kunwar Durg Vijay Singh Yadav (2008). Vermiculture and vermicomposting using human faeces as feed and
Eisenia foetida as earthworm species. Doctoral thesis: Department of civil engineering. Indian Institute of
Technology, Kampur, India.
Lehmann et al. (2003b) Biochar soil management on highly weathered soils in humid tropics.
http://www.fluxfarm.com/reviews/bio-char-soil-management-highly-weathered-soils-humid-tropics.html
(accessed 3 February 2010)
NETSSAF, 2008: NETSSAF Participatory planning approach. A project supported by the European Union under
the 6th Framework Programme, Thematic Priority: Global Change and Ecosystems.
Pieplow H. (2008). Terra Preta: Ein Modell für regionales Stoffstrommanagement (A model of regional
resources management), Band 215: 2. Aachener Kongress „Dezentrale Infrastruktur Wasser Energie Abfall
(Decentralised infrastructures water energy solid waste)“ GFA an der RWTH Aachen .
Reckin, J. (2003) The Web of Life – Soil’s role in Plant & Animal Health, ACRES U.S.A., Oct., pp 24,
UBA – Umweltbundesamt: Bericht zur Klärschlammentsorgung, (German Federal Agency of the Environment:
Report on Sludge disposal) Berlin, 30.4.2001/4
Shalabi M. (2006) Vermicomposting of Faecal Matter as a Component of Source Control Sanitation. PhD thesis,
TUHH, Hamburg University of Technology, Germany.
www.tuhh.de/aww: http://www.tu-harburg.de/aww/ (accessed 3 February 2010)
www.das-gold-der-erde.de: http://www.das-gold-der-erde.de/ (accessed 3 February 2010)
www.triaterra.de: http://www.triaterra.de/epages/62153231.sf (accessed 3 February 2010)