Session 1
Sludge treatments
and their effects on
pathogens
Treatment and disinfection of sludge using quicklime
A.D. Andreadakis
Department of Water Resources, Faculty of Civil Engineering, National Technical
University of Athens, 5 Iroon Polytechniou St., Zorgafou, Athens 15773, Greece
Introduction
Lime is a readily available alkali, which is widely used in sewage treatment.
Utilisation of lime has the following benefits: a) conditions all types of sludge, b)
precipitates toxic metals and removes nutrients, c) destroys the pathogenic agents, d)
reduces the biochemical and biological oxygen demand and suspended solids, e)
eliminates offensive odours.
Two forms of application of lime are usually distinguished: a) as unslaked (quick-)
lime - CaO and b) as slaked lime - Ca(OH)2. Furthermore during treatment of sludge,
lime can be added to the sludge before thickening, before dewatering, or after
dewatering. No significant difference has been observed between the effect of
hydrated lime and quicklime when treating sludge with high water content, as is the
case of undewatered sludge (Tullander, 1993). In reaction with water CaO within a
few minutes forms Ca(OH)2. For that reason if quicklime is used in sludge with a high
water content the same effect will be attained in practice as with hydrated lime, but
with a lower dose of chemicals (1:1.3). As the handling of quicklime is more
complicated, hydrated lime is normally used in small treatment plants and quicklime
in large plants. When quicklime is slaked to the hydrated form, energy is emitted 1160 kJ/kg CaO. Theoretically, 350-400 kg CaO/m3 water can bring the temperature
near the boiling point. However, as the quantities of lime used in practice normally are
related to the dry solids content of the sludge, the treatment of undewatered sludge
will not result in any significant rise of temperature.
The effects of liming undewatered sludge, usually at the dose of 10-20 kg/m3 can be
summarised as follows: i) improvement of sludge dewatering properties (in some
cases in combination with addition of ferrous sulphate or ferric chloride), ii) pH
increase to about 11.5-12, which lasts for about two weeks, iii) no increase of
temperature, iv) inactivation of bacterial and viral pathogens, but limited effect on
parasites and v) regrowth of bacterial pathogens.
In the case of dewatered sludge, addition of quicklime, results in a significant
temperature rise and high dry solid content due to evaporation. This in turn leads to
improved sludge handling characteristics and long lasting disinfection.
31
This paper deals with quicklime treatment of dewatered sludge and reviews important
aspects of the process involved, such as temperature and dry solids content increase,
pH rise and maintenance, sludge sanitation, sludge handling and physicochemical
characteristics, nutrients availability to the plant and technological possibilities.
Chemical reactions
Calcium can be easily found in the form of CaCO3. However, addition of CaCO3 to
sludge has very limited effect on pathogen destruction, due to the limited pH rise that
can be obtained up to 8.5. For this reason calcium carbonate is thermally treated at a
temperature of 1100-1200 °C, to produce CaO.
CaCO3 + 42.5 kcal → CaO + CO2
(1)
The quality of the final product (quicklime) depends on the quality of CaCO3 and the
thermal process and this quality in term determines the rate of temperature increase of
the subsequent reaction of CaO with water.
CaO + H2O → Ca(OH)2 + 15 kcal
(2)
With addition of the appropriate quantity of CaO a pH increase to approximate 12.5
can be obtained. However, it should be noted that unless excess CaO is used, a
consequent reduction of the pH can be observed due to the reaction of the produced
Ca(OH)2 with the CO2 of the atmosphere or that produced due to biological activity.
Ca(OH)2 + CO2 → CaCO3 + H2O
(3)
CaCO3 + CO2 → Ca2+ + 2HCO-3
(4)
Temperature and solids content increase
The expected temperature rise can be theoretically estimated assuming that 100% of
the heat released (1160 kJ/kg CaO) is utilised and considering the specific heat values
of water, initial solids (TS1) and lime.
∆T=
1160x%CaO
4.16(100%–%TS1) + 0.25x%TS1 + 0.3x%CaO
(5)
Equation (5) gives an almost linear increase of ∆T with CaO dose. For dewatered
sludges with TS content in the range 20-30% respective increases of 3.4 to 3.9 °C for
each percent CaO dose (1% CaO = 10 kg CaO per tonne of sludge) can be
theoretically expected. In practice due to heat losses and quicklime quality (less than
100% active CaO), the observed temperature increases are limited to 60-82% of the
theoretical values. The time needed to effect the temperature increase is
approximately one to two hours after mixing, depending on lime quality.
32
The increase in the solids due to quicklime addition is almost linear. Theoretically the
new solids content (TS2) can be calculated, by considering the chemical reaction (2)
as follows:
%TS2=
%TS1 + [74/56x%CaO]
x100%
100% + %CaO
(6)
A further slight increase of solids can occur due to formation of CaCO3 as a result of
reactions with CO2. The solid content increase results in a more compact sludge,
which can be stored and handled more easily.
pH increase and maintenance
It has been found (Carl Bro S/A, 1997), that in order to raise the pH in sludge from 7
to 12.5, 1.7 mmoles of (OH) are needed per gr TS, so that the buffering capacity of
proteins of sludge is neutralised. Since 56 mg CaO produce 2 mmoles (OH), 50 mg
CaO are needed per gr TS. For sludge with 20-30% TS, 1-1.5% CaO are needed to
effect the increase to 12.5. At this dose there is no excess CaO to neutralise CO2 and
organic acids production. Therefore, a higher dose typically above 2% CaO is
normally needed. It is not unusual to use doses in the range 6-10% CaO in order to
ensure maintenance of the high pH over sufficiently long periods (several months).
One of the main reasons for CO2 production and subsequent pH reduction is
biological activity within the sludge. Due to inefficient mixing and lack of buffering
capacity, at a dose 2% CaO several regions of the sludge are unstable in the sense that
the pH is lower than 12, thus allowing microbial activity and production of CO2. As a
consequence, a very quick reduction of pH in the whole of the sludge can be observed
leading to low pH values to the order of 8-9 within few weeks. The same happens with
a dose of 4% CaO, although at a slower rate (a couple of months). For both doses
odours can develop. Sufficiently long periods (over three months) of stable pH around
12.5 can be ensured with doses in the range 6-10%.
Another factor that has to be taken into consideration is the reaction with external CO2
of the atmosphere and/or CO2 produced by fungi activity at the surface of the sludge.
However, with proper storage (low surface area/volume ratios, low temperatures) this
factor can be minimised.
Sludge sanitation
The effect of CaO on selected bacteriological parameters after mixing is shown in
Table 1, which indicates that substantial reduction of pathogens can be achieved even
for a dose of 2% CaO. However, Tables 2 and 3 show that a reduction of pathogens
over prolonged periods (over 1 day) require a dose of over 4% CaO. The effects of the
duration time under high pH and of the temperature seem to vary with the pathogen
type. Prolonged exposure, over several days, of coliforms to a high pH environment
enhances their removal while increased temperatures appear to be effective only in the
33
Table 1: Influence of CaO on bacteriological parameters 4 hours after mixing, 20 °C (Carl Bro
S/A, 1997).
CaO Dose
Coliforms
%
0
2
4
6
8
10
number/g
23x104
33x101
13x101
33x101
13x101
13x101
Temperature Streptococcus
resistant
Coliforms
number/g
number/g
49x103
11x104
–
<100
–
<100
–
<100
–
<100
–
<100
Clostridium
perfrigens
Salmonella
number/g
30x103
18x102
14x102
13x102
9x102
2x102
number/g
Traced in 10 g
–
–
–
–
–
Table 2: Influence of the CaO dose and storage time on the microbial load [Carl Bro S/A, 1997].
Temperature Coliforms
Temperature
resistant
Coliforms
Clostridium perfringens
Vegetative
Days
1
1
14
1
14
1
14
Salmonella
Spores
1
14
CaO Dose
1
14
Identification in
10 g of sludge
%
°C
0
2
4
6
6
8
8
10
10
15
15
20
20
20
20
26
20
28
20
33
20
39
number/g sludge
10x106
33x102
70x102
70
70
63
26
9
34
33
11
49x104
46
4.9
5.0
5.0
<2
<2
6
5
<2
<2
33x104
2.3
<2
<2
<2
<2
<2
<2
<2
<2
<2
49x104
<2
<2
<2
<2
<2
<2
<2
<2
<2
<2
90x103
30x102
13x102
12x102
60x101
20x102
11x102
18x102
70x101
11x101
<10
10x104
10x102
6x101
10x101
<10
100
<10
<10
<10
<10
<10
90x103
<10x102
<10
<10
<10
<10
<10
<10
<10
<10
<10
40x103
<10x101
100
<10
<10
<10
<10
<10
<10
<10
<10
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
case of vegetative clostridium perfrigens. Quicklime treatment of sludge is also
effective with respect to removal of parasites. Comparative studies of Ascaris ova
growth with (10% CaO, Table 5) and without CaO (Table 4) addition, reveal the
presence of multicell ova and development of fully grown larva up to 71% in the
reference sample, while practically only unicellular ova are observed with quicklime
treatment. Furthermore, Table 6 indicates that the ova can loose the ability to grow,
even under favourable conditions after prolonged exposure to a mixture of sludge CaO (i.e. exposure for five months).
34
Table 3: Reduction of the microbial load in terms of negative logarithms (Carl Bro S/A, 1997).
CaO Dose °C after
1st day
%
0
2
4
6
6
8
8
10
10
15
15
20
20
20
20
26
20
28
20
33
20
39
Coliforms
4 hr
–
2,8
3,2
3,8
–
5,2
–
3,2
–
–
–
1 day
–
3,5
3,2
5,2
5,2
3,2
5,6
6,0
5,5
5,5
6,0
Clostridium perfringens
14 day
1,3
5,3
6,3
6,3
6,3
>6,7
>6,7
6,2
6,3
>6,7
>6,7
Vegetative
1 day 14 day
–
0,1
1,5
2,0
1,8
2,2
1,9
3,0
2,2
4,0
1,5
1,7
3,0
1,9
>4,0
1,2
1,7
>4,0
–
2,1
>4,0
–
2,9
>4,0
–
>4,0
>4,0
4 hr
–
1,2
1,3
1,4
Spores
1 day 14 day
–
0,4
2,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
>4,0
Table 4: Ascaris ova growth (Reference Sample) (Carl Bro S/A, 1997).
GROWTH STAGE
Non fertilised ova
Unicellular
Bicellurar
Four cells
> 8 cell
Premature larva
Fully grown larva
TOTAL
0
1
99
0
0
0
0
0
100
14
9,6
85
1
0
0,7
3,7
0
100
28
11,3
4
0
0
16,7
11,3
56,7
100
DAY
42
9,2
0
0
0
24,6
8,5
57,7
100
56
6,7
8
1
0
19,2
6,7
59,3
100
70
5,3
9,7
0,3
10,3
3,3
71
100
150
3,3
10
1
0,3
13,3
2,8
69,3
100
70
0
99.3
0
0
0
0.7
0
100
150
0
100
0
0
0
0
0
100
Table 5: Ascaris ova growth (CaO 10% treated sludge) (Carl Bro S/A, 1997).
GROWTH STAGE
Non fertilised ova
Unicellular
Bicellurar
Four cells
> 8 cell
Premature larva
Fully grown larva
TOTAL
0
1
99
0
0
0
0
0
100
14
1.3
98
0.7
0
0
0
0
100
28
1.7
96
1
0.3
1
0
0
100
DAY
42
0
92.3
0
0
7.7
0
0
100
56
0
92.7
0.3
0
3
4
0
100
35
Table 6: Ascaris ova cultivation in formaldehyde solution 1% after being isolated out of
sample treated with 10% CaO (Carl Bro S/A, 1997).
GROWTH STAGE
Ova retention time in sample before cultivation (weeks)
2
4
6
8
20
1.7
3.3
0
0
0.4
0.7
7.3
11.7
40.7
63.4
0
1.7
2.3
0
2.3
0
0.3
2.3
0.3
0.6
24
26.7
54.7
37.4
32.7
6.3
4.3
7.7
6.3
0.6
67.3
56.7
21
15.3
0
100
100
100
100
100
Non fertilised ova
Unicellular
Bicellurar
Four cells
> 8 cell
Premature larva
Fully grown larva
TOTAL
Other significant aspects
The long term effect of CaO treatment on sludge characteristics is shown in Table 7.
Upon quicklime addition, an increase in total solids content and pH is observed, while
volatile solids content, phosphorus, nitrogen and ammonia contents are reduced. With
storage over seven months total solids content tends to gradually increase while pH, is
slightly reduced, remain however above 12. It is only after a storage period of two
years that a drastic reduction of pH and alkalinity are observed. The total solids
content increases significantly, wile the ratio of total solids volatile to total solids is
reduced due to degradation of the organic material. This degradation also explains the
higher phosphorus content observed. Sludge obtains a wood-like texture with slight
but not unpleasant odour.
With respect to phosphorous availability to plants, it has been observed (Akrivos et al.,
1999), that upon lime addition and pH increase the available phosphorous is limited to
Table 7: Long term effects of the CaO treatment (10%) on sludge characteristics (Carl Bro
S/A, 1997).
Parameter
Units
pH
TS
g/kg
TS Volat.
g/kg
TSVol/TS
%
Tot-P
g/kg TS
Tot-N
g/kg TS
NH3-N
g/kg
Alcalinity mmol/kg
Odours
* Before CaO addition
36
0*
7.1
168
101
60
17
32
1.5
–
0
12.5
283
87
31
9.8
19
0.27
–
–
14
12.5
292
86
29
9.6
17
0.21
2390
–
Days
45
12.5
291
90
31
–
17
0.27
2700
–
120
12.3
332
93
28
9.3
17
0.31
3080
–
210
12.1
303
104
34
–
17
0.18
2700
–
720
8.4
531
103
19
17
13
–
1340
–
approximately 68%. However, with neutralisation to normal pH (as it happens when
sludge is introduced to the soil) the available phosphorous is increase to over 90%.
Finally, practical experience has shown that both batch or continuous plug flow
systems can be used for quickliming of sludge and that the most important
technological aspect is related to the sludge-CaO mixing efficiency of the system.
Conclusions
Addition of quicklime to dewatered sludge and subsequent storage under a pH of over
12 for at least three months ensures a high degree of sludge sanitation. This sludge can
be used as a soil conditioner and fertiliser without any restrictions as far as pathogens
are concerned. Even after prolonged storage, there is a very limited reduction of
nitrogen, while the availability of phosphorous for plant growth is high (over 90%)
under conditions of neutralised pH, which is very quickly established upon mixing of
the sludge with the soil. Finally, quicklime treatment improves the handling
characteristics of the sludge and allows for long term storage without development of
odour.
Acknowledgements
The results presented in this paper originate from studies conducted by Carl Bro A/S
and the National Technical University of Athens within the framework of a joined EU
project (Hygienic Sludge Management for Agricultural Utilisation), sponsored by the
Directorate-General for Research (formely DG XII) of the European Commission.
The contribution of the other partners of the project namely, NAMA Consulting
Engineers and Planners SA, VKI and the Municipal Enterprises of Water and
Sewerage of Lamia and Rethymnon, is also acknowledged.
References
Tullander V., (1983), “Quicklime treated sludge”, Environmental effects of organic and inorganic
contaminants in sewage sludge, Davis, Hucker & L’Hermite (eds), D. Reidell Publishing Company.
Carl Bro A/S, (1997), “Treatment of Sludge with Lime”, Research Report
Akrivos J., Mamais D., Katsara K., Andreadakis A., (1999), “Agricultural Utilisation of Lime
Treated Sewage Sludge”, Proceedings of the Specialised Conference on Disposal and Utilisation of
Sewage Sludge, Athens, Greece, October 13-15, 1999.
37
Anaerobic digestion of sludge: focusing on degradation of
the contained organic contaminants
Angelidaki, I.1, and Ahring, B.K.1,2
Anaerobic Microbiology/Biotechnology group
Department of Biotechnology, Building 227
The Technical University of Denmark, 2800 Lyngby, Denmark.
Tel. +45 45256187; e-mail: ria@ibt.dtu.dk
1The
2School of Engineering and Applied Science
Department of Civil and Environmental Engineering
University of California
Abstract
Great interest has been devoted in the recent years for recycling of the waste created
by modern society. A common way of recycling the organic fraction is amendment on
farmland. However, these wastes can contain possible hazardous components in small
amounts, which can prevent their use in farming. The objective in our study has been
to develop biological methods by which selected organic xenobiotic compounds can
be biotransformed by anaerobic or aerobic treatment.
Screening tests assessed the capability of various inocula to degrade two phthalates
(DBP, and DEHP), four polycyclic aromatic hydrocarbons (PAHs), four linear
alkylbenzene sulfonates (LAS) and three nonylphenol ethoxylates (NPEO) under
aerobic and anaerobic conditions.
Under aerobic conditions, a large number of the selected xenobiotics could be
degraded by choosing the appropriate inoculum. Aerobic degradation of DEHP was
only possible with leachate from a landfill as inoculum. Anaerobic degradation of
some of the compounds was also detected. Leachate showed capability of degrading
phthalates, and anaerobic sludge showed potential for degrading PAH, LAS and
NPEO.
Furthermore, anaerobic digestion of sludge amended with either pyren and LAS or 4nonyl phenol and DEHP in continuously stirred tank reactors showed anaerobic
transformation of the tested compounds.
The results are very promising as they indicate that a great potential for biological
degradation is present, though the inoculum containing the microorganisms capable of
transforming the recalcitrant xenobiotics has to be carefully chosen.
38
Introduction
Within the European Union the total amount of produced sludge is about 6.5 million
tons per year (Smith 1996). There are several disposal routes for sludge, including
ocean dumping, incineration, spreading on agricultural land, soil incineration, land
spreading in forestry or landfilling. At present the disposal of sludge on landfills is
with 40% the most important outlet in the EU while 37% of the sewage sludge
produced within the EU was used for agricultural purposes in 1994 (Hall 1994). The
amount of sewage sludge requiring disposal is expected to increase significantly in the
future due to recent environmental developments. The Helsinki agreement called for
the banning of ocean sludge dumping by 1987 and the Urban Waste Water Directive
91/271/EEC, required waste water treatment plants with secondary treatment and
nutrient removal in sensitive areas (Kiely 1997). With increasing sludge protection in
the EU larger larger amounts of sewage sludge will be recycled for agricultural
purposes (Smith 1996). This approach seems to be reasonable since agricultural land
can become nutrient deficient due to intensive cultivation.
In addition to sewage sludge other wastes such as organic industrial wastes, manures,
and organic household waste can with great advantage be recycled and used in
farmland as fertilizers and as soil improving components.
However, this type of waste can contain possible hazardous components in small
amounts, which might show adverse effects on the ecosystem, e.g. the farmland
amended with sewage sludge. Indeed, linear alkylbenzene sulfonate (LAS) compounds
such as those used in household detergents, nonylphenols, nonylphenol ethoxylates,
polycyclic aromatic hydrocarbons (PAH), and phthalates have recently been identified
as major anthropogenic organic components in sewage sludge. There is a special public
concern about organic components, which may have a potential for acute toxicity,
mutagenesis, carcinogenesis or teratogenesis or posses estrogenic effects.
The concentrations of LAS in raw wastewater have been reported to range from 3
mg/l to 21 mg/l (Brunner et al., 1988, De Henau et al., 1989, Holt et al. 1995, Ruiz
Bevia et al., 1989). Although LAS and other common surfactants have been reported
to be readily biodegradable by aerobic processes, much of the surfactant load into a
sewage treatment facility (reportedly 20-50%) is associated with suspended solids
(Greiner and Six 1997, and McAvoy et al. 1998) and thus escapes aerobic treatment
processes. LAS is reported not to be biodegraded by anaerobic biological processes
usually employed in sludge stabilization (McEvoy and Giger, 1985; Swisher, 1987),
and it may be found in the gram per kilogram range in anaerobic sludge. According to
Mackay et al. (1996) the emission of LAS to soil is predominant due to sludge
application on agricultural soil and landfilling. The presence of surfactants in sludge
may have undesirable environmental effects since the surfactant molecules may leach
to groundwater contributing to groundwater contamination.
Alkylphenol ethoxylates such as NPnEOs is a group of non-ionic surfactants with
world-wide application and are evidently less biodegradable than LAS (e.g. Swisher,
1970; Steinle, 1964; Pitter, 1968) and a wider range of removals from 0-90% based on
specific analyses such as UV and IR spectroscopy (Swisher, 1970). This suggests that
39
only partial degradation occurs, such as conversion from polyethoxylates to
nonylphenol diethoxylate (NP2EO), nonylphenol monoethoxylate (NP1EO), and
nonylphenol (NP). The latter is the most recalcitrant of the intermediates formed
during alteration of the NPnEO molecules. Mass balances done on treatment plants in
Switzerland (Brunner et al., 1988) support these findings.
Due to the low water solubility and lipophilic properties of PAHs, these compounds
are removed from sewage partly by biodegradation, partly by adsorption on to sludge.
According to Bodzek et al. (1997) PAHs are found in significant amounts in sludge
(up to 2000 mg/kg sludge dry mass). The PAHs are mostly originating from fossil fuel
combustion and industrial processes (Shuttleworth and Cerniglia 1995). Generally the
half-lives of PAHs is increasing with increasing number of aromatic rings, though the
degradation rates is dependent upon the test system. Due to the great adsorption
abilities of PAHs they will precipitate with the particular material in the pre-clearing
tank. The sludge originating from this step has a high content of easily degradable
organic material, which is used for the production of methane. PAH are, however, not
as easily anaerobically degradable as under aerobic conditions and they tend to
accumulate in the digested sludge.
Sewage sludge and compost contains relatively high concentrations of di(2ethylhexyl)phthalate (DEPH) and di-n-butylphthalate (DBP) (Danish Environmental
Agency, 1996). It was reported that these compounds probably originate from both
households and industries. DEHP accounts for 90% of the phthalate production worldwide. At least 95% of the DEHP produced is used as an additive in PVC plastics which
are made into various products such as waterproof clothing, footwear, toys, bloodbags
and heat-seal coatings on metal foils. Plastizisers are poorly soluble substances and
will be removed in the waste water treatment plant by biodegradation, though also
adsorption on sewage sludge is significant. Hence, plastizisers will be transferred to
soil with application of sludge on agricultural land or landfilling of sludge.
Biological treatment of wastes containing toxic compounds could be an effective and
cheap method for detoxifying the wastes. In order to do so, microorganisms that can
degrade the compounds are needed. In the present study we report results from a
screening of microorganisms capable of degrading LAS, nonylphenols,
nonylphenolethoxylates, PAH and phthalates.
Materials and methods
Description of the experimental set-up
The tests were performed in batch serum vials where pressure and/or substance
concentration was followed over time. Two different media were used, one for the
anaerobic (Angelidaki et al. 1990) and one for the aerobic inocula, containing per liter
of water (Milli-Q) 0.8 g K2HPO4, 0.2 g KH2PO4, 0.05 g CaSO4*2H2O, 0.5 g
MgSO4*7H2O, 0.01 g FeSO4,*7H2O, 1.0 G (NH4)SO4.
Three sets of control vials (sterile, substrate-unamended, and uninoculated) were
made in triplicates.
40
Compounds tested
Of PAHs naphthalene, 1-methylnapthalene, fluoranthene, phenanthrene and pyrene
were chosen. The PAHs were added either as a mixture of the five PAHs mixed in
equal amounts or as the individual compounds. Among the phthalates, DBP, and
DEHP were selected. LAS was used as a mixture of LAS with an alkyl chain length of
9 to 13 units and among the nonylphenols 4-Nonylphenol and Nonylphenol mono and
diethoxylate were used. For the mixture of PAH the concentration was counted as the
sum of the five compounds.
Inocula
The inocula originated from several different Danish environments as shown in Table
1, along with redox conditions, and xenobiotics used during the screening test.
Table 1: Inoculum identification (original environment), redox conditions for
biotransformation studies, and xenobiotics used.
Origin
Activated Sludge Lundtofte
Redox conditions Xenobiotics added
Anaerobic, aerobic NPE, LAS, Napthalene,
1-Methylnapthalene, Fluoranthene,
Phenanthrene, Pyrene, DBP, DEHP
Activated Sludge Damhuså
Anaerobic, aerobic NPE, LAS, Napthalene,
1-Methylnapthalene, Fluoranthene,
Phenanthrene, Pyrene, DBP, DEHP
Dewatered sludge Damhuså Anaerobic
NPE, LAS
Biosludge Lundtofte
Anaerobic, aerobic NPE, LAS
Sediment Damhuså stream
Anaerobic
NPE, LAS
Sediment Lake Arresø
Anaerobic, aerobic NPE, LAS, DBP, DEHP
Soil Nr. Herlev, Hillerød
Anaerobic, aerobic NPE, LAS, Napthalene,
1-Methylnapthalene, Fluoranthene,
Phenanthrene, Pyrene, DBP, DEHP
Soil Møllehøj, Arresø
Anaerobic, aerobic NPE, LAS, DBP, DEHP
Soil Colgate-Palmolive
Anaerobic, aerobic NPE, LAS
Soil Oil contaminated, Jutland
Napthalene, 1-Methylnapthalene,
Fluoranthene, Phenanthrene, Pyrene
Soil Hjørring Gaswork
Napthalene, 1-Methylnapthalene,
Fluoranthene, Phenanthrene, Pyrene
Compost AFAV, Hillerød
Anaerobic, aerobic NPE, LAS, DBP, DEHP
Granular sludge UASB reactor, Anaerobic, aerobic NPE, LAS
Eerbeek
Manure Mesophilic CSTR
Anaerobic, aerobic NPE, LAS
Landfill leachate
DBP, DEHP
41
Reactor experiments
Two reactors of total volume of 4.5l were operated with anaerobic digested sludge at a
retention time of 15 days at 37 °C. After the reactors had been operated for a month
with the sludge additional 4-NP or pyren was added to the feed at day 7 to the reactor
R1 and R2 respectively. At day 28 the concentrations of the added compounds were
increased from 50 ppm to 100 ppm. At day 38 was DEHP and LAS was additionally
added to the reactors R1 and R2 respectively.
Analysis
Concentration of PAHs, phthalates, was made by GC-MS. While LAS and nonyl
phenols were measured by HPLC analysis.
Results and discussion
Vials incubated under aerobic conditions showed a decrease of the initial pressure
when degradation was present. In figure 1 the degradation of PAH is shown when
inoculating with sludge amended soil. When PAH was added in a concentration of 20,
100 or 200 ppm the pressure in the vials decreased, indicating degradation of the
added compounds (Fig. 1a). In the PAHs unamended controls the pressure slightly
decreased relative to the sterile control due to digestion of ethanol, though the
pressure remained higher then the PAH amended vials. In figure 1b the concentrations
(in area counts) of the individual PAH are shown at the start and end of the
experiment. There was a significant reduction of the concentrations of the measured
PAH at the end of the experiments compared to the initial concentrations, confirming
that the PAHs were biodegraded.
In figure 2 degradation of DBP under anaerobic conditions is shown. When degradation
occured the pressure in the vials increased due to production of methane and carbon
dioxide. The pressure of the controls without DBP addition also increased due to
mineralization of organics contained in the inoculum and ethanol added in a
concentration corresponding to the DBP amended vials. However, the pressure increase
in the vials with 20 and 100 ppm DBP was higher than in the controls, corresponding to
biodegradation of DBP. Vials with 200 ppm DBP showed no increase of the pressure
indicating that this concentration was toxic to bacteria preventing both degradation of
DBP and of the organics contained in the inoculum. In figure 2b the concentration of
DBP (20 ppm and 100 ppm) is reported and evidence of DBP biodegradation is given
when evaluation the pressure increase together with the DBP concentration decrease.
The results from the screening test are summarized in Table 2. Most compounds were
degraded both under aerobic and anaerobic conditions. Some compounds were easily
degradable with most inocula. Such compounds were naphthalene, DBP, while the rest
compounds showed only degradation with only a few inocula. DEHP is a compound
that has been reported as recalcitrant under anaerobic conditions. Ejlertson (1997) has
reported that DEHP was unaffected, during anaerobic incubation, throughout an
42
16
14
Pressure (psi)
12
10
8
20 ppm PAHs
100 ppm PAHs
200 ppm PAHs
200 ppm PAHs sterile
PAH unamended
6
4
2
0
0
2
4
Time (weeks)
6
8
10
8e+6
begining
end
7e+6
Area counts
6e+6
5e+6
4e+6
3e+6
2e+6
1e+6
0
phenanthrene
fluoranthrene
pyrene
Figure 1: Pressure depletion in PAHs amended aerobic vials and relative concentration in vials
at the beginning and end of the experiment.
43
40
35
20 ppm DBP
100 ppmDBP
200 ppm DBP
200 ppm DBP
200 ppm DBP sterile
DBP unamended
Pressure (psi)
30
25
20
15
10
5
0
0
2
4
6
8
10
Time (weeks)
1e+7
Area counts
8e+6
beginning
end
6e+6
4e+6
2e+6
0
DBP 20
DBP 100
Figure 2: Pressure increment in DBP amended anaerobic vials and relative concentration in
vials at the beginning and end of the experiment.
44
12
Table 2: Results from the screenings test for degradation of xenobiotic compounds.
Group
LAS
NPE
PAH
Phthalate
Tested xenobiotic compound
Linear Alkylbenzene Sulfonate
4-Nonylphenol
Nonylphenolmonoethoxylate
Nonylphenoldietoxylate
Acenapthene
Naphthalene
Phenanthrene
Fluoranthene
Pyrene
DEHP
DBP
Aerobic
+
–
+
+
–
+
+
+
+
+
+
Anaerobic
+
–
+
+
+
+
+
–
+
+
+
+: indicates degradation; -: indicates not degradation;
experimental period of 330 days. However, in our screening experiments we found
indication of degradation of DEHP with inoculum originating from leachate from a
landfill. It is possible that deposition of plastics, and other materials containing DEHP
has favoured the selection of organisms capable of degradation of DEHP. The landfill
percolate did not show capability of aerobic degradation of DEHP.
LAS is known to be easily degraded aerobically, e.g. in activated sludge reactors
during waste water treatment, but is not degraded anaerobically in the waste water
treatment (McEvoy and Giger 1985). In our screening experiments we found inocula
showing anaerobic capability of LAS. Inoculum from lake sediment (Damhuså)
showed capability of anaerobic degradation of LAS. In addition, also inocula that
were found in aerobic environments such as compost and activated sludge (Lundtofte
wastewater treatment plant) showed capability of anaerobic degradation of LAS.
PAHs were found to be degradable under both aerobic and anaerobic conditions,
though a higher number of inocula contained the relevant microorganisms for aerobic
PAH degradation.
From the reactor experiments it can be seen that all the tested compounds were
transformed under anaerobic digestion of sludge. Reduction of the compounds to up
to 100% were observed (Figure 3 and Fig. 4). After additions of the second xenobiotic
compound in the reactors at day 38 initial disturbance of the process with temporary
decrease of the transformation of the PAH and DEHP was observed. However, after
approximately 10 days a good transformation of the compounds was reestablished.
Conclusions
The screening test showed that a range of inocula have a high capacity towards
degradation of recalcitrant xenobiotic compounds. Especially the degradation of
DEHP and LAS under anaerobic conditions is promising considering the importance
45
Figure 3: Anaerobic CSTR reactor experiment (Reactor 1). Feed with anaerobic sludge containing
small concentrations of xenobiotics. At day 6 was the feed amended with 50 ppm 4-pyren which
was increased to 100 ppm at day 28. At day 39 was additionally added 50 ppm LAS12.
Figure 4: Anaerobic CSTR reactor experiment (Reactor 2). Feed with anaerobic sludge
containing small concentrations of xenobiotics. At day 6 was the feed amended with 50 ppm 4-NP
which was increased to 100 ppm at day 28. At day 39 was additionally added 100 ppm DEHP.
of eliminating these compounds during waste water treatment. The discovery of new
bacteria gives the possibility of bioprocessing waste containing toxic compounds by
introducing the appropriate bacteria into, for instance, a biogas reactor system, where
the organic matter of the waste will be converted into biogas with simultaneous
degradation of organic contaminants. The effluent from such a process could then be
applied on agricultural soil as a fertilizer and soil improver component.
46
Acknowledgements
This work was supported by grants from The Strategic Environmental Research
programme 1997-2000 (Subprogramme on sustainable land use).
References
Angelidaki, I., Petersen, S.P. and Ahring, B.K. (1990) Effects of lipids on thermophilic anaerobic
digestion and reduction of lipid inhibition upon addition of bentonite. Appl. Microbiol. Biotecnol. 33,
469-472.
Bodzek, D., Janoszka, B., and Dobosz, C., (1997) Determination of polycyclic aromatic compounds
and heavy metals in sludge from biological sewage treatment plants. Journal of Chromatography A
774, 177-192.
Brunner, P.H., Capri, S., Marcomini, A. and Giger, W. (1988) Occurrence and behaviour of linear
alkylbenzenesulphonates, nonylphenol, nonylphenol mono- and nonylphenol diethoxylates in sewage
and sewage sludge treatment. Water Res. 22, 1465-1472.
Danish Environmental Protection Agency. (1998) Indsamling og anvendelse af organisk
dagrenovation i biogasanlæg. Miljøprojekt 386 (in Danish).
Henau, H. De, Matthijs, E. and Namkung, E. (1989) Trace analysis of linear alkylbenzene sulfonate
(LAS) by HPLC. Detailed results from two sewage treatment plants. In Organic Contaminants in
Waste Water, Sludge and Sediment. D. Quaghebeur, I. Temmerman and G. Angeletti editors.
Elsevier Applied Science, London.
Holt, M.S., Waters, J. and Comber, M.H.I., (1995) AIS/CESIO environmental surfactant monitoring
programme. SDIA sewage treatment pilot study on linear alkylbenzene sulphonate (LAS). Water Res.
29, 2063-2070.
Kiely, G., (1997) Environmental engineering. McGraw-Hill (ed.). pp. 574-583, 605-611.
Mackay, D., di Guardo, A. and Paterson, S. (1996) Assessment of chemical fate in the environment
using evaluative regional and local-scale models: illustrative application to Chlorobenzene and
Linear Alkylbenzene sulfonates. Environm. Toxicol. and Chem. 15,1638-1648.
Mahajan, M.C., Phale, P.S. and Vaidyanathan, C.S. (1994) Evidence for involvement of multiple
pathways in the biodegradaton of 1- and 2-methylnaphthalene by Pseudomonas putida CSV86. Arch.
Microbiol. 161, 425-433.
McAvoy, D.C., Dyer, S.D. and Fendinger, N.J. (1998) Removal of alcohol ethoxylates ,alkyl
ethoxylate sulfates, and linear alkylbenzene sulphonates in wastewater treatment. Environm. Toxicol.
and Chem. 17, 1705-1711.
McEvoy, J. and Giger, W. (1986). Determination of linear alkylbenzenesulfonates in sewage sludge
by high-resolution gas chromatography/mass spectrometry. Environm. Sci. Technol. 20, 376-383.
Quaghebeur, D., Temmerman, I. and Angeletti, G. (editors). Elsevier Applied Science, London.
Pitter, P. (1968) Relation between degradability and chemical structure of nonionic polyethylene
oxide compounds. Surf. Cong., 1, 115-123.
Ruiz Bevia, F., Prats, D. and Rico C. (1989) Elimination of L.A.S. (linear alkylbenzene sulfonate)
during sewage treatment, drying and compostage of sludge and soil amending processes. In Organic
Contaminants in Waste Water, Sludge and Sediment. D. Quaghebeur, I. Temmerman and G.
Angeletti editors. Elsevier Applied Science, London.
Shuttleworth, K.L. and Cerniglia, C.E. (1995) Environmental aspects of PAH biodegradation. Appl.
Bioch. and Biotechnol. 54, 291-302.
47
Smith, S.R., (1996) Agricultural recycling of sewage sludge and the environment. Wallingford.
Steinle, E.C., Myerly, R.C. and Vath, C.A. (1964). Surfactants containing ethylene oxide:
Relationship of structure to biodegradability. Jour. Amer. Oil Chemists Soc. 41, 804-807.
Stringfellow, W.T. and Aitkin, M.D. (1995). Competitive metabolism of naphthalene,
methylnaphthalene and flourene by phenanthrene-degrading pseudomonads. Appl. Environ.
Microbiol. 61 357-362.
Swisher, R.D. (1987). Surfactant Biodegradation. Marcel Dekker, New York.
Volkering, F., Breure, A.M. and Andel, J.G. 1993. Effect of micro-organisms on the bioavailability
and biodegradation of crystaline naphthalene. Appl. Microbiol. Biotechnol. 40, 535-540.
48
Thermal drying – microbiological quality of dried sludge
Jean Paul Chabrier
ENVIRO-CONSULT
32 rue de l’Est, F-68110 Illzach
Tel : 00 33 3 89 53 54 71 - Fax : 00 33 3 89 53 19 20
Introduction
The main use of urban sewage sludge for most European countries is the recycling to
agriculture.
Different pathogens such as viruses, bacteria, protozoa and parasites can be found in
the sludge produced by the waste water treatments plants (Faust, 1976; Schwartzbrod
et al., 1986; Yanko, 1988).
The sanitary risk due to pathogens must be taken into account; however studies on the
capacity of living of these pathogens and parasites (Engelberg, 1985) like Helminth
eggs confirmed the resistance of the eggs to most biological treatments such as
aerobic stabilisation, mesophilic anaerobic digestion and lagooning. After applying
other biological and chemical treatments (composting, liming) the viability of the
parasite eggs clearly shows the necessity of precisely defining the process parameters
(temperature, pH, homogenisation, treatment time, end process heat value…) in order
to efficiently destroy the Helminth eggs.
It is generally demonstrated that heat is a powerful virus killer. The thermophilic
aerobic digestion and pasteurisation remain attractive processes to inactivate viruses
and other pathogens (60 °C for a treatment contact time superior or equal to an hour).
The bacteria indicating faecal contamination are destroyed for processing temperature
treatments superior or equal to 80 °C – that is always the case for drying. However the
need to reach such a temperature involves important energetic costs (in thermal drying
approximatively 900 to 1,200 kWh per tonne of evaporated water – ENVIROCONSULT measurements) if the route is only sludge recycling to agriculture.
Does the sludge microbial quality on its own justify the application of thermal
drying technologies? The author, through his experience, will bring elements to
answer this question.
Reminder of applicable legislation1
The applicable legal texts are very different from one country to another. They can be
summarised as follows:
1Report
Recommendations to preserve and extend sludge disposal routes, CEN/TC 308/WG3 N33.
49
Table 1: Regulations on sanitary sludge quality.
Country
Recommended treatments:
legal guidelines and laws
End product standards
European Union
Sludge treatment by biological,
chemical, thermal treatment, storage
or any other specific process
Sewage Sludge Ordinance (AbfKlärV2)
–
Germany
France
Denmark
Definition in the 97-1133 decree:
Treatment by physical, biological,
chemical or thermal process, for long
chemical or thermal process, for long
term storage, or by any other
appropriate process, in order to
significantly reduce its fermentation
capacity and the correlated sanitary
risks by using it
- Thermal treatment at 70 °C
during an hour or equivalent
combinations of time and
temperature
- Composting at 55 °C during at
least 15 days
- Liming
- Aerobic and anaerobic treatments
Austria
In the three
following three
Länder:
- Burgenland
- Ober Österreich
- Salzburg
Switzerland
–
Italy
Luxembourg
–
–
USA
Application of EPA 92 40 CFR or
PART 503
Process to further reduce pathogens:
PFRP – class A
Process to significantly reduce
pathogens: PSRP – class B
2 Third
50
Only sewage sludge considered
as safe from an epidemic-hygienic
point of view and land for fodder
cultivation
Only for hygienised sludge:
Salmonella: < 8 mpn/10 g dm
Enteroviruses: < 3 cfumpn /10 g dm
viable Helminths eggs:
< 3 per 10 g dm
Thermo-tolerant coliforms: none
–
Salmonellae: none in 1 g dm
Enterobacteriacaee: < 1000 in 1 g dm
Helminth eggs: none in 1g dm
For the agricultural land
producing fodder or vegetables:
Enterobacteriacaee: < 100 in 1 g dm
Infectious parasite eggs:
none in 1 g dm
Salmonellae: < 1000 in 1 g dm
Enterobacteriacaee: < 100 in 1 g dm
Viable parasite eggs: none in 1 g dm
Salmonellae: < 3 mpn or PFU in 4 g dm
Enteroviruses: < 1 PFU in 4 g dm
Viable Helminth eggs: < 1 in 4 g dm
Feacal coliforms: < 2 106 in 1 g dm
Report of the ATV/VKS Working Group 3.2.2 on Disinfection of sewage sludge.
The control of the sludge microbiological contamination can be considered in three
ways:
• Either by applying specific and homologated treatment processes to the sludge
which leads to the non detection (in case of thermal treatment by drying) or the
reduction of pathogenic germs concentration;
• Or by making tests controlling the microbiological quality of the sludge;
• Or else by simultaneously applying both preceding operations.
Analysing the existing situation, we can divide the European countries into four
groups:
Table 2: Classification according to the microbiological quality.
Group Definition of the rules
N° 1
Slight recommended rules:
Little compulsion on the stabilisation and microbiological processes
N° 2
Authorised processes in terms of reduction or removal of pathogens
N° 3
Conformity to a specified microbiological quality and for specific uses
N° 4
Answers related to limited values and authorised processes through
EPA 40 CFR – PART 503
Country
European Union
Germany
Denmark
Switzerland,
Austria
USA
The French rules are placed between group 1 and group 3 because only
microbiological limits are set up for sanitary quality (hygienised sludge). However,
for the determination of pathogens limits, it applies the American analysis methods.
Therefore, in front of these established facts, what is the best approach to consider?
In France, what is the present situation for stabilisation and
microbiological treatments?
The situation is as follows:
The use of mesophilic anaerobic digestion was little developed in the 1980s. Most of
the treatment plants applying this process were built in the 1970s and for thirty years a
few new plants have been equipped. From this specific situation, two major problems
appeared: the lack of sludge stabilisation (smells , harmful effects during storage) and
of microbiological quality even though that one is only partial for the already settled
processes.
To improve the situation, the sludge post-treatment with lime was largely applied in
medium-size plants of 20-150 000 p.e. or even bigger.
In 1997, Article 7 of the 97-1133 decree introduced by a compulsory point of view a
physical, biological, chemical or thermal treatment of the sludge as well as a long
51
storage in order to significantly reduce its fermentation and sanitary risks when used.
The decree application continued to boost the liming process. Lime is generally well
accepted by farmers who actively participate in sludge recycling.
When comparing the French situation of sludge spreading to the US EPA rules on
≤vector reduction” – i.e. management practices that reduce the attraction of insects,
animals, etc – the following table can be made:
Table 3: Process limiting the vector attraction.
Often applied treatments
Treatments sometimes
applied
Ploughing in the soil within
- anaerobic digestion
24 to 48 hours after spreading - lime conditioning
(97-1133 Decree)
Treatments rarely or not yet
applied
- aerobic thermophilic digestion
- soil injection
- increase of dm content to more
than 75 or 90%
Graph 1: Thermal drying in Europe (source: Enviro-Consult).
Note: the thermal drying is in great development in the United Kingdom.
Up to now in France drying is not much developed in comparison with most of other
European countries (number of 'drying lines' in Europe: approx. 400 – in France: 30 of
which 20 in operations and 10 in construction – ENVIRO-CONSULT Data).
52
Present techniques used in France
A last point on the French situation concerns the present methods used in the
laboratories to determine the parasites. According to the opinion of many French
experts, the analysis methods of EPA cannot guarantee that the results obtained will
be validated.
Eight techniques are available to enumerate and test the Helminth eggs viability and
the technique which would lead to the most important number of eggs would be the
one identified by AES3.
In France a big question is being discussed nowadays by experts, which is : “Do
we have to move towards an experimental norm and therefore question the
regulatory texts?”
Parameters and treatments to be used in the control of the
microbiological contamination
The reduction or removal of pathogens is influenced by different factors and by the
applied treatments:
• Setting velocity: for Helminth eggs it varies from 0.65 m/s for Ascaris eggs to 12.5
m/s for Schistosoma (Shuval, 1986).
• Heat and exposition time: graphs 2, 3 and 4 (taken from Sanitation and diseases:
Health aspects of excreta and waste water management) show the influence of
temperature and exposition time for the destruction of
- Ascaris eggs (Feachem et al., 1983)
- Salmonellae (Feachem et al., 1983)
- Enteroviruses (Feachem et al., 1983)
• physical treatments: irradiation, ultrasounds, long term storage
• biological treatments: thermophilic aerobic and anaerobic digestion, composting
• chemical treatments: fertilisers effects and lime conditioning
3
AES = Antiformine, ethylacetate, zinc sulphate: method developed by Gaspard and Schwartzbrod
(1995).
53
Graphs 2: Effect of the relation
temperature/time on the destruction of
Ascaris eggs.
Graphs 3: Effect of the relation
temperature/time on the destruction of
Enteroviruses.
Graph 4: Effect of the relation
temperature/time on the destruction of
Salmonella.
Thermal drying: heat and exposure time
The thermal drying is a physical operation which consists in draining all or part of the
water contained in the dewatered sludge.
Drying is made at the atmospheric pressure and the evaporated vapours are condensed
into a condenser installed on the line (vapour circuit).
There are three main types of thermal drying technologies:
• Indirect drying or by contact: the heat is transferred to the wet sludge which is
deposited on a hot plate, thus allowing the evaporation of the water.
• Direct drying or by convection: the heat is transferred directly from the energy
carrier (flue gas) to the wet prepared sludge, thus the flue gas absorbs the humidity
of the sludge.
• Mixed drying: it is a combination of drying by contact and by convection.
The used heating medium depends on the applied technologies:
For the indirect drying: saturated steam, thermal-oil.
For the direct drying: exhaust flue gas, warm air or gas.
54
The reached temperatures vary according to the processes and the sludge drying phase.
Black (thin) curve: thermogravimetric curve
Grey (thick) curve: sludge temperature increasement curve
Graph 5: ENVIRO-CONSULT Data.
55
Table 4: Temperature of sludge and gas (source: ENVIRO-CONSULT).
Drying technology
Drum dryer
Fluidized bed dryer
Thin film dryer*
Discs dryer**
Temperature of
dried sludge
84 °C
87 °C
95 – 100 °C
112 – 118 °C
Heat of vapours or
flue gas
140 °C
> 110 °C
100 °C
120 °C
End sludge dryness
%
95
95
65
95
*very high transmission coefficient λ = 100 W/mK.
** residence time in dryer: 1h30mn maximum
Microbiological quality results
The following values were found on samples of pre-dried sludge (55% dryness) and
full dried sludge up to 90 % dryness.
Table 5: Results on the sludge microbiological quality – data collected by ENVIRO-CONSULT.
Pathogens
Legal requirements
French
EPA 503
Decree of
8.01.98
Salmonellae < 8 mpn/10 g
MS
Enterovi- < 3 mpn PFU/
ruses
10 g MS
Enterobacteriaceae
Viable
< 3/10 g MS
Helminth
eggs
Thermo–
tolerant
coliforms
*
< 4 mpn/4 g
MS
< 1 PFU/4 g
MS
< 1/4 g MS
< 1000
mpn/g MS
Experimental data
Digested
Digested
dried sludge predried
UWWTP
sludge
Brugges*
UWWTP
Zurich**
Absence /25 g Absence / 10 g Absence / 25 g < 1
Digested
dried sludge
UWWTP
Baltimore
Digested
dried sludge
UWWTP
Nancy
Digested
dried sludge
UWWTP
Zurich***
<1
Undetermined
Absence
Undetermined Undetermined Undetermined
before
thermal
treatment
Absence/25 g Absence/10 g Undetermined < 1
<1
< 10
Undetermined < 10
<1
<1
Digested dried sludge have been stored in bags during one year. One analysis sample was taken
from the storage (Seghers Better Technology Company).
** Indirect drying during 5 minutes residence time at 95-100 °C from sludge side and 160 °C from
heat side. This drying conditions fullfills the EPA rules for class A, however the end dryness is
very low (55%) and a recontamination is always possible except for parasites like Ascaris eggs
(Buss AG, Basel).
*** Indirect drying with a residence time of 45 minutes (Buss AG, Basel).
56
CONCLUSIONS
Important studies on the sanitary quality of the dried sludge have not been made up to
now in France and neither were we able to find data for Germany.
Results obtained were supplied by various dryers and process manufacturers during
specific tests but very few information were given on the methods of making the
sampling or the analysis methods used.
No presence of viable Helminth eggs is established after drying; according to
biologists, their survival is impossible and therefore it can be assumed that the dried
sludge cannot be contaminated again because of the storage conditions, even though
this one can be long.
If the thermal drying is interesting in the stabilisation or microbiological quality,
nonetheless its first advantage is the reduced volume and mass of the sludge in order
to find out new energy and materials routes for recycling in which the microbiological
quality is of little interest.
REFERENCES
CEN/TC 308/WG 3 Report 1999 – 09: Recommendations to preserve and extend
sludge utilisation and disposal
3rd Report of the ATV/VKS Working Group 3.2.2 «disinfection of Sewage Sludge».
ENVIRO CONSULT data board – European sludge drying market analysis – 1998.
Gaspard (1995) THESIS : Environmental parasites contamination : prospective for a
sanitary risks management - .NANCY France.
Richard G. Feachem, David J. Bradley, Hemda Garelick and D. Ducan Mara
Sanitation and disease - Health aspects of excreta and wastewater management.
Schwartzbrod J, Gaspard P, Thiriat L -Volume 1, number 2, 1998 European Water
Management, Pathogenic micro-organisms in sludge and effect of various treatment
processes for their removal.
A survey of E.coli in UK sludges: UK water industry research limited – examples
given by Tim EVANS.
57