Part 2/2009
TFL Eco Guidelines
Salt & Nitrogen Reduction in the Leather Making Process
ECO SOLUTIONS
In harmony with the environment
1
Contents
Introduction
Salt
Ammonium & Total Nitrogen
Types
page 4
Types
page 12
Origin
page 4
Origin
page 12
Where does this salt come from?
Conservation, transport and storage
Chloride content
Opening up
Sulfate content
page 6
Unhairing
Deliming and Bating
Measuring & Guidelines
page 6
Total dissolved solids
Measuring
Electrical conductivity
Analytical methods
Which measurement technique to use?
Total Nitrogen–Kjeldahl method
Water quality guidelines
Total Ammonia
Reduction of salt
page 7
Removal of salts from waste water
Methods to reduce the salt offer
page 9
Salt-free leather chemicals
Function of formulation salt
page 13
Unhairing
page 8
Conservation salt
Pickle salt
Reduction of nitrogen
page 13
Deliming
Bating
page 15
Ammonium salt in relation to COD
page 16
Waste water treatment
page 10
Conclusions
page 17
Glossary
page 18
This publication is the second of a
series that TFL is preparing in order to enhance the understanding
of environmental and ecological
aspects in leather manufacturing.
Worldwide there is a lot of emphasis on operating in a manner that is
compatible with the best ecological
and environmental practices. This
requires many tanneries and supply
industries to have a better understanding of the whole ecological
process of leather manufacturing
from the start to the end.
The tanning industry generates considerable amounts of solid, liquid
and gaseous wastes. Well planned,
clean technology practices, such as
the efficient use of resources like
chemicals and water, as well as recycling and purification of process
floats, allow tanners to reduce environmental costs and comply with
regulations.
ammonium salts used in the leather
making processes. It is clear that
processing of natural materials such
as hides and skins will result in the
release of nitrogen containing wastes. In addition, ammonium salts
are commonly used in the various
process steps to convert the raw
hides and skins to leather. This nitrogen load in the effluent requires
a sophisticated biological treatment
in order to achieve the limits being
required by authorities. Again, if the
nitrogen content of waste water can
be reduced, then there is considerable reduction in environmental
costs for the tannery.
The focus here will be to show, in
a clear and easy to understand
manner, how much and at which
process steps the salt and nitrogen
levels can be reduced by means of
modifications to the process and
the chemicals being used.
The preservation and tanning processes of raw hides and skins typically involves the extensive use
of salt, which is very difficult to remove from the waste water. So it is
logical that for the leather industry this is a major topic of concern.
This publication considers ways of
reducing the amount of salt in the
tannery effluent. Another critical
effluent problem is the nitrogen
from the natural proteins and the
Text prepared by Dr. Campbell Page and Jan-Tiest Pelckmans
2
3
Salt
Salt
Salt
Fig. 2: Sources of salt in waste water
100
Origin
The term “salt” in the tanning industry is typically used
to refer to the 2 commonly used salts, namely sodium
chloride and sodium sulfate. It is quite typical in the
tanning industry to consider both of these inorganic
salts together using the simple term “salt”.
These 2 inorganic salts in the effluent are clearly the
most difficult form of pollution to be dealt with in the
leather industry. Both are very soluble in water and chemically stable, making it effectively impossible to remove them from a mixed effluent in waste water
treatment plants by typical processes such as; sedimentation, oxidation, precipitation or flocculation like
most other pollutants.
If the treated waste water is returned to rivers or used
for irrigation purposes, there is an increase in the soil
salinity, which increases through evaporation and
eventually the crop yields decline. Assuming they do
not contain significant other contaminants, high salt
containing effluents are generally only acceptable if
discharged into the marine environment.
Ammonia and ammonium salts will be dealt with in the
second part of this publication. Fig. 1 shows the relative
amount of salts found in the beamhouse* waste water
of a tannery processing salted hides.
Where does the salt come from?
If salt in the waste water is to be reduced, clearly the
first aspect to be studied is: where does the salt come
from?
Fig. 2 presents the sources of salt found in the waste water for a typical bovine tannery making chrometanned leather from salted hides. Clear to see is that
the vast majority of the salt load occurs in the beamhouse area and this is the area to focus on when considering ways of salt reduction. The wet-end retanning,
dyeing and fatliquoring process have a minor impact on
the total salt load.
Chloride content
If a tannery is processing salted hides, the biggest component in the waste water is always the chloride salt.
Chloride in tannery effluents usually originates from
the large quantities of common salt (sodium chloride)
used in the hide and skin preservation process or in the
pickling process.
Chlorides inhibit the growth of plants, bacteria and fish
in surface waters.
Fig. 1: Salts in beamhouse waste water from processing salted hides
66%
60
40
20
7%
0
conservation
salt
beamhouse
chemicals
13%
12%
2%
pickling
Cr-tanning
wet-end
chemicals
Fig. 3: Chloride – as can be found in the typical spent floats of beamhouse waste water
40
kg salt / ton salted hides
Types
% of total salt
80
30
34
29
20
27
19
10
0
pre-soak main soak
liming
7
6
5
1
wash 1
wash 2
deliming/
bating
wash 3
Fig. 3 shows the chloride salt content in the waste
water for each individual processing step. The dominance of the salt from salted hides in the pre-soak and
main soak is clearly shown. There is also a reasonable
8
wash 4
pickling/
tanning
wash 5
amount of salt in the pickling/tanning step. To give an
idea on the dimensions; the daily output of common
salt from a waste water treatment plant shared by a
group of tanneries in Italy was 120 tons!
150
kg / ton salted hides
135
100
50
48
5
0
chloride
sulfate
ammonium
*) i n this paper beamhouse means all process steps from (pre-)soaking to tanning
4
5
Salt
Salt
Measurement & Guidelines
Sulfate content
Sulfate salts in tannery effluent come from the use of chemical products containing the sulfate ion and the many
powder chemicals, which can contain a considerable
amount of sodium sulfate as an inert standardising salt.
The sulfide containing waste stream from the liming
process is normally separated from the other waste
streams in the tannery and kept alkaline to avoid the
formation of toxic hydrogen sulfide (H2S) gas. With
the help of manganese sulfate catalysts and aeration,
the sulfide is oxidised to form thiosulfate and some
sulfate. However, this waste stream is normally treated
separately and the residuals are precipitated before
the waste water is returned virtually sulfide-free to the
normal waste effluent. Since the sulfide waste water
is typically not treated with the normal tannery waste
water, it will not be considered in this publication.
There are two reasonably simple methods for determining the total salt content of tannery waste water:
1. Total dissolved solids
Total Dissolved Solids (TDS) is measured by evaporating to dryness a water sample, then weighing the solid
residue remaining. It is obvious that this way measures
not only the salts but any other soluble solids as well.
The method is very simple and easy to carry out although it may not be optimal for some industrial waste
waters. TDS is recorded in milligrams of dissolved solid
in one litre of water (mg/l). (Another regularly used unit,
parts per million (ppm) is equivalent to mg/l).
2. Electrical conductivity
The Electrical Conductivity (EC) of the water can be
measured by a conductivity meter. The conductivity of
water increases as the salt content increases; in fresh
water the increase is nearly linear with the increasing salt
content. With this technique, only the salts in the water
are measured. Practically, this means that the more dissolved salt is in the water, the higher the conductivity.
The step from measured conductivity to salt content is
a little complicated, since the conductivity varies with
temperature and also to some extent with the type of
salts dissolved in the water. By making measurements
at 25°C, the temperature effects can be minimised.
The standard EC unit is micro Siemens per centimetre
(µS/cm) at 25°C. As a guideline the EC value can be
High levels of sulfate ions in the waste water can cause
damage to concrete piping and tanks.
kg sulfate / ton salted hides
Fig. 4 shows the sulfate salt content in the waste
water for each individual processing step. Clear to see
is the dominance of the ammonium sulfate used in deliming and bating. Later, there is a reasonable amount of
sulfate from the sulfuric acid used in pickling and from
the chromium sulfate used for tanning.
6
40
Fig. 4: Sulfate - as can be found in the typical spent floats of beamhouse waste water
Which measurement technique to use?
The salt content measurement technique differs depending on the country. For example in India, the TDS is
very often the preferred measurement. It is important to
remember that for effluent prior to treatment the TDS
value is a combination of the total dissolved salts and
the COD. A reasonable waste water treatment plant will
remove most of the COD from the effluent, but not the
salt content. In South Africa, it is the conductivity of the
waste water that is measured.
In many countries, it is quite common that additional
restrictions are specified in terms of the specific ions
involved, e.g. chlorides (Cl-), sulfates (SO42-).
Water quality guidelines
EC range (µS/cm)
Uses for water
0–800
Drinking water for humans and
animals.
800–2’500
Can be consumed by humans
although most would prefer water
in the lower half of this range.
Suitable for animals and the
irrigation of most plants.
2’500–10’000
Not recommended for human
consumption. Only at lower levels
is it suitable for some animals and
salt tolerant crops.
Over 10’000
Not suitable for human
consumption or irrigation.
possibilities have their advantages and disadvantages.
For instance, in India, solar-pan evaporation combined
with recycling of the salt residue is often practiced. This
process has a low efficiency, the salt-water is slow to
evaporate and often large areas of land are needed.
Not to mention the problems with monsoon rainfalls
washing the salt into the nearest river. The advantage
with this type of approach is, that it has low start-up
cost, although it does involve a considerable amount of
manual labour. In addition, it has to be mentioned that
the salt residues recovered from the solar-pans can be
too contaminated to be re-used.
In Australia, with close monitoring, salt containing
treated waste waters have been sprayed on farmland.
It also has a reasonably low start-up cost, but the mid
to long-term build-up of salt in the soil (salinity) and the
possibility of contaminated salt residues require close
monitoring to be undertaken.
In the South of India, the local government has introduced ultra-filtration and reverse osmosis treatments
Solar pan evaporation as practiced in India.
Reduction of salt
30
20
20
14
10
0
converted to TDS for natural waters by the following
relationship: TDS (mg/l) = EC (µS/cm at 25°C) x 0.6
Typically untreated tannery waste water can reach conductivity values on the order of 10’000–12’000 µS/cm.
0.5
0.4
pre-soak main soak
3
liming
0.5
0.3
wash 1
wash 2
8
2.5
deliming/ wash 3
bating
wash 4
pickling/
tanning
wash 5
Removal of salts from waste water
The disposal of salt containing waste water or the removal of these salts from waste water typically involves
very elaborate treatment methods and does not always
solve the problem, but rather moves the responsibility
to someone else. In different regions of the world the
problem is being tackled with different methods, all the
of the tannery waste waters. The idea is that after the
removal of the salt, a large portion of the purified water
can be recycled and re-used in the tanneries.
However, apart from the high operating costs, the disposal of all the concentrated salt effluent has created a
new problem. Another alternative that is feasible in
some coastal areas is to directly discharge salt containing treated waste water into the sea. Some new
tannery clusters have been located close to the sea to
7
Salt
Salt
Since the conservation salt is the major contributor of
the total salt freight in the tannery waste water, it is
worth looking at it in some detail.
take advantage of this disposal method for their treated effluents. In some parts of the UK and Italy, the
treated tannery waste water is able to be diluted with
domestic waste water prior to discharge into rivers or
the sea. This is possible in some areas where tanneries
are situated close to bigger cities and the water authorities are able to handle the streams of domestic and
industrial waste waters efficiently. But often this can be
difficult to arrange and control. Treatment processes for
other pollutants in waste water often break them down
to harmless substances or to a form where they can be
precipitated or flocculated and collected.
Is this possible with chloride and sulfate salts? For sulfate containing waste waters, a biological conversion in
sophisticated bioreactors to reduce sulfate to elemental
sulfur is possible, but the costs are too high for typical
tannery effluents. For chloride containing waste waters,
there is no such solution available.
Functions
To start with we have to explain why salt is used to
preserve hides and skins. The conservation effect of
salt is well-known. Basically, the salt dehydrates the
hide from 60–70% water to a level of about 14%, where
micro organisms have difficulties to grow. In addition,
common salt is a low cost chemical.
Important to know is the hydrotropic effect that conservation salt has on the hide material. During conservation, the salt is denaturising the non-structural protein
(which later on in soaking and liming is easier released)
and if not dosed in sufficient quantity (above 20%), it is
even attacking the collagen.
1. Conservation salt
kg salt / ton salted hides
100
8
Desalting drum
115
100
85
50
35
2. Pickle salt
The salt used in the pickling process is the second biggest contributor of the total salt freight in the waste
water. Common salt is the source for the chlorides,
while sulfuric acid is the major source for sulfates and
sulfate masked chrome as the second source.
15
losses
(appl. &
transp.)
removed
handling
in tannery mechanically
1st soak
2nd soak
carried
over
shifts the equilibrium in favour of the positive charges in
the amino sites. If there would be no salt in the pickle,
this would generate repulsive forces within the structure, forcing the polypeptide chains away from each
other. This would then create empty spaces in which
water would penetrate rapidly. The very strong osmotic
pressure would then destroy the pelt from within. Salt
avoids this swelling process in the pickle by reversing
the osmotic pressure. In a good pickle, these two forces
are balanced out.
Pickle salt has a hydrotropic effect on the pelt that is
increased substantially at a pH under the iso-electric
point of 5.5. This effect is very important to give the
leather inner softness, but in excess can lead to loose structured leather. Sulfates in pickling and tanning
can have a positive effect on the tightness of the final
leather since it stabilises collagen.
TFL recommendations for reducing or
salt in the pickling process are as follows:
Fig. 5: Mass balance of salt applied in curing
50
0
Mechanical de-salting
The simple step of mechanical removal of the conservation salt will have a big effect reducing the amount
of salt going into the waste water. When salt is mechanically removed, it should be noted that it is then
contributing to the solid waste. With this mechanical
de-salting process, special care must be taken not to
damage the hides and skins.
The impact of the mechanical removal can be seen in
diagram, Fig. 5.
Systems and products allowing elimination of salt
For every 1000 kg of raw hides, about 400 kg salt is
applied to preserve them. Typically some 50% of this
amount of salt ends up in the tannery waste water.
Therefore, wherever possible, the use of fresh hides
and consequently the elimination of the conservation
salt, means that the total salt freight of the waste water
is already reduced up to 60–70%.
Very common is the direct processing of hides when a
slaughterhouse is close to the tannery and the logistics
of delivery are well organised.
As an intermediate solution, hides coming from further
away are treated with a bactericide like ARACIT® KL
new. In this condition, they can be transported for
up to one day to the tannery. Another commonly
Methods to reduce the salt offer
There are various ways to reduce the common salt freight
in the waste water. Looking at the total salt freight, the
option with the biggest impact is to use fresh hides.
However, the use of fresh hides is not possible in many
countries, so the tannery must use salted hides.
Other ways of significantly reducing the salt levels in
the effluent are to consider application processes
such as low-salt pickling. A further factor that can be
considered is the use of salt-free chemical products,
although this method has less impact.
150
practised way of working is chilling the hides either by
chipped ice, by cooling in a refrigerated warehouse or
by using ice-cold water. The effect can be improved by
using a bactericide like ARACIT® KL new in the water to
produce the ice.
Function
The main function of salt in the pickle is to avoid the
pelts from swelling in the acid condition. The pickle acid
eliminating
a) Low salt pickling with SELLATAN® P liq.
In the low salt pickling process SELLATAN® PA liq., a
modified polysulfonic acid, replaces the commonly
used pickle acids and allows a substantial reduction
of the common salt used in the pickle. In practice, the
skins remain non-swollen with growth marks in the neck
staying decidedly flat.
In vegetable tannages, SELLATAN® PA liq. gives a faster and a better distribution of the tanning agent. In a
syntan tannage, highly astringent replacement tanning
agents can be used without having stability problems
in the bath. In combination with SELLATAN® WL-G,
the offer of chrome can be reduced even further, thus
reducing sulfate and chromium in the waste water.
Some common salt has to be offered to the pickle to
avoid a more rapid drying-out of the wet-blue.
An electrostatic bond is formed between the collagen
and the polysulfonic acid similar to that of a syntan.
This can be seen as intermediate between a traditional synthetic auxiliary and a vegetable or synthetic replacement tanning agent. A further advantage is that
the chromium salt (or other tanning agents) can diffuse
more easily within the hide. If extra protection of the
pelt against higher pickle temperatures is required,
SELLATAN® P liq. is the preferred product to be used.
Compared to SELLATAN® PA liq., it provides more
skin stabilisation resulting in a further increase of the
9
Salt
b) Recycling
Recycling of the pickle and/or chrome tanning floats
has probably been considered by most tanneries but
the technology is often only partially implemented or
not at all. There are some basic guidelines required for
recycling to be successful:
a continuous, relatively constant through-put. Fluctu•
ations in volumes and variations in processing conditions make it difficult to carry out recycling;
i
nstallation
of extra infrastructure like holding tanks,
•
pumps and monitoring equipment;
• analytical facilities to check the chemicals in the float;
• skilled staff.
If these guidelines are not followed, there is a substantial risk that float recycling could cause a loss in the
quality of the leather produced. There are several types
of recycling systems, from more simple open (flow-back
or counter-stream) type recycling to the closed
recycling involving re-using floats after cleaning procedures. This technique is too detailed to be explained
here. Considerable savings in water and basic chemicals can be expected. Salt and acids are only given to
“top-up” as needed, instead of a full amount being added for each fresh float as in the conventional process.
Impressive savings of 50–80% in salt and 20–25% in
acids have been achieved.
10
3. Salt-free leather chemicals
In both the beamhouse and wet-end processes, extensive use of powder leather chemicals can noticeably
increase the sulfate load in the waste water. Powder
products are often standardised at particular concentrations, using inert salts like sodium sulfate.
Functions of formulation salt
The main purpose of formulation salt is to reduce the
concentration of the active ingredient(s) to a workable level. This means that the amount of the product
used in practice is such that it is easy to weigh. At the
same time, it avoids problems with homogeneity of the
process because of poor distribution within the float.
Fig. 6: Typical waste water values from the pickle and chrome tanning of 3.5 mm lime split pelts
16
kg salt / ton salted raw hides
shrinkage temperature. In Fig. 6 typical waste water
values achieved with low salt pickling are shown.
Salt
14
12
14
13
12
12
10
10
8
6
7
6
4
6
2
0
TFL recommendations for reducing the salt from
chemicals are as follows:
Chloride
Sulfate
Chromium
0.5
Std prickle
+ 6% Cr
0.2
SELLATAN® PA
+ 5% Cr
0.1
SELLATAN® PA
+ WL-G + 4% Cr
0.1
SELLATAN® P
+ 5% Cr
In many situations, it can be worth considering the
use of equivalent salt-free versions of these products.
One simple way is to use liquid formulations; a good
example here is the MAGNOPAL®, SELLASOL®,
SELLATAN® and TANNESCO® liquid retanning agents
and auxiliaries. Just this change to liquid products can
reduce the salt being added from powder chemicals
by up to 50%.Another is to use more concentrated
powder versions of the leather chemicals. However,
working with highly concentrated chemicals can have
it’s own problems for practical and application reasons.
Small weighing errors with highly concentrated chemicals can result in significant application problems. Also
concentrated chemicals can react too quickly with the
leather, not allowing time for a proper distribution in
the drum. Bating enzymes are an example where lower
concentrations are advisable for these reasons.
11
Ammonium & Total Nitrogen
Ammonium & Total Nitrogen
Ammonium & Total Nitrogen
Origin
Measuring
If we look at the leather making process, we can see
that many steps are releasing nitrogen into the spent
process floats. Washing off the dung, grease and blood
adhering to the hides, removal of the hair, the openingup of the hide structure; all lead to nitrogen being released. In general, we can say that nitrogen in the waste
water is the result of making leather out of an animal
skin.
Conservation, transport and storage
The conservation and transport of hides and skins are
usually not in the hands of the tanners. In order to keep
costs low, these processes are often not carried out
optimally. As a result, the hides arriving at the tannery
can have already suffered from autolysis, as well as bacterial attack. Subsequently, more hide proteins will be
released in the (dirt) soaking and washing procedures,
which increase the nitrogen load.
Analytical methods
Total Nitrogen – Kjeldahl method
Determines the nitrogen from proteins and ammonia in
the solution
•
use sulfuric acid and catalyst to break down proteins
to CO2 and NH4+
•NH4+ is collected in an acid solution with steam
and titrated.
There is also the nitrogen from the chemicals used in
the leather making process. The main source is the ammonium salt used to delime the pelt. Other nitrogen
sources, such as amines used in liming auxiliaries, are
smaller in comparison.
In addition, in re-tanning of leather, various nitrogen
containing syntans, polymer products and fillers are
often used; however their contribution to the nitrogen
balance is comparatively small.
Fig. 8: Typical nitrogen load found in the
spent floats of beamhouse waste water
9000
8000
8000
7000
N mg/l
6000
6000
5000
4000
3000
2000
1000
0
1000
soaking
12
liming
deliming
Opening-up
One of the major sources of nitrogen, as shown in
Fig. 8, is the liming process. Since the hides need to
be opened-up to be able to make leather, we cannot
change this too much. One can observe that the addition of proteolytic enzymes to the soaking and liming
processes leads to a slight increase of nitrogen in the
effluent. At the same time, it will lead to softer, better
opened-up leather.
In bating and in pickling, the hides are further
opened-up (softened) leading to the additional release of
nitrogen. It must be pointed out that reducing nitrogen
in the waste water by reducing the opening-up of the
hide normally leads to noticeable changes in the leather quality.
Unhairing
The keratin of hair and epidermis being burned off in
liming, is the other big source of nitrogen originating
from the hide proteins. The nitrogen load largely depends on the degree of pulping the hair.
Deliming and Bating
Another major source of nitrogen is the ammonium salts
used in the deliming and bating processes to perform
the deliming. The total nitrogen found is a combination
of hide protein and ammonium related nitrogen. This
means that even if no ammonium salts are used in deliming and bating, there is still a basic amount of nitrogen in the waste water. The additional nitrogen can be
calculated from the ammonium amount in the chemical
products.
Total Ammonia
Determines the ammonia in the solution
•NH4+ is collected in an acid solution with steam under
vacuum not exceeding 40°C and titrated using MgO
or MgSO4 (to avoid de-amination of amino acids).
Fig. 9: Nitrogen release in liming
7000
6000
N mg/l
Types
6000
5000
4000
4000
3000
2000
1000
0
hair burning
hair saving
Reduction of nitrogen
1. Unhairing
To reduce hide proteins from getting into the process
water, the only practical method is to run an unhairing
process allowing separation of the hair from the hide
before running the liming process for opening up the
hide structure.
TFL recommendation for reducing the release of
Nitrogen in the unhairing process:
a) Hair-Save liming process
By running the patented TFL Hair-Save System with
liming agents like ERHAVIT® HS2 or ERHAVIT® EF, the
nitrogen level can be reduced considerably, see Fig. 9.
In a standard unhairing process the hair is being pulped with sulfides and the nitrogen from the destroyed
hair-keratin is entering the waste water treatment plant.
The TFL Hair-Save System removes the hair without
pulping, thus reducing the nitrogen load on the
waste water. The hair can be filtered from the bath and
disposed of separately.
b) Painting and shearing
For skins often a painting process is practiced and
Hair filter and separated hair
shearing the hair off before processing is another
method practiced, especially when the hair has a value
of its own in the form of wool or as bristles, e.g. for
paint brushes.
2. Deliming
Reducing the ammonium salt in deliming is more
complicated than it may seem. Ammonium salts
are economical and apart from having a low deliming value, they have a high buffering capacity and
13
Ammonium & Total Nitrogen
TFL recommendations for reducing ammonium salts
in the deliming process:
a) Ammonium salts and organic acids
b) Organic acids
They have a very low buffering capacity and therefore
have to be given in several additions to avoid the pH
from dropping below 5.5, which can create acid swelling
and promote draw and scud fixation. The advantage
is, that the products do not contain ammonium salts,
have a low deliming value and provide very good lime
solubility. Sometimes these expensive acids are mixed
with the low cost boric acid. It is often overlooked that
boric acid also works as a fungicide and bactericide.
This can lead to the problem that it has a negative effect on the biological water treatment step. Naturally
the more boric acid is used, the worse the situation. In
extreme cases (e.g. when only boric acid is used), it is
possible that the complete biological water treatment
process stops.
To make organic acids easier to apply, they are often
partly neutralised thus creating neutral salts. The problem is that this neutralisation reduces the deliming
capacity of the product, leading to an increased offer
necessary to perform a complete deliming. Buffering
these acids is only possible with ammonium salts, which
is already described above.
Fig. 10: US steer hides lime split at 2.5 mm
30
25
15
10
10
5
9
8
1.5
2
1
1% DERMASCAL® ASB new
2
0.03
2% (NH4) 2SO4
0.5
1.2 %DERMASCAL® CD
0.3 0.5
2% DERMASCAL® F
Fig. 11: Full substance US steer hides
40
COD (kg O2)
Ammonium (kg)
Nitrogen (kg)
38
35
30
25
20
15
10
15
13
9
5
0
4
3
2% DERMASCAL®
ASB new
5
5
3% (NH4)2SO4
c) Functional esters
The best example for this technology is DERMASCAL®
CD. It cannot be compared with standard deliming
products directly, since it works in a completely different way. In addition, the methods to measure the
deliming value, the buffering capacity and the lime
solubility cannot be applied. Practical experience has
shown that this technology provides a good deliming
with an offer similar to the ammonium salts. Because
the pelts become very flaccid, lime is removed very
well. The product is completely nitrogen-free and has
14
COD (kg O2)
Ammonium (kg)
Nitrogen (kg)
25
20
0
in kg per ton of wet-salted rawhides
CO2 deliming
CO2 deliming is often mentioned in connection with
ecological systems for tanneries as it is ammonium-free,
relatively inexpensive and easy to apply. It has to be
noted, that CO2 deliming is not free of problems in
application. The penetration can be very slow and inconsistent, which makes it mostly suitable for thin pelts
only. Because of the poor pelt relaxation, the lime
removal is very poor. The risk of lime blast is substantial
and can only be countered by applying a complexing
agent like BORRON® LB or BORRON® NF. CO2 deliming has a low process pH, creating a higher risk of
H2S formation. CO2 gas needs special equipment for
storage and application.
The effect is similar to that of just ammonium salts but
with the additional benefits that the combined products
have a better lime removal because of their complexing
ability and a lower ammonium content. The good lime
removal leads to a fine grain and uniform dyeing. The
penetration of both pickle and chrome is improved.
The best known examples are DERMASCAL® ASB new
and DERMASCAL® F.
in kg per ton of
wet-salted rawhides
clean the pelt very well. They de-water and relax the
pelt very well, allowing a good removal of lime. On
the other hand, ammonium salts are toxic in surface
waters and are an additional load for the biological
waste water treatment. To decide which chemistry is
suitable to replace ammonium salts, we have to look at
all their aspects. Trials were carried out to compare the
pollution levels of various deliming and bating systems,
always using the same amount of process water.
This is important since the COD values are measured
in mg O2/liter and the nitrogen in mg/l and will change
with the amount of water used. We calculated all COD
and nitrogen values measured per ton of raw hides.
Ammonium & Total Nitrogen
0.05
0.8
2% DERMASCAL® CD
0.6
1
1% DERMASCAL® F
an excellent buffering capacity since the pH never
drops below 8.5, making it very easy to be applied. It
can be applied on full substance pelts in one addition.
Complete through deliming can be conducted in an
acceptable time. In addition, it leads to flaccid and very
clean pelts. The diagrams, Fig. 10 and 11, show the
amount of pollutants found in practical trials (combining
the effluent from washing after fleshing, deliming,
bating and washing after bating).
15
Ammonium & Total Nitrogen
Conclusions
chemicals that increase the COD makes no sense.
But it has to be realised that ammonia is an aquatic
toxin and has a high biological oxygen demand (BOD5)
whereas DERMASCAL® ASB new, DERMASCAL® F and
DERMASCAL® CD are relatively easy biodegradable.
3. Bating
When considering the ammonium salts, we need to
look more closely at the bating process as well. Most
commercially available bating enzymes are formulated
with ammonium salts. In a normal process, this makes
sense since the formulation salt is adding to the deliming effect, whereas a neutral salt would only add to
the salt load of the waste water. If the amount of ammonium salts in the effluent needs to be reduced as much
as possible, the use of ammonium-free bating agents
like OROPON® ON2, OROPON® ANZ or OROPON® W
will be necessary.
The diagram, Fig. 12, shows the waste water values
achieved in practical trials (combining the effluent from
washing after fleshing, deliming, bating and washing
after bating).
Waste water treatment
Ammonia, and in general nitrogenous compounds, can
be broken down in the biological treatment process
of a waste water plant by combining intensive aerobic
and anaerobic biological treatments. The oxygen demand of the bacteria is very high (about 40% of the
total demand), thus leading to correspondingly high
operational and energy costs. In addition, the bacteria build-up in the biological treatment process is slow,
requiring a constant flow of waste water with a very
similar composition and a reasonably long residence
time in each biological treatment tank. Consequently, the size and investment mean that this biological
treatment step is often made in communal or collective
water treatment plants rather than at the tannery.
Ammonium salt in relation to COD
It is important to understand that a lot of the alternative chemicals for ammonium based products
are increasing the COD of the deliming system.
Some may say that replacing ammonium salt with
Pollution parameter
Raw tannery waste water
mixed effluent, before
treatment (mg/l)
Tannery waste
water after primary
treatment (mg/l)
Discharge to surface waters
after primary & secondary
treatment (mg/l)
100 - 600
100 - 400
20 - 50
Nitrogen (TKN)
Fig.12: Typical waste water values of combined effluents fleshing until bating for unsplit pelts
40000
COD (mg O2/l)
Ammonium (mg/l)
Nitrogen (mg/l)
37600
35000
30000
1537
740
13380
1537
4600
740
5000
3200
10000
3200
15000
12700
20000
15900
mg/l
25000
0
2 % DERMASCAL®
ASB new
16
3% (NH4)2SO4
2% DERMASCAL® CD
• A large reduction of salts in the waste water can be
achieved in many cases by simple means. The best
solution from the environmental point of view is to
process fresh or chilled hides. These hides might have
a short-term, salt-free preservation with bactericides
– but the slaughterhouse has to be near the tannery.
• Whenever salted hides have to be processed,
mechanical removal of the curing salt is the most economical and easiest first step.
• Of the leather production processes the pickle
offers the best opportunity to reduce salt in the waste
water. Recycling processes can be used to achieve salt
reduction in the pickle, but this involves some investment and good process control. An effective alternative, as no investments in infrastructure are needed, is
using a low salt pickling system with SELLATAN® PA
liq. It offers a substantial reduction of sulfates, chlorides and chrome salts in the effluent.
• Reductions of ammonium and nitrogen content in
effluent focuses mainly on the deliming process. If
we compare the basic deliming methods and products, we see that from the point of view of the waste
water CO2 deliming would be the best way to go. But
the system is not free of problems, so that in practice
most tanneries prefer to use conventional methods.
A practical approach to reduce ammonium salts in
deliming and bating is to use DERMASCAL® ASB
new, DERMASCAL® F or DERMASCAL® CD in combination with a bate like OROPON® ANZ, OROPON® W,
or OROPON® ON2.
•
The TFL HS system, removes the hair without destroying it. It allows a large reduction in total nitrogen
dissolved in the waste water due to the much lower
amounts of protein in the effluent. The nitrogen reduction comes along with a significant reduction of
COD (and sulfide) as reported in the first volume of
the TFL ECO Guidelines on COD and BOD5.
• Salt-free chemicals, by comparison, have a relatively
small impact on the total salt load in the waste water
from a typical tannery.
1.5% DERMASCAL® F
17
Glossary
ARACIT® KL new
Liquid bactericide
Highly effective, low-odour preserving agent; quantity applied for fresh hide preservation: 0.3 - 0.5%; as
bactericide in soaking: 0.05% in short-soaks and 0.1%
in overnight soaking
BORRON® LB
Complexing agent
Based on phosphonic acids prevents the formation
of lime blast, calcium sulfate and iron stains; helps to
remove lime blast and calcium in deliming; quantity
applied: 0.2 - 0.5% on salted or fresh weight in the last
wash after liming and /or 0.2 - 0.5% on pelt weight at
the start of deliming
BORRON NF
Lime dispersing and complexing agent
Based on polyphosphates; helps to keep calcium hydroxide in suspension during the liming process; prevents the formation of lime blast, calcium sulfate and
iron stains; helps to remove lime blast and calcium in
deliming; quantity applied: 0.2 - 0.5%
®
DERMASCAL ASB new
Universal deliming agent
Mixture of dicarboxylic acids and ammonium salts; has
a buffering effect without the risk of swelling; prevents
the pH from dropping below the critical limit of 5 (fix
ation of scud and residual hair!); provides a fine, clean
grain; quantity applied: 2 - 4% (depending on rawstock
and thickness)
®
DERMASCAL CD
Ammonium-free deliming agent
Based on functional esters, highly concentrated and
free of acids and nitrogen; delimes gently and homogeneously and buffers the pH around 8.5; effectively
reduces growth and neck wrinkles; imparts very good
grain cleaning, less hair roots and short hair; quantity
applied: 1 - 2% (depending on rawstock and thickness)
®
Text written by Dr. Campbell Page and Jan-Tiest Pelckmans
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DERMASCAL® F
Low-ammonium containing deliming agent
Based on a mix of organic components with masking
ability; delimes gently and homogeneously without big
pH drops; enhances chrome uptake and exhaustion;
quantity applied: 0.6 - 2.5% (depending on rawstock
and thickness)
ERHAVIT EF
Enzymatic liming agent with reductive-effect
Free of amines, nitrogen and sulfides; accelerates
the diffusion of liming chemicals, regulates swelling,
loosens scud, increases yield and quality, permits reduced offer of sulfide; suitable for the Hair-Save (HS)
process; quantity applied: 0.6 - 1%
®
ERHAVIT HS2
Liming agent with reductive-effect for hair saving
Sulfide-free; has a swell-regulating and grain-cleansing
effect; suitable for the Hair-Save (HS) process; attacks
the hair-root area; provides very smooth and clean
pelts; improves area yield; quantity applied: 0.7 - 1.5%
®
MAGNOPAL®
Salt-free, liquid polymer retanning agents
MAGNOPAL® DP, liquid polymer retanning agent with
a dispersing effect to improve penetration and levelness of co-applied retans, fatliquors and dyes; quantity
applied: 2 - 4%.
MAGNOPAL® PGN, liquid polymer retanning agent
with a softening and filling effect for shoe upper leathers; quantity applied: 2 - 6%.
MAGNOPAL® TGR, liquid amphoteric polymer retanning agent for up-grading loose-structured hides and
skins; quantity applied: approx. 1%.
OROPON® ANZ
Salt-free, concentrated liquid bating agent
Based on modified bacterial protease with the specificity of pancreatic protease enzymes; suitable for saltfree bating of any type of leather, but especially for soft
leathers; ensures a good scud-loosening and imparts
an elastic-grain without loosening it; quantity applied:
0.1 - 0.4% depending on the final article
OROPON® ON2
Concentrated ammonium-free bating agent
Pancreatic bate suitable for any type of leather but especially for soft leathers; ensures a good scud-loosening
and imparts an elastic-grain without loosening it; quantity applied: 0.1 - 0.5% depending on the final article
OROPON® W
Ammonium-free bating agent
Pancreatic bate suitable for any type of leather; ensures a good scud-loosening and imparts an elastic-grain
without loosening it; provides an extra fine grain pattern; quantity applied: 0.2 - 1.5% depending on the
final article
SELLASOL®
Salt-free, liquid retanning agents and auxiliaries
SELLASOL® HFN liq., amphoteric liquid filling agent
imparting a high degree of softness; quantity applied:
3 - 6%
SELLASOL® NG liq., liquid neutralizing agent with a
pH buffering effect and mild retanning action; quantity
applied: 1 - 4%
SELLASOL® TN-FF liq., liquid dispersing agent for all
anionic tanning and retanning materials; improves dyelevelness and dye-penentration; quantity applied: 1 - 3%
SELLATAN® CF new
Wet-white tanning agent
Based on aliphatic polyaldehydes; non-ionic; highly suitable for manufacturing wet-white leather; can also be used
for producing shrunk leather; quantity applied: 2 - 3%
SELLATAN
Salt-free, liquid tanning and retanning agents
SELLATAN® FB liq., liquid replacement tanning agent
with good filling properties, medium softness and a
tight-grain; quantity applied: 4 - 6%
SELLATAN® FL liq., liquid syntan for white and pastelshade leathers with high light fastness, good handle
and softness properties; quantity applied: 4 - 8%
SELLATAN® GS-B liq., liquid low astringency tanning
agent for automotive and upholstery leathers, effective pre-tanning agent for wet white leathers; quantity
applied: 5 - 20% for wet-white, 4 - 10% for wet-blue
®
SELLATAN® LV conc liq., liquid highly astringent syntan for white and shrunken-grain leathers, imparts good
fullness with a round handle; quantity applied: 4 - 6%
SELLATAN® RL liq., liquid syntan for light fast automotive and upholstery leathers with good filling and a
soft round handle; quantity applied: 4 - 6% for retanning wet blue, 20 - 25% for tanning/retanning wet white
automotive leather
SELLATAN® P liq.
Non-swelling pickle acid for low-salt pickling
Based on modified polysulfonic acids; anionic; suitable
for combining with wet-white tanning; allows reducing
pickle salt, pickle acid and chrome tanning agent; chrome-tanning auxiliary to improve the fullness of hides;
quantity applied: 1 - 2%
SELLATAN® PA liq.
Pickle acid for low-salt pickling
Based on modified polysulfonic acids; anionic; able to
replace both mineral and organic acids in the pickle
of chrome and chrome-free tanning; allows reducing
pickle salt and chrome tanning agent; quantity applied:
2.0 - 2.2% on split hides, 2.5 - 2.7% on unsplit hides
SELLATAN® WL-G
Wet white tanning agent with extra masking ability
Based on aliphatic polyaldehydes; non-ionic; highly
suitable for manufacturing wet-white leather; can also
be used for producing shrunk leather; quantity applied:
1.5 - 3%
TANNESCO® HN liq.
Liquid tanning and retanning agent
Liquid, chrome containing syntan with low astringency,
evens out differences between wet-blue leathers, improving dyeability and batch-to-batch reproducibility,
imprves softness, gives a fine and smooth grain; quantity applied: 2 - 4%
ARACIT®, BORRON®, DERMASCAL®, ERHAVIT®, MAGNOPAL® OROPON®, SELLASOL®, SELLATAN® and TANNESCO® are registered or filed
trademarks, owned by or licensed to TFL in most countries. Our application recommendations are in line with our present state of knowledge.
They do not, however, exempt the customer from performing his own tests to determine the suitability of the supplied products for their intended
purpose. Application of the products lies outside the scope of our control and therefore comes within the customer’s sphere of responsibility.
We guarantee the satisfactory quality of our products subject to our general terms of sale and delivery.
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For further information have a look at www.tfl.com or contact ecology@tfl.com
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