Salt in wastewater from leather manufacturing

The tanning industry generates considerable amounts of solid, liquid and gaseous wastes. However, 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 disposal costs and comply with environmental waste discharge regulations.
The preservation and tanning processes of raw hides and skins often involves the extensive use of salt, which is very difficult to remove from the wastewater. So it is logical that for the leather industry this is a major topic of concern. This article considers ways of reducing the amount of salt in the tannery effluent, especially how the levels can be reduced by means of modifications to the manufacturing process and the chemicals being used.

Why salt?
The term ‘salt’ in the tanning industry is typically used to refer to the two inorganic salts, sodium chloride and sodium sulphate. Salts in the effluent are clearly one of the most difficult forms 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 wastewater treatment plants by typical processes, such as sedimentation, oxidation, precipitation or flocculation like most other pollutants.
If the treated wastewater is returned to rivers or used for irrigation purposes, there is an increase in the salinity. Assuming the wastewater does not contain other contaminants, high salt containing effluents are generally only acceptable if discharged into the marine environment.

Where does this salt come from?
If salt in the wastewater is to be reduced, then where does the salt come from? The graph (Figure 1) shows the sources of salt for a typical bovine tannery making chrome tanned leather from salted hides. It is clear to see 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 only a minor impact on the total salt load.

Chloride salt load
If a tannery is processing salted hides then the biggest salt component in the wastewater is always the sodium chloride from hide and skin preservation.
The graph (Figure 2) shows the chloride salt content in the wastewater for each step in the beamhouse processing for a tannery using salted hides. It is clear to see the dominance of the salt from the hides in the initial pre-soak and main soak. Later there is a reasonable amount of salt in the pickling/tanning step.

Sulphate salt load
Sulphate in tannery effluent comes mostly from the use of chemical products containing the sulphate ion and the many powder chemicals, which can contain a considerable amount of sodium sulphate as an inert standardising salt.
The graph (Figure 3) shows the sulphate salt content in the wastewater in each beamhouse processing step. Note that it is quite different from the profile for chloride salt shown above. It is clear that the ammonium sulphate used in deliming and bating and in the pickling/tanning the sulphate from sulphuric acid and chromium sulphate.

Measurement of salt content in wastewater
There are two reasonably simple methods for determining the total salt content of tannery wastewater. However, often the wastewater discharge limits are also given in terms of the specific ions involved, eg chlorides (Cl–) and sulphates (SO42–).

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, so it may not be optimal for some industrial wastewaters.

2. Electrical Conductivity
The Electrical Conductivity (EC) of the water can be measured by a conductivity meter. In fresh water the increase in conductivity is effectively linear with the increasing salt content, so this is an easy technique for measuring just the total salts in the water.

Which measurement technique to use?
The total salt content measurement technique differs depending on the country. For example in India the TDS is normally the preferred measurement, but in South Africa it is the conductivity of the wastewater that is measured. 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 components. But after the wastewater has been treated most of the COD components will be removed leaving just the salt content.

Methods to reduce the salt offer
There are various ways to reduce the total salt load in the wastewater. From the graphs (Figures 1-3), it is clear the option having the biggest impact is to avoid processing salted hides and skins. 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, but with less impact, is the use of salt-free chemical products.

1. Reducing conservation salt
The conservation salt contributes some 60-70% of the total salt freight in the tannery wastewater.

Mechanical de-salting
The simple step of mechanical removal of the conservation salt can significantly reduce the amount of salt going into the wastewater. Care must be taken not to damage the hides and skins.

Elimination of salt for conservation
When the slaughterhouse is reasonably close to the tannery and the logistics of delivery are well organised the direct processing of fresh hides can be undertaken. Hides coming from further away can be transported chilled to the tannery.

2. Reducing salt in the pickling process
a) Low-salt pickling with polysulphonic acids
In the low-salt pickling process for lime split pelts a modified polysulphonic acid, Sellatan P liq, can be used to replace some of the mineral acids and it allows a substantial reduction of the common salt used in the pickle. An electrostatic bond is formed between the collagen and the polysulphonic acid similar to that of a syntan.
The exhaustion of the non-swelling acid is excellent. In practice the hides remain non-swollen with growth marks in the neck remaining decidedly flat. It is also noted that a stabilisation occurs and this results in a rise in shrinkage temperature of about 10°C. So this low-salt technique offers improved handling for subsequent mechanical processes such as sammying, splitting and shaving. A further advantage is that the chromium salt (or other tanning agents) can diffuse more easily within the hide.
With the new modified polysulphonic acid, Sellatan PA liq, it is possible to completely replace the mineral acids and keep a 40% reduction in the common salt used during pickling.

b) Recycling
Recycling has probably been considered by most tanneries but the technology is often only partially implemented or not at all. It works best when there is relatively constant throughput and requires the installation of extra holding tanks, pumps and controlling equipment. To avoid build-up of chemicals in recycled floats it is necessary to regularly analyse the floats.
The savings from recycling are in considerably reduced amounts of water, as well as using much less of the basic chemicals like salt and acids. These are only used to ‘top-up’ as needed instead of a full amount being added for each fresh float. Savings of more than 50% in salt and 20% in acids have been reached in well-controlled systems.

3. Salt-free, liquid chemicals
In both the beamhouse and wet-end processes the extensive use of powder leather chemicals can noticeably increase the sulphate load in the wastewater. Powder products are often standardised at particular concentrations using inert salts like sodium sulphate. In many situations the use of equivalent salt-free liquid products, such as the Sellatan and Sellasol syntans and auxiliaries can significantly reduce the salt load in the wastewater.

Conclusion
Large reductions in the total salt load in the wastewater can be achieved, especially for tanneries with a beamhouse. Obviously the best solution from the environmental point of view is to process fresh or chilled hides, but the proximity of the tannery to the slaughterhouse has to be possible.
Whenever salted hides have to be processed, an efficient mechanical removal of the curing salt is the first step. During the processing the use of low-salt pickling system with modified polysulphonic acid products is the best opportunity to reduce salt in the wastewater. This does not require additional investment in infrastructure as for recycling processes.

More information on this topic will shortly be available in a new TFL Eco Guidelines brochure, Part 2, from www.tfl.com