| Enzymatic Dehairing of Hides and Skin | The Netherland | - | Lab Scale |
TANNING AND DRESSING OF LEATHER # 15
Background
The sodium sulphide traditionally used for dehairing of hides and skin is replaced by an enzyme, protease. A filtration system with recirculation is added. The producer of the enzyme is Novo Nordisk. The process is being developed by the British Leather Confederation (BLC). Research for full commercialization is still underway. A few tanneries are using the enzyme on a commercial scale [3]. However, details of the users are not available.
Cleaner Production Principle
Process modification
Cleaner Production Application
Conventional process
Lime blended with sodium sulphide is traditionally used to loosen wool and hair and dissolve these into a pulp. Additionally the process opens the fiber structure and 'plumps' the hide. The duration of the process may vary from 18 hours (drum) to 7 days (pit). This process is responsible for the major part of the COD load from a tannery. Chemicals include: 2-10 % calcium hydroxide (lime), 1-4 % sodium sulphide and sodium sulphydrate. Some caustic soda may also be used [1]. The water can be polluted by BOD, COD, alkalinity, sulphides, organic N and ammonia. There will be air pollution with H2S and solid wastes with hair, lime and organic matter forming a sludge. Traditional soaking and unhairing account for well over 50% of the BOD and COD load in typical tannery effluents.
Modified process
NUE 0,6 MPX is a liming assisting enzyme, an alkali-stable bacterial protease, which is on the market today. One characteristic is its ability to break down the proteoglucans in the hide which provides opening-up of the fiber structure but at the same time NUE also confers depilatory effects. These abilities are exploited in the most optimal way in the Enzyme Assisted Liming as developed by the BLC. NUE 0,6 MPX can be applied through a number of process set-ups depending on the type of advantage required. According to Novo Nordisk the Hair Saving Enzyme Assisted Liming is as close as you can get when speaking about environmentally safe unhairing with reduced organic pollution. What is needed in this case is to mount filtration systems with pumps to circulate the float from the liming drum. The float should be circulated through the filters as frequently as possible to remove the hair from the float as soon as it is loosened from the hides. In this set-up NUE 0,6 MPX is applied in the relime where it removes the residual hair from the hides while also assisting in the opening-up of the hide structure [2]. The saved hair can be used as fertilizer.
Environmental and Economic Benefits
The benefits of using modified process will result in:
| Reduced sulphide requirements by up to 50%; |
| Improved area yield; |
| Reduced liming time to 6-18 hours; |
| Increased the opening-up of the fiber structure of the hide; |
| Cleaner grain surface; |
| Increased softness; |
| Reduced growth and mottle of pelt; |
| Increased strength of the grain leather; |
| Reduced COD by 20-25%. |
| Reduced BOD and |
| Reduced nitrogen by 40% in the wastewater [5]. |
The recirculation system is expensive. Novo Nordisk estimates that the investment costs for a 5-6 tons drum are 500,000 DKK. This is about 115,000 DKK more than without circulation.
Sufficient data are now available on reduction of input costs or pollution costs, and on the operational costs of the enzyme. Information about payback time is not available.
Constraints
Protease can accentuate irregularities in the leather when it is treated in a wrong manner.
Contacts
Type of Source Material: Articles, interview.
Citation
Review Status
This case study was submitted to UNEP IE by the Institute for Applied Environmental Economics (TME) of the Netherlands on the part of the UNEP IE working group on Biotechnology. The case study was edited for the ICPIC diskette in August 1995.
Subsequently the case study has undergone a technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.