Due to increased legislation geared towards environmental protection and preservation of water quality, tanneries are being forced to implement continuous improvement strategies, including:
* the adoption of clean technologies which minimise the organic/inorganic loading in the wastewater
* the use of reclaimed sewage water as tannery ‘fresh’ water and the recovery and recycling of treated process water
* investment in sophisticated on-site effluent treatment plants, including the use of membrane technologies such as UF, RO and MBRs as a supplement to conventional physico-chemical processing.
Largely economics and the specialised requirements of each tannery dictate the selection of specific options.
Tanneries, and perhaps more importantly, legislative powers have recognised that the ultimate aim of water resource management is to achieve the sustainable use of water for the benefit of all users and to protect the quality of water to ensure the sustainability of valuable national industries.
In addition, the increased cost of raw water, increased costs for effluent treatment by local authorities and high costs for disposal of solid wastes have resulted in a major drive towards using cleaner technologies (waste minimisation), reducing water consumption, maximising water recycling and installing on-site wastewater treatment plants.
In most cases, the quality of reclaimed water is more than adequate for industrial use, although occasionally it may be necessary to use chemical precipitation or membrane filtration techniques to reduce the level of undesirable impurities.
Domestic sewage often contains high levels of phosphate which can, if required, be removed by precipitation with ferric chloride, while recovered industrial wastewater may contain higher levels of metal ions, such as chromium.
The need for stricter environmental protection has been progressively emphasised and legislated for, resulting in industries being forced to treat and re-use vast quantities of stored polluted water. For example, saline mine waters are being used as highly purified boiler feed water streams after passing through sophisticated reverse osmosis plants.
The advent of these new technologies offers new opportunities for water recycling for water-intensive industries such as the leather industry, which generates significant volumes of high TDS (total dissolved solids) effluent, with high organic loads.
Membrane technologies, such as ultrafiltration and reverse osmosis (RO), are potential supplements to conventional tannery wastewater treatment processes. The relatively high cost of the capital equipment and consumables (membranes) has been a major deterrent in the past, but with the global growth in this segment of the water treatment industry, prices have decreased significantly within the past five years. One of the advances is the use of reverse osmosis.
This is a water filtration technology, which utilises a semi-permeable membrane. Semi-permeable membranes allow water to pass through or permeate readily, but are impermeable to everything else. In general, semi-permeable membranes will reject charged or relatively large uncharged substances (>100 molecular weight). The bigger or more highly charged the ion or molecule, the greater the rejection percentage.
Osmosis, Figure 1, is the tendency of water to pass through a semi-permeable membrane into a solution of higher salt concentration in order to equalise the concentrations on either side of the membrane.
Reverse osmosis, Figure 2, involves the application of external pressure to the salt solution side of a semi-permeable membrane to cause a solvent flow to the pure water side, ie the reverse of osmosis. The driving force of the reverse osmosis process is applied pressure. The amount of energy required for osmotic separation is directly related to the salinity of the solution. Thus, more energy is required to produce the same amount of water from solutions with higher concentrations of salt.
Scientists have known about osmosis and the potential benefits of reversing osmosis for hundreds of years. Experiments were conducted in the 1950s using naturally occurring membranes such as animal bladders and intestines.
The commercialisation of the reverse osmosis technology occurred when synthetic semi-permeable membranes were developed in the 1960s. Cellulose acetate was the first commercially available membrane material. Improvements in the 1970s allowed for the manufacture of much thinner polyamide semi- permeable membrane layers. These are called thin-film or thin-film composite membranes.
A comparison of the general characterisation of cellulose acetate and thin-film polyamide flat sheet membranes is given in Table 1.
The RO membrane essentially rejects all suspended substances and 95-99% of dissolved substances.
Pre-treatment or pre-filtration is essential to prevent fouling of RO elements with suspended particles. Particle filtration removes particles of 1-1000mm. Other separation processes that are used to separate macro molecules and molecules include microfiltration, ultrafiltration and nanofiltration, with reverse osmosis separating the smallest metal ions as illustrated in Figure 3.
There are few reported cases of the application of membrane technologies in the leather industry worldwide. Cassano et al 1999, from the department of Chemical Engineering, University of Calabria, Italy, describe the use of pressure driven membrane operations to improve chromium recovery from spent chromium tanning baths and to desalinate water discharged from filter presses after Cr(III) precipitation.
Nanofiltration was employed to recover concentrated chromium (III) and water which could be re-used in pickling while RO was employed to desalinate water from the precipitated chromium hydroxide filter presses.
RO permeate water was of a high enough quality for re-use in washing operations.
In December 1999, the BLC published a report entitled: ‘The application of membrane technologies in the tanning industry.’ This report described the research being conducted there on the use of novel membrane technologies for the recycling of spent tannery liquors and the recovery of useful tannery byproducts.
Tubular ultrafiltration (UF) membranes were chosen for testing because they require less stringent pre-screening and can be used for higher recovery concentrations than hollow fibre or spiral type membrane designs.
UF trials were conducted on:
* main soak liquors
* spent unhairing liquors
* organic tanning liquors
A summary of these results is provided in Table 2.
Recovery of fats and oils
One trial that BLC carried out was an investigation of the recovery of fats and oils from the wastewater of sheepskin scouring plants. A waste aqueous degreasing liquor was passed through a variety of tubular UF membranes to separate the fats and oils from the effluent. Ultrafiltration is used for separation of fine particles in the 0.01-0.1mm range and is typically used for separation of macro-molecules, oil and grease, suspended solids, proteins, colloids, bacteria and viruses.
Operating pressures for UF are typically in the 20-100psi range. Ultrafiltration is a well-known operation that has historically been widely used in the food, dairy, environmental control and potable water industries.
Typical applications include the concentration of whey protein during cheese-making and removal of emulsified oils and particulates from wastewaters. In UF, small molecules such as water and all ionic species pass through the membrane while larger molecules and emulsified fats and oils are retained.
The BLC results for the sheepskin degreasing liquor showed a 100% rejection of oils, fats and suspended solids, with zero concentration of these compounds in the clear permeate. Because of the passage of degreasing surfactants through the membrane, COD reduction by UF was limited to 50-60%. The quality of the permeate was, however, considered to be good for recycling back into the tannery for use in washing or soaking operations.
Post-tanning dyeshop wastewater treatment
A combination of membrane filtration and adsorption processes were used to produce a clear permeate with 99.6% colour removal and >99% chrome reduction. The permeate could be re-used as process or wash water instead of incurring costly disposal charges.
Membrane bioreactor (MBR) processes
The membrane bioreactor process is a modification of a conventional activated sludge process where specialised microorganisms are concentrated with organic loaded wastewater in a bioreactor. This is connected to a membrane filtration unit.
Advantages of the MBR include:
* enhanced degradation of difficult pollutants
* 10 fold reduction in removing sludge than conventional biological plants.
Very promising results were obtained with 80-90% COD and 90-98% BOD reductions. Complete re-use of the permeate is possible.
Water minimisation
The leather industry produces large volumes of highly polluted wastewater. Estimates of the volume of water used per ton of hide, range from 13.9-56m³(average 49,5m³/ton hide) according to Huber and Doane, (1980), to 3.5-89.5m³ (average 38m³/ton hide) according to US EPA (Tünay et al, 1999). Significant reductions in water consumption volumes can be achieved by:
* combining sequential processes without dumping
* re-using final wash effluent for the first wash (cascading)
* re-use of water from end-of-line treatment plants.
In addition to reducing the volume of effluent generated, the quality of the water or the level of dissolved solids (pollutants), can be reduced by applying clean technology processes.
Conventional physio-chemical and biological treatment processes together with the selective application of membrane technologies, Table 3, can substantially reduce the net pollutant load discharged. However, the increased cost of effluent treatment due to stricter environmental control requirements has resulted in an increased financial burden on an already profit stressed industry.
Conclusion
In addition to the application of the technological processes described in this paper, a radical change in the mindset of production personnel is needed. Each tannery and department must avoid wasteful use of water and chemicals.
Implementation of environmental management systems, such as ISO14001, allows for the self-audit and continual improvement projects so that companies can progressively improve their co-ordinated waste management programmes.
With increased environmental legislation being introduced worldwide, designed to limit discharges of toxic pollutants, industries such as the tanning industry will have to turn to novel methods of water treatment if they are to remain in business.
