In-plant control and clean technologies

Best management applications and good housekeeping practices are important and common issues to all in-plant control efforts and programs (US EPA 1982;

EC 2009). Water conservation is among other common applications. General approaches for water conservation are: using batch instead of running water washes and modifying existing equipment to use short floats. Among the reuse applications, use of wash water for solution make-up, wash/soak water reuse are important yet easy practices. Another in-plant control tool is substitution of material to increase treatability of wastewater and to reduce the environmental impact. There are numerous applications in this respect in the leather tanning and finishing industry such as substitution of alkylphenol ethoxylates with alcohol ethoxylates and substitution of halogenated organic compounds which have been used in many process steps. Other practices including specific substitution of substances and application of new and emerging technologies are summarised on the process basis as follows.

Curing of leather and leather receiving operations in a tannery are important to protect leather quality and facilitate treatment. Substitution of biocides and salt, and mechanical removal of salt are the important control applications to be accounted for.

Beamhouse: Use of soaking liquor in pre-soaking and green fleshing before liming are advised as general applicable practices. However, the important control alternatives in the beamhouse are about the unhairing process. Use of hair-save process, particularly for bovine leather unhairing, is important and advantageous, since a significant organic-matter reduction can be realised through separation of undestroyed hair which may also be recovered. There are several processes developed for hair-save, the most common one is use of only alkali without the use of sulphide. Another application is low-sulphide unhairing which is based on using organic sulphur compounds including thioglycolate, mercaptoethanol, and thiourea derivatives. Enzymes and amines can be added to facilitate unhairing and reducing sulphide consumption. Recycling of sulphide liquor by filtration is another method which requires close inspection of the composition. Enzymes may be added at the end of the process to facilitate clarification. Use of solely enzymes is another application tested for a long time. Lime-free fibre opening methods using strong alkalis and enzymes are also considered. Lime splitting for heavier hides may considerably reduce the chemicals used. In-situ sulphide oxidation is another application which also facilitates the subsequent wastewater treatment operations (US EPA 1982; UN 1997; Thanikaivelan et al. 2005; EC 2009). Nazer et al. (2006) conducted an experimental study to recycle unhairing effluent four times without affecting the quality of final product, by using several

reactors. Thanikaivelan et al. (2002) proposed a new enzyme unhairing method assisted with very low amount of sodium sulphide. Li et al. (2010) tested a cleaner beamhouse process involving the use of thioglycolate and urea as well as swelling agents such as sodium silicate, sodium hydroxide, guanidine, and protease enzymes. Jian et al. (2010) used ultrasound to accelerate the diffusion of enzymes through the skin enhancing the efficiency of enzyme unhairing. There are also methods for recovery of sulphide as elemental sulphur by chemical, microbiological electrochemical, and photocatalytic methods (Sabate et al. 1990;

Steudel 1996; Waterston et al. 2007).

Tanyard: There have been several methods developed to replace the ammonia salts at the deliming step. The most common replacement method is carbon dioxide. Carbon dioxide is used with a low amount of ammonia, its reaction is slower, auxiliary agents such as organic acids are used for particularly thick leathers. Uses of boric acid and magnesium lactate in deliming have long been considered. Deliming with magnesium sulphate is another method. Use of hydrochloric acid in conjunction with sodium hydrogen carbonate is a subject of interest. Sodium hydrogen carbonate can be used as the main chemical for deliming. Lactic acid was the first organic acid used for deliming. Formic acid, acetic acid, and esters of organic acids have been used as deliming agents (US EPA 1982; UN 1997; Thanikaivelan et al. 2005; EC 2009). Recently developed ammonium-free bating methods utilizing new enzyme preparations are also available (UN 1997).

Salt-free/reduced pickling methods using non-swelling polymeric sulphonic acids have been developed. Pickling liquor recycling with filtration and fungicide addition was also practiced (EC 2009). Bes-Piá et al. (2008) studied reclamation of pickle liquor using nanofiltration. Reuse of chrome tanning liquors in the pickling baths is another reuse method. Waste control methods for degreasing include: (i) substitution of nonyl phenyl ethoxylate-based surfactants;

(ii) substitution of chlorinated solvents; and (iii) use of closed-loop machines for solvent degreasing (UN 1997; EC 2009). Tanning process is a key to both quality of leather and the impact of its waste on the environment. There are several methods employed as in-plant measures to increase the process efficiency and to minimise chromium discharges. The first measure is increasing the efficiency of chrome tanning which can be realised by the optimisation of process parameters such as pH, temperature, time, and using short floats. High-exhaustion chrome tanning is an effective method for chromium control. The method can be applied either by increasing the collagen activity using aromatic dicarbon acids such as glyoxylic acid or by incorporating reactive groups into the chrome-tannin complexes (EC 2009). Another high-exhaustion method uses an aluminium based syntan together with chrome and recycles the spent solution for pickle

float (closed pickle-tan loop (Sreeram and Ramasami 2003)). There are other external chromium exhaust aids such as polyamides, polycarboxylates, long chain alkanolamines, polyelectrolytes, and ion-exchange resins (Sreeram and Ramasami 2003). Combination of chrome with other tanning agents like aluminium, zirconium, titanium, or glutaraldehyde was also proved to be effective in decreasing chromium content in spent tanning solutions. Use of monoethanolamine as catalyst to increase the exhaustion of chromium was also proposed. Recycling of tanning liquor can be applied directly or after chromium separation. Direct recycle has been considered a technically viable process (Sreeram and Ramasani 2003; Thanikaivelan et al. 2005). Recycling of tanning liquor to pickling is another alternative, however, it may cause surface fixation of chromium. Chrome recovery and reuse is an advantageous process in terms of both economy and reducing the environmental impact. The process is well defined. It is based on precipitation of chromic hydroxide at alkali pH, separation of the chromic hydroxide solid phase, and redissolving by acidification. A three-step process involving precipitation, extraction and electro-deposition process as well as a solvent extraction technique has also been developed (UN 1997; Sreeram and Ramasami 2003; Thanikaivelan et al. 2005; EC 2009). Electrochemical methods have also been used for the removal and recovery of chromium from tanning liquors and leather tanning wastewaters. Sirajuddin et al. (2007) recovered chromium from tannery wastewater using electrolysis process with lead anode and copper cathode, and obtained efficiencies over 99 %. Ouejhani et al. (2008) studied electrooxidation to convert Cr (III) to Cr (VI) over a titanium-platinum anode in an electrolysis unit using tanning liquor as electrolyte for further selective recovery of Cr (VI) in tributylphosphate phase. Pretanning with aluminium salts sometimes combined with polyacrilates, glutaraldehyde derivatives, syntans, titanium salts, or colloidal silica is considered an alternative to improve chrome uptake (EC 2009). Use of organic tanning agents to produce chrome-free leathers has been considered for specific applications. Among these applications, vegetable tanning, mostly applied as Liritan process (Sreeram and Ramasami 2003), syntans, and aldehyde based tanning agents are promising.

In addition, mineral tanning agents such as Al (III), Si (VI), Fe (III), Ti (IV), Zr (IV), and Ce (III) or (IV) have been studied as alternatives to chrome tanning.

Some process modifications such as two-stage chrome tanning and pickless tanning were also introduced to improve chrome-tanning process (Sreeram and Ramasami 2003; Thanikaivelan et al. 2005; EC 2009). On the other hand, avoiding formation of hexavalent chromium during and after tanning is an important issue (EC 2009). In-plant controls applicable in the finishing stage are mostly based on process selection to increase the fixation of chemicals.

Reduction in metal content can be realised with the use of high-exhaustion

chrome tanning in the retanning process. Replacement of metal-complex dyes with acid dyes also helps reduction of metal content of wastewaters. The organic matter discharges can be reduced by providing high-exhaustion of chemicals in post-tanning operation. High-exhaustion of dyes using dyeing auxiliaries, use of high affinity retanning chemicals, use of low phenol and low formaldehyde syntans, and high-exhaustion of fatliquors by the use of amphoteric polymers are among the applicable alternatives (EC 2009).

In addition to the aforementioned in-plant controls and process modifications, there are recent developments and new process approaches. Biocatalytic fibre opening, three-step tanning technique (Thanikaivelan et al. 2005), use of ultrasound technology to facilitate the leather processing (Sivakumar et al. 2009), reversal of leather processing sequence (Saravanabhavan et al. 2006) and new approaches to cleaner production are among these developments.