Cleaner Production Techniques for Mitigation of Tannery Waste: Case Study ABC

Transcription

1 National University of Science and Technolgy NuSpace Institutional Repository Industrial and Manufacturing Engineering Industrial and Manufacturing Engineering Publications 2010 Cleaner Production Techniques for Mitigation of Tannery Waste: Case Study ABC Mhlanga, Samson Engineers Without Borders International Conference Downloaded from the National University of Science and Technology (NUST), Zimbabwe

2 Cleaner Production Techniques for Mitigation of Tannery Waste: Case Study ABC * Samson Mhlanga, 1 William M Goriwondo and 2 Cephas Tapedzisa National University of Science and Technology, Department of Industrial and Manufacturing Engineering, P. O. Abstract Box AC939, Ascot, Bulawayo, Zimbabwe Tel Ext.2269, Fax , Cell , *smhlanga@nust.ac.zw 1 wgoriwondo@nust.ac.zw The need for sustainable use of resources has come to the fore in recent years as the globe has realised that there are finite resources available on the earth that must also be utilised by future generations. It is within this scenario that manufacturing has evolved to incorporate environmentally friendly practices and so called green technology. Smaller firms especially in developing countries have however not been exposed to these new technologies making them liable for legislative penalties and less efficient than they otherwise could be. This paper presents a tool that will enhance the ability of a medium scale tannery to manufacture in line with current international environmental standards through the implementation of an environmental management system. The paper focuses on the application of cleaner production principles in the mitigation of identified environmental risks and the ensuing development of alternative processing strategies that will both increase efficiency as well as reduce the environmental impact of the leather tanning operations of the given firm. Keywords: cleaner production, economic, tannery, waste 1. Introduction Manufacturing firms have traditionally pursued ascendency over competition through attempting to make the best quality products at the highest speed available in the market, with the greatest reliability known to customers at a lower cost than competitors with unprecedented flexibility. Globalisation has however increased the level of competition forcing companies to seek alternative key competencies that will give an edge in the market. However, increased concern over the environmental impact of human activities on climate change as well as the ecosystem has led to a paradigm shift of manufacturing philosophy towards more sustainable and environmentally friendly industrial practices. In addition, legislation globally is continually evolving in the area of environmental management demanding swift response for compliance by industry. The advent of international standards such as the ISO series has set a competitive benchmark allowing customers to compare the performance of various firms within this area. 1

3 Emerging economies such as China and India have emerged strongly riding on the development and ensuing success of small to medium-scale enterprises (SMEs). It therefore follows that economic recovery in Zimbabwe potentially hinges on the successful and sustainable development of the SME sector. Industrial and manufacturing engineering principles should be used to aid in the development of environmentally conscious and sustainable productive systems that can support SMEs. This paper will look at applying Cleaner Production (CP) principles (instead of just focussing on pollution prevention) to a local SME. The case study is used in an effort to mitigate the various wastes resultant of production processes and as an illustration of the practicability of CP techniques. SMEs are expected to play a vital role in economic development but they also contribute significantly towards overall industrial pollution. Ramjaewon (2004) supports the view that large-scale industries produce more pollution than SMEs but because of their preferential access to capital investment and to new technologies; it is comparatively easier and more economical for them to control their pollution. SMEs lack access to adequate resources which hinders investment in pollution control. They operate on low level of technology, lack of space, non-availability of trained personnel, and the unwillingness of management to invest in environmental protection. Furthermore, the processing of hides and skins and converting them into leathers has long been an important industrial activity. The negative environmental impact of the processing has been regarded as an inevitable consequence of that activity. However, with the increasing concern over the pressure applied to the environment due to industrial emissions, it is imperative that mitigation efforts be made in developing countries such as Zimbabwe in pollution intensive industries such as tanning. There are a number of potential benefits of implementing CP that affect the entire organization as detailed below 1. Financial benefits in increased profits due to reduced resource usage, less waste, low insurance premiums and high stakeholder support. Table 1 is a summary of nine U.S. industrial case studies in cleaner production and shows financial gains. Table 1 Financial Gains Obtained Through Cleaner Production Implementation (Overcash, 1991) Industry Category of the Plant Process Change Capital Cost (to nearest $500) Annual Savings (to nearest $500) Fine Chemicals Heat recovery $7,500 $5,000 50% Chemical Manufacturing Vapour loss reduction $5,000 $275, % Small Appliance Solvent recycling Manufacturing & substitution $3,000 $20,500 85% Metal Finishing Spray paint loss reduction $874,000 $642,000 33% Textile Printing Solvent recovery $7,500 $90, % Furniture Manufacturing Hazardous waste reuse $1,500,500 $905,000 0% Textile Manufacturing Effluent heat reduction $100,000 $50, % Brewing Waste as fertilizer $88,000 $88,000 0% Food Canning Stream recapture $15,000 $45, % % Savings from Improved Efficiency 2

4 2. Improved Productivity due to process control and less waste. 3. Competitive advantage which increases sales and lessens the burden on marketers 4. Management burden is lessened as there is :- Structured approach to environmental issues and continual improvement Keeping ahead of environmental legislation Better relations with regulators 5. Public relations Improved relations with local community and environmental groups Improved public image 6. Personnel and training Improved working environment Reduced potential for environmental incidents Increased employee motivation and environmental awareness 7. Peace of mind Conforming to legal requirements Avoiding penalties for pollution Avoiding bad publicity from pollution incidents On a broader scale, CP can help alleviate the serious and increasing problems of air and water pollution, ozone depletion, global warming, landscape degradation, solid and liquid wastes, resource depletion, and acidification of the natural and built environment, visual pollution and reduced bio-diversity. 2. Cleaner Production Techniques Cleaner production is of substantial importance in changing the environmental approach within advanced industrialized countries. The critical principles involve fundamental understanding of diverse industrial processes, adherence to the earliest techniques of a hierarchy for reducing wastes, and utilization of an underlying thought process to achieve pollution prevention successes that are both technically feasible and cost-effective Evolution of Environmental Strategies Figure 1 illustrates the development of environmental strategic thinking. 3

5 Figure 1 Development of Environmental Strategies 2.2. Definition of Cleaner Production 2.3. Cleaner Production Principles CP has a number of terms that can be understood to convey the same meaning and these include waste reduction, clean technology, source reduction, environmentally benign synthesis, environmentally-conscious manufacturing, industrial ecology, sustainability and pollution prevention. The United Nations Environment Programme (UNEP) defines cleaner production as, A continuous application of an integrated preventive environmental strategy to processes, products and services to increase efficiency and reduce risks to humans and the environment. It is the application of know-how, improving technology and changing attitudes. CP is thus a business strategy for enhancing productivity and environmental performance for overall socio- economic development. CP follows three basic principles, namely (APINI); 1. Precaution -: these are measures taken in advance in the design stage through assessing the possible environmental impacts within the entire lifecycle of a given product so as to eliminate waste before inception. 2. Prevention -: this entails the avoidance or reuse of wastes produced through processes, products or services. These wastes are chemical losses from the vast diversity of industrial conversions that occur between chemicals in the natural state found around the world and the state of those chemicals in the products or services which reflect the gross domestic product of the all countries. 3. Integration -: waste can however not be reduced to zero hence integration looks at converting to less- or non-hazardous materials Cleaner Production Procedures 4

6 The procedures involved in the implementation of CP can be best understood in the form of a flowchart due to their hierarchical nature. These are summarised in Figure 2. Figure 2 Cleaner Procedures 3. Overview of Leather Making Process Leather is produced from hides/skins through the series of processes shown in Figure 3. The tanning processes can be seen to be falling mainly into two meta-processes (Blackman, 2005): wet blue production and finishing. The former involves removing unwanted substances (salt, flesh, hair, and grease) from a rawhide, trimming it, treating it to impart the desired grain and stretch, and finally soaking it in a chrome bath to prevent decomposition. Finishing involves splitting, shaving, re-tanning, and dying the wet blue. The wet blue and finishing processes are technologically and economically separable, and many tanneries throughout the world specialize in one or the other. The wet blue process is far more polluting than finishing, generating 90% of the water pollution associated with leather tanning. Two sub- stages of this process are particularly dirty: de-hairing, in which rawhides are soaked in a bath of lime and sodium sulphide to dissolve hair and flesh, and chrome tanning, in which hides are soaked in a chrome bath to render them biologically inert. 5

7 Figure 3 Sequence of Tannery Operations 3.1. Tannery Pollution The discharge of solid waste and wastewater containing chromium is the main environmental problem. Chromium is a highly toxic compound and its dumping is in most countries restricted to a few special dumping grounds. Reduction of chromium discharge is therefore essential. Emissions into the air are primarily related to energy use, but also the use of organic solvents and dyes causes emissions into the air. Of the raw stock of hide only a fifth can in practice be converted into reliable leather, the remains form waste or by-product. Some of the raw materials, such as hair, soluble proteins and fat have to be removed during processing to prepare the collagen fibre structure of the hide for tanning. Some parts of the leather also have to be trimmed or shaved during the production process. Residual chemicals from the leather manufacturing process, such as sulphide used for un-hairing and chromium employed for the tanning process, can contribute to the waste. Micropollutants, such as insecticides from raw stock, are also of growing concern. 6

8 Water Pollution Waste water is classified according to whether contaminants are oxygen demanding, algae promoting, infectious, toxic or simply unsightly. Waste water is made up of micro-organisms, solids, inorganic and organic matter. The typical constituents and their harmful effects are highlighted below Solids These can be suspended or gross solids which are large pieces of leather cuttings, trimmings and gross shavings, fleshing residues, solid hair debris and remnants of paper bags Oxygen demand Biochemical oxygen demand (BOD) Chemical oxygen demand (COD) Nitrogen Total Kjeldahl nitrogen (TKN) ammonia (from de-liming materials) and the nitrogen contained in proteinaceous materials (from liming/unhairing operations) Sulphide (S 2- ) The sulphide content in tannery effluent results from the use of sodium sulphide and sodium hydrosulphide used in the breakdown of hair in the unhairing process. Sulphates (SO 4 2- ) Chlorides (Cl - ) Neutral salts Oils and grease Chromium compounds Chrome 3 + (trivalent chrome, chrome III) Chromium is mainly found in waste from the chrome tanning process; it occurs as part of the retanning system and is displaced from leathers during retanning and dyeing processes. Chrome 6 + (hexavalent chrome, chrome VI) 7

9 Other metals such as aluminium and zirconium Clean Tanning Technologies Blackman (2005) highlights five main CP tanning techniques pioneered through the UNEP as follows: 1. High exhaustion Using special inputs and procedures to ensure that more of the chrome in the tanning bath actually affixes to the hide and less ends up in waste streams. Although this technique requires a more expensive type of chrome (self-basifying) and a longer soaking period, it offers significant cost savings due to reduced overall chrome use (UNEP 1991). 2. Enzymes in the dehairing bath Substituting biodegradable enzymes for lime and sodium sulphide in the dehairing float. 3. Precipitation of chrome Using alkalis to precipitate out the chrome in the tanning bath, then collecting the resultant sludge and processing it with sulphuric acid to recover the chrome. 4. Recycling the dehairing bath Saving and reusing the contents of the dehairing bath instead of discharging it all into the sewer after a single use. This simple technology requires only fixed investments in a holding tank, a pump, and a filtering system to remove suspended solids (usually a simple wire mesh screen). Because the chemical inputs into the dehairing bath are relatively inexpensive, only minor cost savings accompany the environmental benefits. According to UNEP (1991), a tannery that produces 1,000 wet blues per day could expect to save only US$8,000 per year. 5. Recycling the chrome tanning bath Entails reusing contents of the tanning bath instead of discharging them into the sewer after a single use. Like recycling the dehairing bath, this simple technology requires only fixed investments in a holding tank, a pump, and a simple filter. It can reduce chrome use by up to 20% (UNEP 1991). In the environmental guidelines geared towards SME s in Africa (USAID, 2009) CP techniques are applied to the leather making process with a focus on: Chemicals Water use Worker health hazards Odour Additionally the International Union Environment (IUE) Commission (2008) recommended cleaner tanning methods discussed below; 1. Chromium-free tanning 8

10 These are: Vegetable tanning is the traditional alternative to chrome tanning: conducted by a dry drum process, or in closed circuit vats, Tanning with organic tanning agents, using polymers or condensed plant polyphenols with an aldehydic crosslinker, can produce mineral-free leather, matching the high hydrothermal stability of chrome leather Semi-metal tanning can produce chrome-free leather, with equally high hydrothermal stability. It is a combination of a metal salt, preferably but not exclusively aluminium(iii), and a plant polyphenol containing pyrogallol groups, often in the form of hydrolysable tannins. 2. Wet-white pre-tanning The rationale behind this notion is to pre-tan or pre-treat the hide, in order to be able to split and shave prior to chrome tanning, so that less tanned waste is created. The rationale is to confer resistance to the frictional heating of the pelt surface during shaving. Ideally, the pre-treatment should be reversible, so that chrome tanning is conducted on unchanged pelt. This process can be considered as a cleaner technology if the chemicals used are neither toxic nor cause adverse environmental impact. Aluminium (III), titanium (IV) and zirconium (IV) have been suggested for this role: they are not listed as hazardous, although restricted in several countries, but their degree of reversibility depends on how they have been applied. Aldehydic tanning agents can be considered as leading to a cleaner process, according to local regulations, but their reactions are completely irreversible, so contribute to a different character in the leather. Syntans are an option, because their action is more reversible. The alternative approach is to change the properties of the pelt, to make it less prone to distort when the surface is struck by the shaving blade. This can be achieved by reducing the ability of the fibre structure to slip over itself: this is best achieved with hydrated silica, used in the fabric industry for the same purpose. Silica interacts weakly with collagen, in a non-tanning manner, and the effect can be reversed: any discharged silica has negligible environmental impact. 4. Use of Materials Balance in Pollution Load Calculation Within the assessment phase of carrying out CP an important tool to be used is that of material and energy balances. The material and energy balances are not only used to identify the inputs and outputs of mass and energy but their economic significance is related to costs which should ultimately be reduced Mass Balance Creation of material balance is based upon the law of conservation of matter. Figure 4 summarizes mass and energy balance. 9

11 Figure 4 Mass and Energy Balance (Buljan et al., 2000) 5. Environmental Audit of ABC Analysis is based mainly on the elephant hide tanning process as current operations are centred on elephant leather production. Unless otherwise stated, analysis is based upon a starting batch of one metric ton of dry elephant hide. ABC is an SME currently with a staff compliment of 37. Operations at ABC are raw materials and labour intensive with raw materials accounting for 70 % of production costs, labour 10 %, chemicals about 15 %, energy 3 %. Environmental costs account for less than 1% of the annual turnover. Operations at ABC are pollution intensive due to the nature of the tanning operation with wastewaters discharged containing pollutants from the hides, products from their decomposition, and chemicals and various spent solutions used for hide preparation and during the tanning process. Solid wastes arise from the fleshings, trimmings, splits, coal ash and sludge. Atmospheric emissions also arise from the combustion of coal in the boiler, diesel in the generator, hydrogen sulphide from the dehairing process and chlorine from the pickling process. Wastewater is however treated on site in an effluent treatment plant with the resulting discharge released into the municipal sewer system Leather Processing Operations 1. Wet End One Operations The operations carried out in this section are: soaking of the, preserved hide in water to return hide to original condition and to remove dirt, blood, dung, curing salt and water-soluble and saline-soluble proteins; unhairing (completee dissolving of all hair) by immersion in lime and sodium sulphide and subsequent reliming; trimming and mechanical removal of extraneous tissue from the flesh side of the hides deliming by treatment with a weak acid (lactic acid) and bating with an enzyme- acidity to the based chemical to remove hair remnants and degraded proteins; pickling using salt and sulphuric acid solutions to give the required skins to prevent subsequent precipitation of chromium salts on the skin fibres Chrome tanning is carried out using chromic sulphate. The tanning process stabilises the proteineous (collagen) network of the hide 10

12 2. Wet End Two Operations After the tanning process the hides are called wet blue as the chrome imparts a light blue colour during process. The post-tanning operations are: pressing (sammying) to remove moisture; a second levelling by shaving; secondary tanning using synthetic tannins (syntans) and tanning extracts; dyeing and softening of the tanned hide with emulsified oils (fat liquoring) 3. Finishing Finishing operations impart the final qualities that are required by the customer and fall under the following; drying and final trimming; dry-cleaning surface coating buffing (finishing) polishing or glazing 5.2. Pollution Loads of Individual Processes The pollution loads of the different processes were established by analysis of wastewater and estimated in comparison to effluent discharges from similar tanneries. These are based on the processing of elephant hide and data collected from production records. The main pollution indicators under consideration from the authorities are: Suspended Solids, SS Chemical Oxygen Demand, COD Biochemical Oxygen Demand, BOD Ammonium, NH 3 -N Total Kjeldahl Nitrogen, TKN Chloride concentration, Cl - Sulphate ion concentration, SO 4 2- Trivalent Chrome ions, Cr 3+ In order to make a comparison with requisite effluent standards, it is imperative that the concentration of individual pollutants be ascertained within the discharged effluent. As such, the water usage during elephant hide processing was monitored and the results of the analysis are shown in Figure 5. 11

13 4000 Water Consumption Estimation Figure 5 Water Consumption in litres per tonne of Elephant Hide The individual pollution loads and their resulting concentrations in the effluent discharge are shown in Table 2. Table 2 Pollution Load per tonne of Elephant Operation SS COD Soaking Liming Deliming Tanning 10 9 Operation SS COD Post-tanning 9 32 Finishing 1 3 Total Soak Liming Deliming Tanning Post-tanning Finishing Pollution load kg/t raw hide BOD Cr S 2- NH 3 -N TKN Cl - 2- SO Pollution load kg/t raw hide BOD Cr S 2- NH 3 -N TKN Cl - 2- SO Comparison is made against the requisite effluent standards in Figure

14 Comparison of ABC Effluent Discharge against Municipal Standards Concentration mg/l SS COD BOD Cr S2- NH3-N TKN Cl- Pollutant SO42- Municipal Sewer Effluent Standards Figure 6 Comparison of ABC Effluent Discharge against Municipal Standards The tannery discharge is failing to meet the regulatory standard for the following: Chrome (Cr); the recorded figure of mg/l is over 11 times than the requisite standard of 10mg/l Sulphate (SO 2-4 ); the recorded figure of mg/l is over five times greater than the requisite standard of 300mg/l BOD; the recorded value of mg/l exceeds by mg/l the requisite standard of 1000 It is therefore imperative that greater attention is given to the processes that make contribution to the specified pollution loads. From the analysis of Table 2 the processes under question can be identified and are illustrated in Figure 7. The processes that require urgent remediation as they make the organisation liable for penalty and/or prosecution are the tanning and liming processes. The tanning and retanning processes contribute 100% of the chrome load in effluent as well as 65.5% of the sulphate concentration. The liming process further contributes 51% of the BOD. It is upon these processes that cleaner production technique will be applied in order to achieve process improvement that will result in environmental and overall efficiency gains. 13

15 2- Figure 7 Comparison of Process Contribution to BOD and SO Materials Balance of ABC Chrome Tanning Process In order to analyse the efficiency of material use and ascertain opportunities for input substitution as well as reuse and recycle of process waste there is need for the construction of a material balance. The case in study is that of elephant hide processing and Figure 8 indicates the inputs, processes, output and nature of waste produced. The following assumptions will hold for the proceeding calculations: 1. Total weight of pelt (hide in process) is 1100kg made up entirely of collagen (leather fibres) and water. 2. The presence of other chemicals is regarded as negligible. 3. The ratio of water : collagen by percentage mass is 76% : 24% 4. Gaseous emissions have negligible mass 5. Approximately 75% of the chrome added during the process is retained in the leather grain structure. (Ljudik, 2000) 14

16 Figure 8 Elephant Hide Process Flowchart 15

17 Table 3 highlights the mass balance in the chrome tanning process. Table 3 Material Balance in Elephant Chrome Tanning Process COMPONENT INPUT OUTPUT kg kg Pelts Process water Effluent NaCl H2SO4/HCOOH 11 0 Chrome extract (25% Cr2O3) MgO/NaHCO3 8 0 Reaction salts 19 Grain leather ( we t blue) Split leather (wet blue) 0 88 Unusable split Trimmings 0 20 Shavings 0 99 Sammying water TOTAL The primary concern of the above balance is the chrome balance. A typical distribution of chrome in the conventional chrome tanning process is shown in Table 4. Table 4 Chrome Balance in Tanning Process (Ludvik, 2000) Chrome offer calculated as: Chrome input % kg In grain leather kg In usable split kg In solid waste kg In effluent kg Basic chrome sulfate Bi-nuclear complex Cr2O (34%) 2.5 (11%) 6.5 (30%) 5.5 (25%) The significant environmental impacts of ABC are arising from the chrome tanning process that sees carcinogenic (potentially causing cancer) effluent released into the environment. This process accounts for approximately 40% of the total environmental impact of ABC operations. The ABC effluent was analysed and compared against regulatory standards. Three main pollutants namely BOD, SO 4 2-, and chrome levels were found to exceed the regulatory limits with the chrome concentration being over ten times the required level. 16

18 6. Cleaner Technology Application 6.1. Input Substitution The inputs considered are alternatives to the chrome tanning process and result in a different intermediate state of the leather. Evaluation was made of using glutaraldehyde along with other organic tanning agents and syntans instead of chrome tanning agents Cost Comparison of Glutaraldehyde and Chrome Tanning Processes The cost analysis shown in Table 5 is based on a standard batch as stated in the preceding chapter. The costs of the chrome tanning process is significantly affected by the cost of disposal hence it might compare favourably if a recycling or recovery process step is added Table 5 Cost Comparison per Batch Between Conventional Chrome Tan & Glutaraldehyde Tan Conventional Chrome tanning Glutaraldehyde Process Process Cost Process Cost 8 % Chrome tanning agent $ % Glutaralaldehyde $55 Disposal of chrome shavings (> 10% of base-weight) 100 kg x 0,13 $/kg Disposal of chrome-containing sewage sludge (> 70% of base-weight) 700 kg x 0,13 $/kg Transport of shavings and sewage sludge (100 kg kg x 0,05 $/kg) $14 $95 Disposal of shavings (chromefree- may be for sale) 100 kg x 0,02 $/kg 4% Organic Tannins and Syntans $2 $70 $36 Transport of sewage sludge $32 Total $231 $ Processing Steps The sequence of processing steps was assessed so as to analyse the differences that arises from using different materials. As is shown in table below, the differences created are marginal as it is only required that the inputs to the pretanning and retanning processes be changed. Hence the existing ABC facilities and processing capacities can be effectively utilised in the alternate processes 17

19 Table 6 Comparison of Processing Steps Glutaraldehyde tanning for the production of crust leather Tanning by 2.5% glutaraldehyde, 1 % sodium meta bi- sulphite, 50% water, 2% naphthalene based syntan, 0.6% sodium bi-carbonate and 0.2% fungicide. Chrome tanning for the production of crust leather Tanning by 6% basic chromium sulphate and 2% chrome syntan. Then sammying, splitting and shaving operations were done for the production of wet white leather. Neutralization by 1.5% Neutralizing syntan. Retanning by 4% acrylic syntan, 6% vegetable tannin and 4% resin syntan. Then fat-liquoring and dyeing were done for the production of crust leather. Then sammying, splitting and shaving operations were done for the production of wet blue leather. Neutralization by 1.5% Neutralizing syntan. Retanning by 4% acrylic syntan, 6% vegetable tannin and 4% resin syntan Then fat-liquoring and dyeing were done for the production of crust leather Quality of Leather 1. Physical Characteristics The quality of the leather was assessed according to some physical as well as chemical characteristics. The physical parameters of both glutaraldehyde tanned and chrome tanned leathers are listed in Table 7. From the table it is found that the physical properties of the glutaraldehyde tanned leather is comparable or in some cases better than the leather tanned by conventional chrome. Table 7 Comparison of Physical Characteristics Physical properties Glutaraldehyde tanned leather Chrome tanned leather Thickness (inches) Tensile strength (psi) Extension (%) Ball brust (lbs) Chemical Characteristics The chemical properties of the leather tanned by glutaraldehyde and chrome are shown in Figure 9. From the graph it is found that in wet white leather percentage of Cr2O3 is null because no chrome is used in wet white leather. In wet white leather ash and hide substance is less than chrome tanned leather but percentage of volatile matter is higher in wet white leather. 18

20 Chemical Component Cr2O3 Hide substance Sulphated total/ash 4.80% % 1.30% 29.50% 75.20% Chemical composition of chrome-tanned leather after shaving (wet blue) Chemical composition of pretanned leather after shaving (wet white) Volatile matters 3.00% 9.80% 0.00% 20.00% 40.00% 60.00% 80.00% Percentage Composition Figure 9 Comparison of Chemical Characteristics Comparison of Pollution Loads The discharge from chrome tanning and glutaraldehyde tanning were analyzed for the parameters which are represented in Figure 10. From the comparison, it is found that though the BOD and COD in the discharge of glutaraldehyde tanning is high but the total solids and chloride are much less in glutaraldehyde tanning discharge. From the graph it is revealed that there is no discharge of chromium in the effluent of glutaraldehyde tanning. So in environmental aspects the glutaraldehyde tanning is much more acceptable than the conventional chrome tanning. 19

21 Concentration mg/l Chrome tanning Glutaraldehyde tanning Discharge Measure Figure 10 Comparison of Pollution Discharge 6.2. Addition of Pretanning Step with Aldehyde Reagents One major contribution to waste was hides that are split and shaved after the chrome tanning process. This led to the investigation of possible process changes leading to splitting and shaving of hides being done before the chrome tanning process. Figure 11 illustrates a possible variation. Figure 11 Pretanning with Glutaraldehyde The process change incorporated the glutaraldehyde processing step as a pretanning process instead of as the main tanning process. The process requires an extra drum though this is absorbed in the already existent ABC capacity. The costs of the Glutaraldehyde pretan process compared against the conventional tanning process are shown in Table 8. 20

22 Table 8 Cost Comparison Between Conventional and Proposed Innovation Conventional Chrome tanning Process Cost Process 8 % Chrome tanning agent $ % Glutaralaldehyde Disposal of chrome shavings (> 10% of Disposal of shavings (chrome free- may $14 base-weight) 100 kg x 0,13 $/kg be for sale) 100 kg x 0,02 $/kg Disposal of chrome-containing sewage sludge (> 70% of base-weight) 700 kg x 0,13 $/kg $95 6 % Chrome tanning agent on shaved weight shaved weight (wet-white) = 40% of base-weight = 400kg Transport of shavings and sewage sludge (100 kg kg x 0,05 $/kg) $36 Transport of sewage sludge Drum Operating Cost (Labour + Utilities) Total $231 Glutaraldehyde Pretan Process Cost $55 $2 $25 $32 $75 $189 The cost reduction is due to a reduction in chrome tanning reagents usage as the mass of the wet white is reduced by 40% from the sammying, splitting, shaving and trimming processes occurring before the chrome tanning process. The percentage of chrome addedd is also reduced from 8% to 6% as the pretanning process aides in the collagen fibres adsorption of chemicals Reduction in Pollution Discharge The pollution discharge is compared against municipal effluent standards in Figure 12. As shown, chrome discharge is 9.8mg/l which is marginally lower than the standard of 10mg/l. The reduction is due to the reduced weight of pelt as well as the lower percentage of chrome required in the float. Concentration mg/l Comparison of Estimated Discharge of New Process against Municipal Standards 9.80 SS COD BOD Cr S2- NH3-N TKN Cl- SO42- Pollutant Estimated Pollution Dicharge of New Process Municipal Sewer Effluent Standards Figure 12 Comparison of Pollution Load against Effluent Standard 21

23 The mechanical as well as thermal properties associated with chrome tanned leather are still retained due to the chrome tanning step Chrome Recovery Additionally the processing could be made more resource efficient by adding a recovery step that aids in the recycling by precipitating chrome in a used float. The process evaluated entails slow precipitation with magnesium oxide, settling of the suspension, decantation of the supernatant (no need for a filter press) and subsequent acidification of the relatively dense precipitate. Figure 13 represents the probable system by a flow diagram. Figure 13 Chrome Recovery Process Flow Chrome Recovery Efficiency In conventional chrome tanning, the chrome oxide content in the recovered liquor is usually g Cr 2 O 3 /l. Experience in India has shown that leather tanned with 70 % fresh chrome and 30 % recovered chrome has more or less the same quality as leather tanned with 100 % fresh chrome (Buljan, 2000). ABC tanning efficiencies were approximated at 68% and a model of chrome recovery was created for that efficiency and the results tabulated and presented in Table 9. 22

24 Table 9 Chrome distribution between leather and spent floats Distribution Efficiency 68 % Offer - tanning 83 - retanning 17 Leather - tanning retanning 11.9 Spent tanning float - recoverable unrecoverable 1.5 Residual water recoverable - sammying draining 0.3 Spent retanning float - recoverable unrecoverable 0.9 Recovered - tanning sammying/draining retanning 4.2 Total - to reuse to discharge 2.4 There was also need to investigate the distribution of chrome throughout the process so as to enhance the recovery process. The results of the distribution are shown in Figure 14 Figure 14 Chrome distribution scheme 23

25 Cost Ratios of Operating a Chrome Recovery Plant Chrome recovery techniques have a direct bearing on capital and running costs. The choice between an alkali sodium salt and MgO is decisive in terms of capital costs since as a rule a filter press is not needed to dewater the chrome oxide precipitated with MgO. For a daily chrome recovery capacity of 12-15m 3 spent floats, capital costs of the following order can be expected (Ludvik, 2000): Chrome recovery with magnesium oxide precipitation: US$ 60,000-80,000 An Indian plant of similar capacity to ABC faces the following annual running costs (Rajamani, 1995) shown in Table 10. Table 10 Annual Running Costs of Chrome Recovery Plant Item Cost/US$ Maintenance 1,500 Labour 1,000 Chemicals 9,000 Electricity 500 Miscellaneous 2,000 Total operating costs 14,000 Financial costs 7,800 Depreciation 5,200 Total annual costs 27,000 It follows from the data on the Indian chrome-recovery plant that total annual costs related to processed hides amount to US$ 9 /t raw hides. Taking into consideration the operating costs associated with mechanical dewatering, total annual costs related to one tonne of processed hides will be higher in most cases when the mechanical process is applied. Drawing on operational experience, the profitability of chrome recovery plant based on the use of MgO can be calculated. The profitability expressed in terms of the payback time is shown in Figure 15. Figure 15 Payback Time as a Function of Recovery Plant Capacity 24

26 Assuming a reasonable payback period of three years, a chrome recovery plant based on the use of MgO would be profitable at a capacity as low as 2.5m3/day. Table 11 highlights the benefits and limitations identified for each evaluated technique. Table 11 Associated Benefits and Limitations Technique Advantages Limitations Substitution of Chrome Use of Chrome eliminated with Glutaraldehyde significantly reducing Introduction of Pretanning Process Introduction of Chrome Recovery Process environmental impact Process Sequence unchanged No need for ne equipment or processes Significant reduction in processing cost due to less waste Combines positive characteristics of aldehydes and chrome in leather Shavings from leather can be used as fertiliser or stock feed Savings in chrome used Minimum procedure change in tanning Reduced level of chrome in waste stream Unpredictable effect on leather mechanical and thermal properties Staff would require extensive training Imparts odour due to volatile substances Increased lead time Increased labour and overheads cots from extra process Mechanical pre-treatment of waste stream required Additional chemicals and manpower required Initial capital investment required 7. Conclusion Our present generation has a responsibility to the future generations to use resources to meet our needs in a manner that does not threaten the future generation s ability to meet their own needs. CP techniques provide a means for this to be achieved whilst challenging process engineers to greater levels of innovation. ABC is set to benefit from adaptation of CP technologies through the resultant reduction of environmental impact as well as improved resource utilisation and process efficiency. Of paramount importance is a reduction in the use of chrome including a greater control in the discharge levels. The organisation is also set to benefit from greater management control of environmental aspects of its activities through organised environmental management. There however remains much ground to be covered as chromium is not the only environmentally causing agent in the tannery s operations. Investigation into sulphate as well as organic matter discharge is also required in order for the organisation to continue to achieve compliance with environmental regulations. The organisational philosophy must shift towards source reduction and this can be enhanced by annually allocating budget for research and development into new cleaner materials and emerging options in cleaner processing. 25

27 The case study chosen however also limited the analysis of the subject matter due to lack of historical processing data that would have aided in formulation of mathematical models and trends. On the whole, CP techniques are proving a useful tool and require a greater sustained effort towards implementation. References 1. Blackman A, Adoption of Clean Leather-Tanning Technologies in Mexico. Washington. Retrieved from 2. Bosnic M, Buljan J and. Daniels R P, Pollutants in Tannery Effluents: Definitions and Environmental Impact - Limits For Discharge into Water Bodies and Sewers. Pollution Control, UNIDO 3. Chakraborty D, Quadery A H and Azad M A K, 2008 Studies on the Tanning with Glutaraldehyde as an Alternative to Traditional Chrome Tanning System for the Production of Chrome Free Leather. Chemical Research Institute Division, Leather Research Institute, Bangladesh Council of Scientific & Industrial Research, Dhaka Institute of Environmental Engineering, Introduction to Cleaner Production (Cp) Concepts and Practice. Kaunas University of Technology. UNEP 5. LASRA and Grierson H, An Environmental Management for the Leather Industry: Guidance Document for the Implementation of ISO Wellington, LASRA and Harrison Grierson Consultants Limited 6. Ludvik J, 2000a. Chrome Management in the Tanyard. Pollution Control, UNIDO 7. Ludvik J, 2000b. The Scope for Decreasing Pollution Load in Leather Processing. Pollution Control, UNIDO 8. Ramjeawon T, A Case Study of Cleaner Production Opportunities in Small and Medium Enterprises on the Island of Mauritius. Electronic Green Journal, 1(20). Retrieved from: 9. Schill and Seilacher, Increased Tannery Flexibility. Aktiengesellschaft, D Boeblingen Schoenaicherstr. 10. Stapleton P J, Glover A M, Environmental Management Systems: An Implementation Guide for Small and Medium-Sized Organizations 2 nd Edition. Grasonville. NSF 11. Tinsley S, Environmental Plans Demystified: A Guide to Implementing ISO Retrieved from Voice T C et al., Evaluation of Chromium Recovery Opportunities in a Leather Tannery. Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI