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RETRIEVAL OF BIODEGRADABLE POLYMER AND VALUE ADDED PRODUCTS FROM LEATHER INDUSTRY WASTE THROUGH PROCESS BIOTECHNOLOGY: A PROGRESS REVIEW

by S. N. MUKHOPADHYAY* Dept. of Biochemical Engineering & Biotechnology Indian Institute of Technology HAUZ KHAS, NEW DELHI-110016 INDIA and N. SAHA, L. SAHA, P. SAHA AND K. KOLOMAZNIK Faculty of Technology, Tomas Bata University in Zlin TGM 275,CZ 76272 ZLIN CZECH REPUBLIC products include biodegradable polymer and its derived ABSTRACT materials, biogas, enzyme, inhibitor, recyclable chromium and bio-fertilizer/compost. The availability of these byLeather industry wastes can provide biodegradproducts largely depends on the biodegradability of the able polymer (BDP) and value added products waste resource. Thus understanding of the fundamentals of through process biotechnology. As the wastes are biochemical/chemical phenomena involved in biodegradahighly proteinaceous, a wide range of value added tion and biodeterioration is very important. This review, products could be retrieved making the leather therefore, focuses on the possible by-product retrieval from industry an integrated business with value creation leather industry wastes using integrated process biotechnolopportunities. These are described in this progress ogy. review.

INTRODUCTION Leather tanning industry waste/waste water is not an ordinary waste; it has a treasure of value-added materials, which need retrieval. This industry generates substantial quantities of waste.1-11 Wastewater from different sections12 of leather tanning when mixed with its solid waste forms sludge with chrome shavings, which is greenish black in colour. Many possibilities to the chrome-tanned waste treatments have been indicated in literatures.13-17 The major aims of all solutions has been abatement of environmental pollution and by-product retrieval in order to economize the leather industry as well as to create new job opportunities by gaining more economic returns in the industry. A scheme of the total recycle process integration approach, which should economize leather industry business and value creation opportunities by biological process, is shown in Figure 1. It indicates many retrievable value added products which could be obtained by various biotechnologies and provide a higher economic return. As depicted in this figure, the by-

Tannery waste sources and characteristics. Tannery wastes are proteinaceous biopolymeric materials generated in various steps (Figure 1) in leather production. In the process many chemicals are used, among which chlorides, sodium carbonates, ammonia, calcium and sodium dichromates, and sulfuric acid are the major ones. In the finishing step organic dyes are used. It has been estimated that per kilogram of finished leather nearly 35 litres of wastewater is generated. This wastewater along with solid wastes needs an adequate treatment/biological processing before disposal to avoid nuisance/pollution of the environment. Furthermore, raw tannery wastes/waste water corrodes sewer lines, causes ground and surface water pollution and imparts odour problems. In order to lower these adverse effects tannery wastes must be treated to safe-guard the receiving bodies/sinks and surface and ground water sources. It must be kept in mind that animal skin has several layers,12 out of which only inner corium layer is useful in leather making. The corium is made up of collagen, which is a proteinaceous material.

* Visiting Scientist (on leave) at Tomas Bata University in Zlin, Czech Republic Contact E-mail address: [email protected] or [email protected] JALCA, VOL. 99, 2004

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LEATHER INDUSTRY WASTE

Figure 1. - Total recycling process integration approach for economizing leather industry business and value creation opportunities

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Table I, represents the composite of tannery effluent when it reaches to the plant for treatment. The nature of waste generated during different steps of tanning is shown in Tables II and the quantity of biological composition of settled and filtered composite tannery waste is given in Table III. These tables are evidence that tannery waste has high protein content and is suitable as substrate resource in recycling through process biotechnology (biological treatment methods) in value added products recovery.

TABLE I Tannery Composite Wastewater Characteristics12 pH D. Vs TDS soluble COD soluble BOD 5 TKN T.P as PO4

7-10.4 2700-4400 mg/l 15000-35000 mg/l 2700-5000 mg/l 1400-2500 mg/l 200-270 mg/l 3-22 mg/l

TABLE III Biological Composition of Settled and Filtered Composite Tannery Waste12 Item

Concentra- % on dry tion, mg/l weight basis Dissolved protein 1000-1700 Volatile fatty acids as acetic acid 200-500 Ether solubles 80-140 Tannin as tannic acid 530-900 Moisture 80-90 Ash 20-40 Nitrogen 30-60 Chromium oxide 20-30 Magnesium oxide 10-20

Fundamentals of biodegradation. Structurally and composition wise leather industry wastes are polymeric materials which are biodegradable. The fundamental concept for chemical/biochemical representation of these waste materials biodegradation under aerobic and anaerobic conditions is given below. Under Aerobic Condition: Polymer/Biopolymer + O2

Biocells/Bioconversion

CO2 + H2O + Biomass + Residue (1)

Under Anaerobic Condition: Polymer/Biopolymer

Biocells/Bioconversion

CH4 + CO2 + H2O + Biomass + Residue (2)

Based on the above basic approaches investigators have conducted biodeterioration /biodegradation of various wastes-/refuse materials for their reuse by recycling as well as treatment for cleaner environment.8,18-19. Some of the wastes undergo photodegradation as well as biodegradation simultaneously but slowly. In such cases the combined incremental degradation (dD) of polymeric materials has been described by the following equation.20 dD = (

δD δD δD δD ) MPC dB + ( ) PCB dM + ( ) BMC dP + ( ) BMC dC (3) δB δM δP δC

In this formula, B represents total biomass, M is microorganism, P represents photons and C means chemical. In these cases degradation of the four independent contributions, as given by above partial differential terms in brackets, interact and combine with each other to provide biodegradation end results. In order to observe the contributions of individual terms of equation (3) in biodegrad-

TABLE II Tanning Step and Originated Wastewater Nature12 Source step Soaking Liming Unhairing & Fleshing Spent bating & deliming Spent vegetable tan liquor

. . . . . .

Spent chrome

.

Fat liquoring & dyeing operation

.

Nature Olive green in colour, contains dirt, dung, blood, hair & salt. Highly putrescible & has obnoxious smell. Highly alkaline, one of the heaviest of waste fractions in terms of SS & BOD. Hairy, fatty & fleshy particles in suspension. Contains significant concentration of dissolved organic matters & has high BOD. Probably the strongest fraction in the composite tannery effluent. It is acidic in nature and has a high BOD & persistent colour which is difficult to remove by chemical and biological treatment. Also highly acidic and greenish in colour. It contains a high concentration of chromium & salt which render the effluent highly acidic & toxic. The BOD value is generally low because of the presence of chromium. Colours and oil emulsions.

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ability test the necessary conditions are stated. For δD = (

TABLE IV Relative Cost Break-up in Leather Industry23

δD ) δB MPC

needs to be done in absence of sunlight, reactive chemicals and microorganisms or with photons. Likewise for δD = (

δD ) δP

MBC

UV lamps are used in the absence of microbes, insects or autooxidants. In such tests it is usually presumed that one of the degradation mechanisms dominates and a standard is utilized with the assumption. In biodegradation by equations (1) and (2), one needs to keep in mind that all natural living polymeric materials are composed mostly of carbohydrate units that are catabolized by providing energy for a reaction (respiration) very similar to combustion (CH2O)n + n O2

nCO2 + nH2O + energy

(4)

This catabolic biodegradation is required by a naturally acquired design engineering (NADE) process like photosynthesis, providing energy of living cellular materials of plant origin from solar energy to renew the biomaterial resource via the following chemical reaction nCO2 + nH2O

(CH2O)n + nO2

(5)

It appears that depending on the extent of biodegradation and bioresource material improves the residue composition in equations (1) and (2) varies considerably. This demonstrates that biopolymer degraded residues are neither pollutants, nor are they noxious to agricultural crops. That is why biodegradable film packages have been extensively used in foils for agro-seedling sowing to provide the best solution to the disposal problem in plasticulture.20 Value added products from waste through process biotechnology Figure 1 shows that the chrome sludge is the major waste resource, which is in the connection of problems of real application in or out of the tannery industry. Daily availability of this waste is of huge amount as stated earlier. The capital-intensive nature of the leather industry has been reported to be due to high costs of raw materials, and the cost of labour is a relatively small component. From the cost structure breakup, as given in Table IV, in order to get more economic return from this capital intensive industry experts have suggested integration of its waste recycling for retrieval of value added products.3,22 The major of this value added products include as described in following sections.


Items

Cost com% share ponents 1. Raw materials/ supplies Raw hides 62-65 Chemicals 15-18 Labour 12-14 Environmental 5-10 Energy 3-4 2. Overhead Organization/admin- 10 istration/selling Capital 5-10 * The cost break down is likely to vary with the location Improved Biodegradable Polymer (IBDP) The production of IBDP is done by the following two steps. 1.) Chrome sludge to hydrolyzed protein (HP) As the waste resource has high protein content investigators have attempted to hydrolyze the protein by enzyme (commercial grade) to produce hydrolyzed protein (HP) and subsequently to polymerize it by polycondensation reaction11,14,24-25 with some aldehydes /gluteraldehydes. In hydrolyzing the proteinous waste the enzymatic process biotechnology was carried out in stages shown schematicaly in Figure 2. For enzyme bioreactor operation the hydrolysis reactor is fed with 10 metric tonne of water, 90 kg of cyclohexylamine or its equivalent amine compounds and 60 kg of MgO. Another 3 metric tons of chrome shaving waste is gradually added under constant stirring to homogenize the mixture and heating is started to carry out hydrolysis. The hydrolysis is run at the optimal 70 oC temperature and pH 9; to adjust pH, organic base is added. After 3-5 hrs of the reaction the hot heterogeneous mixture is filtered. The filtrate is collected in a metering storage tank and repumped into a continuously working 3 stage vacuum evaporator. The filter cake is transferred to an enzyme reactor situated under the vacuum filter. In this reactor, after pouring 3 metric tons of water, the pH is adjusted to 9 and 0.9 kg of an alkaline proteolytic enzyme (ALCALASE 2 5 LD X of Novo Nordisk, Denmark) is added. This enzyme reaction mixture is maintained at 70 oC under constant stirring for 23 hours. Next it is filtered to give hydrolyzed protein (HP) and filter cake. The HP clear solution is passed through the metering storage tank to a vacuum evaporator. As shown in Figure 2, the filter cake is recycled to the reactor and diluted in water, and the heterogeneous mixture is fed into a hot solution of Na-dichromate and H2SO4 in the reactor for producing tanning salt either in liquid or in solid states. HP containing free amino acid fractions is transformed to improved


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Figure 2. - Enzymatic process biotechnology followed in Tannex Co. plant15

TABLE V IBDP Preparation Composition22 Components(%) Polyvinyl alcohol (PVA) Protein hydrolyzate (HP) Glycerol Stearin Talc

Film type F5 F6 70.8 58.6 11.5 23.9 17.7 17.7 0.3 0.3 0.3 0.3

biodegradable polymer (IBDP) in a blend with polyvinyl alcohol (PVA) or starch by heating in an extruder as described in reference 15. 2.) HP to IBDP Protein hydrolyzate (HP) obtained by above enzymatic process is ready for producing biodegradable material based on suitable synthetic polymer like PVA (polyvinyl alcohol). Although PVA is relatively poor in degradation under aerobic conditions, its modification with HP provides several advantages. HP modified blow extruded PVA film is a better biodegradable plastic and has special advantages. The function of HP in PVA is not only as a filler but it also makes the material more economical. This is because HP powder is 50 % cheaper than PVA. Moreover, in terms of biodegradability and mechanical strength this HP-modified material has improved features.

In producing IBDP blends were prepared by mechanical mixing of several components in different proportions. The components included PVA, HP, glycerol and stearin. A typical proportion of these components in IBDP film production is given in Table V. However this composition needs optimization to obtain the best performance of IBDP. Biogas Biogas production using process biotechnology requires the following: a. Substrate and Inoculum Chrome shavings/chrome sludge solid waste could be successfully used as substrate in anaerobic digestion for methane (biogas) production.26,27 The maximum rate of biogas production has been reported to be at a mixture of chrome sludge: inoculum: water = 3:1:1. The inoculum consisted of mixed anaerobic bacteria taken from a continuously operated anaerobic wastewater treatment plant (Zlin Malenovice, Czech Republic) and stored at 4oC under anaerobiosis. Before set up the biogas production experiments the culture slurry was sieved using a thin cloth filter and the supernatant was used as inoculum. b. Substrate treatment Raw chrome sludge was denatured for 30 mins by adding 1% (v/v) of suspension of MgO and heating at 85 oC. On JALCA, VOL. 99, 2004

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Figure 3. - SEM image of anaerobic bacteria involved in biogas production [Left view at 480x and right view at 4800x]

cooling to room temperature the pH of the suspension was adjusted using isopropic amine to 8.5 - 9.5. Then 1 % (w/w) of a commercial proteolytic enzyme (Alcalase, Novonordisk, Denmark) was added and enzymatic hydrolysis conducted together with good stirring at a temperature of 70 oC for 4 hours. Next, the hot mixture was filtered. The residue, a grey - green viscous matter was used as substrate. It had TS 20.9 %, VS - 17.5% and Ash-3-4%. c. Anaerobic Digestion (AD) The treated substrate obtained above was used in a small scale in different runs with various ratio of sludge - inoculum - water (4:1:0, 3:1: 1, 2:1:2 and 1:1:3). The SEM image of active anaerobic bacteria which were involved in AD in biogas generation is shown in Figure 3. The AD was carried out at 50 oC for 15 days and the maximum biogas production was 1 ml /g substrate (17.5% VS). From these encouraging results it was recommended that pilot scale studies for the production of biogas from chrome sludge could be carried out.16,26-27 Enzyme Enzyme treatment of leather industry waste to produce HP as described in the previous section has been reported to be a viable biotechnological process. However, to treat the large volume of waste produced in the industry, bulk quantities of enzyme are required. It demands that the allied enzyme production technology should not be high tech and expensive. Solid-state fermentation (SSF) process biotechnology has been stated to be suitable for the production of bulk enzyme at economical cost10 Thus in HP production from leather industry waste the enzyme availability may be from two sources: recovering the used enzyme and reusing JALCA, VOL. 99, 2004

it, or producing bulk enzyme by microorganisms through cheap SSF technology. Recyclable chromium The chrome tanning solution in the leather industry contains chromium(III) salts. In spite of introducing methods and agents that increase exhaustion of chrome salts, modern tanning technologies are not able to comply with the requirements of environmental protection. High levels of chrome in tannery effluents make the effluent treatment difficult and yield tannery sludge that is hazardous and requires special attention of disposal.19,28 Reported literature information indicates that the tanning industry generates about 60000 tonnes of solid chrome tanned wastes each year worldwide. Also, processing one- metric ton of hide generates 200-kg chrome-tanned leather 50,000 kg wastewater containing 5 kg chromium, 250-kg non-tanned solid waste and 200 kg tanned solid waste containing 3 kg of chromium.29 This chromium in the effluent is in trivalent state and is nontoxic. However, after using the process biotechnology of enzymatic hydrolysis of chrome sludge the chrome cake obtained as a residue is further treated chemically as per reported method 30 to give a recyclable chrome cake containing primarily trivalent chromium, which is non toxic. Chromium cake contains protein or resin which make difficulties in subsequent solution in sulfuric acid and for this reason it can be removed easily by chemical treatment for chromium recycling process as described in reference.15 Biofertilizer/Compost If HP is available in excess or HP-based biodegradable polymer film needs disposal one of the simplest way of its disposal has been stated to be the utilization in agriculture as

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Figure 4. - Closed loop chromium recycling in beam house processing of leather

an organo - nitrogenous fertilizer (Figure 1). With the cooperation of the Agricultural Control and Testing Institute of the Czech Republic, Regional Department of Soil Agrochemistry and Plant Nutrition, the suitability of enzyme process biotechnology generated HP and biodegradable polymer as biofertilizer was checked by conducting a vegetation test. The Czech Institute ran a comparative test on HP, commercial name EKO-N-HP, and available commercial fertilizer LOVODAN, which is a blend of ammonium nitrate and urea in a 1:1 ratio. The tested crop was lettuce. Both fertilizers were fed in such a way that the mass of nitrogen for all tested plants remained the same. A simultaneous comparative test on unfertilized soil was also conducted. This vegetation test result is given in Table VII and indicates a substantial difference in the yields and nitrate contents in the consumable parts of lettuce.

TABLE VI Comparative Vegetation Tests on the Suitability of HP as a Biofertilizer15 Type of fertilizer LOVODAN EKO-N-HP Unfertilized

Yield (g/test) 187 164 105

Nitrate content (ppm NO3) 519 23 52

As the vegetation tests showed positive effect of EKO-NHP as biofertilizer on the growth and development of the agro food product, its use was extended in agriculture. It was also successfully tested in an effort to avoid the thinning of growing plants consists in employing sowing tapes

bearing firmly fixed, specially treated seeds at optimal distance in biodegradable foil tape. Machine sowing requires that the sowing tape has good mechanical properties and meets soil conditions for optimal seed sprouting. PVA mixed with 20-40 % HP and processed by blow molding can provide modified foil having higher mechanical strength and good composting characteristics. In addition, organic nitrogen from this biodegradable foil being slowly liberated acts as a fertilizer and creates a favorable soil microclimate for growth of sprouts. For this reason the largest volume of HP is channeled into agriculture as an organic nitrogenous biofertilizer.

CONCLUSIONS Process integration of leather manufacturing industry has high potential for larger economic return through enzymatic retrieval of biodegradable polymer (BDP) and other value-added products using process biotechnology. The retrieved hydrolyzed protein (HP) and BDP have already gained lots of agroindustrial applications in different countries. Biogas generation by anaerobic digestion (AD) of chrome sludge has also been shown to have a high potential in terms of a low cost and ecofriendly process biotechnology. However, it needs further studies on scale up for industrial processing. Recyclable chromium, enzyme and biotransformation of HP and IBDP as biofertilizer/compost have also great potential to provide higher economic return to the leather industry. Finally, it may be said that in enzymatic process biotechJALCA, VOL. 99, 2004

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nology the enzyme reaction provides advantage for the production of HP of a relatively good quality and chrome sludge. In turn it has a place in the treatment of chromium containing tannery waste by AD using microbial process biotechnology that gives value-added product - biogas. Thus the funds that are expended in this area of research having brought satisfactory field results definitely support the idea that the process integration approach by total recycling of leather industry wastes using process biotechnology is a profitable business. However, its thorough economic evaluation is necessary.

ACKNOWLEDGMENTS The authors acknowledge with great appreciation the financial help received through the grant of Ministry of Education of the Czech Republic (Grant No: MSM 265200015). Also the technical help received from Dr. Kishor Goswami and Mr. Marek Bouma, in developing the PC generated figures is gratefully acknowledged.

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