1.1 SUMMARY AND OVERVIEW
Over the past few decades, increasing importance has been attached, by all branches of industry worldwide, to the ecology alongside economy and quality considerations. The leather industry is no exception. The relationship between price and quality used to be the dominant consideration in times of rapid industrialization, but consumers and manufacturers have since become much more conscious of health, safety and environmental protection issues.
Tanneries can only survive in the future if they manufacture leather of the required quality, discharge treated effluents that comply with authorized limits and appropriately manage the disposal of wastes. Since the advent of legislation prohibiting the untreated tannery waste disposal emanating from leather manufacturing operations, pressure has been put on tannery related personnel.
Continuous observations from past few years have introduced various types of wastes producing from tanneries and their sources. From raw stock treatment to final finished leather there emanated a huge amount of wastes.
Different altered and surprising parameters are found which suggest the planned and successful systems to recover or treat these vast effluents. Contributions of several organizations as well as individuals worldwide have also introduced us various fruitful treatment methods which can verily relieve or minimize the load of a series of waste products.
However, all these treatments though can easily be practiced in the developed countries; we can not make a best use of them all for the lack of our limited and broken economic infrastructure. Yet the proposed methods and systems stated in this book can once be effective in course of time in future when tanneries will be set up as per environmental safety issue.
The tanneries in Hazaribagh have already been proposed to be transferred in Savar. Universally accepted treatments can be practiced as much as possible then.
Suggestions have also been included here for a typical small and large Tannery as per their location and criteria of systematic waste management,
Tanning hides and skins is being practiced from ancient time which has become an inevitable industry in this modem age of glamour and fashion as because there is no material in the world like natural leather in the midst of variety of synthetic and artificial materials which can be used to make outstanding goods specially dresses, bags and foot wears.
But as a matter of irony, tanneries are isolated from human habitations as for their characteristic odor and mode of pollution since a tannery deals with tanning raw hides where a series of chemical operations are undergo to convert putrefying raw hides into non putrefying leather.
There is no single process for producing leather. Depending on the circumstances prevailing, different options for unit operations will be used. Accordingly, different wastes will be emanated and different possibilities will exist for reuse or conversion of residues as waste management systems.
The potential environmental impact of tannery effluents is widely acknowledged. It has a long-term negative impact on the growth potential of a country. From an economic viewpoint the discharge of residues is a waste of scarce resources, whether chemicals, energy or raw materials.
1.3 HISTORICAL BACKGROUND OF WASTE MANAGEMENT
The historical development of waste treatment and disposal has been motivated by concern for public health. The industrial revolution between 1750 and 1850 led a massive expansion of the population living in towns and cities migrated from rural areas which gave rise in the volume of wastes. This led to an increasing awareness of the link between public health and environment.
To deal with this potential threat to human health, legislation was introduced on a local and national basis in many countries. For example in the UK, throughout the latter half of the nineteenth century, a series of Nuisance Removal and Disease
Prevention Acts were introduced which empowered local authorities to set up teams of inspectors to deal with offensive trades and control pollution within city limits.
Following the Second World War, a series of incidents in the late 1960s and 1970s highlighted waste as a potential major source of environmental pollution. A series of toxic chemical waste dumping incidents led to increasing awareness of the importance of waste management and the need for a more stringent legislative control of waste-Besides, the massive adverse publicity and public outcry led to pressure for the problem of waste disposal to be more strictly controlled by the legislature. Still now environmentalists are researching and inventing newer cleaner technologies to Minimize and reuse wastes. In Bangladesh also waste minimization techniques are thought to be launched though it was negligible in previous times. But some tanneries like Apex and BATA have already started these projects in their leather industries.
1.4 DEFINITIONS OF WASTE
The definition of waste can vary as what represents waste to one person may represent a valuable resource to another. It is clear from several standpoints that accurate definitions and classifications of waste are required. By its very nature, waste is a heterogeneous material and difficult to describe, define and classify. In many instances the waste will be a mixture of different types, or may be on the border between two categories.
However, the term ‘waste’ can lexically defined as any material that has no use and is thrown away. (Oxford)
According to Environment Act 1995, a waste is defined as any substance or object which the holder discards or intends to discard. A ‘holder’ means the producer of the waste or the person who is in possession of it. This Act sets out different categories of waste to which the definition of waste applies.
- Production or consumption residues not otherwise specified below.
- Off-specification products.
- Materials contaminated or soiled as a result of planned actions.
- Residues of industrial processes.
- Residues from pollution abatement processes.
- Residues from raw materials extraction and processing.
- Any materials whose use has been banned by law, etc.
Some particular waste definitions are stated below according to the EC Waste Framework Directive (91/156/EEC):-
Household, industrial and commercial wastes those can be controlled by any means are known as controlled waste.
Some of wastes emerged from industrial, commercial or agricultural Sources are known as uncontrolled waste.
The list of generic wastes which if they contain properties is rendered as hazardous, such as, corrosive, toxic, reactive, carcinogenic, infectious, irritant or harmful to human health and environment.
1.5 CLASSIFICATIONS OF WASTE
As stated earlier the classification of waste is difficult since there can be great variation in composition between different loads of waste, However, according to Porteous (1992) waste can be classified as their-
- a) Origin- e.g., household or urban solid wastes, industrial wastes;
Form- e.g., liquid, solid, gaseous, slurries, powders;
- Properties- g., toxic, reactive, acidic, alkaline, volatile,
- Legal definition- g., special, controlled, industrial, commercial.
According to the Development of a National Waste Classification Scheme 1995 (UK) waste is classified into six categories:-
- a) The industry producing the waste:
What kind of industry is taken under consideration includes this type.
- b) The process producing the waste:
The mode of processes involved in generation of waste includes this category.
- c) The waste category:
The key area of flu’s classification is the waste description and detailed composition.
- d) The properties of the waste:
This includes the properties like explosives, oxidation, irritation, toxicity, infection, odour, etc.
- e) The physical form of the waste:
This includes whether the waste is solid, liquid or sludge etc.
0 The information label:
The labels like S, special waste; H, hazardous waste; P, packaging waste; C, clinical waste show their information.
1.5 Beam house operation:
“The term “Beamhouse” refers to the processes in the tannery between the removal of the skins or hides from storage and their preparation for tanning. This includes trimming, soaking, green fleshing, unhearing liming, lime fleshing, lime splitting and trimming, deliming, bating, pickling and chrome tanning. The term dates back to the time when the hair was removed from the skins by means of a hand beam, i.e. on a sloping, curbed table or large fog using a two-handed knife. This working if the most modern and sophisticated facilities, some hand work on the beam is needed occasionally for quality improvement.
The beam house operation has the distinction of being the most disagreeable step in learner manufacture. It involves the use of bad smelling concoctions which have been responsible for much of the poor name of the leather industry in its community relations. The beam house operations are also of tremendous importance in the ultimate quality of the leather. Indeed, in the opinion of most practical tanners, ‘leather is made in the beam house/ Beam house operations also employ complex principle of biochemistry and inorganic chemistry and are the most difficult areas of leather manufacture of the uninitiated to understand. Practical beam house operation, however, can be reduced to relatively simple steps, and good quality leather can be made by close attention to detail and empirical observation of cause and effect.
1.6 SOURCES, QUALITY AND CHARACTERISTICS OF BEAMHOUSE WASTES
The typical wastes generating from beam house can be categorized as three significant forms:
- a) Solid wastes
- liquid wastes and
- c) Gaseous wastes
1.7. Waste generated during Beam house operation from tanneries in Bangladesh
|Liquid Waste||Process Sequence||Solid/Gaseous Waste|
|Blood flesh, proteolyses and unused Sodium Chloride||Soaking||Flesh, hair etc.|
|Unused Calcium Hydroxide, Sodium Sulphide and Sodium bi sulphide||Unhearing and Liming||Fat and flesh, hair
|Unused Sodium meta-bi-sulphide, Sodium sulphide, salt of Ammonia, Alkali etc.||Deliming and Bating
|Sulfur-di-oxide and Ammonia gas.|
|Unused Sulphuric acid. Formic acid. Sodium Chloride.||Pickling||Chlorine.|
|Unused Chromium sulfate, Sodiurn-bi-carbonatc, Sodium carbonate and Sodium formate.||Chrome Tanning|
Methodology for the production of Shoe upper from wet blue cow hide
Wet blue cow hide two pcs
Samming by samming machine
Splitting by splitting machine (thickness-1.5 cm)
Trimming by hand knife.
Shaving by Shaving machine (thickness-1.1cm)
Take shaved weight. (All based on this shaved wt)
Name of Operation %of chemical used Time Analytical checking.
Wet Back: 200% water
0.2% Supratan UF (wetting agent)
0.25% Atlasol 177C ( Fat) Run 45’
: Drain wash
100% Water at N.T
0.2% Formic acid Run 10’
Check PH 3.3
+ 2.0% Relugan GT-5 Run 30’
+ 1.0% Black PNT Run 10’
+ 8.0% Chrome powder
1.0% Atlasol 177C Run 30’
+ 2.0% Chrome syntan Run 60’
+ 2.0% Relugan RF Run 30’
+ 3.0% Sod. formate Run 30’
+ 1.0% Sod. Bicarbonate Run 60’
Check PH 3.8-3.9
Leave 0/N. in the bath
Next day Drain wash well
150% Water at 450C
+ 2.0% Sellasol NG
0.3% Sod. Bicarbonate Run 60’
Check PH 4.5-4.6 Drain wash
Retanning & Dyeing-
100% water at 450C
4.0% Relugan RE Run 15’
+1.0% Ingrassante VFM Run 15’
+ 4.0% Intan 806
4.0% Basyntan N
4.0% Butan 7813
2.0% Dye stuff Run 90’ Check penetration.
2.0% Butan 7818 (Filler) Run 10’
3.0% Ing. VFM
1.5% Ing. VA2
1.0% E-123 (Natural oil)
0.05% Biocide C-3 Run 45’
+ 2.0% TP-340 ( Resin) Run 30’
+2.0% Formic acid Run 10+30’
200% Water at 500C
1.5% Dye stuff
1.0% Chrome Run 20’
+ 1.5% F. acid Run 20’
+ 0.25% Cat S Run 10’
Check PH 3.6/3.7
Drain wash, Hours up o/n
Basic Finishing Procedure
Metal complex dye(liquid) 200p
Spray 2x , dry well.
Polymer binder 150p
Casein binder 30p
Spray 3x .dry well
Polymer binder 100p
Spray 3x .dry well
Formaldehyde (30%) 300p
Acetic acid 50p
Spray 1x .dry well
P.U lacquer component 100p
Organic solvent 150p
Cross-linking agent 50p
Spray 2x .dry well
Different types of wastes with their salient features are discussed below-
Soak liquor contain soluble proteins like albumin and are a major source of proteolytic and other bacteria. Suspended matters like dirt, dung and blood adhering to the hides and skins are discharged intermittently with the soak liquor. The salinity of soak liquor is very high varying from 15,000 to 20,000 mg/L (as Cl) soak liquors undergo putrefaction very rapidly since. They contain a large amount of impurities and a favourable pH for the growth of bacteria. The biochemical oxygen demand (BOD) of the soak liquor is usually between 1100 to 2500 mg/L and the quantity of the soak liquor discharged will be 250 to 400 litres per 100 kg of hide or skin tanned.
Lime liquors contain suspended and dissolved lime and sodium sulphide. The discharged of the spent lime liquor in intermittent and its average quantity is about 650 to 1000 litre per 100 kg of highly processed. It is highly alkaline and is one of the heaviest of the fractions in terms of biochemical oxygen demand (BOD) and suspended solids. This waste contains high concentrations of sulphides when sodium sulphide is used in the liming process. This waste contains high ammoniacal nitrogen content. The BOD varies from 6000 to 9000 mg/L.
Unhairing and fleshing effluent:
The effluent from the unhairing operation is more or less continuous and contains mostly air and sulphides, fleshing. This operation gives rise to an effluent which is also more or less continuous and contains fatty and fleshy matter in suspension.
Spent deliming which are discharged as waste also carries a significant pollution in terms of BOD, The BOD of this waste varies from 1000 to
2000 mg/L. The quantity of waste discharged is about 700 to SCO liters per 100 kg of hide processed
Estimated volume 3-7 M3/1000 Kg
Parameter mg/L Kg
Total solid 40.000-50,000 200- 230
Solid) 5,000 -10,000 25 – 50
Total Dissolved 30,000 – 40,000 50 – 200
Biochemical Oxygen 1,200-2,000 6-10
COD 3,000-5,000 15-20
Alkalinity 1,000-1,500 5-7.5
Sulfide None none
Chromium None None
Spent bate liquor:
It has a high amount of organic mailer and ammoniacal nitrogen due to the presence of soluble skin proteins and ammonium sails used in bating BOD is usually low.
Spent pickle liquor:
Chrome tanning effluent-
The spent chrome liquor is greenish in colour and highly acidic. The waste contains high concentrations of trivalent chromium (Cr3+) ranging from 100 to 200 mg/L. Hexavalent chromium is not generally present in the waste chrome liquor because of the reducing agent used and one bath process utilized. The average discharge of the waste chrome tan liquor is about 400 to 500 litres per 100 kg of hide or skin tanned. BOD of the waste is usually about 1000 mg/L.
CHROME TANNING WASTES
Estimated Volume 3-7 M3/1000kg
Parameter mg/L Kg
Total solid 30,000 – 60,000 150-300
Total Suspended Solid 1,000 -1,500 5-11.5
Total Dissolved Solid 9,000-37,500 145 – 287
BOD 400-800 2-4
COD 1,000-2,000 5-10
Oil and Grease 600-1200 3-6
Acidity 2,000-5,000 10-25
Chromium 2.000-5.000 10-25
Sulfide none none
The composite effluent from the tannery is highly colored and bad smelling. It is highly alkaline with high amount of suspended and dissolved impurities. BOD of the effluent varies from 2000 to 3000 mg/L.
VEGETAIILE TANNINC WASTES
Estimated volume 3 – 6 M3/ 1000 Kg
Parameter mg/L Kg
Total Solids 25,000 – 60,00O 125 – 300
Total Suspended Solid 5,0000 – 10,000 25 – 50
Total Dissolved Solid 20,000 – 50, 000 100 – 250
Biological Oxygen Demand 6,000- 18,000 30 – 90
Chemical Oxygen 15,000 – 40,000 75 – 200
Oil and Grease 200-400 1-2
Acidity 2,000 – 4,000 10-20
Chromium none none
Sulfide none none
Composition of typical unreacted combined tannery effluent . Units are mg/L unless otherwise indicated (Source – UNIDO)
pH 9 9
total solids 10000 10000
Total ash 6000 6000
Suspended solids 2500 1500
Ash in suspended solids 1000 500
Settled solids 100 50
BODs 900 1700
Potassium permanganate 1000 2500
COD 2500 3000
Sulphide 160 160
Total nitrogen 120 120
Ammonia nitrogen 70 70
Chloride 2500 2500
Sulphate 2000 2000
Phosphorous 1 1
Ether extractable 200 200
1.7 WASTES AND THEIR IMPACT ASSESSMENT ON environment-
Though it is a very common trend all over the world that tanneries are the culprits of producing strict odor, today the impact of their wastes discharged is the most concerning matter. It should be noted that in high density industrial areas, the environment can no longer assimilate some natural substances such as nitrogen and phosphorous. For aquatic life this is a serious problem for eutrophication. This level of wastes as effluent requires immediate treatment as being urged by the environmentalists upon the tanners.
Now we will take a look over the impact assessment of various wastes on
1) Effects on Land:
When the tannery waste gains access to cultivable lands or when the land Is irrigated with such waste, losses its fertility. The effluent may. change the characteristics of the soil and also may interfere with the water uptake of the plant. The presence of chromium in the effluent influences the metabolic activities of the plants, resulting into reduced yield.
2) Effects on Surface Waters:
The disposal of effluents from a tannery into any water coarse interferes with the physical, chemical and biological characteristics of the receiving water. The high chloride and sulphide content of the waste affect the quality of the water courses and impart bad taste and odour. The presence of suspended matter as lime, hairs, flesh etc makes the water turbid. Due to the excessive organic content of the waste, it exerts a high BOD demand depleting the oxygen content of the receiving water. The colour of the waste is highly persistent and presents an unsightly appearance. The alkalinity, high pH and the presence of sulphides are detrimental to fish and other aquatic life. Due to conversion of Cr3+ to Cr6+ the waste makes the receiving water much more toxic.
3) Effects on Ground Waters:
Tannery waste may gain access to ground water strata through percolation, when the raw waste gets stagnated or is used for irrigation. The washings from the waste salt dust dumps outside the tanneries and the soak and pickling dump waste cause increased salt content in the ground water. Liming and Deliming wastes cause increased hardness.
4) Effects on Sewers:
Tannery effluent is known to cause depositions of calcium carbonate inside a receiving sewer and choke it. Lime is converted to calcium carbonate by the carbon dioxide produced by decomposition of the organic matter present in the effluent and the hair and fleshing help to form a binder with the calcium carbonate which finally where and build up gradually on the inside surface of sewer and ultimately choke it.
5) Effects on Air Quality:
Biological decomposition of organic materials as well as sulphide emissions from waste waters, are responsible for the characteristic objectionable odours from tanneries. Ammonia emissions from unhairing and delime liquors and fleshings are also important potential sources of odour.
6) Waste Dumps:
Industrial waste dumps containing hazardous chemicals are highly noxious owing to odorous wastes. The presence in dumps of unwashed containers from industry may result in the poisoning of people who try to reuse such containers.
7) Other Effects:
A variety of other minor effects such as air emissions from open burning or other operations and unsightly visual impacts may unnecessarily make
The tanning industry a bad neighbour.
1.8 WASTES AND THEIR IMPACT ON HUMAN HEALTH:
Direct contact with some industrial chemicals can cause disability, illness and death. Even relatively minor exposures, if they occur frequently, can eventually build up to toxic levels. Following health hazards may occur in tanning industries in Beamhouse operation :
- Skin Afflictions
Some serious health hazards due to chemical wastes from tanneries in Beamhouse operation
|Health Hazards||Responsible chemicals||Diseases|
|1)Chemical poisoning||Sodium sulphide, caustic solution, mineral acids, certain solvents etc||Irritation, allergy, nausea, vomiting to unconsciousness|
|2) Skin affliction||Salt, sod. Sulphide, caustic solutions, chrome powder, certain dyes, preservatives||Rashes, itches, redness, swelling|
|3) Inflammation and burning of lungs||Chrome powder, pigments, shaving dust, etc||Bronchitis|
|4) Carcinogens||Basic chrome powder, preservatives, benzidine based dyes||Cancer|
1.9ENVIRONMENTAL PARAMETERS WITH RESPECT TO TANNERY WASTES AND THEIR EFFECTS:
There are certain parameters which go with the environmental influence by means of those we can assess the lethal impact of the wastes on our environment as well as our health.
The high chloride and sulphide content of the tannery waste affect the quality of the water and impart bad odor. Organic impurities in tannery effluents give off strong odors which considerable smell nuisance. Strong odor sometimes causes vomiting to the passers-by.
Tannin substances, sodium sulphide, dye stuffs etc, give particular colour 10 the receiving water. The colour of the waste is highly persistent and presents an unsightly appearance. Colour imparted to receiving water by such tanning substance, dye stuff etc. is not tolerated by the public who use this receiving water for drinking and bathing purpose,
- SUSPENDED SOLID;
Due to the heavy suspended solids present in tannery effluents sludge banks are formed in the receiving stream and present an unsightly appearance. The suspended matter gets deposited on the bed of the stream and kills aquatic organism in the stream bottom. The floating solids may interfere with the streams ability for self purification by re-oxygenalion, by causing mechanical interference with the absorption of atmospheric oxygen and by interfering with the photosynthetic activity of the streams plankton and aquatic plants. Turbidity caused by the suspended solids may cut down the penetration of sunlight into the water thus reducing the photosynthetic activity.
IX DISSOLVED SOLIDS:
- Sodium chloride:
Chloride is one of the important indicators of pollution. Common salt adds to salinity of water. The salt concentration level more than 2100 mg/L is harmful to plant life. Salt reduces the fertility of agricultural land. The salt build-up in soil exceeds the trees or crops salinity tolerance, reduction in growth or even death. The high content of salt in sludge might prevent its use for composting and soil conditioning. Also the accumulation of Na in the soil can cause deterioration in soil physical properties especially in porosity and permeability to water. Chloride corrodes metal parts also. High concentration of sodium affects the fertility of the soil. This is called “Sodium hazard” to the soil.
- Chromic oxide:
The soluble chromium salt present in the tannery wastes exert an inhibitory action on the activity of fish and microorganism present in the stream and also render the water unsuitable for domestic or other purposes. Chromium which is directly toxic to biological life of the stream may inhibit bacterial activity and thus delay the decomposition of organic matter and may cause profound changes in the content of native flora ‘and fauna. High concentration of chromium in soil might inhabit the germination of seeds and retardation in the growth of plants. The Merck Index (1989) states that chromium (III) compounds are generally non-toxic, non irritant, non carcinogenic and immobile. But hexavalent chromium is highly toxic. Cr3+ is converted to Cr6+ by oxidation in the process of heating or by means of enzymes or by favorable pH condition. Chromium can also be oxidized in an environment or during drinking water chlorination. Hexavalent salts of chromium are irritants, corrosive and in some instances carcinogenic. All chromium compounds- are sensitizers and may cause contact dermatitis, some may be a cause of occupational asthma. Long term exposure to chromium compounds may cause1 perforation of the cartilaginous nasal septum and occasionally lead to chromic rhinitis and chronic bronchitis.
Hexavalent chromium compounds also cause coughing, wheezing, aspirator, pain, fever and loss of weight. Prolonged skin contact may lead to local irritation and if skin damage is extensive, sufficient compound may be absorbed to cause renal damage and death. Cr6+ is carcinogenic which can cause lung cancer and sino nasal cancer. Various Cr6+ compounds induce a variety of gene-toxic effects, including DNA damage, point mutation system, chromatid exchange, chromosomal aberration, aneuploidy, germ line mutation, Cr3+consumes oxygen when converted into Cr thus reduces dissolve oxygen in water.
can readily be reduced to sulphide by an aerobic bacteria in sludges and liquid effluents. The presence of Na and MgSO4 In drinking water beyond the prescribed limit may cause cathartic action. Sulphate destroys concrete,
- Sulphide content:
Sulfides are odour producer and toxic if the concentration exceeds 5 mg/L. They combine with metal to form black precipitate rendering unsightly appearance. Sulphide is the gaseous form is an asphyxiant, Toxicity of hydrogen sulphide gas is
similar to hydrogen cyanide. Sulphide poisoning may cause death to the people. Hydrogen sulphide gas affects the human nervous system, It can cause respiration difficulties, bronchitis, skin disease etc. Sulphide is also a known corrosive element eating the metals. Crown corrosion, incrustation and blockade of pipes are consequences. Micro organisms capable of reducing organic matters are also destabilized and biochemical decomposition arrested by sulphide exceeding 200 mg/L .
Tannin both synthetic and vegetable are toxic if present in the effluent in high doses. Toxicities on Tubules, Daphnia and Assails species are 200 mg/L for vegetable tannin and 1-10 mg/L for syntans
The inorganic soluble chemicals like acid, salts, alkalis etc. change the pH of the receiving water which brings the aquatic condition unfavorable for aquatic life. Alkaline pH affects the aquatic population adversely. Acid condition of the receiving stream will liberate H from dissolve salts-Effluents with low pH will corrode metals and concrete in sewerage system and have a sterilizing effect reducing inhibiting microbial activity,
Total acidity and free mineral acidity:
Acidity of the effluents lowers the pH of the receiving water which can cause the death of aquatic life. Strong acid like sulphuric acid causes wounds on the human skin and may cause cancer ultimately. Weak acids both organic; and inorganic are more toxic than strong acids. Weak acids can kill fish even if the pH of water is within the safe range 5.0-9.0.
BOD (Biological Oxygen Demand):
BOD of tannery wastes represents their pollution potential. The tannery wastes are characterizer by a high BOD. High BOD usually indicates high concentration of organic matter. If wastes containing high BOD are discharged into a stream, it results in development of the growth of masses of sewage fungus that cover the bed and sides of the stream and prevent growth of animals and plants which serve as food for fish.
- COD (Chemical Oxygen Demand):
COD represents chemically oxidisable load of organic matter in water. High COD of receiving water indicates the high strength of organic matter and low DO of water. In high COD aquatic life can not exist high value of COD in the pollutants can starve aquatic animals,
Toxicity of nitrogenous compounds is considered to be one of the most serious problems in the present aqua culture. High levels of ammonia can results in nitrite which is toxic to fish- Ammonia gas causes headache, nausea and drowsiness. Ammonia salts are harmful for the reproduction of fish. Ammonia- nitrogen content stimulates growth in plants and causes water weed problems. High level of nitrogen in water is also deleterious to health. When people inhale ammonia gas it inflames upper respiratory passage.
WASTE MANAGEMENT SYSTEMS IN BEAMHOUSE OPERATION
2.1 CLEAN TECHNOLOGY- REMEDIAL STEPS FOR BEAMHOUSE EFFLUENTS-.
Clean technology in a tannery is a coordinated effort of planned processes for proper disposal, reduction or recycling of wastes created at different stages of leather production. It involves the following leather processing techniques:
- Disposal of liquid waste
- Cleaning the solid waste found in the liquor
- Filtration and reuse of lime liquor
- Treatment and use of lime liquor
5} Use of chrome liquor, split and shaving
6) Disposal of organic soluble and other vapours.
The choice of treatment system depends on the location of the plant with respect to neighboring land uses. If several tanneries exist closely, a cooperative treatment operation will lower the cost of treatment of each tannery. The Final choice depends on the tannery and discharge standards.
Specific technological choices will fall under the following headings:
- Pre-treatment, consisting of mechanical screening to remove coarse
material, Primary treatment, which includes sulphide removal from
beamhouse effluents; chrome removal from tanning effluents; flow
equalization; physical- chemical treatment for BOD removal and Neutralization.
- Secondary treatment usually biological.
- Tertiary treatment, including nitrification and denitrification.
- Sedimentation and sludge handling.
- Pre-treatment Screening
Prior to treatment it is essential to screen all effluent streams to remove the large fragments which needs regular cleaning and maintenance. Up to 30-40% of total suspended solids can be removed by a properly designed routing screen.
A preliminary settling operation can remove up to 30% of COD,
Flow equalization (balancing)
Flow balancing and combining of the effluents are needed to deal with peak flows. After sulphide and chrome removal, liquor of volume of less than 30% is taken in equalizing tank. As equalization promotes neutralization and precipitation, liquors are well-mixed.
- Primary treatment technologies
In catalytic oxidation of sulphides, Mg catalyst is used in aeration. Approximately 60 Quebec meter of air is needed for each Quebec meter of effluent. Bui this is not a suitable process. In Direct precipitation, ferrous sulphate and ferric chloride can be used to remove sulphides from solution.
Removal of Chromium
Here major chrome bearing liquors are used to precipitate. To obtain an almost chrome free supernatant, pH should be greater than 8 taken in a sedimentation vessel which is further treated for BOD reduction.
Physical-chemical treatment for BOD and solids removal relatively simple technology allows the removal of up to 95% of suspended solids and around 70% of BOD through this treatment. Ferrous sulphate is advantageously used in this method.
- Secondary treatment technologies
In case of high effluent quality requirement, a secondary treatment is necessary. The choice of treatment systems will be among:
- biological filters
- activated sludge (oxidation ditch)
- activated sludge (conventional)
— lagooning (aerated, facultative or anaerobic).
- Tertiary treatment technologies
Water may require reduction of the nitrogen load discharged by large installations. Where replacement of ammonium salts is not feasible, nitrogen removal through nitrification or denitrification will be needed. In sedimentation and sludge handling horizontal and vertical flow tanks arc used. Sludge from primary sedimentation may be only 3-5% solids and a clarification efficiency of 95% gives a removal of 1250 Kg/day of dry solids. Dewatering also reduce sludges.
- Best treatment option
The best treatment option under any given circumstances depends on both technical and economic factors. The available technologies economically achievable by new plant are —
- Recycle of chrome and vegetable tanning solutions
- Fine screening
- Sulphide oxidation
- Chrome removal
- h) Sludge handling and disposal
2.2 OPTIONS FOR CLEANER TECHNOLOGY IN LEATHER PRODUCTION:
The minimization of pollution load due to leather processing can be summarized by prevention, reduction, recovery, reuse and regeneration. The first approach is to address the source of generation by using or developing cleaner process. In feather industry, the use of some sorts of recycling systems has already begun especially for economic reasons. Nowadays, nearly every part of the tanning process has several cleaner and waste water management systems.
Possible technological solution for clean technologies in leather production
|Preservation||# Green processing
# Drying # Dry pickling
|Green fleshing||Best when fleshing rendered at abattoir|
|Soaking||# Mechanical desalting to reduce salt content in waste water. # For surfactants alcohol ethoxylates replace nonylphenol ethoxylales|
|Unhairing||Sulphide reduced liming to minimize sulphides and COD content. Direct liming floats or hair saving unhairing used in bio filter systems.|
|Deliming||# Reduction of nitrogen content in waste water by adopting ammonia free operations like carbon dioxide deliming. # Elimination of hydrogen sulphide.|
|Pickling||# Salt free pickling * Recycle of pickle liquor.|
|# Reduction in chrome wastage by direct liquor recycle process, # Precipitation and reuse process # High exhaust system # Best result for combining both direct and precipitation for reuse, # Split limed hides can be used in chrome waste reduction, #More effective fungicides are needed.|
|Alternative tannages||# Gluteraldehyde tanning|
|# Vegetable tanning
#Organic tanning materials
|Retnning, dyeing. Falliquoring||# High exhausting
Use of non toxic dyes
# Amphoteric polymers are very
|Finishing||# Aqueous systems
#Equipments to reduce wastage
# Less toxic cross linking agents
# Use of non toxic dyes and
|Utilization of waste||1 Green flesh: fertiliser and
compost 2) Chrome shavings: isolation of chromium and collagen and their reuse.
3) Leather wastes: Incineration
However, it must be noted that the above mentioned options are just annexed at a glance and there are so many choices of clean technologies worldwide as newer and modified methods are being invented and practiced every day by either any organization or an individual,
2.3VARIOUS WASTE MANAGEMENT SYSTEMS IN
THE TANNING PROCESS (BEAMHOUSE):
TREATMENT OF SULPHIDE WASTES BY PRECIPITATION:
In any unhairing system using sulfides the concentration of the sulfides in the combined spent solutions will be near 1000 mg/1 more or less depending on the details of the system. Because of dangers of toxic accidents from sulfides3 treatment for the destruction of the sulfides is strongly advised even if not required by the Local regulations.
The most common method used in the industry is the oxidation with air using a manganese catalyst. All of the sulfide bearing wastes are collectedg wastes, and pumped into a treatment tank. The catalyst is added to the tank The amount is not critical, but 5 -1 Kg of Manganese Sulfate pr M of combined wastes is normal. In any system the optimum cost effective quantity of catalyst should be done by performance tests.
The reaction is the same as has been described in the addition of sulfide bearing wastes into a river the difference is that the reaction is speeded by the catalyst and the oxygen is replenished by the spurges.
Theoretically, the amount of oxygen needed is half the weight of the sulfide to be oxidized- The actual efficiency of the reaction will be somewhat less than 30 %. An example of the calculations is as follows; –
The reaction tank can be steel corrosion protected or concrete, and should have a volume twice the expected volume of wastes to be treated. The freeboard of this open topped tank is desirable to prevent overflow due to foam formation. The air is introduced into the reaction tank through a pipe and spargers, as in secondary treatment. A simple pipe with holes about .5 mm in diameter is often used in small reactors. The better the spargers system the more efficient the oxygen transfer and the shorter the reaction time needed. In most systems about four hours are needed to bring the concentration of the sulfides to below 5 mg/1. To reach less than 1 mg may take assume that the combined sulfide bearing wastes from 1000 K.g ot hides is 20 cubic meters containing 5Kgof sulfide (S ion).
The reaction tank- would be 2 X 20, or 40 cubic meters. With a depth of the solution of 2 M. the tank could be. 3M X 3. X 4M this would give the desired freeboard and allow for some over-loading in an emergency
The Oxygen required would be 2.5 Kg. Since air is about 20% oxygen, 12.5 Kg of air would he needed. The efficiency of the system will be, at best, 25-30%. The design should provide for 40-50Kg of air. Air weighs L2 g per liter so 40,000 L of air in 4 hours is a reasonable design level. The air rate would be 10,000 per hour or 166 L 1m,The efficiency of a system cannot be precisely determined in advance so some over engineering is advised-Completion of the oxidation can be qualitatively observed with Lead Acetate papers. For quantitative data the Iodine titration as described in the methods section of this handbook can be used. It is necessary to test all batches for completion of the reaction. Common practice is to collect all of the sulfide wastes for the day into one batch rather than treat several small batches. The treated lime wastes may then be mixed with other tannery wastes to neutralize acids and aid in precipitation of the combined wastes.
One of the problems often encountered with treated tannery wasted is that there may be a continued breakdown of the organic and inorganic sulfur compounds with the formation of additional free sulfides. Samples to be analyzed must be stabilized by the addition of Zinc Acetate if the analysis is not to be done immediately. The delayed release of sulfides is usually only a very small amount and seldom causes a problem in the receiving stream or the municipal collection and treatment system.
In the following figure-1 we can have a clear idea on the mechanism cited above:
RECYCLING CHROME TANNING MATERIALS BY PRECIPITATION:
In this most common method of chrome recovery, the separation of the precipitated chromium from the solution is usually done by decanting the supernatant liquid which is drawn off and chromium hydroxide slurry pumped to a dissolving tank. The precipitation with calcium or magnesium hydroxide works for the formation of a precipitation that will settle well. The next figure-2 shows an illustration on the mechanism of the chrome recover, through precipitation:
Chrome Recycling Drum System
A WASTE WATER TREATMENT:
A prescribed treatment for waste water has been elucidated below by a block diagram where the waste water storage is treated through mechanical, chemical and biological treatment consequently. After any of the operations, sludge is come out at the end. Separation, preclarificalion, setting and removal of sediment take place in mechanical operation while colloidal removal with pit adjustment is added with chemical operation. In biological treatment through biochemical degradation main drain is discharged after clarification.
Now we will observe the figure-3 which will depict the above process-
WASTE WATER TREATMENT:
Juwel er ETP plant set korte hobe
AN ETP PLANT
Screening (Bar screening)
At first the raw effluent come from the process section through the screen chamber to equalization tank. The suspended coarse solids are separate from waste water by the bar screen
Fig. Bar screen chamber
Equalization and Aeration: The effluent is then passed through a grit chamber and collected in equalization tank provided with submerged ejector aerators. The aerators sucks air from atmospheric medium and passes into bottom of the tank and it facilitates to disturbance to settling, reduction of BOD and COD in subsequent treatment units. The ejector aerators homogenize the effluent, besides oxidizing sulfides present in the raw effluent and facilitate the bacterial breakdown of oxygen demanding wastes.
Fig. Equalization tank
The purpose of equalization is to minimize the wide fluctuation in effluent flow rate and variaton in composition of the effluent. No treatment is achieved in equalization itself. However, the uniformity of effluent produced by this process improves the consistency of performance in subsequent treatment. Here the pollution load are given below-
|Parameters||BOD mg/l||COD mg/l||TSS mg/l||TDS mg/l|
It is homogenised character of effluent obtained from time to time during the flow and also the filtrate from dewatering unit that mixes with the equalization tank. Here the PH values lies within a range of 5.8-6.5
Chemical treatment (coagulation, flocculation)
The equalized effluent is then pumped to the flash mixer, where alum, lime and polyelectrolyte is added.
Chemicals are added in order to improve and accelerate the settling of suspended solids, especially of fine and colloidal matter. In wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water. These terms are often used interchangeably, or the single term – be it “coagulation”
or “flocculation” – is used to describe both; sometimes “flocculation” is understood as the second stage of “coagulation”. In fact, they are two distinct processes usually carried out in sequence as a combination of physical and chemical procedures. Finely dispersed solids (colloids) suspended in wastewater are stabilized by negative electric charges on their surfaces, causing them to repel each other. Since this prevents
these charged particles from colliding to form larger masses, called flocs, they do not settle.
Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. Cationic coagulants provide positive electric charges to reduce the negative charge (zeta potential) of the colloids. As a result, the particles collide to form larger particles (flocs). Rapid mixing is required to disperse the coagulant throughout the liquid. Care must be taken not to overdose the coagulants as this can cause a complete charge reversal and thus re-stabilize the colloid complex.
Flocculation is the action of polymers to form bridges between flocs and bind particles into large agglomerates or clumps. In this process it is essential that the flocculating agent be added by slow and gentle mixing to allow for contact between the small flocs and to agglomerate them into larger particles.
The newly formed agglomerated particles are quite fragile and can be broken apart by shear forces during mixing. Care must also be taken not to overdose the polymer as doing so will cause settling/clarification problems. Once suspended particles are flocculated into larger particles, they can usually be removed from the liquid by sedimentation, filtration, straining or floatation. The flocculation reaction not only increases the size of floc particles in order to settle them faster, but also affects the physical nature of flocs
making them less gelatinous and thereby easier to dewater.
The inorganic coagulants are compounds that break colloidal suspensions and help floc forming. The most frequently used coagulants in tannery effluent treatment are:
– alum: industrial aluminium sulphate Al2(SO4)3 · 18H2O
– lime: industrial calcium hydroxide Ca(OH)2
Coagulant aid – flocculants – are water-soluble organic (anionic) polyelectrolytes that support agglomeration
of colloidal and very fine suspended matter thus enhancing the impact of coagulation.
Figure 16. Schematic view of the coagulation and flocculation system
For optimal results, appropriate dosing is essential; It can be adjusted by jar test.
Optimization of concentration of chemicals at Flash mixture
Capacity of lime dosing tank 3.30 m3 (3300L)
Amount of lime dosing at a time 40kg
Concentration of lime solution 12.12 gm/L
Capacity of alum dosing tank 1.21m3 (1210L)
Amount of alum dosing at a time 50kg
Concentration of alum solution 41gm/L
Capacity of polyelectrolyte dosing tank 500 L
Amount of polyelectrolyte dosing at a time 125g
Concentration of polyelectrolyte solution 0 .36 g/l (360 ppm)
Flow rate of raw effluent 150 L/mins
Flow rate of Lime solution 15 L/mins
Flow rate of alum solution 7 L/mins
Flow rate of polyelectrolyte 230 ml/mins
|SL||Raw effluent(ml)||lime(ml)||alum(ml)||polyelectrolyte(ml)||Coagulation & flocculation|
From above experiments it is clear that 1.5ml polyelectrolyte gives better result
|2||1000||120||50||1.5||good clear but settle slowly|
|3||1000||120||60||1.5||good clear & settle fastly|
|4||1000||120||70||1.5||poor clear & not settle|
From above experiments it is clear that if 60ml alum is added better result is obtained
|2||1000||120||60||1.5||8.0||good clear but settle slowly|
|3||1000||130||60||1.5||8.5||good clear & settle fastly|
|4||1000||150||60||1.5||9.5||poor clear & not settle|
From above experiments it is clear that if 130ml lime is added better result is obtained
So, the ratio of dosing effluents, lime, alum and polyelectrolyte is 1000: 130: 60:1.5 i.e, 16.67:2.16:1:0.025.
- Biological (secondary) treatment
2.1 Objective and basic principles
The main objective at this stage is to further reduce the amount of organic (expressed as BOD and COD) and other substances still present in the effluent after the primary treatment and thereby satisfy the standards/limits for discharge into surface waters (rivers, lakes).
The biological treatment duplicates processes that take place in nature, but under controlled conditions and, especially, at a highly accelerated pace; however, the efficiency of this treatment largely depends on the biodegradability of the polluting substrate, i.e., its inherent capacity to decompose by biological processes. The remaining suspended and colloidal solids are removed by flocculation and adsorption. While biological treatment may be aerobic, facultative or anaerobic (or some combination thereof), in practice, almost only aerobic systems are used; exceptionally, in countries with a hot climate and where a lot of land is available, facultative (preferably aerated/facultative) lagoons are also used.
Due to the inherent characteristics of tannery effluents, primarily their sulphide/sulphate content, in practice, anaerobic treatment is used only in sludge digestion. Among many variations of the aerobic process, the most widely used method is (complete-mix) activated sludge treatment with extended aeration; despite some very interesting features, membrane bioreactors (MBRs) have not made significant inroads in the tanning sector.
The activated sludge process is an aerobic, biological process, which uses the metabolism of microorganisms to remove substances causing oxygen demand. The qualitative biochemical reaction taking place in the organic matter stabilization process can be summarized in the following manner:
Inert matter + organic matter + oxygen + nutrients + micro-organisms =
new micro-organisms + CO2 + H2O + additional inert matter.
Simply said, we stimulate micro-organisms to convert (eat and digest) harmful, oxygen-demanding organic compounds into an environmentally more acceptable form (micro-organisms) and low-energy, stable compounds like water and carbon dioxide. The microbial community that does that job comprises various species of bacteria, fungi, protozoa,
Figure28. A simplified flow diagram of the activated sludge process
Generally, the biological stage is the most complex part of the overall effluent treatment process, with highest investment and operational costs, its day-to-day running requiring considerable skills and experience.
- Advanced (tertiary) treatment
Often, wastewater from secondary treatment receives tertiary treatment. Depending on the quality on the treated effluent, a second chemical dosing and settling may be applied to reach the required standards and color removal before discharge. Aluminium salts and polyelectrolyte are commonly used, forming very fine flocks. These can be settled out using specially designed settling tanks, but often large shallow lagoons are employed. Effective biological treatment can also be provided by reed beds. Other techniques include sand filtration activated carbon.
3.1 Pressure Sand Filter: The treated water is pumped through a pressure sand filter to remove any fine solids that might have been carried over the water. The water from the pressure sand is collected in collection sump-II.
3.2 Activated Carbon Filter: The water of collection sump-II is slightly colored. To remove this color we transfer it through the activated carbon filter.
Fig. Sand filter & Activated carbon filter
After all the process we check the parameters of the treated water. If it complies with the specification the treated water is discharged into inland water. Otherwise it is recycled into equalization tank to treat again.
Characters of tannery effluent in different units of ETP
|Parameters||Raw||Chemical treatment||Biological treatment||Tertiary treatment|
|Parameters||Raw||Treated water||DOE Standards|
Sludge from primary clarification, sludge from secondary clarification and tertiary clarification are transferred to sludge thickener through pump. For drying of sludge it is transferred from sludge thickener to sludge drying beds. In case bad weather it is transferred to Centrifuge for mechanically dewatering. When the sludge dry it is used for land filling and with mixing some additives this sludge can be used for making bricks..
RECOVERY AND REUSE
3.1 RECOVERY AND REUSE: SOME DEVELOPED TECHNIQUES OF THE BEAMHOUSE EFFLUENT TREATMENT SYSTEMS:
There are several reuses and recovery options of the beamhouse effluents. Here is a chart which will give us some useful ways of this regard.
Sources and uses of solid wastes from tanneries:
|Solid Place of production Possible application waste|
Non proteinious wastes
|1 . Used salt||Salt dusting||Regeneration for salting and pickling|
|2. Lime sludge||Lime pelts||As material of construction and soil conditioner|
|3. Spent tan bark||Tannin extraction||Carton industry as fuel|
|4. Tan liquor sludge||Vegetable tanning||Reduction of chrome tan liquor, boiler compounds|
|5. Fat||Defatting of hides or skins||Soap industry|
|Treatment of tannery effluents||Tipping or composting|
Non collagenious protein
|1 . Lime protein||Lime yard||Substitute of casein, animal food|
|2. Pig bristle||Beam house||Brushes|
|3. Tail and body hair||Beam house||Drugget and carpet industry, cushions|
|1 . Untanned trimmings and shavings, lime split, etc||Beam house||Glue, gelatine, protein degradation products|
|2. Fleshings||Beam house||Glue and gelatine manufacture|
|1 . Chrome tanned
|Chrome tanning||Glue, gelatin, protein degradation products|
|2. Vegetable tanned leather shavings||Vegetable tanning||Artificial fibre leather, leather boards|
|3. Formaldehyde tanned leather||Sulphochloride tanning||Artificial fibre leather|
|4. Whitening||Finishing of russet
|Recovery of fat, artificial fibre leather|
|5. Tanned splits||Splitting after finishing|
|6. By-products skiving leveling by splitting and fabrication||Stitching and cutting of leathers||Fibre leather, sole patches
Removal of Chromium from Tannery(Beamhouse) Effluent Using Powdered Leaves:
- Suseela, M. Shivaparvathi and S. Nandy showed a preliminary investigation where dried fallen leaves were used for the removal of chromium from tannery effluent. Effects pf factors like the concentration of leaf powder, treatment period, pH of the effluent liquor, temperature and the concentration of in the spent chrome liquor were studied. Chromium removal was unaffected by different types of leaves but when dry leaf material was used in powder form the removal effect was improved. Leaf material could also absorb other heavy metals like copper, iron, sodium and nickel. Using this simple technique it may be possible to treat exhausted chrome-tan baths and minimize the disposal problem to a considerable extent.
Exhausted Chrome Tanning Solution Regeneration Method:
Some Russian scientists have developed a method where exhausted tanning solution containing toxic chromium compounds are first settled and filtered to remove suspended and fat substances after which solution is treated with oxidant on heating until dissolved organic impurities are destroyed. Organic free exhausted solution is then mixed with fresh tanner and returned into process.
Characterization of tannery(Beamhouse) effluents and study of the water hyacinth in chromium recovery:
Alvarez, Maldonado, Gerth and Kusehk established a method where four types of effluent were sampled from a tannery having a treatment plant. After four weeks effluents were evaluated in phytoremediation assays using water hyacinth for the efficiency of chromium removal under tannery water injection conditions of 4 L/day and 8 L/day. This affected the life of water hyacinth. It is concluded that the high concentrations of the parameters measured made phytoremediation impossible for this type of effluent, making primary treatment necessary in order to decrease the load of compounds present.
Sulfur recovery from beamhouse waste water:
Tannery “waste water was treated in pilot plants in India and Netherlands. The plants comprised an aerobic biological treatment system and a sulfur recovery system. Those removed a large part of COD, sulfate and sulfide, while biogas and elemental sulfur were produced. An Upflow Anaerobic Sludge Blanket reactor was used in which COD was eliminated and converted into biogas while all sulfur compounds were converted into sulfide. In this method 70% of COD and 90% of sulfur compounds can be removed. Apart from these two countries Bangladesh is also interested in this practice as the report says.
Centriquip- A case study of simple solids removal:
In Scotland, Ireland, England, South Africa, Brazil this system was practiced. Centriquip were approached by a tannery producing wet-blue from bovine hides and having an effluent discharge of approximately 700 m3/day needed to process in 16 hours. The average suspended solids were 8000 mg/litre and average COD of 9000 mg/litre. Effluent costs were more than 600000 GBP per annum.
An initial one week test with a 2 m3 /hour mobile pilot plant indicated that a centrifuge would clarify the effluent resulting in a dry almost crumbly cake and water suitable for direct discharge to the sewer. The larger trial unit was installed which could process 40 m3 /hour. This was successful and led to the client placing an order to purchase the trial unit as well as a new unit capable of 50 m3 /hour. Suspended solids were reduced to 400 mg/litre; COD reduced to 4500 mg/litre resulting in effluent charges being reduced to less than 200000 GBP per annum. The unexpected benefit of residual hydrogen sulfide was seen.
The utilization of soaking liquors for irrigation using silicates:
Former Principal Dr Fazlul Karim of Bangladesh College of Leather Technology, being one of the fellow researchers of a team of Munz, Banaszak, Chandra Babu, Quadery (BCSIR) and others was involved in this research.
A series of carefully controlled investigations was performed to study the effects of both sodium chloride and silicates on crop irrigation. Salinity levels of tannery effluent were 10,000 mg/1 for Sodium Chloride.and 1700 for Sodium Silicate adjusted to pH 7.0 using formic acid.
Five major crops were grown in controlled fields trials over 2 years. One crop (tomatoes) was grown under green house conditions. Four other crops were grown outdoors. Each crop was sub-divided into three sections, each being irrigated with water, silicate and saline solutions. The outdoor crops showed common trends. Findings showed that silicates can replace salt as a solution of effluent treatment and be used for irrigation on a wide variety of crops.
Gasification of tannery solids and generation of power by the Pyro Arc Process:
Developed by a Norwegian company a gasification process is coupled with the verification of inorganic residues into insoluble slag. The technology enables both heat recovery’ and generation of electricity.
Here solid material is fed into the gasifier through a lock-hopper arrangement where the level in the feed pipe below the hopper controls the feed sequence. The upper, middle and lower zones of the gasifier involve temperatures of 400-600 °C, 1000 °C and 1500 °C respectfully.
A resultant combustible gas is evolved which after cleaning can be used as a fuel for gas engines for the production of electricity. The solids being disposed is a mixture of dewatered sludge from the effluent treatment plant, fleshings, chrome shavings, leather trimmings, pallets, plastics, buffing dusts, etc , amounting to about 840 tonnes each month,
Biomethanation of fleshings and sludge from tannery and effluent treatment plants:
A joint venture of CLRI, Chennai and UN1DO in January 2000 has installed a 130m3 capacity two-digester Biomethanation plant which was designed to take 5 t/d of waste – about 3 t/d of fleshings and 2 t/d of primary sludge to generate 312 m3 of gas and 700 kWh of electric power daily.
The plant consists of a mincer for preparing the fleshings, feed chamber digesters, gas holder, H2S scrubber and a dual-fuel power generator, with necessary pumps and control mechanisms. Fleshings are delivered to the sump and minced. The ground fleshings are delivered to the second sump and blended with sludge from the Common Effluent Treatment Plant (CETP). The blended solids are delivered to the feed container as 1 tonne lots by volume. The solids are pumped to one of the inter-Hnked reaction vessels for the biological breakdown to generate methane. A unit volume is drawn off from the reaction vessels to equalize the volume within the biomethane unit.
Industrial composting of solid organic tanning wastes:
This is a process known as ‘Silo-Cage Continuous Flow Thermophilic Composting System’ which is a continuous aerobic composting operation with low energy requirements. This system holds 32 m3 of mix. Bulking agents complementary to the target waste are used in the mix to provide an aerobic structure for active composting. An overhead feeder shreds and feeds 2.5 m3 of the mix into each cage daily.
In the first temperature zone, rapid degrading of the waste begins at 70-80 °C. After 24-36 hours the composting mix passes into a second zone at 60-65 °C for 6-7 days, then exists at 50 °C after a processing time of 14 days. The volume is reduced to 70 and the solids removed by a slowly rotating auger which traverses beneath the bank of Silo-Cages.
Salt recovery from hide preservation brine and the drying of sludges:
This system is practiced by Sibus Ltd., UK with Rapid Air Swept (RAS) dryers where the slurry is introduced along with hot air in a co-flow design. A range of sludges has been dried, in many cases together with a dry fibrous waste, to produce either a fertilizer or secondary fuel source. This project also developed a treatment process to convert blood contaminated brine into clean, white, reusable salt. The first stage of this process involves the pretreatment of the red/brown brine at 30% solids to remove the discolouration thus providing clear liquor.
The second stage is to premix the liquor with a quantity of dried salt via a Sibus Continuous Turbo Blender. The resulting slurry is then dried in the RAS drying system as the third stage. This produces a dry, white, clean and reusable salt.
Low temperature conversion of tannery sludges:
Lederinstitut Gerberschule Reutlingen (LGR), Germany, established a process of effluent management system where production of Cr+6 due to oxidation of Cr3+ was prevented by a pilot project comprising a direct fluid bed dryer and a conversion system. The direct flue gas dryer has a maximum water evaporation capacity of 90 kg/hr with an energy demand of 3 MJ/kg of evaporated water. The conversion system consists of two converters having a maximum throughput of 20 dry kg/hr. The sludge is heated in the absence of oxygen to around 400 °C. The solid residue is converted to char and usable oil.
It is noted that the oil produced from tannery sludge range from 4.3 to 14.7%, whereas undigested domestic sewage has range from 20 to 30%. The main product of this system is the char amounting to 55 to 77% of the processed sludge. Another product is oil containing only a small portion of non-volatiles and 90% evaporates at 350 °C. In this project 0.167 kg of diesel is required to produce 1 kg of dried sludge,
Reducing chemical consumption and waste water COD loadings by recycling in conjunction with membrane filtration:
This system was governed by a German scientist, K. Hellinger, where separation is continued in two phases; in the first phase, called retentate. the usable materials are concentrated for recycling.
The second phase, known as the permeate, is low in these usable products and either discharged or used with fresh water in making up the process float during the recycling procedure. After preliminary tests on model floats, the reduction in COD of the permeate was found significant, especially leathers based on chrome-free processing. After a series of recycling, it was found that 75% retentate recycling provides 50% savings in fatliquor and syntans without any loss of quality or differences in physical properties. Recycling six times caused no problems in process or to leather quality, producing an effluent reduction of 20% and materials savings.
Total dissolved solids (TDS) management in tanneries:
UNIDO, Chennai, has recently developed practices regarding TDS management systems in tanneries which suggested various desalting procedures as common salts are responsible for TDS problem in tanneries. The quantity of common salt used in curing is about 40% of hides weight most of which is absorbed by hides, yet some remains stuck to the surface. Four techniques are so far successful to remove this salt.
In the perforated drum, holes made of 85mm diameter spacing of 130mm is effective for desalting where 15% of salt in curing can be removed and recovered. A brush-type desalting machine having a cylindrical roller with hard nylon bristles with widths of 1500mm, 1800mm and 2100mm can recover 6-7% curing salt. Though labour-intensive, hand brushing is also effective in this case. The Dodeca Wooden Frame can recover 7% of this salt; it is has a dome-like top frame over which the salted hides are shaken off to loose the extra salts.
Also in soaking, solar evaporation reduces salt discharge up to 50%. Besides, if soaking is done in two installments of 150%, same result will produce. Again 80% of salt and 25% of formic/sulfuric acid can be saved if pickle float is recycled before mixing with tanning float. If chrome powder is used rather than liquor, a good result will come. Direct recycling of spent chrome liquor not only reduces chrome salts but also recovers salts.
Salt balance as applied to one tonne of raw hides/skins
|1 . Applied during curing||400|
|2. Discharged as leachate during curing as a result of dehydration of hides||60|
|3. Present on the surface||160|
|4. Fallen from the raw stock during handling||40|
|5. Fallen during cutting, trimming, etc||15|
|6. a) Manual desalting||70|
|b) Mechanical desalting||60|
|c) Brush type desalting||60-70|
|7. Washed off in first soaking||120-145|
|8. Washed off in second soaking||50-55|
|9. Carried over by hides to further operations||30-40|
Leather production — Environmentally friendly processes and products
|Soaking||– Using fresh raw hides free of salt – Biodegradable surfactants|
|Liming||– Low-sulfide or sulfide-free processes – Hair-saving processes|
|Deliming/Bating||– Low-ammonium or ammonium-free processes – Low-ammonium bating|
|Degreasing||– Replacement of solvent degreasing by aqueous processes|
|Pickling||– Low-salt or salt-free processes|
|Tannage||– Chromium recycling – Improved chrome exhaustion and fixation – Alternative tanning or pretanning techniques|
|– Retanning agents and processes with high exhaustion – Retanning agents with low residual monomer and salt contents|
|Dyeing||– Dyes dyeing auxiliaries leading to high
|Fatliquoring||– Fatliquors with high exhaustion|
|Finishing||– Aqueous finishing systems – Pigments free of heavy metals|
Utilization of tannery by products:
Raw trimmings can be unhaired then dried to recover both protein and grease.
Quality grease and proteins can be recovered without use of chemicals.
Grease and proteins can be extracted from limed fleshings for animal food stuffs and recovered greases as a replacement for boiler fuel.
Can be used to manufacture industrial glue.
The trimmings can be processed to obtain gelatin, proteins, hydrolysates. dog chew products and casings. Additional uses are found in cosmetics and medical products. Extracted amino acids are used as nutritional supplements.
Sludge following solvent degreasing:
Waste obtained from degreasing sheep skins after solvent recovery can be used as boiler fuel.
Wet blue trimmings shavings:
These materials can be pulverized and dried, then used in the manufacture of light boards for insulation, or as a polishing material for metals. Major uses include leather board production for shoe insoles and fancy goods. Hydrolysates are being produced in significant volumes as fertilizers, animal foodstuff supplements, and for various industrial applications. Due to properties including mould ability, sound proofing and fire retardancy gear box tunnels for automotives have been formed, and roof linings and door panels for truck cabs. An unusual line of investigation has been the manufacture of brake linings. Fresh opportunities may be found by the use of additives to the latex or specialized binders.
Vegetable tanned shavings:
This can be utilized as slow release fertilizers.
4.1 THE CONDITION OF THE TANNERIES IN BANGLADESH WITH RESPECT TO WASTE MANAGEMENT:
In the midst of multifarious industries in Bangladesh, tannery industry restores a place of dignity as per a first row foreign currency earner whose contribution in building national revenue with a strong earning in a fiscal year. But as an irony, though this is a dominant industry with a prospective alleviated future for the development of Bangladesh, due to various problems like mismanagement, inferior technology, negative approach towards industrialization and last but not the least, lack of proper waste management systems emerging a series of detrimental effects on environmental balance, leather industry is facing threatening to hold its position in the world market.
As for international awakening in the protection of our environment declaring ISO 14000, we can not produce any leather which is harmful to the environment. So if our tanneries do not manufacture environmentally friendly finished leathers, it will extinct in the long run.
There are about 270 tanneries in this country. Of them 90% tanneries are situated at Hazaribagh in Dhaka. 6% tanneries are in Chittagong and rest of 4% are in other places. Total 8.47 million liter liquid waste and 98 MT solid wastes are generated from all tanneries.
7.70 million liter liquid waste and 88 MT solid wastes are emanated daily from Hazaribagh tanneries. The rest 0.77 million liter liquid and
10 MT solid wastes are produced from the tanneries situated in other places of the country. This huge amount of wastes has become a direct threat to the envkonment of our country for the lack of proper treatment plant. All the liquid wastes are discharged into the Burigonga river without treatment which cause severe envkonmental pollution.
Liquid waste generated daily
Total – 8.47 million liter
Hazaribagh – 7.70 million liter Other Places – 0.77 million liter
Solid waste generated daily
Total – 98 Metric Tons
Hazaribagh – 88 Metric Tons Other Places – 10 Metric Tons
Some of the important matters which have been concluded in the report are laid down here below:
# An assessed calculation of total amount of beam house waste is not specifically checked out even today for the lack of which there can not be estimated the degree of environmental and health hazards in a technical way.
# The treatment plant must be set up by sufficient prior investigation and in a well planned way.
# The beamhouse operational obsoletes should be properly reused or made into various useful products.
Tanneries and the Environment Technical Guide Tannery
Thomas C. Thorstensen, Fundamentals of Pollution Control for
the Leather Industry
S.S. Dutta, An Introduction to the Principles of Leather
P.R. Trivedy and Gurdecp Raj, Environmental Industrial
Pollution Control, Vol-4
P.R. Trivedy and Gurdeep Raj, Environmental Industrial
Pollution Control, Vol-5
R.K. Trivedy, Advances in Environmental Science and Technology
World Leather, Vol-12, No-7, November 1999
World Leather, Vol-13,No-5, September 2000
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