Assignment on E & S Management

Introduction:

Tannery is one of the oldest industries of the world. During ancient times, tanning activities were organized to meet the local demands of leather footwear, drums & musical instrument. With the growth of population the increasing requirements of leather & its products led to the establishment of large commercial tanneries. Two methods are adopted for tanning of raw skin/hide- a) Vegetable tanning    b) Chrome tanning

The production process in tannery can be split into four categories: a) Skin storage & beam house operations B) Tanyard operations C) Post- tanning operations D) Finishing operations

Manufacturing of leather, leather goods leather boards and fur produces numerous by-products, solid wastes, high amounts of wastewater containing different loads of pollutants and emissions into the air. The uncontrolled release of tannery effluents to natural water bodies increases health risks for human beings and environmental pollution. Effluents from raw hide processing tanneries, which produce wet blue, crust leather or finished leather contain compounds of trivalent chromium Cr) and sulphides in most cases. Organic and other ingredients are responsible for high BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) values and represent an immense pollution load, causing technical problems, sophisticated technologies and high costs in concern with effluent treatment.

The objective of this paper is to review on wastages generated in the leather industry and the useful technologies developed to overcome the wastages problem.

POLLUTANTS:

All the three categories of waste solid, liquid, and gaseous, are emitted by the leather industry in the form

of:

1. Wastewater

2. Solid Wastes

3. Air Emissions

The results of environmental auditing are described below:

Wastewater:

Water is used as the carrier for chemicals to render the cleaning of raw hides and skins. The water after completion of the process is drained out in the same quantity as it was used in the process. Normally water consumption of 50 liter/kg is recommended for tanneries but it is found that tanneries generally consume more water and in some cases it is found to be as high as 150 liter/kg. Samples of water were taken from different processes of leather manufacturing and were analyzed. Tannery wastewater is highly polluted in terms of biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids, total Kjeldhal Nitrogen, conductivity, Sulphate, Sulphide, and Chromium.

Wastewater of each tannery process consists of pollution of varying pH values. Similarly a large variation exists in every parameter BOD, COD, Chloride, Sulphate, etc. Discharge of these chemicals into wastewater is hazardous for the environment.

Table 1 provides an overview on water consumption in individual processing operations during the tanning process. Depending on the type of applied technology (conventional or advanced) the water consumption varies extremely. Technologies that can be regarded as advanced in comparison to conventional methods involve processes usually termed low-waste or cleaner technologies (high exhaustion, chrome fixing). Mainly in dry regions, where water supply is limited, this factor plays an important role.

Table 1 : Water consumption in individual processing operations (C-conventional technology, A-advanced technology )

Operation	C (m3/t raw hide)	A (m3/t raw hide)
Soaking	7-9	2.0
Liming	9-15	4.5
Deliming, Bating	7-11	2.0
Tanning	3-5	0.5
Post-tanning	7-13	3.0
Finishing	1-3	0
Total	34-56	12

Figure 1. Overview of the tanning industry

Solid Wastes:

Out of 1000kg of raw hide, nearly 850 kg is generated as solid wastages in leather processing. Only 150 kg of the raw material is converted into leather.

The major solid wastes consist of dusted curing salt, wet trimmings, dry trimmings, wet shaving, buffing, etc. These wastes are generally separated at the source. During handling of raw skins, adhered dusted salt, which is contaminated with blood, hair, dirt and certain type of bacteria is removed and dumped. Trimmings are cuttings of edges of raw skins. Fleshing is the flesh material of the limed skins generated during fleshing operation. Chrome wet shaving is produced when skins are shaved for proper thickness after chrome tanning. Except dusted salt most of the solid wastes are sold in the local market to the poultry feed manufacturers

due to the protein content of the solid wastes. The main problem with these wastes is their high Chromium content. The Chrome tanned waste contains Chromium in trivalent form but it was found that when the solid wastes are used in making poultry feed the Chromium is converted to hexavalent form which is carcinogenic. The mixing of this metal in poultry feed could produce serious human health problems.

Air Emissions:

Typical gaseous pollutants from a tannery are chlorine, formaldehyde, toluene, xylol, metha

nol, magnesium sulphate, chromium |||, sulphuric acid, ethylene glycol etc. In tanneries air emissions are produced from the stacks of boilers and generators and during the processing of leather. Emissions from the stacks are well within the limits and pose no serious environmental impact. But hydrogen Sulphide and ammonia gases which are emitted during different processes are a health hazard for the workers.

ENVIRONMENTAL IMPACTS OF POLLUTANTS:

Out of the three wastes mentioned, air emissions are very low as compared to the standards mentioned in NEQS whereas the solid wastes are used in preparation of poultry feed which pose health problems. The wastewater is a serious threat to the environment. Tanneries are disposing of their wastewater into drains, which finally find its way into natural water bodies. Therefore major environmental problems are linked with the wastewater of tanneries. The pH of directly discharged tannery effluent varies between 3.5 to 13.5. Water with a low pH is corrosive to water-carrying systems and can lead to metal dissolving in the water. The high pH water can cause scaling in the sewers. Also large fluctuation in the pH value is detrimental to some aquatic species. The large quantities of proteins and their degrading products forming a major part of the wastewater can effect Biochemical Oxygen Demand (BOD).. The high value of BOD in extreme cases can kill natural life in the effected area. Tanneries discharge water having 10-190 times the recommended value of BOD by

NEQS. Chemical Oxygen Demand (COD) value in wastewater is in a range of 1000-43000 mg/liter which is 25-275 times more than the NEQS standard. Sulphide in the wastewater releases hydrogen Sulphide gas which has an objectionable smell even in trace amounts. It is highly toxic for many forms of life. In higher concentrations fish mortality may also occur. NEQS recommends a value of 1g/liter for Sulphide where as tanneries' wastewater contains 290mg/liter. Trivalent chromium is released from chrome tanning process. This is much less toxic than Hexavalent Chromium. For plants and animals the toxicity of chromium is variable. Algae have been shown to very sensitive to the chromium levels. At present tanneries are discharging chromium 133mg/liter whereas NEQS standards recommend a value of 1mg/liter. Suspended solids discharged in the wastewater forms a layer on the bottom of watercourse and covers natural fauna on which aquatic life depends. This can lead to localized depletion of oxygen supplies in the bottom waters. Suspended solids also reduce light penetration and thus photosynthesis in the water. Apart from these Sulphate and natural salt is also found in the wastewater. This can affect fresh water aquatic life if their concentration becomes too high. There is no economic way of removing them. Poultry feed manufacturers collect these materials from tanneries and use it due to the protein content of the solid wastes e.g. fleshing, raw trimming, chrome shaving, buffing dusts etc. These wastes contain chrome in the range of 14-26 gm/kg. Chromium in the waste is in trivalent form which is less toxic than hexavalent form. A recent survey under Bangladesh Tanneries Association showed that poultry feed made using solid wastes contain hexavalent form of chromium also. It seems that during feed preparation the transition of trivalent chromium to hexavalent chromium takes place. These pose a serious threat to human health.

Decomposition of Wastages

Treatment:

Before turning to treatment itself, it is important to bear in mind the following:

• The design of an effluent treatment plant (ETP) is always tailored to the requirements of a specific site; thus, there are no two identical ETPs.

• Pollutants contained in effluent cannot disappear; they are only converted into something which is

environmentally more acceptable or easier to dispose of (sludge).

• Somewhat paradoxically, the obvious is often overlooked: the same amount of pollutants at lower water consumption means lower hydraulic load (volume) but higher concentration – not always easy to treat.

• It is important for a tanner to understand the relation between the leather technologies applied and

wastewater treatment in order to reduce the overall cost of treatment. Wastewater treatment is a multi-stage process to purify wastewater before it enters a body of natural water, or it is applied to the land, or it is reused. The goal is to reduce or remove organic matter, solids, nutrients, Cr and other pollutants since each receiving body of water can only receive certain amounts of pollutants without suffering degradation. Therefore, each effluent treatment plant must adhere to discharge standards

– limits usually promulgated by the relevant environmental authority as allowable levels of pollutants, for practical reasons expressed as BOD5, COD, suspended solids (SS), Cr, total dissolved solids (TDS) and others. The three main categories of tannery wastewater, each one having very distinctive characteristics, are:

• Effluents emanating from the beam-house – liming, deliming/bating, water from fleshing and splitting machines; they contain sulphides, their pH is high, but they are chrome-free.

• Effluents emanating from the tanyard (tanning and re-tanning, sammying) – high Cr content, acidic.

Preliminary treatment

Typically, in the case of common effluent treatment plants (CETPs) servicing tannery clusters often found

in developing countries, it is essential to have pre-treatment units installed in individual tanneries. Their

role is to remove large particles, sand/grit and grease, but also to significantly reduce the content of chrome

and sulphides before the effluent is discharged into the collection network.

               

Physical-chemical treatment (primary)

The objective here is the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25-50% of the incoming biochemical oxygen demand (BOD5), 50-70% of total suspended solids (SS), and 65% of the oil and

grease are removed during primary treatment. The effluent and sludge from primary sedimentation are referred to as primary effluent and sludge.

Biological treatment (secondary)

In most cases, secondary treatment follows primary treatment, its goal being the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is carried out in the presence of oxygen by aerobic micro-organisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more micro-organisms and inorganic end products (principally CO2, NH3, and H2O). Several aerobic biological processes are used for secondary treatment and the differences among them have to do primarily with the manner in which oxygen is supplied to the micro-organisms and with the rate at which organisms metabolize the organic matter

Figure . Sources and types of pollutants generated in leather processing

Advanced (tertiary) treatment

Tertiary or advanced wastewater treatment is employed to reduce residual COD load and/or when specific

wastewater constituents are not removed by previous treatment stages.

Sludge handling and disposal

Effluent treatment plants produce treated, “cleaned” effluent and sludge because inherently the primary

aim of wastewater treatment is the removal of solids and some potentially hazardous substances from the

wastewater. Furthermore, biologically degradable organic substances are converted into bacterial cells,

and the latter are removed from the wastewater.

Removal of grit and floating matter:

A simple, non-aerated grit-and-floating-matter removal chamber is usually placed in a horizontal

gravity channel immediately after the rough screen.

Large ETPs

­

Figure. Rough bar screen, operation principle

Figure . Simple, non-aerated grit-and-floating-matter removal chamber

Self-cleaning screens

Chemical treatment (coagulation, flocculation)

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.

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.

The most frequently used cogaulants in tannery industries are followings:

Alum-                 AlSO4. 18H2O

Iron sulphate-    FeSO4. 7H2O

Iron choride-      FeCl2.  6H2O

Figure 16. Schematic view of the coagulation and flocculation system

Settling – primary sedimentation

The main objective at this stage is the removal of suspended solids; however, various constituents such as

fats, waxes, mineral oils, floating non-fatty materials, etc. (“grease”), not already removed in the grit-andoil

chamber (usually positioned between screening and equalization), are also separated here.

Figure . Cross section of a rectangular sedimentation tank with travelling

     

Photo. Sedimentation tank (empty)                                                     Photo. Sedimentation tank in operation

Biological Treatment:

After the waste water has been collected from the tanning process and polymer has been added, the decanters continuously separate all organic solids such as hair from the thickened sludge. The separated solids can be composted and find application, for example, in horticulture or forestry. The liquid phase is then subjected to biological treatment. The resultant sludge is recycled into the dewatering process; the waste water can be sent safely to conventional municipal waste water treatment plants.

The advantages of decanters provide optimum dewatering capacity and use very little rinsing water. The closed system prevents aerosols from escaping; such aerosols would otherwise pose a hazard for humans and the environment.