In this article we will discuss about the water intensive industries.
Major Water Intensive Industries:
(i) Tanneries:
The various processes employed at tanneries are:
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(a) Soaking
(b) Liming and removal of hair,
(c) De-liming;
(d) Bating
(e) Pickling
(f) Tanning (vegetable and chrome) and
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(g) Finishing (fat liquoring and dyeing).
The quantity of waste-water let out from Indian tanneries works out to 3008 to 3324 liters per 100 kg. of hides processed. BOD varied from 4 to 8 kg. per 100 kg. of hides processed.
Objectionable constituents in a tannery waste are suspended solids, colour, high BOD, tannin, trivalent, chromium, hair and fleshings. Imparities are present in suspension solution and colloidal dispersion.
(ii) Pulp and Paper Industries:
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Water is one of the main raw materials in the manufacture of pulp and paper although it does not appear in the final product. It is estimated that about 273 to 455 M3 of water is required per ton of paper made and practically the entire quantity reappears as effluent requiring treatment.
Effluent discharged from different mills in their volumes which depend on the process and raw material used and the type of paper material.
Most of the paper and straw board mills use lime and caustic for pulping and do not recover the chemicals and hence the BOD and suspended solids are invariably high. On an average 70-80 kg BOD/tonne of product is discharged by these mills. The suspended solids are also very high (2500- 4000 mg/1).
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Another important Toxic pollutant (that is yet to be assessed) in pulp and paper industry is mercury. This is so because some of the mills make their own caustic and chlorine in mercury cells and it has been reported that about 0.25 kg. of mercury is lost per tonne of caustic made.
Further, some organic mercury compounds such as phenyl mercuric acetate (50-100 ppm of dry weight of pulp), methoxy ethyl mercury and pyridyl mercury salts are being used as slimicides on paper machines which finally find their way into the effluent
(iii) Textile Industries:
In the textile mill raw cotton is corded, spun, spooled and wrapped, sized drawn and woven into cloth before being sent into the finishing unit. No water-borne pollution originates into this section of operation.
The major wet process in the mills are:
Sizing, desizing, bleaching, mercerizing, fancy dyeing, screen printing, khaki dyeing, yarn dyeing and finishing. The quantities and the characteristics of the waste-water discharged by mill vary highly from one mill to another and depend upon water usage and average daily production and the process involved.
The combined waste-water discharged by the factory is large in volume and show sharp variation in colour, pH, total dissolved solids, etc.
It will be seen from the above data that the characteristics of the combined waste-water discharged by the mills are similar to domestic sewage in BOD strength. But the wastes have a high pH, alkalinity and total dissolved solids.
(iv) Breweries and Soft Drinks:
(v) Dairy and Dairy Products:
The nature and composition of the wastes from a dairy depends upon the quantity of milk processed and the types of products manufactured. Various processes involved in dairies are milk receiving station, pasturization, cheese, butter-milk and skimmed milk, ghee, baby-foods, powder milk etc.
Industrial wastes arising out of dairy plants include wash water from milk cans, equipment, bottles and floors. They also include portions of spilled milk, skimmed milk, Ghee and butter-milk.
These wastes arise out of:
(i) Rinsing and washing of cans and bottles, tanks or drums, equipment, pipe line and floors;
(ii) Overflows, spillage and leakages from pumps and other equipment entrainment during evaporation.
The nature and composition of waste from the dairy depends upon the quantity of milk processed and the type of products produced.
It is well known that milk contains carbohydrates, protein and fats in addition to minor organic and inorganic materials.
The composition of the combined waste from a dairy depends upon the quantity of milk processed and the type of products produced.
Table 17- Characteristics of waste from three different dairies:
(vii) Steel Mills:
The production of iron and steel involves large quantities of water (246 M3 per ton of steel). About 70% water in this industry is used for cooling, most of which is re-used. The major waste from steel mills comes from the unit, blast furnace, pig casting, steel melting shop, rolling mills, acid pickling, head treatment, cock oven and by product plant, sintering plant and power plant.
(a) Blast Furnace:
The wash-water of the gas cleaning plant and the cooling water of the blast furnace constitute the waste-water. The wash- water will have high suspended solid mostly fine particles of coke, iron ore or sinter.
The wash water may also have impurities such as dissolved iron and to lesser extent ammonia, phenol, cyanide and carbon monoxide. Blast furnace cooling water leaves the furnace essentially as received except for the heat added.
(b) Pig Casting:
In the pouring of cast iron into the pigs, lime solution is sprayed on the moulds of the pigging machine to prevent sticking of pig iron in the moulds. More water is sprayed into the moulds and the pigs for cooling. This cooling water contains solid particles (scales) with high settling velocity.
(c) Acid Pickling:
The discharge from the pickling area generally includes spent acid and acidic reuse water. In addition, there may be effluents from degreasing and other ancillary processes together with floor washings and spillages.
The spent pickle liquor may contain about 1-10 percent of free acid and 2-8 percent of ferrous iron. Strong acidic effluents can be reused and ferrous salt can be recovered. The volume of waste-waters from this section will be about 250-450 litres/tonne of steel pickled.
(d) Coke Oven and By-Products Plant:
The bulk of water used in a coke oven plant is for quenching which can be reused after recovery of coke dust. Water is used in the by-product plant for cooling, washing and scrubbing of coke oven gas and for different by-product processes. The wastewater from by-product section contains tar, ammonia, phenols, hydrogen sulphide, cyanides, oils, etc. the actual concentrations depending upon the recovery system.
(e) Sintering Plant:
Wastewater from the sintering plant contains mostly suspended solids, i.e„ ore and coal dust in the effluent from air washers.
(f) Power Plant:
The operation of power plants and turbo blower station involves the generation of heat from coal, oil or other fuel to produce steam from demineralized water. Apart from the heated cooling water, the following types of waste -water may emanate from the power plant.
Hot, concentrated water salines from the boiler and evaporator with pH around 11.0 and total solids content of about 6,000 mg/1. It may also contain antifoam materials and high-phosphate organic agent. Acid and alkaline chemical solutions (hydrochloric/sulphuric acid, hydroxene, phosphates, sodium hydroxide) are used for flushing and cleaning boilers.
Acid, brine and caustic solutions resulting from regeneration of ion exchange softerners and demineralisation water treatment plants which are used for conditioning boiler feed water.
(g) Steel Melting Shops:
Open hearth furnaces use large quantity of cooling water which leave the furnace essentially as received except for the heat added and hence can be reused after cooling. The air blast used in the blast furnace carries large amounts of suspended solids.
To avoid air pollution, this gaseous effluent is scrubbed with water resulting in effluent loaded with flus dust containing iron oxide, alumina, silica, carbon, lime and magnesia. If proper sedimentation does not take place the waste can contain suspended solids upto 1600 mg/1. BOD 245 mg/1 and COD 1300 mg/1.
Wastes from steel melting shop and rolling mills contain suspended solids in the form of broken refractories, and rust from the furnaces and the rolling item respectively. Waste is let out at a higher temperature.
(h) Rolling Mills:
In the hot rolling mills, oil in the water from this mill is generally lubricating oil from bearings and usually non- emulsified. The scale pit over flow is quite low in suspended solids about 100-120 mg/1. with about 60-100 mg/1. of oil. In the cold rolling mills oil problem is considerably more difficult to handle since emulsified oils are added directly to the water sprayed on the metal as it is being rolled.
(i) Heat Treatment and Electroplating:
This is already explained in the category of Electroplating and Heat Treatment Industries.
Moderate Water Consuming Industries:
(1) Canning Industries:
(2) Roller and Flour Mills:
In the roller and flour mills large quantity of water is used for a various purpose such as process, washing, flushing and digestion.
Raw effluent gives a bad smell during the process of digestion of grains. It also has a severe problem of husk in suspension. Though Waste- Water with husk does not have much of BOD, but interferes in the biological treatment.
(3) Industries using Water for Sanitation:
Water required in these types of industries is mainly for sanitary facilities meant for factory workers and for the residential quarters in the premises. Water requirement for industrial purpose is minimal sometimes a little quantity of water is required in the manufacturing process.
(4) Engineering Industries:
Small quantity of water is used in this type of industry. Waste-water from the industry does not have much Biochemical oxygen demand and volatile suspended solids. Water may have insoluble suspended solid which can be separated by screen etc. The higher concentration of oil and grease are sometimes observed in the waste water.
The industrial waste is of two types:
(1) Solid wastes and
(2) Liquid wastes.
Solid wastes:
The greatest portion of factory is wood, cardboard and paper etc. which occurs as a result of packing operations. In case of chemical industry waste material consists of plastics, rubber, textile, polymers and organics of many descriptions. In case of metal industry the waste is unused metal, metal oxide and other compounds.
Liquid wastes:
Liquid wastes may be organic material both fluids and torry or viscous or a class which can be called contaminated waters i.e., contaminant may be incompatible with biological treatment due to toxicity.
Liquids such as waste solvents and waste oils are highly combustible and can be treated as liquid fuel source. From industry large quantities of sludge’s are produced. All sludge’s have high water content in the range 90-95%. They give odour and contaminate the ground water.
Sludge’s are of four types:
(1) Flocculent Sludge’s:
From primary sedimentation of effluent such as paper mill etc.
(2) Biological Sludge’s:
From the secondary sedimentation of biological treatment processes.
(3) Chemical Sludge’s:
From neutralization and precipitation processes.
(4) Oil and hydrocarbon Sludge’s:
From mineral and petrochemical industries.
