1. Introduction

Solid wastes generated from industrial sources are heterogeneous ranging from inert inorganics as in those produced in mining, collieries, to organics from those producing basic consumer products, and may include even hazardous wastes as in nuclear industry.

Waste products from industry may get recycled and reused in the same industry or maybe a source of raw material for another industry. At times, a by-product may not find a ready market within a short distance. Consumers located farther away may find it uneconomic due to transport cost and taxes. In such cases, concessions (tax incentive and benefits, reduction in transport costs) could be granted when the secondary materials are being reused.

The problem of disposal of solid wastes varies from industry to industry and each case need be studied separately. The wastes can be grouped as i) biodegradable, ii) non-biodegradable, and iii) hazardous.

2. Biodegradable Wastes

2.1. Fruit Processing: These industries are seasonal and the wastes produced are organic in nature. The simplest method of disposal will be to feed it to cattle. It could also be added to municipal refuse or such other low moisture, biodegradable material for composting.

2.2. Slaughterhouse Waste: In a slaughterhouse, wastes are produced in every operation, most of which are reused. A number of useful products such as glue, gelatin, glycerine, trypsin, etc. are produced which have a ready market.

2.3. Cotton Ginning: In the ginning process the cotton fibre is separated from foreign matter and seed. Some cotton fibre remains in the waste when the ginned cotton is further processed through a lint cleaner, some small pieces of cotton occur as waste. The total waste thus produced varies from 5% in handpicking to 20% in machine scrapped, of the material processed.

2.4. Textile Mills: Cotton textile mills produce a large amount of cotton dust in blowrooms to the extent of about 20-50 tonnes/year/25000 spindles. The textile mills in India produce about 30,000-33,000 tonnes/year of this waste; 20% of which is produced in Bombay and 15% each in Ahmedabad and Coimbatore. The waste essentially consists of unrecoverable cotton fibres and broken cotton seed coats. Normally this waste is disposed of along with other sweepings from mill or used as fuel in boilers or sometimes a portion is used as a cheap filling in quilt blankets

3. Non-biodegradable Wastes

3.1. Colliery Wastes: Colliery wastes include coal and stone, timber and metal scrap, rejected brattice and belting, sludges and other miscellaneous wastes. Heaps of these wastes start burning causing air pollution ($CO, H_2S, SO_2$) and Ieachates from them may cause water pollution. The wastes can be disposed of by emplacement and utilisation. Emplacement involves disposition of material elsewhere away from the mine. Utilisation involves fluidized combustion process and burning; spoil material can be used as a road base and embankments.

3.2. Solid Wastes from Refineries: Crude oil contains some basic substance and water (BS & W) constituting a mixture of water, iron, rust, iron sulphide, clay, etc. produced with the crude oil or accumulated during transit of crude oil. Wastewater from refineries contains metal ions such as Fe, Al, Cu and Mg from corrosion of refinery equipment, chemicals used in treating cooling water, salts in intake water and chemicals used in processing.

Solid wastes from refineries can be grouped as:

• i) inert dry solids - trash, silt, spent catalysts
• ii) combustible dry solids - trash, waste paper, scrap lumber
• iii) sludge from water softner and sanitary sludge
• iv) sludge containing oil such as spent clays
• v) sludge containing oil, water and solids - from water separator bottoms

Treatment and Disposal:

When the sludges containing solids and water come in contact with oily water, solids get coated with oil. Hence it is desirable to avoid mixing of the two. The sludge should not be allowed to flow in oily water drains but dewatered by gravity in sludge thickeners or pond. The sludge may also be reused for neutralizing some wastes or in cement manufacture, etc.

The first step consists of gravity settling in a continuous operating thickner or in a batch opt-rated hopper bottom settling tank. In the case of wastes containing solids, oil as well as water, centrifugation is necessary.

Before the sludge is sent to the centrifuges, it is screened and then subjected-to primary centrifugation in scroll type centrifuges. After the process, the oil water mixture contains about 5-10% solids and is subjected to secondary centrifugation. Secondary centrifugation can be carried out either by using a basket type or a disc type unit. The basket centrifuge is used in intermittant method while disc type is used for continuous operation. The solid cake obtained after centrifugation is taken to the landfill site or incinerated.

In the case of sludges which do not contain oil, filtration is carried out either on sand beds or in filter press in vacuum filtration. When the sludge contains materials which tend to clog or blind the filter medium precoat filtration is used. The sludges after filtration can be incinerated or dumped on landfill sites.

3.3. Steel Plants: Blast furnaces produce hot metal for steel making and pig iron. The amount of slag produced is about half a tonne per tonne of pig iron produced. A large amount of dust is also produced from the blast furnace. In the basic oxygen furnace, approximately 22 kg of dust is produced per tonne of steel. The electric furnace method of steel making uses recyclable scrap and generates large quantities of dust.

Part of the wastes is recycled and reused in the following ways:

• i) Air cooled slag is produced in pits or when slag is allowed to flow down an embankment. It can be used as aggregate for portland cement concrete. It can also be used as aggregate for road construction, rail-road ballast, and roofing material.
• ii) When the slag is cooled in a controlled quantity of water a porous or light product is obtained which is crushed, graded and used as lightweight aggregate for concrete. It can also be used as a good filling material for insulation purposes.
• iii) When chilled in large quantity of water, granulated slag (slag sand) is obtained which is used for slag cement manufacture and as glass sand for manufacture of glass.

3.4. Thermal Power Plants: Most of the thermal power plants in India used lump coal in the boilers and were disposing coal residue as bottom ash commonly referred to as furnace clinker. Pulverised coal has now replaced lump coal which produces fine residue known as fly ash.

The fly ash generated from thermal power plants would need about $0.035 m^3$ of dumping space per tonne and its transportation to such sites will add to the cost. The first known attempt at using fly ash was as pozzolona in mass concrete constructions, e.g., in Rihand dam.

3.5. Lead-Zinc Industry: The solid waste produced from these industries consists of i) dust from lead blast furnace and ii) slag. Dust has to be collected from hoods placed over sinter machine furnaces and other equipment. The sludge contains Zn, Pb, Cd, As & CN and hence care should be taken during its disposal.

Presently most of the slag produced is stored in dumps. These dumps contain valuable ingredients. In Belgium, slags from lead are reduced to give copper, silver matts, iron slags and zinc and lead oxide which is used for byproduct recovery such as germinium in the semiconductor industry.

3.6. Paper Industry: Solid wastes are produced from the following sources:

• i) Boiler cinder
• ii) Chip screen dust
• iii) lime mud from soda recovery plant
• iv) Sludge from bleach making plant

The boiler cinder is used for landfilling and brick making. The chip screen dust has high fuel value and hence is used as an auxiliary fuel. The lime mud can be used for the manufacture of portland cement, provided the caustic alkali is reduced to a minimum by use of proper washing and dewatering system. The sludge can also be used in certain ceramic industries.

3.7. Aluminium Industry: Aluminium is produced by Bayers process from bauxite or from secondary aluminium. Secondary aluminium is produced by remelting scraps and is mainly used for making alloys for foundry work as well as aluminium ingots.

• i) Red mud: In the Bayers process red mud, the leached residue of bauxite is produced.
• ii) Waste from secondary aluminium industry: Maximum amount of waste is produced from the pot roor's where the electrolytic cells are housed and remelt and cast into ingots.

These wastes are generally recycled. A process has been developed by National Metallurgical Laboratory, Jamshedpur for recovery of $AI_2O_3$ from red mud.